HR: 16:15h
AN: V54A-02    [Abstracts]
TI: Compositional Convection Trumps Silicate Liquid Immiscibility (and Other Matters)
AU: * Morse, S A
EM: tm@geo.umass.edu
AF: Geosciences, Univ. Mass., 611 North Pleasant St., Amherst, MA 01003-9297, United States
AB: Published data for the Skaergaard (Skd) intrusion through MZ-UZc suggest that all the average cumulates are hyper-mafic and poor in felsic components relative to a cotectic condition. The "lost" felsic components presumably reside in the SHR and UBS. Reports of conjugate mafic-felsic liquids in Skd have always run afoul of the difficult extrapolation of scale from the microscopic emulsion to the megascopic separation of two liquids in the intrusion. This is a false question. When Mt+ occurs below the base of MZ, the cumulates are exceedingly dense and are nearly perfect adcumulates (shown by least plagioclase zoning: Toplis et al. ms CMP 07). The resulting rejected solute (RS) is, therefore, clearly felsic, buoyant, hot, and evolved relative to the resident bulk magma. This RS will rise freely in plumes and collect near the roof. The challenge to the 2-liquid hypothesis is, then, how to distinguish it from classical cumulate theory. From current evidence, this challenge cannot be met. Compositional convection is a well-understood and quantifiable property of freezing liquids, from the formation of sea ice to the crystallization of the inner core of the Earth and the corrosion of troctolite above thin dunite sills at Rum. Its power has been quantified to a first approximation (Morse 1986 JPet 27:1183) and shown to be appreciable. In contrast, the ability of emulsions with high variance of NBO/T to separate megascopically has never been shown. The probability of solute rejection and pluming of felsic residua is 1.0, whereas the contrary hypothesis of liquid separation has no known probability. Even if it occurred, it would be redundant in the face of assured compositional convection. The issue is moot at best, and unworthy of further exploration. Both ideas produce an evolving felsic melt near the intrusion roof, and that leads to increasing polymerization and inability to achieve adcumulus growth, so the UZ of the intrusion has greater plagioclase zoning as evidence of more orthocumulus growth. Multiphase Rayleigh fractionation and the internal reservoir of large magma chambers (Morse 2007 abstr Frontiers Min Sci, minersoc.org p. 228) may explain the lower Mg No. of sub-cotectic (i.e., scarcer) pyroxene in a Bushveld drill core: the local depletion factor overwhelms any recharge from within the magma reservoir (if present).... Other matters concern sawtooth compositional profiles in some intrusions: these represent occasional or regular involvement of the internal reservoir, alternating with isolation from it; they therefore represent magma-dynamic oscillations rather than clever (Mg-wise) external recharge events....The "M&M" packing fraction of PL+Ol troctolites may be as high as 0.8 or more, rather than the 0.6 obtained from Al tablets in water by Wager et al. (1960)....The chief and now archaic problem of layered intrusions is to refine the cooling history with time using realistic boundary conditions. A recent primary advance has been that of Holness and her group at Cambridge (JPet submitted), who demonstrate that cumulate maturation at Skd reflects aspects of the liquidus history of the magma body. In short, the enthalpy budget goes discontinuously from dominance by specific heat to latent heat at each addition of a cumulus (liquidus) crystal phase, to an isothermal end point where the enthalpy transfer to the surroundings is all latent heat and no sensible heat. These findings represent true advances in the understanding of the evolution of mafic magma bodies, of which layered intrusions are our most precious relic.
DE: 3618 Magma chamber processes (1036)
DE: 3640 Igneous petrology
DE: 3643 Layered magma chambers
DE: 8145 Physics of magma and magma bodies
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2007 Fall Meeting