The microstructure of minerals in coarse-grained Ca-Al-rich inclusions from the Allende meteorite

00167037/84/f3.00

Geochimica n Cosmochimica Acta Vol. 48. pp. 769-783 8 Pcrgamon Pms Ltd. 1984. Printed in U.S.A.

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The microstructure of minerals in coarse-grained Ca-Al-rich inclusions from the Allende meteorite D. J. BARBERand P. M. MARTIN Departmentof Physin, University of Essex,Wivenhoe Park, Colchester Co4 3SQ, United Kingdom and

Enrico

I. D. HUTCHEON* Fermi Institute, University of Chicago, Chicago, IL 60637

(Received August 2, 1983; accepted in revised form January 16, 1984) Aimtract-The

microstructure and microchemistry

of minerals in Ca-Al-rich, coamegnu ‘ned inclusions

(CAI) from the Allende meteorite have been investigatedwith transmissionelectron microscopy(TEM). Spinelscontain only low to moderate dislocation densities and are characterized by a ubiquitous, fine black spotty texture believed to originate from a slightly non-stoichiometriccomposition. Ti-Al-pyroxenesate relativelyfeatureless,but contain veins of secondary phases apparently deposited in unhealed cracks Chromite has been identified in the veins, suggesting transport of oxidized iron during alteration. Melilites exhibit the greatest variety of microstructures and are the most heavily altered phase in CAI. High dislocation densities are common and crystab exhibit considerable intemal strain, indicating that they have not been annealed. Alteration occurs both as veins along cm& and in fronts extending across several grains. Plagioclase is commonly twinned, but dislocations are rare. The size and morphology of antiphase domains suggest a high temperature of formation with significant low-temperature annealing. Submicron pyroxene precipitates are a ubiquitous and unusual feature of Allende plagiochtse whose properties are most consistent with prolonged slow cooling and equilibration after plagioclase crystallization. The precipitates appear to be sufficiently abundant to contain the majority of Mg present in plagioclase but do not easily account for Na and Ti abundances. Wollastonite needles from a cavity in a “flut?)” Type A inclusion exhibit the growth habits and relatively perfect external surfaces indicative of direct condensation from a vapor. Alteration products are predominantly crystalline and alteration of melilite appears to have proceeded primarily by

solid-state diffision at a temperatun of approximately 920°K. Overall, the TEM observations suggestthat CA1 formed under near equilibrium conditions characterized by slow cooling and that solid-state bulk diffusion was the major pmcess alfecting their post-crystallization history. INTBODUCIlON

et al.. 1975; CHOU eI al., 1976; HASHIMOTOet al., 1979; WOOD, 198 I; WARK and LEVERING, 1982). The CALCIUM-ALUMINUM-RICH inclusions (CAI) in Allende cooling and metamorphic history of high-temperature and other carbonaceous chondrite meteorites are inclusions is equally uncertain. Despite the number widely believed to be representative of the earliest maof preserved isotopic effects indicating the primitive terial formed at high temperatures in the solar nebula nature of these objects (BEGEMANN,1980), the abun(GROSSMAN,1980). Despite the considerable effort exdance of low-temperature secondary phases (CLARKE pended on these inclusions for more than a decade, et al., 1970, FUCHS,1974) and mineralogically distinct the petrographic, chemical, and isotopic evidence has rims (WARK and LOVERING, 1977; MACPHERSONet not provided a clear, unambiguous picture of the origin al.. 1981) indicates that refractory inclusions were not of refractory inclusions and major questions regarding isolated from the nebula at high temperature but contheir origin and metamorphic history remain unretinued to react with the nebular gas at temperatures solved. Many of the minerals found in CA1 have combelow 1OOO’K (HUTCHEON and NEWTON, 1981: positions similar to those predicted by equilibrium WARK, 198 1). The role of planetary metamorphism condensation theory (GROSSMAN,1972) and some CA1 in the formation and modification of CA1 has also are believed to have formed by vapor-to-solid conbeen recently reevaluated and requires further attention densation in the nebula (GROSSMAN, 1975; ALLENet (ARMSTRONG et al., 1982; MEEKERet al.. 1983). al., 1978; GROSSMAN,1980). The textural and chemical The ubiquitous presence of fine-grained alteration data from CA1 have, however, also been interpreted phases in CA1 underscores the importance of postas indicating origin by crystallization from a liquid (BLANDER and F~JCHS, 1975; MACPHERSONand formation processes in the history of refractory incluGROSSMAN, 198 1) or as distillation residues (KURAT sions and the chemical and mineralogical complexity of these objects requires the use of analytical techniques with high spatial resolution to distinguish between primary and secondary phases. The transmission electron * Present addmssz The Lunatic Asylum of the Charles Arms microscope (TEM) coupled with an energy-dispersive Laboratory, Division of Geological and Planetary Sciences, x-ray analyzer (EDX) offers the capability to resolve California Institute of Technology, Pasadena, California 91125. and chemically characterixe fine-scale features (cg., 769

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D. J. Barber. P. M. Martin and I. D. Hutcheon

