Alteration of Ghromite from the Twin Sisters Dunite, Washington

American Mineralogist, Volume 59, pages 608412' 1974

Alterationof Ghromitefrom the Twin SistersDunite, Washington

ANrnoNv C. ONYracocne'

Department of Geological Sciences, Uniuersity of llashington, Seattle, Washington 98 195

Affiact

Chromite in the Twin Sistersdunite, an alpine-type ultramafic body some 90 km2, occurs as minor disseminatedeuhedral-subhedral crystals or in clusters forming pods and lenses.Modal proportions of chromite in the pods and lensesare frequently as high as 80 percent.Ferritchromite occurs as rims on chromite crystalsthat are bordered by chlorite and (where still available) olivine' Microprobe analysesof chromite, ferritchromite, chrome-magnetite,olivine, and chlorite show that the chromite-ferritchromite boundary is sharp (lessthan l0 microns) and that, relative to chromite' ferritchromite is enriched in Cr, Fe, Ni, and Mn and impoverished in Mg and Al' However, when chrome-magnetite,found only in association with serpentine,is the alteration product, there is a marked decreasein chromium. Chromite alteration appearsto take place above the upper stability limit of serpentineand below that of chlorite.

Inhoduction Two types of chromite alteration were recognized in this study.In one,the alterationproduct is chrome- Alteration of chromite to opaque highly reflecting bearingmagnetite which is associatedwith serpentine. bordershas beenreported by severalauthors (Weiser, In the other, the alteration product is ferritchromite 1967;Kern, 1968;Mih6lik and Saager,1968; Beeson which is in no way associated with serpentine. and Jackson,1969; Frisch, 1971).Because the altera- Serpentineassociated with the magnetite was not tion product is rich in iron, it has been called "gray studiedin detail. magnetite" (Horninger, l94l) or "ferritchromit" (Spangenberg,1943). Several quantitative chemical PehograPhY descriptions of this alteration product have been Chromite crystals in the Twin Sisters dunite worked out (Table l). Most workers, however, (Ragan, 196?)occur either as disseminated,euhedral- investigated only the distribution of the major to-subhedralgrains which constituteless than I per- elements between the chromite and its alteration of the rock, or as concentrationsin bands or product. In this investigation,the behavior of four cent schlieren.They also occur in clustersforming pods major elements(Mg, Fe, Al, and Cr) and also six and lensessome 0.3 metersor more thick. Thesepods minor elements(Ni, Mn, Co, Ti, Zn, and V) were and lensesare distributed irregularly over the entire studied. body. Modal proportions of chromite in these Beesonand Jackson(1969) reported that chromite rock pods and lensesare frequently as high as 80 percent. alteration accompanies the change of associated The chromite crystals in the pods and lenses are olivine and orthopyroxeneto chlorite. Many of the anhedral and some display gran- reported instancesof chromite alteration have been characteristically margins. The dominant, although restricted, related to serpentinizationperhaps caused by late ulated in the original chromite pods and lensesis the pneumatolytic-hydrothermalsolutions (Kern, 1968). change breakdown of chromite plus olivine and enstatite to From a fresh dunite nodule from a basaltic tuff, ferritchromite and chlorite. Frisch (1971) reported chromite alteration that The chromite alteration product in three specimens appears to have resulted from high-temperature, from chromite lenses from locations 2, 5, and 7 late-magmaticprocesses prior to eruption. (Figure 2, Onyeagocha,1973)were studied in detail. developedferrit- I Present address: Department of Mineral Sciences,Smith- Severalchromite crystalswith well sonian Institution, Washington, D. C. 20560. chromite rims were studied, but data are presented 608 ALTERATION OF CHROMITE 609 for only a few that illustrate the extent and nature of Tnsrp 1. Pattern of Chromite Alteration this alteration. These samplesconsist of unaltered chromiteand olivine with local zonesof ferritchromite No Pattern Enrlched in chromite Enriched in Ferrl.tchromlte and light purple chlorite. The chromite grains are Al, Mg Fe, Cr ((reiser, Case I,1967) A1, Mg, Cr fe (Weiser, Case II, 1967) anhedralto subhedral.Chlorite is kinked, representing A1, Mg, Cr Fe (Mlhaltk & Saager, 1968) an adjustmeritto forces exertedby the surrounding A1, Mg. Cr Fe (Beeson & Jackson, 1969) chromite during chlorite growth. Cr At, Mg Fe (Lapham, 1964) There is some A1 Fe (whlte,1966) textural intergrowth of ferritchromite and chlorite Cr Fe (Mulr & Ttlley, 1964) Cr, A1, Mg, Zn Fe, Ti, Ni (Frlsch, 1971) (identified as chlorite by partial microprobe analysis) AI, Mg Fe, Cr, Nl, Mn, Co, T1, much like that shown by Golding and Bayliss(1968, Zn, V (ThIs study) p. 174, Figure 8) which they called "coarsely pitted Cr, A1, Mg Fe, Co, Zn, Ni. (Enriched ln wgnetlte) (Thls study) alteredchromite." At the boundary between a chromite lens and dunite,chromite alteration did not yield ferritchromite and later re-equilibratedat lower temperature.This but instead chrome-bearingmagnetite and books of is corroborated by the data in Table 3 for chrome- kinked green chlorite. The magnetite is associated bearing magnetite where apparent equilibration with serpentine.Instead of the increasein Cr observed temperatures,for what they are worth, are negative. in the other samples,the alteration product showsa No difference in temperature is'apparent between tremendous decreasein Cr. The lower Cr content olivine-chromite and olivine-ferritchromite in this noted in magnetite relative to ferritchromite is study (Table 3) nor did Frisch (1971) notice any accompaniedby a lower Cr content of coexisting, difference.Since the olivine is, in general,homoge- greenchlorite relativeto the purple chlorite. neous, an alternateinterpretation of re-equilibration Discussion with olivine richer in iron is not applicablehere. In fact, judging from the equation written for alteration Microprobe analyses of phases associatedwith of chromite to ferritchromite using the actual analyt- chromite alteration (Table 2) show that olivine is ical data obtained in this study, it appearsthat the particularly homogeneous.No grain-to-grain-varia- initial olivine is richer in iron. The equation'assumes tions wereobserved, nor did stepscans across individ- that Al, Cr, and Mg wereconserved. ual grains reveal any inhomogeneity. Unaltered go. - * chromitein thesespecimens exhibits no variationsthat 4. 4 4(M zrFe' r uAI0. z u F eBo. oe) On " :lf :-rtr."r could not be accounted for by count statistics. Ferritchromiteand chrome-bearingmagnetite showed * 3.52(Mg.r,.€,Fe'*'. le Sion) slight inhomogeneity.Cr, Fe, and Al in chlorite are remarkably variable within grains and from grain to f 2.54(Meo.:LT:,:.0,SiO3)+ 8 H,O + 0.27O' grain. Within grain a single the Cr-content increases * toward : 3( M 96.55 F"' lo.08 F es o.a u) On the unaltered chromite core and decreases *.::11"".."1l?A towards olivine. For example,specimen 5-5A (near chromite) has 3,2 percent CrrO, whereas 5-5B * ( Mg,o. rrCr6.nrAlr. rTFe'*0.lex Si6.06Alr. s4) O2o o Hr6 (near olivine) only contains 0.10 percent CrrO, (Table 2). ExcessFe and Si were partitioned to form forsterite and Figure I showsstep scansacross a crystal in which enstatitewith the sameFe/Mg ratio. Written in the chromite-ferritchromiteboundary is shdrp (less this manner, the equation showsthat only HrO and than l0 microns). The scansalso show that, relative O, need be introduced. The alteration, therefore, to the chromitc, ferritchromite is enrichedin Cr, Fe, appearsto be a responseto changingoxygen fugacity (Irvine, Ni, and Mn, and impoverishedin Mg and Al. The 1965)and changing/rr,o. pattern is not clear for Co, Zn, andY. A volume-for-volumereplacement of the outcrop as postulatedby Beesonand Jackson(1969, p. 1096) The decreasein the ratios, Mg/(Mg f Fe,*) and Al/(Cr + Al + Fe'*), and the simultaneousincrease z In in Fe'*/(Cr + Al + Fe'*) observedin both types of writing this equation, specimen 2-4Afr1 was used for chromite, ferritchromite, and chlorite. No found chromite alteration olivine was according to Jackson (1969), within severalcm ofthis crystal. Olivine used in the equation is indicatesthat the chromite formed at high temperature from specimen2-4 and is some l0 cm away from2-4A. 610 ANTHONY C, ONYEAGOCHA

