American Mineralogist, Volume 6l , pages 725-731, 1976

Pressuresand temperatures calculated from chromium-rich compositions of megacrystsand peridotitexenoliths, Black Rock Summit.Nevada

JlNr E.NrrlsoN Prrr U.S. GeologicalSuruey, Menlo Park, California 94025

Abstract

Olivine-rich spinel-peridotitexenoliths from basanitoidflows and pyroclastics,Black Rock Summit, Nevada, contain peculiarly Cr-rich orthopyroxenes(average lVo CrzO') and low-Ca chromian clinopyroxenes( I 5- l87oCaO). Electron probe analyses,including a scan acrossan orthopyroxenite veinlike structure in one sample,reveal that there are significantcompositional variations within megacrystsand pyroxenesof over distancesof only a few millimeters.These variations lead to large apparent gradients in pressureand temperature when calculatedby current methodsand plotted on the widely-usedP-T diagramsof Maccregor (1974) and Boyd (1973). The chemical variations in the scannedspecimen are systematicand thus are not due to analytical error, but probably are causedby metasomaticand other reactionsin the source areas of the ultramafic rocks. The presenceof these variations brings into question the assumptionsupon which current petrogeneticgrids are based,and strongly suggeststhat the interpretationof P-T curvesas fossil geothermsis premature.

Introduction to ultramafic host rocks. The megacrysts have Clinopyroxene megacrystsand spinel-faciesper- rounded shapes,are rimmed by border zonesof in- idotite xenoliths collectedfrom Black Rock Summit, cipient melting, and are usually surrounded by host Nevada(Trask, 1969)are -rich.Many contain lavas of alkalic basalt. These large crystals contain a distinctive bottle-green pyroxene that resembles strain lamellae,and zonesof granulation are concen- neither the apple-green(Cr-diopside) nor the black trated along the lamellae. (Al-augite) pyroxene-bearingspinel-facies peridotite xenoliths at the same location. The bottle-greenpyrox- Compositionsof rocks and minerals eneshave been distinguishedin peridotitesat a num- Of about 80 bottle-greenperidotite xenoliths in this ber of other localities in the United States (H.G. collection from Black Rock Summit, most were iden- Wilshire, personalcommunication, 1974). tified as wehrlite and only four samplesas lherzolite, The collection of xenoliths from Black Rock Sum- basedon the appearanceofthe pyroxenesin the field. mit consists predominantly of small rounded frag- In ,much of the presumedclinopyroxene ments with an averagediameter of 27 mm, although has low birefringence and small extinction angles some are angular, and a few appear to have broken (3-12"), but optic-axisfigures yield positive2Vs of along a preferred direction producing a tabular about 60o. Severalthin sections contain a few pyrox- shape. textures range from unshearedal- ene grains with higher birefringence and slightly lotriomorphic-granular types with relatively large larger extinction angles. In eight rock sections se- grain-size(up to 2 mm) to highly-shearedmylonites lectedfor electron probe analysis,the larger propor- of very fine grain-size. Most allotriomorphic-gran- tion of has very low calcium content ular xenolithscontain zones of incipientgranulation, (l-2Vo CaO) compositionally resembling enstatite. especiallyat grain boundaries. High-calcium clinopyroxene(17-l8To CaO) is ex- Megacrysts are single and twinned pyroxenes of tremely scarce. In addition, several rocks that ap- comparablesize to the xenoliths.None of thosestud- peared to contain two pyroxenes(based on birefrin- ied is a mineral aggregate,nor is any found attached gence and extinction angles),in fact have only one 725 726 JANE E. NIELSON PIKE

Teslr l. Pyroxenecompositional parameters and calculated values of pressureand temperature