exsolution, precipitates, anti-phase domains) which cannot be observed by light-optical methods or the scanning electron microscope (SEM) (see CHAMPNESS ( 1977) for a review of TEM applications to geological materials). We initially undertook the study of coarsegrained, calcium-aluminum-rich inclusions from Allende with the anticipation that TEM microstructural and microchemical information would provide unique new information about the conditions of inclusion formation and subsequent thermal and metamorphic history. Some preliminary TEM results on minerals in CA1 from the Leoville (CV3) meteorite have also been reported by MUELLER and WLOTZKA (1982). In this paper, we present our initial TEM observations of Ca-Al-rich Allende inclusions and discuss how the new information gleaned from this study points to formation of Allende Type BI inclusions under conditions close to thermal equilibrium. EXPERIMENTAL TECHNIQUES Our investigation began by examination of whole Ca-Alrich inclusions extracted intact from slab surfaces using diamond dental tools. The inclusions were then potted in a lowviscosity epoxy (Araldite, household 2-pack formulation warmed to -SOY) and, whenever feasible. sliced using a thin-bladed diamond saw to yield a number of sections. Using techniques described in BARBER ( 198 I), these sections were mechanically ground and polished to a thickness of 20-30 pm and then finally thinned to a thickness of - 1 pm with ion beam bombardment. Thermal alteration induced by the argon ion beam was not a severe problem with these inclusions and accelerating voltages of 5-5.5 kV were used. The minerals found in CA1 exhibit a wide range of sputtering rates and we encountered problems thinning the polymineralic inclusions due to differential sputtering. Fine-grained areas containing abundant secondary phases, in particular, eroded very rapidly. These problems and some additional difficulties in mineral identification (due to limited access to TEM/EDX facilities) made progress on the whole inclusion samples slow. Mineral separates from CA1 offered a solution to these sample preparation problems and became the subject of most of the work reported here. Most of the mineral separates were originally prepared by a combination of hand-picking, magnetic, and heavy-liquid separation techniques for an oxygen isotope study (CLAYTONet ol., 1977). Aliquots from the samples studied by Clayton ef al. were characterized by SEM/EDX and cathodoluminescence and specific grains selected for TEM observation. Individual grains were mounted with epoxy in “single-hole” electron microscope grids after the grid aperture had been enlarged so that its diameter was approximately equal to the largest dimension of the grain. The upper and lower surfaces of the grains were subsequently carefully polished (dry) on fine-grained abrasive papers in order to reduce their thickness and to improve the prospects of satisfactory ion thinning. Since epoxy (and the copper grid) is sputtered more quickly than oxide and silicate minerals, the grains needed bequent and car&t1 resupporting during ion-thinning. To minimize this problem, the peripheries of grains were supported either prior to or after some ion beam thinning with epoxy cement heavily loaded with grains of Linde gamma polishing alumina. A fairly thick fillet of this mixture could be applied to the grains without fear of it flowing over their surf&es. Such a mixture is much more resistant to ion erosion than normal epoxy cement and has a thinning rate comparable to that of the minerals studied (BARBER, I97 1). Investigation of thinned specimens was carried out using several different TEM instruments over a period of about four years. Most of the results were obtained with an AEl

EM7 (HVEM) microscope operated at 1 MeV. The mam advantage of operating at such high accelerating voltages is the additional penetration of the electron beam (- I pm). permitting study of relatively large volumes and in overcoming imaging difficulties presented by regions of variable thickness. In the later stages of the work, high resolution studies on a JEOL IOOBand analytical work on JEOL 200 CX, Philips EM 300 and EM 400 microscopes fitted with EDX were carried out in order to elucidate some of the features observed bv HVEM. In all cases. TEM results were asses& in the liaht of prior optical petrographic and SEM examinations. fhe SEM studies were performed with a JEOL JSM-35 fitted with a Kevex EDX system.

SAMPLE CHARACTERIZATION An important aspect of this study was the characterization of all samples by optical microscopy, SEM/EDX. electron microprobe. and cathodoluminescence techniques prior to TEM observations. By following this approach, we hoped to relate TEM observations of microsamples (i.e., scale length ofa few tens of nanometers) directly to relatively macroscopic features studied with other techniques. Mineral separates Born five Allende refractory inclusions representing three of the four basic classes of coarse-grained inclusions were selected for TEM study. The Type Bl inclusions. A13S4 (TS34) and A115 (TS23) (nomenclature from CLAYTON er ol., 1977:thin section numbers (TS) used in GROSSMAN, 1975 and HUTCHEON,1982). are very similar petrographically consisting of a thick outer mantle of melilite with minor spine1 surrounding a core of Ti-Al-pyroxene, anorthite, and more akermaniterich melilite. Spine1 occurs as euhedral crystals poikilitically enclosed by the other major phases. The petrographic evidence suggests that both of these inclusions passed through at least a partially molten stage. Both inclusions contain I60 and 26Mgexcessestypical of Allende Type B 1 inclusions (CLAYTON et al.. 1977; HUTCHEON, 1982). (A photomicrograph of inclusion Al3S4 is shown in CLAYTONet al. (1977)) Two “fluffy” Type A inclusions. TS25 Fl and CGI I. and one “compact” Type A inclusion, Al I-16, were also sampled. MACPHERSONand GROSSMAN(1979) distinguished between “fluffy” and “compact” Type A inclusions on the basis of morphology, degree of alteration, and melilite composition. “Flutfy” A inclusions have very contorted, irregular shapes. are heavily altered. and contain meliite with lower akermanite content. (Inclusion CGI I was described in detail by ALLEN et al. (1978)) Individual spine1 crystals from A13.W(Type Bl) and Al l16 (Type A) previously studied with the SEM (HUTCHEON, 1977) and bv cathodoluminescence [HUTCHEONet al.. 1978) were hand-picked for TEM analysis (Fig. I). Spinels were grouped according to parent inclusion, chemistry, and luminescence as follows: A: Spine] from “compact” Type A inclusions; extremely euhedral with abundant epitaxial perovskite overgrowths; op tically colorless. N: “Normal” spine] from Type Bl inclusions; pure MgA120,; euhedral but with rounded apexes and without perovskite; optically colorless; red luminescence. F: Fe-rich spine1 from Type Bl inclusions containing up to -8% FeO; morphology similar to “N” spinels: blue luminescence; optically pink to black. P: Spine1from Type Bl inclusions with included perovskite; usually located around the periphery of inclusions. In contrast to spine], the other major phases in the Type Bl inclusions are not chemically homogeneous and exhibit chemical zoning often correlated with variations in cathodoluminescence color. Plagiociase grains from inclusion Al3S4 are uniformly AnW to AnW.J, but exhibit large correlated variations in Na and Mg content within individual grains. Mg and Na vary together from - 100 to 1000 ppmw with up to a factor of four variation on a 10 rrn scale (HUTCHEON

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FIG. 1. (a) Type A spinet from the “compact” A inclusion Al I-16. The spine1 apexes are decorated with perovskite rods. (b) Type A spine1 from inclusion Al I- 16 with numerous epitaxial perovskite overgrowths. (c) Type N spine1 from the Type Bl inclusion Al3S4. No perovskite is present. (d) Type F spine1 from inclusion Al 3W, some rounding of crystal apexes is visible.

