Origin of primitive ultra-calcic arc melts at crustal conditions - Experimental evidence on the La Sommata basalt, Vulcano, Aeolian Islands

摘要

To interpret primitive magma compositions in the Aeolian arc and contribute to a better experimental characterization\udof ultra-calcic arc melts, equilibrium phase relations have been determined experimentally for the La\udSommata basalt (Som-1, Vulcano, Aeolian arc). Som-1 (Na2O + K2O = 4.46 wt.%, CaO = 12.97 wt.%, MgO =\ud8.78 wt.%, CaO/Al2O3 = 1.03) is a reference primitive ne-normative arc basalt with a strong ultra-calcic affinity.\udThe experiments have been performed between 44 and 154 MPa, 1050 and 1150 °C and from NNO + 0.2 to\udNNO + 1.9. Fluid-present conditions were imposed with H2O–CO2 mixtures yielding melt H2O concentrations\udfrom0.7 to 3.5wt.%. Phases encountered include clinopyroxene, olivine, plagioclase and Fe-oxide. Clinopyroxene\udis slightly earlier than olivine in the crystallization sequence. It is the liquidus phase at 150 MPa, being joined by\udolivine on the liquidus between 44 and 88MPa. Plagioclase is the third phase to appear in the crystallization sequence\udand orthopyroxene was not found. Experimental clinopyroxenes (Fs7–16) and olivines (Fo78–92) partially\udreproduce the natural phenocryst compositions (respectively Fs5–7 and Fo87–91). Upon progressive crystallization,\udexperimental liquids shift towards higher SiO2 (up to ~55 wt.%), Al2O3 (up to ~18 wt.%) and K2O (up to\ud~5.5wt.%) and lower CaO,MgO and CaO/Al2O3. Experimental glasses and natural whole-rock compositions overlap,\udindicating that progressive crystallization of Som-1 type melts can generate differentiated compositions such\udas those encountered at Vulcano. The lowpressure cotectic experimental glasses reproduce glass inclusions in La\udSommata clinopyroxene but contrast with glass inclusions in olivine which preserve basaltic melts more primitive\udthan Som-1. Phase relations for the La Sommata basalt are identical in all critical aspects to those obtained\udpreviously on a synthetic ultra-calcic arc composition. In particular, clinopyroxene+olivine co-saturation occurs\udat very low pressures (≤100 MPa). Ultra-calcic arc compositions do not represent primary mantle melts but result\udfrom the interaction between a primary mantle melt and clinopyroxene-bearing rocks in the arc crust. At\udVulcano, primitive ultra-calcic end-member melts were generated between 250 and 350 MPa in the lower\udmagma accumulation zone by reaction between hot primitive melts and wehrlitic or gabbroic lithologies. At\udStromboli, golden pumices and glass inclusions with an ultra-calcic affinity were also generated at shallow\udpressures, between 150 and 250 MPa, suggesting that the interaction model is of general significance in the\udAeolian arc