HR: 14:00h
AN: V43G-02 INVITED     [Abstracts]
TI: Hot, warm, cold; wet, damp, dry; peridotite, pyroxenite, eclogite; do petrologists know anything about mid-ocean ridge and ocean island basalt sources?
AU: * Asimow, P D
EM: asimow@gps.caltech.edu
AF: California Institute of Technology, 1200 E. California Blvd. M/C 170-25, Pasadena, CA 91125 United States
AB: Igneous petrology can offer essential constraints on models of spreading center and intraplate volcanism to complement information drawn from seismology, geophysics, and geochemistry. However, as with all these other disciplines, inferences from petrological data are often non-unique and model-dependent. Petrology will be most useful to the general plume debate when and if it can uniquely invert for the temperature, volatile content, and major element composition of the mantle sources of erupted basaltic lavas. When instead there are ambiguities it is important to acknowledge these lest preferred models be taken as fact. Or, rather than attempting to invert for source information, petrologists might content themselves with running forward models to test hypotheses proposed by others. This is a well-defined task, free from ambiguity, and consistent with a conservative falsification approach to science. It is also of the first importance for all parties to avoid over-generalization of their arguments and false grouping of different localities into one category; proposed mantle plumes must be evaluated one at a time, rather than collectively. Furthermore, in order to be generally accepted, models must be able to explain all the observable features of a volcanic chain: the longevity and fixity (or not), the magma and buoyancy fluxes, the trace element and isotopic (lithophile, noble gases, and stable) character, and the distribution (in time and space) of major-element lava types. Such models must also be consistent with mass and energy conservation and known phase equilibria. Two tasks that are quite straightforward at present are (1) inference of the MgO content of the most primitive demonstrable parental lava in a suite from observed liquid and olivine phenocryst compositions and (2) the estimation of source parameters assuming dry peridotite melting. Although in principle an arbitrary amount of $H_{2}O$ in the primary magma might depress the liquidus temperature at any particular MgO as much as desired, in practice there are generally limits on parental $H_{2}O$ contents from melt inclusions or submarine eruptions. Among the remaining model-dependent uncertainties are the estimation of potential temperature from liquidus temperatures and the estimation of source parameters when wet melting, eclogite sources, or mixed lithology sources are considered. It is important to emphasize that a parental liquidus temperature can never be anything but a lower bound on source potential temperature. Any number of processes, most notably adiabatic melting and near-surface cooling, lower the temperature and may leave no record in the phenocryst population. To actually determine the potential temperature it is necessary to find a unique and self-consistent forward model that generates the appropriate parental melt compositions and at the same time sufficient melt volume. In this talk, I will focus on wet melting and the maximum effect that water might have on increased magma production and on over-estimation of liquidus temperatures. I will use the mid-Atlantic ridge near the Azores, the Reykjanes ridge and Iceland, the Galapagos Spreading Center, and Hawaii as examples.
DE: 8121 Dynamics, convection currents and mantle plumes
DE: 8124 Earth's interior--composition and state (old 8105)
DE: 3640 Igneous petrology
DE: 1025 Composition of the mantle
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2004 AGU Fall Meeting