ALBERT

Description: Sperrylite (PtAs 2 ) is one of most common Pt minerals, but the processes whereby it forms are not clearly established. Most commonly it is associated with the major-component base metal sulfide minerals (pyrrhotite, pentlandite, and chalcopyrite), which are believed to have crystallized from magmatic sulfide melts. Hence, sperrylite is thought to have formed by crystallization from a sulfide melt or by exsolution from sulfide minerals. However, sperrylite is also found associated with silicate and oxide minerals where it is thought to have formed by crystallization from the silicate magma. To investigate the conditions under which sperrylite could crystallize from a magmatic sulfide melt we investigated sperrylite saturation in Fe-Ni-Cu-S sulfide melts under controlled $${f}_{{\mathrm{O}}_{2}}$$ and $${f}_{{\mathrm{S}}_{2}}$$ at 910–1060 °C and 1 bar. The As and Pt concentrations in the sulfide melt at sperrylite saturation increase from 0.23–0.41 to 2.2–4.4 wt% and from 0.36–0.65 to 1.9–2.8 wt%, respectively, as the iron concentration in the sulfide melt decreases from 50 to 36 wt% at 910–1060 °C. We show that transitional metal concentrations, particular iron and nickel, as well as sulfur and oxygen fugacities influence As and Pt concentrations in the sulfide melt at sperrylite saturation. These intensive variables appear to effect sperrylite solubility by influencing the oxidation state of As in the sulfide melt. The measured concentrations of As and Pt in sperrylite-saturated sulfide melts produced in our experiments are much higher than that in most natural sulfides, implying that arsenides and sulfarsenides will not reach saturation in natural magmatic sulfide melts at high temperatures unless the magma has been contaminated with an exceptionally As-rich rock. This suggests that the observed arsenides and sulfarsenides in natural sulfide ores were not formed by crystallization from unfractionated sulfide melts at high temperatures above 900 °C, but might form at low temperatures below 900 °C.

Description: X-ray computed microtomography is a non-destructive imaging technique recognized in the geosciences as a powerful tool to investigate rock textures directly in three dimensions (3D) at the micrometer and sub-micrometer scale. The quantitative morphological and textural analysis of images requires segmentation and characterization of phases in the reconstructed volume based upon their gray levels (related to their relative X-ray attenuation) and/or morphological aspects. Often the differences in X-ray attenuation of some phases are so small that no contrast is observed in the reconstructed slices or, although the human eye can discern the differences between these phases, it is difficult, or sometimes impossible, to reliably segment and separately analyze these phases. Facing this challenge, we propose an experimental and computational procedure that allows the segmentation of phases with small density variations in geomaterials. By using an experimental protocol based on phase-contrast synchrotron X-ray microtomography combined with a customized 3D image processing procedure, we successfully segmented feldspar from the glassy matrix in both a natural volcanic sample and a synthetic analog. Our results demonstrate that crystallized natural volcanic rocks and synthetic analogs can be characterized by synchrotron X-ray phase-contrast microtomography and that phase-retrieval processing is an invaluable tool for the reconstruction of 3D multiphase textures.