(1973) Effect of pressure on the composition of co-existing pyroxenes and garnet in the system. CaSiO.-MgSiO,-FeSiO"-CaAlTi,Ou Carnegie Inst lilash. year.
American Mineraolgist, Volume 61, pages 616-625, 1976
Subsolidus equilibriabetweenpyroxenesin the CaO-MgO-SiO, systemat highpressures and temperatures TerrsHr Monr nNo Devrn H. GnrBN Research School of Earth Sciences Australian Nqtional Uniuersity, Canberra, Aust ralia 2600
Abstract The extent of pyroxene solid solutions has been experimentally determined in the CaO-MgO-SiO2 systemat 30 kbar and 1500"C. The resultsshow the enstatiteand diopside mutual solid solutions and also definethe limited solid solutionstowards olivine and quartz. Although the enstatite-diopsidesolvusis wider when olivine rather than quartz is a coexisting phase, the difference does not seriously affect the pyroxene solvus as a geothermometer. Thus, this geothermometer can be used without taking the coexisting phases other than pyroxenes lnto account. The study of the enstatite-diopside solvus in the MgrSi,Ou-CaMgSirOu system has been extended above 1500'C at 20 and 30 kbar. These pyroxenes show almost complete solid solution at solidus temperatures.The iron-free pigeonite-diopside solvus which was previously found at 20 kbar and inferred to be stableat 30 kbar did not appearat thesepressures. The phase equilibrium data in this paper are combined with those in the literature to make a compilation of the enstatite-diopsidesolvus over a wide temperature-pressurerange. Some difficultiesin use of the pyroxene solvus geothermometerare discussed,taking the garnet lherzolite nodules from Lesotho kimberlites as examples.
Introduction The past few years have seen much interest in the phase equilibrium studies of pyroxenes (Lindsley el al., 1974a; Warner and Luth, 1974; MacGregor, 1974;Smyth,1974; Nehru and Wyllie, 1974;Howells a n d O ' H a r a , 1 9 7 5 ; M o r i a n d G r e e n , 1 9 7 5 ;a n d C h e n and Presnall, 1975). We note particularly Howells and O'Hara's (1975) treatment of pyroxenesas solid solutions towards forsterite and quartz. The present study, although a continuation of our earlier work, was in part undertakento evaluatetheir suggestions. The enstatite-diopsidesolvushas long beenusedas a geothermometerand applied to temperature estimates of natural rocks. An implicit assumption in using the geothermometeris that pyroxeneshad no solid solutions towards forsterite and/or quartz. Thus temperatureshave been estimatedwithout taking coexistingphasesinto account. However, Howells and O'Hara (1975)suggestedthat pyroxeneshave solid solutionstowards forsteriteand quartz and that these solid solutions greatly affect the degree of mutual solid solution. Since all minerals are solid solutions in a strict sense,it is important whether or not
the magnitudes of solid solution towards forsterite and quartz are enough to alter the practical application of the solvus as a geothermometer. In this paper we describe experiments made to define these solution limits. The second problem treated in this paper is a reexamination of the stability of iron-free pigeonite' at high pressures.Kushiro (1969) presenteda phasediagram in the systemMgrSirO.-CaMgSirO. at2Okbar. Pigeonite was thought to be stable above 1450"C, producing two solvi betweenenstatiteand pigeonite, and pigeonite and diopside. Howells and O'Hara (1975) reexaminedthis system at the same pressure, and contrary to Kushiro's experimentsfound homogeneous clinopyroxenes well within the pigeonite-diopside solvus as determined by Kushiro. We have reexaminedthis systemat 20 kbar and obtained ' Hereafter, this mineral is called pigeonite for convenience.We use this term for low-calcium clinopyroxene which is distinguished from diopside (high-calcium clinopyroxene) by a solvus between them This definition is also applicable to iron-bearing pigeonite. Note that if the two minerals have the same crystal structure of C2/c, it is not necessaryto call them by different names
616
6t7
SUBSOLIDUS EQUILIBR]A BETWEEN PYROXENES
results supporting Howells and O'Hara's conclusions. The third experimental problem discussedin this paper is the enstatite-diopsidesolvusat temperatures a b o v el 5 0 0 o Ca t 2 0 a n d 3 0 k b a r .T h e d a t aa r es u p p l e mentary to those previously obtained at lower temperaturesby us (Mori and Green, 1975). Throughout this paper we use mole percent rather than weight percent in describingpyroxenecompositions in terms of their end members.Data referredto from the literature have also been converted to mole percent. .