Tnsle 2. Electron Microprobe Analysesof Chromite, Ferritchromite, Magnetite, Olivine, and Chlorite fro'm the Twjn SistersDunite, Washington

CHROMITE FERRITCI{ROMITE MAGNETITE

2-4 2-4A#L 2-4Lll2 5-4B 5-4c 5-4D 5-5 5-5D 5-7 z4Alll 2-4All2 5-5 5-4A 5-48 5-4C 5-7

Tto2 0.03 0.05 0.05 0.10 0.13 0.08 0.08 0.08 0.05 0.12 0.90 0.13 0.02 0.03 0.02 0.02 AL2o3 2r.3 20.6 2t.l LL.7 10.7 16.6 17.5 16.8 18.1 1.8 2.7 9.9 0.48 0,57 0.67 0.56 Cr2O3 47.O 47.7 46.5 51.s 52.7 47.L 51.1 50.8 50.4 56.0 54.9 55.5 0.07 0.25 1.9 4.2 Fe203* 3.8 3.7 4.O 9.7 10.4 5.8 4.L 4.6 3,9 13.1 13.0 1,9 69,5 59.6 67.7 64.9

FeO* 11.0 11.3 10.8 13.9 L3.7 L7.9 t2,4 13.5 12.6 L4.7 t3.7 14.r 25.i 25.5 26,4 27.2 lfnO 0.23 0,24 0.41 0.50 0.58 0.52 0.44 0,47 0.4 o.47 0.63 0.55 o.22 0.18 0.18 0.45 MsO 15.5 L5.2 r5.4 L2.6 12.8 10.3 14.3 L3.4 L4.2 10.1 11,5 L2.4 2.4 2.4 2.O L.7 CoO 0.04 0.03 0.04 0.08 0.11 0.11 0.07 0.08 0.05 0.04 0.08 0.10 o.25 0,23 0.22 0.21

ZnO 0.05 0.07 0.03 0.14 0.14 0,27 0.05 0.08 0.05 0.20 0.14 0.10 0.29 0,29 0.23 0.24 N10 0.10 0, L0 0.09 0.r1 0.15 0,09 0.08 0.09 0.09 0.49 0.34 0.13 1.4 1.5 L.4 0.50 vZ03 0.11 0,12 0.12 0.16 0.19 0.21 0.16 0,18 0.12 0.16 0.16 0.19 0.05 0.07 0.04 0.13

Total 99, 16 99. 11 98.54 100.49 101.60 99.98 100.28 100.18 99.99 97.18 98.05 101.00 99.93 L00.62100.75 100.21

IONIC PROPORTIONSONTHE BASIS OF 32 OXYGENATOMS

Ti .006 .oo9 .oo9 .019 .O2s ,015 .015 .015 ,009 .026 .187 .025 .005 .007 .005 .005 A1 6.156 5,982 6,L37 3,554 3.23L 5.032 5.129.4.972 5,309 .600 .880 3.019 .171 .202 .237 .200 g.rr2 9,292 9,O74 LO.496L0.676 9.579 10.04710.085 9.9L6 12,s30 12.000 11.354 .017 .059 .451 1.004 Fe3* .6ss .684 .747 1.878 2.004 1.315 .762 ,877 .732 2.182 2.7rL 1.538 15.79115.709 L5.294 14.756

