TRACE-ELEMENT AND ISOTOPIC CHARACTERISTICS OF SMALL-DEGREE MELTS OF THE ASTHENOSPHERE - EVIDENCE FROM THE ALKALIC BASALTS OF THE ANTARCTIC PENINSULA

Miocene-Recent continental alkalic basalts were erupted along the Antarctic Peninsula as a result of decompressional melting of the asthenosphere caused by the formation of slab-windows beneath the continental margin following the cessation of subduction. The basalts appear not to be related to a period of major lithospheric attenuation, nor were they formed as a result of the influence of a mantle plume. They exhibit strong trace-element and isotopic affinities with OIB, Sr- and Nd-isotope compositions ranging from 0.70269 to 0.70343 and 0.512863 to 0.51300, respectively, similar to the composition of HIMU OIB. However, new Pb-isotope analyses show that Pb-206/Pb-204 ratios (18.79-19.28) fall within the range for E-type MORB with DELTA8/4 and DELTA7/4 varying from -28 to +26 and from +1 to +10, respectively. DELTA8/4-values, Sr-isotope ratios and some LILE/HFSE ratios exhibit negative covariations with La(n)/Yb(n) and Nb/Y ratios implying some control of degree of partial melting on geochemical composition. Nb/U ratios (14-40) are considerably lower than most OIB and MORB. The basalts also have unusually low absolute abundances of Rb and Ba and high K/Ba and K/Rb ratios (50-140 and 400-1500, respectively). Correlated Pb-Sr-Nd isotope and trace-element behaviour suggests that the asthenosphere from which these basalts were derived was subjected to multiple melt extraction/depletion events. One period of melt extraction was ancient (approximately 1.7 Ga) and similar to that affecting MORB source mantle, and was followed by a more recent (?Mesozoic) event. This more recent event resulted in increased U/Pb, U/Nb and U/Th ratios and further depletion in ultra-incompatible element such as Rb and Ba, causing high K/Rb and K/Ba ratios in the erupted lavas. This implies that the asthenosphere beneath the Antarctic Peninsula is heterogeneous on a small scale. Small-degree melts are capable of sampling geochemically, and possibly mineralogically, distinct mantle domains from larger-degree melts. During larger degrees of partial melting, the scale of melting approaches the scale of heterogeneity and integration of melts from different geochemical domains occurs.