Experimental method and results
All the starting materials used in the experiments were mechanical mixtures of two or three synthetic mineralsamong diopside(CaMgSirOu),clinoenstatite (MgrSirO.), forsterite (MgzSiOr), and quartz (SiOr). The mixtures were dried and loaded into Pt capsules. We used a piston-cylinderpressureapparatus (Boyd and England, 1960a), and the furnace assemblage included alumina sleeveand alumina thermocoupleinsulator to minimize thermocouple contamination and pyrex glass instead of boron nitride outside the graphite heater.A pressurecorrection of - l0 percent of nominal load pressure,using piston-in technique, was applied. No pressure correction was made to thermometry by WrrRer/WrrRe2uthermocouples. All the run products were analyzedby means of the
TPD-electron microprobe. Phaseswere almost free from inclusions and were large enough to avoid chemical contamination from surrounding minerals: the sizes of crystals were up to 60, 40, 20, and 20 microns for enstatite,diopside,forsterite,and quartz, respectively,exceptfor quartz in trace amount (grain size around 4 microns). The details of experiments are listed in Table l. Chemical analysesof the run products are shown in Tables 2 and 3. The extentsofpyroxene solid solutionsin the CaO-MgO-SiO2 sYstem Figure I is an isothermal, isobaric diagram of the Di(CaM gSi,Ou)-Fo(2Mg,SiOn)-Qz(2SiO')system which is itself a portion of the CaO-MgO-SiOz system. Though solid solution fields are much exaggerated and are schematic only, two aspectssuggestedby Howells and O'Hara (1975)are illustrated:firstly, the diopside limb of the enstatite-diopsidesolvus is less calcic when pyroxenes are coexisting with forsterite rather than with quattz, and secondly, enstatite-diopside tie lines cross the stoichiometricEn-Di join from upper left to lower right. Among the six runs (2B1-2Gin Table l) made at 30 kbar and 1500'C, two had bulk compositions within the three-phasefields, namely, one in the forsterite-enstatite-diopside field and the other in the quartz-enstatite-diopside field, and the remaining
Tner-E l. Results of experiments in the systemsDi(CaMgSizOe)-Fo(2Mg,SiO,)-Qz(2SiOz) and En(Mg,SirO.)-Di(CaMgSirOu) Run No.
Starting
Bulk compositions (note %)
materials
P (kb)
T (oc)
hrs
Products
2BL
Fo, C-En
F o 6 g . 3Q ' s 6 . 2
30
1500
4
Fo, En
282
C-En, Qz
Fo2t.tQt72.g
30
1 50 0
4
En, Qz
283
Di, Qz
Disz.g Q'qz.z
30
1 50 0
4
Di, QZ
284
Fo, Di
Di73.o Fo27.o
30
1500
4
Fo, Di
2F
Fo, C-En, Di
D i z 9 . 5 F o + 0 . 9Q ' 2 9 . 6
30
1500
20
Fo, En, Di
Qz, C-En, Di
Digo.
30
l5 00
24
Qz, En, Di
Foz9. Q'+0. E z o
YI
C-En, Di
Fn
20
I 600
6
Di, Qz (tr)
243
C-En, Di
Et87.o Dits.o
20
16 0 0
8
En, Di
Y2*
C-En, Di
En83.rDito.9
30
1700
2
En, Di
Y3*
C-En, Di
I 600
4
En, Di
2A4
C-En, Di
""83.1 "'16.9 Etzt. Diz8. s s
50 30
I 600
l0
ni oo-6
JJ.Z
En, Di, Qz (tr)
C - E n : c l i n o e n s t a t i t e , E n : e n s t a t i t e , D i : d i o p s i d e , F o : f o r s t e r i t e , Q z : q u a r t z . The ninerals in the starting m a t e r i a l s a r e o f s y n t h e t i c end menber conponents and those in the run products a r e s o l i d s o l u t i o n s . *: disequilibrim.
TAKESHI MORI AND DAVID H, GREEN Tlsl-s 2. Electronprobeanalyses ofolivine,pyroxenes, and quartzin thesystemDi-Fo-Qz at 30kbar and I 500"C 281
Run No.