Ee2* 2.256 2,339 2.227 3.003 2.931 3.861 2.583 2.851 2.6L4 3.490 3.L79 3.055 6.402 6.406 6.623 6.873 Mn . 048 .050 .086 .109 .L26 .113 .093 .100 .091 .113 .148 .rzL .056 .046 ,046 .115 Mg 5.665 s.582 5.665 4.841 4.888 3.949 5.301 5.015 5.267 4.260 4.739 4.782 1.080 1 .073 .894 .766 Co .008 .005 .008 .0r7 .o23 .o23 .014 .016 .010 .009 .018 ,ozL .06r .055 ,053 .051

Zt .009 .013 .005 .o27 .026 .051 .009 .015 .009 .o42 .029 .019 .065 .064 .0s1 0.54 Ni j020 ,020 .018 .O23 .031 .019 .016 .018 .018 .LL2 .076 .027 .340 .362 .338 .L46 .oLz .017 .010 .032 1t .o22 .024 .024 .033 .039 .043 .032 .036 .O24 .036 .035 .039

OLIVINE CITLORITE

)-4 )-) )-) 2-4Al!L 2-4A#2 5-5A 5-58 5-7 ch1

St02 4r.4 4L.2 41,4 4L.7 5102 33.4 32.3 33.1 31.4 32.L 33.0 3L.4 Fe0 4.4 8,2 5.7 5.1 A1203 13 . 5 14.1 14.s 15.8 15.4 16.2 17.0 Mn0 0.05 0.13 0.10 0.08 cr203 0.56 2.9 1 ,8 0.01 3 .2 0.10 0 .46

Mgo 53.1 50.7 52.4 52.6 FeO 1.5 1.2 1.3 2.3 1.5 r.7 1.8 Ni0 0.71 0.43 o.52 0.62 lho 0.03 0.04 0.04 0.01 0.02 0.01 0.01 uau 0,00 0.01 0.01 0.01 MgO 37 .0 36 .9 36.4 37.2 35.8 35.8 36.5 N10 0.2L 0.24 0.30 0.24 0.36 0.34 0.r.6 Total 99.66 100.67 100.13 100.11 Toral 86,30 87.58 87.44 86.85 88"38 88.15 87.33

Ionic proportions on the basis of 4 oxygen atons Ionic proportlons on the basls of 28 oxygen atons

sl .996 .997 .996 1.00r sl 6.317 6.062 6.L96 5.928 5.985 6.107 5.883 .089 .166 .115 ,L02 Alrv 1.683 1.938 1.804 2,072 2.Or4 1.893 2.Lr7 }tn .001 .003 .002 .002 Alvr L,126 l-.1-81 1.395 t.443 L.37L 1.640 r.$7 cr .099 .430 .266 .001 .472 .015 .068 Mg 1.904 1.829 1.880 r.882 Ni .014 .008 .010 .oL2 Fe .237 .188 .204 .363 .234 .263 .282 .000 .000 .000 .000 Mn .005 .006 .005 .002 .003 .002 .002 R2+ 2.008 2.006 2.00't 1.998 Ms 10.430 10.322 l0 .156 10.458 9,951 10,1s0 10.193 Nl .032 .036 .045 .036 .054 ,051 .O24

*Feo and Fe203 computed fron total iron aasualng perfect sptnel stolchloDetry.

