Major and trace element and isotopic constraints on magma systematics in lavas from the Mid-Cayman Spreading Center

Abstract:

The Mid-Cayman Spreading Center (MCSC) is an ultra-slow spreading mid-ocean ridge system (15-17mm/yr) located at the North American and Caribbean plate boundary. Here, we present new microprobe major element contents, new solution ICP-MS and LA-ICP-MS trace element compositions and new Sr-Nd-Pb isotope ratios on eight lavas erupted at the northern end of the MCSC to assess melting systematics, mantle source variations, and fractional crystallization processes at an ultraslow spreading mid-ocean ridge. The basalts are all relatively evolved, with < 7 wt.% MgO and have abundant olivine and plagioclase phenocrysts. Additionally, the eight basalts all have similar trace element patterns, indicating a homogenous source for the MCSC, and a similar percent melting. As observed in previous studies (Thompson et al., 1980), the lavas erupted here at the MCSC also have elevated TiO₂, Zr, Y, Sr, and Ba compared to normal mid-ocean ridge basalts (NMORB) and indicating that the MCSC lavas are more evolved. The basalts show overall enriched rare earth element (REE) patterns compared to NMORB with minor Eu anomalies. In comparison, the LA-ICP-MS data for the same samples also show enriched REE patterns, however, there are no Eu anomalies present. This suggests the presence of micro-plagioclase crystals in the glass during dissolution. Combining the new major and trace element data with radiogenic isotope ratios will provide insight into the mantle composition beneath the MCSC, including mantle enrichment and potential mantle zonation.