CJ-inopyroxene o"thopyToxene

Calculated";7qi;-- ** x+ r\ u/ '*2-3or . .."'2-3 o .+*+ smD]e no. CaO*+ Mgoxx ca/(Ca+Me) Sampl-e no. P(kbar) 'r Lherzolite A-Qr 1 17.95 20.r5 0. 3904 r8o Lherzolite A-@ 1 2.47 I Jrl -cp 2 tB. oh 20.l-o o 20)) ].eBl+ -Op2 2.23 f ,05 43.6

-lA Lherzofite B-Cp l- in aa ,o U. +JJJ rlTo LherzoJ-r-le-6-14) J- >.+3 u.oo ttTq -op 2 4.98 24,r- w5 -o! 3 )+.79 o.79 nl, a 1173 -op 4 5.06 o.72 r? oT l-l-oo -oP5 ,'26 u. oo cc A- 1 1nA -Opb U. OO ,.35 .| 'r180 -op 7 6.26 q f9.u' r-2t-B -@ 8 5'39 )Aa -op o.67 A' 1f7o 9 5.o4 "" J-I76 -op t0 5.3O o.66 'r r\6ro^lita f-6 1 o. ro8 Lherzolite C-Qr 1 r-.oB 17.26 17.9' 3968 '12o8 3.0f -(1- ) 18.53 2a.25 o.399r -ap 2 2.99 1.rt 3Y,O 1ra6 -op 3 2.99 1. 09 J9.o 1 )O7 -op 4 3.01 r.w

16.68 tB.T6 o 3899 lI262 T,herzolite D-@ I l+.BB u. oy 27.8 -cp 2 L6.72 18.64 o.3927 r256 -op 2 4.84 1.01 29.3 1 tqn _@ 3 4.79 1.Ol_ 29.7 )262 -op 4 4.85 1.OI too -oP 5 l+'Bl 1.Ol_ 29.6

LIahTlif.a A-arn 1 tq ot 18.3\ o. 3842 -cp 2 r\.69 19.B' 1 2)7 -c! 3 tq 70 l_B. 05 o. 3861 l r?q -cp 4 l_5. 04 rA 60 a aAvn 1280 r A 22 -vy ) o.3BoB ].250 -Q6 I Jr 1v An U, "c JOOU LzB5 -u! rq RR tR n I "AJrl l-240 aa "n -VP U 1a 12 o.4o9t Jtoo

Tt6L?1i+a ?-h 1 r-6.oo l.8,76 o.38o1 r25O -cp 2 lE qq 1A R2 0.3275 1360 -cP 3 15.87 18.87 03768 w& -cb 4 r,.95 r-8.64 o.3BoB r25o -cP 5 rr.n 18.92 0.37\7 126'

Megacqfst A-f a7.5o L7.24 O.4zZ4 1127 -2 17.\2 r7.28 o.42O2 ll35 -3 17.\2 17.62 0.4154 r-15o -4 17.16 17.74 O.t+I57 1150 -) L7.28 ]-.7,46 o,I+IjY ljt'o

Megacryst B-1 18.45 L6.25 0.4493 1050 -2 t8.l+6 t6.99 o.4385 1080 -3 r8.rrB 16.29 o.\\92 105O

Megacr:rst C-l 17.\3 r7'L6 o,4220 1130 -2 t7.74 17.o9 o.4238 1't25 ao Lo 1o ^a -3 !1.+I LI..e o.42oB 1f35

Megac"yst D-l tz tA lz oa) o. l+o8o r]70 ^ f,.1^^ 11An -2 17.oo 77.59 w.'p>> svr -3 L7.26 fi.95 0.4087 1170

