Mantle amphibole trace-element and isotopic signatures trace multiple metasomatic episodes in...

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Lithos 75 (2004) 141–171

Mantle amphibole trace-element and isotopic signatures trace

multiple metasomatic episodes in lithospheric mantle,

western Victoria, Australia

William Powella,*, Ming Zhanga, Suzanne Y. O’Reillya, Massimo Tiepolob

aGEMOC ARC National Key Centre, Department of Earth and Planetary Sciences, Macquarie University, Sydney NSW 2109, AustraliabC.N.R.-Istituto di Geoscienze e Georisorse-Pavia, Via Ferrata 1, I-27100 Pavia, Italy

Received 11 February 2003; received in revised form 19 September 2003; accepted 18 December 2003

Available online 12 April 2004

Abstract

Peridotite xenoliths from western Victoria, Australia, contain varying proportions of amphibole (F apatite). The xenoliths

also exhibit varying degrees of metasomatic enrichment as indicated by the rare-earth element (REE) patterns of clinopyroxene

and amphibole analysed in situ by laser-ablation microprobe ICP–MS and by whole-rock trace-element patterns. Trace-element

patterns allow the samples to be divided into three groups. Group A is characterised by flat REE patterns for whole-rock,

clinopyroxene and amphibole, and small negative or absent high-field-strength element (HFSE) anomalies; Group B features

high levels of light-REE enrichment, pronounced negative HFSE anomalies and low Ti/Eu in whole-rock trace-element patterns;

and Group C shows positive HFSE anomalies in whole-rock patterns. The different trace-element patterns are consistent with

metasomatism by different agents or combinations of agents, which have acted upon the already modally metasomatised sub-

continental lithospheric mantle (SCLM) beneath western Victoria. Geochemical signatures suggest that Group A samples have

undergone silicate melt metasomatism, Group B samples have undergone metasomatism by a carbonate-rich or carbonatitic fluid

and Group C samples may record metasomatism by a fractionated silicate fluid with hydrous component.

D 2004 Elsevier B.V. All rights reserved.

Keywords: Mantle metasomatism; Mantle amphibole; Lithospheric mantle composition; Carbonatite metasomatism; Mantle fluids

1. Introduction the appropriate stability field), amphibole, phlogopite,

Metasomatic effects have been widely recognised

and described in mantle samples, and can be broadly

divided into two types (e.g., Dawson, 1980). Modal

metasomatism adds phases to the assemblage: Typical

examples of phases formed during metasomatism of

spinel lherzolites include clinopyroxene, garnet (in

0024-4937/$ - see front matter D 2004 Elsevier B.V. All rights reserved.

doi:10.1016/j.lithos.2003.12.017

* Corresponding author. Fax: +61-2-9850-8943.

E-mail address: [email protected] (W. Powell).

more rarely apatite, sulfides, carbonate and oxides

(e.g., Gregoire et al., 2000a). Cryptic metasomatism

affects the trace-element abundances of the phases

present by interaction of the wall-rock and metaso-

matic fluid, but no new minerals are formed. Modal

metasomatism is obvious due to changes in whole-

rock major- and trace-element geochemistry, petro-

graphic evidence of reactions or new mineral growth,

and the presence of metasomatic minerals (e.g.,

Gregoire et al., 2000b; Moine et al., 2001; O’Reilly

W. Powell et al. / Lithos 75 (2004) 141–171142

and Griffin, 1988; O’Reilly et al., 1991). The meta-

somatic signatures of cryptic metasomatism are more

subtle, and are evidenced by the trace-element and

isotopic characteristics of the various phases present.

Such signatures reveal much about the nature of the

metasomatic agent, but can be complicated by over-

printing of multiple episodes of cryptic and/or modal

metasomatism.

Clinopyroxene commonly is the main host phase

for trace elements in the four-phase mantle assem-

blage olivine–orthopyroxene–clinopyroxene–spinel

(in the spinel lherzolite field), which makes it ideal

for identifying signatures of various metasomatic

agents (e.g., Xu et al., 1999). In a mantle volume

affected by modal metasomatism, however, new

phases provide more residence sites for trace ele-

ments, dramatically changing the whole-rock trace-

element budget (O’Reilly et al., 1991).

In order to identify specific metasomatic agents that

have affected the lithospheric mantle beneath western

Victoria, Australia, and to further study the relation-

ship between amphibole structure and geochemistry, a

new suite of modally metasomatised xenoliths contain-

ing varying amounts of amphiboleF apatiteFmica

has been collected from the Gnotuk and Bullenmerri

maars. Previous studies of xenoliths from these and

other xenolith localities in western Victoria have de-

scribed several metasomatic events with distinct geo-

chemical characteristics (e.g., Frey and Green, 1974;

Griffin et al., 1988; Norman, 1998; O’Reilly and

Griffin, 1988; Yaxley et al., 1991). The samples in this

study present an opportunity to identify the effects of

multiple metasomatic events, and to attempt to unravel

the metasomatic signatures imparted by each of the

events. Identification of metasomatic signatures in

these xenoliths, where the metasomatism is extensive,

can also assist in the identification of metasomatic

agents in mantle domains where metasomatic effects

are weaker.

In addition to whole-rock major- and trace-ele-

ment data, this study presents the first trace-element

data for coexisting clinopyroxene, amphibole, and

apatite from this locality, determined in situ by

laser-ablation ICP–MS with mineral isotopic data

(Nd, Sr) for the same xenoliths. In addition, amphi-

bole crystal chemistry parameters have been deter-

mined by structural refinement (SREF) analysis. This

integrated data set has identified three groups of

xenoliths with differing petrogenetic histories, which

record a history of interaction with different types of

metasomatic agents.

2. Geological setting and previous work

Mantle xenoliths are known from a large number

of localities in the Newer Volcanic Province of west-

ern Victoria. Previous studies document a variety of

xenolith types, many of which record episodes of both

cryptic and modal metasomatism (e.g., Frey et al.,

1978; Griffin et al., 1984; O’Reilly and Griffin, 1988;

Yaxley et al., 1998). Mantle-derived xenoliths used in

this study are from Lakes Bullenmerri and Gnotuk,

Quaternary maars of the Newer Volcanic Province

located adjacent to one another ( < 1 km apart). The

close proximity of the localities and similarities be-

tween the xenoliths suggest a similar mantle domain

was sampled at both sites, so xenoliths from both

localities are considered together here. The suite of

xenoliths present at these localities is compositionally

diverse and covers the whole depth interval repre-

sented by the spinel lherzolite field (i.e., f 24–55

km, O’Reilly and Griffin, 1985). Rock types include

spinel lherzolites and harzburgites, garnet-bearing

granulites and pyroxenites, ultramafic cumulates and

composite xenoliths (Griffin et al., 1984). Samples

used in this study were chosen for the presence of

modal metasomatic phases, particularly amphibole,

and samples large enough to allow whole-rock tech-

niques and mineral separation were favoured.

Frey and Green (1974) studied six samples from

five localities (none from Bullenmerri or Gnotuk),

presenting their major element and modal composi-

tions, and rare-earth element (REE) abundances for

whole-rocks and mineral separates. They confirmed

the upper-mantle origin of the xenoliths, and conclud-

ed that their formation was a result of mixing between

a residue after extraction of basaltic melt from prim-

itive mantle (which they termed Component A) and a

highly fractionated melt phase enriched in K, Ti, P,

light-REE, Th and U produced by small amounts of

partial melting (Component B).

Griffin et al. (1984) presented mineral and whole-

rock major element compositions for xenoliths from

Bullenmerri and Gnotuk. As part of this study pres-

sure and temperature estimates for garnet-bearing

W. Powell et al. / Lithos 75 (2004) 141–171 143

xenoliths (websterites and pyroxenites) were com-

bined with data from the literature to define the South

Eastern Australia geotherm. Calculated temperatures

for spinel lherzolites indicated that both hydrous and

anhydrous xenoliths were entrained over approxi-

mately 30–50 km depth with no observable spatial

pattern.

O’Reilly and Griffin (1988) presented petrologic

data and major element data for whole-rocks and

minerals, and trace-element data for whole-rocks from

Bullenmerri and Gnotuk. They emphasised the com-

plexity of the metasomatic process, and the importance

of crystal/fluid partitioning and crystal chemistry of

primary and metasomatic phases on its effects. Three

types of metasomatism were recognised in the perido-

tite xenoliths: cryptic metasomatism recorded in the

anhydrous samples; amphiboleFmica metasomatism;

and apatite metasomatism thought to be independent

of the previous amphiboleFmica addition. Griffin et

al. (1988) presented isotopic data for these lherzolite

xenoliths as well as two distinct suites of Al-augite

pyroxenite xenoliths (whole-rocks and amphibole,

clinopyroxene and apatite separates), providing further

constraints on the timing of the metasomatism and

the nature of the metasomatising agents. The fluids

that led to amphiboleFmica addition to the lherzo-

lite wall-rocks were isotopically similar (low eNd andhigh 87Sr/86Sr) to the melts which crystallised in the

mantle to form the pyroxenites and metapyroxenites

(which reequilibrated to garnet-bearing assemb-

lages), probably around 300–500 Ma ago. The fluid

associated with the apatite metasomatism is isotopi-

cally distinct from that of the amphiboleFmica meta-

somatism (although it too may have introduced new

amphiboleFmica), and is inferred by Griffin et al.

(1988) to represent a younger, distinct metasomatic

event which wiped out the previous Sr and Nd isotopic

signature. A young metasomatic event formed wehr-

litic (Type II of Frey and Prinz, 1978) veins, with Sr

and Nd isotopic compositions overlapping the range

for the Newer Basalts (4.5 Ma to recent) and the Older

Basalts Province (60–38 Ma, Day, 1988).

Stolz and Davies (1988) presented Sr, Nd and Pb

isotopic abundances for whole-rock and batched cli-

nopyroxene, amphibole and mica separated from

spinel lherzolite xenoliths from Gnotuk and Bullen-

merri. They used the isotopic data, along with whole-

rock major- and trace-element abundances to infer

three distinct styles of metasomatism. The first is

represented by a single anhydrous sample with light-

REE enrichment, with a Nd isotopic model age of

f 1.5 Ga. Group 2 spinel lherzolites contain Ti-rich

amphiboleFmica (amphibole TiO2 between 2.79 and

3.99 wt.%), thought to be the result of reaction

between infiltrating alkaline basaltic melt (or possibly

C–O–H fluids derived from alkaline basaltic melt)

and the spinel lherzolite wall-rock. Similarities be-

tween the isotopic composition of the host basalt and

the amphibole (Fmica) suggested the metasomatis-

ing agent may have been related to the Newer Vol-

canics. Group 3 samples contain amphiboleF apatite

thought to be a result of metasomatism by a H2O-rich

fluid < 100 Ma ago.

Eleven xenoliths from Mts Leura and Shadwell

(also part of the Newer Basalt Province in western

Victoria, approximately 5 and 34 km, respectively,

from the localities studied here) were studied by

Yaxley et al. (1991). The samples included lherzolites

and wehrlites selected for their unusual petrographic

characteristics: ten of the xenoliths contained around

2 vol.% apatite, and the majority were free of

orthopyroxene. Evidence for interaction with a car-

bonatitic agent included extreme large-ion lithophile

element (LILE) enrichment with low Ti abundance

(in whole-rocks), presence of apatite, whole-rock

major-element chemistry indicative of equilibration

with a Ca-rich Al2O3-poor phase, and petrographic

evidence of orthopyroxene replacement by jadeitic

clinopyroxene and forsterite (which had previously

been proposed as a consequence of carbonatitic

metasomatism by Green and Wallace, 1988). Sm–

Nd and Rb–Sr isotopic data for whole-rock xenoliths

and apatite and clinopyroxene separates was used as

evidence for a petrogenetic link between the carbo-

natitic metasomatic event and the host volcanics, and

thus its recent occurrence.

Major- and trace-element data for olivine, ortho-

pyroxene, spinel and clinopyroxene in five anhydrous

(apart from a single grain of pargasitic amphibole in

one sample) lherzolite xenoliths from Mt. Shadwell

were presented by Norman (1998). Depletions in trace

elements in diopside and increase in Ni and Ni/Co in

olivine with increasing mg were considered to be

the result of progressive extraction of partial melt.

Subsequent cryptic metasomatism was invoked to

account for clinopyroxene enriched in light-REE,


Th, U, Pb, Sr and Nd, although specific metasomatic

agents were not identified.

