Bland, PA

Krot, AN

Petaev, MI

Spiral - Imperial College Digital Repository

GROWTH OF FERROUS OLIVINE IN THE OXIDIZED CV CHONDRITES DURING FLUID-ASSISTED THERMAL METAMORPHISM. A. N. Krot1, M. I. Petaev2, and P. A. Bland3 1HIGP, U. Hawai’i at Manoa, USA (sasha@higp.hawaii.edu); 2CfA & DEPS, Harvard U., USA; 3Imperial College, DESE, UK. The CV carbonaceous chondrites are subdivided into the oxi-dized [Allende-like (CVoxA) and Bali-like (CVoxB)] and reduced subgroups [1], which reflect complex alteration histories experi-enced by CVs [2]. Although recent mineralogical [3-5] and isotopic [6-8] studies show that CV alteration occurred in the presence of aqueous solutions and lasted for at least several Ma, the origin of secondary ferrous olivine (Fa40-60) remains controversial. Two types of models have previously been proposed: (i) gas-solid condensation from an oxidized nebular gas [9] and (ii) dehydration of phyllosili-cates during thermal metamorphism [10, 11]. Here we argue that ferrous olivine in the oxidized CVs formed during fluid-assisted metamorphism by several mechanisms (1-4) described below: (1) Replacement of opaque nodules in CVoxB chondrules by fay-alite (Fa>90) [2, 12] is a two-stage process involving oxidation of metal to magnetite: 3Fe(s) + 4H2O(l,g) = Fe3O4(s) + 4H2(g), and re-placement of magnetite by fayalite: 2Fe3O4(s) + 3SiO2(aq) + 2H2(g) = 3Fe2SiO4(s) + 2H2O(l,g). In MET00430, which is intermediate be-tween CVoxA and CVoxB, the opaque nodules are replaced by ferrous olivine (Fa60-80), whereas those in Allende are replaced by less Fe-rich olivine (Fa40-50). This probably reflects higher alteration tem-peratures experienced by the CVoxA compared to the CVoxB [3]. (2) Direct precipitation of ferrous olivine from a supersaturated fluid in pore space: Fe2+(aq) + Mg2+(aq) + SiO2(aq) = (Fe,Mg)2SiO4(s), is indicated by the presence of euhedral ferrous olivines overgrowing Mg-rich olivines and pyroxenes in chondrules and AOAs in oxi-dized CVs, and by the 16O-poor compositions of the ferrous olivines in Allende AOA [13]. Large compositional variations of the ferrous olivines (Fa50-80) within individual AOAs in Kaba and complex Fe-Mg zoning (Fa50-80) of individual ferrous olivines in MET00430 probably reflect fluctuations of fluid compositions on a local scale. (3) Replacement of low-Ca pyroxene by ferrous olivine is com-monly observed in the CVoxA and CV dark inclusions (DIs) [11, 14]. The presence of phyllosilicates coexisting with these olivines [14, 15] suggests that this process occurred in the presence of aqueous solutions: Fe2+(aq) + MgSiO3(s) +H2O(l,g) = (Fe,Mg)2SiO4(s) + H2(g). (4) Formation of ferrous olivine by dehydration of phyllosili-cates is inferred from fibrous textures of some of the ferrous olivines in the Allende DIs [10], and from the presence of inclusions of pent-landite and poorly-graphitized carbon in the CVoxA matrix olivines [16]. However, the Mg-rich compositions of phyllosilicates in CVs, the rarity of fibrous olivines, and the lack of O-isotopic evidence for extensive hydration-dehydration of CVs [17] suggest that a direct substitution of olivine for phyllosilicates has played a minor role in the origin of ferrous olivine. Growth of ferrous olivine in the pres-ence of fluid released during dehydration of CVs seems more likely. References: [1] Weisberg et al. (1997) MAPS 32, A138; [2] Krot et al. (1998) MAPS 33, 1065; [3] Krot et al. (2001) Geochem. Int. 36, 351; [4] MacPherson et al. (2002) MAPS 37, A91; [5] Coza-rinsky et al. (2003) LPS 34, #1043; [6] Hutcheon et al. (1998) Sci-ence 282, 1865; [7] Pravdivtseva et al. (2001) MAPS 36, A168; [8] Krot et al. (2002) MAPS 37, A82; [9] Weisberg and Prinz (1998) MAPS 33, 1087; [10] Kojima and Tomeoka (1996) GCA 60, 2651; [11] Krot et al. (1997) Meteoritics 32, 31; [12] Choi et al. (2000) MAPS 35, 1239; [13] Imai and Yurimoto (2003) GCA 67, 765; [14] Krot et al. (1999) MAPS 34, 67; [15] Brearley (1997) Science 276, 1103; [16] Brearley (1999) Science 285, 1380; [17] Clayton and Mayeda (1999) GCA 63, 2089. 66th Annual Meteoritical Society Meeting (2003) 5132.pdf

Growth of ferrous olivine in the oxidized CV chondrites during a fluid-assisted thermal metamorphism.

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