PRIMITIVE ANKARAMITIC MAGMAS IN VOLCANIC ARCS - A MELT-INCLUSION APPROACH

Ankaramite is more mafic than basalt, commonly porphyritic, and genera lly interpreted as a variety of picrite or olivine basalt that has bee n enriched in clinopyroxene crystals rather than a product of crystall ization of an ankaramitic magma. Samples of ankaramite from the Ulakan Formation, Bah, and Rinjani volcano, Lombok, both in the Sunda are, a nd from Merelava and Epi, in the Vanuatu are, contain olivine and clin opyroxene phenocrysts with vitreous and crystalline inclusions. Heated and homogenized silicate melt inclusions hosted by olivine with Fo(>9 0) have CaO/Al2O3 (wt%) values > 1, and are rich in Mg (>14 wt% MgO) a nd Ca (>13 wt% CaO), and therefore have ankaramitic affinities and are quite unlike picrites. Although the composition of the inclusions pro vides evidence that primitive ankaramitic melts exist, they are consis tently more silica-undersaturated than the ankaramitic bulk-rock compo sitions. Primitive ankaramitic melts are substantially higher in their normative diopside contents than the compositions of experimental mel ts of dry Iherzolite at all pressures up to 4 GPa, which typically are picritic. Olivine fractionation of primary komatiite-like melts with CaO/Al2O3 (wt%) values > 1, derived by partial melting of Iherzolite a t high pressures (>5 GPa), could produce compositions similar to primi tive ankaramitic bulk-rock compositions. Alternatively, melting of Ihe rzolite in the presence of mantle fluids hearing H2O and CO2 could pro duce compositions similar to those of the silica-undersaturated melt i nclusions trapped in the olivine (Fo(>90)) of ankaramitic samples. Suc h melts would be likely to react with mantle during ascent to become l ess silica-undersaturated and more like ankaramitic bulk-compositions.