Samples from Mt. Noorat, Lake Bullenmerri and

the Anakies (Mt. Noorat and the Anakies are 17 and

110 km from Bullenmerri, respectively) were used by

Yaxley et al. (1998), in addition to the samples used in

their previous study (Yaxley et al., 1991). Samples

were selected on the basis that they contained modal

evidence of inferred carbonatite metasomatism, in

order to characterise the metasomatic process they

record (32 samples in total, containing up to 2 vol.%

apatite). The metasomatic agent was thought to be a

sodic carbonatite, which acted upon refractory harz-

burgite or lherzolite. The observed whole-rock Al2O3

content could not be explained by this model, howev-

er, so a fugitive melt component was thought to have

been lost from the system, possibly to crystallise

elsewhere as alkali-rich feldspar + Ti-oxide zones at

higher levels in the lithospheric mantle. Relatively

small amounts of carbonatite metasomatism were

suggested as responsible for cryptic metasomatism of

clinopyroxene, thus, the entire range of metasomatic

features of the samples were explained by interaction

with a single metasomatic agent.

A series of noble gas studies of the component

minerals in western Victorian xenoliths was undertaken

by Matsumoto et al. (1997, 1998, 2000). The 1998

study was based on olivine separates from anhydrous

lherzolites from Bullenmerri (one sample), Wiridgil

Hill (three samples, 10 km from Lake Bullenmerri) and

Mt. Gambier (five samples, in South Australia 210 km

from Lake Bullenmerri). Samples for the 2000 study

came from Bullenmerri, Mt. Shadwell and Mt. Leura,

and included whole-rocks and amphibole and olivine

mineral separates from garnet pyroxenite and meta-

somatised lherzolite xenoliths. Matsumoto et al. con-

firmed that at least two metasomatic events are

recorded in the samples: one with MORB-like noble

gases contained in CO2-rich fluid inclusions, and one

with plume-like Ne contained in fluid inclusions in

apatite. It was also suggested that most apatite (i.e.,

the CO2-rich chlorapatite compositions of O’Reilly

and Griffin, 2000) in the western Victorian lithosphe-

ric mantle results from a very young event, distinct

from the earlier events which introduced amphiboleFmica. The apatite-forming event also introduced meta-

somatic clinopyroxene and possibly a new generation

of amphibole.

3. Analytical techniques

Whole-rock major-element analyses were carried

out using X-ray fluorescence on a Siemens SRS-1

instrument at Macquarie University, following the

method of Norrish and Chappell (1977). All samples

were analysed in duplicate. Major elements, except for

FeO, H2O�, H2O

+ and CO2 were determined using

glass fusion discs prepared according to Norrish and

Hutton (1969). Calibration was by means of interna-

tional rock standards, and appropriate international

rock standards or well-calibrated internal standards

were included in each run as unknowns. FeO (ferrous

iron) was determined by HF digestion and titration with

Ceric sulphate. An estimate of the precision for the

XRF analyses is given in O’Reilly andGriffin (1988)—

1-sigma errors for the major elements are <F 1%

relative; Na2O F 1%, and the minor elements (Ti,

Mn) F 2% or better.

Whole-rock trace elements were analysed by solu-

tion ICP–MS. Prepared solutions were analysed using

an Agilent HP4500 instrument featuring a shielded

torch. USGS and JGS rock standards were included in

each batch to check the accuracy of analyses.

Electron microprobe analyses were carried out on a

Cameca SX-50 instrument fitted with five fixed-wave-

length dispersive spectrometers. An accelerating volt-

age of 15 keV and a sample current of 20 nAwere the

usual operating conditions, and the spot sizewas around

2–3 Am. Count times were 10 s for peaks and 5 s for

backgrounds on either side of the peak and corrections

were by the method of Pouchou and Pichoir (1984).

Trace-element abundances in clinopyroxene, am-

phibole and apatite were determined in situ in 200-

Am-thick polished sections using laser-ablation ICP–

MS. The laser system used was a Continuum Surelite

I-20, Q-switched Nd/YAG laser with a spot size of

around 50 Am. The ablated material was carried by an

argon–helium mix to the ICP–MS, which was the

HP4500 instrument used for solution analyses men-

tioned above. CaO determined by electron microprobe

was used as the internal standard, and the NIST 610 or

NIST 612 glass was used for calibration and drift

correction. Data reduction was performed using GLIT-

TER (van Achterbergh et al., 2001). More information

on operating conditions of the laser system is avail-

able on the analytical methods section of the GEMOC

website (http://www.es.mq.edu.au/gemoc/).

http:\\www.es.mq.edu.au\gemoc\

Table 1

Characteristics of samples in the three groups discussed in this

study. The geochemical differences are inferred to be a result of

interaction with different metasomatic agents: Group A silicate,

Group B carbonatitic. The petrogenetic affinities of the Group C


Modal abundances were determined by least-

squares fit of the electron-probe data to the XRF-

determined major-element data, and where trace-ele-

ment data are normalised the primitive mantle values of

McDonough and Sun (1995) are used, unless otherwise

stated.

Amphibole structure refinement was carried out at

the C.N.R.-Istituto di Geoscienze e Georisorse-

Sezione di Pavia. One amphibole crystal of suitable

size for X-ray single-crystal structure refinement

(SREF) was extracted from each sample by hand-

picking after grinding. The quality of the crystals

and the absence of inclusions were checked by

microscopy and by the profile of selected Bragg

reflections. The procedures for X-ray analysis, data

collection and refinement are reported in Oberti et

al. (1992).

Methods for the Sr and Nd isotopic analyses of

clinopyroxene and amphibole separates are given in

Appendix A.
samples are less clear
Characteristic Group A Group B Group C

Whole-rock

mg

0.888–0.905 0.888–0.911

(GN-991-0.880,

GN-998-0.852)

0.897,

0.886

Modal

amphibole

< 0.5–6.8

(BM9911–

12.1)

7.2–17.9

(GN-999-2.1,

GN-991-32.5)

17.1, 17.6

Amphibole/

Cpx ratio

0.03–1.39 1.07–5.25 2.8, 4.6

Whole-rock

(La/Yb)N

1.2–7.7 8.2–85.2 10.6, 7.0

Whole-rock

(Ti/Eu)

1935–5216 290–1693 5410,

2670

Apatitea No Yes No

Whole-rock

P2O5

< 0.01–0.03

(av. 0.01)

0.01–0.52

(av. 0.15)

0.03, 0.03

Zr, Hf

anomaly

Negative Strong Negative Positive

(Hf/Sm)N 0.63–1.07 0.026–0.287 3.09, 2.20

Nb, Ta

anomaly

Negative or

absent

Strong Negative Positive

(Ta/La)N 0.109–1.418 0.042–0.742 3.81–9.05

eNd 4.9 � 1.6–0.7 –

Sample

numbers

BM-992, BM-994,

BM-995, BM-996,

BM-9911,

GN-993, GN-995,

GN-9910,

GN-9911

BM-998, BM-999;

GN-991, GN-994,

GN-996, GN-998,

GN-999 (BM-998

and GN-998 are

wehrlites)

BM-993,

GN-992

a High whole-rock P2O5 is inferred to indicate the presence of

apatite, even if it has not been identified in thin section. mg=Mg/

(Mg+ Fetot).

4. Rock types

The selected suite includes 18 xenoliths containing

0.1–32.5 vol.% amphibole. Most of the samples are

peridotites that contain amphibole + orthopyroxene +

clinopyroxene + olivineF spinel. Two samples are

orthopyroxene-free amphibole – spinel wehrlites

(GN-998 and BM-998). Apatite has been observed

in thin section in four of the samples, and phlogopite

is present in one of the wehrlites (GN-998). Whole-

rock mg (mg =Mg/(Mg + Fetot)) and cr (cr =Cr/

(Cr +Al)) in chrome-diopside and low Ti contents in

the whole-rock and silicate phases indicate the xen-

oliths are all mantle wall-rocks (Type I according to the

classification of Frey and Prinz, 1978). The wehrlite

GN-998 has relatively low whole-rock mg (0.852),

high TiO2 (0.18 wt.%) and Na2O (1.05 wt.%) relative

to the other samples, but still lies within the composi-

tional range of Type I xenoliths. Sample GN-991 has

the highest modal amphibole content (32.5 vol.%),

high pyroxene content (clinopyroxene: 18.2 vol.%,

orthopyroxene: 19.4 vol.%) and correspondingly low

olivine content (28.8 vol.%). The modal proportion of

amphibole in this sample is variable on the hand-

specimen scale. This study used the most amphibole-

and pyroxene-rich part.

The samples are divided into three groups (A, B and

C) according to systematic differences in whole-rock

and mineral major- and trace-element geochemistry,

petrography and modal abundances. The differences

are interpreted to have petrogenetic significance (see

below) and reflect the influence of different metaso-

matic agents, thus the samples are discussed here

according to their classification as Group A, B or C.

The characteristics of the groups are given in Table 1,

and they will be discussed in more detail in the

following pages.

Samples in Groups A and B vary from slightly to

strongly foliated. The two samples in Group C contain

a foliation defined primarily by large (approximately


3� 5 mm) clusters of amphibole grains, and also by

tabular olivine and orthopyroxene in the matrix.

Amphibole shows diverse microstructures. In all

groups, amphibole is commonly found closely associ-

ated with spinel, which is typically embayed. Group A

samples have modal spinel >1 vol.% (with one excep-

tion) and modal amphibole < 7 vol.% (with one excep-

tion) resulting in high spinel/amphibole ratios relative

to the other groups. Amphibole is found as small grains

mantling spinel and rarely as strings of small grains. In

Group B samples modal spinel is low, generally < 0.6

vol.% and modal amphibole is generally >7 vol.% so

spinel/amphibole ratios are low. Amphibole grains are

relatively large, sometimes present as clusters of grains

f 5 mm long, although in most cases, trails of smaller

grains are also present. The clusters of amphibole

grains commonly contain skeletal relict grains of spinel

in their centre. In some of the Group B samples, am-

phiboles are partially or completely surrounded by

clear, glassy patches. In Group C samples, amphibole

is present as elongate clusters of grains (up to f 8� 4

mm), which contain relict spinel, and as strings of

smaller grains in the matrix. The elongate clusters of

amphiboles in these samples define a strong foliation,

along with tabular matrix olivine and orthopyroxene.

Apatite is observed in four of the samples in Group

B. In three of the samples (BM-998, BM-999 and GN-

996), it is found as disseminated polygonal grains,

which tend to be grey-black to almost opaque due to

the high abundance of fluid inclusions. In sample GN-

994 apatite is found in a 1–4-mm-wide cross-cutting

vein interspersed with bright green chrome diopside,

and rarely as dispersed polygonal grains located away

from the vein. In this sample, the apatite tends toward

light pink to brown colours. In all cases apatite has a

cloudy or dusty appearance which is attributed to

abundant fine CO2-rich inclusions (Griffin et al.,

1984; O’Reilly and Griffin, 1988; O’Reilly and Grif-

fin, 2000; Yaxley et al., 1998). No apatite has been

identified in any of the Group A or C samples.

A small amount of phlogopite ( < 0.5 vol.%) occurs

in a cluster of amphibole grains in one of the wehrlitic

samples (GN-998, which is in Group B).

Trails of fluid inclusions are commonly found

cross-cutting the assemblage in all three groups. Fluid

inclusions are generally < 50 Am across with negative

crystal shapes, and rarely contain bubbles. Small

sulfide grains may be associated with these trails.

The fluid inclusions are CO2 rich, although evidence

for a significant hydrous component (10%) and traces

of other gases has been detected (Andersen et al.,

1984). Characteristics of the fluid inclusions have

been presented in more detail in Andersen et al.

(1984), Griffin et al. (1984) and O’Reilly et al. (1990).

Electron probe analyses of cores and rims of

multiple grains of each phase indicate both inter-

and intra-grain homogeneity within samples. Strong

systematic correlations are also found between mg in

whole-rocks and constituent olivine, orthopyroxene,

clinopyroxene and amphibole, indicating that they

have undergone major-element reequilibration subse-

quent to metasomatism. Correlations in trace-element

ratios between clinopyroxene and amphibole (e.g., La/

Yb, Ti/Eu) indicate trace-element equilibrium. Textur-

al disequilibrium indicated by glassy patches around

amphibole in some samples is taken to be a result of

decompression melting associated with sampling by

the host basalt, which has apparently had little effect

on the amphibole composition apart from that imme-

diately adjacent to the rim.

