Polybaric melting


Polybaric melting

"Etymology:" International scientific vocabulary"Subject:" Basalt petrogenesis

Fundamentally, this implies that liquids are incrementally separated from residues across a range of pressures and subsequently mix and move through the mantle without equilibrating with surrounding mantle minerals [McKenzie, Dan and O'Nions, R. K. (1991), Partial melt distributions from inversion of Rare Earth Element concentrations, J. Petrol., 32(5), 1021] . This model was developed to better approximate basalt petrogenesis in modeling and experiments. Usually involves polybaric near-fractional melting (e.g., constant intergranular porosity in the rock during melting and/or reactive porous flow in melt extraction [Kelemen, P. B. et al. (1997), A review of melt migration processes in the adiabatically upwelling mantle beneath oceanic spreading ridges, Phil. Trans. R. Soc. Lond. A, 355(1723), 283-318 [http://dx.doi.org/10.1098/rsta.1997.0010 doi 10.1098/rsta.1997.0010] ] ) along an adiabatic path [Asimov, Paul D., and Longhi, John (2004), The significance of multiple saturation points in the context of polybaric near-fractional melting, J. Petrol., 45(12), 2349-2367 [http://dx.doi.org/10.1093/petrology/egh043 doi 10.1093/petrology/egh043] ] .

In practice, petrologic models employ advanced forms of the polybaric concept for greater physical plausibility [Asimov, Paul D., and Longhi, John (2004), The significance of multiple saturation points in the context of polybaric near-fractional melting, J. Petrol., 45(12), 2349-2367 [http://dx.doi.org/10.1093/petrology/egh043 doi 10.1093/petrology/egh043] ] . Such models incorporate interconnected porosity to facilitate buoyant flow of liquids from lherzolitic or harzburgitic assemblages [Kelemen, P. B. et al. (1997), A review of melt migration processes in the adiabatically upwelling mantle beneath oceanic spreading ridges, Phil. Trans. R. Soc. Lond. A, 355(1723), 283-318 [http://dx.doi.org/10.1098/rsta.1997.0010 doi 10.1098/rsta.1997.0010] ] , such as replacive dunite formation in migration channels. The porosity has to consist of at least two or a continuum of size scales to account for U-series disequilibria [Lundstrom, Craig (2000), Models of U-series disequilibria generation in MORB: the effects of two scales of melt porosity, Phys. Earth. Planet. In., 121(3-4), 189-204 [http://dx.doi.org/10.1016/S0031-9201(00)00168-0 doi 10.1016/S0031-9201(00)00168-0] ] and major/trace element chemistry of abyssal peridotites [Asimow, Paul D. (1999), A model that reconciles major- and trace-element data from abyssal peridotites, Earth Planet. Sc. Lett., 169(3-4), 303-319 [http://dx.doi.org/10.1016/S0012-821X(99)00084-9 doi 10.1016/S0012-821X(99)00084-9] ] .

The realization that polybaric near-fractional melting may be the dominant form of basalt petrogenesis was a consequence of difficulties with a simpler paradigm involving only a chemically distinct primary melt, in equilibrium with residual mantle minerals, undergoing fractionation (and transportation) to yield basaltic and Mid-Ocean Ridge Basalt (MORB) lava. The assumption of a unique primary melt led to the expectation that chemical and mineral characterization of primitive glasses associated with a basalt would constrain the residual mantle mineral assemblage, temperature, and pressure of the (presumed) primary melt. However, such "inverse" modeling as well as "forward" peridotite melting experiments failed to fully constrain underlying processes, necessitating the use of polybaric near-fractional melting [Asimov, Paul D., and Longhi, John (2004), The significance of multiple saturation points in the context of polybaric near-fractional melting, J. Petrol., 45(12), 2349-2367 [http://dx.doi.org/10.1093/petrology/egh043 doi 10.1093/petrology/egh043] ] .

It is possible to incorporate polybaric near-fractional melting considerations into predictive algorithms such as pMELTS [Ghiorso, Mark S. et al. (2002), The pMELTS: A revision of MELTS for improved calculation of phase relations and major element partitioning related to partial melting of the mantle to 3 GPa, Geochem. Geophy. Geosy., 3(5), 1030 [http://dx.doi.org/10.1029/2001GC000217 doi 10.1029/2001GC000217] ] and MAGPOX [Longhi, John (2002), Some phase equilibrium systematics of lherzolite melting: I, Geochem. Geophy. Geosy., 3(3), 1020 [http://dx.doi.org/10.1029/2001GC000204 doi 10.1029/2001GC000204] ] .

References


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