Geochemical Variability & Implications for Magma Transport at 14°N on the Mid-Atlantic Ridge

Observations of seafloor bathymetry and gravity surveys indicate that magma focuses in the center of slow spreading Mid-Ocean Ridge (MOR) segments, however; it is not well constrained how magma is generated, stored, and transported to the segment ends. There are two end-member models for magma transport: 1) a focused magma model wherein the magma upwells beneath the entire ridge axis, is focused and pools beneath the center of the segment, and is then transported towards the segment ends via lateral diking in the shallow crust and 2) a distributed magma model wherein magma vertically upwells and is erupted on the seafloor along the entire segment, but there is enhanced focusing in the segment center. (Figure 1). Both models are supported by the bathymetric and geophysical observations but have different implications for the chemistry of lavas erupted along the segment. To test how lava chemistries vary along a slow-spreading MOR, we systematically sampled a segment of the Mid-Atlantic Ridge. The segment (~14°N) (Figure 2) is known to host Popping Rocks, gas-rich basalts which, upon reaching surface pressures, explode. Two expeditions to this region in 2016 and 2018 collected both ship-based bathymetry (75 m gridded resolution) aboard the R/V Atlantis and high-resolution bathymetry (1 m) from the Autonomous Underwater Vehicle (AUV) Sentry. 27 dives from the Human Occupied Vehicle (HOV) Alvin collected 382 lavas all of which have been analyzed for major element contents, and 162 have been analyzed for trace element contents. During these expeditions, samples were collected both along and across axis from the magmatically robust segment center, through a transition region, to a sparsely magmatic region. Analytical results show that there is significant chemical variability along this segment. For example, there is less variability at the segment center (K/Ti ratios from 0.24 to 0.46 and La/Sm from 2.58 to 3.59) compared to the sparsely magmatic region (K/Ti values from 0.06 to 0.42 and La/Sm). This suggests that magmas erupted at the segment center are more homogeneous compared to lavas erupting in the sparsely magmatic region. Major element contents in each region vary, but on average, become more mafic moving southward away from the magmatically robust segment center towards the sparsely magmatic region. Petrologic modeling of fractional crystallization and trace element contents show that fractional crystallization dominates the chemical variability in the sparsely magmatic region, while either extent of melting or differing mantle sources dominates the variability in the transition regions and the sparsely magmatic region. Reconciling these data with both physical and geophysical observations of a slow spreading ridge, we present a model of magma generation, storage, and transport that is a hybrid of the two proposed models.