GEOCHEMICAL VARIATION OF ENRICHED MID-OCEAN RIDGE BASALTS ERUPTED FROM A DYING

RIFT IN NORTHCENTRAL ICELAND

MARIAN KRAMERSmith College

Sponsor: Mark Brandriss

INTRODUCTIONIceland is geologically unique due to theanomalous exposure of the Mid-AtlanticRidge on land and the presence of a hot spot,believed to be a mantle plume, superimposedon the active Mid-Atlantic Ridge. Thisexposure makes Iceland an ideal location tostudy mid-ocean ridge dynamics as well theinteractions between tectonic rift volcanismand hot spot volcanism.The Mid-Atlantic Ridge in Iceland undergoesrift relocation events due to the interplaybetween the northwest drift of the rift axis andthe hot spot, currently located under theVatnajokull Ice Cap (Tronnes, 2002). Riftrelocation occurs as an older rift to thenorthwest is abandoned while a new rift iscreated to the southeast, over the hot spot.These rift relocation events take about 4million years for completion and occur atintervals of 8-12 million years (Hardarson etal., 1997). The rift relocation event thatcorresponds to abandonment of the major riftin the area of the Skagi peninsula (the riftprobably responsible for most volcanism inthe study region) occurred at 7 Ma (Tronnes,2001).As a result of rift relocation, Iceland containsvaluable evidence of the chemical variation ofmagmas associated with the life of a rift fromits inception to its extinction. The purpose ofthis study is to analyze the petrographic andchemical characteristics of basalts eruptedfrom a dying rift adjacent to a hot spot.

METHODSMajor lithologic units in my field area weredetermined through mapping of a 6 km2 fieldarea and sampling along a traverse in theLangadalsfjall mountain range, located east ofthe town of Blonduos in north central Iceland(Fig. 1). A stratigraphic section of thesampling traverse was constructed from fielddata (Fig. 2).

Laboratory work consisted of thin sectionpreparation, whole rock x-ray fluorescence

(XRF) analysis, petrographic analysis,analysis of mineral compositions by scanning-

Figure 2: Section measured along traverse shown in Figure 1. Sample locations are shown on right. Whole-rock MgO weight percent indicated on graph. Photomicrographs are of flows representative of each unit.

electron microscopy with energy dispersivespectrometry (SEM/EDS), and agedetermination using 40Ar/39Ar isotopes. XRFpowders were prepared at Williams Collegeusing a steel jaw crusher and an aluminashatter box. Major and minor element analysiswas carried out by XRF at Franklin andMarshall College. SEM/EDS analyses weredone at Smith College.

RESULTSTwenty of the twenty-one samples areclassified as basalts on an alkali versus silicadiagram, with weight percentages of SiO2ranging between 46.75% and 47.90% (LeBaset. al., 1986). The exception is the lowestsample in the section, which is a basalticandesite with 55.21% weight percent SiO2. Asshown in Figure 3, every sample showsenrichment of trace elements in a patterntypical of enriched mid-ocean ridge basalts,known as EMORB (Sun & McDonough,1989).The flows in the section can be separated intothree distinct lithologic units, referred to as B1,B1.5, and B2 (Fig. 2). The youngest flow is 7.08+ 0.06 million years old (Ma), very close tothe time at which the main rift in the Skagiarea went extinct (Tronnes, 2001). B1 is thelargest section, with twenty-four flows. Flowsfrom B1 contain a cryptocrystallinegroundmass with occasional phenocrysts ofolivine, plagioclase, and augite. Volcanic glasswas present in some flows. The phenocrystswere approximately 0.5 mm wide. In the field,the flows are aphanitic, of variable thickness,with a layer of breccia, tephra, or oxidized ashcommonly found between flows. B1.5comprises two flows. Flows from B1.5 consistof a microcrystalline groundmass withabundant olivine and rare plagioclasephenocrysts, both roughly1 mm wide. B2comprises five flows. In thin section, B2 ischaracterized by a microcrystallinegroundmass with abundant plagioclasephenocrysts ranging in size from 1 to 4 mm inlength. Occasional phenocrysts of augite andolivine are also found. Breccia and oxidizedash layers are commonly found betweenflows.

