Melting relations of Ca–Mg carbonates and trace element signature of carbonate melts up to 9 GPa – a proxy for melting of carbonated mantle lithologies
<p>The most profound consequences of the presence of Ca–Mg carbonates (CaCO<span class="inline-formula"><sub>3</sub></span>–MgCO<span class="inline-formula"><sub>3</sub></span>) in the Earth's upper mantle may be to lower...
| Main Authors: | , , , , |
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| Format: | Article |
| Language: | English |
| Published: |
Copernicus Publications
2022-10-01
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| Series: | European Journal of Mineralogy |
| Online Access: | https://ejm.copernicus.org/articles/34/411/2022/ejm-34-411-2022.pdf |
| Summary: | <p>The most profound consequences of the presence of Ca–Mg carbonates
(CaCO<span class="inline-formula"><sub>3</sub></span>–MgCO<span class="inline-formula"><sub>3</sub></span>) in the Earth's upper mantle may be to lower the
melting temperatures of the mantle and control the melt composition.
Low-degree partial melting of a carbonate-bearing mantle produces
CO<span class="inline-formula"><sub>2</sub></span>-rich, silica-poor melts compositionally imposed by the
melting relations of carbonates. Thus, understanding the melting relations
in the CaCO<span class="inline-formula"><sub>3</sub></span>–MgCO<span class="inline-formula"><sub>3</sub></span> system facilitates the interpretation of
natural carbonate-bearing silicate systems.</p>
<p>We report the melting relations of the CaCO<span class="inline-formula"><sub>3</sub></span>–MgCO<span class="inline-formula"><sub>3</sub></span> system and
the partition coefficient of trace elements between carbonates and carbonate
melt from experiments at high pressure (6 and 9 GPa) and temperature
(1300–1800 <span class="inline-formula"><sup>∘</sup></span>C) using a rocking multi-anvil press. In the
absence of water, Ca–Mg carbonates are stable along geothermal
gradients typical of subducting slabs. Ca–Mg carbonates
(<span class="inline-formula">∼</span> Mg<span class="inline-formula"><sub>0.1–0.9</sub></span>Ca<span class="inline-formula"><sub>0.9–0.1</sub></span>CO<span class="inline-formula"><sub>3</sub></span>) partially melt
beneath mid-ocean ridges and in plume settings. Ca–Mg carbonates
melt incongruently, forming periclase crystals and carbonate melt between 4
and 9 GPa. Furthermore, we show that the rare earth element (REE) signature
of Group-I kimberlites, namely strong REE fractionation and depletion of
heavy REE relative to the primitive mantle, is resembled by carbonate
melt in equilibrium with Ca-bearing magnesite and periclase at 6 and
9 GPa. This suggests that the dolomite–magnesite join of the
CaCO<span class="inline-formula"><sub>3</sub></span>–MgCO<span class="inline-formula"><sub>3</sub></span> system might be useful to approximate the REE
signature of carbonate-rich melts parental to kimberlites.</p> |
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| ISSN: | 0935-1221 1617-4011 |