282
Phase
283 Di
Qz
si0^
s9.ss(63)
42.70(20)
60.0s(s0)
Mgo
39.98(54)
s6.49(13)
39.33(33)
1 0 0 . 3 (7r 7 ) 0 .4 1( s )
CaO Total
284
9e.s3(96)
99.19(r2)
Number of i o n s ( o l i v i n e :
99.39(70)
4 2. 3 2 ( 3 0 )
9e.1(1.e)
s 4 .e s( s 1 ) r9.3s(43) 2 4. 8 2( s 7 ) 99.12(7r)
s s . 2 3( 6 3 ) r 8 .r 8( 3 1 )
e 8 . 7( 1 . 8 )
2s.98(32)
0 . 2 8( 4 )
r00.78(22) s e . 4 ( 1 . 1 )
0 . o s( 4 )
s 6 .r 0 ( 1 7 ) 0 . 7 9( 9 ) 9 9 . 2 0( 4 6 )
0=4, p)'roxene: O=6, q u a r t z : 0 : 4 )
Si
2.000(3)
r.007(4)
2.016(6)
r . 9 9 4( 1 )
Mo
2.001(s)
1.e86(7)
r.968(r2)
0.0r2(r)
Ca
2. 003(2) 0.e83(r0) I . 0 1 0( 9 )
1 . 9 9 6( 1 )
r . e 9 4( s )
0 . 0 0 2( 1)
| . 0 4 7( 2 3 )
r . o o r( 3 ) r . 9 7 8( s )
0 . o 0 6( r )
0 . 9 6 s( 2 2 )
0 . 0 2 0( 3 )
r00.8(e) -0.7(s) - 0 . 1( s )
0 . 6( 1 )
e6.8(2.3)
- 0 . 1( r )
2 .r ( 1 . 1 )
s s .s ( 1 )
r.2(1.3)
4 .0( s ) s 7 . 8( s ) - r . 8( s )
Molecular percent ( D i : C a M g S i r O 6F , o : 2 M g Z S i O OQ, z : 2 S i O r ) Di
s0.0(2) s0.0(2)
Fo Qz
98.6(7)
48.8(s)
0.3(1)
r.4(7)
s1.2(s)
9 s . 7( r )
Run No. Phase
Di
Qz
si02
59.l0
5 6. 5 7
42.79(40)
s9.04
MgO
37.96
26.46
s6.78(45)
37.s6
26.66
CaO
2.68
16.13
0 . 3 s( 2 )
2.84
99.74
9 9 .r 5
99.92(81)
Total
En s 7. 6 0
15.51
100.26(18) 60.27 0 . 2 r( 6 ) 3 7. 3 L 0 .l s ( s ) 2.86
99.44
99.54
r00.6r(2s) 100.44
9 9 .6 9
2. 0 L 4
26.94 15.15
Nunber of ions si
1.996
1.998
1 . 0 0 3( 2 )
2.000
2. 0 7 2
l . s s s( r )
2.0L8
Mg
I.911
I
r.897
1.394
0.006(2)
L.862
1.404
Ca
0.097
0.6r0
r . 9 8 s( s ) 0.009(r)
0 .1 0 3
0.583
0.003(r)
0.103
0.568
'>-€
a(
D D:< >>>DXa
20 kb ,t600
a
50
MgrSirOo
CoMgSi206
MOLE % F t c . 3 . M i c r o p r o b e a n a l y s e s o f p y r o x e n e s . A l l a r e o f h o m o g e n i z a t i o n e x p e r i m e n t s .D i r e c t i o n o f triangles shows direction of chemical reaction Decimal fractions in the raw data have been rounded to the n e a r e s ti n t e g e r .
example, the diopside limb of the solvus at 1600'C and 20 kbar was estimated to be at EnuoDiuo. Alternatively, Lhe20 angle of the 3l I peak can be more directly usedin determining the pyroxene compositions (McCallister, 1974). 20 values read from Kushiro's Figure 4, combined with Mccallister's 2dcomposition equation, which we found very satisfactory, allowed us to estimatethe diopside limb of the solvus to be on EnrrDir", EnruDiur, or EnrrDiu, at 1 6 0 0 ' C a n d 2 0 k b a r , a n d o n E n . r D i u ,a t 1 6 5 0 ' C a n d 20 kbar. All the data locate more calcic pyroxenes than the diopside limb as determined by Kushiro at correspondingtemperatures.The differenceis about 20 mole percent CaMgSirOu. In addition, according to Kushiro's Figure 3, liquid, instead of clinopyroxene, is stable for the last composition at 1650'C and 20 kbar. These facts show diffifficulties in the use of paired X-ray reflection to infer the presenceor absenceof the pigeonite-diopsidesolvus. We also cast doubt on the pigeonite-diopsidesolvus at 17.5 kbar as inferred by Kushiro and Yoder ( 1 9 7 0 ,F i g . l 9 ) , w h i c h w e b e l i e v ew a s c o n s t r u c t e db y an analogy to the solvus at 20 kbar by Kushiro (1969). Thus, our experimentsrefute the existenceof the pigeonite-diopside solvus at 1600"C and 20 kbar, and the X-ray data of Kushiro (1969), reinterpreted by McCallister's (1974) calibration, can be shown to be inconsistent with the interpretations placed on them by Kushiro (1969).However, there still remains a possibility that a very narrow pigeonite-diopside solvuscould exist at 20 kbar and a temperaturelower than 1600"C. This possibility is discussedin the next section.
