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A Migroprobe Study of Antigorite and Some

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A Migroprobe Study of Antigorite and Some CanadianMineralogist Vol. 17, pp.77l-784 (t979) A MIGROPROBESTUDY OF ANTIGORITEAND SOMESERPENTINE PSEUIPMORPHS MICHAEL A. DUNGAN Department of Geological Sciences, Southern Methodist (ltriversity, Dallas, Texas 75275, U.S.A. AssrRAcr tine. There are two major petrological types of serpentinite, each distinct in its ,mineralogy, Microprobe analyses of serpentine pseudomorphs texture and petrogenesis (Coleman 1971). The after olivine, orthopyroxene and clinopyroxene most abundant, type consists of serpentinites demonstrato partial inheritance of compositional peridotites, characferistics from the parent phases. Bastites have formed by the hydration of referred higher chromium and aluminum contents than to here as pseudomorphic serpentinites. T\eir mesh-textured serpentine. Qualitative estimates of serpentine mineralogy is lizardite plus minor elemental mobility during serpentinization are chrysotile, and they are characterized by tex- Fe))Al)Cr. The observed persistence of lizardite tures that are strictly indicative of retrograde bastites in progressively metamorphosed serpenti replacement of olivine, orthopyroxene, clino- nites suggestsa compositional control. The presence pyroxene and, more rarely, tremolite, talc and of aluminum in solid solution may increase the anthophyllite. Antigorite serpentinites are thermal stability of lizardite with respect to anti- formed largely by recrystallization of pseudo- gorite or to partly dehydrated assemblages. The microprobe analyses confirm the compositional morphic serpentinites during progtessive meta- differences betw€en pseudomorphic serpentines (liz- morphism (Coleman 1971, Wicks & Whittaker aldite and chrysotile) and antigorite that are pre- 1977), These relatively coarse-gtained rocks dicted by their slightly different structural formulae: consist of aggregatesof interlocking antigorite antigorites have lower HrO, lower octahedral oc- blades. If thermal upgrading proceeds to suf- cupancies and relatively high SiOz. ficiently elevatedlemperatures (f.e. 40O-600'C), antigorite serpentinites may become further SoMMAnE reconstituted to forsteritrantigorite rocks or completely dehydratedto form secondaryperido- D'aprds les analyses d la microsonde, les serpen- (Peters Vance & tines pseudomorphes d'olivine, orthopyroxbne ou tites 1968, Matthes 1971, clinopyroxEne h6ritent certaines caract6ristiques de Dungan 1.977). la composition originelle. ks bastites contiennent Microprobe analyses in conjunction with plus de Cr et d'Al que les serpentines i texture mineralogical and textural evidence indicate r6ticul6e. Pendant la serpentinisation, la mobilit6 that different factors control the compositions relative des 6l6ments est repr6sent6e, i l'estime, of pseudomorphic serpentines and antigorite. par La persistance de la le sch6ma Fe))Al)Cr. Contrasts in the nature of the equilibria as' bastite fzardite du m6tamorphisme pro- a au cours are gressif des serpentinites t6moigne de l'influence du sociated with the two tyPes of serpentinite facteur composition. La pr6sence d'aluminium en evaluated in relation to the compositions of the solution solide dans la lizardite pourrait en ac- phases analyzed. The persistence in antigorite cro?tre la stabilit6 thermique par rapport i fanti- serpentinites of lizardite bastites after ortho- gorite ou d des assemblages partiellement d6shy- pyroxene and clinopyroxene is also discussed drat6s. On confirme i la microsonde des diver- in the light of compositional differences be- gences de composition entre les esplces pseudo- tween mesh-texturedserpentine and the bastites. morphes (lizardite et chrysotile) et I'antigoriten divergences que font pr6voir des formules struc- turales l€gdrement diffdrentes: les antigorites con- Pnnvrous Wom tiennent moins d'eau, plus de lacunes en position (1962), (1967) and octaEdrique et une t€neur en SiOz relativement Faust & Fahey Page 6lev6e. Whittaker & Wicks (1970) have attempted to Clraduit par la R6daction) define the compositional characteristics of the serpentine minerals through an evaluation of (1962) INrnoouctroN published analyses.Faust & Fahey com- piled the serpentine analy$es available at that The objective of this study is to analyze time. Page (1968) added new data (Page & with the electron microprobe some mineralogical Coleman 1967) but discarded many old' ques- and textural varieties of rock-forming serpen- tionable analysesfrom the Faust & Fahey list- 77r 772 THE cANADIAN MINBRALOGIST ing. Page also pres€nted a discussion of the Secondary lizardite or six-layer serpentine compositional differences among the serpentine that has replaced pseudomorphic serpentine minerals based on this new list. Whittaker & superficially may resemble antigorite: both can Wicks .