Examining Variability Along a Single Segment of the Mid-Atlantic Ridge at 14°N Using Geochemistry, Petrologic Modeling & Cluster Analysis

Volcanism at slow spreading Mid-Ocean Ridge segments is variably distributed along-axis. Evidence of this includes segment centers that have thicker basaltic crust relative to the segment ends. This distribution may result from changes in the sub-ridge melting systematics, variations in mantle source, and/or transportation of magma down the ridge axis in the shallow crust, each of which may produce a distinct geochemical signature in erupted lavas. Thus, understanding how lava compositions vary along a single segment can provide great insight into the magmatic plumbing system underneath a ridge axis. To evaluate chemical variability along-axis, we systematically mapped and sampled lavas erupted along a single segment at the Mid-Atlantic Ridge (14°N). Samples were collected using DSV Alvin from 5 regions which span the magmatic gradient from the magmatically robust segment center to the more sparsely magmatic non-transform offset. Major elements were analyzed for 382 basaltic glasses, 197 were analyzed for trace element contents, and Pb, Nd, and Sr isotopes of 32 lavas were analyzed to determine mantle source heterogeneity. Our results show significant geochemical variability in lava compositions erupted along the axial valley. MgO varies from 4.99-9.44, K/Ti from 0.06- 0.51, La/Sm from 1.06-4.14, and 206Pb/204Pb from 18.45-19.76.

To statistically quantify the geochemical variability, we utilized a hierarchical clustering analysis. This analysis combined with high-resolution bathymetric data from AUV Sentry allows us to examine along-axis sameness on the segment scale, as well as within the 5 aforementioned regions to look at inter-regional variability. Additionally, we utilized petrologic modeling of mantle melting to investigate the origin of geochemical variability along axis. We show that chemical variability increases moving away from the segment center, and the southern portion of the segment requires multiple depths of crystallization, greater variability in extent of melting, as well as multiple sources to explain the chemical variability. This combination of high-resolution bathymetry, dense sampling, petrologic modeling, and statistical analyses can be used to better understand, quantify, and map individual eruptive units, morphologies, chemical variability, and magma transport.