The presentstateof knowledgeis that the pigeonite-diopsidesolvusis definitelystableonly at I atmosphere(Yang, 1973; Yang and Foster 1912, and Kushiro 1972),and the maximum pressurefor the stabilityof the solvusremainsunknown.
50 MgrSirOo
MOLE %
CoMsSirOo
F t c 4 . C o m p a r i s o n o f e n s t a t i t e - d i o p s i d es o l v i a t 2 0 a n d 3 0 k b a r Dots are from Table 3 Stars and solidus at 30 kbar are from Howells and O'Hara (1975). The solvi below l500oC are after M o r i a n d G r e e n ( 1 9 7 5 ) .S o l i d i a r e o n l y p a r t l y d r a w n . S o l i d u sa t 2 0 kbar is from Kushiro (1969).
622
TAKESHI MARI AND DAVID H. GREEN
I r o n - b e a r i n g p i g e o n i t e h a s b e e n o b s e r v e da t l 5 l o n g e d r u n d u r a t i o n ( 1 0 h o u r s ) . M o r e c l o s e l y c l u s kbar and 980'C (Lindsley et al. 1974b), coexisting t e r e d c o m p o s i t i o n so f d i o p s i d er e s u l t e d .T h e c h e m w i t h m o r e c a l c i u m - r i c hp y r o x e n e a n d h y p e r s t h e n e . i c a l c o m p o s i t i o n s o f p y r o x e n e s i n t h o s e r u n s I f p i g e o n i t e i s u n s t a b l e i n t h e i r o n - f r e e s y s t e m a t i n c l u d i n g e n s t a t i t eo f Y 2 ( 3 0 k b a r a n d 1 7 0 0 ' C ) a r e this pressure,then the pigeonite""-diopside""-hyper- l i s t e di n T a b l e 3 , a n d a r e c o m p a r e dw i t h t h e d a t a b y s t h e n e "t"h r e e - p h a s ter i a n g l em u s t c o n v e r g et o a d i o p - H o w e l l s a n d O ' H a r a ( 1 9 7 5 )i n F i g u r e4 . O u r d a t a a r e side""-hypersthene"" tie-line with decreasingFeSiO, in good agreementwith theirs and further confirm the (associated with increasing temperature) large pressureeffect on the enstatite-diopsidesolvus content w i t h i n t h e m a g n e s i a np a r t o f t h e p y r o x e n e q u a d - at high temperatures. rilateral. A t t h e s o l i d u s b e t w e e n2 0 - 3 0 k b a r , t h e e n s t a t i t e a n d d i o p s i d e s h o w a l m o s t c o m p l e t es o l i d s o l u t i o n , The enstatite-diopsidesolvusin the b u t m e l t i n g o c c u r s j u s t b e l o w t h e t e m p e r a t u r e sa t MgrSirO.-CaMgSirO. system which the enstatite and diopside (with their different F o u r r u n s ( Y 2 , Y 3 , 2 A 3 a n d 2 A 4 i n T a b l e I ) a t c r y s t a ls t r u c t u r e s )l i m b s w o u l d i n t e r s e c t . A peculiar feature observedin Figure 4 is that the t e m p e r a t u r e sa b o v e 1 5 0 0 " C a r e r e l e v a n tt o d e t e r m i n a t i o n o f t h e e n s t a t i t e - d i o p s i d es o l v u s . T h e s e a r e d i o p s i d el i m b s a t b o t h 2 0 a n d 3 0 k b a r s h o w a s l i g h t c o m p l e m e n t a r yt o o u r p r e v i o u sr e s u l t sa t a n d b e l o w i n f l e c t i o n b e t w e e n 1 5 0 0 a n d 1 6 0 0 " C . A l t h o u g h w e 1 5 0 0 ' C( M o r i a n d G r e e n ,1 9 7 5 )A . g a i n i n t h i s s t u d y , prefer to regard the figure as an equilibrium phase , ss u g g e s t e d we experienced sluggishnessof chemical reaction, d i a g r a m ,t h e r es t i l l r e m a i n sa p o s s i b i l i t y a e v e n a t t e m p e r a t u r e sa s h i g h a s 1 7 0 0 " C .I n t h e r u n s b y t h e p r e s e n c eo f t h e i n f l e c t i o n , t h a t t h e p i g e o n Y2 (30 kbar and 1700'C) and Y3 (30 kbar and ite-dionside solvus exists around the inflection 1 6 0 0 ' C ) , a l t h o u g h c o m p o s i t i o n so f e n s t a t i t e w e r e p o i n t a t 2 0 a n d 3 0 k b a r . I n t h i s r e g a r d ,i t i s i n t e r e s t rather well clustered,those of diopsideshowedexten- i n g t o n o t e t h a t E g g l e r ( 1 9 7 4 ) s u g g e s t e da c o e x i s t sive hysteresisin the change in chemistry from the e n c e o f t w o c l i n o