ef al., 1978). Twinning is common in plagioclase and included spinels are common. Ti-Al-pyroxenes from inclusions A13S4 and Al I5 are nearly identical, exhibit a range in Ti02 content from 4 to 10% typical of pyrcxenes in Allende Type B CA1 (GROSSMAN,1975), and contain abundant included spine]. Melilites from Al3S4 and Al I5 are blocky crystals spanning the compositional range Ak 15-60. Individual melihtes are commonly zoned with Mg content increasing from core to rim. Both pyroxene and melilite from A115 contain deformation lamellae and areas of wavy extinction, probably due to mild shock (MARVIN t-f al., 1970). Wavy extinction is particularly noticeable after ion thinning Melilite from the “fluw Type A inclusion TS25 had a very different optical appearance. In incident light it had a cloudy whitish color and in transmitted light abundant kinkbands were visible. These observations, together with the patchy extinction exhibited by very thin (510 pm) crystals between crossed polars, suggest a very strained and inhomogeneous internal structure. All of the minerals except spine1 exhibit varying degrees of alteration. Alteration in Type B 1 CAI occurs predominantly at melilite-melilite and mehlite-plagioclase grain boundaries, although veins of secondary minerals running through grains (especially mehlite) are also observed. Optically, alteration appears as very tine-g&ted opaque regions in which individual components am unresolved even after ion thinning to a thickness of only I to 2 pm. Altered regions take on a dark brown

color in polarized light in ultra-thin specimens. In Type Bl CA1 alteration occurring at melilite-plagioclase. grain boundaries is coarser grained and predominantly comprised of grossular garnet and monticellite (HUTCHEONand NEWTON, 198 I ). Although the observation of secondary phases was not a primary objective of this study, several heavily altered melilites were selected with the SEM and prepared for the TEM, especially to look at melilite-plagioclase alteration interfaces. ELECTRON

MICROSCOPE

OBSERVATIONS

Spinels Nearly all of the spine1 separates examined contain only moderate to low dislocation densities (510” cm-‘). These dislocations usually occurred singly (i.e., they did not interact with others within the electron-transparent regions) and are gently curving. Sub-grain boundary arrays are absent. The low dislocation densities and general lack of microstructure in the spinels contrast sharply with the TEM observations of melilite and plagioclase described below and point to steady, near-equilibrium growth conditions with freedom horn subsequent shock and deformation (SCH&R et al.,

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D. J. Barber, P. M. Martin and 1. D. Hutcheon

198 I). A few atypicaI areas with above-average dii location densities were found, but these appear to be associated with unidentified second phase particles. Common to all of the spinels is the ubiq~tous fme black spotty texture shown in Fii. 2. Some differences in density were noted between specimens but, since the intensity and appearance of the black spots depend on the partictdar diffraction conditions used for TEM imaging, no correlations have been established between the density of black spots and spine1 group. We have previously noted similar spotty texture in spine1 and chromitc from gas-rich residues from Allende prepared by acid dissolution (SRINIVASAN et al., i978). (TEM sampies of these residues were also prepared by ion thinning) Similar textures have been seen in terrestrial spinel-strnctured minerals (SMITH, 1979,198O; PRICE, 1979, 19@0) and SMITH (1980) has argued that the spotty texture is an artifact of the ion-beam thinning process. PUTNIS and PRICE (1980) have shown, however, that spotty contrast is also found (although less pronounced) in samples prepared by fracturing, and suggest that the spotty contrast is associated with small defects characteristic of spinels and is not wholly an artifact of specimen preparation. Most natural spinels are probably non-stoichiometic, a property which can result in clusters of point defects. MITCHELL et al. ( 1979) have reported that non-stoichiometry in various oxides is accommodated by defect clustering, while L~wrs (1966), in his work on synthetic, stoichiometric Mg-spine1 does not mention black, spotty contrast, nor do his TEM images show it. Any defect clusters in natural spine1 could act as nucleation sites for riuther point defects produced by ion thinning and electron beam induced displacements and thus give rise to the spotty contrast. In an attempt to obtain more detailed information about the defects responsible for the spotty contrast, we carried out hip-~lution lattice imaging on a few spine1 separates which showed the effect strongly. No discontinuities in the f 1111 and [220] lattice planes or marked changes in inter-planar spacings were detected, although some bending of the planes was evident.

FIG,2. Spotty contrast due to small strain centers, probably clusters of point deftcts, in type F spin& imaged at 1 MV.

Marked changes in contrast in the lattice images were also absent. These observations suggest that the defects are small in size compared with the thickness of the regions of the crystals imaged (-25 nm) and, together with the absence of the lobed image contrast normally associated with large diameter (-20 nm) defect clusters, point to the existence of very small defect clusters in spine& probably of the order of 2-3 nm in size. The variations in chemical composition, morphology, and luminescence characteristic of Allende spinets on a macroscopic scale are not reflected in the TEM observations. Dislocation densities and defect structures show no correlation with spine1 group and the microstructure of spine1 from Type A CA1 is indistinguishable from that of spine1 from Type B CAI. The most distinguis~ng feature of spine1 from “compact” A CAI, perovskite overgrowths (Fig. I ), was not observed in this study. Titaniferom pyroxenes In contrast to the other mineral separates, the pyroxene grains are somewhat featureless. Most of the pyroxenes, like the spinels, contain only low to moderate dislocation densities. Numerous polygonalshaped voids occur in some regions, usually in strings and often linked by dislocations apparently associated with interfaces. The dislocations do not appear to have been generated by the voids, as might be anticipated if at some stage the voids had contained gas or fluid. The relationship between the voids and dislocations, as typified in Fig. 3, suggests that the voids and the associated dislocations arose from the healing of cracks initially formed at fairly low temperatures, i.e., temperatures less than -0.557’, (TP - 1500°K is the melting point of Ti-Al pyroxene (BECKETT and GROSSMAN, 1982; STOLPER, 1982)), but later subjected to higher temperatures where signifi~nt bulk diffusion occurred. The polygonal void shapes clearly indicate equilibration by surface diffusion. Dislocations also occur in the proximity of narrow, unzoned veins of fine-grained, fairly compact secondary phases filling cracks in the pyroxene. In contrast to the healed cracks described above, the cracks filled with alteration products arc not healed at any point and probably formed at a different time than the “dean” (free of alteration) healed cracks. The dislocations bordering the veins may have arisen from lo& stresses produced either during the formation of secondary phases or in the process of vein filling. The sharp boundaries between the host pyroxene and the secondary phases suggest the laner mechanism, since in situ alteration of pyroxene by prolonged gaseous diffusion and attack along the cracks would be expected to produce more porous, possibly zoned, veins. An example of such a zoned alteration vein, albeit on a macroscopic scale, is shown in Fig. 1 of MACPHERSON ef al. (198 1). In contrast to the gaseous transport discussed by those authors, filling of fine, open cracks with low melting point, fluid alteration products from

