Earth and Planetary Science Letters, 117 (1993) 169-180 169 Elsevier Science Publishers B.V., Amsterdam

[PT]

In v a c u o crushing experiments and K-feldspar thermochronometry

T. Mark Harrison, Matthew T. Heizler and Oscar M. Lovera Department of Earth and Space Sciences and Institute of Geophysics and Planetary Physics, 3806 Geology Building,

University of California, Los Angeles, CA 90024, USA

Received August 17, 1992; revision accepted February 28, 1993

ABSTRACT

Using in c, acuo crushing methods, both C1- and K-correlated components of trapped 4°At have recently been identified in hypersolvus alkali feldspar and K-feldspar-bearing 'chert ' and stromatolite samples. If these components re-emerge during the late stages of thermal degassing, interpretation of 4°Ar/39Ar age spectra could be complicated. We have extended these observations by performing similar measurements on a well-characterized low-temperature K-feldspar, MH-10. A plot of 4 °Ar* /K against C I / K confirms the presence of a chlorine-related excess argon component from sites accessible by crushing MH-10, although it amounts to only 0.078% of the total potassium-derived 39Ar from this sample. No potassium- correlated component of excess argon could be clearly identified. Surprisingly, isothermal duplicate heating steps reveal a Cl-correlated component (4°ArE/Cl = 2.7 _+ 0.2 X 10 -4) that appears to be related to the modification of very small (< 1 /xm) inclusions. This method appears to provide a basis with which to correct for the excess argon that is commonly observed during the initial stages of step heating of K-feldspars. The product of the 4°ArE/CI and the C1/K (determined for each step via the 3SAr/39Ar ratio) yields an "excess age" that is simply subtracted from the measured 4°Ar*/4°K to yield an age corrected for the presence of excess argon. Crushing appears to produce an artifact that seriously affects the step heating age spectrum but does shed light on possible complexities in the internal distributions of 4°Ar and 39At. On balance, we find that any adverse consequences arising from these effects on interpreting the 4°Ar/39Ar results in terms of thermochronomet- tic models are exceedingly small.

1. Introduction

Coupled with a reputation for yielding ages younger than rock forming events, the recogni- tion of excess radiogenic argon (Ar E ) in the early 1960s [1] contributed to a dampening of enthusi- asm for the K - A r method in establishing base- ment chronologies [2]. Interest was rekindled fol- lowing development of the 4°Ar/39Ar step heating variant which can, in principle, test underlying assumptions in K - A r dating [3]. Recently, two interesting and provocative studies [4,5] have uti- lized an additional dimension, in v a c u o crushing, for the examination of argon isotopes from K- feldspar-bearing samples. Their results suggest an approach which has the potential of providing new insights into an old problem: the incorpora- tion of excess argon in minerals.

Turner and Wang [4] presented 4°Ar/39Ar re-

sults, obtained by a combination of crushing and step heating from fine-grained metasedimentary rocks, that led them to suggest caution in inter- preting K-feldspar age spectra in terms of ther- mochronometric models [e.g., 6-8]. Their results point to potentially important and problematic aspects regarding the siting of excess argon in these composite materials and the implications for more commonly used samples (e.g., sub-solvus K-feldspars of igneous origin) need to be directly addressed. In v a c u o crushing of their two sam- ples prior to thermal degassing yielded 1% and 4.8% of the total 39ARK and well-correlated plots of 4°Ar*/39ArK versus 38ArcI/39ArK, suggesting the presence of both Cl-correlated (4°ArE/C1) and K-correlated (4°Are/K) components of ra- diogenic 4°Ar (4°Ar * ).

Although it was possibly hosted by fluid inclu- sions, Turner and Wang [4] explored several al-

0012-821X/93/$06.00 © 1993 - Elsevier Science Publishers B.V. All rights reserved

170

TABLE 1

Argon isotopic results, experimental conditions and diffusion coefficients

T . M . H A R R I S O N E T A L

( rush nf lcml) ( ° ( ) 41) \Mm "w~ 38 W( 1/30 'WKh t6,kr]t9ArZxll) a3~:'xrxlO~Smol3qAr~l~d(51 40At* ( q ) c 41l ~,r-/3~ ~,rK ~,ppa~nl age ± I o Iim¢ (mini IIII10/1 (K) log l)/r2 ts !

MH lOu K h, t d w a r (J = 0 0 0 7 9 4 2 We&ht = 0 00968 e) a

I 1214 0 6d~f) 308 6 0.036 0 (~)2 ~1 ~ 11)07 4101~ ~3 • I o65 o 7186 114 7 11 ~1O5 OOO ~ , ~ v (~71 5 ~ ' ~ , ± ')6

1252 ,) 754~, 451 7 OO67 0 . t~7 59 ~ II 19 4 1 U ± ~4 4 I 124 0 5269 3 6 t (1 01152 (I 0 i / ~ 1 4 IOIh ~';78 ~ ~ g 5 t t t I 0 7285 3 7 9 3 0 (~)9 0 0 [ 2 8 9 8 1019 ~083 ± 40 6 t051 (150~9 318 2 0 O28 0 f)14 i~ )0 n57 t ]881 1: I I

/ [49 (I 5204 41~ 5 t) 020 O I) 15 80 2 1026 t99~ ± /5 g 1044 O 496] ~47 1 0 017 DO] 6 '~) 1 ~ 4 1 4 3856 ± 36 o 11182 (14964 ~242 01)32 I) 1118 91 I 0 8 5 0 1 0 3 0 1 : 4 6

i o u52 (, 04O38 232 6 0 082 O024 92 " 8 ~ 0 ~756 ± 20 1 t 010 4 0 4 6 2 0 270 v 0 IR)5 0 0 2 8 '11. ~ g ~94 t675 ± t " 12 9 6 2 6 0 ~ 9 6 224 2 0 070 0 033 92 3 -96 4 ~502 ± 20 13 8 9 2 1 0 4 ] ' 5 2 3 1 0 0 0 5 5 OO~7 923 823.9 3 6 4 6 ± 4 1 ~4 815 I 0) 3971 274 6 0 0 4 7 0 .040 tX)(I 733 q ~46"6 ± 21 15 go~ i) I) 4274 18t 4 0 039 0.043 9 3 ~ 7 5 4 8 ~51~1 ± 6 t6 767 6 D4144 2 4 4 3 O 118 0 I)51 ~) 6 6 9 5 5 3383 ± 13 [- 18118 0 1 5 7 0 1604 (} 1~)4 I) 1151 71 1 190 8 1285 ± IO0 18 ~(m I) 0 M 5 9 t~132 0 0(H 0 051 ~ ~ 3 7 9 3 251),6 ± 295 19 7 3 0 ] 0 t 6 9 7 31)1 2 0 2 0 9 0fh56 g 7 8 fMI 1 t26[) ± 21 211 465 3 0 1 ] 97 468 4 0 020 0 067 7(1 I ! 26 9 2gGq ± 21 21 "65 8 0 3860 41)5.o 0 l 17 01175 8118 61 q 1 3208 ± 7 22 7 1 6 4 0 ~ 7 9 4 47118 0 0 2 2 0 [176 81) 5 5 ~ t 3103 ± 26 2 t 720 4 0 3687 423 0 0 020 0 O78 82 6 505 5 ~ 149 ± ~q