*Tem9retatutest lhetzolltes- corrections of wood and Banno, 1973i wehrlites and Megactqsts- minumm vaLues frcm Davis and Botld, 1965, **weight petcent *r*Pressu.lies.. cotretions of Metciet and Cartet, 1975. a plot within spinel-FEridotite fieLd. PYROXENE COMPOSITIONS OF MEGACRTSTSAND XENOLITHS 727 pyroxene,either high-or low-calcium.By theseanaly- olivine ) orthopyroxene ) clinopyroxene.One of ses,the eight rock sectionswere redefinedas lherzo- the four (lherzolite C, Table 1), does not contain lite (4), wehrlite (2), and harzburgite (2). spinel in the section,and another (lherzoliteB, Table The analytical resultsreveal that low-calcium pyrox- I ) appears to be an olivine-rich harzburgite with a enesin the rocks have a high chrome content (about veinlike concentration of orthopyroxenite. This lvo Cr2Os,Table l), which apparently produces the "vein" containslarge clumps of spinel and a few tiny bottle-greencolor and the unusualoptical properties. clinopyroxenegrains. This specimenalso has a grain Basedon the chemical compositions,these minerals of pigeonite (CaO - 5Vo,see 9: Figs. l, 2A; and will here be referred to as orthopyroxene.These or- Table 3) in the center of the olivine-rich portion. thopyroxeneshave alumina contents that vary from Figure 2 is a graph of an electron probe traverse 2.2-6.2 percent in rocks with coexistingclinopyrox- acrossorthopyroxene and spinel grains of lherzolite ene, but are constantat about 3 percentin harzburg- B. Figure2A showsthat the compositionalvariations ite xenoliths (Table l). Clinopyroxenes are small in the pyroxenesof this rock are systematicacross the and scarcein all lherzolite nodules.The clinopyrox- "vein" and into the harzburgite;the largestcomposi- enes in xenoliths and megacrystsall have calcium tional gradients occur in the area transitional be- contents lower than the diopside range (CaO tween pyroxenite and harzburgite.Spinels (Fig. 28) 18.6-15.57o)(Fig. I ) and contain 1.5-1.2 percent show a typical variation in Cr2O3and AlrO' (Wilshire CrrOr. Figure I shows that the analyzedclinopyrox- and Shervais,1975), but chromium and aluminumin enes in wehrlites have lower calcium and the the orthopyroxenesdo not have the expectedinverse megacrystshigher calcium contentsthan clinopyrox- relation. This is difficult to explain unlesssome un- enesin lherzolite. completed mass transfer of ions is frozen into the The four lherzoliteshave modal compositions of sample.This type of relation has beenfound in some amphibole veins from peridotite (C. E. Meyer, per- sonal communication. 1975). The in the lherzoliteshave lower forsterite contents(Foro-Fo'.) COMPOSITIONOF COEXISTING compared to most lherzolites,and have little varia- PYROXENES IN LHERZOLITES tion within samples (e.g., there is no variation in olivine compositions across the traverse of or- thopyroxenes in Fig. 2,A). Spinels, however, vary greatly in alumina and chrome contentsbetween xe- noliths (18-44VoAl2O3, 50-20Vo CrzOa; see Table 2). Of the two wehrlites (olivine ) clinopyroxene) and two harzburgites,one (wehrlite A,Table 2) con- tains spinel.Both wehrlitesare mylonitized. Wehrlite A (Table l, Fig.3) hasa largevariation of Ca-content in the clinopyroxene(Table I ). Since the xenolith is small and the content of clinopyroxeneis very small and spatially concentrated (clustered within a dis- tance of I cm) in the wehrlites, even the smaller variation of Ca-content in wehrlite B is significant (Fig. 3). It seemsunlikely that this variationcould be due to cumulate fractionation processes. The lack of spinel in some of the rocks is thought not to be significantin terms of phaserelations. The samplesare so small that only one thin sectioncan be made from most of the xenoliths, and spinel grains are not abundant in any of the rocks (exceptlherzo- lite B). Thus, all the samples are treated here as spinel-faciesrocks. Ftc. l. Compositions of coexisting pyroxenes in lherzolites. Calculation of pressureand temperature Symbofs: fherzolite A-A, lherzolite B-O, lherzolite C-l l, lherzolite D-O (Table l); Area Z-composition of pyroxenes Boyd (1973),Nixon and Boyd (1973)and MacGre- from wehrlites, Area M-composition of megacrysts. gor and Basu (1974)have used compositionaldata 728 JANE E. NIELSON PIKE