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FIG. 3. Voids and dislocations in pyroxene, apparently delineating the locations of cracks which have healed by diffusion. 100 kV. grain surfaces by capillarlty

seems a more plausible mechanism to explain the TEM observations in pyroxene. The fine-grained nature of the unzoned veins makes total mineral constitution difficult to determine; although individual grains can be resolved by TEM, analyzing each one is prohibitively time-consuming. Sub-micron grossular and chromite have been identified from selected area diffraction patterns and the composition of chromite confirmed by TEM/EDX analysis.

Melilite Melilites exhibit the greatest variety of microstructures among the CA1 minerals and the interpretation of the TEM images is not straightforward. In agreement with optical and SEM results, we found that melilite is the most extensively altered of any of the primary phases. Even in regions which optically show no evidence of alteration, we commonly observed high dislocation densities as shown in Fig. 4. This dislocation microstructure, consisting of arrays of dislocations, sometimes cross-grids or nets, defining small cells of material appears to be characteristic of melilite from Type Bl CAI. Many of the dislocations are slightly dissociated and the splittings are very obvious at some nodes. MULLER and WLOTZKA (1982) have also reported that high dislocation densities (- 10g/cm2) are commonly observed in melilite from Leoville CAL Most Allende melilites are clearly considerably strained, i.e., are not recovered, and have not been annealed since the dislocations were produced. The strain could arise from the effects of alteration at some distance through change of volume requirements, but the apparent lack of correlation between the dislocation microstructure and the degree of alteration makes this possibility unlikely. In principle, particles of second phase (such as spinel) could be responsible for some dislocation generation; in practice, the dislocation walls, although occurring in the proximity of exsolved

particles, do not seem to be related to them. Figure 4b shows spine1 in melilite, close to dislocation arrays. Interfacial dislocations occur in the interface between the spine1 and melilite but no dislocations appear to have been created by differential stress effects. Although much of the melilite from Type Bl CA1 is partially altered, high dislocation densities and character&+ dislocation arrays also occur in optically clear melilites with no visible alteration. Since neither cracks nor healed cracks are common, the dislocation microstructure must be largely attributed to deformation, most probably at elevated temperatures. Alteration appears to predominate where the highly strained regions are found adjacent to the external grain surfaces. In this context, it should be noted that some evidence for shocked melilite in Allende inclusions has been reported by BUNCH (pea-s. commun.) and by MARVIN et al. (1970). Kinking of melilites in Allende has been noted by GROSSMAN(1975) and MACFHERSONand GROSSMAN( 198 1) and the dislocation walls and subboundaries which we observe are consistent with kinking and a small degree of annealing. Alteration in melilite from inclusion A13S4 was observed in two characteristic morphologies: (1) vein alteration extending inwards along preexisting cracks and grain boundaries and (2) surface alteration occurring in fronts extending across grain boundaries. It appears that dislocations act as easy diffusion paths for the alteration process because., in addition to occurring in considerable numbers close to alteration fronts, dislocations are often decorated with second phase particles, as shown by the characteristic “beaded” dislocation strain field images in Fig. 5a. Alteration of melilite was thus at least partly diffusive and occurred at elevated temperatures. The alteration products in veins are densely packed and fine-grained and have many features in common with altered regions in finegrained refractory inclusions. In areas of surface alteration the contact boundaries between melilite and the alteration products are smoothly corrugated and

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FIG. 4. (a) Typical high dislocation density in melilite with dislocations forming irregular walls and loose networks. I MV. (b) Spine1 precipitates (arrowed, A) and dislocation substructure (B) in melilite. Few, if any, of the dislocations appear lo have been generated by the precipitates, although interfacial dislocations

are apparent for precipitate A. I MV. not crystallographically-facetted, suggestive of a highly reactive, possibly liquid, agent of alteration, rather than vapor-phase attack. In contrast to the vein alteration, the secondary minerals found in surface alteration zones are more variable in grain size and highly porous on a scale ranging from microns to nanometers. Both micron and sub-micron voids are common, creating the frothy appearance shown in Fig. 5b. Frequently, the porosity suggests a foam-like structure in which the grain shapes and surfaces of large voids suggest that some of the alteration products were probably liquid at one stage. The abundance of disseminated porosity ahead of the main alteration front in a plagioclase grain suggests that alteration proceeded largely by diffusion in the solid state. Melilite from the *‘flu@” Type A inclusion TS25, was very different in both optical and TEM appearance from the “blocky” melilite separates described above. HVEM images of TS25 melilite revealed a fragmented, although compact, nonporous structure with cracks separating grains which clearly once were integral. Other grains, some rounded and approximately micron-size, were more difficult to interpret, bestowing to the whole a fine-scale, brecciated appearance. Good images of the internal structures of the grains were

difficult to obtain because of intense elastic strains, but investigation of a number of grains showed them to contain very high dislocation densities (> 10” cm-*). High dislocation densities appear to be ubiquitous in TS25 melilite and are consistent with the appearance of selected area diffraction patterns, which typically exhibit numerous reflections extending over -5 to 10” arcs corresponding to highly strained single grains. The overall impression given by the melilite from “fluffy” A inclusions was that of a mildly shocked, brecciated and heavily deformed assemblage, perhaps partly altered but not annealed. Alteration of “fluffy” Type A inclusions on a macroscopic scale is pervasive (GROSSMAN,1980), but SEM/EDX characterization of the individual melilites studied by TEM showed them to be relatively free of alteration. Plagioclase feldspars