25[~ 441 I) 0 1483811 1) I I I 0 fig5 0 7 ~ ? 2 32 412 ± 024 250 "94 8 0 I)232 257-; 0 0 0 3 5 0 0 8 8 4 16 ~4 24 4~4 ± 236 3OO 5.~9 I 0 0078 1"225) 0 183 0 I11O " 2 6 41135 5O2 1:61 t25 15"~ 2 0 01('~5 426-_ 0 }14 0 122 17 2 27 IO 352 ± ~ 1 371) 67 21 0 t )133 138 7 I).881 0 182 ~8 2 26 2O 1141 I ± 7 ; 40O 4 8 5 ~ () @~88 85 2 t I 24 I) 266 47 1 21 36 t l)" () 1 : 1 4 450 (~184 0 0301 51 IJ 1 4 2 0 3 6 3 74 I 45 ~2 5 5 8 8 ± ~tl 450 3 6 6 2 011022 4 t 57 3 22 0 5 8 3 6 4 I 2~ 72 41 I 3 ± 2 3 5(30 ~ 6 9 6 0 0 1 8 0 3 17o 1 4 6 (}68~ 95 2 ~61K) 45~ 6 ± 1 2

0 8 6 8 51~q 2 2 9 8 O(DI5 1 586 2 71 9 6 O 2 2 4 0 2 9 6 5 ± I 3 350 44 42 0 (K)OI 4 2 5 0 O085 0 8 - 4 1 6 8 31 84 4 ( ~ 7 ± IO 2 550 25.07 fl 0{)59 1258 7 7 3 1 4 0 ')7 g 24 6 ~ 3 2 2 8 ± 0 4 550 1')63 0 I)2';12 0 28411 8 0 2 [ 9 5 98 v 10 52 2 6 0 0 1 :0 2 (z)0 21 (11 0 (X)I8 0 230~1 12 ~ 2 811 9 9 1 21) 02 2 7 7 3 ± 0 5 600 2 0 1 7 0 (XI02 0 0 8 3 6 9 6 1 3 45 9O4 21) 12 2 6 7 5 ± 0 3 650 2 I 19 OOOO8 0 3053 3 4 6 5 82 q ~ I 2 1 0 4 2 7 8 8 1:0 2 650 21 l 0 0 0 0 ~411 55 2 0 5,) ,~o2 21) 08 278.11 ± 0 4 65O 21 61 I) (IOO2 0.4394 7 1 7 14 5 09 2 2 1 4 6 2 8 3 0 ± 116 ~(Xl 2 I 97 0 ~XX)~ 114210 3 0 4 ] 7 2 t)o ~ 21 ,R 2 2~8 3 ± 0 4

700 2 2 2 2 0 (~C(~ 0 6 4 6 8 32.2 194 9911 22 O3 290 5 ± 0 4 750 22 53 011111311 0 2050 70 (I 24 2 0 9 5 2 2 ~ 2 205 6 ~ 0 3 801) 23 61 0~101 0.1014 2O ~ 38 3 0 9 4 2 3 4 7 3 0 8 4 ± 0 4 81X) 2-* 71 D 0~)1 0 3854 21 8 89 8 99 4 24 57 3 2 1 6 ± I 1 85O 25 05 0OOO2 0 5667 41 2 42 6 ocl 2 24 86 ~25 I ± O4 ~ ) 2 5 ~ S o ~ a o 2 I) 6~,48 8 8 5 ~8 7 99.~ 25 - 6 335 8 ± 0 8 950 2" 50 110002 115OO0 88 I 5.:. 7 9 9 3 2 ~ t2 3 5 4 3 ± 0 5

10fK) 2 9 2 5 I) 110¢)5 0 ~ 1 2 2 66( , 5 9 3 , m / 2 0 0 2 3 7 4 1 ± 1 ) 5 1009 2~ 61 o ( ~ ) 5 i 152 4 4 8 6 2 3 9 8 7 20 25 ~ 7 6 8 ± 0.5 1050 2 9 6 2 O.O013 1 275 t 6 5 0 4 8 0 8 5 20 22 3 7 6 5 ± 13 1050 29.70 I ) .~21 1 931 ~2 I 6 7 0 9 7 8 2 o 11 175 2 ± D 5 1100 30 29 011030 1 018 20 6 69 / 07 4 2 9 7 0 3 8 2 1 ± D t 1100 3 0 6 4 0 (K,W.2 2 9 4 4 3 3 4 7 1 t 9 6 5 2 9 7 4 3 8 2 6 ± 1 ) . 8 11 (X) ~0 86 0 (KI49 3 764 3 4 6 73 ~ 0 5 3 29 7 ~ 3 8 2 4 ± D.5 1150 ~ I 62 00O63 2 931 27 6 75 t) 9 5 9 10 7 ~ 1 9 4 0 ± 0.7 1200 31 57 0OO67 2 163 52 v 7 9 2 9 7 2 I110/ 3 0 6 0 ± O 5

1250 311 (1~ o f105~ 0 8 1 5 3 185 91 9 98 5 29 .~ t82 9 ± 0 7 13110 291~1 0 (I~M7 t f)(M 115 99.8 05,1 2851 t 6 8 2 z 0 3 1400 55 0 8 00f)6di 95 68 3 17 09 09 16 2 26 78 t 4 " 9 ± 2 1500 416.5 0.0 133] [I 0 3l)5 100 2 82 2 2 4 8 2O6 ± 55

16 1')12 1 1 t 2

18 i ')12 H 52

. . . . . . . . . 2 12 / 6=2 ,I 17 1 555 0 16 24 1 486 8 8 v 14 1 ~83 8 37 87 1 383 8 58 n l 294 7 05

14 1 2 9 4 7 6 t

Iv 1215 ,

30 1 2 1 5 7 11 /2 I 145 6 3 ~ iN 1 1 4 5 6 5 t 14 1.083 5.60 29 i t )~ i 5 5 ~ 70 1 08 g 5 66 22 IO28 5 RI 24 1 0 2 8 5 3 6 21 0 978 4 ~O ~o f) 032 4 53 1~ 0 8 ~ 1 4 ~ )

i i 81~1 4 4 2

i1: . . . . . . . . . . I) 8 1 8 4 17 10 11786 4 17 16 0 7 8 6 4 4 4 It) 0 756 4 33 16 I) 756 4 56 10 O ' 2 8 4 W 20 I) 728 4 5 9 40 f) "28 4 8 4 13 O7[)I 4 4 1 1 I O 679 4 0 1 16 1t657 ~ 4 0 12 0 6 ~ 6 2 71) 14 0 5 o 8 2 91