Based on weight percentages of MgO, themost primitive flows are found in B1.5, themiddle of the section (Fig. 2). Flows in B1 andB2 have a narrow range of MgO contents, withthe exception of the basaltic andesite (whichcontains the lowest weight percentage). OnHarker diagrams (e.g. K2O vs. MgO), B1 andB2 typically plot along a single chemical trend,while B1.5 does not plot near this trend (Fig. 4).

DISCUSSIONThe enrichment of trace elements, similar totypical EMORB suggests that the magmasources derive either from lower enrichedmantle, or from upper depleted mantleenriched by hot spot magmatism (Winter,2001). The latter hypothesis is the mostplausible because, according to Tronnes(2001), in Iceland “the bulk of the plumematerial will be deflected at shallower levels,where feeding of plume material into themelting zones under the rift zones becomes animportant process.” Thus, in Iceland it is

common to find chemically depleted riftvolcanism enriched by hot spot material,creating EMORB (Tronnes, 2001).Chemical data strongly suggest that B1 and B2may derive from a single magma source, whileB1.5 appears to be derived from a chemicallydistinct source. The presence of relativelyprimitive B1.5 flows (based on weight % MgO)in the middle of the section implies that thesection cannot be derived from a single,closed-system fractionating magma (Fig. 2).Harker diagrams, such as the K20 vs. MgOplot shown in Figure 4, further supports thishypothesis. Flows originating from a singlefractionating magma generally plot along acommon trend line, showing increasingconcentration in incompatible elements withdecreasing MgO content. This is the case forB1 and B2 (Fig. 4). However, B1.5 does not plotnear this trend, nor does it plot at a point onthe graph where chemical variation due tomagma mixing would easily allow for acommon origin with B1 and B2 (Fig. 4).Figure 5 is a Pearce diagram showinghypothetical chemical trends for olivine,plagioclase, and augite fractionation, the threemain mineral phases in lavas from this area.Also plotted are the compositions of whole-rock samples. In Figure 5, samples from B1and B2 plot along a single trend while B1.5plots on a separate trend, providing furtherevidence that B1 and B2 are from a differentmagma source than B1.5.Both sample trends plot between the olivine,augite, and olivine fractionation trends (Fig.5). This suggests that the magmas producingeach trend were fractionating somecombination of plagioclase, olivine, and/oraugite. It is also important to note that the B1-B2 trend is of constant slope, suggesting thatthe flows originated from a single magmacrystallizing phases in a fixed proportion. TheB1.5 trend is also of constant slope, indicatingthat these flows originated from anothermagma fractionating in a similar manner.The chemical dissimilarity between B1.5 andthe other lavas indicates that B1.5 is most likely

from a different source. This raises thepossibility that B1.5 represents magma derivedfrom a new source associated with theincipient abandonment of the major Skagi rift.Comparison with flows from this area and thesurrounding region will help test thishypothesis.

REFERENCES CITEDHardarson, B.S., Fitton, J.G., Ellam, R.M., Pringle,

M.S., 1997, Rift relocation - a geochemical andgeochronological investigation of a palaeo-rift innorthwest Iceland: Earth and Planeary ScienceLetters, v. 153, p. 181-196.

LeBas, M.J., LeMaitre, R.W., Streckeisen, A., andZanettin, B., 1986, A chemical classification ofvolcanic rocks based on the total alkali silicadiagram: Journal of Petrology, v. 27, p. 745-750.

Russell, J.K., and Stanley, C.R., eds., 1990, Theory andapplication of pearce element ratios to geochemicaldata analysis: Geological Association of Canada, v.8, p. 1-18.

Sun, S.S., and McDonough, W.F., 1989, Chemical andisotopic systematics of oceanic basalts-implicationsfor mantle compositions and processes: GeologicSociety Special Publication, n. 42, p. 313-345.

Tronnes, R.G., 2002, Geology and geodynamics ofIceland: Nordic Volcanological Institute, Universityof Iceland, p. 1-19.

Winter, J., 2001, Igneous and metamorphic petrology,1st edition: Prentice Hall, 699 p.