p y r o x e n e sa t 2 0 k b a r a n d a r o u n d p u r e d i o p s i d e( C a M g S i r O . )o f t h e s t a r t i n gm i x e s .T h e 1 5 4 0 " C b a s e d o n s y n t h e s i s d a t a i n t h e s y s t e m r u n a t 3 0 k b a r a n d 1 6 0 0 ' C w a s r e p e a t e dw i t h p r o - C a O - M g O - S i O r - C O r . H o w e v e r , t h e p i g e o n i t e diopside solvus, should it exist at temperatures lower than 1600'C at 20 kbar. must be much narr o w e r t h a n i n d i c a t e db y K u s h i r o ( 1 9 6 9 ) . a T h e d a t a i n F i g u r e 4 w e r eu s e dt o g e t h e rw i t h t h o s e o f M o r i a n d G r e e n ( 1 9 7 5 ) i n c o n s t r u c t i o no f t h e pressure-compositionsection (Fig. 5) which shows enstatiteand diopside limbs as a function of temperature and pressure. 40
-o J
ltJ d l
Application to temperatureand pressureestimatesof garnet lherzolite nodulesfrom Lesotho kimberlites' and constraintson the pyroxene-solvus geothermometry
Garnet lherzolite nodules from Lesotho kimberlites have been extensivelystudied ( e.g. Boyd, 1973). P h y s i c a cl o n d i t i o n so f e q u i l i b r a t i o nf o r t h e s en o d u l e s can be estimated by solving two independent geol0 thermo-barometers.For this purpose, the MacGregor (1974) and Davis and Boyd's (1966)grids or modifications of them have been commonly used ( B o y d , 1 9 7 3a n d M e r c i e r a n d C a r t e r , 1 9 7 5 ) . ln this section, we will also try to estimate pres50 Mg,Si,O" CoMgSi,q and temperaturesof thesenodules using Boyd's sures MOLE% chemicaldata for the minerals of the garnet extensive F r c . 5 . C o m p i l a t i o n o f e n s t a t i t e l i m b s ( t h e l e f t - s i d eb a t c h o f l h e r z o l i t en o d u l e s( B o y d , i n N i x o n , 1 9 7 3 ) .A l t h o u g h l i n e s ) a n d d i o p s i d e l i m b s ( t h e r i g h t - s i d eb a t c h o f l i n e s ) b a s e d o n the data from Fig. 4 and Mori and Green (1975) the principle of the method is the same,we will emtt, 2 .tt Br e, o-
SUBSOLIDUS EQUILIBRIA BETWEEN PYROXENES
ploy the enstatite-diopside solvuspresentedin the previoussectionratherthan that of Davis and Boyd (1966).Ca/(Me * Ca) ratio of diopsidefrom the ToC nodules gives one temperature-pressure equation. AlrOr contentsof enstatitecoexistingwith diopside 1600 and garnetandwith or withoutolivinewerecompiled from the experimentaldata by Akella and Boyd (1972, 1973,and 1974),Hensen(1973),Mysen and Boettcher(1975),Green(1973),and Mori and Green r 400 (unpublisheddata on the pyrolite system).All the chemicalsystemsincludeat leastSiO2,TiO2,Al2Os, FeO, MgO, and CaO. Their temperatures and presr 200 suresvary from 900 to 1410'C and from 15 to 45 kbar. Consistency among the data is, however,not fully satisfactory.In addition,both geothermo-bar 000 rometershave to be greatly extrapolatedtowards //* higher pressuresand temperaturesin order to be .93 -!-appliedto the garnetlherzolitenodules.Thesetwo * featuresgreatly limit reliabilityof this method of 800 temperature-pressure estimates. Figure 6 showsa graphicmethod of solvingthe two equations.Diopside-component isoplethscross the AlrOr (enstatite) isoplethsgivinguniquesolutions 0204060kb of temperatureand pressure. Boyd'sanalyticaldata (Boyd, in Nixon 1973)show that AlrO, contentin Frc. 6. Method of temperature and pressureestienstatiteis rather constant through the nodules mates. Compositions of diopside limbs (broken ( A l r o a : l l + 0 . 