10

(r~h#[lemp(°C} 40Ar~qAr ~ 38Ar(I/39ArK b 36Ar#t�ArSx]0~ 3qAr x l 0 5mol 39At ~ l e ~ d ( % ) 4 0 A r ' ( % ) c 411Ar*,#�ArK A p p ~ n t a $ e ± l o l ] ~ ( m l n ) I(X~J/Y(K~ l o g l ) / r 2 ( s "~ )

M H lOu' (>150 t l~) K4elds~ar (I = 0 007942: Weight = 0.0006 ~,~

~50 6 9 0 7 00O82 221)6 0 0 1 9 0 0 2 1 t 80 47 67 580±920 22 i 6115 1057 41X) 12~9 0 0 265 7 0 I)38 O(ff~4 t t 5 45 34 555±88 34 1 486 9 86 45O 1914 0 1934 2(10 2 0 023 0 0 9 0 3 5 7 [ 2O 6 1277±33 I I 1 183 0 32 45O 7 7 19 0 0187 2111 7 01) I8 0 I If) ~ 37 1757 236~z'~4 16 1 38~ 9 48 5()q IL~} 5 0 0 6 0 ~ ~.221 0 I01 0 222 48 0 8801 056113 13 1 2 0 4 g 42 51~) 32 97 0 0 4 2 6 2 0 1 ~6 0 ~52 34 6 2 0 3 5 270~1 I 28 I 294 g 46 51~) 3O DI 0 0 81 6 6 O 027 0 382 1 0 6 14 86 201±25 14 t 294 8 6 8 55O 33 I)2 (I 0218 I I 77 11233 06-13 77 3 29 5! 3 7 9 9 ± 6 0 14 1 2 1 5 7 N~ 55O 21 9~ 0 tl~ ~ t i 1.78 O IV6 II 8 ~ b 9 6 18 4~ 2 4 0 4 ± 8 8 20 / 215 7"2 (~1~1 2 9 9 2 OO2O3 2 823 0 3 6 2 I 2 a 9115 2 9 0 6 ~74 7 ± 3 0 10 I 145 6 96 61111 20 85 o (X~)2 6 82~ 0 422 1 72 8 2 7 18 81 251 2 ± 1 8 17 / 145 o q " 65o 2 2 9 8 0 0 0 2 6 7 , )07 0 7 3 0 2 5 3 85 5 2 0 5 o 2 " 3 3 ± 1 6 10 1 083 6 t4 650 20 o v O 0 ~ 2gR 0 962 3 6 t 86 7 1o 97 265 6 ± 1 2 19 I I)8t 6 34 7t)O 2 2 4 6 0 0 0 2 0 4 0 7 2 0 9 0 8 4 63 G,0 I 2 i 23 281 1±24 7 1 0 2 8 5 80 7OO 2 1 5 3 O.O(D6 2 023 2 0 2 6 8 9 ')2 9 20 91 2 7 7 1 ~-0.8 2O I 0251 5 7 7 ~50 2241 0 01Kt7 2 ffa6 3 72 I 1 I 92 7 2t 78 287 O±l I i9 O ° 7 8 5 29 75O 23 37 0(} 4 386 2 55 1 3 9 85 v 22115 2 9 1 1 ± 1 7 26 t) 07~ 5 4 4 8(FO 23 86 0 0 4 3O4 2 5 4 16 7 8 7 4 2 2 5 6 2 9 7 3 ± 2 5 14 O 932 5 O9 8[g) 24 65 0 0 4 ~ 8 5 3 4 0 2O 6 89 ~ 2 t 33 Ic4~ 7 ~ 9 ~2 0 932 5 24 850 2 6 2 2 0 0 5 534 3 4~ 2 4 4 8 8 2 24 56 ~215±2 7 10 (I 81~) 4 0 2 850 26 89 0 ( ~ ) 2 5 2f~ 2 55 27 2 87 - 2 5 t l 3 t 0 4 z 1 4 27 0 88~ 5 15 o(xI 26 80 11111)13 2 825 2 83 3 0 4 ,.~ 8 25 o4 3 ~8.0±1 ~ 18 11853 4 88 950 28 33 0 Of XR) 2 ~62 5 {X) 3 6 . 80 5 2" 61 ~57 7±1 2 2 Q II81 g a 72

10011 2q 02 0 ~ 5 6 8 6 9 2 3 8 ~9 1 86 a 27 I% ~(~) 7±t 6 g 11786 4 48 I00~) 29 12 0 f~)10 ~ ~1~ 5 al) 45.4 97 3 28 12 361.6~0 9 94 D 78h 4 71 1051) 20 DI 0 (/4)18 I ()~0 31)0 48 7 92 9 28 67 ~70 0 ± 2 0 14 O756 4 53 11(~) 20 5 ] O Of)lq I (M,O a 55 5 3 R 93 1 2~ [" t 7 6 0 - 2 2 0 14 I) v28 4 3 I [ [(~) 2 8 4 6 0 (~)32 f) 053I ~ 87 58 i R t) 9 2R /6 ~6~ l ± l 6 21 I)~28 4 52 t 100 29 I t 0 ~X142 1 069 2 7 2 6 1 2 86 q 2851) ~ 6 8 1±2 7 2 ~ I) 2 8 4 62 1 I(R) 29 45 O(~M ~ 4 54(I 4 811 66 5 7O 7 28 OR 363 2 ~ ) H 76 O 728 4 88 I I(1~? 2 9 6 4 DO(M4 6 t 2 1 5 87 73 I . i i1 2 7 " 4 ~59 2 ± 2 4 224 D - 2 8 5 18 1[(#) 31 57 0 01)38 9 473 5 9 g 7o 8 57 4 28 "5 3 7 l I):t:63 625 0 728 5 51 12(#) 31129 0 f~)8N 5 214 1 4~ 8 1 4 ~ 4 2 2 8 " 2 t70 7 .1 v 10 0 679 4 25 : 25o 29 82 0 004S 4 23O - (~ 81)~ 89 2 28.55 ~68 6 ~ ) : 16 I) 657 ~ 59 I ~G~I 28 N) 0 (1~47 0 IXI(×) 7 51 9 8 ~ '~15 28 87 3 7 2 4 ~ ) 7 81 0 636 2 ')2 / 4(1~1 ~7 27 0 0041 44 71 1118 r~) 5 1 7 2 24 (~t t / 5 21: g 5 t l /I 598 ~ 18 15511 l q 6Q 0 (I 44 ~9 0 4 IN 100 2 ~o 6 5 5 t 01 5 ~ 3 0

I N V A C U O C R U S H I N G E X P E R I M E N T S A N D K - F E L D S P A R T H E R M O C H R O N O M E T R Y 171

ternative explanations for the K-correlated com- ponent, including the effects of recoil and voids in the K-feldspar, but were unable to reach a definite conclusion. The late release of additional 3SAra during thermal degassing, together with rising apparent ages, led them to infer that argon, released from these composite materials by an incompletely understood mechanism, had af- fected the age spectrum. We have extended their approach to a K-feldspar sample more typical of those used in our thermochronologic studies and can confirm the release of Cl-correlated argon during crushing. However, we conclude that these effects have negligible consequences on the inter- pretation of the age spectrum in terms of ther- mochronometric models. Observation of a consis- tent relationship between the release of Cl-de- rived argon and excess 4°Ar* among isothermal duplicate heating steps provides a key to seeing through the effects of the excess argon commonly observed in the initial heating steps of K-feld- spars.