for the Stillwater alteration is not necessarilyappli- tion (Misch, 1936) and in periclase dehydration cable to this study. There are no continuousbeds or (Burnham,1959; Turner and Weiss,1965; Carpenter, bands recognizablein the Twin Sistersdunite, but r96',7). the bending and kinking of some chlorite crystals It shouldbe emphasizedthat the chromitealteration represent growth in a confined environment and observedin some lensesis restricted to only small thereforeare an expressionof increasein volume.The fractions of the total lens. Specimenscollected a-few samephenomenon is known to occur in mica forma- centimetersaway from the alteration do not exhibit ALTERATION OF CHROMITE 611

any alteration at all. The alteration characteristically is concentratedin the chromite-rich rocks whereas chromite and olivine in the adjacent dunite are E unaltered. It would thus appear that the alteration * occurs above the upper stability limit of serpentine ' but below that ofchlorite. Frost (1973)has shown that in progressivemetamorphisrn of serpentinite,magnetite co-exists with serpentine. With increase in grade, the oxide becomes richer in chrome until chromite is formed; This change in composition of the spinel phase is at least partially related to the coexistingphyllosilicate. Using the data of Kitahara, Takenouchi,and Kennedy (1966),the upper stability of serpentineat 10 kbar water pressurewould yield temperaturesof about 570"C. Using the data of Fawcett and Yoder (1966),the upper limit of chlorite E is around 800'C. The 570"C to 800oCtemperature :l interval is wide enoughfor all metamorphicreactions :l in the Twin Sistersdunite reported in Onyeagocha (re73\.

Acknowledgments The author would like to thank Drs. Bernard W. Evans and Melvin H. Beeson for critically reviewing this manuscript. This work was supported by NSF grant cA 29767 (8. W. Evans). References Flc. 1 Step scansacross altered chromite. BEEsoN,M. H., lxo E. D. JecrsoN (1969) Chemical composition of altered chromites from the Stillwater Complex, TlaI.r 3. Derivation of Apparent Equilibration Temperature, Montana. Am. Mineral. 54, 1084-1100. Using the Equation of Jackson (1969)* BunNnarrr,C. W. (1959) Contact metamorphism of mag- nesian limestone at Crestmore. California. GeoI. Soc.

speclnen Ms/Mc+Fe Ms/Ms+Fe d B S 5"r_".2+ l.c Am. Bull. 70. 879-920. CrnreNrnn, A. B. (1967) Mineralogy and petrology of 0Lidne-Chrcnite the system CaO-MgO-CO,-H,O Ollvtne ChroEtte at Crestmore, California. 2-4 .955 .775 .57L .386 .044 8,45 Am. Mineral. 52. 134l-1363. 2-4^11 .955 .705 ,5A2 .375 .043 8.89 rt38 Fewcrrr, J. J., rNo H. S. Yoopn (1966) Phase relations 2-4^ll2 .955 .718 .569 .385 .047 8.34 1163 5-4B .9L7 .6L7 .659 .223 .1r8 6.85 1660 of chlorites in the system MgO-ALOrSiO,-H,O. Am.