3 w t . V ow lines, in CaMgSirO. mole fraction) are from Mori ) , h i l e t h e C a / ( M g+ C a ) and Green (1975) and this paper. AlrO. contents in ratio variesextensivelyfrom 0.481 to 0.287.This e n s t a t i t e( l i n e s w i t h d o t s , i n w t . V o ) a r e f r o m A k e l l a meansthat the temperature-pressure trendis depenand Boyd (1972, 1973, and 1974), Hensen (1973), dent on the AlrO, isoplethsand is linearwithin * 3 G r e e n ( 1 9 7 3 ) ,M y s e n a n d B o e t t c h e r( 1 9 7 5 ) ,a n d o u r kbar, which roughlycorresponds to the variationof unpublished data. The stars represent temperature-pressure estimates for garnet lherzolite no* 0.3 percentAlrO3.If the nodulesareclassified into dules (1572, 2302A, 15598, 1859N, 2273, 83, and granularand shearedtypes(Boyd,1973),we canfind 1924) from Lesotho kimberlites (Nixon, 1973). In that the shearedtype is more aluminousthan the plotting the natural diopsides we have used the ratio granulartype by 0.3 weightpercent.The diopside2Ca/(Ca + Mg). Construction of this figure incomponentisoplethsonly limit the extentof the temvolved large extrapolation of the data and neglect of some inconsistencies among AlzOs data. The deperature-pressure trend determinedby the AlrO, isoduced P,7 conditions may be far from the true valpleths. ues (see text). A few data points from Boyd (in Nixon, 1973)are plotted in Figure 6. The highest temper(up to 1430"C)by Boyd(1973)is mainly ature-pressure rock (1597,not plotted in Fig. 6) is temperature estimatedat about 85 kbar and 1950"C,and other due to his use of the enstatite-diopside solvusof shearednodulesapproachtheseconditions(Fig. 6). Davis and Boyd (1966)at 30 kbar, which showsa The granular nodules yield temperaturesup to more calcic diopsidelimb below 1000"Cand less 1170"C.Many granulartype nodulesplot at lower calciclimb aboveit comparedto our solvus.Mercier temperaturesthan 900oC, but the diopside-com- and Carter's(1975)revisionshowstemperatureand ponent isopleths become temperature-insensitive. pressure valuesup to 80 kbar and 1600"C.Again,the Thus, the lowesttemperature-pressure valuescannot differencefrom the trend in Figure 6 is mainly dueto be reasonably solvus,which estimated. their schemefor the enstatite-diopside very pressure Our estimatesare different from those by hasless effecton the solvusthan ours. Boyd (1973)and Mercierand Carter (1975).The I t m i g h t b e c o n s i d e r e dt h a t t h e . t e m p e r (up to 70 kbar) and ature-pressure rathernarrow rangeof pressure trend in Figure6 is too steepto repre-
TAKESHI MORI AND DAVID H GREEN
oz+
Tlsr-B4. Accuracyof TPD-probeanalysis JG-1
JB-I
reconnended value
TPD-probe
reconunended value
TPD-probe
si02 Ti02
72.9s
73.06(41)
1 .3 8
s3.64(2r) 1 . 3 3( 4 )
A1203
14.36
14 . 42 (3o)
14.87
1 4 .8 3 ( 1 3 )
1.99
I . 8 e( 4 )
8.31
8 . s 3( 8 )
F"oaoa"l
.26
.06
MnO Mgo
.75
Ca0
2.22
Na20
3.4L
Kzo
4 .0 0
.26(4)
n .d .
. ee(4) 2.2r(s) 3 . 2 s( e ) 3.e3(s)
5 3 .s 3
.15 7.9L 9.50 2.87 L.47
. 0 3( 7 )
7.s3(r2) s . 4 7( 7 ) 2.7sG4) 1 . 4 7( s )
The recomnended values for the standard rocks are frorn Ando et al. (L974). Both sets of data are recalculated to total of 100. The numbers in parentheses represent estinated standard errors (1 signa) of the probe analysis, and refer to the last decinal place(s).
sent a palaeogeotherm.From presentknowledge,we do not know whether the nodule trend is an artifact due to lack of data for phaseequilibria at very high pressuresand temperatures.In addition, we do not know that the temperature-pressure thus estimated representsa palaeogeotherm. The principal difficulty in temperatureestimateson the basis of the pyroxene solvus is, as shown in the example above, the fact that we have to know pressure values by an independentmethod. In addition, the solvus is almost uselessat temperatures below -900"C. We emphasizethat the pressure-temperature estimates of Figure 6 are an illustration of the effect that the new solvus data have on the methods of pressure-temperature estimation used by Boyd (1973). We consider that the estimates of pressure-temperature in Figure 6 have very large uncertainties indeed.
