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  • 1
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    The energetics of melting fertile heterogeneities within the depleted mantle (2011)
    Wiley
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    Publication Date: 2011-10-21
    Description: To explore the consequences of mantle heterogeneity for primary melt production, we develop a mathematical model of energy conservation for an upwelling, melting body of recycled oceanic crust embedded in the depleted upper mantle. We consider the end-member geometric cases of spherical blobs and tabular veins. The model predicts that thermal diffusion into the heterogeneity can cause a factor-of-two increase in the degree of melting for bodies with minimum dimension smaller than ∼1 km, yielding melt fractions between 50 and 80%. The role of diffusion is quantified by an appropriately defined Peclet number, which represents the balance of diffusion-driven and adiabatic melting. At intermediate Peclet number, we show that melting a heterogeneity can cool the ambient mantle by up to ∼20 K (spherical) or ∼60 K (tabular) within a distance of two times the characteristic size of the body. At small Peclet number, where heterogeneities are expected to be in thermal equilibrium with the ambient mantle, we calculate the energetic effect of pyroxenite melting on the surrounding peridotite; we find that each 5% of recycled oceanic crust diminishes the peridotite degree of melting by 1–2%. Injection of the magma from highly molten bodies of recycled oceanic crust into a melting region of depleted upper mantle may nucleate reactive-dissolution channels that remain chemically isolated from the surrounding peridotite.
    Electronic ISSN: 1525-2027
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Published by Wiley on behalf of American Geophysical Union (AGU).
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  • 2
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    Combined 147,146Sm-143,142Nd constraints on the longevity and residence time of early terrestrial crust (2014)
    Wiley
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    Publication Date: 2014-05-08
    Description: Primordial silicate differentiation controlled the composition of Earth's oldest crust. Inherited 142 Nd anomalies in Archean rocks are vestiges of the mantle-crust differentiation before ca. 4300 Ma. Here, we report new whole-rock 147,146 Sm- 143,142 Nd data for the Acasta Gneiss Complex (AGC; Northwest Territories, Canada). Our 147 Sm- 143 Nd data combined with literature data define an age of 3371±141 Ma (2 SD) and yield an initial ε 143 Nd of -5.6±2.1. These results are at odds with the Acasta zircon U-Pb record, which comprises emplacement ages of 3920-3960 Ma. Ten of our thirteen samples show 142 Nd deficits of -9.6±4.8 ppm (2 SD) relative to the modern Earth. The discrepancy between 142 Nd anomalies and a mid-Archean 147 Sm- 143 Nd age can be reconciled with Nd isotope reequilibration of the AGC during metamorphic perturbations at ca. 3400 Ma. A model age of ca. 4310 Ma is derived for the early-enrichment of the Acasta source. Two compositional end-members can be identified: a felsic component with 142 Nd/ 144 Nd identical to the modern Earth, and a mafic component with 142 Nd/ 144 Nd as low as -14.1 ppm. The ca. 4310 Ma AGC source is ~200 Myr younger than those estimated for Nuvvuagittuq (northern Québec) and Isua (Itsaq Gneiss Complex, West Greenland). The AGC does not have the same decoupled Nd-Hf isotope systematics as these other two terranes, which have been attributed to the crystallization of an early magma ocean. The Acasta signature rather is ascribed to the formation of Hadean crust that was preserved for several hundred Myr. Its longevity can be linked to 142 Nd evolution in the mantle and does not require slow mantle stirring times nor modification of its convective mode.
    Electronic ISSN: 1525-2027
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Published by Wiley on behalf of American Geophysical Union (AGU).
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  • 3
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    Compaction around a rigid, circular inclusion in partially molten rock (2014)
    Wiley
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    Publication Date: 2014-06-14
    Description: Conservation laws that describe the behavior of partially molten mantle rock have been established for several decades, but the associated rheology remains poorly understood. Constraints on the rheology may be obtained from recently published experiments involving deformation of partially molten rock around a rigid, spherical inclusion. These experiments give rise to patterns of melt segregation that exhibit the competing effects of pressure shadows and melt-rich bands. Such patterns provide an opportunity to infer rheological parameters through comparison with models based on the conservation laws and constitutive relations that hypothetically govern the system. To this end, we have developed software tools to simulate finite strain, two-phase flow around a circular inclusion in a configuration that mirrors the experiments. Simulations indicate that the evolution of porosity is predominantly controlled by the porosity-weakening exponent of the shear viscosity and the poorly known bulk viscosity. In two-dimensional simulations presented here, we find that the balance of pressure shadows and melt-rich bands observed in experiments only occurs for bulk-to-shear-viscosity ratio of less than about five. However, the evolution of porosity in simulations with such low bulk viscosity exceeds physical bounds at unrealistically small strain due to the unchecked, exponential growth of the porosity variations. Processes that limit or balance porosity localization should be incorporated in the formulation of the model to produce results that are consistent with the porosity evolution in experiments.
    Print ISSN: 0148-0227
    Topics: Geosciences , Physics
    Published by Wiley on behalf of American Geophysical Union (AGU).
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