Pericline twins are common within Allende plagioclase grains (the coarser ones can be seen optically) but in general, dislocations are rare. A few dislocations occur in association with precipitates, discussed below, and with micron-size voids. The voids commonly occur in strings, sometimes along twin boundaries. and have

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PIG. 5. (a) Contact Lxtween alteration products and melilite surface. Note the “beaded” appearance of subsurface dislocations caused by strain fields around the small precipitates decorating the dislocations. 100 kV. (b) Polycrystalline altered zone in melilite with considerable finescale porosity. Note the fine twin lamellae (arrowed) indicative of strain. 100 kV.

crystallographic shapes indicative of equilibration. These observations, taken in isolation, would be con-

sistent with a well-equilibrated, slow cooling history. In common with all plagioclase feldspars, however, Allende anorthites contain antiphase domain boundaries (APB’s) which carry additional information about the thermal and transformational history of the host phase. Recent TEM work on feldspars has revealed several varieties of APB’s associated with phase transformations and the effects of substitution and ordering of atoms (for reviews see MCLAREN, 1974; HEUER and NORD, 1976). Two types of APB’s can occur in anorthite-rich plagioclases,associated respectively with the b antiphase. domains first observed by CHRISTIEet al. (1971) in lunar basahs and the c antiphase domains seen in both lunar and terrestrial plagioclase shortly thereafter (CZANKet af., 1972; HEUER ef nl.. 1972; MOLLER et 01.. 1972). Three structutai forms of plagioclase occur, namely, a high-temperature form (Ci) where tetrahedrahy coordinated Al and Si have a random distribution, a bodycentered form (Ii) where these atoms are ordered, and a lowtemperature primitive form (Pi). The b antiphase domains am related to the Ci - Ii transformation and b APB’s accordingly indicate out-of-phase effects in the regularalteration of Al and Si atoms. The c domains result from the lowtemperature Ii - Pi transition, which occurs at temperatures below -250°C (BROWNet al.. 1963: ALDHARTet al., 1980). In TEM, APB’s he observed by forming images with appr& priate superstructure reflections: b reflections (h + k = odd, I= odd) reveal b APB’s; c retlections (h + k = even, I = odd) reveal c APB’s. Several studies have indicated that the characteristics of the b domain structure are related to thermal history. HEUER ef al. (1972) noted that there appeared to be a correlation between domain size and cooling rate in lunar hasahs. More

recently, WENK and NAKAJIMA(1980) have shown that b domains in plagioclascs formed at high tempcratums typically have curved APB’s, while APB’s composed entirely of straight sections are characteristicof plagioclaseeither formed at lower temperaturesor having a history of low temperatureannealing. In an experimental study of synthetic plagioclases devitrified from glassesat high temperatures,KROLLand MOLLER (1980) showed that the size of b domains increased with time of isothermal annealing up to a size plateau of I pm. The size of c domains is also influenced by the hi-temperature history even though c domains appear only below 200-300°C. The c domains were largest in the crystalswith the largestb domains and their position, shape, and size appear to be determined by the pattern of local Al, Si disorder created at high temperatures. From this study and other work on An-rich plagioclases, it is clear that the size and morphology of b domains, and to a lesser extent the c domains, too, are indicators of thermal history.

The Allende plagioclase separates contain both b and c APB’s, as illustrated in Figs. 6a and 6b, taken from regions of the same grain a few microns apart. It can be seen that the b domains are considerably larger than the c domains, typically by a factor of between 5 and 10. The b APB’s are not ubiquitous and could not be found in some regions of the grains, behavior which may be linked with the nonuniform distribution of Na discussed in “Sample Characterization”. Parts of the b APB’s are gently curving, but they also exhibit straight and parallel sections on opposite sides of the domain. The c APB’s, in contrast, are irregular and somewhat jagged in appearance and enclose domains with a variety of shapes and sixes. The b domain walls are more regular than those in some lunar basalts and the straightening of portions

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D. J. Barber, P. M. Martin and I. D. Hutcheon

FIG.6. (a) Type b antiphase domain boundaries (APB’s) in Allende plagioclase. The APB morphology suggests a high temperature origin for the plagioclase with equilibration through slow cooling. 200 kV dark field image using 031 reflection. (b) Type c APB’s in a nearby region of the same grain illustrated in Fig. 6a. The siz and shape of the domains outlined by the c APB’s also indicates that the plagioclase is well equilibrated. 200 kV dark field image using 02 I reflection.

of the b APB’s strongly suggests that plagioclase from inclusion A13S4 formed at high temperature but was subjected to significant low temperature annealing. The sharpness of the b reflections is a further indication of the high anorthite content and near equilibrium growth conditions. While the c domains are also comparatively large, the observations of c APB’s cannot yet be unambiguously interpreted. Although ALDHART et al. (1980) have shown that the Ii - Pi transformation responsible for forming c APB’s occurs below -250°C MULLER et al. (1973) have also shown that the size and shape of c domains is not affected by heating up to -500°C. The form of c domains thus appears to be determined by events occurring above 500°C and not by the cooling history at lower temperatures. An unusual yet ubiquitous feature of plagioclase from Type B I CAT is platelet-shaped precipitates, -0.1 pm long, which form preferentially on twins and twin boundaries. EDX microanalysis of the precipitates reveals a composition strikingly different from that of the host plagioclase, containing abundant Mg and Ti and occasionally detectable Fe, in addition to Ca, Al, and Si. The precipitates are thus qualitatively similar in composition to the Ti-Al-pyroxene which occurs as a major phase in Type B CAL Micro-diffraction data collected with an electron beam - 10 nm in diameter

also point to pyroxene rather than any other phase, although positive identification was inhibited by the degradation of the precipitates with loss of Mg during micro-diffraction and EDX analysis. Pyroxene precipitates have also been observed in the well-documented lunar anorthosite 154 15 and the lunar troctolite 76535 (LALLY et al., 1972; NORD, 1976). The precipitates also affect the distribution of the fine-scale spotty contrast observed in Allende plagioclase (Fig. 6). These defects are beam sensitive and their density and nature change during observation. After irradiation for a minute or so at typical beam currents, the centem of contrast become enlarged, have increased in number by a factor of - 5, and the manner in which the contrast reverses across thickness fringes indicates that the defects are now voids within the specimen. Additionally, TEM images taken after some time show (Fig. 7) that there are many surface pits where the spherical voids produced during electron irradiation have grown to intersect the surfaces of the specimen. At first sight these beam-induced effects might appear to be the whole story. However, images taken quickly of virgin material reveal that individual spots frequently show a double-lobed contrast typical of small platelike precipitates. These spots are not uniformly distributed, but zones denuded of the contrast centers occur around the larger pyroxene precipitates

