Exploring the mineralogical heterogeneities of the L ouisville S eamount T rail

Abstract Diopside phenocrysts of the Louisville Seamount Trail show an increase in Ti, Al, and Na with decreasing Mg/(Mg + Fe) as is typical for clinopyroxene in alkalic basalts. Chondrite‐normalized REE patterns of calculated liquids from LA‐ICPMS analyses are comparable to whole‐rock and glass values. Exceptions are clinopyroxene crystals from the Rigil Seamount, the second oldest seamount drilled at the northern end of the chain. Some crystals from this site are strongly zoned with distinct compositional boundaries between cores and mantles. The cores have high Mg/(Mg + Fe) and low Al and Ti concentrations compared to the mantles and phenocrysts. Major element, clinopyroxene discrimination diagrams indicate that the clinopyroxene mantles and phenocrysts crystallized from alkalic basalts. In contrast, the Mg‐rich cores have tholeiitic affinities. The REE abundances of the cores are similar to that of clinopyroxene from transitional tholeiites of the Kerguelen Archipelago. Calculated liquid La/Yb values for the cores have ratios that are similar to transitional tholeiites in Hawaii, whereas the mantles have higher La/Yb values similar to Hawaiian alkalic basalts. The major and trace element compositions of clinopyroxene cores from the Rigil seamount suggest that a transitional tholeiitic magma was present, but no evidence for Hawaiian shield‐type tholeiites was found. Plagioclase crystals from the Rigil seamount have 86 Sr/ 86 Sr from 0.70306 to 0.70363, within the range of FOZO. The transitional tholeiitic signature of the Mg‐rich clinopyroxene cores probably did not have a distinct source compared to other Louisville magmas, but more likely indicates a higher degree of partial melting (2–5%) of that FOZO source.