Immiscible transition from carbonate-rich to silicate-rich melts in the 3 GPa melting interval of eclogite + CO2 and genesis of silica-undersaturated ocean island lavas

Rajdeep Dasgupta, Marc M. Hirschmann, Kathryn Stalker

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Abstract

We explore the partial melting behavior of a carbonated silica-deficient eclogite (SLEC1; 5 wt % CO2) from experiments at 3 GPa and compare the compositions of partial melts with those of alkalic and highly alkalic oceanic island basalts (OIBs). The solidus is located at 1050-1075 °C and the liquidus at ∼1415 °C. The sub-solidus assemblage consists of clinopyroxene, garnet, ilmenite, and calcio-dolomitic solid solution and the near solidus melt is carbonatitic (<2 wt % SiO2, <1 wt % Al2 O3, and <0·1 wt % TiO2). Beginning at 1225 °C, a strongly silica-undersaturated silicate melt (∼34-43 wt % SiO2) with high TiO2 (up to 19 wt %) coexists with carbonate-rich melt (<5 wt % SiO2). The first appearance of carbonated silicate melt is ∼100 °C cooler than the expected solidus of CO2-free eclogite. In contrast to the continuous transition from carbonate to silicate melts observed experimentally in peridotite + CO2 systems, carbonate and silicate melt coexist over a wide temperature interval for partial melting of SLEC1 carbonated eclogite at 3 GPa. Silicate melts generated from SLEC1, especially at high melt fraction (>20 wt %), may be plausible sources or contributing components to melilitites and melilititic nephelinites from oceanic provinces, as they have strong compositional similarities including their SiO2, FeO*, MgO, CaO, TiO2 and Na2O contents, and CaO/Al2O3 ratios. Carbonated silicate partial melts from eclogite may also contribute to less extreme alkalic OIB, as these lavas have a number of compositional attributes, such as high TiO2 and FeO* and low Al2O3, that have not been observed from partial melting of peridotite ± CO2. In upwelling mantle, formation of carbonatite and silicate melts from eclogite and peridotite source lithologies occurs over a wide range of depths, producing significant opportunities for metasomatic transfer and implantation of melts.

Original languageEnglish (US)
Pages (from-to)647-671
Number of pages25
JournalJournal of Petrology
Volume47
Issue number4
DOIs
StatePublished - Apr 2006

Bibliographical note

Funding Information:
R.D. acknowledges support from V. R. Murthy & J. Noruk fellowship and a Doctoral Dissertation fellowship of the University of Minnesota during course of this study. We thank Rachel Dwarzski and Dave Draper for permission to cite their unpublished data, and Cyril Aubaud and Fred Frey for valuable discussions. Bob Luth, Peter Ulmer, and an anonymous reviewer are gratefully acknowledged for their constructive and thorough reviews. This work is supported by National Science Foundation grant EAR-0310142 to M.M.H. Kate Stalker’s summer internship at Minnesota was funded through the University of Minnesota, Department of Geology & Geophysics, REU site, National Science Foundation grant EAR-0243526.

Keywords

  • Carbonated eclogite
  • Experimental phase equilibria
  • Liquid immiscibility
  • Ocean island basalts
  • Partial melting

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