2. Results

MH-10 K-feldspar, described in detail else- where [9], has been used extensively in the devel- opment of an interpretive model for 4°Ar/39Ar age spectra that assumes the presence of a distri- bution of diffusion domain sizes [6-10]. Its use in a test of the Turner and Wang proposal is ideal, because it has been the subject of thorough ki- netic, XRD, TEM and visible light investigations, with the result that its microstructure and argon diffusion behavior are very well understood [9,10]. This sample contains regions of pristine K-feld- spar 50-100/xm in size, separated by zones char- acterized by a structurally modified feldspar. This latter material makes up ~ 5 vol.% of the sample and contains minor, apparently not connected, fluid-filled (?) inclusions that range from 0.05 to 1-2 /xm in diameter. About 20% of the K-feld- spar domains defined by these features contain micron to sub-micron perthite development, but otherwise the K-feldspar is remarkably pristine

0.001

0.0008

0 . 0 0 0 6

0.0004

0.0002

i

M H - I O'~

~ + O o

(y°~ o

o 0 0.0'005 0.001 0.0'015

[ ' 1200

(=J 1100

1000

900

800

~.~ 700 t ~

600

500

l 400

_ _ 300

200

100

0 0.002

i i I i i i i i i

0

0.2,1 ~ .K -a~ " 6 C~ / 'K I ,

0 0.1 0.2 0.3 0.4 015 0.6 017 018 0.9

SaAT=/S~IT X

1.0

Fig. 1. (a) Isochron plot of argon isotopes evolved from twenty crushing and three blank steps of irradiated MH-10.u K-feldspar. No linear array is apparent in these results, suggesting the presence of additional component(s). (b) Correlation plot of 4°Ar*/39Ar~ against 38Arcl/39ARK for the same results as in (a) in which 'atmospheric' 4°Ar has been removed by 36Ar'295.5. Note that 4 ° a r * / K = 4°Ar */39ARK. 9.72 × 10 s and C1/K = 38Arcl/39Ari,:. 0.277. The fit shown incorporates sixteen of the twenty crushing data but does not include the three 'blank' steps. Despite a reasonably good correlation (4°ARE/CI = 1.02 × 10-3), the meaning of

this array remains obscure.

172 T.M. H A R R I S O N E T AL.

and undeformed. For example, MH-10 contains negligible dislocations and no 'voids' observable by TEM [9].

We have performed an in v a c u o crushing ex- periment on MH-10 using a simple arrangement in which the driver of an all-metal mini valve displaced a stainless steel shaft into a blank flange, shaped as a mortar. This mortar contained 24.3 mg of K-feldspar ~ 0.5 mm in size. Following baking over night, argon was released in 23 steps by progressively torquing the valve driver, eventu- ally reaching 10 N-m. This method appears to be at least as effective in reducing grain size as 1000 crushes using a magnetically operated pestle [5]. Microscopic examination of the fine powder cre- ated by crushing reveals that about two-thirds of the grains contain at least one edge as large as 50-100 Ixm, with the rest mostly in the form of angular fragments up to 100 ixm in size. Only 4% of the sample was in the form of 150-200 Ixm sized particles and no grain was larger than 200 /xm. As relatively little gas was lost over the last two crushing steps, we infer that most of the argon accessible by this approach was released.

Details of the irradiation environment, extrac- tion system, mass spectrometry and correction factors are given in [8], with the exception of (38Ar/39Ar)K = 1.20 _+ 0.03 × 10 -2. The C I / K ra- tios were calculated using the relationship 38ArcI/39ArK • 0.277, determined by analysis of ir- radiated KC1. The results from both the crushing of the whole sample and step heating of an un- fractionated aliquant of the resulting powder were designated MH-10.u. Analyses performed on the > 150 /xm split of the remaining powder were labelled MH-10.u'. All isotopic results are pre- sented in Table 1.37Ar/39Ar ratios are not given because the sample was irradiated more than 1 yr prior to analysis. No correction was necessary for 36Ar produced b y / 3 - decay of 36C1.

An isochron plot of the argon ratios obtained from the crushing experiment (Fig. la) yields a complex, J-shaped pattern. However, when atmo- spheric corrected (see caption of Fig. 1), sixteen of the 23 crushing steps yield a moderately well correlated array on a plot of 4OAr*/39ArK versus 38Arc|/39Ar K that extrapolates to a negative 4°Ar*/39ArK value for zero C1/K (Fig. lb). Not included in this fit are the three lowermost data on the left ( 'blank' steps in which the sample

remained under stress but the piston was not further advanced, including an overnight blank) and four of the five initial crushing steps (in the right uppermost corner), that fall off this line.

In strong contrast to earlier results [4,5], the total 39At liberated during this process (4.42 × 10-15 mol) amounts to only 0.078% of the sample total. This small fraction, however, does account for 7 Ma of the total integrated K - A r age and appears to correspond broadly to the anoma- lously old ages found in the first few tenths of a percent of 39Ar released from conventionally heated splits [10]. We have since crushed two contrasting K-feldspar samples by this method (Cooma Granodiorite K-feldspar and Fish Canyon sanidine), and obtained only 0.084 and 0.11% of their total 39Ar, respectively. The similarity be- tween these three results suggests that a value of ~ 0.1% may be typical of many alkali feldspars.

A 9.68 mg split of the crushed powder was wrapped in Sn foil and heated at temperatures from 250 ° to 1500°C in a double-vacuum resis- tance furnace [8]. The resulting age spectrum has a pronounced peak at about 80% 39Ar released and yields an integrated total fusion age of 341.3 Ma, or 348.3 Ma when the 4°Ar* /K released during crushing is added back. This latter age is indistinguishable from the average K - A r age of 347 + 2 Ma previously obtained for MH-10. The clearest differences between these results relative to the uncrushed material are: both the signifi- cantly lowered 38ArcJ39ArK values and the lack of plus ages over 109 yrs in the initial gas release, anomalously old ages at around 80% gas release that then drop sharply, and a decrease in argon retentivity (see diffusion coefficients in Table 1 compared to [10]). Only this latter effect was previously observed in fractions of MH-10 re- duced in size to tens of microns prior to irradia- tion [10].