5-4C .9L7 .625 .67r .203 .726 6.62 1756 Mineral. 51, 353-380. 5-4D ,971 .506 .602 .316 ,083 10.80 1090 (1971\ ,949 .672 .630 .322 .048 9.07 1210 FRIscH,T. Alteration of chrome spinel in a dunite 5-5D ,942 .638 .633 .3t2 .055 9.23 L2L2 nodule from Lanzarote,Canary Islands. Lithos,4,83-97. 0 l,i uine - Fe rpi tch rc fr it e Fnosr, B. R. (1973) Contact Metamorphism of the Ingalls Ollvlne Fetrltchroblte Ultramafic Complex at Paddy-Go-Easy Pass, Central 2-4A{l .955 ,550 .187 .038 .175 L7.39 I 188 2-ttAll2 .955 .599 .770 .056 .r74 14.24 1298 Cascades, Vlashington. Ph.D. thesis, University of Wash- 5-5 .949 .610 .7t4 .r90 .o97 11.89 1225 ington, Seattle, 185 p. Olirifle-I4agnetite GororNG, H. G., eNo P. Bryrrss (1968) Altered chrome Ollvlne Chlone-Mgnetlte ores from the Coolac Serpentine Belt, New South Wales, 5-4A .9I7 ,744 .001 .011 .988 6s.47 5-4B .917 .144 .004 . 013 .984 55.93 -81 Australia. Am. Mineral. 53, 162-183. 5-4C .9L7 ,119 .028 . 015 .957 81,8I -62 HonNIncen, G. (1941) Beobachtungenam Erzinhalt von * Thts equatton (Jackson, 1969, p. 63, equarion 9) reads Gesteinenund am Chromerz aus Tampadel in Schlesein. Schweiz. Mineral. Petrogr. Mitt. 52, 316-345. 0,90a + 2.568 - 3,086 - 1.47 + I.987 rn lq -!,ls-Fe-' Invrrvn, T. N. (1965) Chromian spinel as a petrogenetic where d, B, and 6--as clted ln lhe above rable--are rhe fractlons of trl- indicator. Pt. I. Theory. Can. l. Earth Sci. 2, 648-672. valent callons, Cr, el, te3+ respectlvely, 1n spinel; and where + + = r.0 a B 6 Iecrsox, E. D. (1969) Chemical variation in coexisting * r.2*)}or K- .. - {u*l(u" !""2*/(u** r"2*)}'}t chromite and olivine in chromitite zones of the Still- "ug-r." {rezrl(Ms + r"'*))"' * r.'*)}.n. {ug/(rlg water Complex. Econ. Geol. Illonogr. 4, 4l-71. 612 ANTHONY C. ONYEAGOCHA

KEnN, H. (1958) Zur Geochemie und Lagerstiittenkunde ONvsecocru, A. C. (1973) Petrology and Mineralogy of des Chroms und zur Mikroskopie und Genese der the Twin SistersDunite, Washington' Ph'D. thesis' Uni- Chromerz, Cla'usthaler Helt z. Lagersti;ttenkunde Geo' versity of WashingJon,Seattle, 135 p. chem.Minerol. Rohstofte,6,236 p, ReclN, D. M. (1957) The Twin Sistersdunite, Washington. Knerr n,l, S., S. TexpNoucHl, ANDG. C. KsNNppv (1966) In, P. J. Wyllie, Ed., Ultramofic and Related Rocks, Phase relations in the system MgO-SiO,-H,O at high New York, London, Sydney, John Wiley and Sons, Inc., temperaturesand pressures.Am. l. $ci.2641 223-233' p. l6Fl67. LmHlrvr, D. M. (1964) Spinel-orthopyroxene compositions SpANcENspRc,K. (1943) Die Chromitlagerstbttevon Tam- and their bearing on the origin of the serpentinite near padel in Zobten. Z. Prokt. Geol. 51, 13-35. Mayagtez, Puerto Rico. In, C. A. Burk, Ed', A Study Tt-rnxpn, F. J., rNl L. E. WErss (1965) Deformational ol Serpentinite. NAS-NRC Pub. 11E8' Washington, p. kinks in brucite and gypsum. Proc. Nat. Acad, Sci- U'S' 134-144. 54. 359-364. Mrnir-rx, P., eNr R. Seecr.n (1968) Chromite grains show- Wrrsrn, T. (1967) Untersuchungen mit der Electronen- ing altered borders from the Basal Reef, Witwatersrand mikrosonde iiber die Zusammensetzung von Chromiten. System. Am. Mineral. 53' 1543-1550. Neues lahrb. Mineral. Abh. 107' ll3-143. MIScH, P. (1936) Ein gefalteter junger Sandsteinim Nord- WHrrE, R. W. (1966) Ultramafic inclusions in basaltic west-Himalaya und sein Gefiige. In, "Stille Festschrift," rocks from Hawaii. Contrib. Mineral. Petrol. 12, 245- Verlag Ferdinand Enke, Stuttgart, p. 258-276. 314. Murn. I. D...r.No C. E. Ttwev (1954) Basaltsfrom the northern part of the rift zone ol the Mid-Atlantic Ridge. Manurcript receiued, October 29, 1973; accepted L Petrol. 5, 409-434. lor publication, FebruarY 1, 1974.