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Titanian Chondrodite- and Titanian Clinohumite-Bearing Metadunitefrom the 3800 Ma Isua Supracrustalbelt West Greenland: Chemistry, Petrology, and Origin

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Titanian Chondrodite- and Titanian Clinohumite-Bearing Metadunitefrom the 3800 Ma Isua Supracrustalbelt West Greenland: Chemistry, Petrology, and Origin Ameican Mineralogist, Volume 73, pages 547-558, 1988 Titanian chondrodite- and titanian clinohumite-bearing metadunitefrom the 3800 Ma Isua supracrustalbelt West Greenland: Chemistry, petrology, and origin Ronnnr F. Dvnrnr Department of Earth and Planetary Sciencesand McDonnell Center for the SpaceSciences, Washington University, St. Louis, Missouri 63130, U.S.A. ARcooil andGas.",,o",,jr1llil: -t; ?i*erd, carirorniae3302, u.s.A. Slu C. Bnornnns Department of Geology, University of New Mexico, Albuquerque, New Mexico 87131, U.S.A. Ansrnlcr Titanian chondrodite (Ti-Ch) and clinohumite (Ti-CD have been discovered in a ser- pentinite from the 3.8 Ga Isua supracrustalbelt, West Greenland, where they occur with olivine (= Fonr),magnesite (X*" = 0.99), magnetite,and Ni-Co-Pb sulfidesin a matrix of antigorite. This serpentinite has the bulk composition of a dunite (Niggli value mg x 0.89), with high Ni (-2300 ppm) and Cr (-950 ppm), and its REE pattern is broadly similar to those reported for cumulate dunites of igneous origin. The highly magnesian phase compositions indicate that the metadunite evolved from a serpentinite precursor, which experiencedadditional late serpentinization. Two different generationsof olivine and antrgorite, both with slightly different compositions, indicate an extremely complex history for the sample.However, the titanian humites locally occru as inclusions in olivine, indicating that they formed relatively early. Ti-Ch (-9 wto/oTiOr) occrus as separategrains and also forms lamellar intergrowths with Ti-Cl (-6 wto/oTiOr). These phasesdiffer significantly only in terms of TilSi, as X', (-0.98), X*u X-o, Xrt (-0.45), and X, (-0.05) are similar. The high content of Ti and low content of F indicate nearly complete TiOrMg ,F-, exchange,whereas the high X'n values make thesethe most magnesianTi-rich humites yet reported. The two-phaseintergrowths may representthe reaction Ti-Ch + forsterite : Ti-Cl. The sample lacks geikielite or any magnesian ilmenite, indicating that the terminal breakdown of titanian humites by the reaction titanian humites : forsterite + geikielite * HrO was not approached.Chemographic relations for the systemMgTiO,-MgrSiO4-HrO reveal that Ti-Ch should be the high-pressurehumite phase,on account of the experimentally deter- mined location of the Ti-Cl breakdown reaction (Engi and Lindsley, 1980). The only other known occurrence of a Ti-Ch and Ti-Cl intergrowth is in kimberlite (Aoki et al., 1976),and the possibility that Ti-Ch is a mantle phaseshould be reconsidered. However, we do not regard the Isua sample to be of mantle origin, although the inter- growths may record decompressionduring the polymetamorphic history of the region. INrnolucrroN the Ruby Range, Montana (e.g., cimmino et al., 1979; Natural occlurencesof titanian clinohumite (Ti-Cl) are Trommsdorffand Evans, 1980; Desmarais, 1981; Evans rare but have attracted considerableattention in recent and Trommsdortr, 1983).The work of Trommsdorffand years becausethey are found principally in ultramafic Evans (1980) is particularly important since it seemsto rocks, some of which are demonstrably of mantle origin. establish on petrographic grounds that the stability field McGetchin et al. (1970) noted the presenceof TiCl in for Ti-Cl is contained fully within that of antigorite. Ex- ultramafic nodules found in kimberlite from the Colo- perimental work (Engi and Lindsley, 1980) has shown rado Plateau and speculatedthat this phasecould repre- that Ti-Cl is unstable above 475 "C at 3.5 kbar and 675 sent an important upper-mantle reservoir for volatiles, "C at 2l kbar, which placessevere limitations on the im- especiallywater. A number of reports have also described portance of this mineral to mantle petrology, while at the occurrencesof Ti-Cl in peridotites and serpentinitesfrom same time confirming its stability within the P- Z regime orogenic belts that include the Alps, the Appenines, and relevant to "ordinary" crustal metamorphism. 0003-o04x/88/0506-0547$02.00 547 548 DYMEK ET AL.: CHONDRODITE AND CLINOHUMITE IN METADUNITE TABLE1. Chemicalcomposition of lsua metadunite1W809-4 tiquity of the Isua belt and establishit as the oldest dated group ofrocks on Eanh. Cation % A variety of ultramafic rocks are found at Isua, includ- sio, 41.30 Si 34.0 ing (l) massive metaperidotite-metadunite (olivine + Tio, 0.09 Ti 0.1 + Al203 0.82 AI 0.8 chlorite + Cr-rich magnetite t tremolite cumming- FeO 9.92 Fe 6.8 tonite or anthophyllite + dolomite + magnesite, with MnO 0.16 Mn 0.1 relict greenspinel + orthopyroxene found at one locality; Mgo 47.07 Mg 57.8 CaO o.25 Ca 0.2 Brothers and Dymek, 1983);(2) ultramafic schist derived Naro 0.06 Na 0.1 from metaperidotite (talc + magnesite+ anthophyllite + K"O 0.07 K <0.1 P.o. <0.03 P <0.1 