Geochemistry of lavas from the Garrett Transform Fault: insights into mantle heterogeneity beneath the eastern Pacific

Young intra-transform lavas erupted as a result of extension within the Gar rett Transform Fault on the southern East Pacific Rise, are more porphyriti c, less evolved, have lower concentrations of incompatible trace elements, and lower ratios of more incompatible to less incompatible elements (e.g. l ow K/Ti and La/Sm) compared to lavas from the adjacent East Pacific Rise ri dge axis. Sr, Nd and Pb isotope compositions overlap with the depleted end of the field for Pacific mid-ocean ridge basalts, but extend to lower Sr-87 /Sr-86 (0.702137), Pb-206/Pb-204 (17.462), Pb-207/Pb-204 (15.331), Pb-208/P b-204 (36.831), and higher Nd-143/Nd-144 (0.513345) than any lavas previous ly reported from the Pacific. Peridotites from the Garrett Transform have N d isotope compositions within the range of the intra-transform lavas. The u nusual major and trace element compositions of the Garrett lavas appear to be characteristic of other intra-transform lavas from elsewhere in the Paci fic. The chemical and isotopic features of the Garrett lavas can be explain ed by remelting, beneath the transform, a two-component upper mantle which was depleted in incompatible element-enriched heterogeneities during meltin g beneath the East Pacific Rise ridge axis (within the past 1 Ma). Our data place new constraints on the trace element and isotope composition of the depleted mantle component that contributes to magmatism in the Pacific, and show that this component is heterogeneous, both on the scale of a single t ransform fault, and on the scale of an ocean basin. (C) 1999 Elsevier Scien ce B.V. All rights reserved.