The formation and alteration history of a forsterite-bearing Type C CAI from Allende: Evidence for a Type B CAI precursor, and implications for fluid-assisted metasomatism on the CV chondrite parent body

Abstract

Type C CAIs are a rare group of refractory inclusions in carbonaceous chondrites that are compositionally and isotopically distinct from the more commonly observed igneous CAIs (i.e., Type As and Type Bs). We have investigated a forsterite-bearing Type C CAI ALNH-04 from the Allende CV3 chondrite. This CAI has both textural and compositional similarities to some of the Type C CAIs previously reported; however, there are notable differences that imply that ALNH-04 may have formed from a different precursor from other Type C inclusions. Based on the bulk composition of ALNH-04 and the minor element contents of forsterite, we suggest that the forsterite grains were inherited from a Forsterite-bearing Type B CAI (FoB) precursor. The presence of augite on the periphery of ALNH-04 implies a re-melting event that probably occurred in a chondrule-forming region.Another interesting feature of ALNH-04 is the secondary iron-alkali-halogen zoning sequence as manifested by varying proportions of nepheline, sodalite, fayalitic olivine, and sulfides in different regions of the CAI. Nepheline ± sodalite have replaced anorthite in the outer part of the inclusion, giving way to the presence of ubiquitous sodalite with minor nepheline, partially replacing anorthite at grain boundaries and fractures in the interior of the inclusion. Sulfides and Fe-bearing olivine form an iron-rich alteration zone. The textural relationships between nepheline and sodalite show no evidence of a direct replacement relationship between the two phases. Combined with the SEM observations, the microstructures are most consistent with a two-stage fluid alteration process: (1) nepheline replaced anorthite in the outer part of the CAI via a fluid with aNaCl(aq) within the stability range of nepheline; (2) a later-stage fluid, with elevated aNaCl(aq) that could preferentially stabilize sodalite, penetrated further into the CAI interior, replacing anorthite with sodalite. The lack of a nepheline-sodalite replacement relationship indicates that the conditions and fluid chemistry were suitable for nepheline and/or sodalite to be stable. Together with other Fe-rich secondary phases, fayalitic olivine may have recorded an increase in pH and aSiO2(aq) of the fluid. These changes were probably induced by the extensive alteration of the outer part of the CAI by feldspathoids. The observed alteration microstructures are consistent with a coupled dissolution-precipitation alteration mechanism. The fluid alteration was also responsible for the formation of Na- and Ca-rich halos in the matrix surrounding the CAI.We compared ALNH-04 with other CAIs and chondrules showing alkali-halogen-(iron) zoning sequences in Allende, and found that the observed zoning structures are consistent with the two-stage fluid-assisted metasomatic process mentioned above. The different distribution patterns of nepheline and sodalite in plagioclase-rich CAIs, chondrules, and melilite-rich CAIs may be explained by different chemical potential gradients in SiO2 in the fluid. Precipitation of nepheline and sodalite may require a higher SiO2 activity compared to grossular and dmisteinbergite (±secondary anorthite), which controlled the formation location of sodalite during the second fluid alteration event. Fluids with different compositions may be produced by fluid percolation along different directions and pathways, changing convection patterns, or release of water from a differentiated asteroidal interior.