Evidence for low‐temperature growth of fayalite and hedenbergite in MacAlpine Hills 88107, an ungrouped carbonaceous chondrite related to the CM‐CO clan

Abstract

Abstract-The carbonaceous chondrite MacAlpine Hills (MAC) 88 107 has bulk composition and mineralogy that are intermediate between those of CO and CM chondrites. This meteorite experienced minor alteration and a low degree of thermal metamorphism (petrologic type 3.1) and escaped post-accretional brecciation. The alteration resulted in the formation of fayalite (Fago-loo), Al-free hedenbergite (-Fs~oWo50), phyllosilicates (saponite-serpentine intergrowths), magnetite, and Ni-bearing sulfides (pyrrhotite and pentlandite). Fayalite and hedenbergite typically occur as veins, which start at the opaque nodules in the chondrule peripheries, crosscut fine-grained rims and either terminate at the boundaries with the neighboring fine-grained rims or continue as layers between these rims. These observations suggest that fayalite and hedenbergite crystallized after accretion and compaction of the fine-grained rims. Fayalite also overgrows isolated forsteritic (Fa]-5) and fayalitic (Fa204 olivine grains without any evidence for Fe-Mg interdiffusion; it also replaces massive magnetite-sulfide grains. The initial 53Md55Mn ratio of (1.58 2 0.26) x 10-6 in the MAC 88107 fayalite corresponds to an age difference between the formation of fayalite and refractory inclusions in Allende of either -9 or 18 Ma, depending upon the value of the solar system initial abundance of 53Mn used in age calculations.Formation of secondary fayalite and hedenbergite requires mobilization and transport of Ca, Si, and Fe either through a high-temperature gaseous phase (Hua and Buseck, 1995) or low-temperature aqueous solution (Krot et al., 1998a,b). The high-temperature nebular model for the origin of fayalite (HIM and Buseck, 1995) fails to explain (a) formation of fayalite-hedenbergite assemblages after accretion of fine-grained rims that lack any evidence for high-temperature processing; (b) extreme fractionation of refractory lithophile elements of similar volatility, Ca and Al, in hedenbergite; and (c) absence of Fe-Mg interdiffusion along fayaliteforsterite boundaries. We conclude that fayalite and hedenbergite in MAC 88107 formed during late-stage, lowtemperature (approximately 150-200 "C) aqueous alteration. The data for MAC 88 107 extend the evidence for an early onset of aqueous activity on chondrite parent bodies and reinforce the conclusion that liquid water played an important role in the chemical and mineralogical evolution of the first chondritic planetesimals.

Early aqueous activity on the ordinary and carbonaceous chondrite parent bodies recorded by fayalite

“…Here we report on the mineralogy, petrography and oxygen-isotope compositions of the fayalite-bearing assemblages and 53 Mn- 53 Cr ages of fayalite in type 3 L, LL, CO and CV chondrites. Our data confirm the origin of fayalite during low-temperature aqueous alteration on the L, CV and CO chondrite parent bodies [10][11][12] , and allow us to constrain the accretion ages and the accretion regions of these bodies. Fig.…”

“…3 and Supplementary Table 4). The observed differences in RSF values between fayalite (Fa 90-100 ) and San Carlos olivine (Fa 10 ) suggest that all previously reported 53 Mn- 53 Cr dating of fayalite in carbonaceous chondrites [10][11][12] are in error and yield systematically young ages 14 .…”

“…Carbonates, however, are virtually absent in weakly aqueously altered ordinary (H, L and LL) and several carbonaceous chondrite groups, including CV (Vigarano-like) and CO (Ornans-like). Instead, the least metamorphosed meteorites of these groups contain aqueously formed Mn-rich, Cr-poor fayalite (Fa 490 ; Fayalite (Fa) number ¼ atomic Fe/(Fe þ Mg) Â 100 in olivine) suitable for in situ Mn-Cr isotope dating with SIMS [10][11][12] . As for carbonates, SIMS measurements of 53 Mn- 53 Cr systematics in fayalite require a proper standard to determine the RSF and calculate 55 Mn/ 52 Cr ratios.…”

“…As for carbonates, SIMS measurements of 53 Mn- 53 Cr systematics in fayalite require a proper standard to determine the RSF and calculate 55 Mn/ 52 Cr ratios. Because natural fayalite contains virtually no chromium, it cannot be used as a standard for MnCr isotope measurements; San Carlos olivine (Fa 10 ) was typically used instead [10][11][12] . However, it has been recently discovered that the RSF in olivine changes as a function of its fayalite content 13,14 .…”

“…However, it has been recently discovered that the RSF in olivine changes as a function of its fayalite content 13,14 . Therefore, all previously published 53 Mn- 53 Cr ages of CV and CO chondritic fayalite, acquired with a San Carlos olivine standard [10][11][12] , need to be corrected.…”