Investigating the lid effect on the generation of Ocean Island Basalts: 1. Geochemical trends

Abstract

Ocean island basalts (OIBs) are generated by mantle plumes, with their geochemistry controlled by a combination of source composition, temperature, and thickness of overlying lithosphere. For example, OIBs erupting onto thicker, older oceanic lithosphere are expected to exhibit signatures indicative of higher average melting pressures. Here, we quantitatively investigate this relationship using a global data set of Neogene and younger OIB compositions. Local lithospheric thicknesses are estimated using theoretical plate-cooling models and Bayes factors are applied to identify trends. Our findings provide compelling evidence for a correlation between OIB geochemistry and lithospheric thickness, with some variables (SiO2, Al2O3, FeO, Lu) showing linear trends that can be attributed to increasing average melting pressure, whereas others (CaO, La, lambda 0, and lambda 1) require a bi-linear fit with a change in gradient at similar to 55 km. Observed variations in highly incompatible elements are consistent with degrees of melting that decrease with increasing lithospheric thickness, as expected. Nevertheless, at thicknesses beyond similar to 55 km, the implied degree of melting does not decrease as rapidly as is suggested by theoretical expectations. This observation is robust across different lithospheric thickness estimates, including those derived from seismic constraints. We infer that at thicknesses exceeding similar to 55 km, weak plumes fail to effectively thin overlying lithosphere and/or produce insufficient melt to erupt. This is supported by independent estimates of plume buoyancy flux, indicating that OIB magmatism on older lithosphere may be biased toward hotter plumes. In addition, we find evidence for a "memory effect" of incomplete homogenization of melts during their ascent.Most of Earth's volcanoes occur at tectonic plate boundaries, but some emerge within plate interiors in so-called intra-plate settings. These volcanoes are believed to mark the surface expression of mantle plumes: hot, buoyant columns that rise from the core-mantle-boundary toward the surface. As they rise, lower pressures near the surface facilitate melting. However, the lithosphere-Earth's rigid outermost shell-limits plume ascent, and therefore controls the final (lowest) melting pressure of mantle plumes (the "lid effect"). Here, we collate and analyze a global geochemical data set of oceanic island basalts-the products of plume melting-to test this hypothesis. Using a range of diagnostics and a novel probabilistic analytical approach, we find that some geochemical parameters either linearly increase or decrease with lithospheric thickness, whereas other trends exhibit abrupt changes. We propose potential explanations for these patterns, focusing on factors such as the degree of melting (which is sensitive to temperature and pressure) and variations in mantle mineralogy at different depths. Notably, we suggest that there is a higher chance of observing volcanism above hotter plumes in regions of thicker lithosphere and identify a "memory effect", whereby their geochemistry to some extent preserves information from the initial melting process.We quantify the relationship between lithospheric thickness and ocean island basalt geochemistry: the so-called lid effect Observed trends are controlled by pressure-related changes in the degree of melting, mineral assemblage, and spinel-garnet phase transition Magmatism beneath older lithosphere may be biased toward hotter plumes that more effectively thin and penetrate overlying lithosphere