The composition of mesosiderite olivine clasts and implications for the origin of pallasites

Abstract

Centimeter-sized olivine clasts from the mesosiderites Emery, Mincy and Pinnaroo have been analyzed using the electron microprobe. Instrumental neutron activation analysis (INAA) has been performed on one olivine each from Emery and Pinnaroo and on an olivine separate from the Brenham pallasite. Nine olivines from Emery have a range of compositions from Fa18 to Fa28 and appear to come from a heterogenous source. This source was formed either by differing degrees of partial melting, yielding dunitic residues of variable composition or by fractional crystallization of one or more ultramafic magmas. Olivines from Mincy and Pinnaroo are more MgO-rich; Fa10 and Fa8 respectively. They require essentially total melting of a source composition that is rich enough in FeO to produce the basaltic clasts found in mesosiderites. Evidence is presented to support the hypothesis that the mesosiderite olivines were formed in the outer few kilometers of their parent body. The range of mesosiderite olivine compositions completely overlaps the range of pallasite olivines. It is proposed that pallasite olivines also had a near-surface origin. A model is presented where external heating of a chondritic parent body yielded pallasites as a by-product of igneous differentiation. The olivine fraction was initially a dunitic residue from extensive partial melting or a cumulate dunite (solidus temperature ∼ 1600−1700°C) that was invaded by molten metal (solidus temperature ∼ 1500°C) from the upper portion of the parent body. The metal phase that was ultimately mixed with the dunite was probably the residual liquid after extensive fractional crystallization. The pallasites were formed where the dunite was cool enough to allow the metal to crystallize thereby preventing it from sinking further. This could have occurred in the outer few tens of kilometers of a 102-km-sized asteroid.