Abstract
The thermoelastic properties of K0.7Na0.3AlSi3O8 hollandite and NaAlSi2O6 jadeite, synthesized from a (K, Na)-felspar (microcline), were investigated by a combination of in situ energy dispersive synchrotron X-ray radiation and multi-anvil techniques at high pressure (P) and temperature (T) up to 21 GPa and 1700 K. The second-order phase transformation was found to occur in hollandite at ~16 GPa from tetragonal I/4m (hollandite-I) to monoclinic I2/m (hollandite-II), which confirms the previous report that the incorporation of Na in the hollandite structure decreases the transformation pressure. Fitting the pressure–volume–temperature data to the Birch–Murnaghan equation of state yielded estimates of the thermoelastic parameters for jadeite as well as the K0.7Na0.3AlSi3O8 hollandite-I and -II phases, which indicate that the incorporation of Na is likely to decrease the bulk moduli of both hollandite phases. The obtained thermoelastic parameters were combined with those of other mantle minerals reported previously to estimate the density of continental materials along an average mantle geotherm. Based on our results, continental crust and sediment become, respectively, 11% and 15% denser than the pyrolitic mantle at pressure >10 GPa, suggesting that once pulled down to the critical depth of ~300 km, the continental portions of the slab can subduct further into the deep mantle, down to the lowermost part of the mantle transition region.
Highlights
Geological observations suggest that the subduction of continental crust may have occurred through time and transported extensive amounts of continental crust and sedimentary materials into the Earth’s mantle [1], which can be traced by the diamond inclusions of minerals such as K-rich hollandite [2,3]
At the mantle transition region (MTR) depths, petrological works have shown that continental crust materials transform to a mixture of garnet, stishovite, pyroxene, hollandite, and Ca-rich aluminosilicate (CAS) phase [4,5]
There have been only a few works that addressed the effect of cationic substitutions on the elastic properties of those minerals under the relevant pressure and temperature conditions, which hampers precise estimates of the density of the continental crust in the deep mantle. (K,Na)AlSi3 O8 hollandite is the solid solution in the KAlSi3 O8 liebermannite [13] and NaAlSi3 O8 lingunite [14] joint system, and jadeite is a Na-rich clinopyroxene
Summary
Geological observations suggest that the subduction of continental crust may have occurred through time and transported extensive amounts of continental crust and sedimentary materials into the Earth’s mantle [1], which can be traced by the diamond inclusions of minerals such as K-rich hollandite [2,3]. The density estimates made by previous studies were, mostly based on assuming the thermoelastic properties of the endmembers of continental crust minerals [11,12] To this day, there have been only a few works that addressed the effect of cationic substitutions on the elastic properties of those minerals under the relevant pressure and temperature conditions, which hampers precise estimates of the density of the continental crust in the deep mantle. There have been only a few works that addressed the effect of cationic substitutions on the elastic properties of those minerals under the relevant pressure and temperature conditions, which hampers precise estimates of the density of the continental crust in the deep mantle. These two phases are very specific to continental crust compositions where they represent up to ~40 vol.% of the phase assemblages at high pressure, while they do not appear in pyrolite or mid-ocean ridge basalt (MORB) compositions at the MTR pressures [15,16,17]
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