NEW INSIGHTS INTO USING KAERSUTITE (TI-RICH AMPHIBOLE) AS A PETROGENETIC TOOL: AN EXPERIMENTAL INVESTIGATION

Kaersutite is found in mantle-derived peridotites and as phenocrysts in alkalic rocks with obvious high-pressure histories. This has led many workers to believe that kaersutite is a high-pressure mineral phase. However, its presence as quench crystals in ferropicritic lavas from the Kola Peninsula, Russia [Hanski, E. J. (1992) Geological Survey of Finland #367.] and in some of the Martian meteorites for which low-pressure histories have been interpreted, calls into question the requirement of high-pressure. Can kaersutite be stable at low pressure, and if so, what are the distinguishing characteristics of low-pressure kaersutite?

Crystallization experiments on a fluorinated kaersutite composition have been conducted to assess kaersutite stability at 1 atm. Starting materials consisted of a mixture of powdered oxides and CaF₂. All experiments were conducted in a Deltech furnace in Fe-capsules inserted into silica-glass tubes which were evacuated and then sealed. All charges were first melted at 1200^oC for 2 hours before dropping to the final crystallization temperature of 1050^oC. Run products consisted of fluor-oxy kaersutite, ilmenite, spinel, glass, and feldspar; phases were analyzed by electron microprobe and micro-FTIR.

Kaersutite stability does extend to 1 atm. However, 1 atm kaersutite differs from naturally occurring “low-pressure kaersutite” because it is mostly a fluor-oxy-kaersutite. Additional experiments with added Cl (2.54 wt% Cl in the liquid) produced kaersutite with 0.10 wt% Cl suggesting that Cl is not an important factor in stabilizing kaersutite at low pressure.

Ongoing experiments are assessing (i) whether greenschist facies metamorphic conditions (i.e., those to which ferropicrites have been commonly subjected) can induce significant F:OH exchange and (ii) the minimum pressure under which kaersutite with significant OH is stable in a magmatic system.