LIP-PRINTING: A GEOCHEMICAL PROXY APPROACH TO LIP FORENSICS (Invited Presentation)

One effective way to carry out forensic examination of LIPs is by using geochemical proxies for parameters such as mantle temperature, mantle composition, nature and degree of partial melting, crustal composition, and assimilation-fractional crystallization. In the ideal world, each geochemical proxy would highlight just one petrogenetic variable. Here, we investigate in detail the utility of three immobile element proxies: Th/Nb as a proxy for crustal input (via subduction or assimilation); Ti/Yb in basic rocks as a proxy for the role of garnet in the melting process; and either Sc or Co (depending on melting history) as a proxy for the degree of fractional crystallization. The Th/Nb ratio distinguishes LIP lavas with no crustal input and hence low ratios typical of MORB and OIB, from those with crustal involvement and hence higher ratios. Of the latter, the crustal input may represent a subducted component stored in sub-continental lithospheric mantle (SCLM) or continental crust assimilated by fractionating magma. Combining the crustal proxy with the fractionation proxy, provides an opportunity to distinguish these options in many cases. The Ti/Yb proxy is based on the fact that high ratios are indicative of significant residual garnet, which in turn is a function of mantle potential temperature and depth range of melting. A further consequence is that unusually low Ti/Yb ratios result from remelting of mantle preconditioned by earlier melting in the garnet facies. There are wide variations in the values of these proxies, both within and between individual LIPs. Between LIPs, differences in crustal assimilation, SCLM involvement, and mantle and melting parameters highlight differences in setting and geodynamics. Within LIPs, spatial and temporal variations in both crustal and mantle variables are common and, where exposure and reconstructions allow, these can be highlighted through geochemical mapping of dike swarms and lava sequences.