cenlervein lronsiiion horzburgite | 2 3 4 56 7 A 9

GI .o L -a.-.-_^__r' + ol

=CD

o o

;e ro o = 29 N lo o ^l s s - ](, ' lo 6.O l.o o o (\t (\l () ;e ;e

!! i! o.5 ' 4.O +4

5lo1520uc (mm) Distonce ocross somple (mm) Longifudinol distonce (opx) (sPinel) Frc.2 Compositionalvariation in (A) orthopyroxenesand (B) spinels(Traverses across lherzolite B).

from pyroxenesin xenoliths, in conjunction with ex- sent pressureand temperatureconditions of equilib- perimental data from idealizedsystems, to construct rium crystallizationor recrystallizationfor the xeno- pressure-temperaturecurves that they interpret as lith's pyroxenephases at various levelsof the mantle, ancientgeotherms. These curves are thought to repre- sampled by the host basalt or kimberlite. Wilshire and Jackson (1975) presenteddata which indicate T,rrnlr 2. Spinelcomposittons that problems exist in this approach. Data collected in this researchpresent similar problems.

Sanple no. Mg/(U€tFe+2) Ar203* Cr203* According to the method of plotting "geotherm" diagrams, temperatures for a rock are determined Lherzolite A-l 0.6624 18. 49 50.03 from the Di-En solvus,and thus dependon the Ca- 0 .6012 18.34 50. 10 -3 0 .6000 18.49 49.9s content of clinopyroxenes in equilibrium with or- Hueb- Lherzolite B-1 0.6491 44.52 20.o2 thopyroxene(Davis and Boyd, 1966;Ross and -2 0.6405 43.40 20.84 ner, 1976),while pressuresare determinedfrom the -3 0.6448 41.13 2r.87 -4 0.6495 42.52 20.60 Al-content of the orthopyroxene, and may be cor- -5 43.87 L9.96 0.6441 rected for CrzO, and Na (Boyd, 1973; Wood and Lherzollte IF1 o.6923 40,45 26.54 Banno, 1973;Akella and Boyd, 1974;MacGregor, wehrlite A-1 o.6932 42.L8 20.38 1974;Wood, 1914;Mercier and Carter, 1975).Be- 0 .6718 36.28 23.20 -3 0 .7051 33.62 20.90 causedetermining pressures by this method requires -4 0.7205 29.33 L6.47 that the rocks contain two pyroxene phases,wehr- -5 o.722L 29.L8 20.24 -6 0 .6531 38,11 26.70 lites, harzburgites,and megacrystsmay not be used -7 27.28 19.50 0.7165 to construct geotherms.Figure 3 is a plot of Ca-Mg ratio us. minimum temperaturefor wehrlitesand cli- ,ueight percat nopyroxene megacrysts(data, Table 1), using the PYROXENE COMPOSITIO|{S OF MEGACRYSTS AND XENOLITHS 729

TlsI-r 3. Microprobe analyses(matrix corrected)of phases listed in Tables l, 2; Figures l-4

Sample Name Mineral si02 AIZ03 Naro *zo Ti02 Cr Ni0 Total 203

Lherzolite A cp-t 52,06 2.27 5.37 20.t5 l7 .95 0.06 0.00 0.01 2.08 - 99.95 cp'2 5L,42 2.73 3.45 20.t0 lA.O4 0.32 0.05 0.02 2.05 - 9818

op-l 55,9r 2.47 5.58 32.45 2.14 0.06 0.02 0.01 1.41 - 100.06 op-2 53.83 2.23 5.41 33.57 2.11 0.06 0.01 0.00 1.05 - 98.04 op-3 53.48 2.23 5.31 33.44 2.09 0.06 0.01 0.01 1.06 - 97.69

sp-1 18.49 15.04 13. 66 50.03 - 97.22 sp-2 18.34 15.04 17. 78 50.01 - 101.17 sp-3 18.49 14.88 17.68 49,95 - 101.00

