In Situ Growth of Ca-Rich Rims around Allende Dark Inclusions*

Transcription

1 In Situ Growth of Ca-Rich Rims around Allende Dark Inclusions* A. N. Krotl, M. I. Petaev2, A. Meibom3, and K. Kei11y4 Hawaii I~~.~titiite of Geoplz)1sics & Planetology, School of Ocean & Earth Science & Technology, University of Hawaii at Manoa, Honolulu, HI 96822, USA Harvard-Smitlz.sonian Center for Astrophysics, Cambridge MA 02138, USA ' Geological & Environmerrtal Sciences, Stanford University, Building 320, Loirzita Mall, CA 94305, USA ~awaii Centerfor Volcanology Received May 10,2000 Abstract-Four heavily altered dark inclusions (4301-2, IV-1, 3b-1, 1%) in the Allende meteorite are composed exclusively of secondary minerals, including fayalitic olivine (Fa40-45), salitic pyroxenes ( FS,~~~WO~~), nepheline, sodalite, and Fe-Ni sulfides. Chondrules in the dark inclusions are replaced by fayalitic olivine and nepheline. The chondrule pseudomorphs in and IVa are surrounded by salitic pyroxene rims which are cornmonly interconnected. Matrices of these dark inclusions are crosscut by multiple veins of salitic pyroxenes and Fe-Ni sulfides. The inclusions are surrounded by continuous Ca-Fe-rich rims with variable widths and show roughly symmetric mineralogic zoning. The innermost zone consists of diopside-salitic pyroxene (FS,,~~WO~~-~~). The central zone is composed of hedenbergite ( F s ~ ~ ~ ~ wollastonite ~ W ~ ~ (CaSi03), ~ - ~ ~ andra- ), dite (Ca?Fe2Si,012),. and kirschsteinite (CaFeSi04). The outermost zone consists of salite-hedenbergitic pyroxenes ( F.Y~~-~~WO~~-~~). The rims are commonly intergrown with divine in the Allende matrix and enclose fragments of forsteritic olivine chondrules and lumps of the Allende-like matrix material. Abundant rounded objects composed of salite-hedenbergitic pyroxenes, andradite, and wollastonite are observed in the Allende matrix near the Ca-rich rims. There are progressive Ca depletions from to IV-1 to 3b-1 to IVa. The degree of depletion correlates to the rim thicknesses, which increase in the same order. We conclude that the Allende dark inclusions experienced at least two stages of alteration in the presence of an aqueous solution after lithification and aggregation. One episode of alteration resulted in the replacement of primary chondrule minerals (forsterite, enstatite, anorthitic mesostasis, kamacite f troilite nodules) by secondary fayalitic olivine, nepheline, sodalite, and Ni-rich sulfides. Calcium removed from the chondrules was redeposited as salite-hedenbergitic pyroxene veins, as rounded objects in the matrix, and as rims around chondrules. Following this alteration, the dark inclusions were excavated from their original location (most likely the CV3 asteroidal body) and incorporated into the Allende host. The last episode of alteration took place in situ and resulted the in dissolution of Ca-Fe-rich pyroxenes in the dark inclusions and redeposition of Ca-rich minerals as rims around dark inclusions at the interfaces with the Allende host. The proposed mechanism of rim growth involvcs diffusional exchange of Ca and Fe between the dark inclusions and Allende, driven by thc compositional difference between their matrix olivines. The precipitation of rim minerals started at -250 C at an iron-rich geochemical barrier (the Allende host), and continued at lower temperatures until the aqueous solution dried out. INTRODUCTION Dark inclusions in Allende are lithic chondritic clasts which are genetically related to the oxidized CV3 chondrites of the Allende-like subgroup; both have similar chemical and oxygen isotopic compositions and secondary mineralizations, including fayalitic olivine, Ca-Fe-rich silicates, nepheline, sodalite, and Fe-Ni sulfides [3, 6-16, 20, 25, 261. Although there is a general consensus that the Allende dark inclusions experienced alteration prior to incorporation into the host meteorite, there is no agreement on the place and conditions of this alteration. Two types of models were proposed: (I) nebular (e.g., [S, 16, 20, 251) and (2) asteroidal (e.g., [6,9-151). According to the nebular models, the Allende dark inclusions are aggregates of *This article was submitted by the authors in English. nebular condensates that experienced multiple, hightemperature gas-solid, metasomatic reactions of different degrees with a very oxidized, nebular gas. According to the asteroidal model, the Allende dark inclusions are fragments of CV3 chondritic materials that experienced various degrees of alteration in the presence of aqueous solutions at relatively low temperatures (~300 C) on the CV asteroidal body. It was previously found that heavily altered dark inclusions in Allende (those containing virtually no primary minerals and abundant chondrule and CAI pseudomorphs) are surrounded by Ca-rich rims (e.g., [6-8, 1 1, , which are mineralogically similar to the outermost Ca-rich rims around heavily altered CAIs in Allende (e.g., [I, 17-19]. Largely based on these similarities, Johnson et al. [S] concluded that Ca-rich rims around Allende dark inclusions formed by direct

