Abstract
The composition of spinel lherzolite xenolith KLB-1 from the Kilbourne Hole volcanic crater, United States, which is close to the composition of the Earth’s primitive mantle, was used for thermodynamic modeling of phase relations in the Na2O–CaO–FeO–MgO–Al2O3–SiO2 system (NCFMAS) using the Perple_X software package in the temperature range of 900–2000 °C and pressures of 0.0001–30 GPa. The calculated phase diagram is in good agreement with published thermodynamic data on KLB-1 composition and reveals the peculiarities of mantle mineral assemblages at P–T parameters on which experimental data are insufficient or absent. The results showed that the mineral assemblage of garnet wehrlite (garnet + olivine + clinopyroxene), the least common type of mantle peridotite on the Earth’s surface, prevails in the upper mantle since the Archean. Mineral assemblage of garnet lherzolite (garnet + olivine + clinopyroxene + orthopyroxene), which is a variety of mantle peridotites most widely found on the Earth’s surface, is formed in the lithospheric mantle because its temperatures are lower than those of the convecting mantle. Thermodynamic modeling reveals a ringwoodite-free field in the P–T diagram (located at the bottom of the mantle transition zone), which is crosscut by Archean adiabats and by the geotherms of Archean and the hottest Phanerozoic plumes. This area causes a change, from negative to positive, in the slope of the boundary between the lower mantle and the mantle transition zone. A positive slope of the boundary in the Archean should have stimulated the ascend of lower mantle plumes through the transition zone. Conversely, this boundary has a negative slope for most Phanerozoic plumes, rising from the lower mantle, and as a result, the plumes either slow down or stop.
Highlights
Geodynamic, petrological, and geochemical processes are largely controlled by phase relations in peridotites
Note that much of the diagram is covered by the field of the mineral assemblage of garnet wehrlite (Grt + Ol + Cpx)1, which occurs between the modern adiabat and the dry liquidus line (Fig. 1)
An olivine polymorph, mark the upper boundary of the mantle transition zone by a line positively sloped in P–T diagram at T > 1100°C
Summary
Geodynamic, petrological, and geochemical processes are largely controlled by phase relations in peridotites. Petrological studies of metamorphic complexes are lately often carried out with the application of the Perple_X (Connolly, 2009) and THERMOCALC (Powell et al, 1998) software packages. Their usage, together with thermodynamic data on deep mantle minerals (Jennings and Holland, 2015; Holland et al, 2013; Stixrude and Lithgow-Bertelloni, 2011), opens new avenues for studying phase relations in peridotites and other lithologies in different mantle layers (e.g., Klein et al, 2017). The average compositions of the mantle currently most widely used by various researchers can be classified into the following three major groups (Table 1). The first one comprises model compositions of the so-called primitive mantle (Primitive Mantle or Bulk Silicate Earth) that had occurred during the early Earth’s evolution, Component
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