Geochemical heterogeneity within mid-ocean ridge lava flows: insights into eruption, emplacement and global variations in magma generation

Compositional heterogeneity in mid-ocean ridge (MOR) lava flows is a powerful yet presently under-utilized volcanological and petrological tracer. Here, it is demonstrated that variations in pre- and syn-eruptive magmatic conditions throughout the global ridge system can be constrained with intra-flow compositional heterogeneity among 10 discrete MOR flows. Geographical distribution of chemical heterogeneity within flows is also used along with mapped physical features to help decipher the range of conditions that apply to seafloor eruptions (i.e. inferred vent locations and whether there were single or multiple eruptive episodes). Although low-pressure equilibrium fractional crystallization can account for much of the observed intra-flow compositional heterogeneity, some cases require multiple parent magmas and/or more complex crystallization conditions. Globally, the extent of within-flow compositional heterogeneity is well correlated (positively) with estimated erupted volume for flows from the northern East Pacific Rise (EPR), and the Mid Atlantic, Juan de Fuca and Gorda Ridges; however, some lavas from the superfast spreading southern EPR fall below this trend. Compositional heterogeneity is also inversely correlated with spreading rate. The more homogeneous compositions of lavas from faster spreading ridges likely reflect the relative thermal stability and longevity of sub-ridge crustal magma bodies, and possibly higher eruption frequencies. By contrast, greater compositional heterogeneity in lavas at slower spreading rates probably results from low thermal stability of the crust (due to diminished magma supply and greater hydrothermal cooling). Finally, the within-flow compositional variations observed here imply that caution must be exercised when interpreting MOR basalt data on samples where individual flows have not been mapped because chemical variations between lava samples may not necessarily record the history of spatially and temporally distinct eruptions.