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fbtwitterlinkedinvimeoflicker grey 14rssslideshare1
Lindgren, P.; Lee, M.R. (2015)
Languages: English
Types: Other
Subjects:
The earliest stages of CM carbonaceous\ud chondrite aqueous alteration are very poorly\ud understood as mildly altered CMs are extremely rare.\ud The Paris meteorite (CM2.7) [1-3] and QUE 97990\ud (CM2.6) [4,5] are among the least aqueously altered\ud CMs described to date. However, neither of them contain\ud the pristine attribute of chondrule mesostasis glass.\ud Glass is highly reactive and so among the very first\ud phases to undergo aqueous alteration [6]. Therefore,\ud the CM carbonaceous chondrite EET 96029 is very\ud unusual as it has been shown to have retained\ud mesostasis glass in at least one chondrule [7]. According\ud to the new CM classification scheme of [8], which\ud is based on H content, EET 96029 has an index of 2.0\ud (data in [9]), meaning that it is less altered than all but\ud one of the fifty CMs analysed by [8]. A caveat is that a\ud low H content could be due to mild heating as well as a\ud low degree of aqueous processing [9]. However, the\ud bulk O oxygen isotope composition of EET 96029 (as\ud determined by [10]) is consistent with a low degree of\ud alteration as it is slightly closer to that of the CO3 falls\ud (possible representatives of the anhydrous progenitors\ud of the CMs) than even the least altered lithology of\ud Paris (Fig. 1).

\ud To better understand the earliest stages of CM\ud aqueous alteration and its impact on mesostasis glass,\ud we have undertaken a detailed study of chondrule\ud mesostasis textures and compositions in the mildly\ud aqueously altered CM chondrite EET 96029.

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    • cronstedtite with a high density of stacking faults. However, this mesostasis has a microgranular texture and Fe-enrichment suggesting that it also contains grains of Fe-oxide (Fig. 2e). Chemical analysis (Fig. 2f) shows that aqueous alteration of chondrule mesostases is accompanied by export of Si+Al, and import of Fe and Mg.
    • TEM imaging of the meteorite matrix has revealed the presence of a compact amorphous material, finegrained phyllosilicates and organic nanoglobules. As an amorphous groundmass also occurs in the matrix of Paris [1] and Y-791198 [13], it would appear to be a characteristic of mildly altered CMs.
    • CAIs are relatively abundant in the thin section (0.2 CAI/mm2) and at least one of them contains gehlenite as confirmed by X-ray microanalysis and Raman spectroscopy. Melilite (solid solution between √•kermanite and gehlenite) is thought to have been a common constituent of CAIs that were accreted by the CM parent body(ies), but has been lost owing to its susceptibility to aqueous alteration [14]. Melilite is extremely rare in CMs and have only been reported in mildly altered CMs including Paris [1], Murchison [15,16] and LEW 85311 [17].
    • Conclusions: EET 96029 may be the least altered CM yet described. Indicators of its near-pristine nature include (i) chondrule mesostasis glass; (ii) gehlenitebearing CAI; (iii) an amorphous groundmass to the matrix. The mesostases of most chondrules have been altered, although twelve out of twenty chondrules have preserved a quench crystallite texture in their mesostasis phyllosilicates (ranging from well-defined needles to lath-shaped crystals and voids of remnant crystallites), indicating a very early state of aqueous alteration [5]. EET 96029 provides another rare example of a mildly aqueously altered CM, recording the earliest stages of CM alteration.
    • References: [1] Marrocchi et al. (2014) MAPS 49, 1232- 1249 [2] Hewins et al. (2014) GCA 124, 190-222 [3] Rubin (2014) LPS XLV Abstract #1130 [4] Rubin et al. (2007) GCA 71, 2361-2382 [5] Maeda et al. (2009) J. Min. Pet. Sci. 104, 92-96 [6] Burger & Brearley (2004) LPS XXXV Abstract #1966 [7] Lindgren & Lee (2014) MetSoc LXXVII Abstract#5276 [8] Alexander et al. (2013) GCA 123, 244-260 [9] Alexander et al. (2012) Science 337, 721-723 [10] Tyra et al. (2007) GCA 71, 782-795 [11] Clayton & Mayeda (1999) GCA 63, 2089-2104 [12] Lauretta et al. (2006) MESSII, 431-459 [13] Chizmadia & Brearley (1998) GCA 72, 602-625 [14] Rubin et al. (2007) MAPS 42, 1711-1726 [15] MacPherson et al. (1983) GCA 47, 823-839 [16] Simon et al. (2006) Am. Min. 91, 1675-1687 [17] Simon et al. (2005) MAPS 40: A141. Acknowledgements: NASA Antarctic meteorite collection and UK-STFC.
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