2 c.3-q.)..i- KROT et a!. Fig. 1. X-ray elemental maps in Ca (a), Na (b), Fe (c) and S K, (d) of the Allende dark inclusion sample Dark inclusion contains abundant chondrule pseudomorphs (indicated by stars), which are enriched in Na, depleted in Ca, and commonly surrounded by interconnected Ca-rich rims. Chonrule pseudomorphs are less Fe-rich than the surrounding matrix; the latter is less Fe-rich than the Allende matrix.the dark inclusion is crosscut by multiple Ca-rich pyroxene* andradite veins (indicated by double arrows) and surrounded by a Ca-rich, Na-free rim. Veins crosscutting the chondrule pseudomorphs are rare (one is labeled as "C"). The outer portion of the inclusion is depleted in Ca. Coarse Ca-rich objects (labeled as "hed-andr-wol") composed of hedenbergitic pyroxene. andradite and wollastonite are observed in the Allende host near the Ca-rich rim. Outlined regions labeled A and B are shown in detail in Figs. 6 and I I. respectively. condensation of the Ca-rich minerals from the hightemperature oxidized nebular gas followed by sequential iiccretion onto distinct surface layers of the dark inclusions. Kurat et al. [16] inferred that the Ca-rich rims around the Allende dark inclusion All-AF formed when a larger piece of rock was broken and vapors from inside the fragment reacted with the ambient nebular gas. In contrast, Krot et nl. [I 53 suggested that Ca-rich rims around Allende dark inclusions resulted from the metasomatic alteration within the CV3 asteroidal body. In this paper, we present new observations on the Ca-rich rims around four heavily altered Allende dark inclusions. We argue that these inclusions experienced at least two stages of alteration after aggregation and lithification in an asteroidal environment. The last episode of alteration took place in situ after incorporation into the Allende host, which resulted in the growth of Ca-rich rims around the inclusions. SAMPLES AND ANALYTICAL PROCEDURES Five polished thin sections of four Allende dark inclusions, 3b-1, IV-1, -2, IVa, and AMNH , were studied using optical microscopy, back-scattered electron (BSE) imaging, X-ray elemental mapping, and electron microprobe analysis. The mineralogy, petrography, bulk chemistry, and oxygen isotopic compositions of the dark inclusions ,3b-1, and IV- I have been previously described [6,8,13,14]. BSE images were obtained using a Zeiss DSM 962 scanning electron microscope. Electron probe microanalyses were performed with a Cameca-microbeam SX-50 electron microprobe using a 15 kev accelerating voltage, na beam current and beam size of -1-2 pm. For each element, peak and background counting times were 30 s. Standards included chromite for Cr, rutile for Ti, Cr-augite for Ca and Si, San Carlos olivine and

3 t,r airu GROWTH OF Ca-RICH RIMS AROUND ALLENDE DARK INCLUSIONS S353 hypersthene for Mg and Si, fayalite for Fe, Verma garnet for Mn, and jadeite for AI. Matrix effects were corrected using PAP procedures. X-ray elemental maps of the dark inclusions and neighboring Allende were RESULTS Mineralogy and Petrography of the Allende Dark Inclusions acquired using SX-50 at 15 k e accelerating ~ voltage, The Allende dark inclusions studied are mineralog na beam current and pm beam size. ically similar to the heavily altered Allende dark inclusion AII-AF [9, 10, 16, 201 These inclusions consist largely of an Allende-like matrix and chondrule-like obiects:.,, both are com~osed of secondarv favalitic oliv-.i, ine, nepheline, sodaiite, Fe,Ni-sulfides, Ca-Fe-rich pyroxenes, and andradite (Figs. 14). It has been shown that the chondrule-like objects in the Allende dark inclusions are indeed chondrule pseudomorphs and not aggregates of nebular condensates (see discussion in [9-141). Thus, we refer to these objects as chondrules or chondrule pseudomorphs throughout the text. In the dark inclusions, the chondrule phenocrysts and mesostases are replaced by anhedral and lath-shaped fayalitic olivine, nepheline, and minor sodalite (Figs. 5, 6). Relict grains of forstentic olivine and low-ca pyroxene are absent in chondrules and matrices of the dark inclu- sions; relict grains of diopsidic pyroxenes were found ~ in IVa (Fig. 5a in [13]). Opaque nodules in chondrules I are rare and consist largely of fine-grained Fe-Ni sulfides intergrown with fayalitic olivine and salitic pyroxene (Figs. 5a, 5b in [13]); magnetite is absent. The matrices are composed of lath-shaped fayalitic olivine, fine-grained Fe-Ni sulfides, nepheline, sodalite, and rounded or irregularly shaped inclusions mostly composed of salitic pyroxenes; magnetite grains are absent (Figs. 5-6). All four dark inclusions studied are surrounded by continuous Ca-rich rims (Figs. la, 2a, 3a, 4a). Dark inclusions and IVa are crosscut by multiple salitic pyroxene veins (Figs. la, 4a, 6); the former also contains abundant sulfide-rich veins (Fig. 4c). Before we characterize the mineralogy of the Carich objects (matrix pyroxenes, veins, and rims) observed inside and around the Allende dark inclusions, we describe the distribution of Ca, Na, Fe, and S in them and in the neighboring Allende matrix. Distribution of Ca The dark inclusions are surrounded by continuous Ca-rich rims with highly variable widths (Figs. la, 2a, 3a, 4a). The inclusions and IVa are crosscut by Fig. 2. X-ray elemental maps in Ca (a), Na (b), and Fe K, (c) of the Allende dark inclusion sample IV-I. The dark inclusion consists of the Allende-like matrix and rare chondrule pseudormorphs (indicated by stars); the former is less Ferich than the Allende matrix. The chondrule pseudomorphs are less Fe-rich than the surrounding matrix. The dark inclusion is surrounded by a Ca-rich and Na-free rim. Coarse Ca-rich objects (indicated by arrows) composed of hedenbergtic pyroxene, andradite, and wollastonite are observed in the Allende host near the Ca-rich rim. The outer zone of the dark inclusion is depleted in Ca relative to its core; the depletion in Ca is not correlated with Na distribution.

