Lau Basin basalts (LBB): trace element and Sr [sbnd]Nd isotopic evidence for heterogeneity in backarc basin mantle

Abstract

Diverse 87Sr/ 86Sr and 143Nd/ 144Nd isotopic compositions among basalts from the Lau Basin (LBB), an active backarc basin in the southwest Pacific, indicate heterogeneity in the underlying mantle. Isotopic compositions display bimodal distributions which are related to geographic location. Type I LBB ( 87/Sr 86Sr ⩽ 0.70366; 143Nd/ 144Nd ⩾ 0.51297) include tholeiites from the central basin, Peggy Ridge, and Rochambeau Bank, while Type II basaltic and andesitic glasses from the northeastern portion of the basin, near Niua Fo'ou island, have higher 87Sr/ 86Sr ( ⩾ 0.7038) and lower 143Nd/ 144Nd ( ⩽ 0.51288). Both depleted (e.g. N-MORB) and enriched (e.g. E-MORB) trace element abundances occur among Type I and Type II LBB. Covariation between trace element and isotopic ratios among Type I LBB is consistent with mixing between depleted mantle similar to the source for MORB and relatively enriched peridotite similar to the source for E-MORB. Relative to MORB, uniformly high 87Sr/ 86Sr ( +0.0005) among all Type I LBB for given Nd isotopic compositions ( ε Nd = +8 to +12) may reflect a lithospheric component, such as ancient recycled altered ocean crust. Type II LBB have Sr Nd isotopic compositions which are gradational between enriched mantle similar to the source of OIB and a component with distinct Sr isotopic composition such as that observed in Samoan post-erosional basalts. Isotopic and geographic discontinuity between Type I and Type II LBB, and isotopic affinity of Type II and Niua Fo`ou island basalts with those from Samoa suggests that volcanism in the northeastern portion of the basin is tapping deeper mantle beneath the adjoining Pacific plate, as well as Indo-Australian mantle overlying the Pacific lithosphere that is subducted into the Tonga Trench.