Crystallization experiments of intercumulus melts for nakhlites under QFM +/- 2 at 1 bar

Abstract

Crystallization of parent melts for nakhlites was experimentally studied under QFM +/- 2 at one bar. Isothermal experiments suggest that melts having parent magma composition for nakhlites crystallize both augites and titanomagnetites at liquidus temperatures of 1144-1154 degrees C. Compositions of the augites are identical to those of phenocrystic core augites (En36-38Fs22-25Wo39-40) in nakhlites. No olivines crystallize from the isothermal runs, and solidus temperature is about 1000 degrees C. Linear-cooling experiments were carried out at various cooling rates (1-17 degrees C/h) ranging from liquidus to solidus temperatures under similar pressure conditions to the isothermal runs. Augites, titanomagnetites, and fayalites crystallized in the cooling runs, but magnesian olivines never crystallized there. Magnesian core augite in the cooling runs has the same composition as those of nakhlites. Rims of augite crystals from the cooling runs of 1-4 degrees C/h consist of two layers, ferroan augite inner rim and hedenbergite outer rim, which are very similar to those in the Miller Range (MIL) 03346 nakhlite. Small amounts of pyroxferroite crystallized in mesostasis and augite rims from two cooling runs. Titanomagnetites from cooling runs never accompany ilmenite lamellae as seen in nakhlites, suggesting that the subsolidus cooling rate of the cooling runs was much more rapid than those of nakhlite intercumulus melts. The cooling experiments reproduce the crystallization processes of pyroxenes and the compositional change of residual melt for a rapidly cooled magma such as MIL 03346.