4 S354 KROT et al. Fig. 3. X-ray elemental maps in (a) Ca, (b) Na, and (c) Fe K, of the Allende dark inclusion sample 3b-1. The dark inclusion consists of secondary fayalitic olivine, Ca-Fe-rich pyroxenes, andradite, and abundant nepheline. Chondrule pseudomorphs are common (indicated by stars) and primary magnesian silicates are absent. Fayalitic olivines in the inclusion matrix are less Fe-rich than those of the Allende matrix. The chondrule pseudomorphs are depleted in Na relative to the surrounding matrix (see b). Ca-Fe-rich pyroxene + andradite objects are largely concentrated in a small central region of the inclusion (outlined by dashed line); outside this region, the Ca-rich objects are very rare. The observed distribution of Ca is not correlated with Na distribution. The dark inclusion is surrounded by a Ca-rich, Na-free rim. The rim contains abundant isolated forsteritic olivine grains, chondrule fragments (labeled as "chd"), and regions of fine-grained material (labeled A) similar to the Allende matrix (see Fig. 10 for details). multiple Ca-rich veins; some veins are connected to the rims; no veins were found to extend into the Allende host (Figs. la, 44. Chondrules in all four dark inclusions are depleted in Ca. Rare Ca-rich minerals inside the chondrules are secondary salitic pyroxenes closely associated with Fe-Ni sulfides (Figs. 2c, 5a in [13]). Chondrules in are surrounded by Ca-rich rims (Figs. la, 5). The rims around neighboring chondrules are commonly connected and look similar to the Carich veins. Calcium-rich veins crosscutting the chondrules are rare (Fig. la). Although no Ca-rich rims or veins are observed around chondrules in 3b-1, the finegrained rims around these chondrules are slightly enriched in Ca (Fig. 3a). Matrices in , IV-1 and 3b-1 contain irregularly shaped, porous Ca-rich inclusions largely composed of salitic pyroxenes (Fig. 9r in [13]), but such objects are nearly absent in the matrix of IVa (Fig. 44. There is a progressive depletion in Ca from to IV-I to 3b-1 to IVa. The calcium depletion is zoned in such a way that the Ca concentration is lowest near the rim and increases towards the core of the inclusions. It appears that the Ca depletion is positively correlated with the rim thickness which also increases in the following order: , IV-1, 3b1, and IVa. The Ca-rich rims around dark inclusions are accompanied by Ca-rich inclusions in the Allende matrix just outside the rims (Figs. la, 2a, 3a, 44. Distribution of Nu Chondrule pseudomorphs in contain abundant Na-bearing phases, nepheline, and minor sodalite, and the matrix in is depleted in Na (Figs. lb, 5a-5d). Chondrule pseudomorphs in 3b-1 are depleted in Na, whereas its matrix is uniformly enriched in Na (Fig. 2b). Sodium-bearing phases in IV-1 are relatively minor and no apparent correlation between these phases and the rare chondrule pseudomorphs is observed in this inclusion (Fig. 2b; see also Fig. 6c in [13]). The Ca-rich rims around all four dark inclusions lack Na-bearing phases (Figs. I b, 2b, 3b, 4b). Similar depletion in Na is observed along sulfide-rich veins in IVa (Fig. 4b). Although the most Ca-depleted inclusions, 3b-1 and IVa (Figs. 3a, 4a), have the highest con-

5 Irl sit11 GROWTH OF Ca-RICH RIMS AROUND ALLENDE DARK INCLUSIONS S355 tents of Na-bearing phases (Figs. 3b, 4b), there is no apparent correlation bctwecn the distribution of Ca-rich and Na-rich phases in the dark inclusions. Distribirtion of Fe Minor differences in Fe contents are observed (1) between chondrule pseudomorphs and matrices of the dark inclusions, (2) between the central and outer portions of the inclusions, and (3) between the matrices of the inclusions and host Allende (Figs. lc, 2c, 3c, 7). Chondrule pseudomorphs are slightly less Fe-rich (more magnesian) than the surrounding matrix. The central portions of the dark inclusions are on average less Fe-rich (Fa4, +,, 12 = 23) than their outer zones (Fa,, I, n = 19) and the Allende matrix near Ca-rich rims (Fa47,,, n = 13); all values are for (Fig. 7). Distribution of S The X-ray elemental maps in S K, were acquired for only two inclusions, and IVa (Figs. Id, 4c). Chondrules in contain very rare sulfide nodules, whereas the matrix of the inclusion contains abundant fine-grained sulfides. The inclusion IVa is crosscut by multiple sulfide veins (Fig. 4c), but sulfide veins in are very rare (Fig. 60. The relatively few distinct chondrules in IVa are depleted in S and surrounded by sulfide rims which are commonly connected to sulfide veins (Figs. 4c, 6a, 6c). It appears that the sulfide veinslrims generally follow the paths of Carich pyroxene veinstrims in this inclusion, and occasionally form combined pyroxene-sulfide veins (Figs. 4% 4c, 6a-6d). No crosscutting relationships between sulfide and pyroxene veins are observed. In contrast to the Carich pyroxene veins in IVa, which are generally connected to a Ca-rich rim around the inclusion, the sulfide veins always terminate at the Ca-rich rim. The Ca-rich rims around and IVa are depleted in sulfides, whereas the Allende matrix adjacent to the rims shows no depletion in S (Figs. Id, 4c). Mineralogy of Ca-Rich Pyroxene Objects in Matrices and Chondrctle Pseudon~.orplzs of the Dark Inclusions Matrices and some chondrule pseudomorphs in inclusions and IV-1 contain various amounts of rounded and irregularly shaped Ca-rich inclusions largely composed of salitic pyroxenes (Figs. 2, Sa, 9r in 1131). Although these pyroxenes are texturally similar to pyroxenes in the matrices of the oxidized CV3 chondrites (Fig. 2 in [14]), the former have a narrower com- Fig. 4. X-ray elemental maps in (a) Ca, (b) Na, and (c) S K, of the Allende dark inclusion sample IVa. The dark inclusion consists of secondary fayalitic olivine, Ca-Fe-rich pyroxenes, andradite, abundant nepheline, and Fe-Ni-sulfides; chondrule pseudomorphs are rare (indicated by stars).the dark inclusion is crosscut by multiple veins of Fe-Ni sulfide and Ca-rich pyroxenes. The chondrule pseudomorphs are surrounded by sulfide rims, which are connected with the sulfide veins. The dark inclusion is surrounded by a Ca-rich and Na-free rim. There are abundant Ca-rich objects (indicated by arrows) in the Allende matrix near the Ca-rich rim.

6 KROT et al. Fig. 5. Back-scattered electron images of chondrule pseudomorphs in the Allende dark inclusion sample (see outlined regions labeled A in Figs. la-lc). (a-c) The chondmle pseudomorph is composed of lath-shaped fayalitic olivine (fa) and nepheline (nph). Outlined regions in (a) are shown in detail in (b) and (c). The chondrule is surrounded by a continuous rim composed of Carich pyroxene (dark gray) and sulfide (white); the sulfide is concentrated in the central zone of the rim. (d-f) Two chondrule pseudoniorphs (labeled chd 1 and chd2) are composed of anhedral fayalitic olivine and rare nepheline grains. The chondrules are surrounded by an interconnected Ca-rich pyroxene rim (indicated by black arrows). positional range and a lower Fs content than the latter characterized in detail in our earlier paper [I31 and is (Fig. 8; Fig. 2 in [14]). shown in Fig. 8a. Mineralogy of Ca-Rich Veins and Rims aro~~nd Clzondr~~les Calcium-rich rims around chondrule pseudomorphs and Ca-rich veins crosscutting the matrices of the IVa and inclusions are mineralogically very similar; both largely consist of salitic pyroxenes and minor sulfides (Figs. 5,6). The veins and rims around chondrules are commonly interconnected (Fig. 5). The mineral composition of pyroxene veins and chondrule rims was Mineralogy of Ca-Rich Rims around Dark Inclusions In contrast to the salitic pyroxene veins and rims around chondrules, the Ca-rich rims around dark inclusions are mineralogically zoned and more complex (Figs. 8, 9). The innermost thin layer of the rims consists of diopside-salitic pyroxene (Fs,,,,Wo,,-,,). It is followed by a thick central zone composed of the nearly end-member hedenbergite (FS~~-~~WO,,-~~), wollastonite (CaSiO,), and andradite (Ca,Fe,SiROI,). Wol-

