MELT HARZBURGITE REACTION IN THE PETROGENESIS OF THOLEIITIC MAGMA FROM KILAUEA VOLCANO, HAWAII/

We use the results of elevated pressure melting experiments to constra in the role of melt/mantle reaction in the formation of tholeiitic mag ma from Kilauea volcano, Hawaii. Trace element abundance data is commo nly interpreted as evidence that Kilauea tholeiite is produced by part ial melting of garnet Iherzolite. We experimentally determine the liqu idus relations of a tightly constrained estimate of primary tholeiite composition, and find that it is not in equilibrium on its liquidus wi th a garnet Iherzolite assemblage at any pressure. The composition is, however, cosaturated on its liquidus with olivine and orthopyroxene a t 1.4 GPa and 1425 degrees C, from which we infer that primary tholeii te is in equilibrium with harzburgite at lithospheric depths beneath K ilauea. These results are consistent with our observation that tholeii te primary magmas have higher normative silica contents than experimen tally produced melts of garnet Iherzolite. A model is presented whereb y primary tholeiite forms via a two-stage process. In the first stage, magmas are generated by melting of garnet lherzolite in a mantle plum e. In the second stage, the ascent and decompression of magmas causes them to react with harzburgite in the mantle by assimilating orthopyro xene and crystallizing olivine. This reaction can produce typical thol eiite primary magmas from significantly less siliceous garnet Iherzoli te melts, and is consistent with the shift in liquidus boundaries that accompanies decompression of an ascending magma. We determine the pro portion of reactants by major element mass balance. The ratio of mass assimilated to mass crystallized (M-a/M-c) varies from 2.7 to 1.4, dep ending on the primary magma composition. We use an AFC calculation to model the effect of melt/harzburgite reaction on melt rare earth and h igh field strength element abundances, and find that reaction dilutes, but does not significantly fractionate, the abundances of these eleme nts. Assuming olivine and orthopyroxene have similar heats of fusion, the M-a/M-c ratio indicates that reaction is endothermic. The addition al thermal energy is supplied by the melt, which becomes superheated d uring adiabatic ascent and can provide more thermal energy than requir ed. Melt/ harzburgite reaction likely occurs over a range of depths, a nd we infer a mean depth of 42 km from our experimental results. This depth is well within the lithosphere beneath Kilauea. Since geochemica l evidence indicates that melt/harzburgite reaction likely occurs in t he top of the Hawaiian plume, the plume must be able to thin a signifi cant portion of the lithosphere.