American Mineralogist, Volume 6l , pages 725-731, 1976 Pressuresand temperatures calculated from chromium-rich pyroxene 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 peridotite xenoliths 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 olivine-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 thin section,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. Xenolith 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 pyroxenes 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 olivines 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.