Origin of Grossular-bearing Assemblages in Cais from Cv Carbonaceous Chondrites

Grossular is a secondary mineral commonly observed in Al-lende CAIs associated with monticellite, wollastonite, forsterite, anorthite, and wadalite [1–6]. Studies of the 26 Al– 26 Mg systemat-ics in grossular yield (26 Al/ 27 Al) 0 ranging from ~5×10 –5 to <3×10 –6 suggesting grossular formation started almost contempo-raneously with crystallization of CAIs and lasted > 3 My [1–3]. This interpretation has been recently questioned by [4]. Grossular has three main textural occurrences in Allende CAIs. (i) Lacy Åk-rich melilite+anorthite in Type C CAIs are pseudomorphed by a porous aggregate of grossular, monticellite, and forsterite [4]. (26 Al/ 27 Al) 0 in this grossular has not been measured. (ii) In cores of Type B1 CAIs, Åk-rich melilite + anorthite are replaced by coarse-grained intergrowths of grossular, monticellite, wollas-tonite, and wadalite. This grossular has high 27 Al/ 24 Mg ratio (50−100), but shows no evidence of excess 26 Mg (26 Mg*) [(26 Al/ 27 Al) 0 <5×10 −6 ]. Our preliminary data indicate that melilite and grossular have nearly identical O-isotope compositions (Δ 17 O = −2.7±0.8‰ and −3.0±0.8‰, respectively). We suggest both occurrences of grossular resulted from a metamorphic reaction between melilite and anorthite. (iii) In compact Type A CAIs and melilite-rich mantles of Type B1 CAIs, fine-grained grossular forms thin veins crosscutting Åk-poor melilite and closely associated with secondary anorthite. Secondary anorthite shows low or unresolvable 26 Mg*, whereas grossular (27 Al/ 24 Mg<50) contains 26 Mg* corresponding to (26 Al/ 27 Al) 0 ~4.5×10 −5 [3]. We suggest these grossular veins resulted from a two-stage process during fluid-assisted thermal metamorphism: replacement of melilite by secondary anorthite, followed by reaction between anorthite and melilite to form grossular. (26 Al/ 27 Al) 0 approaching the ca-nonical value in this vein grossular reflects 26 Mg* inherited from melilite, following 26 Al decay. A metamorphic origin of grossu-lar-bearing assemblages in Allende CAIs is consistent with the absence of grossular in CAIs from the less metamorphosed CV chondrites – Leoville, Efremovka, and Kaba. The Cl-bearing secondary minerals in Allende CAIs, sodalite (Na 4 Al 3 Si 3 O 12 Cl) and wadalite (Ca 6 Al 5 Si 2 O 16 Cl 3), have different textural occurrences and yield distinct (36 Cl/ 35 Cl) 0 of (<1.6– 4)×10 –6 and (1.72±0.25)×10 –5 , respectively [6–9]. Wadalite occurs as intergrowths with coarse-grained grossular, monticellite and wollastonite in Type B1 CAI cores, whereas sodalite together with nepheline …