7 Irl situ GROWTH OF Ca-RICH RIMS AROUND ALLENDE DARK INCLUSIONS Fig. 6. Rack-scattered electron images of the Ca-rich pyroxene and sulfide veins and rims around chondrule pseudomorphs in the Allende dark inclusion samples IVa (a-d) and (e, f). (a) The chondrule pseudomorph (chd) consists of fayalitic olivine and nepheline and is surrounded by a Fe-Ni sulfide (sf) rim. (b-d) Veins crosscutting the matrix of the inclusion consist of salitic pyroxene (sal) and Fe-Ni sulfide. The matrix consists of lath-shaped fayalitic olivine and nepheline (nph) grains; the latter enclose fayalitic olivine grains. (e) Salitic pyroxene vein (indicated by arrows) crosscutting the matrix of the dark inclusion. The matrix consists of lath-shaped fayalitic olivine and Fe-Ni-sulfides (white). (f) A sulfide vein (indicated by arrows) near the chondrule pseudomorph. lastonite and hedenbergite seem to replace andradite (Figs. 9b, 9d). Rare grains of kirschsteinite (CaFeSi04) were found in the central zones of the IV-2 and rims. Kirschsteinite in IV-I appears as an overgrowth on andradite (Fig. 9c). The outermost rim layer consist of salite-hedenbergitic pyroxenes (FS,,-~~W~,,~~~). This layer is commonly intergrown with fayalitic olivine of the Allende matrix (Fig. 13f in [13]). It also encloses isolated forsteritic olivine grains, chondrule fragments, and regions of the Allende-like matrix material consisting of lath-shaped fayalitic olivine, nepheline, and sulfides (Fig. 10). The chondrule fragment shown in Fig. I Of is surrounded by a fine-grained matrix rim that is texturally and compositionally similar to finegrained chondrule rims in Allende. Mineralogy of Ca-Rich Objects in the Allende Matrix around Ca-Rich Rims The calcium-rich rims around dark inclusions are commonly accompanied by rounded and irregularly shaped Ca-rich objects in the neighboring Allende matrix (Figs. la, 2a, 3a, 4a, 11). Some of these objects are intergrown with the Ca-rich rims of the inclusions. The mineralogy of Ca-rich objects is similar to that of the rims. Their cores consist of andradite, wollastonite, and hedenbergite and are surrounded by a salitic pyrox-

8 S358 KROT et al. '* X Allende matrix Dark inclusion matrix near Ca-rich rim Dark inclusion matrix in core Dark inclusion chondrule pseudomorphs Fayalite (mol %) Fig. 7. Compositions of fayalitic olivine in the matrix Allende (a) and (b-d) the dark inclusion samule (b) ~atrix b~ivin; in a peripheral portion of the'dark inclusion near the Ca-rich rim. (c) Matrix olivine in a central portion of the dark inclusion. (d) Olivine in chondrule pseudormorphs. ene layer (Fig. 1 I). This layer is often intergrown with the Allende matrix olivine and forsteritic olivine grains (Figs. 1 lc, I Id). DISCUSSION Iiz Siru Growth of Ca-Rich Rims In our previous papers [13, 141, we showed that salite-hedenbergitic pyroxenes in matrices of the oxidized CV chondrites and Allende dark inclusions are secondary minerals and resulted from the alteration of the primary Ca-rich phases in chondrules (anorthitic mesostases, pyroxenes) and CAIs (melilite, anorthite). We suggested that the secondary Ca-rich phases formed at relatively low temperatures (<300 C) in the presence of aqueous solutions. The presence of the complete chondrule pseudomorphs, which are depleted in Ca and surrounded by interconnecting pyroxene rims, and the pyroxene and sulfide veins crosscutting chondrule pseudomorphs and matrices of the inclusions suggest that the Allende dark inclusions experienced alteration after agglomeration and lithification. The significant differences in the degrees of alteration and in the compositions of the matrix olivines between the dark inclusions and the host Allende indicate that the former had been altered prior to incorporation into Allende. This conclusion is consistent with the observations of Kurat et al. [16], Palme et al. [20], Johnson et al. [8], Kojima and Tomeoka [9, 101, and Krot et al. [ Due to the presence of continuous Ca-rich rims surrounding the dark inclusions, we conclude that the rim formation postdates the alteration and excavation of the inclusions from their original location, most likely the CV asteroidal body. The observed progressive depletion of Ca in the outer portions of the inclusions and the corresponding increase in the thicknesses of the Ca-rich rims (Figs. 1 a, 2a, 3a, 4a) indicates that Ca removed from the inclusions was redeposited in the rims. The outer pyroxene layers of the rims are commonly intergrown with the lath-shaped fayalitic olivines of the Allende matrix, indicating in situ formation of the rims. This conclusion is consistent with the commonly present isolated forsterite grains rimmed by fayalitic olivine and trapped portions of the Allende-like matrix material in the Ca-rich rims (Fig. 10). The observed coarse salitehedenbergite-andradite-wollastonite objects intergrown with the Allende matrix material just outside the rims of and IVa (Figs, 1 a, 4a, 1 1 ) suggest that Ca mobilized from the interiors of the inclusions was deposited at the interface between the inclusions and the Allende host and in Allende itse8 We note that these petrographic observations are inconsistent with the high-temperature nebular models of the Ca-rich rim formation either by direct condensation of the rim minerals from an oxidized nebular gas [8] or by metasomatic reactions between nebular gases and the inclusions [16]. The nebular models are also inconsistent with the lack of S and Na depletions in the outer portions of the dark inclusions; these elements are much more volatile than Ca even under highly oxidizing conditions [23, 271. These elements would have been completely removed if the observed depletion of Ca in the dark inclusions had occurred at high temperatures. Based on the petrographic observations summarized above and the lack of S and Na depletions in the rimmed dark inclusions, we infer that the in situ growth of Ca-rich rims must have occurred at relatively low temperatures. This conclusion is consistent with the presence of talc and biopyriboles in Allende chondrules, which would not survive high temperatures [4]. Because the growth of Ca-rich rims around dark inclusions requires significant transport (remobilization) of Ca, Fe, Si, and Mg, which is impossible at low temperatures in a gas phase, we infer that aqueous solutions must have played an important role in this process [13, 14, 221. The laboratory experiments by Brearley and Duke [5] show the rapid mobilization of Ca during low temperature (-200 C) aqueous alteration of

