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Hot mantle geotherms stabilize calcic carbonatite magmas up to the surface

Tahar Hammouda, Julien Chantel, Geeth Manthilake, Jeremy Guignard and Wilson Crichton
Hot mantle geotherms stabilize calcic carbonatite magmas up to the surface
Geology (Boulder) (October 2014) 42 (10): 911-914


The eruption of calciocarbonatites at Earth's surface is at odds with them being equilibrated with the mantle at depth because high-pressure experimental studies predict that significant magnesium contents should be expected. Here we report on new high-pressure experiments that demonstrate extreme calcium enrichment of carbonatites en route to the surface. We have monitored the decompression of partially molten carbonated peridotite using a multianvil apparatus coupled to synchrotron radiation. The experimental charge was molten at high pressure and high temperature, before being decompressed along a path that avoided the so-called "carbonate ledge" (a boundary that prevents carbonatitic melts from reaching the surface). Reaction with clinopyroxene yields calcium enrichment and magnesium depletion. The resulting Ca/(Ca + Mg) of the quenched melt reaches 0.95, which compares well with the composition of erupted calcic carbonatites [Ca/(Ca + Mg) approximately 0.96-0.99] and of calcic melts trapped in mantle xenoliths from ocean islands [Ca/(Ca + Mg) approximately 0.84-0.97]. Our results demonstrate that it is possible to bring carbonatites very close to the surface, without breakdown, and therefore without catastrophic CO (sub 2) release. Such occurrence appears to be favored by hot geotherms, meaning that higher temperatures tend to stabilize carbonatitic melts at shallow mantle pressure. Carbonatitic magmas are usually associated with low temperatures, because of the assumed low melting degree or low eruption temperature of the only active carbonatite volcano (i.e., Oldoinyo Lengai, Tanzania). Here we show that emplacement of carbonatites at or near the surface necessitates a hot environment.

ISSN: 0091-7613
EISSN: 1943-2682
Serial Title: Geology (Boulder)
Serial Volume: 42
Serial Issue: 10
Title: Hot mantle geotherms stabilize calcic carbonatite magmas up to the surface
Affiliation: Universite Blaise Pascal, Laboratoire Magmas et Volcans, Clermont-Ferrand, France
Pages: 911-914
Published: 201410
Text Language: English
Publisher: Geological Society of America (GSA), Boulder, CO, United States
References: 38
Accession Number: 2014-082865
Categories: Geochemistry of rocks, soils, and sedimentsIgneous and metamorphic petrology
Document Type: Serial
Bibliographic Level: Analytic
Illustration Description: illus. incl. 1 table
Secondary Affiliation: European Synchrotron Radiation Facility, FRA, France
Country of Publication: United States
Secondary Affiliation: GeoRef, Copyright 2017, American Geosciences Institute. Reference includes data from GeoScienceWorld, Alexandria, VA, United States. Reference includes data supplied by the Geological Society of America, Boulder, CO, United States
Update Code: 201443
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