By sieving the remaining ~ 14 mg through a 100 mesh screen, 0.6 mg of > 150 txm grains were recovered. The separated fraction was wrapped in Sn foil and step heated at tempera- tures from 350 ° to 1550°C using procedures iden- tical to those described above (Table 1). This analysis, designated MH-10.u' , contrasts with MH-10.u in yielding both a similar age spectrum (Fig. 2) and Arrhenius data (Table 1) to the many results previously obtained for MH-10 [e.g., 6 -

I N V A C U O C R U S H I N G E X P E R I M E N T S A N D K - F E L D S P A R T H E R M O C H R O N O M E T R Y 173

lo-'J I

10-~ [ , 40O

"6" 35o

o 300

t~

& < 250

200

i : K ' t"d" ~

MH-IO.t MH-IO.u'

- , , , ,

o ~o 2o 3o +o so +o 7o ~o 9o ~oo

Cumulative %39Ar released

Fig. 2. Age spectra for the thermal degassing results of MH-10.u (unfractionated sample) and MH-10.u' ( > 150 /xm grains) following in vacuo crushing. Also shown for reference is an age spectrum of MH-10 (MH-10.t grain size ~ 140/xm [10]) that was crushed prior to irradiation. The molar C1/K ratio for each heating step of MH-10.u is shown in the accompanying box. Although the age spectrum of the split crushed after irradiation has been profoundly modified com- pared to the control spectrum and that of the > 150/zm split,

the K - A t age is unchanged.

8,10]. The incremental total fusion age, excluding the crushing results, is 346 Ma. This is within the expected bounds for MH-10, indicating that these > 150 ~ m crystals have not lost 4°ArE amounts comparable to that removed from the split con- taining particles well below the largest diffusion domain size.

3. Discussion

3.1 Cl-correlated 4°Ar from crushing

Although restricted to a much smaller fraction of total argon release, the 4°ArE/C1 = 1.02 _+ 0.17 × 10 -3 (chlorine-correlated excess 4°Ar*) sug- gested by the crushing results from MH-10.u (Fig. lb) is similar to the highest value reported by Burgess et al. [5] and a factor of four higher than Turner and Wang's sample, JX04 [4]. While Burgess et al. [5] attributed direct chronological significance to the age intercept of the 4°Ar*/

39ARK versus 38Arcl/39ArK plot, Turner and Wang [4] were more cautious. Several factors, including discordance between apparent ages derived in this way and by step or laser heating [5], large uncertainties [4,5], or the lack of an independent benchmark with which to assess concordancy [4], prevent definitive interpretation of the chrono- logical meaning of these results. Our result (Fig. lb) suggests caution in the a priori interpretation of correlations arising from crushing as having significance for the age of the K-feldspar and associated features.

Although even the linear array of data from the crushing experiment (Fig. lb) contains scatter in excess of analytical precision (MSWD = 7.9), the projected line is nearly 4 s away from passing through the K - A r age. A plot of all [4°Ar*] against [3SArcl] results (not shown) yields a broadly linear relationship that passes through the origin, suggesting no K-correlated excess 4°Ar*, but this line is heavily weighted by the 'blank' steps. All data on an 4°Ar/39ArK against 36Ar/39ArK against 38Arcl/39ArK plot (not shown) yield a positive 4 ° A r / 3 9 A r K intercept (correspond- ing to an age of ~ 600 Ma), but contain a rela- tively large dispersion. A similar 3-D analysis of only those results used in the 2-D regression (i.e., Fig. lb) results in a significantly better fit and necessarily also yields a negative intercept on the 4°Ar/39ArK axis. Possibly because of the domi- nance of the Cl-correlated argon, we have not been able to establish the existence in MH-10.u of a K-correlated component of 4°Ar* that is accessible via crushing.

The 39Ar released during crushing may be de- rived all or in part from the K-feldspar structure. This is because the highly linear and reproducible Arrhenius plots obtained from MH-10 over the initial gas release (fig. 4a in [10]) do not suggest derivation of significant 39ARK from sources other than the K-feldspar structure. Consider the case in which the K-feldspar has been reduced in size to 20 x 20 × 20 /zm cubes. What fraction of the K-derived 39Ar will be exposed by the creation of these new surfaces? Given that the K-feldspar unit cell is ~ 10 .~ on edge, approximately 0.03% of the 39mr would be directly adjacent to a freshly created boundary. This small fraction is in the order of that lost from MH-10 during crushing and we speculate that the 39Ar at these bound-

174 T . M . H A R R I S O N E T A L .

aries might be readily lost during the fracturing process. Although the effective particle size of the crushed aggregate is undoubtedly greater than 20 p~m, we expect the fractured surfaces would have a fractal character, thereby exposing a higher proportion of unit cells than that estimated by the calculation above.

Even in the unlikely case in which all the 39ARK evolved during the crushing experiment was hosted by, for example, inclusions in the modified feldspar zones [9], the effect of a non-volume distribution of < 0.1% of the potassium on the Arrhenius plot would still be very small as this tiny fraction of 39Ar is usually released in the first one or two steps [10]. It is often these initial steps that yield diffusion coefficients slightly higher than those predicted by extrapolation of the higher t empera tu re data [7,11], suggesting that many other K-feldspars may also be minimally affected by this potential complication. Provided K-feldspar samples yield robust and linear Arrhe- nius relationships in the initial stages of gas re- lease, it appears unnecessary to perform in vacuo crushing routinely to assess the fraction of potas- sium held in non-volume sites.

3.2 Cl-correlated 4°mr from thermal degassing

To establish whether trends on Arrhenius plots are meaningful or only apparent, we have adopted the practice of performing isothermal duplicate heating steps [7]. During the step heating of MH- 10.u (and MH-10.u') in this fashion, we noticed a relationship b e t w e e n 38Arcl/39Ar K and apparent age. From 450 ° to 600°C, the first heating step at a given temperature yields both a significantly higher age and 38Ara/39ArK than the subsequent, duplicate step (note that the 250°C pair could not be used due to very high atmospheric contamina- tion). Above 600°C (i.e., at > 3.5% of cumulative 39mr release), this correlation is no longer seen because C1/K values have dropped to back- ground levels. To understand this relationship requires some additional background.