9 In sirrt GROWTH OF Ca-RICH RIMS AROUND ALLENDE DARK INCLUSIONS Allende, which supports our interpretation. In order to understand the mechanism of Ca redistribution and growth of Ca-rich rims around dark inclusions, we used thermodynamic analysis of the mineral assemblages observed in the rims. The rinodynanzic Analysis qf Rim-Fonning Reactions Simple rim mineralogy (Figs. 8, 9) indicates that only Ca, Fe, Mg, and Si were largely involved in the rim-forming process. Taking into account the necessity of having an aqueous solution as a medium for redistributing Ca, Fe, Mg, and Si, the thermodynamic system of interest can be restricted to five chemical elements: Ca, Fe, Si, 0, and H. Magnesium, which is a constituent in all mineral solid solutions, can be excluded from the thermodynamic analysis by the addition of the activity terms for the fayalite and hedenbergite dissolved in the olivine and pyroxene solid solutions, respectively. The ideal models of Fe-Mg exchange in olivine and pyroxene solid solutions have been used. In thermodynamic analysis of the rim-forming reactions, we use an approach developed in our previous papers [I 3, 141. We assume a complete equilibrium between the aqueous solution and the gaseous phase at any given temperature and pressure. This allows us to use such intensive parameters of the Ca-Fe-Si-0-H system as the H,O/H, ratio in the gaseous phase (this ratio describes redox conditions in the system instead of.p2 typically used for such purposes) and the Fe2+/Ca2+ ratio in the aqueous solution as independent variables. The mineralogical reactions observed in the Ca-rich rims, i.e., the replacement of andradite by hedenbergite and wollastonite (Fig. 9d) and the rare kirschisteinite and wollastonite overgrowths on andradite (Fig. 9c), can be described by chemical reactions (1) and (2), respectively: Ca3Fe2Si30,2(,) + 2Si02(,,) + HZ(,) andr = 2CaFeSi2O,(,, + CaSiO,(,, + H20(g) (1) hed wol Ca3Fe2Si30,2(s) + H2(g) andr = 2CaFeSiO,(,, + CaSiO,(,, + H20(,,, (2) kirsch wol where the subscripts (s), (aq), and (g) denote a solid phase, an aqueous species, and a gas, respectively. Additional constraints could be placed on the stability fields of andradite and hedenbergite by the reactions Fig. 8. Compositions of Ca-rich silicates in the (a) matrix of the dark inclusion sample 3b-1 and (b) rims around dark inclusions sample 3b- I and IV-l(c). describing the substitution of these minerals by fayalite or magnetite in the Fe2+-rich portion of the system [(reactions (3)-(5)] or by wollastonite under Ca2+-rich conditions [reactions (6) and (7)]:

10 KROT et al. Fig. 9. Back-scattered electron images of Ca-rich rims around dark inclusions 3b-1 (a, b) and IV-1 (c, d). Both rims show similar mineralogical zoning. Their central portions consist of andradite (andr), wollastonite (wol), and hedenbergitic pyroxenes (hed). A subheclral grain of kirschsteinite (kr) overgrows andradite in the rim around IV-I (e). The innermost and outermost portions of the rims are composed of diopsidic (di) and salitic pyroxenes (sal), respectively. 3Ca.?Fe,Si.?O,,(,) + 9~e:~:,) + 2H,O(,, andr (3) Finally, the lack of magnetite, which in principle could, but in practice does not replace fayalitic olivine in the dark inclusions, Ca-rich rims around them or in the = 5Fe,0,1,, + 9Ca:.f,, + 9Si0, + 2H2,,, Allende matrix adjacent to the rims, occurs by the reaction: mgt 3Fe2Si0,,,, + 2H,O(,, 2Ca3Fe2Si3012(s) + 6Fe::q) + 2H2(g) fa andr = 5Fe,Si04(,, + SiO,[,, + 6ca$, + 2H20,,, mgt fa CaFeSi20,(,, + ~ e&, hed = Fe2Si04(,, + SiO,i,,, + ~a:~:,, fa Ca.?Fe2Si30,2(s) + 2Si02(q) + 2ca::q) + andr = 5CaSi03,,, + 2Fe:;,, + H20(,, wol (4) = 2Fe304(s) + 3SiO,(,,) + 2Hqg1 (5) (6) CaFeSi,O,,,, + ca:';, = 2CaSi0,,s, + Fe,. (7) hed wol We note that there is no evidence for the newly formed magnetite and fayalitic olivine in the Ca-rich rims. 2+ (8) and constrains an upper limit on the H20/H2 ratios characteristic of the rim-forming reactions. The values of the equilibrium constants of reactions (1)-(8) were calculated from the standard thermodynamic properties of reactants and products at a total pressure of 100 bar and temperatures of 50, 100, 150, 200,250, and 300 C; data are [22]. The corresponding equations linking the equilibrium constants of reactions (1)-(10) with the intensive parameters of the alteration process are as follows: log K, = 2logX,,, + log [H,O/H,] - 210g [SiO,] (9) log& = log [H,O/H,] (10) log K, = 9log[SiO,] - 910g [ ~e~+/ca~+ ]- 210g [H20/H2] (1 1)

