Melting phase relations of model carbonated peridotite from 2 to 3 GPa in the system CaO‐MgO‐Al₂O₃‐SiO₂‐CO₂ and further indication of possible unmixing between carbonatite and silicate liquids

Abstract

Melting phase relations of model carbonated peridotite in the system CaO-MgO-Al 2 O 3 -SiO 2 -CO 2 from 2 to 3 GPa are reported. Experimentally produced melts, which are model carbonatites, with approximately 36-40 wt % CaO, 12-17 wt % MgO, 0.2-1.5 wt % Al 2 O 3 , 1-4 wt % SiO 2 , and 40-42 wt % CO 2 (carbon dioxide) are present at all pressures investigated. At 2.8 and 3 GPa, carbonatitic melts are seen experimentally at temperatures that are very close to the vapor-free (CO 2 ) peridotite solidus and are found in equilibrium with forsterite, orthopyroxene, clinopyroxene, and garnet. Solidus phase relations with isobaric and pressure-temperature invariant points, defining the so-called carbonated peridotite solidus ledge, are also reported from 2.1 to 3 GPa. A divariant region exists from 2 to 2.6 GPa wherein two, compositionally different melts are present. In this region, these two melts, carbonatitic and silicate in composition, coexist with crystalline phase assemblage and free vapor. The silicate liquid has approximately 30-48 wt % SiO 2 and approximately 6 to 20 wt % of dissolved CO 2 . The presence of carbonatitic and silicate liquids is interpreted to be due to liquid immiscibility. On the basis of melting phase relations reported here, we conclude that (a) the ledge is a feature along which model carbonatitic liquids are produced by reaction of silicates and CO 2 vapor and (b) alkali-free carbonatites and silicate melts can form through melt unmixing at depths of~60-80 km in the Earth's mantle.

A window in the course of alkaline magma differentiation conducive to immiscible REE-rich carbonatites

“…The D Ca CL/SL model is calibrated on a large experimental database (Freestone and Hamilton, 1980;Hamilton et al, 1989;Kjarsgaard et al, 1995;Jones et al, 1995;Brooker, 1998;Kjarsgaard, 1998;Veksler et al, 1998;Veksler et al, 2012;Martin et al, 2012;Martin et al, 2013;Massuyeau et al, 2015), which regroups 120 experimental data of immiscible carbonate and silicate liquids in natural-like systems, at pressure and temperature respectively ranging from 0.04 to 3.2 GPa and 700 to 1400 °C (Table 6). Synthetic systems (Kjarsgaard and Hamilton, 1988;Brooker and Hamilton, 1990;Lee and Wyllie, 1996;Brooker and Kjarsgaard, 2011;Keshav and Gudfinnsson, 2013;Novella et al, 2014) are excluded from this calibration because of their compositions being at odds with the alkaline magma systems. All the silicate melts from this experimental database (including data from this study) have composition evolving from strongly peralkaline to slightly metaluminous melts (ASI varies from 0.06 to 0.84; AI varies from -0.35 to 0.03; see Fig.…”

The Link between the Physical and Chemical Properties of Carbon-Bearing Melts and Their Application for Geophysical Imaging of Earth’s Mantle

“…41,42 The link between a two-subnetwork atomic structure and macroscopic separation in a two-liquid system remains unclear despite its major geochemical importance. 43,44 Finally, the transport properties of such a melt are difficult to predict a priori, as the silicate component implies high viscosity while the carbonate component implies low viscosity. 45 Figure 7.4 MD-generated snapshots of a carbo-silicate melt (17 wt.% SiO 2 and 28 wt.% CO 2 ) at 8 GPa and 1727 K. In the left panel, all atoms are depicted (SiO 4 in yellow and red, CO 3 in cyan and red, Mg in green, Ca in cyan, Na in blue, K in pink, and Fe in purple, with Al and Ti not being represented for clarity reasons).…”

Orthopyroxene survival in deep carbonatite melts: implications for kimberlites

“…This reaction is the same reaction that forms the "ledge" at ~2.8 GPa in the model lherzolite system CaO-MgO-SiO 2 -CO 2 (Eggler 1975;Huang 1975a, 1976;Eggler 1978;Moore and Wood 1998;Lee et al 2000), and the idea that exsolution of CO 2 from ascending carbonatitic and kimberlitic melts upon intersecting this reaction would take place dates back to the 1970s as well (e.g., Wyllie and Huang 1975a, b). The analogous reaction in the CaO-MgO-Al 2 O 3 -SiO 2 -CO 2 is of the form: (Dalton and Presnall 1998a;Novella et al 2014). This reaction lies at lower pressures than does (1), and our run conditions were such that our experiment was above this reaction.…”