Low-Pressure Water-Undersaturated PT Phase Relations of Mafic High-Alumina Basalt: Preliminary Results

A. Dana Johnston (Dept. Geological Sciences, University of Oregon, Eugene OR 97403-12721 USA, +1-541-346-5588 email adjohn@oregon.uoregon.edu) and David S. Draper (School of Earth Sciences, Macquarie University, Sydney NSW 2109 Australia, +61-2-9850-8347, email david.draper@mq.edu.au)

We report preliminary near-liquidus phase relations for the mafic (Mg number 63, 7.3 wt% MgO) high-alumina basalt (HAB) SSS.1.4, studied under anhydrous conditions by Johnston (1986, CMP 92:368). Experiments were conducted at 0.2, 0.5, and 0.7 GPa in internally heated pressure vessels (IHPVs) and at 0.6-1.5 GPa in piston-cylinder apparatus. In all runs, 2 to 3 wt% H2O was added and oxygen fugacity was buffered to NNO. In the IHPVexperiments, sample was sealed into graphite-lined Pt inner capsules with oxalic acid dihydrate as the fluid sourcein an amount calculated to yield the desired water content. We assume that the low solubility of CO2 in basaltic melt at the low pressures in the IHPV runs had little effect on phase relations. In the piston-cylinder experiments, distilled water was loaded directly into the Au80Pd20 capsules, which were used to minimize Fe loss to the capsule. This approach was largely successful, with at most 25-30% relative Fe loss, and typically less. As such, these experiments are the first on primitive arc HAB in which the relevant PT space has been explored under water undersaturated conditions at buffered oxygen fugacities that are appropriate to arc settings and with minimal Fe loss.

Olivine is not on the anhydrous liquidus of SSS.1.4 at any pressure (Johnston 1986) and this result has been cited in support of the slab-melting model for generation of HABs. Draper and Johnston (1992, CMP 112:501) showed that olivine-liquid reaction could not account for this result and noted that low water contents might displace plagioclase as the liquidus phase of HABs in favor of olivine. The water-undersaturated liquidus of SSS.1.4 is depressed by 50-100°C and the near-liquidus region is characterized by cpx + plag, implying that the multiple saturation point with these two phases has shifted from approx. 1.7 GPa (dry) to <1.0 GPa. Orthopyroxene is a near-liquidus phase at 0.6 GPa; this phase did not appear near the liquidus under anhydrous conditions. Its appearance weakens the argument against mantle phases in equilibrium with HAB, and ongoing work will be presented to clarify these initial results.