11 In sitli GROWTH OF Ca-RICH RIMS AROUND ALLENDE DARK INCLUSIONS Fig. 10. Back-scattered electron images of Ca-rich rims around dark inclusions 3b-1 (a-d) and 12b-I (e, 0. The rims consist of porous regions of salite-diopsidic pyroxenes (sal-di) and more massive regions of hedenbergite + wollastonite (hed + wol). Rims commonly contain fragments of forsteritic olivine (01) grains, regions of the Allende-like matrix material (mx), and rare chondrule fragments (chd). The chondrule fragment shown in d is surrounded by a fine-grained matrix material. log K, = SlogX,,, + 210g [H20/H2] + log [SiO,] - 610g [~e'+/ca~+] IogK, = IogX,, - log[fe /Ca ] + log [SiO,] - logx,,, log K, = log[h20/h2] + 210g [Fe2'/~a2'] - 210g [SiO,] (1 2) (13) (14) log K, = log [Fe2+/ca2'] - log X,,, (1 5) log K, = 3log [SiO,] - 3logX, - 210g[H20/H2],(16) where Ki is the equilibrium constant of reaction (i); [H20/H2] = XH,O/XH, ; XHZO and XH2 are the mixing ratios of H20 and H2 in the gaseous phase equilibrated with an aqueous solution; [SiO,], [Ca2+], and [Fe2+] are the activities of Si02, Ca2+, and Fe2+ in an aqueous solution; and X,,, and X,, are the Fe/(Fe + Mg) ratios in pyroxene and olivine, respectively. Please note that values X,,, used hereafter are 50 times lower than the Fs content in the reported analyses of salitic pyroxenes, i.e., X, = 2Fs/100, because in salitic pyroxenes, Mg/(Mg + Fe) = 1/2Mg/(Mg + Fe + Ca). At any given temperature, there are five independent parameters in Eqs. (9)-(16), i.e., XHed in pyroxene, X, in olivine, H20/H, ratio in the gaseous phase, and

12 KROT et al. Fig. 11. X-ray elemental map in Ca K, (a) and back-scattered electron images (b-d) of Ca-rich objects in the Allende matrix (mx) near Ca-rich rim around dark inclusion (see outlined region labeled B in Fig. 1). Regions outlined in (a) and (b) are shown in detail in (c)and (d), respectively. Calcium-rich objects consist of andradite (andr), wollastonite (wol), hedenbergitic (hed) and salite-diopsidic (di) pyroxenes. The salite-diopsidic pyroxenes are intergrown with isolated forsteritic olivine grains (fo) and the matrix olivine. Forsteritic olivine shown in (d) is partly surrounded by fayalitic olivine rim (fa). Fe2+/Ca2+ ratio and Si02 activity in aqueous solution, which define the stability fields of andradite, kirschsteinite, wollastonite, magnetite, fayalitic olivine, and hedenbergitic pyroxene in the Fe-Ca-Si-0-H system. Two of these parameters, the H20/H, ratio in gaseous phase and the Fe2+/Ca2+ ratio in the aqueous solution, are used as variables in the phase diagrams of the Ca-Fe-Si-0-H system (Fig. 12). The XHed in pyroxene and X,, in olivine can be calculated from the chemical analyses of the rim minerals (Fig. 8). The activity of Si02 in aqueous solution deserves special consideration because of its significance on the stability fields of andradite and magnetite, as well as on the hedenbergite-fayalite reaction boundary in the Ca-Fe-Si-0-H system, which is discussed in Appendix I. The phase relations described by reactions (1)-(8) at temperatures of 200 and 250 C are shown in Fig. 12 as a function of the Fe2+/Ca2+ ratio of an aqueous solution and the H,O/H, ratio of a gaseous phase. The activities of SiOz shown in Fig. 12 are those of an aqueous solution equilibrated with the Allende matrix at corresponding temperatures and a total pressure of 100 bar. In the phase diagrams, we accepted X, values of 0.5 and 0.4 as representative of the matrix olivines in the Allende host and in the central zones of the dark inclusions, respectively (Fig. 7). Since andradite in the central zones of the Ca-rich rims coexists with essentially pure hedenbergite, XHed = 1.0 was accepted. There are five stability fields in the diagrams of the Ca-Fe-Si-0-H system (Fig. 12). Fayalitic olivine is stable in equilibrium with the relatively reducing gaseous phase (low log [H,0/H2] values) and the Fe2+-rich aqueous solution (high log[fe2+/ca2+] values). Wollastonite is stable in the presence of a Ca2+-rich aqueous solution regardless of the redox conditions. Magnetite requires an oxidizing gaseous phase (high log [H20/H2] values) and an Fe2+-rich aqueous solution. The stability fields of andradite (oxidizing conditions) and hedenbergite (reducing conditions) lie between those of wollastonite from the Ca-rich end of the diagrams, and magnetite and fayalitic olivine from the Fe-rich end. Wollastonite shown in the lower middle fields of both diagrams, in addition to hedenbergite, is an excessive phase included to account for the decomposition of andradite by reaction (I). Kirschsteinite is unstable relative to hedenbergite at all temperatures due to the high concentration of Si02 in the aqueous solution.

