A Simple Thermodynamic Model for Melting of Peridotite in the System NCFMASOCr

Abstract

A new thermodynamic model is presented for calculating phase relations in peridotite, from 0Á001 to 60 kbar and from 800 C to liquidus temperatures, in the system NCFMASOCr. This model system is large enough to simulate phase relations and melting of natural peridotite and basaltic liquids. Calculations in the program THERMOCALC illustrate mantle phase relationships and melting conditions, specifically for the peridotite composition KLB-1. The garnet-spinel transition zone intersects the solidus at 21Á4-21Á7 kbar, where both Fe 3þ and Cr increase spinel stability, expanding the width of the transition. Orthopyroxene is lost at the solidus at 42 kbar in KLB-1, although this pressure is very sensitive to bulk composition. Calculated oxidation states are in excellent agreement with measured log f O 2 for xenolith suites with mantle Fe 2 O 3 contents in the range 0Á1-0Á3 wt %. It appears that mantle oxidation state is not just a simple function of P and T, but depends on phase assemblage, and may vary in a complex way within a single assemblage. The liquid model performs well, such that calculated solidus, melt productivity and liquid compositions compare favourably with those of experimental studies, permitting its use in interpolating between, and extrapolating from, experimental P-T conditions. Experimentally challenging but geologically useful regimes can be explored, such as subsolidus samples and very low melt fractions, with application to both mantle xenoliths and the origin of basalt.

Quantitative Relationships Between Basalt Geochemistry, Shear Wave Velocity, and Asthenospheric Temperature Beneath Western North America

“…However, Green et al (2012) and Jennings and Holland (2015) argue that the pressure of the garnet-spinel transition was overestimated in previous experimental studies, largely due to the simplicity of the phase systems used (i.e., Mg-Al-Si rather than Ca-Mg-Al-Si). Jennings and Holland (2015) demonstrate that their model compares well to existing studies, provided that simplifying corrections are applied, notably allowing for Ca activity within garnet. Samples from RGR include 671, 672, 674-676, 678, 695, 699, 860, 864-866, 869, 870, 875, 879, 882, 883, 888, 894, 895, 898, 6100, 6102-6104, 6108, 6110, 6130, 6140, 6143, 6151, 6152, 6155, 6157, 6158, 6185, 6187, 8101, 8103, 8107, 8109-8112, 8128, 8129, 8134, 8136, 8138-8140, 8144, 8157, 8159-8161, and 8164 from R. Thompson et al (2005).…”

“…2, 11, 13b, 18, 19b, 22, 23b, 26, 27, 30, 31b, 32-35, SC 07 03, and SC 07 05 generously provided by T. Plank (personal communication, 26 September 2015) and Plank and Forsyth (2016). By contrast, increasing concentrations of Cr and Fe 3+ within peridotite tend both to increase the thickness of the transition and to cause a shift to greater pressures due to the greater stability of spinel (Jennings & Holland, 2015;Klemme & O'Neill, 2000). Jennings and Holland (2015) demonstrate that their model compares well to existing studies, provided that simplifying corrections are applied, notably allowing for Ca activity within garnet.…”

Application of thermodynamic modelling to natural mantle xenoliths: examples of density variations and pressure–temperature evolution of the lithospheric mantle

“…Among the available sets of endmember properties and solution models for Cr-bearing phases (e.g. Klemme et al 2009;Ziberna et al 2013, Jennings andHolland 2015), we adopted the model suggested by Klemme et al (2009) which was later on refined by Ziberna et al (2013). This model shows good agreement with the available experimental data in both Cr-poor and Cr-rich mantle Compositions from LS1 to BN4 were taken from Stern et al (1999) and were normalized to 100 wt% after removing TiO compositions (e.g.…”

“…However, the recent advances in thermodynamic modelling techniques and refinements of thermodynamic databases may help to investigate subsolidus phase relations in simple (e.g. Klemme 2004;Green et al 2012) and complex (Asimow and Ghiorso 1998;Klemme et al 2009, Ziberna et al 2013, Jennings and Holland 2015 mantle compositions. In two previous papers, Klemme et al (2009) and Ziberna et al (2013) focused more on methods and calibration of a thermodynamic model for Cr-bearing mantle rocks, so the aim of this paper is to show how this thermodynamic model may be applied to investigate processes in the upper mantle.…”

Petrological Geodynamics of Mantle Melting I. AlphaMELTS + Multiphase Flow: Dynamic Equilibrium Melting, Method and Results

“…The chemical thermodynamic approach developed over several years by Ghiorso (Ghiorso, 1985;Ghiorso and Sacks, 1995;Ghiorso et al, 2002) specifically includes the melt phase and provides a more rigorous understanding of the petrological evolution at different P,T,X,fO 2 conditions. Recently new thermodynamic descriptions of mantle melting have been presented (Ueki and Iwamori, 2014;Jennings and Holland, 2015). An advantage of the thermodynamic formulation is that, within certain limits, it creates a framework that allows also to make predictions at conditions not interested by experimental studies.…”