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Oxygen Isotopic Compositions of Secondary Ca-Fe-Rich Silicates from the Allende Dark Inclusions: Evidence Against High-Temperature Formation

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Oxygen Isotopic Compositions of Secondary Ca-Fe-Rich Silicates from the Allende Dark Inclusions: Evidence Against High-Temperature Formation Lunar and Planetary Science XXXI 1463.pdf OXYGEN ISOTOPIC COMPOSITIONS OF SECONDARY CA-FE-RICH SILICATES FROM THE ALLENDE DARK INCLUSIONS: EVIDENCE AGAINST HIGH-TEMPERATURE FORMATION. Alexander N. Krot1, Hajime Hiyagon2, Mikhail I. Petaev3, and Anders Meibom1. 1Hawai'i Institute of Geophysics & Planetology, School of Ocean & Earth Science & Technology, University of Hawai'i at Manoa, HI 96822, USA; 2University of Tokyo, Dept. of Geophysics and Planetary Physics, Yayoi, Tokyo, Japan; 3Harward-Smithsonian Center for Astrophysics, Cambridge, MA 02138, USA Abstract: We report in situ measurements of O-isotope Allende and DI, and continued at lower temperatures until compositions of secondary Ca-rich minerals (salite- an aqueous solution dried out. Here we report in situ hedenbergitic pyroxene, wollastonite, andradite), which measurements of O-isotope compositions of Ca-Fe-rich occur as concentrically-zoned objects and veins in matrix silicates in Allende DIs. and rims around heavily-altered Allende dark inclusions Mineralogy and Petrography: The Allende DIs #3529 (DIs). O-isotope compositions of these minerals show a and #IV-1 consist of chondrules replaced to various large range in d18O (from -11‰ to +20‰) and d17O degrees by fayalitic olivine and Allende-like matrix (for (from -8‰ to +6‰) and plot near terrestrial fractionation details see [7,10]). The DI matrix consists of lath-shaped 17 (TF) line (-3‰ < D O < -1‰; 1s = ±2‰). The observed fayalitic olivine (Fa40-45), nepheline, sodalite, and Ni-rich range in O-isotope compositions is consistent with a two- sulfides, and concentrically-zoned objects composed of stage low-temperature formation of secondary Ca-rich andradite core and salite-hedenbergitic pyroxene (Fs10- minerals in the Allende DIs in the presence of limited 30Wo50) mantle. Both inclusions are surrounded by amount of aqueous solution [1], but is inconsistent with continuous Ca-Fe-rich rims (Fig. 4 in [7]; Fig. 1) which their high-temperature condensation origin proposed by have variable widths and show roughly symmetric [2]. Very similar D17O values have been previously mineralogical zoning (Fig. 2). The innermost zone of a reported for secondary fayalite and magnetite from the rim consists of diopside-salitic pyroxene (~Fs10-15Wo48- oxidized CV chondrites Kaba and Mokoia [3] and 50). The central zone is composed of hedenbergite (Fs48- magnetite from Allende [4]. We infer that O-isotope 50Wo48-50), wollastonite, and andradite. The outermost composition of the oxidizing reservoir responsible for the zone consists of salite-hedenbergitic pyroxenes (Fs15- formation of secondary mineralization in the Allende DIs 35Wo48-50), which are commonly intergrown with the and oxidized CV chondrites was near the TF line. Allende matrix olivines. Previous studies: Dark inclusions (DIs) in the Allende Results and Discussion: O-isotope compositions of the CV3 carbonaceous chondrite are lithic chondritic clasts Ca-Fe-rich silicates analyzed are listed in Table 1 and which are genetically related to the CV3 materials: both plotted in Fig. 3. In #IV-1 only rim minerals, including have similar chemical and oxygen isotopic compositions andradite, wollastonite and salite-hedenbergite pyroxenes and show similar alteration features [1,2,5-8]. The most were analyzed; the pyroxene-andradite objects in the #IV- heavily-altered Allende DIs containing partially-to- 1 matrix are too fine-grained and porous to be analyzed completely pseudomorphed chondrules are surrounded by by ion probe. O-isotope data show relatively small range Ca-Fe-silicate rims composed of salite-hedenbergite in d18O (Table 1) and plot near terrestrial fractionation pyroxenes, wollastonite, andradite, and kirschsteinite [7]. (TF) line (-3‰ < D17O < -1‰; 1s = ±2‰), to the left These rims are texturally and mineralogically similar to from both carbonaceous chondrite anhydrous mineral the outermost Ca-Fe-rich rims around heavily altered (CCAM) line [11] and "primary" 16O-line [12]. In #3529 CAIs in Allende [9]. Largely based on these similarities, the measurements were done on 3 pyroxene-andradite it was concluded that Ca-Fe-rich rims around DIs formed inclusions in matrix. O-isotope data also plot near TF line by direct condensation of the Ca-Fe-rich minerals from a (-4‰ < D17O < -1‰), but largely to the right from highly-oxidized nebular gas produced by evaporation of "pristine" 16O-line (Fig. 3). Andradite cores of all three nebular regions with enhanced dust/gas ratio [2]. Kurat et inclusions have