Equilibrium temperatures for the xenoliths were

calculated using the geothermometers of Sachtleben

and Seck (SS, 1981) and Witt-Eickschen and Seck

(WES, 1991). Results from the two methods are

broadly similar, although temperatures could not be

calculated for the wehrlites, as orthopyroxene is

missing from the assemblage in these samples. Group

A xenoliths give equilibration temperatures ranging

from 840 to 990 jC (SS method; WES method: 830–

925 jC), Group B xenoliths range from 805 to 870 jC(SS method; WES: 820–870 jC). The Group C

xenoliths lie at different temperatures: BM-993 at

925 jC (SS; WES: 890 jC), sample GN-992 at 860

jC (SS; WES: 810 jC). By reference to the SEA

geotherm (O’Reilly and Griffin, 1985), these temper-

atures indicate derivation from approximately 30–42

km (Group A) and 27–32 km (Group B) depth.

5. Geochemical characteristics

5.1. Whole-rock major- and trace-element

compositions

Whole-rock mg for most of the xenolith suite lies

in the typical range for young sub-continental litho-


spheric mantle (SCLM) such as the Tecton classifi-

cation of Griffin et al. (1999) (which includes

samples from continental areas with incipient or little

extension, oceanic islands and convergent margin

Fig. 1. Whole-rock major element compositions for the xenolith suite. Prim

by the cross-hairs at 4.55 wt.% Al2O3. The compositional fields for xeno

(small dashes), Proton (large dashes) and Tecton (solid line), as shown in

settings), with all but one sample in the range

0.880–0.911 (the exception is GN-998 from Group

B, considered previously, with mg = 0.852). When

plotted against Al2O3 (an index of depletion or

itive mantle composition of McDonough and Sun (1995) is shown

lith groups defined by Griffin et al. (1999) are as follows: Archon

the SiO2–Al2O3 plot. mg is Mg/(Mg+Fetot).


extraction of basaltic components) the major-element

oxides follow the normal trends for SCLM as seen in

large data sets (e.g., Griffin et al., 1999; Maaløe and

Aoki, 1977; Yaxley et al., 1991), with a few notable

exceptions (Fig. 1). Whole-rock TiO2, Na2O and

K2O (not plotted) in Group C samples are higher

than the normal range based on the database of

Fig. 2. Whole-rock trace-element abundances for the xenolith suite, norma

black patterns are those of the group indicated in the top right corner of the

Group A, for example, the grey patterns are those of the Groups B and C

Griffin et al. (1999) and a number of samples also

have CaO, MgO and mg outside the normal trend for

SCLM, but the range is similar to that reported for

xenoliths from western Victoria by O’Reilly and

Griffin (1988). Sample GN-991, with its unusually

high amphibole and pyroxene content (and corre-

spondingly low olivine content) has the highest

lised to primitive mantle values of McDonough and Sun (1995). The

plot, and the grey patterns represent the ‘other’ groups: In the plot for

samples.


Al2O3 of the suite (6.17 wt.%), lying above the

range defined by the Tecton classification of Griffin

et al. (1999) and primitive mantle composition

(McDonough and Sun, 1995). The CaO, Na2O and

Cr2O3 contents of this sample are high relative to the

rest of the suite and the Tecton classification and

primitive mantle, and the MgO is low. These char-

acteristics are a result of the high amphibole and

clinopyroxene content. Group A samples tend toward

high SiO2 and Al2O3 (Al2O3: 1.87–3.64 wt.%)

relative to Group B (Al2O3: 0.61–1.86 wt.%, except

sample GN-994 at 3.28) and Group C (Al2O3: 2.73

and 3.00 wt.%), although some overlap is evident.

The major element trends against Al2O3 for Group B

are offset to higher values for Cr2O3, FeO, Na2O and

K2O compared to Group A, and lower values for

MgO and mg.

Whole-rock P2O5, although variable, is consistent-

ly higher in Group B samples compared to the other

groups (Fig. 1). O’Reilly and Griffin (2000) sug-

gested 0.4 wt.% P2O5 in the whole-rock is equivalent

to f 1 vol.% apatite. The strong partitioning of

many trace elements into mantle apatite means that

even very small modal percentages have significant

effects on the whole-rock trace-element budget and

the resultant metasomatic signature (O’Reilly et al.,

1991). As noted above, apatite has been identified in

thin section in four samples from Group B, and the

elevated P2O5 in Group B samples (0.01–0.53 wt.%,

Fig. 3. Amphibole major element compositions.

average: 0.15) is taken to indicate the presence of

trace amounts of apatite on the hand-specimen scale,

even though it may be absent from a single petro-

graphic section. The presence of apatite is thus taken

to be a characteristic feature of samples in Group B.

Apatite has not been identified in any of the samples

from Group A, and their low P2O5 content ( < 0.03

wt.%) suggests apatite is absent from the assem-

blage. Apatite has not been found in thin sections of

the Group C samples, but the P2O5 content (0.03

wt.% in each sample) suggests trace amounts may be

present.

Group A samples are characterised by relatively

flat whole-rock REE abundance patterns with (La/

Yb)N values from 1.2 to 7.7, and REE concentrations

between f 0.3 and 2 times primitive mantle (Fig. 2).

Patterns are slightly concave upward for La and Ce

relative to the middle REEs. Group B samples have

comparatively steep REE patterns ((La/Yb)N ratios

8.2–85.2) and high levels of light REE enrichment

(LaN: 1.6–82.0). Heavy REE concentrations are

similar to those found in the Group A samples,

although a greater range in concentrations is ob-

served, which reflects variation in modal amphibole

and clinopyroxene. The two samples in Group C

have REE abundance patterns overlapping those of

Groups A and B ((La/Yb)N: 7.0, 10.6; LaN: 6.2, 7.1).

In the extended trace-element patterns differences

between the groups are found in high-field strength

Axes are in atoms per formula unit (apfu).

Table 2

Selected amphibole structural parameters determined by SREF

Sample Group hT(1)–Oi hT(2)–Oi hM1–Oi hM2–Oi hM3–Oi hM4–Oi hA–OiBM-992 A 1.669 1.636 2.072 2.052 2.057 2.489 2.915

BM-994 A 1.671 1.635 2.070 2.049 2.058 2.492 2.914

BM-995 A 1.670 1.636 2.071 2.052 2.055 2.486 2.911

BM-996 A 1.671 1.635 2.072 2.052 2.057 2.486 2.914

BM-9911 A 1.671 1.635 2.072 2.107 2.052 2.488 2.919

GN-993 A 1.673 1.635 2.072 2.053 2.057 2.490 2.922

GN-995 A 1.672 1.635 2.072 2.058 2.052 2.488 2.913

GN-9910 A 1.670 1.635 2.073 2.054 2.059 2.489 2.919

BM-998 B 1.662 1.636 2.072 2.065 2.062 2.505 2.917

BM-999 B 1.668 1.635 2.073 2.060 2.060 2.497 2.918

GN-991 B 1.669 1.637 2.074 2.057 2.061 2.496 2.919

GN-994 B 1.670 1.637 2.074 2.059 2.062 2.497 2.923

GN-996 B 1.667 1.636 2.074 2.060 2.061 2.498 2.919

GN-998 B 1.664 1.633 2.073 2.065 2.067 2.517 2.913

GN-999 B 1.667 1.635 2.072 2.059 2.061 2.495 2.917

BM-993 C 1.668 1.636 2.072 2.051 2.065 2.494 2.923

GN-992 C 1.669 1.636 2.073 2.060 2.059 2.499 2.922

Mean interatomic distances are in angstroms. SREF measurements were not made for sample GN-9911.


element (HFSE: Zr, Hf, Nb, Ta, Ti), Th, U and Sr

abundances. Group A samples exhibit small negative

anomalies for Ti ((Ti/Eu)N: 0.53F 0.13), Zr and Hf

((Hf/Sm)N: 0.81F 0.16) and Nb and Ta ((Ta/La)N:

0.74F 0.53). Group B patterns feature large negative

anomalies for Ti ((Ti/Eu)N: 0.11F 0.08), Zr and Hf

((Hf/Sm)N: 0.15F 0.10), and Nb and Ta ((Ta/La)N:

0.36F 0.28), and in most Group B samples, strong

enrichment in Th and U. In contrast, samples in Group

Fig. 4. Clinopyroxene major element compositions

C have positive anomalies for Zr and Hf ((Hf/Sm)N:

2.20, 3.09) and for Nb and Ta ((Ta/La)N: 3.8, 9.0), and

a small negative anomaly for Ti.

5.2. Mineral major- and trace-element compositions

Amphibole crystal-chemical formulae were calcu-

lated on the basis of 24 cations (O, F, Cl) with the A-site

K +Na content in agreement with single crystal struc-

. Axes are in atoms per formula unit (apfu).


ture refinements. According to the classification of

Leake et al. (1997), the amphiboles are pargasites, with

the exception of the amphiboles in thewehrlitic samples

GN-998 and BM-998 (Group B), which contain Si >6.5

atoms per formula unit (apfu) and are thus shifted

towards edenites. The amphiboles in all groups are

characterised by a significant oxy component at the

O3 site (O3O2�= 0.2–0.5 apfu). Amphibole in Group

A samples has high [4]Al (1.76–1.85 apfu) and [6]Al

Fig. 5. Amphibole trace-element patterns normalised to primitive mantle o

group indicated in the top right corner of the plot, and the grey patterns r

(0.56–0.76 apfu) compared toGroupB (1.27–1.70 and

0.29–0.72 apfu, respectively), and Ti in amphibole

from Group A xenoliths (0.08–0.22 apfu) is generally

higher than in amphibole from Group B (0.03–0.12

apfu) (Fig. 3). Amphiboles in Group C samples have

major element compositions overlapping those in

Groups A and B, with the exception of K, which shows

the highest values in the suite (0.17–0.20 apfu). The

edenites in the wehrlitic samples have the highest Na

f McDonough and Sun (1995). The black patterns are those of the

epresent the ‘other’ groups.

Table 3

Average major- and trace-element abundances for amphibole determined in this study, and for repeat analyses of USGS reference material BCR-

2 over the same time period

Group Amphibole

BM-992 BM-994 BM-995 BM-996 BM-9911

A A A A A

n= 19 n= 11 n= 12 n= 14 n= 9

SiO2 43.01 42.71 43.37 43.17 43.21

TiO2 1.21 1.86 1.75 2.03 1.56

Al2O3 14.32 14.71 14.49 14.52 14.50

Cr2O3 1.52 0.92 1.02 0.89 0.89

FeO 3.43 4.91 3.44 3.82 3.69

MnO 0.04 0.06 0.05 0.04 0.04

MgO 17.30 16.76 17.84 17.59 17.67

CaO 10.91 10.41 11.05 10.95 11.32

Na2O 3.59 3.90 3.77 3.66 3.51

K2O 0.65 0.26 0.21 0.34 0.29

NiO 0.10 0.10 0.11 0.11 0.12

P2O5 n.a. n.a. 0.07 0.02 n.a.