We can write the total atmospheric-corrected 4 ° A r * released in the laboratory as the sum of two components:

4°Ar * = 4°ARE + 4°ArR (1)

TABLE 2

Isothermal duplicate results used in Fig. 3

Temperature CI/K 4OAr*/K 4OAI-R/K A4OAI'*/K ACI/K (°C) (10- 3 ) (10-6) (10.6) (10-6) (10-3)

450 8.338 4.442 2131 2.137 7.728 450 0.609 2.304 2.135 800 4.986 3.497 2 116 1 313 4.570 500 0.415 2.185 2.069 550 1.634 2.397 1,944 0500 1.580 550 0.054 1.896 1.881 600 0.499 2.032 1.893 0.078 0.444 600 0.054 1.955 1.939

where 40ArR is in situ radiogenic 4°Ar. The ratio 4°Ar*/K can be expressed as:

4°Ar*/K = (4°ArE/CI • C1/K ) + 4°ArR/K (2)

Assuming that 4°ArE/C1 is constant across some interval of degassing, the difference be- tween isotopic results from any two heating steps of equal age (i.e., A4°)krR/K = 0) is given by:

m4°mr * / K = 4°ArE/C1 • AC1/K (3)

and thus a plot of A4°Ar*/K versus AC1/K should yield a linear relationship with a slope equal to the 4°Arz/Cl ratio.

We have recast our isothermal duplicate data as follows: the 4°Ar * / K ratio of the initial step in the pair is subtracted from the second 4°Ar*/K ratio. This value is then plotted against AC1/K, calculated from the difference in 38Arcl/39ArK ratios of the contiguous isothermal steps. The

2.5

MH-10.U 450OC 4O -4

2.0 Are/Cl = (2.7-+0.2)x10

L S 5 0 0 0 C

1.0

550~C MH-10.U'

0.5 00°C ~ 0 0 o C (~(2.3-+0.3)x10" 4

0.0 / ~ ' ' , , r 0.0 2.0 4.0 6.0 8.0

ACI/K (xl0 "3)

Fig. 3. Plot of A4°Ar*/39ArK against ACI /K (molar) for the four isothermal step-heating pairs between 400 ° and 600°C for MH-10.u. The slope of the line corresponds to an 4°ARE/Cl = 2.7_+ 0.2 X 10-4, indicating the presencc of a Cl-correlated component of excess 4°At*. The initial isothermal duplicates from MH-10.u' (450-600°C)yield an average value of 2.3 _+ 0.3

x 10 -4, which is indistinguishable from that of MH-10.u.

IN VACUO CRUSHING EXPERIMENTS AND K-FELDSPAR T H E R M O C H R O N O M E T R Y 175

duplicate results corrected in this way are highly correlated and indicate an 4°mrE/C1 ratio of 2.7 ± 0.2 x 10 -4 (Table 2; Fig. 3). Although less pre- cise because of their smaller total signals, the isothermal duplicates in the temperature range 450-600°C from the > 150 /xm split (MH-10.u', Table 1) yield an essentially identical average 4°ArE/C1 ratio of 2.3 + 0.3 X 10 -4. Isothermal duplicate results from two previously measured 'virgin' splits, MH-10.q and MH-10.r [10], were re-examined and found to contain 4°Ar~/C1 ra- tios of 3.1 x 10 - 4 and 3.5 x 10 -4, respectively. These somewhat higher values are not unex- pected as both samples presumably still con- tained the elevated 4°ArE/CI component (i.e., 1.02 x 10 -3) found during in vacuo crushing of MH-10.u.

Why is the first step of the isothermal dupli- cate pair disproportionately affected by the C1- correlated excess argon component and why are the crushing and step-heating 4°mrE/C1 ratios different by a factor of four? The answer to the first question appears to be straightforward and we address it here, returning later to the second question. Attainment of a new peak temperature during laboratory degassing appears to be the key element in the release of Cl-correlated 4°Ar. The subsequent step at the same temperature appears to have little or no effect on releasing these isotopes. The potential for reaction due to expo- sure at a certain temperature is largely realized upon reaching that threshold and leads us to conclude that these correlated argon isotopes are derived from a system containing a spectrum of thermal stabilities. Heating at 450°C causes the more susceptible portions of this system to re- lease their contained argon, while a second step at 450°C releases a higher fraction of volume-sited argon. When the temperature is raised to 500°C, features that had barely resisted deterioration now fail, releasing Cl-correlated argon. This pro- cess continues until all accessible features are exhausted.

Both the degassing behavior during isothermal replicate steps and microstructural observations are consistent with this effect, arising from the decrepitation of fluid inclusions. Many factors control the strength of an inclusion, including size, shape, fluid density and composition, and heating (strain) rate [17-19]. As temperature is

raised, fluid pressure within sealed pores in- creases while the strength of the confining medium drops, eventually causing the host to fail. For a given geometry, larger pores will tend to rupture first. For a distribution of geometries containing a similar fluid pressure, irregularly shaped inclusions will tend to have higher stress in their tips compared to more equant ones and preferentially fail [20]. The most likely candidates to host the Cl-correlated 4°Ar are the apparently unconnected inclusions, typically with diameters of 0.05-0.5 t~m, that inhabit zones of 'modified' feldspar [9]. Although some of these features appear to persist when heat treated to 950°C, a new generation of pore trains, ~ 20 nm in size, is created by laboratory heating [9], and are proba- bly the result of the decrepitation of larger inclu- sions. There is some indication that an extremely small-scale feature, making up no more than 0.1 vol.% of the sample, is produced by laboratory heat treatment [10]. This might reflect nano-scale fracturing of the modified zones.

Further insight into the siting of the Cl-corre- lated component comes from results of samples of MH-10 heated in the laboratory prior to irra- diation (e.g., MH-10.n in [10]). Despite experienc- ing temperatures as high as 1100°C, initial step heating results yield similar 38Aro/agArK values to those observed from the in vacuo crushing. That is, pre-irradiation heating has caused 4°Ar* to be released but the C1/K ratios in the early stages of thermal degassing are not significantly reduced. In vacuo crushing of a sister split pre- heated to 1100°C for 71 min prior to irradiation (MH-10.v) releases a similar fraction (0.11%) of agAr, a much greater (5 times) quantity of 4°At*, and about half the O-derived argon compared to MH-10.u (Table 1). It appears that physical changes to the sample during laboratory heat treatment have both permitted some of the 4°Ar* mobilized during heating to be captured in fea- tures accessible by crushing and resulted in the loss of C1. Although this particular agency would have no relevance to the capture of 4°Ar* during slow cooling, we do not rule out a related natural mechanism having such a property.