13 In siru GROWTH OF Ca-RICH RIMS AROUND ALLENDE DARK INCLUSIONS, C; log[si02] = I Mt --- Ecpilibrium H20/H2 ratio H20/H2 ratio Dark inclusions Uendeh LVul Wol -Qg S? i 01 : - X log [ ~e*+/~a~+] in aqueous solution Fig. 12. Phase relations among Ca, Fe-rich minerals in the Fe-Ca-Si-O-H system at (a) 200 C and (b) 250 C. And, andradite; Hed, hedenbergite, Mr. magnetite; Wol, wollastonite; 01 olivine. Stability fields of olivines in the dark inclusions (Fa4,) and the Allende host (FajO) are shown by solid ancl dotted lines, respectively. Hedenbergite in the lower middle fields of both diagrams is the only mineral equilibrated with fayalitic olivine; wollastonite shown in addition to hedenbergite is an excessive phase included to account for the observed breakdown of andradite to wollastonite and hedenbergite by reaction (1). Dashed lines show H20/H2 ratios of the gaseous phases equilibrated with the aqueous solutions. At a temperature of -250 C, olivines from both the dark inclusions and Allende are in equilibrium with anclradite, whereas at -200 C the equilibrium mineral assemblage consists of fayalitic olivine and hedenbergite. I I I I 1 The temperature increase from 200 C to 250 C enlarges the stability field of wollastonite at the expense of andradite and hedenbergite, but it has little effect on the other phase boundaries (Figs. 12a, 12b). The only important difference between the diagrams is the value of the equilibrium H,O/H, ratio in the gaseous phase, which has a significant effect on reaction (1). At 250 C, the gaseous phase is oxidizing enough to stabilize andradite. At 200 C, if equilibrium in the Ca-Fe- Si-O-H system is maintained, andradite would break down to hedenbergite and wollastonite. pyroxene of -0.75, which can be in equilibrium with andradite, wollastonite, and the Allende matrix olivines. The temperature at which andradite, hedenbergite, and wollastonite are stable together can be found by plotting H20/H2 ratios of both reaction (1) and the gaseous phase equilibrated with aqueous solution in the log [H20/H2] vs. temperature plot (Fig. 13). This plot Temperature, OC 'Is0 an H'~OIH, of Fig. 13. Temperature stabiliiy of Ca-Fe-rich minerals in the olivine to magnetite). End-member hedenbergite coex- Fe-Ca-Si-O-H system. And andradite; Faso, Allende matrix ists with andradite, wollastonite, and fayalitic olivine olivine; Hed hedenbergite; MI, magnetite: Wol. wollastonite. (Fa,,) at 242 C. A decrease of X,,, in pyroxene moves Thin lines show the Fhase boun&ries of the reaction (1) (And = the phase of reaction ) toward higher Hed + Wol) at different hedenbergite contents in salitic pyroxenes indicated by numbers. The dotted line shows variperatures until the maximum stability temperature ations of the equilibrium H20/H2 ratios in the gaseous phase. (277 C) of fayalitic olivine (Fa,,) is reached. Since the Intersection of the dotted line with the andradite-heden-bergdark inclusions, the Ca-rich rims and the Allende ite-wollastonite phase boundary at 242 C defines the lower matrix adjacent to the rims lack magnetite, this temper- tempenture limit of andradite stability. At temperatures above 2770C, Allende matrix olivine becoma unstable md is places an upper temperature limit On the rim- replaced by magnetite if equilibrium among the gaseous phase, forming event. It also defines the minimum XHed in aqueous solution, and solid phases is maintained.

14 S364 KROT et al. Because the sizes of the mineral stability fields shown in Fig. 12 strongly depend on the log[si02], whereas the equilibrium H20/H2 ratios in the gaseous phase depend only upon temperature and total pressure in the system (for details, see [13]), the estimated temperatures of the andradite-(hedenbergite + wollastonite) and magnetite-olivine phase boundaries (Fig. 13) are very sensitive to the accepted log[si02] values. An increase of log [SiO,] will move the andraditeihedenbergite + wollastonite) and magnetite-olivine phase boundaries toward higher H20M2 ratios in Fig. 13 [see Eqs. (9) and (16)l. This will result in increase of the estimated temperatures of andradite-(hedenbergite + wollastonite) and olivine-magnetite equilibria. However, the temperature can not exceed 310 C, which is the upper temperature limit for the existence of an aqueous solution at 100 bar (for details, see [13]). Mrchanisrn and Physicochemical Parameters of the Rim Growth Our mineralogical observations indicate that the Ca-rich rims coexist with the matrix olivines in the host Allende and dark inclusions with different compositions (FaS, and Fa,,, respectively; Fig. 7). The stability fields of olivine with fayalite contents of 40 and 50 are plotted in Fig. 12. Although the sizes of these stability fields change slightly with temperature, the offset between Fa,,,-hedenbergite and Fas,-hedenbergite and Fa,,-andradite and Fa5,-andradite phase boundaries remains the same (Fig. 12). This means that if equilibrium among minerals and aqueous solutions is locally maintained, the aqueous solution that would equilibrate with an assemblage of fayalitic olivine and hedenbergitic pyroxene in the dark inclusions would have an Fe2+/Ca2+ ratio lower by a factor of about 1.2 than that in the Allende host. It can be shown that the aqueous solution that equilibrated with the dark inclusions would be richer in Ca2+ and poorer in Fe2+ compared to the solution equilibrated with the Allende host. These compositional gradients cause diffusional fluxes of Ca2+ ions from the dark inclusions toward Allende and Fe2+ ions in the opposite direction, which drives the reactive system toward equilibrium. We note that the compositional gradients and diffusional exchange of Ca and Fe between the solutions exists only if the solutions are stagnant in the pores of the Allende and dark inclusions; the solutions can mix together in the cavities along the Allende - dark inclusion interface. Mixing of solutions with slightly different compositions at the Allende-dark inclusion interface would increase the concentrations of both Ca2+ and Fe" ions above the solubility limit of a Ca-Fe-rich phase, which would precipitate as a rim in the cavities along the interface. We suggest that the higher concentration of Fe2+ ions in aqueous solution that has equilibrated with the Allende host has acted as a geochemical barrier for Ca2+ ions dissolved from the dark inclusions. Precipitation of Carich minerals would deplete the solution at the Allendedark inclusion interface in both Ca2+ and Fe2+ ions; however, these ions can subsequently be supplied to the solution again from the inclusion and Allende, respectively. Rim formation would continue until the matrix oliv- ine~ in the dark inclusion and Allende equilibrate, or the Ca-rich objects in the dark inclusions are exhausted, or when the aqueous solution dries out. Our petrographic observations indicate that neither complete exhaustion of the dark inclusions in Ca, nor complete equilibrium between the dark inclusions and Allende have been achieved. We suggest that growth of the Carich rims was terminated by the disappearance of the aqueous solution. Formation of Central Andradite-Wollastonite- Hedenbergite and Outer Hedenbergitic Pyroxene Zones in Ca-Rich Rims Based on our mineralogical observations, we infer that the central portions of the Ca-rich rims, composed of andradite, wollastonite, and hedenbergite, predate crystallization of the outer salite-hedenbergitic pyroxene layers. Andradite, which is one of the major minerals in the central portions of the rims, is replaced by wollastonite and hedenbergite (Figs. 9b, 9d), and we thus infer that andradite was the first to crystallize. The sequence of mineral deposition in the Ca-rich rims will largely depend on the starting temperature of the rim-forming process. According to the phase diagrams of the Fe-Ca-Si-0-H system, andradite growth begins at high temperatures in the presence of an oxidizing gaseous phase equilibrated with an aqueous solution (Fig. 124, and continues at gradually decreasing temperature until the andradite-(hedenbergite + wollastonite) reaction boundary is reached, at 242OC (Figs. 12b, 13). Below this temperature, andradite becomes unstable and should be replaced by hedenbergite and wollastonite [(reaction I)]. The ongoing equilibration of Ca and Fe between the dark inclusion and Allende will continue to supply Ca and Fe for the growth of hedenbergite in the outer rim. Pure hedenbergite will continue to be deposited in the outer rim until the Fe2+/Ca2+ ratios of an aqueous solution on both sides of a Ca-rich rim reach equilibrium. However, the absolute concentrations of Ca2+ and Fe2+ ions on the opposite sides of the rim would not be equal until the chemical compositions of both olivine and residual hedenbergitic pyroxene of the dark inclusions become the same as those of Allende. According to Eq. (14), the equilibrium Fe2+/Ca2+ ratio in the aqueous solution at a given temperature and activity of SiO, will be achieved when X,,(Allende) - XF,(dark inclusion) = XHed(Allende) - XHed(dark inclusion). After X,,, in pyroxenes in the dark inclusion