Cl n.a. n.a. 0.03 0.03 0.03

F n.a. n.a. 0.06 0.07 0.07

Total 96.08 96.60 97.24 97.25 96.90

mg 0.900 0.859 0.902 0.891 0.895

n= 6 n= 4 n= 6 n= 6 n= 5

Sc 38.3F 3.7 40.8F 1.7* 46.0F 2.3 47.1F 3.6 54.1F 2.9

Ti 6770F 520 11690F 400* 10560F 720 12140F 530 13450F 860

V 242.9F 8.6 340F 15* 290F 16 264.3F 9.1 384F 12*

Co 27.6F 1.0 47.1F 4.9 29.96F 0.87* 28.12F 0.91 40.0F 1.3*

Ni 614F 41 915F 71 642F 20* 578F 19 832F 27*

Rb 14.56F 0.53 2.83F 0.20* 1.35F 0.15 1.60F 0.10 4.16F 0.18

Sr 108.5F 5.9 425F 15* 105.7F 3.0* 58.1F1.5* 329F 33

Y 15.2F 1.5 19.45F 0.88* 23.0F 1.6 25.4F 2.6 26.2F 2.5

Zr 59.1F 9.2 32.7F 2.6 33.1F1.3* 35.3F 6.7 39.7F 3.7

Nb 40.3F 3.3 6.6F 2.4 33.7F 4.0 43.4F 2.6 45.6F 2.8

Ba 172F 23 76.0F 6.7 113.7F 7.9 257F 36 150F 16

La 9.99F 0.80 8.78F 0.43* 6.33F 0.29 4.41F 0.26 11.3F 1.7

Ce 18.68F 0.49* 27.1F1.2* 12.28F 0.48 7.46F 0.27 22.5F 2.5

Pr 2.117F 0.078 3.15F 0.26 1.620F 0.055 1.096F 0.046 2.72F 0.25

Nd 9.23F 0.43 12.1F1.8 8.03F 0.28* 5.92F 0.39 12.5F 1.0

Sm 2.36F 0.15 2.83F 0.44 2.49F 0.10* 2.16F 0.16 3.50F 0.19

Eu 0.842F 0.037 1.09F 0.14 0.926F 0.032* 0.848F 0.050 1.288F 0.081

Gd 2.74F 0.23 3.28F 0.28* 3.31F 0.20 3.36F 0.38 4.36F 0.30

Dy 2.95F 0.29 3.55F 0.35* 4.09F 0.27 4.43F 0.49 4.84F 0.44

Ho 0.606F 0.060 0.725F 0.063* 0.872F 0.051 0.96F 0.10 0.956F 0.087

Er 1.70F 0.20 2.15F 0.23* 2.47F 0.15 2.87F 0.34 2.86F 0.43

Yb 1.52F 0.14 2.13F 0.20* 2.33F 0.15 2.63F 0.22 2.64F 0.15

Lu 0.202F 0.022 0.298F 0.038* 0.327F 0.030 0.367F 0.040 0.370F 0.052

Hf 1.13F 0.25 0.840F 0.098* 0.786F 0.062 0.86F 0.17 0.78F 0.12

Ta 2.76F 0.39 0.115F 0.042 1.51F 0.15 2.25F 0.45 0.722F 0.088

Th 1.38F 0.13 0.218F 0.041* 0.880F 0.065 0.688F 0.063 1.37F 0.13

U 0.256F 0.021 0.058F 0.018* 0.1938F 0.0097* 0.1046F 0.0066 0.300F 0.021


(continued on next page)

Table 3 (continued)

Group Amphibole

GN-993 GN-995 GN-9910 GN-9911 BM-998

A A A A B

n= 11 n= 11 n= 11 n= 14 n= 12

SiO2 43.49 43.84 43.47 43.09 46.48

TiO2 1.21 1.16 0.78 1.45 0.40

Al2O3 14.07 14.98 14.25 14.83 10.77

Cr2O3 1.58 1.06 1.38 1.08 1.88

FeO 3.25 3.34 3.67 3.32 3.32

MnO 0.03 0.04 0.04 0.05 0.02

MgO 17.71 17.52 17.77 18.13 19.09

CaO 11.48 11.67 10.83 11.36 10.25

Na2O 3.41 3.69 3.53 3.78 4.18

K2O 0.97 0.14 0.96 0.27 0.79

NiO 0.10 0.14 0.10 0.13 0.10

P2O5 0.05 0.04 n.a. n.a. 0.07

Cl 0.03 0.03 0.07 0.03 0.09

F 0.13 0.05 0.10 0.07 0.10

Total 97.51 97.69 96.96 97.58 97.55

mg 0.907 0.903 0.896 0.907 0.911

n= 6 n= 6 n= 6 n= 4 n= 5

Sc 49.6F 2.7 57.4F 3.3 42.3F 2.4* 52.3F 4.6* 36.2F 1.7

Ti 9770F 300* n.a. 4870F 320 8690F 400* 2950F 110*

V 400F 11* 425F 16* 249.3F 9.8* 315F 22* 238F 12

Co 43.8F 2.5 39.9F 1.8* 29.1F1.2* 29.1F1.8* 35.4F 1.4*

Ni 1008F 66 940F 46* 660F 26* 679F 35* 784F 37*

Rb 8.7F 1.3 2.08F 0.21 10.92F 0.60 2.43F 0.22 3.93F 0.18*

Sr 280.5F 7.4* 132.4F 4.2* 401F 20 102.2F 6.4 601F18*

Y 15.12F 0.96 23.62F 0.78 15.7F 1.5 25.4F 2.9* 18.20F 0.62

Zr 64.5F 5.7 18.60F 0.89 72.6F 8.1 26.3F 3.0* 56.6F 4.8

Nb 37.3F 2.1 11.4F 1.2* 39.4F 3.9 48.1F 2.7* 174F 25

Ba 167.7F 6.0 109.9F 6.6 260F 36 128.8F 6.8 487F 41

La 12.32F 0.33* 5.15F 0.21* 18.38F 0.89* 4.18F 0.48 18.94F 0.61

Ce 27.33F 0.73* 10.45F 0.33* 36.4F 1.3 8.08F 0.75 57.8F 2.0*

Pr 3.233F 0.082* 1.30F 0.042* 4.38F 0.20 1.19F 0.13 8.18F 0.27*

Nd 13.08F 0.41* 6.25F 0.22* 17.98F 0.78* 6.18F 0.67 36.7F 2.0

Sm 2.80F 0.11 1.957F 0.075 3.58F 0.21* 1.98F 0.23* 7.40F 0.34

Eu 0.960F 0.041* 0.807F 0.035* 1.122F 0.069 0.765F 0.068* 2.34F 0.12

Gd 2.97F 0.12 2.867F 0.082* 3.45F 0.22 3.03F 0.35* 6.04F 0.27

Dy 3.00F 0.20 4.02F 0.16 3.18F 0.24 4.28F 0.46* 4.20F 0.22*

Ho 0.588F 0.055 0.887F 0.037* 0.608F 0.060 0.985F 0.093* 0.698F 0.037

Er 1.61F 0.11 2.717F 0.082 1.63F 0.14 2.83F 0.36 1.690F 0.084*

Yb 1.387F 0.071 2.567F 0.082 1.38F 0.12* 2.60F 0.41 1.396F 0.077*

Lu 0.183F 0.010 0.358F 0.018* 0.185F 0.021* 0.348F 0.046 0.190F 0.015*

Hf 0.86F 0.14 0.612F 0.030 1.75F 0.35 0.75F 0.13 0.316F 0.038

Ta 1.59F 0.17 0.49F 0.12 1.07F 0.17 2.87F 0.38 1.87F 0.41

Th 1.37F 0.12 0.757F 0.054 1.69F 0.17 0.608F 0.066* 1.02F 0.12

U 0.280F 0.018 0.235F 0.016 0.312F 0.022* 0.138F 0.018* 0.175F 0.027


Table 3 (continued)

Group Amphibole

BM-999 GN-991 GN-994 GN-996 GN-998

B B B B B

n= 12 n= 14 n= 13 n= 15 n= 12

SiO2 44.69 44.55 44.07 45.19 47.32

TiO2 0.24 0.75 0.74 0.25 1.13

Al2O3 12.19 13.27 14.40 12.27 9.30

Cr2O3 2.35 1.00 1.31 2.08 1.32

FeO 3.49 4.08 3.51 3.59 4.90

MnO 0.03 0.07 0.03 0.02 0.06

MgO 18.34 17.96 17.60 17.84 18.41

CaO 10.92 10.81 11.36 10.63 10.15

Na2O 3.55 3.85 3.57 3.72 4.40

K2O 0.92 0.50 0.71 0.89 0.68

NiO 0.09 0.09 0.11 0.10 0.10

P2O5 n.a. n.a. 0.05 0.06 n.a.

Cl 0.04 0.03 0.07 0.05 0.06

F 0.13 0.09 0.11 0.14 0.12

Total 96.98 97.06 97.65 96.84 97.95

mg 0.904 0.887 0.899 0.899 0.870

n= 4 n= 6 n= 6 n= 4 n= 5

Sc 54.8F 1.8* 42.8F 3.9 51.0F 2.0* 50.5F 3.4 32.4F 3.0

Ti 2020F 340 3950F 300 n.a. n.a. n.a.

V 298.9F 9.9* 270F 17 318F 12* 338F 16* 207.5F 7.9*

Co 36.7F 1.3* 37.7F 1.1* 39.8F 1.7* 39.2F 2.1* 36.6F 1.5*

Ni 825F 29* 813F 25* 855F 40* 896F 54* 747F 37*

Rb 4.03F 0.13 7.42F 0.35 12.2F 1.2 4.28F 0.20* 3.72F 0.22

Sr 441F14* 319.4F 8.4* 346F 80 456F 18* 541F 25

Y 18.33F 0.78 14.6F 1.4 17.25F 0.93 11.88F 0.50* 15.6F 1.1

Zr 128F 34 19.13F 0.97 145.3F 8.7 41.3F 1.9* 165.9F 8.2

Nb 81.8F 7.8 0.1090F 0.0088* 38.2F 1.3 35.2F 3.1 106F 7.7

Ba 95.5F 3.2* 202F 20 270F 120 119.3F 9.4 97.3F 4.4

La 16.13F 0.53* 9.72F 0.31 12.5F 2.4 21.08F 0.85* 18.92F 0.64*

Ce 49.5F 2.0 28.7F 2.9 30.7F 7.3 59.4F 2.3* 57.2F 1.8*

Pr 7.10F 0.25* 3.69F 0.52 3.62F 0.94 7.43F 0.30* 7.58F 0.24*

Nd 33.4F 1.1* 16.1F1.5 14.4F 3.6 29.6F 1.1* 32.50F 0.98*

Sm 7.45F 0.30* 2.99F 0.53 3.10F 0.55 5.23F 0.20* 6.58F 0.23*

Eu 2.345F 0.090* 1.21F 0.11 1.13F 0.20 1.668F 0.070* 2.223F 0.078

Gd 6.53F 0.23* 2.80F 0.23 3.27F 0.23 4.10F 0.20* 5.42F 0.25

Dy 4.68F 0.20* 2.82F 0.15 3.37F 0.15* 2.83F 0.17 3.90F 0.18

Ho 0.730F 0.030* 0.555F 0.051 0.658F 0.035 0.475F 0.030* 0.618F 0.035

Er 1.613F 0.088* 1.57F 0.22 1.88F 0.13 1.123F 0.063* 1.473F 0.093

Yb 1.110F 0.060* 1.54F 0.18 1.80F 0.14 0.930F 0.050 1.103F 0.076

Lu 0.140F 0.010* 0.212F 0.019 0.248F 0.018 0.129F 0.010* 0.145F 0.013

Hf 0.41F 0.12 0.537F 0.035 2.98F 0.34 0.470F 0.040* 2.13F 0.19

Ta 0.93F 0.13 0.0078F 0.0027* 2.29F 0.33 0.278F 0.067 4.43F 0.37

Th 1.028F 0.045* 0.605F 0.042 1.34F 0.14 1.478F 0.063* 1.120F 0.069

U 0.178F 0.022 0.1357F 0.0079* 0.290F 0.040 0.300F 0.020* 0.222F 0.017


Table 3 (continued)

