HR: 11:50h
AN: V52A-07    [Abstracts]
TI: Compositional Variation in Arc Lavas and their Associated Melt Inclusions: Implications for Magmatic Volatile Contents and Degassing Behavior
AU: * Moore, G M
EM: gordon.moore@asu.edu
AF: Dept. of Chemistry and Biochemistry, Arizona State University, Tempe, AZ 85287-1604 United States
AU: Roggensack, K
EM: kxr@asu.edu
AF: Dept. of Geological Sciences, Arizona State University, Tempe, AZ 85287-1404 United States
AU: Vennemann, T
EM: Torsten.Vennemann@unil.ch
AF: Institut de Min‚ralogie et G‚ochimie, University of Lausanne, Lausanne, CH-1015 Switzerland
AB: Recent studies have suggested that the compositional variation found in arc-related melt inclusions can be much greater than that reflected in the variation found in the eruptive products. The occurence of high-Ca melt inclusions (containing up to 19 wt% CaO) in the Bataan arc was recently presented by Schiano et al (2000), and similar melt compositions have been found in inclusions from the Nicaraguan portion of the Central American arc. While the origin of these high Ca melts and what they represent has been debated, the effect of such extreme compositional variation on the solubility behavior of mixed volatiles (H2O and CO2) in the melts is unconstrained, making it difficult to determine the intensive parameters for the melt inclusions. Given the importance in unravelling the significance of such inclusions, not only for the origin of arc lavas but also for understanding the degassing history of their host volcanoes, an experimental study to define the solubility behavior of such melts as a function of CaO content was undertaken. Superliquidus experiments (0.4 to 0.7 GPa; 1200 deg C) were conducted in four related basaltic compositions that vary in their CaO content (approx 9 to 18 wt% CaO). A non end-loaded piston cylinder apparatus was used along with a double capsule method. An outer Pt capsule containing an undersaturated hydrous rhyolite was used to prevent the brittle failure of an inner Au-Pd capsule containing basalt and added volatiles (variable H2O:CO2 ratios; XH2O(fluid) equal to 0.3 to 0.8). Oxygen fugacity was monitored in the inner capsule using a thin Pt strip and measuring the resulting Pt-Fe alloy, and select runs were also buffered at Ni-NiO. After equilibration and quenching, fluid compositions were measured by vacuum manometry, and the volatile content of the glasses was measured by high T vacuum manometry, secondary ion mass spectrometry, and infra-red spectrometry. The resulting solubility determinations show that the CO2 solubility increases by a factor of two over the CaO content investigated, while the H2O solubility increases only slightly. Non-ideality in the system was not identifiable outside the scatter of the data, perhaps due to the high temperature conditions of the experiments. The solubility data have also been modelled as a function of P, T, and Ca content to allow the assessment of intensive parameters for the high Ca content melt inclusions, and to allow a comparison with previous work from the literature.
DE: 8412 Reactions and phase equilibria (1012, 3612)
DE: 8413 Subduction zone processes (1031, 3060, 3613, 8170)
SC: Volcanology, Geochemistry, Petrology [V]
MN: Fall Meeting 2005