Year: 2017
Type: article
Abstract: 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 °C, which is 250-450 °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 °C.Modeling the liquid line of descent along the calcite surface requires a total fractionation of ~48% calcite, ~12% apatite, and ~2 wt% clinopyroxene.SiO 2 solubility only increases from 0.2 to 2.9 wt% at 750-1200 °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.
Source: Geology
DOI: 10.1130/g38801.1
Institutions: University of California, Los Angeles, ETH Zurich
Language: English
Subfield: Geochemistry and Petrology
Field: Earth and Planetary Sciences
Domain: Physical Sciences
Open Access status: hybrid
OpenAlex ID: W2600861237