antigorite, with rare multiple chain silicates;O'Leary and Dymek, 1987); and, (3) antigorite serpentinite. Late li- LOI 10.77 zardite and/or chrysotile, either as pseudomorphs after v14 Rb <1 Ce 0.925 olivine or in small veins, occur in many samples. Cr 958 Sr <2 Nd 0.615 The sampleof interest here (IW809-4) is a serpentinite, Ni 2357 Y<1 Sm 0.143 having the bulk composition of a dunite (seebelow). It Zn 59 Zr <2 Eu 0.019 Ga <1 Nb <1 Gd 0.171 is fine-grained, dense, and very fresh, with the typical Ba <5 Dy 0.181 blue-graycolor of the other Isua ultramafic rocks.IW809-4 Pb 13 Er 0.145 red-weathering Th <1 Yb 0.196 was collected from one of a seriesof out- crops that locally contain bladed (metamorphic) olivine Niggfivafue mg: O.894 (Mg + Fe)/Si:1.903 crystals up to several centimeters long. These ultramafic outcrops form pods (on the order of l0 x 50 m) within (1984) Titanian chondrodite (Ti-Ch) is even more rare. a mafic schist that Nutman et al. termed the se- Trommsdorffand Evans(1980) remarked upon its pres- quence A-lower amphibolite formation. Out of about ence in their Alpine samples as a possible breakdown 50 samplesstudied from severallocalities throughout the product of Ti-Cl. The only published analysesthat we are Isua belt, Iw809-4 is the only one in which humite min- aware of are those from the Buell Park, Arizona, kim- erals were found, attesting to the relative rarity of these phases. berlite (Aoki et al., 1976;,cf. Smith, 1977; Fujino and Tak6uchi, 1978),where the Ti-Ch forms intergrowthswith Crmurcar, coMPosrrloN Ti-Cl. The exactparagenesis ofthis occurrenceis unclear, as the humites were not recognized "in situ" but were Major elements recovered through mineral separation on crushed rock The major-element composition of sample IW809-4 fragments. was determined by microprobe analysisof glassprepared In the course of investigating ultramafic metamorphic by fusion of 1.0 g each of sample powder and LirBoO' rocks from the 3.8 Ga Isua supracrustalbelt, West Green- flux using the same proceduresapplied to the analysis of land, we have discoveredTi-Ch in a serpentinizedmeta- the North American Shale Composite (NASC) by Gro- dunite, where it occurs as independent grains and also met et al. (1984).Results are listed in Table I on a vol- forms intergrowths with Ti-Cl. The purpose of this paper atile-free basis and representthe weighted averageof 15 is to document the chemistry of these humites and to replicate determinations. evaluate their petrogenesisin light of available data on The analysisshows the sample to be highly magnesian other occurrences.The present paper follows upon pre- (Niegli value mg = 0.9) with MgO, FeO, and SiO, being liminary resultsreported earlier (Dymek et al., 1983) and the only oxides present in amounts exceeding I wt0/0. is expanded here to include a thorough account on the [(Mg + Fe)/Si]","-,.is close to 2, and a cation-norm cal- phasepetrology of the host metadunite, togetherwith ad- culation yields an olivine content of -95o/o. Thus, the ditional information on its major- and trace-element bulk composition corresponds closely to a dunite. The composition. small quantities of NarO, CaO, KrO, and AlrO, perhaps representvestiges ofaccessory pyroxene, spinel, or mica Gnolocrc SETTTNGAND sAMpLE DESCRrprroN originally present in the protolith, or they were added The Isua supracrustalbelt (lat 65"15'N; long 50'00'W) during metamorphism or alteration. is located in the central part of the West Greenland Ar- Loss-on-ignition (LO! representsweight loss upon fu- chean gneisscomplex. The geology of the Isua area has sion for 15 min at 1000"C on 1.0-gsample powder pre- been studied by, among others, Bridgwater and Mc- viously dried overnight at I l0 'C. The value thus deter- Gregor(1974), Allaart (1976),and Nutman et al. (1984), mined (- I I wto/o)is quite high and indicative of the large who recognized a diverse suite of clastic and chemical amounts of serpentineand magnesitepresent in the sam- metasedimentary rocks, mafic metavolcanic rocks, and ple (seebelow). stratiform ultramafic rocks. Various radiometric dating techniqueshave yielded agesfor the supracrustalrocks in Trace elernents the 3700-3800Ma range[see Baadsgaard et al. (1984)for Abundancesof V, Cr, Ni, Zn, Ga, Rb, Sr, Y, Zr, Nb, recent summaryl, which leave little doubt as to the an- Ba, and Pb were determined by X-ray fluorescenceanal- DYMEK ET AL.: CHONDRODITE AND CLINOHUMITE IN METADUNITE 549 ysis of pressedpowder pellets at the University of Mas- sachusetts,Amherst, using methods describedin Rhodes Soopslore et al. (1978). Tabulated results represent the averageof (Finlond) analyseson two separatealiquants of the sample. Values for most elementsare very low, in severalcases \ below practical limits of detection for the method em- ISUA METAOUNIfE ployed. The high concentration of Ni (-2350 ppm) is I consistent with the inferred original olivine-rich nature of the sample, whereashigh Cr (-960 ppm) is probably due to spinel in the protolith. Of particular note is the high concentration of Pb (- 13 ppm), which is contained in the rare sulfide mineral shandite (Dymek, 1987).
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