Lherzollte B Cp-l 6.74 4,35 76.2917.33 1.06 0.06 0.61 | .24 - 99.10

" oP-l 5.43 7.85 3r.L6 1.55 0.13 0.04 0.41 o,66 - 100.39 " Op-2 53.39 4.98 7.49 31.51 1.62 0.L4 0.03 0.39 - 100.29 " op-3 54, 10 4.79 7,52 3t.83 1.67 0.13 0.03 0.40 o.19 - 100.90 " op-4 53.62 5.06 7.67 32.L3 I.66 0.I4 0.03 0.40 0,72 - 101.07 " op-5 52.71 5.26 7.97 32.22 1.63 0.15 o.o2 0.42 0.66 - 101.10 " op-6 53.05 5.35 7.54 3l 36 1. 67 0. 16 0.04 0.41 o.66 - r00.24 ,, op_7 53,t4 6.26 7.t2 30.52 L 63 0.13 0.02 0.38 - 100.45 " op-8 52.70 5.39 6.78 2A.73 5.53 0.35 0.02 0.51 0.75 - 100.76 " op-9 53.10 5.04 7.73 32.24 1.59 0.14 0.02 0.39 0,67 - r00.92 " Op-Io 53.11 5.30 7.45 31.26 1.55 0.14 0.02 0.40 o ,66 - 99,89

" o1_t 39.38 13.59 46.04 0.L6 _ 0,15 o.24 99.49 ,, 01-2 39.33 L3.24 45.04 0.L6 - 0.15 0.24 99.L6 - " o1-3 38.50 t3.33 46.34 0.t5 0.14 0.22 9a.19 ', ot-4 39.84 13.4L 46.L2 0,L'l - 0.15 0.23 99.92 " 01-5 39.40 13.03 45,98 0. 14 - 0.16 0.24 9A.95

Lherzolite C Cp-I 52,33 3.25 3.34 20.2518.53 0.06 0.03 0.03 - 99.16 ,, cp_2 53.01 3.42 3.42 20.L4 18,61 0.05 0.03 0.04 1.36 - 99.94

" op-l 55.59 3.01 5.35 32,65 2,t4 0,07 0.00 0.01 1.08 - 99.87 " op-2 q<

" o1-1 39.56 9.48 49.09 0.25 - 0.11 0.29 98.91 " ot-z 40.59 9.65 49.O8 0,24 - 0.13 o.26 99.98 ,' o1_3 40.59 9.59 4A.85 0.24 - o.l2 0.29 99.'10

Lherzollte D Cp-1 50.69 5.46 4.25 18.76 L6,68 0,77 0.04 0. 17 r.47 98.31 " cp-z 57,74 5,49 4.09 18.6416.77 0,83 0.04 0,14 L.49 9a.54

" op-l 53.96 4.88 4.87 79.L9 1,31 0.11 0.02 0.04 0.69 100.0r " op-2 53,37 4.a4 6.24 3L.66 r .86 0. 14 0.03 0.07 1.01 ao t, " op-3 53.17 4.79 5.09 32.54 I .85 0. 14 0,03 0.07 1.01 99.79 " oP-4 53,79 4.85 5.95 32.52 I .83 0. 13 0.02 0.06 1.01 100.19 " op-5 53.51 4.87 6.23 32.96 1.84 0.14 0.02 0,05 1.01 100.55 ,, o1_1 40.63 - 1I. 11 47.43 0.22 o.L4 0.25 99.78 " o1-2 39.56 - 11.tl 47.58 0.27 0.12 0.25 98.83 " o1-3 39.67 - 11.21 47.69 0.24 0. 15 0.22 99.18 " o1_4 39.90 - 11.19 47.63 0.L8 o.t2 0.26 99.49 " o1_5 39.49 - 10.87 41.6t 0.20 o.L3 0.24 99.55

