METHODS OF EXPERIMENTAL APATITE GROWTH AND THEIR APPLICATIONS TO MAGMATIC SYSTEMS

Apatite, Ca₅(PO₄)₃(F,Cl,OH), is a ubiquitous trace mineral in many volcanic and magmatic systems. As F, Cl and OH are important components in volcanic systems, we can compare the abundances of these volatile components in natural apatites from volcanic rocks to those grown experimentally under known conditions in a lab, and then estimate what the volatile contents of a magma may have been before eruption.

To grow large apatites experimentally, hydrothermal experiments were run in an internally heated pressure vessel and cold-seal pressure vessels at temperatures between 900 and 920°C and pressures ranging from 0.5 to 2.0 kbar. Our first goal was to develop methods for growing large, stable, homogeneous apatite crystals under experimental conditions in three weeks or less. Prior experiments involving apatite seed crystals could take months to achieve chemical equilibrium between apatite, felsic melt and fluid at these conditions. We found that adding 2 to 4-micrometer apatite seed crystals, phosphoric acid, CaCl₂, distilled H2O, and NaF to a starting charge of powdered rhyodacite from Mt. Mazama while cycling temperature over a range of 10°C every day seemed to produce the best results; euhedral apatite crystals with diameters as large as 100 micrometers were grown.

We will use the volatile contents of the synthetically prepared apatite to interpret natural apatite compositions from Augustine volcano, Alaska, and other similar volcanoes. This is useful as the volatile content of a magma has significant influence over how explosive an eruption will be, and it will allow for more accurate predictions of the explosivity of future eruptions. Also, as only limited experimental data are available for apatite-melt-fluid equilibria at pressures < 2 kbar, our work will allow for better predictions of the behavior of these phases in shallow volcanic magma chambers.