The Chemical Nature of Kaersutite Experimentally Produced at 0 Kbar

Introduction: Kaersutite is a Ti-rich amphibole defined as having between 0.50 and 0.80 Ti atoms per formula unit [1, 2]. It is commonly found in terrestrial mantle-derived peridotites, in alkalic rocks with obvious high-pressure histories [3, 4], and in some SNC meteorites for which the exact petrogenetic histories are unknown [5, 6]. Because kaersutite is found in rocks with high-pressure histories on Earth, some workers have suggested that the presence of kaersutite in the SNC meteorites implies high-pressure crystallization histories for the magmas that produced those meteorites, while others have argued on separate grounds for low-pressure crystallization histories. A significant amount of experimental work has been conducted assessing the high-pressure stability of kaersutite [e.g., 7, 8, 9], however, less is known about its low-pressure stability [10, 11]. This work focuses on experiments aimed at evaluating the low-pressure stability limits of fluorinebearing kaersutite, the chemical nature of any lowpressure kaersutite, and any distinguishing differences between low-pressure and high-pressure kaersutite. Special attention is given to the O(3) and M(1-3) crystallographic sites because they are the most likely sites to record meaningful chemical variations in amphibole. Experimental Technique: First a powdered mix of oxides and CaF2 was created based on the kaersutite compositions for Chassigny reported by [10, 12]. An Fe/Fe ratio of 3.21 was used, and sufficient CaF2 added so that F could fill the O(3) site in the amphibole. Next the powder was put in Fe capsules and dried under vacuum (in the presence of an Fe oxygen getter) at 800C for 20 minutes in silica-glass tubes to drive off absorbed and structural H2O. The evacuatedsilica glass tubes were sealed and placed in Pt-wire quench furnaces where 0 kbar crystallization experiments were carried out. The sample was first melted at 1200C for 2-3 hours and then crystallized at temperatures ranging from 750C-1050C over 4-10 days. Based on the experimental results from the original powder, two additional synthetic powders were created. One was the original mix but with a lower Mg#; the other was the original mix but with 75 mol% less fluorine (every 2 F replaced by one O in the mix). Experiments using these powders are the same as described above for the original powder. Chemical compositions of all three powders are reported in Table 1. Table 1. Compositions of synthetic powders used for experiments obtained by fusing the synthetic powders and analyzing the resulting glasses by electron microprobe.