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Journal of Petrology | Volume 45 | Number 3 | Pages 531-554 | 2004
Journal of Petrology 45(3) © Oxford University Press 2004; all rights reserved.
Ultra-calcic Magmas Generated from Ca-depleted Mantle: an Experimental Study on the Origin of Ankaramites
1 INSTITUTE OF MINERALOGY AND PETROLOGY, ETH ZURICH, 8092 ZURICH, SWITZERLAND
2 RESEARCH SCHOOL OF EARTH SCIENCES, ANU, CANBERRA, A.C.T. 0200, AUSTRALIA
* Corresponding author. Telephone: xx41-1-632-7988. Fax: xx41-1-632-1088. E-mail: max.schmidt{at}erdw.ethz.ch
Ultra-calcic ankaramitic magmas or melt inclusions are ubiquitous in arc, ocean-island and mid-ocean ridge settings. They are primitive in character (XMg > 0·65) and have high CaO contents (>14 wt %) and CaO/Al2O3 (>1·1). Experiments on an ankaramite from Epi, Vanuatu arc, demonstrate that its liquidus surface has only clinopyroxene at pressures of 15 and 20 kbar, with XCO2 in the volatile component from 0 to 0·86. The parental Epi ankaramite is thus not an unfractionated magma. However, forcing the ankaramite experimentally into saturation with olivine, orthopyroxene and spinel results in more magnesian, ultra-calcic melts with CaO/Al2O3 of 1·21–1·58. The experimental melts are not extremely Ca-rich but high in CaO/Al2O3 and in MgO (up to 18.5 wt %), and would evolve to high-CaO melts through olivine fractionation. Fractionation models show that the Epi parent magma can be derived from such ultra-calcic experimental melts through mainly olivine fractionation. We show that the experimental ultra-calcic melts could form through low-degree melting of somewhat refractory mantle. The latter would have been depleted by previous melt extraction, which increases the CaO/Al2O3 in the residue as long as some clinopyroxene remains residual. This finding corrects the common assumption that ultra-calcic magmas must come from a Ca-rich pyroxenite-type source. The temperatures necessary for the generation of ultra-calcic magmas are 
1330°C, and their presence would suggest melting regimes that are at the upper temperature end of previous interpretations made on the basis of picritic magmas.
KEY WORDS: ankaramite; refractory lherzolite; ultra-calcic magma
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