The journey of K-MORBs, told through geological, geochemical and geophysical data

The compositional variability of mid-ocean ridge basalts (MORBs) stems from a combination of the heterogeneity of the mantle source, magma mixing, and partial crystallisation of magma in the lower crust. These variations have been mainly explored at the global and ridge scales, with only a few studies investigating them at the kilometre scale and below. Here, we focus on a series of tholeiitic to K-rich basalt samples collected during a single submarine dive near the eastern intersection between the Mid-Atlantic Ridge and the Romanche transform fault, in the equatorial Atlantic. The geochemical and petrological variations, attributed to variations in melting conditions, are extreme and consistent with the geological features. Clinopyroxene phenocrysts present in certain K-rich basalts, recorded the history of magma storage. We calculated the crystallisation pressures of clinopyroxenes and compared them to microseismicity depths recorded in the area, thus providing constraints on the lithospheric structure. This multidisciplinary work highlights the interest in exploring the composition of MORBs at high resolution to better understand the construction of the oceanic crust.