Thermal history of Pallasite olivines.

Abstract

There has long been a discrepancy between models for the formation of pallasites due to lack of geochemical data that comes with the depleted nature of olivines and the simple two-phase structure. There are three over-arching hypotheses that have been offered as a model for the formation of pallasites: one hypothesis states that olivines form at the silicate-metallic melt boundary; another states that the olivines form at the bottom of a crust which is then fractured and intruded by liquid melt. A newer hypothesis states that an initial impact event fractures olivine crust and forces the olivine into a liquid melt and then a secondary impact fractures the body and causes quick cooling. The lack of geochemical data and limited numbers of phases means that it has been hard to provide conclusive data to show which hypothesis is the best model for pallasite formation. In this study the markers of each hypothesis have been investigated along with the hypothesis that oxygen data, taken from a previous study by Greenwood et al. (2015), show no bimodal spread that suggests the samples are from different parent bodies. Although this study has concluded that the Springwater pallasite is possibly from another body due to systematic differences in geochemical data from the other studied samples, but the mode of formation is a similar process. The data from this study supports the hypothesis of mantle olivine formation due to differing Al levels and REE patterns, coupled with at least two violent mixing events, that forced the mixing of liquid metallic melt and then broke the parent body apart creating a rapid cooling effect.