Parameterization of the Melting Regime of the Shallow Upper Mantle and the Effects of Variable Lithospheric Stretching on Mantle Modal Stratification and Trace-Element Concentrations in Magmas

Abstract

Parameterization of melting phenomena in the upper mantle has primarily focused on two basic themes, namely the physical and chemical processes that govern partial melting. Parameterization of physical processes mainly refers to establishing relationships between parameters such as the temperature, pressure, matrix and melt flow geometry, lithospheric stretching, and volume of magma. By contrast, parameterization of chemical processes largely implies unravelling the relationships between type and degree of melting, and source and melt composition. Few attempts have been made, however, to interrelate the two processes. The present work is an effort to provide a link between physical and chemical parameters associated with mantle melting and to allow in-depth modelling of partial melting processes in upwelling asthenosphere in a rigorous yet simplified manner. Several correlations among the most important physical parameters (e.g., equilibration and extrusion temperature and pressure of magma, melt fraction and thickness, stretching factor, etc.) are explored. On this basis, a model for the compositional stratification of the lithosphere is proposed, and its bearing on the nature of intra-oceanic arc magmatism is emphasized. Trends of melting residues in terms of modal olivine and clinopyroxene are calculated for a wide range of possible potential temperatures that may be applied to xenolith or abyssal peridotite suites to constrain further their original depth of upwelling. Dry solidus equations for depleted peridotite compositions are also derived that may be used to infer the effects of volatiles on the melting of refractory supra-subduction zone mantle. The sensitivity of certain elements to temperature variations during melting in a column of ascending mantle is highlighted using Ni as an example, and the dangers of using single-value distribution coefficients to predict concentrations of transition metals in magmas are emphasized. MORB-normalized multi-element profiles calculated using a variety of sources, mantle potential temperatures, and stretching factors are presented, and the differences between instantaneous and pooled melts are discussed. A technique to calculate mineral proportions during transformation of garnet lherzolite to spinel lherzolite, together with estimates of the modal composition of fertile spinel and garnet lherzolite are included. Selected trace-element abundances in various sources [bulk silicate Earth, depleted MORB (mid-ocean ridge basalt) mantle, N-MORB] and distribution coefficients for common rock-forming minerals are also tabulated.