As is often the case, apparent ages over the first few percent of gas release are anomalously old and highly variable. Using the relationship 4°ArE/C1 = 2.7 x 10 -4, we have applied a 4°ARE

176

k~

10 "2

10 a

10 "~

600

550

5OO

450

400

350

~. 300

250

200

150 0

/ -

i L t - - - M H - 1 0 . U ( u n c o r r e c t e d )

f - - M H - 1 0 . u ( C I c o r r e c t e d a g e s ) i i i i i i i

i i

i _ i i

i i I _ l r . . . . . . -

_ F -

7 -

I I

1 2

Cumula t i ve % 39 A r re leased

Fig. 4. Age spectrum of the first 3% of gas released from MH-10.u showing the results corrected (solid line) and uncor- rected (dashed line) for the presence of Cl-correlated excess 4°Ar. The product of the 4°ArE/CI (Fig. 3) and the C1/K (determined for each step via the 38Ar/39Ar ratio) yields an "excess age" that is simply subtradted from the measured 4°Ar*/4°K to yield an age corrected for the presence of excess argon. The corrected results suggest an age distribution

consistent with continued cooling.

correction to the ages between 0.12 and 2% of 39Ar release (see caption in Fig. 4). Uncorrected, the age spectrum over the first few percent of gas release is highly variable but, following correc- tion, reveals a geologically reasonable and broadly increasing pat tern of ages (Fig. 4). We interpret this spectrum as recording the continuation of cooling of the host pluton to temperatures as low as ~ 125°C [6]. This apparent success supports the view of Turner and Wang [4] that correction for a Cl-correlated component should be straight- forward, although, in this case, identification of the function arose from thermal degassing rather than in vacuo crushing. We have recently repli- cated this observation using a K-feldspar (XR-2A) from our studies of southern Tibet. This sample contains substantial Cl-correlated excess argon (4°ArE/CI = 1.45 __+ 0.04 × 10 -4) over the first 9% of gas release, with ages ranging between 16 and 71 Ma. However, using this approach, all eleven ages correct to a geologically meaningful value of 9_+2 Ma.

T . M . H A R R I S O N E T A L .

Although the method described above was piv- otal in our recognition of this effect, it is not absolutely necessary to perform isothermal dupli- cates to make the correction for Cl-correlated excess argon, provided that the 4°ArE/CI and 4°ArR/K ratios are constant throughout the con- taminated portion of the age spectrum. However, because we have no a priori knowledge of whether an in situ radiogenic age gradient is present, it is advisable to restrict calculation of A to adjacent isothermal steps.

3.3 Cl-correlated 4°Ar and its effect on the age spectrum

Midway through the thermal degassing of JX04 that followed in vacuo crushing, Turner and Wang [4] observed an increase in the 4°Ar* /K ratio towards values associated with the K-correlated component, and later increased C I / K ratios. They speculated that the same reservoirs that had been tapped during the crushing were releasing extra- neous argon late in thermal degassing. It was this possible relationship that inspired their caution- ary statement with regard to thermochronometric interpretations. In contrast, our results for MH-10 do not indicate the presence of a K-correlated 4°ARE component released by crushing (section 3.1) but, rather, suggest that the origin of the increased C I / K is volume-sited chlorine and that the anomalous form of age spectra from crushed samples is due an artifact produced by the crush- ing experiment.

C1/K ratios increase in the last 40% of gas release from values of 0.00005 to 0.0019 but even this latter C1/K is still more than two orders of magnitude lower than the peak values obtained during crushing (Table 1). Over this same range of 39Ar release, ages rise well above that of pluton emplacement. Worth noting are both the jumps in age associated with temperature increases be- tween 1050-1150°C and the opposite trend with respect to the AC1/K of isothermal duplicates in this temperature range compared with the 450- 600°C steps. Could the anomalously old ages be due to either of the two distinctive 4°ArE/C1 components already identified? The answer ap- pears to be no for two related reasons. Assuming a 'background' 38Aro/39ArK and age of 0.0002 (Table 1) and 373 Ma [21], respectively, a plot of

IN VACUO C R U S H I N G E X P E R I M E N T S AND K-FELDSPAR T H E R M O C H R O N O M E T R Y 177

Aa°Ar*/K versus AC1/K for the last 40% of gas release yields what could be interpreted as yet a third Cl-correlated component of 4°ARE (4°ArE/ CI = 8.0 + 0.2 X 10-5). We seriously doubt that this latter relationship is meaningful as we ob- serve a similar increase in C1/K over the same portion of gas release in a ~ 140/xm split crushed prior to irradiation (MH-10.t in [10]) and in the > 150 /xm fraction of the crushed sample (MH- 10.u'). Both these samples yield plateau ages in the expected range of 365-373 Ma. In fact, log (r/r o) plots for both 38Ar o and 39ARK released from MH-10.u show similar behavior over the last 80% of 39Ar release, suggesting that the associ- ated chlorine is volume sited.

The inference we make from these results is that the late released chlorine from MH-10.u, amounting to 50 ppm or about 75% of the sample total, is distributed within oxygen sites in the K-feldspar lattice. This conclusion is consistent with measurements of insoluble CI in K-feldspar, which suggest volume chlorine contents at least as high as we have measured [15]. Under this

interpretation, 38Arft is released from these sites only at high temperatures, due to their low acti- vation energy a n d / o r the very sluggish nature of anion vacancy transfer under anhydrous condi- tions [12-14]. Direct ion imaging of the chlorine distribution in MH-10 confirms that high levels of C1 occur both within and adjacent to the zones of modified feldspar (Fig. 5) but that the vast major- ity of the feldspar contains relatively low (0.1-1 ppm) and uniformly distributed chlorine concen- trations. As can be seen from Fig. 5, the regions of heterogeneous C1 are typically 5 x 15 /xm in size. However, TEM imaging [9] has shown that even the largest fluid-bearing inclusions in MH-10 are only 1-2 /xm in size and are more typically much smaller, indicating that the vast majority of the chlorine in this feldspar must indeed be vol- ume sited.

The close association between the 5 x 15 /xm regions of high chlorine concentration and the sub-micron inclusions, containing the Cl-corre- lated component of 4°mrE released during the initial stages of step-heating, raises a question.

0 ' 9RP

25 24 23 21 2e 18

15 13

i0

Fig. 5. Scanning ion image of the 35C1- distribution in a modified zone of MH-10 K-feldspar. The image was acquired by a CAMECA 4f ion microprobe using a primary probe of 10 kV Cs ÷ at a mass resolving power of 2000 and using the normal incidence electron gun to neutralize sample charging. The imaged field is 75 x 75 tzm and the lateral resolution is 0.7 txm. Chlorine concentrations were quantified using an NBS glass containing ~ 50 ppm CI. In this image, relative chlorine abundances are shown by the grey scale where the background concentration is approximately 1 ppm. CI is heterogeneously distributed, with high concentrations persisting over 5 × 10 tzm regions. TEM imaging reveals that the largest fluid-bearing inclusions are 1-2/~m in size, and typically much smaller. Other ion images (e.g., 28Si) and previous TEM observations indicate that these regions are alkali

feldspar and thus the vast majority of the chlorine is volume sited.

