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A common origin of carbonatite magmas
2017 (English)In: Geology, ISSN 0091-7613, E-ISSN 1943-2682, Vol. 45, no 6, p. 507-510Article in journal (Refereed) Published
Abstract [en]

The more than 500 fossil Ca-carbonatite occurrences on Earth are at odds with the only active East African Rift carbonatite volcano, Oldoinyo Lengai (Tanzania), which produces Na-carbonatite magmas. The volcano’s recent major explosive eruptions yielded a mix of nephelinitic and carbonatite melts, supporting the hypothesis that carbonatites and spatially associated peralkaline silicate lavas are related through liquid immiscibility. Nevertheless, previous eruption temperatures of Na-carbonatites were 490-595 degrees C, which is 250-450 degrees C lower than for any suitable conjugate silicate liquid. This study demonstrates experimentally that moderately alkaline Ca-carbonatite melts evolve to Na-carbonatites through crystal fractionation. The thermal barrier of the synthetic Na-Ca-carbonate system, held to preclude an evolution from Ca-carbonatites to Na-carbonatites, vanishes in the natural system, where continuous fractionation of calcite + apatite leads to Na-carbonatites, as observed at Oldoinyo Lengai. Furthermore, saturating the Na-carbonatite with minerals present in possible conjugate nephelinites yields a parent carbonatite with total alkali contents of 8-9 wt%, i. e., concentrations that are realistic for immiscible separation from nephelinitic liquids at 1000-1050 degrees C. Modeling the liquid line of descent along the calcite surface requires a total fractionation of similar to 48% calcite, similar to 12% apatite, and similar to 2 wt% clinopyroxene. SiO2 solubility only increases from 0.2 to 2.9 wt% at 750-1200 degrees C, leaving little leeway for crystallization of silicates. The experimental results suggest a moderately alkaline parent to the Oldoinyo Lengai carbonatites and therefore a common origin for carbonatites related to alkaline magmatism.

Place, publisher, year, edition, pages
2017. Vol. 45, no 6, p. 507-510
National Category
Geochemistry Geology
Identifiers
URN: urn:nbn:se:uu:diva-375153DOI: 10.1130/G38801.1OAI: oai:DiVA.org:uu-375153DiVA, id: diva2:1283100
Available from: 2019-01-28 Created: 2019-01-28 Last updated: 2019-01-28

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