Tracing mantle sources in the northern Lau Basin by independent component analysis of basalt isotopic compositions

Isotopic variability in oceanic basalts could indicate possible interactions among multiple mantle components, while independent component analysis (ICA) is a statistical method suited for extracting independent features in multivariable compositional space. Here we employed the ICA to investigate the end-member reservoirs of mantle sources in the northern Lau Basin based on a complied geochemical dataset (764) from the northern Lau Basin (NLB), central-southern Lau Basin (C-SLB), Pacific and Indian Ocean ridges, and Samoa Islands. Newly-identified three independent components (ICs) in the five-dimensional space of Sr-Nd-Pb isotopic ratios in this study can account for 96.5% of the isotopic variance and explain the major signatures of the compositional space.

The correlations between the newly-identified ICs and incompatible trace elements ratios were further used to examine the origin of these ICs. The first IC (IC1) shows negative correlation with La/Sm ratios, and has tendency geographically to increase from Samoa Islands, to the NLB, to the C-SLB, to the Indian Ocean ridge, and to the Pacific Ocean ridge. The second IC (IC2) discriminates mid-ocean ridge basalts (MORB) from Pacific (IC2>0) and Indian (IC2<0) Ocean ridges, as well as the C-SLB (IC2>0) and NLB (IC2<0) lavas. Furthermore, IC2 presents a positive correlation with Ba/Th ratios. The third IC (IC3) distinguishes Pacific and Indian MORBs (IC3>0) and Lau Basin lavas (IC3<0), which also displays a negative correlation with Th/Nb values. On a regional scale in Lau Basin, there is an increase in IC1, IC2 and IC3 from north to south. On the other hand, IC2 decreases and IC3 increases from the east to the west of the Lau Basin, i.e., the increasing distance from the arc.

The geochemical nature and the statistical properties of the ICs suggest that IC1 is related to an enriched mantle component most possibly from the nearby Samoan plume, whereas IC2 and IC3 correspond to subduction components. This study further supports that the relatively recent mixing of Samoan plume materials with the subduction-metasomatized Indian-type mantle may be responsible for the observed geochemical compositions in NLB lavas (Tian et al., 2011).

Reference: Tian et al. (2011), JGR-Solid Earth 116, B11201.