Coupling trace element mapping and high resolution in-situ oxygen isotopic measurement in olivine crystals to constrain magmatic conditions at Axial Seamount (Juan de Fuca Ridge)

Axial Seamount is the most extensively monitored submarine volcano, located in the Pacific Ocean on the Juan de Fuca Ridge. Its voluminous edifice is related to the interaction of a hotspot with a convergent margin. Axial Seamount had recent eruptions occurring in 1998, 2011, and 2015 from a 3 km × 8 km caldera. Recent 3D seismic reflection imaging (Kent et al., 2025) has highlighted funnel-shaped structures beneath the axial magma lens, interpreted as magma assimilation fronts. These structures are thought to result from the assimilation by Axial Seamount magmas of hydrothermally altered sheeted dykes and gabbroic rocks. The assimilation fronts are produced during periods of high magmatic activity, whereas their solidification and accretion occur during low magmatic phases, implying cyclicity in the magmatic activity of Axial Seamount.

Here, we present new high-resolution petrological data focused on olivine phenocrysts from Axial Seamount lavas to better constrain the organisation of its magmatic reservoir and its associated assimilation dynamics. We conducted major and trace element mapping (Mg, Cr, Ni, Al, Ca, and P) of olivine grains using electron probe microanalysis (EPMA) specifically optimized for trace element measurements. These elemental maps were coupled with high spatial and analytical resolution in situ oxygen isotope measurements performed on the same grains using secondary ion mass spectrometry (SIMS).

Elemental mapping of olivine grains allows investigation of specific magma dynamics such as differentiation, early rapid growth events, and the presence and potential destabilization of long-lived mushy environments, which are notably recorded by slowly diffusing elements such as phosphorus and aluminum. In contrast, oxygen isotopic composition is a powerful tracer of the nature of potential assimilated components. We therefore use oxygen isotopes to search for additional evidence of assimilating phases inferred from geophysical data at Axial Seamount and link these observations to reservoir dynamics by coupling isotopic data with elemental crystal mapping.

Our results provide the first ultra-high spatial resolution measurements of oxygen isotopic compositions within single olivine grains, with analytical uncertainties below 0.25‰. Across the full range of magmatic processes recorded by olivine trace element compositions, we find no evidence for assimilation of hydrothermally altered components. Although the analyzed samples span a wide range of ages, none of them are from the historical eruptions. This may indicate that the samples record periods of former relatively low magmatic activity at Axial Seamount. Alternatively, they may represent different magmatic styles that have not been described yet, or assimilation may occur at depths or under conditions that are not recorded by olivine crystallization.

Kent, G.M., Arnulf, A.F., Singh, S.C. et al. Melt focusing along lithosphere–asthenosphere boundary below Axial volcano. Nature 641, 380–387 (2025).