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FIG. 7. Voids and surface pits in plagioclase produced by prolonged (-5 min.) electron irradiation in TEM. Three pyroxene precipitates, also a&ted by the irradiation. are visible close to the central twin boundary. 120 kV.

and along twin boundaries and dislocations, as illustrated in Fig. 8a. With further irradiation, not only does the density and nature of the centers change as the rounded voids develop, but voids grow directly adjacent to and around the precipitates, with some voids appearing to collapse to form dislocation loops adjacent to the precipitates (as shown by the arrows

in Fig 8b). We suggest, therefore, that vacancies cluster and voids nucleate not only on the larger (0.1 pm) pyroxene precipitates, but also to some extent on the population of smaller (30 nm) precipitates indigenous to the plagioclase. Members of the finer population do not occur in the proximity of the larger precipitates, a typical consequence of stages of heterogeneous and

c

FIG. 8. (a) Non-uniform distribution of strain centers in plagioclase. There is a zone denuded of the centers adjacent to pyroxene precipitates along a twin boundary (center) and an isolated pmcipitate (edge, top le!t). 1 MV. (b) Denuded zones around pyroxene precipitates on which defw have clustered and then collapsed to give dislocation loops (the arrowed example is most clearly visible). This segregation, coupled with the surface pitting visible in Fig. 7, suggests that the clustering defects are lattice vacancies. 100 kV.

D. J. Barber. P. M. Martin and I. D. Hutcheon

778

homogeneous nucleation of a second phase occurring concurrently. Ti-pyroxene precipitation (exsolution) therefore appears to be an integral part of plagioclase crystallization in Allende Type Bl CA1 and is much more common than in terrestrial plagioclase. Wollastonite needles

A prominent feature of the “fluffy” Type A inclusion CG 11 is cavities 0.1-0.2 mm in diameter lined with euhedral crystals of grossular garnet (ALLEN et al., 1978). Sprays and mats of acicular wollastonite crystaB (- 1 pm* in cross-section) are perched on top of grossular crystals and project into the cavities. Several wollastonite needles were carefully plucked from a cavity and glued to the rim of the aperture in a TEM grid. Electron diffraction and EDX analysis gave agreement with earlier SEM/EDX identification as wollastonite. The form and internal structure of the needles and their diffraction patterns were studied to investigate the mechanism by which and environment in which the needles grew. Figures 9a and 9b show two fairly typical needles, the larger one apparently having a more complex cross-section than the smaller one

(which is ribbon-like, judging from the equal thickness contours). A high density of stacking faults parallel to the needle’s long axis with corresponding streaking in the diffraction pattern (producing the fringes visible in Fig. 9b) is a common feature of wollastonite from CG 11. The needles grew parallel to a ( 100) zone axis. SO that streaking is parallel to the a* directions and the stacking faults are similar to those described by WENK ef al. (1976) and THOMAS er al. ( 1978-9) in terrestrial wollastonites. Reflections characteristic of both wollastonite and twinned Ii wollastonite are present. At the top right comer of Fig. 9b a set of fine fringes occurs (arrowed) which appears (inset) to be a set of periodically spaced stacking faults parallel to the (100) planes. The external surfaces of the needles are generally very smooth and slightly tapered, although at a few points there is additional material grown on. rather than adhering to, the surfaces (as in Fig. 9a). The relatively perfect external surfaces, the growth habits (principally ribbons) and the paraxial internal faults (perhaps suggesting a varying growth environment) collectively point to a vapor-phase mechanism of growth, rather than a melt origin. Taken in conjunction with the observations of ALLEN et al. C1978)

FIG. 9. (a) Ribbon-like wollastonite needles extracted from a cavity in the “fluf@” Type A inclusion CG 1I. 1 MV. (b) Wollastonite needle with complex cross section from the same location as Fig. 9a. Inset shows fringes due to sets of stacking faults. I MV.

CA1 from Allende that invade needles are bent through 90” and pro ject into void space, the TEM observations provide conclusive evidence that wollastonite condensed directly from a trapped vapor phase within the vesicles. DISCUSSlON Even though we have employed HVEM techniques which permit study of thicker samples than does conventional TEM, only small amounts of material have been sampled and our observations ideally require further elaboration. Nonetheless we believe that this initial TEM study has provided new insights into the formation and metamorphic history of invade minerals and their parent inclusions. The lack of dislocation microstructures indicates that the minerals reached a high degree of ~~libmtion after a stage which we assume to be that of the initial inclusion formation. This view is supported by the void shapes and presence of healed cracks in several minerals and by the morphologies of the b APB’s in plagioclase grains. With the exception of melihte, the minerals contain low densities of dislocations, grain boundaries am absent, and there is no evidence of significant deformation subsequent to inclusion formation. The presence of high dislocation densities throughout some melilites and in particular regions of selected melilite grains, however, remains slightly enigmatic. These unrecovered dislocation configurations may be evidence for post-formation deformation, possibly produced by slow compaction or even mild shock, while the grains were still hot and somewhat plastic (there is no associated cracking). The possible role of shock in the alteration of CA1 has been examined by MULLERet al. ( 1982), who speculate that melilite may have been deformed by shock waves of rather low peak pressure while hot (7’ > 500°C). While this interpretation generally agrees with optical observations that Allende melilites show wavy extinction, the growing TEM evidence that many terrestrial melilites exhibit similar dislocation structures (G. L. NORD;W. F. MOLLERprivate ~mmun.) suggests that further study of melilite is needed. No relic pyroxene crystals were observed within any of the melilites, suggesting that if melilite did form in a rn~rno~~c reaction replacing pyroxene (MEEKERet al., 1983), the reaction consumed all of the precursor pyroxene. Cooling rate The prop&es of the second phase particles observed in Allende plagioclase are most consistent with prolonged slow cooling and equilibration after plagioclase crystallization. The preferential nucleation of pfecip itates on dislocations, grain boundaries, and twin boundaries can occur only after crystallization, indicating that the pyroxenes are the result of solid-state precipitation (~lution) and are not melt droplets trapped during crystaBization. There are distinct parallels between our observations and those of LALLY et al. (1972) and NORD (1976) in two lunar sampta