Group Amphibole

GN-999 BM-993 GN-992 BCR-2

B C C

n= 11 n= 12 n= 8

SiO2 44.99 43.72 44.63

TiO2 0.51 1.61 0.79

Al2O3 12.50 12.92 13.03

Cr2O3 2.29 1.67 1.46

FeO 3.07 3.80 3.76

MnO 0.03 0.04 0.03

MgO 18.33 17.51 17.75

CaO 10.62 10.55 11.17

Na2O 3.93 3.48 3.62

K2O 0.51 1.09 0.96

NiO 0.13 0.10 0.08

P2O5 n.a. n.a. 0.07

Cl 0.15 n.a. 0.04

F 0.10 n.a. 0.18

Total 97.16 96.51 97.57

mg 0.914 0.891 0.894

n= 6 n= 6 n= 4 n= 19

Sc 78.7F 3.7* 37.8F 2.4 43.3F 1.7* 35.3F 2.8

Ti 3180F 210 9080F 480 5780F 550 16100F 1200

V 316F 12 184.2F 5.4 252.7F 8.8* 407F 23

Co 27.08F 0.82* 27.3F 0.73* 37.7F 1.4* 36.1F 2.4

Ni 666F 25* 568F 16* 776F 36 11.52F 0.92

Rb 4.73F 0.16* 4.85F 0.30 4.08F 0.17 44.3F 3.9

Sr 541F18* 174.6F 5.7 263.0F 8.7* 330F 12

Y 12.63F 0.58 11.85F 0.48 15.05F 0.77 33.3F 3.7

Zr 30.7F 6.5 213F 11 180.3F 8.1* 172F 19

Nb 41.2F 3.2 64.1F 6.3 83.0F 8.3 12.06F 0.57

Ba 535F 98 199F 14 31.7F 2.0 639F 22

La 27.28F 0.92 14.70F 0.68 9.98F 0.40* 24.7F 1.4

Ce 62.0F 1.8* 29.11F 0.77* 28.65F 0.90* 50.3F 2.1

Pr 8.20F 0.29* 3.705F 0.098* 4.13F 0.10* 6.42F 0.27

Nd 34.6F 1.1* 16.13F 0.46* 19.30F 0.70* 28.1F 2.1

Sm 6.48F 0.24* 3.72F 0.11* 4.58F 0.20* 6.39F 0.55

Eu 1.867F 0.082* 1.202F 0.037* 1.600F 0.068* 1.87F 0.14

Gd 5.05F 0.24* 3.48F 0.11* 4.25F 0.20* 6.49F 0.70

Dy 3.05F 0.24 2.757F 0.090* 3.45F 0.30 6.30F 0.72

Ho 0.485F 0.033* 0.480F 0.017* 0.613F 0.036 1.26F 0.15

Er 1.083F 0.082* 1.202F 0.056 1.55F 0.10* 3.52F 0.43

Yb 0.728F 0.080 0.967F 0.036* 1.343F 0.078* 3.44F 0.38

Lu 0.097F 0.018 0.126F 0.010 0.180F 0.014 0.503F 0.068

Hf 0.407F 0.055* 7.13F 0.29 1.81F 0.10* 4.58F 0.58

Ta 0.75F 0.19 6.51F 0.41 2.31F 0.57 0.82F 0.16

Th 3.88F 0.18 1.791F 0.086 0.748F 0.040* 5.86F 0.62

U 1.010F 0.078 0.320F 0.016 0.178F 0.010* 1.56F 0.15

Uncertainties for the trace-element data are the greater of either the standard deviation on the average of several analyses or the propagated

analytical uncertainty (the latter is indicated by an asterisk *). The uncertainty on BCR-2 is the standard deviation on the average. Major and

trace elements are reported in wt.% and ppm, respectively. n.d.: not detected, n.a.: not analysed.



content of the group (1.15–1.21 apfu). Structural re-

finement analysis indicates most amphibole in the suite

has hT(2)–Oi larger than 1.635 A, which suggests the

presence of [4]Al in the T2 site and thus a high

crystallisation temperature (Oberti et al., 1995) (Table

2). Furthermore, the relatively small dimensions of the

M3 site in Group A amphiboles suggests dehydroge-

nation of amphibole in these samples is not balanced

solely by incorporation of Ti into the M1 site, but also

by incorporation of Fe3 + into the M3 site (Zanetti

et al., 1997).

Similar differences between the groups are ob-

served in clinopyroxene major-element chemistry as

for amphibole. The clinopyroxene compositions as a

whole show negative correlations of Ti and Al with

Si, and positive correlations of Cr and Na with Si (Fig.

4). Group A clinopyroxenes have relatively low Si

(1.877–1.932 apfu) and Cr, and high Ti and Al in

comparison to those in Groups B and C. The clino-

pyroxenes in the wehrlitic samples BM-998 and GN-

998 have the highest Si (1.979 and 1.992 apfu,

respectively) and Na contents and among the lowest

Ti and Al. Samples in Group C have clinopyroxene

compositions overlapping those of Groups A and B.

Apatite was analysed in samples BM-998 and GN-

994. In sample BM-998 apatite occurs as disseminat-

ed grains, whereas in sample GN-994, it is found in an

apatite–clinopyroxene vein. The apatites contain 1.65

wt.% Cl in BM-998 and 2.09 wt.% in GN-994 with

low F contents (0.22 and 0.20 wt.%), and are thus

hydroxychlorapatites (corresponding to the Type A of

O’Reilly and Griffin, 2000).

Rare-earth-element patterns for amphibole in

Group A samples are relatively flat at the heavy end

and show progressive enrichment in light REEs with

(Ce/Sm)N: 0.8–2.5 (Fig. 5; Table 3). The REE patterns

of Group B samples are relatively steep ((La/Yb)N:

4.7–25.5), with lower heavy REE abundances and

higher abundances of REEs lighter than Dy than

Group A. Group A samples generally have positive

anomalies for Nb, Ta and Ti, negative anomalies for

Zr, Hf and Sc, and a variable anomaly for Sr. Group B

samples generally have a large negative anomaly for

Ti, and negative and positive anomalies for the pairs

Zr, Hf and Nb, Ta respectively. One exception is

sample GN-991, which has the lowest trace-element

abundances of the Group B samples, and a pronounced

negative anomaly for Nb and Ta. Group C patterns lie

between those of the other groups for most elements,

although there are differences between the two group

C samples. Sample BM-993 has positive anomalies for

both Nb, Ta and Zr, Hf pairs, and (Nb/Ta)N and (Zr/

Hf)N ratios < 1. Sample GN-992 has a positive anom-

aly for Nb and Ta, slightly positive Zr and negative Hf

anomalies, and (Nb/Ta)N and (Zr/Hf)N >1.

Clinopyroxene REE patterns for Group A are rela-

tively flat, although a progressive increase in light-REE

enrichment similar to that observed for the amphibole

is present ((Ce/Sm) N: 0.9–2.9, Fig. 6, Table 4). Group

B patterns overlap those of Group A for the heavy-

REEs, and are higher in REEs lighter than Dy. Group A

clinopyroxenes generally have negative anomalies for

Nb and Ta, Zr and Hf, Ti ((Ti/Eu)N: 0.14 to 0.8), and a

positive anomaly for Sc. Similar anomalies are found in

the Group B patterns, although the negative Ti anomaly

in these samples is larger ((Ti/Eu)N: 0.01 to 0.14).

Sample GN-991 has the lowest trace-element abundan-

ces of any of the Group B samples (as observed for

amphibole), including large negative anomalies for the

HFSE. The Group C samples have trace-element pat-

terns overlapping those of the other groups and like

amphibole, the two samples have different patterns for

some elements. Both samples have negative anomalies

for Nb, Ta and Ti, although the anomalies in sample

GN-993 are larger. Sample BM-993 has a negative

anomaly for Sr ((Sr/Nd)N: 0.44), positive anomaly for

Zr, Hf, and (Zr/Hf)N < 1, while sample GN-992 has no

Sr anomaly, a negative anomaly for Zr and Hf and (Zr/

Hf)N >1.

Mass-balance calculations have been carried out to

reconcile the trace-element characteristics of amphi-

bole and clinopyroxene in Group C samples with their

corresponding whole-rock patterns. Trace-element

contents and modal abundances of amphibole and

clinopyroxene were used to calculate whole-rock

patterns for the two samples, which were then com-

pared with the analytical results (Fig. 7). In both

samples, the whole-rock budget for many trace ele-

ments is hosted mainly by amphibole, as the amphi-

bole mode exceeds that of clinopyroxene by a factor

of >3. For sample BM-993, the calculated and mea-

sured patterns match one another closely for all trace

elements except Sr and the transition elements V, Co,

Ni. The mismatch of the patterns for the transition

metals is due to their residence in phases not included

in the calculation: namely olivine, orthopyroxene and


spinel. The calculated and measured patterns for

sample GN-992 are parallel for most elements, with

the exception of Ta and Hf, and to a lesser extent Zr

and Nb. In order to resolve the problem of the

‘missing’ trace elements, addition of other trace-ele-

ment-bearing phases was modelled. Addition of apa-

tite produces a calculated whole-rock composition

with too high a ratio of light- to heavy-REE, due to

its extremely high (La/Ce)N ratio. Tiny amounts of

Fig. 6. Clinopyroxene trace-element patterns normalised to primitive mantl

group indicated in the top right corner of the plot, and the grey patterns r

ilmenite (found in some xenoliths from Gnotuk and

Bullenmerri, Griffin et al., 1984) could provide the

required HFSE addition to sample GN-992, but far

exceeded the required Ti addition. Addition of small

amounts of amphibole and/or clinopyroxene to the

assemblage raises the trace-element abundance with-

out changing the (La/Yb)N ratio. The addition of

approximately 7% amphibole or 9% clinopyroxene

(or f 3% of each) to sample BM-993 results in

e of McDonough and Sun (1995). The black patterns are those of the

epresent the ‘other’ groups.

Table 4

Average major- and trace-element abundances for clinopyroxene determined in this study

Group Clinopyroxene

BM-992 BM-994 BM-995 BM-996 BM-9911

A A A A A

n= 15 n= 10 n= 12 n= 11 n= 10

SiO2 52.75 51.39 52.93 52.66 52.78

TiO2 0.18 0.70 0.38 0.43 0.33

Al2O3 4.76 6.89 5.54 5.68 5.27

Cr2O3 0.81 0.65 0.68 0.65 0.58

FeO 2.44 3.16 2.45 2.61 2.55

MnO 0.08 0.09 0.08 0.07 0.07

MgO 15.28 14.41 15.38 15.38 15.34

CaO 21.27 19.97 21.46 21.26 21.47

Na2O 1.57 1.89 1.52 1.49 1.44

K2O 0.01 0.01 0.00 0.00 0.01

NiO 0.04 0.05 0.05 0.07 0.05

P2O5 n.a. n.a. 0.05 0.02 n.a.

Cl n.a. n.a. 0.00 0.00 0.00

F n.a. n.a. 0.02 0.03 0.04

Total 99.19 99.22 100.56 100.36 99.93

mg 0.918 0.890 0.918 0.913 0.915

n= 6 n= 4 n= 6 n= 6 n= 5

Sc 55.9F 6.4 70.5F 3.1* 66.4F 4.6 63.0F 6.5 75.0F 2.3*

Ti 1095F 95 5160F 180* 2220F 130 2413F 86 2660F 170

V 162.2F 5.9 288F 14* 201F10 189.3F 6.5 282.7F 8.5*

Co 14.15F 0.63 22.3F 1.0* 15.05F 0.77 14.05F 0.40 19.58F 0.73*

Ni 237F 17 353F 20* 244F 19 220F 12 304F 11*

Rb 0.26F 0.18 n.d. 0.037F 0.021 n.d. n.d.

Sr 55.6F 3.2 169.5F 6.3* 42.7F 1.3* 21.63F 0.69 126.4F 3.6*

Y 10.5F 1.5 22.6F 1.7 16.2F 1.4 16.8F 2.0 16.14F 0.83

Zr 52.6F 9.1 39.5F 2.2* 35.6F 2.2 32.5F 6.6 36.1F 3.2

Nb 0.282F 0.020 0.138F 0.025* 0.38F 0.13 0.426F 0.028 0.300F 0.046

Ba 2.7F 2.4 n.d. 0.171F 0.048* n.d. n.d.

