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

Compositional heterogeneity in mid-ocean ridge (MOR) lava flows is a powerf ul yet presently under-utilized volcanological and petrological tracer. Her e. it is demonstrated that variations in pre- and syn-eruptive magmatic con ditions throughout the global ridge system can be constrained with intra-fl ow compositional heterogeneity among 10 discrete MOR flows. Geographical di stribution of chemical heterogeneity within flows is also used along with m apped 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 f ractional crystallization can account for much of the observed intra-flow c ompositional heterogeneity, some cases require multiple parent magmas and/o r more complex crystallization conditions. Globally, the extent of within-f low compositional heterogeneity is well correlated (positively) with estima ted 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 h eterogeneity is also inversely correlated with spreading rate. The more hom ogeneous compositions of lavas from faster spreading ridges likely reflect the relative thermal stability and longevity of sub-ridge crustal magma bod ies, and possibly higher eruption frequencies. By contrast, greater composi tional heterogeneity in lavas at slower spreading rates probably results fr om low thermal stability of the crust (due to diminished magma supply and g reater hydrothermal cooling). Finally, the within-flow compositional variat ions observed here imply that caution must be exercised when interpreting M OR basalt data on samples where individual flows have not been mapped becau se chemical variations between lava samples may not necessarily record the history of spatially and temporally distinct eruptions. (C) 2001 Elsevier S cience B.V. All rights reserved.