The solidus of carbonated eclogite in the system CaO–Al2O3–MgO–SiO2–Na2O–CO2 to 32 GPa and carbonatite liquid in the deep mantle

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Abstract

Melting phase relations have been determined in a model carbonated eclogite (5 wt.% CO2) at 10.5–32.0 GPa and 1300–1850 °C. The assemblage of silicate minerals coexisting with partial melts changes with pressure from garnet–omphacite–kyanite–stishovite at 10 GPa via garnet–corundum–stishovite at 16–20 GPa to Mg–perovskite–Ca–perovskite–CF phase–stishovite at 27–32 GPa. Magnesite is the only carbonate stable in this system through the studied pressure range. The solidus temperature was defined by the appearance of partial melt. The solidus of carbonated eclogite is bracketed at 1380–1460 °C at 10.5 GPa, 1460–1560 °C at 16.5 GPa, 1530–1630 °C at 20 GPa, and 1600–1790 °C at 27 and 32 GPa. The slope of solidus curve is less steep at 10–32 GPa than at lower pressures. The solidus curve of Fe-free carbonated eclogite roughly coincides with an average mantle geotherm. Partial melts formed by melting of carbonated eclogite at 10.5–32.0 GPa have magnesiocarbonatite compositions with Ca/Mg ratios higher than in similar melts in peridotite assemblages, and contain high Na2O-contents. It has been demonstrated that carbonatite-like melt can be generated by partial melting of carbonated eclogites at pressure up to at least 32 GPa, i.e. to lower mantle depths.

Introduction

Subduction of oceanic basalts and sediments plays an important role in creating mantle heterogeneity and in transporting volatile components back to the mantle. Calcium carbonate sequestered in oceanic crust by hydrothermal alteration accounts for a significant flux of carbon into the mantle. A significant part of this carbonate resides in the top portion of basaltic rocks, where CO2 concentrations may exceed 3 wt.% (Alt and Teagle, 1999). Thermal modeling of subducting slabs coupled with thermodymamic and experimental studies of phase relations indicates that most carbonates can be transported to the deep mantle without decarbonation during hydrous melting beneath the island arcs (e.g. Bebout, 1995, Molina & Poli, 2000, Kerrick & Connolly, 2001, Connolly, 2005, Poli et al., 2009). Consequently, relatively dry carbonated eclogite may transport carbonates to the deep mantle. Possible melting of carbonated eclogite in the mantle can affect melt and fluid compositions and enrich source regions of different types of basaltic and ultramafic volcanics with trace elements and volatiles. Thus, determination of the solidus of carbonated eclogite is important for understanding deep carbon dynamics and cycling. Comparison with solidi of carbonated peridotite is also important to determine potential hosts for carbon and carbonate in the convecting upper mantle (e.g. Hirschmann and Dasgupta, 2009).

Carbonatite, lamproite, and kimberlite magmas, originated at pressures of at least 4–8 GPa, undoubtedly prove the existence of a significant source of CO2 in the Earth's mantle. The presence of carbonate in the mantle is also apparent through the occurrence of carbonate in mantle xenoliths and inclusions in diamonds (e.g. Ionov et al., 1996, Wang et al., 1996, Leost et al., 2003). Inclusions of carbonatite melt in fibrous diamonds have also been reported (e.g. Zedgenizov et al., 2004). Recently, Brenker et al. (2007) found carbonate inclusions in ultra-deep diamonds from Juina (Brazil), which have presumably been transported from the transition zone or even from the lower mantle, suggesting that Earth's global CO2-cycle has an ultradeep extension. Due to their low density and viscosity and high reactivity, carbonatite melt and fluid are thought to be amongst the most important metasomatic agents in the sublithospheric mantle (e.g. Bell, 1989) and may be as important at greater depths.

Previous experimental studies of carbonated eclogite systems were concentrated on pressures below 10 GPa. It has been demonstrated that the shape of the eclogite-CO2 solidus curve has a backbend, caused by stabilization of dolomite or calcite, with increasing pressure. The shape of the solidus and position of the maxima vary in different systems. The solidus temperature increases with increasing Ca# [= Ca/(Ca+MgO+FeO)] of the bulk compositions and depends on total CO2 content (Hammouda, 2003, Dasgupta et al., 2004, Dasgupta et al., 2005, Yaxley & Brey, 2004). Dasgupta et al. (2006) reported the existence of liquid immiscibility between carbonatite and silicate melts in carbonated eclogite at 3 GPa and applied the results to the origin of low silica ocean island lavas.

Here we present melting phase relations of simplified carbonated eclogite containing 5 wt.% CO2 from 10 to 32 GPa and temperatures between 1300 °C and 1800 °C to clarify the solidus of carbonated eclogite to lower mantle depths, and discuss near solidus melt compositions with implications for the origins of carbonatite-like magmas in the deep mantle.

Section snippets

Experimental technique

The starting material for the experiments was an oxide mixture, which was very close to average mid-ocean ridge basalt (e.g. Pertermann and Hirschmann, 2003) and simplified to the CaO–MgO–Al2O3–SiO2–Na2O (hereafter CMASN) system according to the procedure described by O`Hara (1968). 5.0 wt.% of CO2 was added as MgCO3, CaCO3, and Na2CO3 (adjusting the proportion of related oxides) (Table 1). We used an Fe-free composition to avoid temperature- and run duration-dependent Fe loss to the Pt capsule

Phase relations and solidus temperatures in carbonated eclogite

The phase relations in carbonated eclogite are presented in Table 2 and Fig. 3. The solidus temperatures were determined using the disappearance of magnesite and the appearance of visible carbonatite melt. We also used data on the temperature gradient inside the sample capsule (see Litasov and Ohtani, 2009a) to estimate solidus temperature more precisely.

The subsolidus phase assemblage at 10.5 GPa and 1300 °C includes garnet, clinopyroxene, stishovite, kyanite, and magnesite. Both magnesite and

Solidus of carbonated eclogite

The melting temperature of volatile-bearing systems is much lower than that of volatile-free systems. The solidus of eclogite + 5 wt.% CO2 is located 400–500 °C below the volatile-free solidus at 10 GPa (Fig. 3, Fig. 8). The solidi of carbonated eclogite at lower pressures were placed at very different temperatures. Hammouda (2003) demonstrated that the solidus of Ca–carbonate-bearing eclogite is located at low temperatures near 1000 °C at 6–10 GPa, whereas Yaxley and Brey (2004) placed the solidus of

Conclusions

Melting phase relations have been determined in a model carbonated eclogite (5 wt.% CO2) at 10.5–32.0 GPa and 1300–1850 °C. The assemblage of silicate minerals coexisting with partial melts changes with pressure from garnet–omphacite–kyanite–stishovite at 10 GPa to Mg–perovskite–Ca–perovskite–CF phase–stishovite at 27–32 GPa. Magnesite is the only carbonate stable in eclogite throughout the pressure range.

The solidus temperature was defined by the stability of magnesite and the appearance of partial

Acknowledgements

We thank R. Dasgupta, G. Yaxley and an anonymous reviewer for thorough reviews and suggestions and M. Weinberger for technical corrections that improve the manuscript. This work was supported by the grants in aid for Scientific Researches from the Ministry of Education, Culture, Sports, Science and Technology, Japan (No 14102009 and 16075202), to EO and grant in aid for young scientists from Japan Society for Promotion of Science (No 17740344) and Russian Foundation for Basic Research grant no

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