2008 Vienna

1. 2008 Vienna Abstract: It is now widely accepted that large-scale mantle melting initiated by energetic bolide impacts must have been a common event during the early history of the Earth, and igneous petrology can be used to address the fate of these mega-melts. In other words, the same 1000 km scale impact craters seen on the Moon did not form on Earth, but instead are envisaged to have formed gigantic melt-filled basins, of the order of >25 to 100 km thick, which almost certainly differentiated during slow cooling, and may have helped to initiate the crustal dichotomy between sialic and basaltic terrains (Therriault, Grieve et al LPSI 2001). Different physical models for large impacts processes differ in detail, but converge on this result that for impact events above some critical size (few hundred kilometres effective crater diameter), the volume of impact melt exceeds the transient crater volume. In addition, the contribution to substantially increasing volumes of impact melting (> 10e6 km3; Jones et al EPSL 2002) derived from decompression of the underlying mantle remains an important but largely unexplored concept. The Sudbury impact melt sheet serves as a newly recognised example of differentiation from super-liquidus melts of re-melted sialic crust, but what about similar events in purely basaltic or peridotitic terrains (ie oceanic)? Igneous petrology provides the philosophical criteria for predicting how such processes as immiscible liquid fractionation and subliquidus phase fractionation will occur during thermodynamic crystallisation over the pressure range of what are effectively discrete small magma oceans. However, there is a huge gap in our knowledge of the high temperature data required,which extends well above the liquidus for silicates (>> 1900 K) at relatively low pressures (< 5 GPa). Hence, a new era of experimental igneous petrology is urgently required to understand these superheated events in the Earth’s ancient mantle. To put this into context, we can imagine the instantaneous formation of a gigantic superheated melt disc whose vertical height surpasses the total depth of even the thickest crust. How did such a superhot lid influence the underlying mantle? Lastly, such nonplume ideas (impact volcanism) need not be confined to the time period of the early Earth, and have been proposed to operate during the Mesozoic (Ontong Java Plateau; Jones Elements 2005) and on other planets, including Venus (Hansen JGR 2006). NEW STUFF OLD STUFF Impact volcanism and upper mantle melting; mega-meltingAdrian P Jones,Department of Earth Sciences, University College London, Gower Street, London WC1E 6BTemail: adrian.jones@ucl.ac.uk NEW Igneous Petrology FRAMEWORK For Impact Magma Oceans Hadean impact craters into basaltic and peridotitic lithosphere producing ~ million km 3 impact melts, as open or crusted small regional magma oceans. Major melt fractionation processes share revolutionary petrological concepts developed for (1) the ~200 km scale Sudbury impact melt sheet (Keays and Lightfoot, 2004) and (2) ~10,000 km (Earth-Moon) scale global magma ocean model (eg Solomatov 2000). Unlike a magma ocean, the walls and floor of the magma body are relatively well confined to ~100-200 km depths. This model ignores large-scale secondary melting or “impact plume” initiation beneath the impact site, and ignores seawater interaction. Stage 1 (~10s s): SUPERHEAT Impact melt volume rapidly increasing, superheated liquids, vapour phase, violent phase disequilibrium, vesiculation from self vapour possible (subsequently "void"). Stage 2 (~ hours): SUPERLIQUIDUS Liquid phase, melt volume ~constant, surface vapour condensed, all liquid phenomena; emulsions, immisicibility, liquid-liquid frationation, gravity sensitive, low viscosity melt systems, cooling still above liquidus at shallow depths. Strongly disequilibrium processes, strirring and reservoir segregations; wall rock digestion. Subcrater melting persists perhaps ~doubling of melt volume. Stage 3 (>>years): SUPERSOLIDUS Crystallisation phase, melt volume thermodynamically controlled, classical igneous petrology starts, P-T gradients Approaching equilibrium processes. Increasing viscosity, slow cooling. The large scale of the magma bodies result in long geological-scale time periods during which conventional igneous petrology may be applied, including phase diagrams for peridotite upper mantle appropriate to a range of ultrabasic to basaltic melt compositions (from komatiite to basaltic). There is very little experimental data to quantify Stages 1 and 2, since most petrological experiments have been guided by the Earth geotherm, and inhibited by convention to avoid conditions of superheat. With the recognition that major sulphide ore-forming processes likely occur under large impact conditions, there is an urgent need to design a new era of very high-T petrological experiments, in order to derive new geochemical and mineralogical data appropriate to liquid-liquid fractionation, and isotopic exchange within vapour-liquid-solid silicate systems. 2 FIGURE 3. Hadean MOHO A simple phase diagram for pyrolite or chondritic mantle peridotite shows the inescapable fact that magma oceans may have triggered the first segregation of low density feldspar-rich melts from higher density feldspar-free peridotite, implying that the Moho and feldspar-rich crust is a natural product of impact magma oceans. 1 FIGURE 2. TOTAL MELT VOLUME produced, including shock melt (~1 million km3) and post-shock melt ((~1 million km3) produced in an Ontong Java simulation (Jones et al 2005). Two apparent rates of melt production observed may be related to the rate of energy to heat/ mass transfer (post shock) and are likely to reflect a transition from vapour present (1) superheated melts and (2) superliquidus melts. A third and much longer supersolidus phase (3) of volume reduction is thermodynamically constrained to perhaps ~millions of years and has not yet been modelled, although all the likely parameters involved are set out in a review of terrestrial magma ocean fluid dynamics by Solomatov (2000). Crystal kinetics denies glass formation, even in the mantle, and predcits grain sizes of ~mm scale (Solomatov 2000). FIGURE 1. Impact model of melt distribution (left hand side) resulting from large bolide impact into young, relatively hot oceanic lithosphere (uniform lithostatic P-T gradients and parameterised peridotite melting model); snapshots cover the period of about 10 minutes duration, during which time approximately >106km3 melt forms, ranging from superheated liquid peridotite (red - yellow: komatiite) to lower temperature melt (blue; basaltic). After Jones et al (2005) Box is 700 km across by 150 km deep. REFERENCES Hansen V L (2006) JGR 111, E11010 Jones AP et al 2002 EPSL, 551-561 Jones A P et al 2005 Geol Soc Am, S Pap 388, 711-720 Jones A P 2005 Elements, 1, 277-281 Keays R R and Lightfoot P C 2004 Mineralogy and Petrology 82, 217-258 Solomatov V S (2000) Fluid dynamics in a Terrestrial magma ocean, in Origin of Earth and Moon, Arizona Press, 323-338.