The glasseswere analyzedby TPD-microprobe using analytical system with the routhe energy-dispersive tine analyticalprocedurein our laboratory (Reed and Ware, 1973) but with defocussedbeam (about 100 microns). The raw data and the recommendedvalues for the standard rocks (Ando et al. 1974) were recalculatedto total of 100.This was necessaryin order to compare the two setsof data sinceHrO+ and HzOetc. evaporateduring the melting process. The resultsare listed in Table 4. The probe data are satisfactoryexceptfor MnO. Concentrationsof MnO in these rocks are nearly the same or just above the detection limit of MnO by TPD-probe (about 0.08 wt.Vo).For those elementswith very low concentramicrotions, the conventional wavelength-dispersive probe system must be used instead of the energydispersiveTPD-microprobe. It should be noted that the standard deviations in Table 4 are due almost entirely to heterogeneiiiesof the glasses.
Appendix : Accuracy of TPD-microprobe analysis Acknowledgments Standard rocks (JG-l and JB-l) of Geological Surwith M. Obata(M.I.T.),G. Brey We benefitted from discussions vey of Japan were melted on an iridium strip heater (A.N.U.), L Kushiro (Geophys.Lab ) and J.R. Smyth (Lunar following the melting procedure of Nicholls (1974). ScienceInst.). Technicalassistance by W.O Hibberson,N G.
SUBSOLIDUS
EOUILIBRIA
Ware and E.H Pedersen was particularly helpful. D H. Eggler ( G e o p h y s . L a b ) a n d A . L . B o e t t c h e r( P e n n S t a t e U n i v . ) r e v i e w e d the manuscript with constructive criticism. These contributions are gratefully acknowledged
References A K E I - L A ,J . e r o F . R . B o v o ( 1 9 7 2 ) P a r t i t i o n i n g o f T i a n d A l between pyroxenes, garnets, and oxides Carnegie Inst. Wash Year Book 11,378-384. AND( 1 9 7 3 ) E f f e c t o f p r e s s u r eo n t h e c o m p o s i t i o n o f co-existing pyroxenes and garnet in the system CaSiO.-MgSiO,-FeSiO"-CaAlTi,Ou Carnegie Inst lilash. year Book,12, 523-526. (1974) Petrogeneticgrid for garnet periodotites. AND Carnegie Inst. VItash Year 9ook,73,269-273. A N o o , A . , H K u n n s a w l , T . O n u o n l A N D E . T A K E D A( 1 9 7 4 ) l 9 j 4 c o m p i l a t i o n o f d a t a o n t h e G . S . J .g e o c h e m i c a rl e f e r e n c es a m p l e s J G - l g r a n o d i o r i t e a n d J B - l b a s a l t . G e o c h e m J. . 8 , 1 7 5 - 1 9 2 Boyn, F R. (1973) A pyroxene geotherm. Geochim. Cosmochim. Acta, 31,2533-2546. A N D J . L . E N c r - l N o ( 1 9 6 0 a )A p p a r a t u s f o r p h a s ee q u i l i b n u m m e a s u r e m e n t sa t p r e s s u r e su p t o 5 0 k b a n d t e m p e r a t u r e s up ro l750oc J. Geophys Res. 65, 741-749. AND (1960b) Minerals of the mantle. Carnegie Inst. llash Year Book. 59. 47-52 CsrN, C-H. AND D. C. PnrsNeu- (1975) The system Mg,SiO4-SiO, at pressures up to 25 kilobars. Am Mineral 60, 398-406. Devrs, B. T. C. rr.ro F R. Bovo (1966) The join Mg,Si,Ou_ C a M g S i r O ua t 3 0 k i l o b a r s p r e s s u r ea n d i t s a p p l i c a t i o nt o p y r o x e n e s f r o m k i m b e r l i t e s .J G e o p h y s R e s . 1 l , 3 5 6 7 - 3 5 j 6 . E c G L E R ,D . H . ( 1 9 7 4 ) E f f e c t o f C O , o n t h e m e l t i n g o f p e r i d o t i r e . Carnegie Inst Wash. Year Book,73,215-224. G n e r N , D H . ( 1 9 7 3 ) C o n d i t i o n s o f m e l t i n g o f b a s a n i t em a g m a from garnet peridotite Earth Planet Sci. Lett 17,456-465. H r r s r N , B J ( 1 9 7 3 ) P y r o x e n e sa n d g a r n e t s a s g e o t h e r m o m e t e r s and barometers. Carnegie Inst Wash. Year Book,12,5Z'l-534. H o w e L l s , S . e N n M . J . O ' H r n n ( 1 9 7 5 ) P a l a e o g e o t h e r m sa n d t h e diopside-enstatite solvus. N alure, 254, 406-408 K u s u r n o , I . ( 1 9 6 9 ) T h e s y s t e mf o r s t e r i t e - d i o p s i d e - s i l i c aw i t h a n d without water at.high pressures.Am J Sci. Schairer Vol 267-A, 269-294. (19'72) Determination of liquidus relations in syrrthetic silicate systems with electron probe analysis: the system forsterite-diopside-silica at I atmosphere. Am. Mineral. 57, 1260-
