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

characterization
of ultra-calcic arc melts, equilibrium phase relations have been determined experimentally for the La
Sommata basalt (Som-1, Vulcano, Aeolian arc). Som-1 (Na2O + K2O = 4.46 wt.%, CaO = 12.97 wt.%, MgO =
8.78 wt.%, CaO/Al2O3 = 1.03) is a reference primitive ne-normative arc basalt with a strong ultra-calcic affinity.
The experiments have been performed between 44 and 154 MPa, 1050 and 1150 °C and from NNO + 0.2 to
NNO + 1.9. Fluid-present conditions were imposed with H2O–CO2 mixtures yielding melt H2O concentrations
from0.7 to 3.5wt.%. Phases encountered include clinopyroxene, olivine, plagioclase and Fe-oxide. Clinopyroxene
is slightly earlier than olivine in the crystallization sequence. It is the liquidus phase at 150 MPa, being joined by
olivine on the liquidus between 44 and 88MPa. Plagioclase is the third phase to appear in the crystallization sequence
and orthopyroxene was not found. Experimental clinopyroxenes (Fs7–16) and olivines (Fo78–92) partially
reproduce the natural phenocryst compositions (respectively Fs5–7 and Fo87–91). Upon progressive crystallization,
experimental liquids shift towards higher SiO2 (up to ~55 wt.%), Al2O3 (up to ~18 wt.%) and K2O (up to
~5.5wt.%) and lower CaO,MgO and CaO/Al2O3. Experimental glasses and natural whole-rock compositions overlap,
indicating that progressive crystallization of Som-1 type melts can generate differentiated compositions such
as those encountered at Vulcano. The lowpressure cotectic experimental glasses reproduce glass inclusions in La
Sommata clinopyroxene but contrast with glass inclusions in olivine which preserve basaltic melts more primitive
than Som-1. Phase relations for the La Sommata basalt are identical in all critical aspects to those obtained
previously on a synthetic ultra-calcic arc composition. In particular, clinopyroxene+olivine co-saturation occurs
at very low pressures (≤100 MPa). Ultra-calcic arc compositions do not represent primary mantle melts but result
from the interaction between a primary mantle melt and clinopyroxene-bearing rocks in the arc crust. At
Vulcano, primitive ultra-calcic end-member melts were generated between 250 and 350 MPa in the lower
magma accumulation zone by reaction between hot primitive melts and wehrlitic or gabbroic lithologies. At
Stromboli, golden pumices and glass inclusions with an ultra-calcic affinity were also generated at shallow
pressures, between 150 and 250 MPa, suggesting that the interaction model is of general significance in the
Aeolian arc.