La 6.96F 0.68 9.38F 0.50* 4.45F 0.19 3.03F 0.22 6.84F 0.20*

Ce 13.28F 0.66 25.4F 1.2* 8.63F 0.32 5.53F 0.18 13.94F 0.54

Pr 1.496F 0.059 2.38F 0.16* 1.168F 0.038* 0.800F 0.024* 1.706F 0.064*

Nd 6.25F 0.46 7.73F 0.53* 5.83F 0.21* 4.23F 0.24 7.98F 0.30

Sm 1.62F 0.14 2.13F 0.20* 1.778F 0.075* 1.55F 0.13 2.30F 0.12

Eu 0.565F 0.045 0.825F 0.078* 0.662F 0.025* 0.597F 0.037 0.882F 0.042*

Gd 1.85F 0.25 3.00F 0.28* 2.48F 0.14 2.28F 0.27 2.76F 0.15

Dy 2.07F 0.30 4.05F 0.43* 2.91F 0.20 3.05F 0.42 3.18F 0.26

Ho 0.416F 0.059 0.863F 0.083* 0.619F 0.051 0.657F 0.098 0.644F 0.031*

Er 1.16F 0.18 2.65F 0.29 1.85F 0.20 1.94F 0.27 1.74F 0.21

Yb 1.11F 0.13 2.73F 0.32 1.73F 0.12 1.91F 0.24 1.746F 0.083*

Lu 0.154F 0.022 0.433F 0.078 0.256F 0.025 0.279F 0.040 0.254F 0.020*

Hf 1.01F 0.22 1.20F 0.13* 0.911F 0.091 0.83F 0.15 0.802F 0.055

Ta 0.073F 0.017 n.d. 0.073F 0.015 0.106F 0.023 0.0275F 0.0087

Th 1.03F 0.14 0.533F 0.074 0.743F 0.074 0.577F 0.089 0.924F 0.045

U 0.196F 0.027 0.128F 0.025* 0.174F 0.011 0.0991F 0.0062 0.224F 0.020*


Table 4 (continued)

Group Clinopyroxene

GN-993 GN-995 GN-9910 GN-9911 BM-998

A A A A B

n= 12 n= 12 n= 12 n= 10 n= 20

SiO2 53.19 53.49 53.02 52.52 54.99

TiO2 0.19 0.24 0.15 0.27 0.03

Al2O3 4.76 4.63 5.13 4.76 3.31

Cr2O3 1.05 0.73 0.96 0.64 1.35

FeO 2.32 2.29 2.52 2.39 2.44

MnO 0.05 0.07 0.07 0.04 0.07

MgO 15.72 15.08 15.32 15.69 15.51

CaO 22.09 22.40 20.81 21.99 20.37

Na2O 1.61 1.28 1.76 1.41 2.25

K2O 0.01 0.00 0.00 0.01 0.01

NiO 0.03 0.06 0.05 0.03 0.05

P2O5 0.02 0.03 n.a. n.a. 0.03

Cl 0.00 0.01 0.01 0.00 0.00

F 0.03 0.02 0.02 0.03 0.03

Total 101.06 100.33 99.81 99.79 100.45

mg 0.924 0.921 0.916 0.921 0.919

n= 6 n= 6 n= 6 n= 6 n= 5

Sc 76.8F 2.1* 78.9F 3.2* 64.1F 3.4* 71.2F 5.4* 57.8F 1.7*

Ti 1522F 97 n.a. 914F 87 1680F 120 235F 16

V 253.6F 6.8* 259.4F 9.9* 170.3F 6.4* 201F12* 186.8F 5.8*

Co 22.30F 0.71 18.35F 0.84* 14.85F 0.63* 13.17F 0.72* 17.10F 0.78*

Ni 384F 12* 315F 16* 251.9F 9.5* 217.3F 9.6* 271F14*

Rb 0.090F 0.040* n.d. n.d. n.d. n.d.

Sr 136.9F 7.6 61.4F 1.9* 206F 38 45.8F 3.8 502F 39

Y 11.17F 0.46 13.85F 0.55 12F 1.0 15.0F 1.5* 20.04F 0.66*

Zr 56.3F 6.2 17.22F 0.68 66.2F 4.6 19.6F 5.4 36.2F 2.2

Nb 0.240F 0.052 0.068F 0.016* 0.217F 0.054 0.298F 0.075 0.346F 0.033

Ba 0.56F 0.18* n.d. 0.09F 0.10* n.d. 0.16F 0.13*

La 7.98F 0.37 3.20F 0.14* 13.3F 2.0 2.65F 0.34 13.9F 1.2

Ce 17.58F 0.58 6.37F 0.26 26.6F 3.8 5.15F 0.59 46.1F 3.0

Pr 2.085F 0.092 0.825F 0.039 3.27F 0.34 0.75F 0.11 6.90F 0.35

Nd 8.83F 0.33* 3.97F 0.20 13.13F 0.86 3.95F 0.43 33.5F 1.6*

Sm 1.865F 0.082* 1.260F 0.056* 2.58F 0.16* 1.32F 0.15 7.42F 0.40*

Eu 0.665F 0.037* 0.480F 0.030 0.813F 0.059* 0.485F 0.047* 2.35F 0.15*

Gd 2.17F 0.14 1.778F 0.081 2.48F 0.17 1.87F 0.18 6.20F 0.32*

Dy 2.20F 0.13* 2.467F 0.082* 2.38F 0.23 2.55F 0.27* 4.56F 0.27*

Ho 0.450F 0.023* 0.538F 0.024* 0.462F 0.042 0.557F 0.059* 0.778F 0.049*

Er 1.18F 0.16 1.628F 0.072* 1.28F 0.14* 1.72F 0.21 1.94F 0.16*

Yb 1.125F 0.082 1.600F 0.066 1.20F 0.11 1.63F 0.16* 1.68F 0.15*

Lu 0.158F 0.015* 0.235F 0.012* 0.158F 0.022* 0.233F 0.033* 0.224F 0.025*

Hf 0.80F 0.13 0.548F 0.032* 1.63F 0.20 0.58F 0.11 0.214F 0.044*

Ta 0.041F 0.012 0.0188F 0.0050 0.033F 0.015 0.095F 0.040 0.027F 0.016*

Th 0.917F 0.099 0.517F 0.029 1.353F 0.088 0.420F 0.045* 0.762F 0.060*

U 0.203F 0.028 0.1563F 0.0080* 0.242F 0.029 0.098F 0.016* 0.122F 0.022

(continued on next page)


Table 4 (continued)

Group Clinopyroxene

BM-999 GN-991 GN-994 GN-996 GN-998

B B B B B

n= 10 n= 6 n= 12 n= 12 n= 11

SiO2 54.41 54.33 53.98 54.29 55.12

TiO2 0.02 0.12 0.09 0.02 0.09

Al2O3 2.76 4.51 4.20 3.48 2.57

Cr2O3 1.39 0.62 0.77 1.36 1.03

FeO 2.37 2.59 2.26 2.68 3.67

MnO 0.06 0.09 0.08 0.06 0.09

MgO 15.95 15.11 15.33 14.70 15.01

CaO 21.33 20.84 21.87 20.74 20.82

Na2O 1.59 2.02 1.53 2.05 2.14

K2O 0.00 0.00 0.00 0.00 0.00

NiO 0.05 0.03 0.05 0.03 0.04

P2O5 n.a. n.a. 0.03 0.03 n.a.

Cl 0.00 0.01 0.00 0.00 0.00

F 0.03 0.02 0.02 0.02 0.03

Total 99.98 100.29 100.22 99.47 100.61

mg 0.923 0.912 0.924 0.907 0.879

n= 4 n= 4 n= 3 n= 4 n= 6

Sc 84.9F 2.8* 77.0F 2.8* 73.2F 4.3 82.9F 4.2* 50.6F 4.6

Ti 126F 34 1050F 150 n.a. n.a. n.a.

V 190.4F 8.1 231.5F 7.9* 197F 11* 239F 12* 181.2F 7.6

Co 18.50F 0.88 22.4F 1.5 18.1F1.1* 18.2F 1.0* 17.52F 0.80*

Ni 310F 11* 372F 16 295F 20* 291F19* 253F 14*

Rb n.d. 0.75F 0.14 0.040F 0.040* n.d. n.d.

Sr 374F 12* 249.4F 8.1* 362F 53 371F15* 578F 35

Y 13.65F 0.90 11.13F 0.40* 13.70F 0.58* 11.50F 0.50* 19.6F 1.3

Zr 65F 16 18.3F 1.6 123.7F 5.7* 26F 1.5 99.3F 8.9

Nb 0.127F 0.020* 0.0207F 0.0089* 0.70F 0.11 0.103F 0.020* 0.220F 0.094

Ba n.d. 16.80F 0.99 0.150F 0.085 n.d. 0.120F 0.060

La 10.35F 0.40* 6.90F 0.84 16.5F 1.1 15.30F 0.68* 16.9F 1.1

Ce 33.1F1.4 20.20F 0.76 45.1F 2.1* 44.2F 1.8* 54.6F 2.4

Pr 4.93F 0.28 2.79F 0.14 5.77F 0.23 5.73F 0.20* 7.74F 0.27*

Nd 23.3F 1.3 12.10F 0.43* 24.63F 0.92* 24.4F 1.0* 34.3F 1.5

Sm 5.30F 0.24 2.50F 0.10* 4.80F 0.23* 4.78F 0.29 7.36F 0.34

Eu 1.68F 0.14 0.928F 0.045* 1.463F 0.069* 1.438F 0.073* 2.516F 0.088*

Gd 4.65F 0.37 2.20F 0.10* 4.07F 0.23* 3.73F 0.22 6.32F 0.38

Dy 3.48F 0.36 2.23F 0.10* 3.20F 0.12* 2.75F 0.24 4.76F 0.29

Ho 0.563F 0.030* 0.430F 0.023* 0.557F 0.031* 0.455F 0.028* 0.772F 0.054

Er 1.213F 0.098 1.21F 0.14 1.43F 0.082 1.113F 0.075* 1.87F 0.15

Yb 0.888F 0.068* 1.228F 0.078 1.29F 0.069* 0.938F 0.063* 1.460F 0.073

Lu 0.123F 0.010* 0.180F 0.013* 0.180F 0.012* 0.138F 0.010* 0.188F 0.013

Hf 0.273F 0.085 0.758F 0.079 2.33F 0.14 0.368F 0.046 1.22F 0.27

Ta 0.0115F 0.0079 0.0075F 0.0036* 0.080F 0.0081* n.d. 0.0180F 0.0043*

Th 0.535F 0.039 0.540F 0.050 1.387F 0.073* 1.083F 0.058* 0.778F 0.082

U 0.100F 0.010* 0.103F 0.011 0.267F 0.012 0.210F 0.018 0.1230F 0.0094


Table 4 (continued)

Group Clinopyroxene

GN-999 BM-993 GN-992

B C C

n= 12 n= 14 n= 7

SiO2 54.51 53.40 54.24

TiO2 0.05 0.25 0.06

Al2O3 3.24 4.51 3.64

Cr2O3 1.37 1.19 0.86

FeO 2.06 2.70 2.51

MnO 0.05 0.07 0.09

MgO 15.56 15.47 15.54

CaO 21.32 20.11 22.08

Na2O 1.85 1.86 1.65

K2O 0.01 0.00 0.00

NiO 0.05 0.05 0.02

P2O5 n.a. n.a. 0.02

Cl 0.00 n.a. 0.00

F 0.02 n.a. 0.05

Total 100.10 99.62 100.77

mg 0.931 0.911 0.917

n= 6 n= 6 n= 8

Sc 114.5F 7.8 57.9F 5.5 65.3F 7.8

Ti 255F 26 1470F 160 403F 62

V 188.3F 6.6* 128F 3.5* 146F 30

Co 12.48F 0.48 14.18F 0.37* 17.3F 3.2

Ni 220.9F 8.3* 220.4F 6.4* 256F 48

Rb n.d. 0.0286F 0.0069 n.d.

Sr 404F 13 88.6F 5.6 157F 18

Y 9.02F 0.61 10.10F 0.57 10.7F 1.1

Zr 17.23F 0.80* 190F 25 82F 22

Nb 0.088F 0.022* 0.71F 0.10 0.140F 0.037

Ba 0.190F 0.077* 1.35F 0.43 n.d.

La 17.30F 0.91 10.59F 0.30 5.54F 0.52

Ce 40.1F 2.2 21.3F 1.4 14.8F 2.0

Pr 5.57F 0.31 2.87F 0.16 2.33F 0.26

Nd 24.2F 1.4 12.56F 0.37 11.1F1.3

Sm 4.58F 0.34 2.923F 0.092 2.81F 0.34

Eu 1.333F 0.077* 0.940F 0.030* 0.938F 0.090

Gd 3.68F 0.29 2.84F 0.18 2.68F 0.29

Dy 2.28F 0.17 2.31F 0.13 2.43F 0.19

Ho 0.353F 0.034 0.411F 0.031 0.448F 0.066

Er 0.783F 0.082* 1.025F 0.075 1.13F 0.14

Yb 0.570F 0.068* 0.914F 0.076 1.03F 0.16

Lu 0.082F 0.016* 0.1199F 0.0094 0.150F 0.021

Hf 0.317F 0.051* 6.90F 0.80 0.82F 0.21

Ta n.d. 0.153F 0.038 0.0195F 0.0078*

Th 1.90F 0.28 1.41F 0.15 0.444F 0.060

U 0.458F 0.088 0.238F 0.011 0.097F 0.019

Major and trace elements are reported in wt.% and ppm, respectively. Uncertainties as for Table 3; n.a.: not analysed, n.d.: not detected.