t z 71 -
AND H. S. YonEn, Jn. (1970) Srability field of iron-free pigeonite in the system MgSiOr-CaMgSi2Ou. Carnegie Inst. Wash Year Book, 68, 226-229. L T N D S T - ED y ,. H , H E . K r N c J n . a N o A C . T u n N o c r ( 1 9 7 4 a ) Compositions of synthetic augite and hyperstheneco-existing at
BETWEEN
PYROXENES
625
8 1 0 " C : a p p l i c a t i o n t o p y r o x e n e sf r o m l u n a r h i g h l a n d s r o c k s . G e o p h y s .R e s .L e t t . l , 1 3 4 - 1 3 6 . lpp J. E. Gnovrn (1974b) Phaserelations in the pyroxene quadrilateral at 980oC and 15 kbar. Geol. Soc. Am. Abstracts with Programs, 6, 846 M r c G n r c o n , I . D . ( 1 9 7 4 ) T h e s y s t e mM g O - A L O ' - S i O z : s o l u b i l ity of AlrO, in enstatite for spinel and garnet peridotite compositions Am Mineral.59, ll0-119. M c C n l l r s r E n , R H . ( 1 9 7 4 ) T h e e x s o l u t i o nk i n e t i c so f a d i o p s i d e solid solution having the composition 54.1 mole 7oCdMgSizOe, 45 9 mole % MgrSirO". Carnegie Inst. Wash. Year Book, 73,
392-396. M r n c t e n , J - C . e N r N . L . C A n r e n ( 1 9 7 5 ) P y r o x e n eg e o t h e r m s .. I Geophys. Res. 80, 3349-3362 M o n t , T . A N D D . H . G n r r N ( 1 9 7 5 ) P y r o x e n e si n t h e s y s t e m MgrSirO.-CaMgSLOe at high pressure.Earth Planet. Sci. Lett. 26,277-286. MvsrN, B. O rr.ro A. L. BoBrrcurn (1975)Melting of a hydrous mantle: II. Geochemistry of crystals and liquids formed by a n a t e x i so f m a n t l e p e r i d o t i t e a t h i g h p r e s s u r e sa n d h i g h t e m p e r atures as a function of controlled activities of water, hydrogen, and carbon dioxide. J Pelrol. 16,549-593. N e H n u , C E l N p P . J . W v L L t s ( 1 9 7 4 )E l e c t r o nm i c r o p r o b e m e a s urement of pyroxenes co-existing with HzO-undersaturated liqu i d i n t h e j o i n C a M g S i z O u - M g , S i , O u - H , Oa t 3 0 k i l o b a r s , w i t h applications to geothermometry. Contrib Mineral. Petrol. 48, 22t-228. N r c H o L L s , l . A . ( 1 9 7 4 )A d i r e c t f u s i o n m e t h o d o f p r e p a r i n gs i l i c a t e rock glassesfor energy-dispersiveelectron microprobe analysis. Chem. Geol 14, l5l-157. NrxoN, P H (Ed.) (1973) Lesotho Kimberlites. Lesotho National D e v e l o p m e n tC o r p o r a t i o n , M a s e r u , L e s o t h o . 3 5 0 p . Onuvn, K. eNo M. Antul (1975) The join MgSiOs-MgAl,SiO. a n d t h e s o l u b i l i t y o f A l r O r i n e n s t a t i t ea t a t m o s p h e r i cp r e s s u r e . J. Japan. Assoc. Mineral Petrol Econ. Geol. 10, 53-60. RErn, S J. B. euo N. G. Wrne (1973) Quantitative electron microprobe analysis using a lithium drifted silicon detector. Xray Spectrometry, 2, 69-7 4. S u y r u , J . R ( 1 9 7 4 ) E x p e r i m e n t a ls t u d y o n t h e p o l y m o r p h i s m o f enstatite. Am. Mineral. 59, 345-352. S o s o r - E v ,N V . J n . , I . K . K u z N r r s o v A A N D N I Z v u z t t t ( 1 9 6 8 ) The petrology of grospydite xenoliths from the Zagadochnaya kimberlite pipe in Yakutia. J. Petrol. 9,253-280. W,c.nNrn, R D. rr.ro W. C. LurH (1974) The diopside-orthoenstatite two-phase region in the system CaMgSLOT-MgrSirC.,6. Am Mineral 59, 98-109 Y,qNc, H-Y (1973) Crystallization of iron-free pigeonite in the system anorthite-diopside-enstatite-silica at atmospheric pressure. Am J Sci 273,488-497 A N D W R F o s r E R ( 1 9 7 2 )S t a b i l i t y o f i r o n - f r e e p i g e o n i t ea t atmospheric pressure. Am. Mineral. 57, 1232-1241.
