Hydrous metasomatic veins in the lithospheric mantle: Implications for the origin of Bermuda alkaline lavas and the HIMU component
Abstract
The peridotitic source of alkaline basalts from Bermuda has recently been interpreted as having been metasomatized by deep (>410 km) partial melts of a recycled component that had been stored in the mantle transition zone for no more than 650 Ma [1]. Here we consider an alternative model to explain the chemical and isotopic characteristics of these lavas, building on previous models of OIB and continental rift magmas that involve partial melting of hydrous metasomatic veins formed at much shallower depths in the lithospheric mantle [2]. Major-, trace-element, and isotopic compositions of the Bermuda alkaline lavas are consistent with such models: e.g., variable K2O/Na2O ratios and high TiO2 contents of these basalts are consistent with the melting of pyroxenite containing varying proportions of amphibole and phlogopite; melts of hornblendite at 2.5 GPa have trace-element patterns similar to those measured by [1] in Bermuda alkaline basalts; Monte-Carlo-based numeric simulations of the formation of metasomatic veins predict that these veins will have U/Pb, Rb/Sr, and Sm/Nd ratios that can produce isotopic compositions similar to those of the Bermuda alkaline lavas after an isolation time of 200-250 Ma (Fig. 1). In our interpretation, metasomatic veins form during the early stage of continental rifting of Pangea—when the supercontinent breaks apart, small blocks of metasomatized continental lithosphere are incorporate in the newly formed oceanic lithosphere. Small thermal perturbations (potentially linked to the production of the associated tholeiites) melt these metasomatic veins producing the alkaline basalts.
Our work suggests that melting of metasomatic veins included in the Atlantic lithospheric mantle is consistent with the major-, trace-element, and isotopic compositions of Bermuda alkaline basalts, and that this hypothesis is a viable alternative to a model that links the alkaline magmas to deep melting of a H2O and CO2 enriched peridotitic source [1]. If further work supports a link between metasomatic veins and the high 206Pb/204Pb ratios of the Bermuda alkaline lavas, this would support the more general hypothesis that the HIMU component also corresponds to domains of metasomatized lithosphere. [1] Mazza et al., 2019, Nature 569, 398-403 [2] Pilet et al., 2008, Science 320, 916-919- Publication:
-
AGU Fall Meeting Abstracts
- Pub Date:
- December 2019
- Bibcode:
- 2019AGUFM.V13B..07P
- Keywords:
-
- 1033 Intra-plate processes;
- GEOCHEMISTRY;
- 7208 Mantle;
- SEISMOLOGY;
- 8137 Hotspots;
- large igneous provinces;
- and flood basalt volcanism;
- TECTONOPHYSICS;
- 8416 Mid-oceanic ridge processes;
- VOLCANOLOGY