W. Powell et al. / Lithos162

matching calculated and analysed trace-element abun-

dances, with the exception of Sr, which could be

explained by trace amounts of apatite. For sample

GN-992 a match required addition of approximately

6% of amphibole and clinopyroxene, although another

phase is required to add HFSE (particularly Ta and

Hf), without Ti.

Apatite, analysed in two of the Group B samples

is highly enriched in REEs with La abundances of

3875 and 4390 times primitive mantle. Heavy REEs

are less enriched (Yb: 12.7, 21.2 times primitive

mantle) and the rare-earth element patterns are

straight lines between these values ((La/Yb)N: 206,

305, Fig. 8).

Fig. 7. Mass-balance calculations for the samples in Group C. The filled sy

solutions by ICP–MS. The hollow symbols represent whole-rock trace-el

trace-element abundances of amphibole and clinopyroxene. Co and Ni in t

the contribution of phases such as orthopyroxene, olivine and spinel were

5.3. Amphibole/clinopyroxene trace-element

partitioning

Trace-element concentration ratios or partition

coefficients (Amph/CpxD) between coexisting amphi-

bole and clinopyroxene are shown in Fig. 9. Values

for most samples are similar to those reported in

previous studies (Chazot et al., 1996; Ionov and

Hofmann, 1995; Vannucci et al., 1995), although

systematic differences between the groups defined in

this study are present. Samples in Group A have

higher Amph/CpxD than the other groups for Sr and

lower Amph/CpxD for HFSE. Group B samples show

more variation, although the patterns remain parallel to

75 (2004) 141–171

mbols represent whole-rock trace-element abundances determined on

ement abundances calculated by combining modal abundances and

he calculated pattern are low relative to the analysed pattern because

not included in the calculations.


one another. The Amph/CpxD values for HFSE for

Group B exceed those of the other groups, and the

values previously reported by Ionov and Hofmann

(1995) and Chazot et al. (1996). One sample in Group

B (GN-991) stands out from the others due to its very

low Amph/CpxD values for HFSE (particularly Nb and

Ta, Fig. 9). As noted previously, the modal abundances

of amphibole and clinopyroxene in this sample

are variable on the hand-specimen scale, with anom-

alously low olivine and high amphibole + pyroxene

content (>70 vol.%) in the part studied.

It is also apparent that Amph/CpxD for REEs heavier

than Sm are higher in Group A than Group B,

although there is some overlap. The values for sam-

ples in Group C are different to one another: sample

BM-993 has Amph/CpxD values closely similar to

Group A, sample GN-992 has Amph/CpxD values over-

lapping those of Group B.

5.4. Sr and Nd isotopic compositions

Preliminary Sr and Nd isotopic analyses were

carried out on amphiboles and clinopyroxenes from

one sample from Group A and three Group B samples

(Table 5; Fig. 10). The clinopyroxene–amphibole

pairs of the three Group B samples have 87Sr/86Sr of

0.70457–0.70505 and 143Nd/144Nd of 0.512555–

0.512674 (eNd + 0.7 to � 1.6). These values are close

to the present-day Bulk Earth (CHUR values) and

overlap those for the most enriched basalts (mainly

Fig. 8. Apatite trace-element patterns. In BM-998, apatite is found as

clinopyroxene. Apatite A analyses are from xenoliths from western Victo

olivine tholeiites) from the Newer Basalts Province

but do not coincide with the inferred plume compo-

sitions for the western Victorian volcanics (e.g.,

Zhang et al., 2001). They all fall in the range of

the published isotopic data for mantle peridotite

xenoliths from the region (Griffin et al., 1988;

McDonough and McCulloch, 1987; Stolz and

Davies, 1988). The coexisting clinopyroxene and

amphibole have similar Sr–Nd isotopic ratios with

a maximum difference of 0.00008 for Sr and 0.8 eNdunits for Nd. The clinopyroxene analysed from

Group A peridotite differs from the Group B xen-

oliths in its higher 143Nd/144Nd (0.512889, eNd + 4.9)although it has 87Sr/86Sr (0.70503) similar to the

latter. It plots outside the ranges defined by both the

basalts and the mantle peridotite xenoliths from the

Newer Basalt Province due to its high 87Sr/86Sr

relative to 143Nd/144Nd.

6. Discussion

6.1. Composition and nature of the metasomatic

agents

The presence of amphibole, mica and apatite

provides clear evidence that the xenoliths from west-

ern Victoria are modally metasomatised. Differences

in trace-element patterns recorded in both whole-rock

and mineral compositions indicate crystallisation from

disseminated grains, in GN-994, apatite is found in a vein with

ria and Alaska (O’Reilly and Griffin, 2000).

Fig. 9. Partition coefficients for amphibole–clinopyroxene pairs. The black patterns are those of the group indicated in the top right corner of the

plot, and the grey patterns represent the ‘other’ groups.


or equilibrium with different metasomatising agents.

The character of the metasomatic agents is recorded

by the trace-element signatures of the whole-rocks

and constituent minerals.

Group B samples have high degrees of light-REE

enrichment, high Na2O and low SiO2 and TiO2 in

whole-rocks and constituent amphibole and clinopyr-

oxene. Apatite is present in four of the samples and

whole-rock P2O5 content indicates it is likely to be

generally present in the Group B-type assemblage.

These features are consistent with metasomatism by a

fluid with high Na-content, high levels of REE

enrichment and low Si and Ti content. Group A

samples have moderate amounts of light-REE enrich-

ment and major-element compositions within the

normal range for young lithospheric mantle, consis-

tent with metasomatism by a moderately REE-

enriched silicate fluid.

Table 5

Sr–Nd isotope ratios for the amphibole-bearing peridotite xenoliths

Sample Phase Group 87Sr/86Sr 2se 143Nd/144Nd 2 S.E. eNd

BM-994 cpx A 0.705032 0.000018 0.512889 0.000019 4.89

BM-999 cpx B 0.704975 0.000018 0.512617 0.000011 � 0.40

BM-999 amp B 0.704926 0.000023 0.512592 0.000028 � 0.90

GN-991 cpx B 0.704984 0.000020 0.512674 0.000011 0.71

GN-991 amp B 0.705051 0.000023 0.512641 0.000020 0.05

GN-999 cpx B 0.704642 0.000019 0.512594 0.000015 � 0.86

GN-999 amp B 0.704566 0.000015 0.512555 0.000015 � 1.61


The contrasting characteristics of the trace-element

patterns for the two groups can be generalised by

plotting the degree of light-REE enrichment against

the ratio of a high field-strength element such as Ti

relative to a REE of similar incompatibility. Diagrams

of this nature have been widely used to discriminate

between silicate- and carbonatite-rich metasomatic

agents in mantle peridotites (e.g., Coltorti et al.,

1999; Rudnick et al., 1993; Yaxley et al., 1998). A

plot of (La/Yb)N against Ti/Eu is shown in Fig. 11.

Groups A and C samples follow the trend of silicate

metasomatism and Group B samples have high (La/

Yb)N and low Ti/Eu, consistent with metasomatism by

a carbonate-rich fluid possibly related to a carbonatitic

Fig. 10. Sr–Nd isotopic diagram for mantle peridotite xenoliths and basalts

for mineral separates, data from Griffin et al. (1988) are for amphibole-be

study. ‘Other WV xenolith data’ includes amphibole-free xenoliths from Gr

and Stolz and Davies (1988).

melt. As noted by Chazot et al. (1996) addition of

apatite, with its high (La/Yb)N and low Ti/Eu may also

give an apparent whole-rock carbonatitic signature.

The Group B samples with the strongest carbonatitic

signatures do contain apatite (e.g., BM-998 contains

1.7 vol.% apatite, and whole-rock (La/Yb)N= 85.2),

and the generally high whole-rock P2O5 of the group

indicates apatite characteristically occurs in the Group

B xenoliths. The presence of high (La/Yb)N and low

Ti/Eu in coexisting amphibole and clinopyroxene in

addition to the whole-rock is strong evidence for

interaction with a carbonate-rich metasomatic agent,

and evidence for interaction with CO2-rich fluids is

also present in the form of CO2-rich fluid inclusions

from the Victorian Newer Basalts province. Data from this study are

aring whole-rocks, classified according to the groups defined in this

iffin et al. (1988), and data from McDonough and McCulloch (1987)

Fig. 11. Whole-rock (La/Yb)N vs. Ti/Eu. These ratios can be used to

discriminate between highly light-REE enriched, low Ti fluids such

as carbonatites, and less strongly light-REE-enriched, Ti-bearing

fluids such as silicate melts (e.g., Coltorti et al., 1999; Rudnick et

al., 1993; Yaxley et al., 1998). The grey symbols (WVIC88) are

previously published data for western Victorian xenoliths from

O’Reilly and Griffin (1988), those with symbols outlined in black

are apatite-bearing samples.


(Andersen et al., 1984; Matsumoto et al., 1997, 2000;

O’Reilly et al., 1990).

Carbonatite metasomatism has previously been

described in xenoliths from western Victoria by Yax-

ley et al. (1991, 1998). These samples were chosen

specifically because they contained modal apatite and

showed reaction textures, and thus were inferred to

contain modal evidence of interaction with a carbo-

natite. Therefore they represent ‘‘end-member’’ meta-

somatic types. In this study, we demonstrate that the

high levels of REE enrichment and low HFSE enrich-

ment associated with carbonate-rich metasomatism

are recorded in amphibole and clinopyroxene as well

as whole-rock trace-element signatures, and thus pro-

vide an identifying geochemical fingerprint even in

the absence of detectable modal apatite in thin section.

Previously published whole-rock data for amphi-

bole-bearing lherzolite xenoliths from western Victoria

(Griffin et al., 1988; O’Reilly and Griffin, 1988) have

also been considered here to determine if they record

the effects of silicate- and carbonate-rich fluids as

found in this study. The samples are from Bullenmerri

and Gnotuk localities, and lie in two distinct fields in

(La/Yb)N–Ti/Eu compositional space overlapping

those of Groups A and B defined in this study (Fig.

11). Most of the samples overlapping Group B are

apatite-bearing, while the samples overlapping Group

A are generally apatite free. Whole-rock Sr and Nd

isotopic compositions of the samples also overlap the

new mineral isotopic data reported in this study for

Groups A and B. It is thus inferred that the influence

of the same metasomatic agents can be identified in

these samples, and the classification into Groups A

and B describes can be universally used to distin-

guish metasomatism of different origins in amphi-

bole-bearing xenoliths from Bullenmerri and Gnotuk.

6.2. Sr and Nd isotopic composition

The Sr and Nd isotopic signature of Group A

xenoliths (including those from Griffin et al., 1988;

O’Reilly and Griffin, 1988) overlap those of garnet

pyroxenites (from the same localities) reported in

Griffin et al. (1988). These pyroxenites are interpreted

to represent crystallised basaltic melts which under-

went reequilibration in the garnet stability field, and

which are inferred from Nd model ages to have

intruded the mantle between 300 and 500 Ma (Griffin

et al., 1988; O’Reilly and Griffin, 2000). The isotopic

similarities suggest that the silicate-rich metasomatic

agent that affected Group A was associated with the

mafic magmatic event that resulted in the garnet

pyroxenites.

Group B Sr and Nd isotopic compositions lie close

to bulk earth values, and are distinct from the plume-

type source associated with the Newer Basalts (Ewart et

al., 1988; Frey and Prinz, 1978; McDonough et al.,

1985). The Group B samples lie toward lower eNd andhigher 87Sr/86Sr values, similar to those of the more

enriched end-members of the Newer Basalts Province.

The event recorded in the Group B samples obviously

preceded eruption of the Newer Basalts, and we infer

that some tholeiites of the Newer Basalts interacted

with lithosphere modified by the Group B-type com-

ponent. Low degrees of partial melting and assimilation

within the lithospheric mantle (as described in Zhang et

al., 2001) imparted the isotopic signature of the meta-

somatised mantle to some of the lavas, resulting in the

trend away from the inferred western Victorian plume

field (Fig. 10). Apatite (+ clinopyroxeneF amphibole),

and a strong signature of carbonatite metasomatism

were added during the event recorded by the Group B


samples, although later addition of more apatite as

suggested by the noble gas work of Matsumoto et al.

(1997, 1998) is not precluded. The differing character-

istics of the groups defined in this sample suite indicate

two distinct metasomatic types, but they do not neces-

sarily constrain the number of events which have

affected the lithospheric mantle.