sp-1 40.46 15.40 L9.44 26.54 101.84 730 JANE E, NIELSON PIKE

tween expected equilibrium conditions and actual pyroxene compositionscan be remediedby account- ing for coexistingspinel compositions,it is still im- possibleto construct a geotherm from rocks which contain significant apparent pressure and temper- a ature variations within a single thin section. Calcu- lated temperature gradients of as much as 40o/mm found in single megacrystsand of 10'/mm in single rock sectionssuggest that evenlarger apparentvaria- tions might be observed,could the entire source te- pressuregradient of I 37 38 39 40 gion be sampled.The apparent Co4CorMg)xlO0 kbar/mm in plotted pressurevalues from lherzolites Frc. 3. Ca-Mg ratio uJ temperaturefor megacrystsand clino- substantiatesthis inferenceand suggestsstrongly that pyroxenesfrom wehrlites(Davis and Boyd,1966 solvus). Symbols: these rocks are not equilibrated,and at least in the wehrlitel-!, wehrliteB-O; megacrystl-O, megacryst veined rocks, classicequilibrium was never reached. B-f, megacrystC-A, megacrystD-V. The systematicchemical changes in lherzoliteB sug- gest that the calculated P-T variations in that rock solvus of Davis and Boyd (1966). lt demonstrates may be due to mass transfer of ions in an open that compositional variations within a xenolith can system,rather than to real variations of pressureand profoundly affect the calculated temperatures.The temperature.Since the bulk composition of a system valuesin Figure 3 show an apparent minimum tem- is a major factor contfolling the composition of perature spread of 20' between analyzed points in phases crystallizing in that system, the high chro- one megacrystand 40o in another. The temperature mium contentsof orthopyroxenesin these rocks in- changeoccurs between points that are no more than I dicate that an unusual bulk compositon may have mm apart in either case. contributed to the failure of current petrogenetic Calculated temperatures for lherzolites seem to grids to define conditions of formation using ideal- show significantvariations also, but there are so few ized systems. clinopyroxenesthat statisticalproof of significanceis not possible(Table l). Many more orthopyroxenes Conclusion are identifiable,and their AlrOr variations were used The complexity of natural systemsrenders difficult to calculate pressures,using the petrogeneticgrid of any comparisonwith simplifiedmodel systems.While MacGregor (1974). The systematicchemical varia- it is necessaryto study the complex systemsby using tions shown in Figure 2Acreate apparentdifferences in calculated pressures(Figure 4), which reveal a spread in pressureof 2000 to 7000 bars for three xenoliths; the 7000-bardifference occurs over a dis- tanceof 3 mm. In addition, only six of the twenty-one calculatedvalues plot within the field of spinel stabil- ity, though this is uncorrected for the variation in spinel compositions. The large spread in pressure

