TY - JOUR
T1 - Trace-element and isotopic characteristics of small-degree melts of the asthenosphere
T2 - Evidence from the alkalic basalts of the Antarctic Peninsula
AU - Hole, M. J.
AU - Kempton, P. D.
AU - Millar, I. L.
PY - 1993/10/25
Y1 - 1993/10/25
N2 - 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 206Pb 204Pb ratios (18.79-19.28) fall within the range for E-type MORB with Δ8 4 and Δ7 4 varying from -28 to +26 and from +1 to +10, respectively. Δ8 4-values, Sr-isotope ratios and some LILE/HFSE ratios exhibit negative covariations with Lan Ybn 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 PbSrNd 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 (∼ 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.
AB - 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 206Pb 204Pb ratios (18.79-19.28) fall within the range for E-type MORB with Δ8 4 and Δ7 4 varying from -28 to +26 and from +1 to +10, respectively. Δ8 4-values, Sr-isotope ratios and some LILE/HFSE ratios exhibit negative covariations with Lan Ybn 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 PbSrNd 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 (∼ 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.
UR - http://www.scopus.com/inward/record.url?scp=0027868557&partnerID=8YFLogxK
U2 - 10.1016/0009-2541(93)90061-M
DO - 10.1016/0009-2541(93)90061-M
M3 - Article
AN - SCOPUS:0027868557
SN - 0009-2541
VL - 109
SP - 51
EP - 68
JO - Chemical Geology
JF - Chemical Geology
IS - 1-4
ER -