6.3. Trace-element partitioning

Partition coefficients for trace elements between

coexisting clinopyroxene and amphibole vary system-

atically between the groups (Fig. 9). In Group A, Sr is

partitioned more strongly into amphibole than clino-

pyroxene compared with Group B samples. In Group

B, HFSE partition into amphibole more strongly than

clinopyroxene compared to Group A samples. Recent

work on both experimentally produced and natural

amphiboles has shown the importance of crystal chem-

ical control on trace-element partitioning behaviour

between mineral and melt (Bottazzi et al., 1999; Oberti

et al., 1992; Tiepolo et al., 2001). In particular, Tiepolo

et al. (2000) showed that, at constant amphibole dehy-

drogenation, Amph/LDNb,Ta is a function of the Ti

content in the system and that the Amph/LDNb/Amph/LDTa

is controlled by the M1 site dimensions. The amphib-

oles from the present work, as revealed by SREF, show

similar dimensions of the M1 site. The observed

variation in their Nb/Ta ratio cannot be ascribed to

theS/L

D variability but is most likely related to the

composition of the infiltrating melts. This hypothesis is

also in agreement with the relatively constant Amph/

CpxDNb/Amph/CpxDTa ratios. Because Nb and Ta enter

amphibole and clinopyroxene via different crystal

chemical mechanisms, melts with different Nb/Ta

ratios are required to produce the same variation in

both mineral phases. The higher Amph/CpxDNb values in

Group B are most likely related to the paucity of Ti in

the carbonatite melt. Partial dehydrogenation is a

prerequisite for amphibole stability at upper mantle

conditions and it is balanced primarily by incorporation

of Ti at the M1 site (Tiepolo et al., 1999). In Ti-poor

systems (e.g., in the presence of a carbonatite compo-

nent) amphibole S/LD values for Ti, Nb and Ta tend to

increase dramatically, in order to attain the required

amount of high charge cations. This can explain the

near order of magnitude difference in Amph/CpxDNb

values between Group A and Group B.

The samples in Group C show positive anomalies

in whole-rock Nb, Ta, Zr and Hf, whereas in the other

groups, anomalies for these elements range from

strongly negative to absent. The other compositional

characteristics of these samples partially overlap those

of the other groups, including Amph/CpxD values:

Sample BM-993 has Amph/CpxD values similar to those

of Group A and GN-992 has Amph/CpxD similar to

Group B. The similarity of Amph/CpxD values suggests

the positive anomalies for Nb, Ta, Zr and Hf reflect

differences in fluid/mineral partitioning rather than

mineral/mineral partitioning.

Experimental work on clinopyroxene–melt parti-

tioning has shown that HFSE partition preferentially

into the clinopyroxene, while rare-earth and large-ion

lithophile elements partition into the melt in the

presence of a hydrous component (Keppler, 1996).

Moine et al. (2001), in explaining positive HFSE

anomalies in vein amphiboles in xenoliths from Ker-

guelen suggested this mechanism may also influence

partitioning between amphibole and hydrous fluid, asAmph/CpxD for REE is f 1, and for HFSE is much

greater than 1. On the basis of available data and

comparison with the work of Moine et al. (2001), the

presence of a hydrous fluid component may explain

the positive anomalies observed for Zr and Hf in

amphibole and clinopyroxene in BM-993 (Group C).

The effect of hydrous fluid on HFSE partitioning may

also be stronger on amphibole than clinopyroxene,

effectively raising Amph/CpxD values as observed.

Moine et al. (2001) explained the presence of a

hydrous component beneath Kerguelen as a result of

fractionation of the metasomatising silicate fluid.

6.4. Metasomatic history of lithospheric mantle

beneath Bullenmerri and Gnotuk

The depleted wall-rock compositions (Component

A of Frey and Green, 1974) including the harzburgites

formed by original melt depletion. Magmas of basaltic

composition with low eNd and high eSr (EM-II com-

position) were emplaced about 300–500 Ma ago

(Griffin et al., 1988), equilibrating in the lithospheric

mantle to form garnet-bearing pyroxenites. Associated

fluids led to modal (F cryptic) metasomatism of the

adjacent mantle wall-rock, with crystallisation of one

generation of amphiboleFmica. Later metasomatism

by a highly evolved, Na-rich, Ti- and HFSE-poor fluid


such as a sodic carbonatite affected some areas, which

were probably already modally metasomatised by the

previous event. Crystallisation of apatite (F another

generation of clinopyroxene and/or amphibole) took

place, and pre-existing amphibole and clinopyroxene

were strongly enriched in REE (without enrichment in

HFSE). This fluid may have been associated with lavas

of the Newer Volcanic Province, although reequilibra-

tion of major- and trace-element compositions was

able to take place prior to sampling by the host basalt.

7. Conclusions

1. Trace-element and isotopic compositions of whole-

rocks and metasomatic phases measured in xen-

oliths from western Victoria record multiple discrete

episodes of modal metasomatism by at least two

different types of fluid, one silicate rich (Group A)

and one carbonatitic (Group B). Different trace-

element partitioning relationships for different

amphiboles suggests that the silicate-rich fluid

fractionated to yield a hydrous component. Al-

though two end-member compositional types of

metasomatic fluids can be distinguished, the total

number of metasomatic events cannot be con-

strained.

2. Isotopic data in combination with mineral geo-

chemical signatures suggest that the observed

silicate metasomatism may be related to the 300–

500 Ma event in which basaltic melts crystallised

in the lithospheric mantle, subsequently reequili-

brating to garnet-bearing assemblages (the garnet

pyroxenite series of Griffin et al., 1988).

3. The isotopic fingerprint of the carbonatitic meta-

somatism is distinct from that of the plume source

of the Newer Basalts, but similar to some of its

enriched end-members. We suggest that the

enriched magmas of the Newer Basalts interacted

with Group B-type (i.e., carbonatite) metasoma-

tised lithosphere, which imparted its isotopic

signature by partial melting and assimilation.

4. In both Groups A and B, metasomatic types the

variation in Nb/Ta ratio of amphibole reflects a

compositionally variable metasomatic agent. The

different major-element composition and volatile

component of the metasomatic agent between

Groups A and B (i.e., silicate fluid vs. carbonate-

rich fluid) causes significant variations in the Amph/

CpxD values, particularly for trace elements such as

Nb and Ta, which enter the two mineral phases via

different crystal chemical mechanisms.

5. Positive anomalies for Zr and Hf in whole-rock

patterns for Group C samples may possibly be

explained by the presence of a hydrous component

derived from fractionation of the silicate metaso-

matic agent, which enhanced partitioning of Zr and

Hf into clinopyroxene and amphibole relative to

REE and LILE.

Acknowledgements

We are grateful to Norm Pearson and Bill Griffin

for helpful discussions and ideas. Suzy Elhou, Carol

Lawson and Norm Pearson provided invaluable

support for the analytical work at Macquarie

University, R. Oberti for SREF at Pavia. The

manuscript was improved by helpful comments

from the two anonymous reviewers and the

handling Guest Editor, R. Vannucci. Financial

support for this project was provided by an ARC

Discovery Grant (S.Y. O’R. and others), the

GEMOC National Key Centre, a Small ARC Grant

(S.Y. O’R.), a Macquarie University Postgraduate

Research Grant (W.P.) and an Australian Postgrad-

uate Award (W.P.). This is publication number 322

in the GEMOC ARC National Key Centre

(www.es.mq.edu.au/GEMOC/).

Appendix A. Analytical techniques

A.1. Sr and Nd isotopic analysis of clinopyroxene and

amphibole separates

Clinopyroxene and amphibole from amphibole-

bearing peridotite xenoliths were first separated using

a magnetic separator and then handpicked under a

binocular microscope. About 100 mg of samples was

leached with warm 6 N HCl for 1–2 h and rinsed with

Milli-Q water before digestion in 17 ml Savillex

Teflon vials using HF +HNO3 +HClO4 acids and then

6 N HCl. The dissolved samples were conditioned

with 2.5 N HCl and loaded on AG W50-X8 cation

resin columns.

http:\\www.es.mq.edu.au\GEMOC\


The separation procedure was reduced to one-step

elution using 2.5 and 6 N HCl to collect the Sr and

light to middle rare earth elements for Sr and Nd

analyses, respectively. Both fractions were diluted into

50–100 ppb solutions in 2% HNO3 for analysis.

These clinopyroxene and amphibole separates

were analysed using a Nu Plasma multi-collector

inductively coupled plasma–mass spectrometry

(MC-ICP–MS) at the Geochemical Analysis Unit,

GEMOC National Key Centre, Macquarie University.

Sample solutions were introduced to the MC-ICP–

MS using a MCN-6000 micro-concentric nebuliser at

a rate of f 60 Al/min and run for sixty 10-s cycles in

three blocks. Total analysis time is about 15 min per

sample and sample consumption is about 1.0 ml.

A.1.1 . Sr isotopes

Masses 83, 84, 85, 86, 87 and 88 were measured

simultaneously in Faraday collectors and all measure-

ments were made in static mode. Corrections for mass

fractionation of Sr and Rb isotope ratios was made

using an exponential law using a normalising value

for 86Sr/88Sr = 0.1194. Any interference of 87Rb on87Sr was corrected by measuring the intensity of 85Rb

and using a 85Rb/87Rb ratio of 0.38632. This value

was obtained by doping the QCD Analysts Sr stan-

dard with Rb (Plasmachem) and by repeated measure-

ment to refine the value of 85Rb/87Rb necessary to

give the true 87Sr/86Sr. The maximum 87Rb/87Sr ratio

of the spiked solutions used in the refinement of the85Rb/87Rb ratio was 0.3977. Correction procedures

for double-charged heavy rare earth elements (Er, Yb

and Lu) were also adopted. However, all these cor-

rections are negligible for these clinopyroxene and

amphibole separates. 83Kr from the gas was monitored

and a correction for 86Kr interference on 86Sr was

made, though the correction is insignificant.

Average values of repeated standard analyses dur-

ing the period of analysis (June 2001–2002) are 87Sr/86Sr = 0.710228F 28 (S.D., n = 19) for SRM 987,87Sr/86Sr = 0.703484F 36 (S.D., n = 14) for BHVO-1

and 87Sr/86Sr = 0.705070F 21 (S.D., n = 4) for BCR-1.

Total procedural blank is f 900 pg (n = 2) for Sr.

A.1.2 . Nd isotopes

The correction of 144Sm over 144Nd without column

separation relies on the ease with which isobaric

interferences can be corrected to high precision in

MC-ICP–MS analysis. This procedure also depends

on the use of an external normalisation procedure

whereby the mass fractionation coefficient measured

on one element is used to correct the mass discrimina-

tion of an element of similar mass (e.g., Nd to correct

Sm) (Luais et al., 1997). The method assumes that the

mass fractionation coefficients for both elements are

equal, and allows the adjustment of the ‘true’ isotope

ratios of either the normalising element or the interfer-

ing element.

Masses 143, 144, 145, 146, 147 and 148 were

measured simultaneously in Faraday collectors and

all measurements were made in static mode. An expo-

nential correction for mass fractionation was applied

using 146Nd/145Nd = 2.07204 to avoid an iterative cor-

rection for the interference of 144Sm on 144Nd. The

value for 146Nd/145Nd is an average of results obtained

on a 100 ppb JMC321 Nd solution and was normalised

to 146Nd/144Nd = 0.7219. The interference of 144Sm on144Nd was corrected by measuring 147Sm and using144Sm/147Sm=0.2070. This value was obtained by dop-

ing the JMC321 Nd standard solution with JMC309

Sm and by repeated measurement to refine the value of144Sm/147Sm necessary to give the ‘true’ Nd ratio.

Average values of repeated standard analysis dur-

ing the period of analysis (June 2001–2002) are 143Nd/144Nd = 0.511108F 20 (S.D., n = 40) for JMC 321

Nd (f 100 ppb solution), 143Nd/144Nd = 0.511114F17 (S.D., n = 13) for JMC321 Nd spiked with JMC309

Sm standard (f 100 ppb Nd and 33.4 ppb Sm

solution), 143Nd/144Nd = 0.512982F 31 (S.D., n= 26)

for BHVO-1 and 143Nd/144Nd = 0.512630F 18 (S.D.,

n= 3) for BCR-1. Total Nd blank is 30–150 pg (n= 5).

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