values occurs in the transition zone of the orthopy- F roxenite "vein" (points 5-7, Fig. 24; point 7 is marked in Fig. 4). Discussion Low-calcium clinopyroxenes in the analyzed lherzolite xenoliths from Black Rock Summit pro- duce high calculatedtemperatures which, combined t0 rs 20 25 30 35 40 45 with relatively low alumina contents in orthopy- P (tbor) roxene, put the pressureand temperatureof equili- Frc. 4 Plot of calculatedpressures and temperaturesfrom co- bration for theserocks into the high-7, high-P garnet existing pyroxenesin lherzolites(P-I grid of MacGregor, 1974; peridotite facies region, according to MacGregor's plotted from valuesin Table l). Symbols:lherzolite,4-A, lherzo- (1974) petrogeneticgrid. Even if the discrepancybe- lite B-O, lherzoliteC-L], lherzoliteD-O. PYROXENE COMPOSITIONS OF MEGACRTSTSAND XENOLITHS 731 approximate compositionsso that conditions can be Boyo, F. R. (1973) A pyroxene geotherm. Geochim. Cosmochim. controlled and variableskept to a manageablenum- Acra, 37, 2533-2546. ber, drawing far-reachingconclusions on such bases Devrs, B. T. C. ,c.NoF. R. Boyo (1966) The join Mg,StO"- CaMgSirOu at 30 kilobars and its application to pyroxenes is hazardous. Data presented by Wilshire and from kimberlites. J. Geophys. Res. 71, 3567-3576. Shervais(1975) and Wilshire (1975), and Jackson as MrcGnrcon, I. D. (1974)The systemMgO-ALOr-SiOz: solubil- well as the presentwork, indicatethat publishedP-Z ity of ALO, in enstatite for spinel and garnet peridotite composi- grids and correctionsare preliminary stepsin defin- Lions.Am. Mineral. 59, I l0-l19. ing the conditions of formation of natural rocks, and - AND A. R. BAsu (1974) Thermal structureof the lithos- much more remainsto be understoodabout variable phere:a petrologiccontribution Science,185, 1007-l0l l. bulk compositionsas well as varying temperatureand MrncrEn, JsnN-Clnuon ANDN. L. Clnrnn (1975) Pyroxenegeo- depth of formation before evolution of natural sys- therms. J. Geophys. Res 80, 1349-3362. tems can be more accuratelydefined. These data in- NtxoN, P, H. lNo F. R. Bovn (1973)Petrogenesis of the granular and sheared ultrabasic nodule suite in kimberlites. Lesotho Kim- dicate that the compositional variations within a berlites, Lesotho National Development Corporation, Maseru, singlesmall xenolith could be as great or greaterthan Lesotho, South Africa the variations in a collection of xenoliths.Calculated Ross, M. eNo J. S. HuEeNnn (1975)A pyroxenegeothermometer pressure-temperaturevariations derived from a given based on composition-temperature relationships between co- xenolith collection must be interpreted cautiously: existing orthopyroxene, pigeonite, and augite ExtendedAbstacts, lines drawn through such points on a P-T plot may Intl. Conf. Geolhermom.Geobarom., Pennsylvania State University, CollegePark. Pennsylvania. not be real fossil geotherms, but may actually be Tnesr, N. J. (1969) artifacts of compositional Ultramafic xenoliths in basalt,Nye County, changes and dis- Nevada. U S Geol. Suru. Prof. Pap.650-D,43-48. equilibrium in the sourceregion. WtLsnrnr, H. G. lNo E. D. Jecrsox (1975)Problems in determin- Acknowledgments ing mantle geotherms from pyroxene compositions of ultramafic rocks. Geol. 83, 313-329 This research was supported by an NRC Postdoctoral Fellow- "/ - AND J. W. Surnvrrs (1975) Al-augite and Cr-diopside ship at the U.S. GeologicalSurvey. Thanks are due to C E. Meyer ultramafic xenoliths in basaltic rocks from western United and L. C. Calk for technical advice and help in data reduction States: structural and textural relationship Phvs. during the probe work; and to J. D Nakata for assistancein Earth Planet Inter. 9, 257-272 samplepreparation. Reviews of this paper by H. G. Wilshire and E D. Jackson are gratefully acknowledged. Wooo, B. J. (1974) The solubility of alumina in orthopyroxene coexisting with garnet Contrib Mineral. Petrol.46, l-15. References - AND S. BnNro (1973) Garnet-orthopyroxene and clino- Arelll, J. eNo F. R. Boyo (1974) Petrogeneticgrid for garnet pyroxene-orthopyroxene relationships in simple and complex . Carnegie Inst. Lltash Year Book, 13, 269-273. systems. Contrib. Mineral. Petrol. 42, 109-124.