Insights into 3 Ma of Mid-Ocean Ridge Mantle Source Heterogeneity from the Gofar Transform Fault, East Pacific Rise

Rarely are mid-ocean ridges sampled across the time domain, meaning that our understanding of source and melting processes beneath mid-ocean ridges are based primarily on along-axis geochemical variability, or lack thereof. Dredging along a transform fault allows for sampling of volcanism over a period of time. We present a comprehensive geochemical dataset (major and trace elements, volatiles, Sr-Nd-Pb isotopes) of recently dredged basalt samples collected from 12 fault zone localities along a 150 km transect of the Gofar transform fault, East-Pacific Rise (EPR). We observe significant variation in dredged lava compositions as a whole (e.g., RSD of K/Ti = 58%), which is greater than the variability in the nearest on-axis lavas (10%; N=25). However, no systematic pattern of compositional or isotopic variability occurs as a function of distance from the ridge transform intersection, nor do single dredges constitute compositionally distinct populations of samples. Thus, we interpret these samples to represent lavas erupted from the adjacent ridge segments over ~3 Ma and juxtaposed together from both sides of the transform valley by movement along the fault. This dataset allows us to resolve the time-scale over which melting processes vary, and in turn the length scale of mantle source variations. Pb isotopic-variations from double-spike corrected measurements by TIMS form a near-linear array in 206Pb/204Pb-207Pb/204Pb. The least radiogenic samples have isotopic compositions (206Pb/204Pb = 17.7617 207Pb/204Pb = 15.4079) representing the low-end of EPR lavas and are correspondingly depleted in highly incompatible trace elements relative to those that are less incompatible in mantle minerals (e.g., La/Sm = 0.85). The most radiogenic samples (206Pb/204Pb = 18.3291 207Pb/204Pb = 15.4948) form a linear array sub-parallel to the trend of the less radiogenic samples but fall below it in 207Pb/204Pb and have a distinct trace element signature (e.g., elevated Th/La = 1.2). These observations indicate that the mantle sourcing the EPR at Gofar was modified by at least two ancient enrichment or depletion events that ended up in close proximity prior to eruption on the seafloor. Our analysis of a longer time-slice of melting reveals small length-scale heterogeneities in the mantle not detectable by on-axis sampling alone.