15 In situ GROWTH OF Ca-RICH RIMS AROUND ALLENDE DARK INCLUSIONS S365 drops below 0.9, the composition of salitic pyroxene in the outer rim becomes progressively depleted in the hedenhcrgite end-member, providing that selective dissolution of salitic pyroxene in the dark inclusions can occur. It is possible that, at later stages of rim growth, dissolution of the most ferrous salitic pyroxenes of the Allende host may also occur. The occurrence of andradite in the rims raises the question of why hedenbergite in the Allende matrix has not been replaced by andradite during an early stage of rim formation. One possible explanation is that andradite formed in Allende, but was subsequently dissolved when the temperature dropped below 242 C. However, the mineralogical shidies by Peck [21] and Krot et al. [I41 show that pure hedenbergite is virtually absent in the Allende matrix and the highest X,,, in salitic pyroxene is -0.9 (Fig. 2 in [14]). According to the phase diagram (Fig. 13), the Allende pyroxenes with such a composition would coexist with andradite and wollastonite at higher temperatures (-255 C) than pure hedenbergite, andradite, and wollastonite in the rims around dark inclusions (242 C). This means that andradite would not form in Allende if rim formation started below 255 C. Thus, the presence of andradite in the Ca-rich rims and lack of it in the Allende host narrows the estimated temperature range at which the rims begin to form from 242OC to 255OC. Forination of the Inner Diopside-Salitic Zone The presence of the inner diopsidic pyroxene layer in the Ca-rich rims is puzzling, because it appears to contradict the proposed mechanism of rim formation, i.e., precipitation of Ca-Fe-rich minerals at the Fe-rich geochemical barrier. The compositional discontinuity between diopside-salitic pyroxenes of the inner zones and end-member hedenbergite of the central zones (Fig. 9) may indicate that the inner zones formed later, at limited exchange of Ca and Fe with the Allende host via the aqueous solution, implying that the growth of the inner rim occurred in a closed system. Our mineralogical observations indicate that there is -5 mol % difference in fayalite content between the central (Fa,,) and peripheral zones (Fa,,) of the dark inclusions (Fig. 7). If an aqueous solution was present in the dark inclusion during formation of the diopsidesalitic inner zone, then no inner zone can form, because the coexistence of more ferrous olivine (Fa,,) in the peripheral zones of DIs with hedenbergite in the central rim zones should result in dissolution of hedenbergite and transport of both Fe2+ and Ca2+ toward the dark inclusion cores, which consist of more magnesian olivine (X,, - 0.4) and salitic pyroxene (XHed = ). Therefore, we infer that the inner salite-diopsidic layer of the Ca-rich rims formed in the absence of an aqueous solution. We suggest that this layer formed by solid-state recrystallization of the residual diopside-salitic pyroxenes left after dissolution of the more hedenbergitic pyroxenes during rim formation. This process is energetically favorable, because it significantly reduces the surface energy of the pyroxene grains. This interpretation is consistent with the lack of Ca in the peripheral zones of the dark inclusions and the observed compositional differences between diopside-salitic pyroxenes of the inner and salite-hedenbergitic pyroxenes of the outer zones (AX,,, = 10 mol %). However, it still remains to be shown by TEM that the peripheral zones of the rimmed dark inclusions with a Mg-rich inner rim lack even tiny grains of Ca-rich pyroxenes, which are most vulnerable to solid-state recrystallization. Formation of Kirschsteinite The origin of rare kirschsteinite grains in the central zone of the rims is unclear. Breakdown of andradite to kirschsteinite and wollastonite by reaction (2) is an isochemical process which requires the change of redox conditions from oxidizing to reducing. At log[si02] values accepted in the paper, this would occur at temperatures below -100 C. However, such temperatures seem to be too low for a solid-state process to proceed effectively. Reducing conditions in the gaseous phase could be achieved at higher temperatures when an aqueous solution becomes unstable (-310 C at 100 bar). If kirschsteinite in the central zones of the rims formed at high temperatures by breakdown of andradite, one would expect to see a replacement of andradite by intergrowths of kirschsteinite and wollastonite. Because the observed kirschsteinite overgrows andradite (Fig. 9c), we infer that it did not form by the breakdown of andradite. According to our calculations, kirschsteinite in equilibrium with wollastonite, andradite and fayalitic olivine can grow from a SO2-poor aqueous solution (log[si02] < at 250 C) instead of hedenbergite. However, it remains unclear how an aqueous solution is depleted with respect to Si02 on a local scale during rim formation. CONCLUSIONS I. Four dark inclusions , IV- 1,3b- 1, and IVa, in Allende are composed entirely of secondary minerals, including fayalitic olivine, Ca-Fe-rich pyroxenes, andradite, nepheline, and Fe-Ni sulfides. The inclusions are surrounded by continuous Ca-rich rims composed of an inner zone of diopside-salitic pyroxene; a central zone of andradite, wollastonite, hedenbergite, and kirschsteinite; and an outer zone of salite-hedenbergitic pyroxenes. 2. The dark inclusions experienced at least two stages of alteration, and both stages postdate the aggregation and lithification of the inclusions. One ep~sode of alteration resulted in replacement of the primary chondrule and matrix minerals by secondary fayalitic olivine, nepheline, sodalite, and Fe-Ni sulfides. Cal- GEOCHEMISTRY INTERNATIONAL Vol. 38 Suppl. 3