REVISITING THE PALLASITE PROBLEM

A pallasite is a stony-iron meteorite composed of olivine crystals encased in a metallic matrix of iron and nickel. These meteorites are believed to represent material from the core-mantle boundary of an early solar system parent body. The formation of pallasites is debated because silicates and iron-nickel melts are strongly immiscible and the mechanism by which olivine and metal liquids can commingle is poorly understood.

We propose a model where gravitational instabilities occur along a thin, solid metal crust that separates the silicate crystals above the core-mantle boundary from the molten metal below. Continuous small-scale mixing occurs when the dense, solid metal crust becomes gravitationally unstable and develops a sagging breach. Olivine crystals in close contact enter the breached boundary, and then become trapped once the molten boundary layer recovers and solidifies.

In the pallasite samples studied, angular fragments of olivine are observed in close proximity to euhedral olivine crystals. Experiments using olivine crystals and high temperature melts demonstrate that olivine shatters when rapidly introduced to molten materials. We attribute the angular nature of some olivine in pallasites to fragmentation along inherent conchoidal fractures from thermal shock upon mixing with molten metal.

Other metal present in the pallasite samples studied consists of iron, nickel, and sulfur, and is distinguished from iron-nickel metal by its yellow appearance and resistance to polish. Textural analysis reveals that this sulfur-rich metal is always in contact with olivine crystals when present in the pallasite. This observed textural relationship may provide valuable clues to understanding the interactions and mixing processes along the core-mantle boundary of early solar system parent bodies.