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- Title
Melting of carbonated pelite at 5.5–15.5 GPa: implications for the origin of alkali-rich carbonatites and the deep water and carbon cycles.
- Authors
Chen, Xueqian; Wang, Meili; Inoue, Toru; Liu, Qiong; Zhang, Lifei; Bader, Thomas
- Abstract
Melting experiments on a carbonated pelite were performed at 5.5–15.5 GPa, 800–1875 °C using multi-anvil apparatuses to determine the melting phase relations and the P–T stability fields of various phases, which may shed some light on the source of silica-undersaturated magmas and the deep Earth carbon and water cycles. The subsolidus assemblages contain garnet, clinopyroxene, coesite/stishovite at all investigated pressures. Phengite, aragonite or magnesite, and topaz-OH occur below 9.5 GPa. Phase egg, K-hollandite, Ti-oxide, and CAS phase appear at 12–15.5 GPa. Phengite is stable up to 6 GPa and 800 °C, with the phengite-out boundary overlapping with the carbonate-out curve. Thus, the initial melt is carbonatitic and extremely potassium-rich, with K2O/Na2O weight-ratios larger than 40 at fluid-present conditions. The melting reaction phase egg + magnesite + aragonite + (clinopyroxene) + stishovite → melt + garnet + kyanite defines the solidus at 9.5 GPa, 1000–1100 °C. With increasing pressure, the composition of the near-solidus melts gradually evolves from potassium-rich to sodium-rich due to the formation of K-hollandite and the destabilization of clinopyroxene, and as a result of the clinopyroxene-out, the near-solidus melt has the lowest K2O/Na2O value and partitioning coefficient of sodium between clinopyroxene and melt D Na cpx/melt at 15.5 GPa. In addition, phase egg remains stable up to 1400 °C at 15.5 GPa. Thus, phase egg is a good candidate as a deep-water carrier during subduction of pelitic sediments. This study concludes that low degree partial melting of carbonated pelite produces alkali-rich carbonatite melts evolving from potassium-rich (6–12 GPa) to sodium-rich (above 12 GPa) with increasing pressure, and if a slab stagnates at depth, and/or subduction slows down, the produced carbonatite melts will be more silicate-rich with increasing temperature. Moreover, the produced melts generally evolve from relatively silicate-rich to carbonatite-rich with increasing pressure. These alkali-rich carbonatite melts are compositionally similar to those in diamond inclusions, which provides strong evidence for the origin of deep-seated silica-undersaturated carbonatitic magma. Such magma is an ideal metasomatic agent that can give rise to mantle heterogeneity.
- Subjects
CARBON cycle; HYDROLOGIC cycle; CARBONATITES; MELTING; GARNET; METASOMATISM; MAGNESITE
- Publication
Contributions to Mineralogy & Petrology, 2022, Vol 177, Issue 1, p1
- ISSN
0010-7999
- Publication type
Article
- DOI
10.1007/s00410-021-01867-5