779

e~biting~rn~~e exsohSiOU,15415 (~0~~~) and 76535 (troctolite). Exsolution features in both Of these rocks have been interpreted in terms of continUOUS~~OW coolin& t?om cry&l&ion to below 8 10°C in the case of 76535. On a more macroscopic scale, pyroxene inclusions appear to be a a common feature of lunar anorthosites (JAMES,1972; STEELE ad SMITH, 1973)and SMITH and !&EELEf1974) have suggested that the pyroxenes formed by solid state precipitation, possibly via the reaction Ca(Mg, Fe)SisOs - Ca(Mg, Fe)Si206 + SiOr. The silica mineral present in the lunar anorthosites has not been observed in Allende plagioclase and the applicability of this reaction to Altende CAI is uncertain. A high-temperature origin of Allende plagioclase with subsequent slow cooling is also suggested by the size and morphology of the antiphase domains. The sire and shape of domains in Allende plagioclase, when compared with those in slowly+zooled lunar and terrestrial rocks, are qualitatively consistent with the slow cooling rate (less than several degrees per hour) inferred by PAQUEand STOLPER ( 1983) on the basis of dynamic crystallization experiments. Na-Mg zoning in plagioclase One of the most striking, yet poorly understood characteristics of Allende phrgioclase is the heterogeneous distribution of Mg and the accompanying covariation of Na (HUTCHEONef al., 1978, EL GORESY et al., 1978). The unusual patchy distribution of Mg together with the fact that Mg is not a stoichiometric constituent of plagioclase, has led to the suggestion that internal Al-Mg isochrons for Allende plagioclase may, in fact, be mixing lines (WASSERBURG and PAPANASTASSIOU, 1982). Our observation of pyroxene precipitates in plagioclase provides a new basis for evaluating the possibility that Mg is concentrated in sub-micron Mg-rich crystals, in this case pyroxene. At present, we can make only a rough estimate of the amount of Mg contained in the pyroxene precipitates since three critical factors are poorly known: (1) the thickness of the electron transparent regions of the sample, (2) the mean density of the precipitates, and (3) the average Mg content of the precipitates. Assuming an average thickness of 0.2 pm for areas of plagioclase such as shown in Fig 8a and assuming the precipitates to be uniformly distributed we estimate that the pyroxenes occupy approximately two volume percent of the plagioclase. Assuming further that the small (30 nm) precipitates are pyroxenes of the same composition as the large precipitates (Fig. 8b) and that the average MgO content is 8%, we estimate that - 1000 ppmw of Mg is concentrated in the pyroxene inclusions. Since Mg contents of plagioclase in Type B 1 CA1 range from - 1000 to < 100 ppmw, our observations suggest that pyroxene is sufficiently abundant to account for the bulk of the nonradiogenic Mg. This model must also be examined in the hght of two additional characteristics of Allende plagioclase:

780

D. J. Barber. P. M. Martin and I. D. Hutcheon

(1) the linear covariation of Na and Mg and (2) the absence of correlated variations of Mg and Ti. Pyroxenes in Type Bl CA1 typically have between 5 and 9% TiOr (GROSSMAN, 1975) and, by following the argument used previously for Mg, we estimate that pyroxene precipitates are sufficiently abundant to account for the Ti content of Allende plagioclase (-200 ppmw) (HUTCHEON el al., 1978). The Mg and Ti abundances in plagioclase from Type Bl CA1 are, however, uncorrelated and concentrating both Mg and Ti in pyroxene precipitates would require that Mg and Ti in the precipitates not exhibit the inverse correlation characteristic of Mg and Ti in Ti-pyroxene in A 115 (I. M. STEELE,unpub. data). Perhaps the most serious shortcoming of this model is that it provides no explanation for the observed linear correlation between Na and Mg contents. To explain the observations, this model requires the ad hoc assumption that (1) either pyroxene precipitation or the Mg content of precipitates is controlled by the local Na abundance in plagioclase or (2) that Na addition (presumably as NaAlSi30s) and pyroxene precipitation are regulated by the same mechanism. Neither of these assumptions is easily verified. HUTCHEON et al. (1978) suggested that Na and Mg may enter the plagioclase together in a unit of Na(Mg, Si)%sOll. This model does not account for the pyroxene precipitates but could be tested by searching for excess Si in plagioclase, the amount of which should be correlated with the Mg content. The TEM observations of precipitates in plagioclase also bear on the interpretation of Mg isotopic data from CAL If the precipitates are predominantly fassaite formed by exsolution from the host plagioclase, the nonradiogenic Mg component, now concentrated in pyroxene, initially must have been present in plagioclase. Normal Mg may have been locally rearranged during exsolution but there is no TEM evidence suggesting large-scale migration of Mg during metamorphism. The Mg isotopic data should, therefore, reflect the crystallization of plagioclase and not the timing of subsequent alteration (HUTCHEON, 1982; WASSERBURG and PAPANASTASSIOU,1982). Alteration As previously stated, melilite is the most heavily altered phase among the mineral separates and most of our conclusions on alteration are based on observations of melilite. AIteration of melilite appears to have been caused (at least on a sub-micron scale) by the action of a very penetrating and mobile, most likely gaseous, medium. The production of WCOndaV liquid fluxes is not, however, excluded by the observations. Secondary phases are predominantly (finely) crystalline and glass may be present only as a (very) minor constituent. The ambient temperature during

1See,for example, discussion on page 167 in WYATTand DEW-HUGHES(1974).