compositions, which are systematically al. [6] inferred that the rimmed Allende DI (All-AF) was heavier than those of salite-hedenbergite rims around broken off a larger piece of rock and that Ca-Fe-rich rim them (Table 1); this could be partly due to contamination resulted from the reaction between the vapors from inside by matrix fayalitic olivines which are closely intergrown the inclusion with the hot ambient nebular gas. Contrary with rim pyroxenes. to the nebular models, other researchers [1,5,7,8] Because at high temperatures the fractionation factors suggested that the Allende DIs experienced open-system between gas and solids are small, the overall large range aqueous alteration in an asteroidal environment. Recently, in d18O for Ca-Fe-rich silicates in the Allende DIs is Krot et al. [10] suggested that Ca,Fe-rich rims around inconsistent with a condensation nebular origin of Allende DIs resulted from metasomatic reactions between andradite, salite-hedenbergite pyroxenes and wollastonite these inclusions and surrounding material, possibly [2]. This range can be explained by a two-stage alteration Allende itself. According to their model, the Allende DIs model in the presence of limited amount of aqueous experienced two stages of alteration after lithification and solutions as proposed by [10]. We notice that similar D17O aggregation in the presence of an aqueous solution. One values have been previously reported for secondary episode of alteration resulted in replacement of primary fayalite and magnetite from CV chondrites Kaba and chondrule minerals by secondary faytalitic olivine, Mokoia [3] and magnetite from Allende [4]. We infer that nepheline, sodalite, and Ni-rich sulfides. Calcium lost O-isotope composition of the oxidizing reservoir (most from the chondrules was redeposited as Ca-Fe-pyroxene- likely water) responsible for secondary mineralization in andradite veins and inclusions in DI matrix, and as rims the Allende DIs and oxidized CVs was near the TF line. around chondrules. Following this alteration, the DIs were References: [1] Krot et al. (1995) Meteoritics, 30, 748; [2] excavated from their original location (most likely the Johnson et al. (1990) GCA, 54, 819; [3] Choi et al. (1999) CV3 asteroidal body) and incorporated into the host M&PS, submitted; [4] Choi et al. (1997) EPSL, 146, 337; [5] Allende. The last episode of alteration took place in situ Kojima T. and Tomeoka K. (1996) GCA, 60, 2651; [6] Kurat et al. (1989) Z. Naturforsch., 44a, 988; [7] Krot et al. (1998) and resulted in dissolution of Ca-Fe-rich pyroxenes in the M&PS, 33, 623; [8] Buchanan et al. (1997) GCA, 61, 1733; [9] DIs and re-deposition of Ca-bearing minerals as rims MacPherson et al. (1981) Proc. LPSC, 12B, 1079; [10] Krot et around DIs at the interfaces with the host Allende. The al. (1998) LPSC, 29, #1555; [11] Clayton et al. (1977) EPSL, 34, precipitation of rim minerals started at ~250°C at an iron- 209; [12] Young E. D. and Russell S. S. (1998) Science, 282, rich geochemical barrier at the interface between the host 452. Lunar and Planetary Science XXXI 1463.pdf OXYGEN ISOTOPIC COMPOSITIONS OF SECONDARY CA-FE-RICH SILICATES ... Krot A. N. et al. Ca K a IV-1 Table 1. Oxygen isotopic compositions of secondary minerals 7.2x7.2 mm from the Allende dark inclusions #3529 and #IV-1. mineral, location d17O (‰) d18O (‰) D17O #IV-1 salite-hed., rim -5.4 ± 2.6 -6.5 ± 2.9 -2.0 ± 2.1 sal.-hed., rim -7.2 ± 2.6 -11.3 ±2.9 -1.4 ± 2.1 Allende andradite, rim -5.4 ± 2.6 -2.1 ± 2.9 -2.7 ± 2.1 wollastonite, rim -5.4 ± 2.6 -12.2 ± 3.0 -1.9 ± 2.1 salite-hed., rim -5.4 ± 2.6 -5.4 ± 2.9 -1.7 ± 2.1 andradite+wol, rim -5.4 ± 2.6 -8.4 ± 2.9 -1.2 ± 2.1 #3529 Ca depletion olivine (chd. fragm.) 0.4 ± 2.7 1.1 ± 2.8 -2.0 ± 2.1 andradite, incl-1 3.4 ± 2.7 12.3 ± 2.8 -2.0 ± 2.1 salite-hed., incl-1 1.5 ± 2.6 6.4 ± 2.9 -2.0 ± 2.1 * * salite-hed., incl-1 3.1 ± 2.7 8.5 ± 2.9 -2.0 ± 2.1 andradite, incl-2 6.4 ± 2.7 19.6 ± 2.9 -2.0 ± 2.1 rim sal.-hed.+ol, incl-2 -4.2 ± 2.8 -3.6 ± 2.9 -2.0 ± 2.1 andradite, mx incl-3 -6.2 ± 2.7 17.9 ± 2.8 -2.0 ± 2.1 sal.-hed.+ol, incl-3 0.6 ± 2.7 3.8 ± 2.8 -2.0 ± 2.1 * 15 Dark inclusion * 10 TF 5 Fig. 1. X-ray elemental map in Ca Ka of the Allende dark inclusion #IV-1. It is surrounded by a Ca-rich rim shown in detail in Fig. 2. The 0 inner part of the inclusion contains abundant Ca-rich objects O (‰) composed of salite-hedenbergitic pyroxenes and andradite; its 17 DI #3529: d d -5 outermost portion is depleted in Ca, suggesting loss of Ca to the rim. Ca-Fe-pyroxene andradite -10 Fa-olivine DI #IV-1: rim Ca-Fe-pyroxene -15 CCAM andradite Y&R-line wollastonite -20 -15 -10 -5 0 5 10 15 20 25 18 d O (‰) Fig. 3. Oxygen isotopic compositions of salite-hedenbergitic Allende Dark inclusionpyroxenes, wollastonite, andradite and fayalitic olivine from the Allende dark inclusions #359 and IV-1.
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