HR: 10:50h
AN: V51L-03    [PDF]
TI: Phase Equilibria and Kinetic Crystallization Experiments on the Inyo Domes Rhyolite
AU: * Castro, J M
EM: jon.castro@oberlin.edu
AF: Oberlin College, Department of Geology, Oberlin, OH 44074 United States
AU: Gardner, J E
EM: gardner@mail.utexas.edu
AF: University of Texas Austin, Department of Geological Sciences 1 University Station C1100, Austin, TX 78712-0254 United States
AB: Textural characterization of volcanic materials is a common approach used to decipher the conditions of magma ascent, prior to and during eruption. Such an approach is based on the well-established link between magma devolatilization and crystallization. Understanding of the functional relations between degassing and the kinetics of feldspar crystallization has advanced through experimental decompression of natural rhyolitic materials (e.g., Hammer and Rutherford, 2002), thus allowing comparisons between natural and synthetic textures to be used as a geospeedometer. Similar parameterizations of the growth of mafic silicates in silicic melts are sparse, and kinetic information (e.g., growth and nucleation rates) on the crystallization of pyroxene in rhyolitic melts are completely lacking. In light of the common presence of clinopyroxene in groundmass glasses of intermediate volcanic rocks and the abundance of this phase in microlite populations of many pyroclastic and effusive obsidians, we investigate the kinetics of clinopyroxene crystallization in response to isothermal decompression of the Inyo rhyolite. Isothermal decompression experiments are conducted on the low-Ba member of the Inyo rhyolite at H$_{2}$O-saturation, $750\deg$C, Pi=200 MPa, and NNO+1; initial conditions are guided both by previous studies and new constraints on the H$_{2}$O-saturated stability regions of the natural mineral paragenesis determined by phase equilibrium experiments run over broad ranges in P (200 to 50 MPa) and T ($700-900\deg$C). Decompression paths involve an equilibration period (4-7 days) followed by multiple-step pressure drops of equal size (16 to 30 MPa) and duration, to varying final pressures (167 to 4.4 MPa). Integrated decompression rates vary from 0.045 MPa/min to 0.004 MPa/min, corresponding to total run durations ranging from 3 to 24 days. The observed crystallization sequence is: Fe-Ti oxide, cpx, biotite, plagioclase. In 3 and 6 day runs, clinopyroxene exhibits both low number densities ($<$10$^{7}$/cm$^{3}$), and average size (l~4 microns); such textures are indistinguishible from those produced in phase equilibrium experiments at initial P and T. By contrast, longer duration runs (12 and 24 days) produce significant shifts in the number and size distribution of clinopyroxene. In 12 day runs, pyroxene number increases by a factor of 9 over the initial value ($<$10$^{7}$/cm$^{3}$), while the mean crystal length increases by approximately 3 microns over the decompression interval. These textural shifts, combined with the temporal scale of experiments, imply an average nucleation rate of approximately 9x10$^{1}$/cm$^{3}$s$^{1}$ and a growth rate of about 3x10$^{-6}$/s$^{1}$. Clinoyroxene number densities in natural Inyo obsidians exceed experimentally-produced values by up to 3 orders of magnitude, suggesting that the undercoolings imposed by the current experiments were insufficient to induce large degrees of supersaturation required for rapid and ubiquitous microlite nucleation. Size distributions however, match well between experiment and nature. The experimental calibration of clinopyroxene growth rate may therefore provide a valuable tool for constraining time scales of magma ascent where detailed measurements of crystal size can be made.
DE: 3630 Experimental mineralogy and petrology
DE: 3640 Igneous petrology
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2003 Fall Meeting