alteration was sufficiently high for solid state bulk diffusion to predominate, but low enough for pipe diffusion along dislocations still to be an important process. Using the concept of homologous temperature,’ this observation suggests a temperature not much in excess of OSST,, where TM - 1675°K is the melting point of melilite (Ak 20) in Type Bl CA1 (BECKETT and GROSSMAN,1982; STOLPER, 1982). This temperature estimate for the alteration process, T,, - 920°K is in reasonable agreement with the temperature inferred by HUTCHEON and NEWTON (1981) T, - 955”K, for the equilibrium grain boundary reaction of melilite and plagioclase to form grossular and monticellite. Grossular was observed in several heavily altered melilites from Type B 1 CA1 but we were unable to confirm the SEM/EDX identification of monticellite by electron diffraction (due to the inability to prepare thinned samples of suitable altered areas). The bulk of the TEM evidence suggests that alteration of melilite occurred primarily by solid-state diffusion, as discussed by Hutcheon and Newton for the melilite-plagioclase grain boundary reaction in Type Bl CAI. The absence of amorphous alteration products argues against the formation of grossular by devitrification of a glass precursor (FUCHS, 1974). An important feature of the Hutcheon and Newton model is that the formation of grossular and monticellite at melilite-plagioclase boundaries in Type Bl CA1 was a closed-system reaction, which did not involve transport of material into or out ofthe inclusion. The identification of chromite by TEM/EDX and electron difhaction in veins of alteration within TiAl-pyroxene suggests, however, that addition of oxidized iron, at least, occurred during alteration of pyroxene. The importance of diffusion-controlled, opensystem reactions in the alteration of CA1 has been emphasized by MACPHERSONet al. ( 198 1) and WARK ( 198 1). In a detailed study of the mineralogy of CA1 rims and alteration veins, MACPHERSONet al. ( 198 1) concluded that alteration is a nonequilibrium process which proceeds by diffusion of sodium, silicon, and oxidized iron from the nebular gas into inclusions. WARK ( 198 1) also suggested that calcium diffused outward and silicon, iron, and alkalies inward during alteration. The TEM observations suggest that both pipe and bulk diffusion occur during alteration and it ag pears likely that the degree of exchange between CA1 and the nebula was not constant but varied with the amount and type of alteration. In CA1 where alteration is not extensive, such as the Type Bl CA1 studied in this work, the grain boundary reaction discussed by Hutcheon and Newton and the metasomatic conversion of melilite discussed by Wark produce very similar effects and the present TEM observations do not unambiguously distinguish between the two mechanisms. Whether alteration was primarily an open- or closedsystem process, though, the TBM evidence that diffusion both within grains and across grain boundaries was the dominant process affecting CA1 after crystallization, lends support to the hypothesis that the het-

781

CA1 from Allende erogeneous oxygen isotopic composition among co existing phases witbin CAI re5ects di5usioncontrolled exchange of oxygen between minerals in CA1 and the nebular gas (CLAYTONet al., 1977).

in CA1 (MEEKER et al., 1983; MACPHERSON et al., 1983), additional studies to investigate the effect of alteration on the mineralogic and isotopic composition of CA1 are needed. Acknowk&ne~~-We

SUMMARY These initial TEM studies have clearly not resolved the question of the mode(s) of origin of Allende CAI, but they do provide some useful constraints and point the way for future studies to address more specific questions concerning CA1 formation and metamorphism. The majority of the TEM observations of minerals from Type B 1 CA1 are most consistent with crystallixation from a melt under near equilibrium conditions (slow cooling of less than a few degrees per hour). With the exception of plagioclase, the primary phases in CA1 do not exhibit any unusual microstructural features diagnostic of a particular mode of origin. Only in unusual cases such as the wollastonite needles in CG-11, do the TEM data umambiiuously suggest vapor-to-solid condensation. Although “5e’ A inclusions have many of the macrosco pit characteristics of nebular condensates, the TEM observations of melilite from Type A CA1 are dominated by high densities of dislocations produced by post-formation deformation, The absence of cracks accompanying high dislocation densities in melilites from all of the CA1 studied indicates, however, that any shock event pro duced only mild overpressures or occurred while the inclusions were still somewhat hot and easily defdrmed. Models of CAI origin which invoke planetary proces.~ (e. g., ARMSTRONGet al., 1982; MEEKER et al., 1983) must not violate this contraint. An additional clue to the origin of CA1 may be contained in the submicron precipitates present in Allende plagioclase. Additional high resolution TEM studies are needed to establish the composition of the small (30 nm) precipitates, but if they are also Ti-Al-pyroxene, the majority of nonradiogenic Mg (and possibly Ti) present in plagioclase may be concentrated in the exsolved pyroxene. Pyroxene precipitates are not common in terrestrial plagioclases but have been observed in several lunar samples, and additional TEM studies are needed to investigate the applicability of the specific mechanisms proposed to explain exsolution in lunar plagioclase for Allen& CAI. Observations of experimentally synthesixed CA1 (e.g., STOLPER, 1982) may be very useful in thii regard. The TEM may be most valuable in deciphering the effects and mechanism qf metamorphism (alter&on) in CAL Many secondary mineral phases are too finegrained and intergrown to permit quantitative analysis even with the SEM. The study of alteration was not a major objective of this paper, but the ubiquitous Presence of secondary phases in CA1 demands attention. Our obsetvations suggest that alteration was primarilya difhionGontrolkd process, but it seems likely that more than one type of alteration occurred. In view of the controversy over the role of metamorphism

hsve benefitted greatly from discus-

sions with many collea8ues including R. N. Clayton, 1. M. Steele, L. Grossman, G. J. MacPlmtson, M. A. Carpenter, W. F. Milller, J. T. Armstrong and T. Holland. We thank the sta8 at Imperial Col&, London, for access to the 1 MV

electron microscope, L. Grossman and R. N. Clayton for providing samples, and J. Earon, L. Finger and S. Mier for manuscript preparation. We are indebted to D. A. Work for a very thoughtful and constructive review. This work was supportedby funds from the Natmal Envimnmental Research Council grant GR3/1668 (D. J. Barber) and NASA grant NGL 14-001-169 (R. N. Clayton); manusctipt pmparation

supported in part by NASA Grant NAG 943 (G. J. Wasserburg). REFERENCES

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