(1970; limited the group even further ociur in radiating aggregates. Several serpenti- by eliminating all analyses that, in their opinion, nites in the Sultan Complex apparently contain were not unambiguously identified or that may secondary s€rpentine that is not antigorite. have included minor amounts of additional These occurrences were tentatively identified phases. Trommsdorff & Evans (1972) and on the basis of petrography and microprobe Frost (1975) included microprobe analyses of data, but chemical analyses are not reported. antigorite and coexisting phases in partly de- Mineral identification of the various textural hydrated, progressively metamorphosed serpen- components of pseudomorphic serpentinites is tinites. far more difficult than in antigorite serpen- Geological studies of the Sultan Complex and tinites. The textural complexity of the Sultan the associated Darington peridotites were re- Complex pseudomorphic serpentinites eliminated ported by Vance (L972), Dungan & Vance whole-rock X-ray diffraction as a viable method (L972), Johnson et al. (1977), Dungan (L974, for establishing the mineralogy of individual 1977, 1979), Vance & Dungan (1977) and textural types except in a few samples. The Vance et al. (198O). The majority of serpen- results of earlier mineralogical determinations tine samples from which analytical data have of serpentine textures were relied upon for been collected are from the Sultan Complex. providing mineral identification of serpentine Howevet, additional samples from several west- pseudomorphs (Wicks 1969, Wicks & Whittaker coast ultramafis bodies have also been analyzed. 1977, Wicks & Zussman 1975). The results The representative analyses tabulated here are of these studies demonstrate that lizardite is abstracted from a larger body of data included tle dominant mineral in serpentine pseudo- in Dungan (1974)t, these have been used in morphs, particularly in bastites. Mesh-textured the discussionand in the preparation of figures. serpentine after olivine may be composed en- Complete tables of serpentino analyses are tirely of lizardite or intergrowths of lizardite available on request. and chrysotile (t brucite) with lizardite pre- dominating. On tle basis of these results, micro- probe analyses of Sultan Complex bastites are Axer,vrrcer, TEcHNIQUES presumed to be lizardite. It is inferred that serpen- Identification of the serpentine minerals microprobe analyses of mesh-textured tine probably reflect lizardite compositions, but Antigorite is the most easily identified ser- it is acknowledged that this has not been speci- pentjne mineral, by both its habit and its com- fically established and that they may also be paratively distinctive X-ray-diffraction pattern bimineralic. However, there is some value in (Whittaker & Zussman 1956, Page & Coleman comparing antigorite analyses with analyses of L967 ) . Antigorite in monomineralic serpentinites lizardite or lizardite plus chrysotile, since there forms a flarne-like or radiating bladed texture. are differences in the structural formulae be- Where it is in textural equilibrium with olivine tween (1) antigorite and (2) lizardite or chry- it generally occurs in a lepidoblastic fabric of sotile. Where this comparison is made, ltzardrte anhedral blades intergrown with granular and chrysotile have been grouped together as olivine. This textural style is, in contrast, pseudomorphic serpentines. atypical of serpentine-olivine relationships in pseudomorphic serpentinites. The contrast in M icroprobe-analy sis techniques textural relationships between coarse-grained antigorite and strictly pseudomorphic serpentine Microprobe analyses were performed by provides an adequatecriterion for distinguishing standard techniquesat 15 kV and O.5 to 1.5 mA the two types. Whole-rock powders of antigorite on an ARL EMX-SM five-channel microprobe. serpentinites lacking olivine were analyzed by Iron, nickel, manganese,aluminum and chro- X-ray diffraction and gave patterns that in- mium were standardized against natural olivines, dicate the absenceof other serpentine minerals. a synthetic aluminous enstatite and a natural Intimate intergrowths between antigorite and chromite, respectively.A natural antigorite sup- olivine prevented the separation of pure anti- plied by R. G. Coleman (94 NA 62' Page & gorite by heavyJiquid techniques. Howevet, Coleman 1967) was used as a'magnesium and the texture of antigorite-olivine intergrowths is silicon standard. This antigorite was reanalyzed so distinct from pseudomorphic textures that using natural olivines for Si, Mg and Fe and misidentification is improbable. a svnthetic Al-enstatite for aluminum with the A MICROPROBE STUDY OF ANTIGORITE 773 following'results: SiOo 44.5, AlzOs 0.64, MgO IABL.EI. NCROPROBEA'IILYSES OF AIITIGORITES 39.7 and FeO 3.1 (all
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