178 T.M. HARRISON ET AL.

Assuming that the chlorine that is now sited in the K-feldspar structure was associated with a given 4°ArE/C1 in the precursor fluid phase, what happened to the related 4°ARE? During the episode at elevated temperature in which fluids were introduced, causing reprecipitation of small portions of the K-feldspar, this 4°ARE may have either been partitioned into the fluid phase (in- creasing the 4°ArE/C1 ratio) or become struc- turally bound and then subsequently lost, due to its very high diffusivity. Once the inclusions formed, continued evolution of t he 4°ArE/CI ra- tio in the fluid may have occurred due to prefer- ential partitioning of CI into the solid relative to 4 ° A r E.

Not surprisingly, the act of crushing has in- duced an artifact into the age spectrum. We carried out twenty age spectrum analyses on coarse splits (> 130/.~m) of MH-10, including the age spectrum from the > 150 ~m split of the crushed sample (MH-10.u') [7,8,10,21], and had never before seen the anomalous rise in 4°Ar/39Ar at 80% gas release (to apparent ages 25 Ma older than pluton emplacement [18]) followed by an abrupt decrease (Fig. 2). We have, however, seen this identical behavior in a split of MH-10 re- duced in size to ~ 54 /xm prior to irradiation [10]. Thus, the cause of this anomalous effect must be due to crushing the particles smaller than the largest diffusion domain size (50-100 p~m in MH-10 [9,10]). The MH-10.u age spectrum (Fig. 2) appears to mimic features of JX04, sug- gesting that features in the JX04 age spectrum may also be a consequence of crushing.

We speculate that the anomalous age spec- trum behavior is a consequence of exposing re- gions of the crystal that were originally remote from a natural diffusion boundary. Small (a few percent) internal heterogeneities in the distribu- tion of 4°Ar a n d / o r 39Ar could be envisaged (e.g., 4°gr* residing in defect clusters, 39Ar recoiled into K-poor zones) that would normally (i.e., dur- ing step-heating of the intact domain structure) be homogenized by the randomizing effects of diffusion or even by the eventual melting of the sample. By artificially creating internal surfaces via crushing, these anomalous zones could permit either relatively enhanced or retarded release of 4°Ar or 39Ar, which could lead to seemingly par- entless 4°Ar* in a manner somewhat analogous

to the 'unsupported' Pb* observed in complexly zoned zircons [22]. Since we see the anomalous age spectrum behavior in samples crushed both before and after irradiation, and because of the substantially enhanced diffusivity of 39Ar that re- sults from crushing, we suspect 4°Ar* residing in higher-order defects [23] may be the culprit. Al- though an additional complication, these isotopic heterogeneities apparently do not materially af- fect thermochronometric reconstructions, pro- vided that natural diffusion boundaries are not disturbed during laboratory analysis.

Turner and Wang [4] express the view that the disturbed step heating pattern of their sample JX04 might conventionally be read as a sample having an age in excess of 1000 Ma that experi- enced episodic argon loss at about 350 Ma. This interpretation would presumably lead to an erro- neous inference regarding the thermal history of such a sample if the increased apparent ages late in gas release were due to the 4°ArlUK compo- nent. Although we do not share their basic premise, having previously argued [11,16] that virtually identical patterns appear to be diagnos- tic of extraneous argon, the potential presence of excess argon is not an impediment to performing calculations such as the multi-domain analysis [6-8]. This is because evaluation of the relation- ship between inflections in the age spectrum and log ( r / r o) plots permits a self-consistency test of the assumptions used in calculating the model age spectrum. Indeed, in at least one case (FA-8 in [11]), we have used the multi-diffusion domain approach as a tool to confirm the presence of excess argon.

4. Summary

Discovery through in vacuo crushing of CI- and K-correlated components of 4°ARE in K- feldspar-bearing samples [4], components that ap- pear to re-emerge late in thermal degassing, stim- ulated us to perform similar measurements on a well-characterized sub-solvus igneous K-feldspar (MH-10). We confirm the presence of a chlorine- related component ( 4 ° A r E / C I = 1.02 × 10 -3) from sites accessible by crushing MH-10, al- though it is associated with only 0.078% of the total sample potassium. However, we could not identify a K-correlated component. Isothermal

IN VACUO CRUSHING EXPERIMENTS AND K-FELDSPAR T H E R M O C H R O N O M E T R Y 179

duplicate heating steps reveal a Cl-correlated component (4°ArE/C1 = 2.7 × 10 -4) that appears to be related to decrepitation of fluid inclusions. There appears to be no adverse consequence arising from these effects that would significantly impact on a thermochronometric interpretation of the 4°gr/39Ar results for MH-10. Rather, iden- tification of the 4°ArE/C1 component released during step heating permits us to see through the effects of excess argon, allowing a previously in- accessible portion of the age spectrum to reveal thermal history information. We strongly urge others to measure and report 3SAr results from K-feldspar samples routinely.

The implications of Turner and Wang's [4] observations are potentially important and only beginning to be exploited. We expect that there is a continuum of behavior between the samples they describe and the one that we have docu- mented here. However, it is possible that a reader may misinterpret their caution, arising from an effect seen in fine-grained sediments, as a criti- cism of the articles they cite as representative of thermoehronometry. One suggestion from our findings is that care should be exercised when extrapolating behavior observed in materials out- side the realm of those commonly used for ther- mochronometry (e.g., chert and stromatolite, hy- persolvus feldspars and authigenic overgrowths [4,5,24]), lest it divert attention from the funda- mental characteristics shared by many K-feld- spars originating at mid to deep crustal levels.

Our results lead us to the following summary model for MH-10. Crushing in vacuo releases 39ARK from K-feldspar sites adjacent freshly bro- ken surfaces, 4°ArE sited in or adjacent to domain boundaries and inclusions, and 3SArcl from inclu- sions ( + domain boundaries). Initial step heating activates loss of 4°Ar* and 39ARK from the K- feldspar structure by diffusion, but also causes decrepitation of fluid inclusions, formed during a sub-solidus alteration event, that release a dis- tinctive 4°ArE/C1 composition. These features form progressively smaller inclusion trails but are eventually exhausted of fluid. Further step heat- ing causes continued diffusive loss of 4°Ar* and 39ARK, but eventually reaches high enough tem- peratures that 3SAra originating in anion sites begins to be mobilized by diffusion and then released from the mineral. A small fraction of the

4°Ar* that may have been trapped in defects during transport in nature is released from these energy wells, diffusively mixed with the associated 39Ar, and ultimately released at domain bound- aries.

Acknowledgements

We thank Marty Grove for his interest in this study and assistance with the isotopic measure- ments, Ben McClellan for fabricating the crush- ing device on short notice, Didier Renard for the ion probe measurements, Kevin McKeegan and Roberto Gomez for providing the 3-D plotting and regression routines, Jeff Fillipone for helpful discussions, and Chris Roddick and Peter Zeitler for constructive reviews. Support for this research was derived from a grant from the Office of Basic Energy Research, Department of Energy.

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