Fe–Mn–Na Phosphates and Al-Free Chromite in the Metal-Troilite Eutectic Nodule

Abstract

An assemblage with metal, troilite, Fe–Mn–Na phosphates, and Al-free chromite was identified in the metal-troilite eutectic nodules in the shock-produced chondritic melt of the Yanzhuang H6 meteorite. Electron microprobe and Raman spectroscopic analyses show that a few phosphate globules have composition of Na-bearing graftonite (Fe,Mn,Na)3(PO4)2, whereas most other correspond to Mn-bearing galileiite Na(Fe,Mn)4(PO4)3 and a possible new phosphate phase of Na2(Fe,Mn)17(PO4)12 composition. The Yanzhuang meteorite was shocked to a peak pressure of some tens GPa and a peak temperature of ~2000 ℃. All minerals were melted after pressure release to form a chondritic melt due to very high postshock heat that brought the chondrite material above its liquidus. The volatile elements P and Na released from whitlockite and plagioclase along with elements Cr and Mn released from chromite are concentrated into the shock-produced Fe–Ni–S–O melt at high temperatures. During cooling, microcrystalline olivine and pyroxene first crystallized from the chondritic melt, and the formed metal-troilite eutectic intergrowths and silicate glass finally solidified at about 950–1000 ℃. On the other hand, P, Mn, and Na in the Fe–Ni–S–O melt combined with Fe and crystallized as Fe–Mn–Na phosphates within troilite, while Cr combined with Fe and crystallized as Al-free chromite also within troilite.