Local and regional variation of MORB parent magmas: evidence from melt inclusions from the Endeavour Segment of the Juan de Fuca Ridge

The development of petrogenetic models of igneous processes in the mantle i s dependent on a detailed knowledge of the diversity of magmas produced in the melting regime. These primary magmas, however, undergo significant mixi ng and fractionation during transport to the surface, destroying much of th e evidence of their primary diversity. To circumvent this problem and to de termine the diversity of melts produced in the mantle, we used melt inclusi ons hosted in primitive plagioclase phenocrysts from eight mid-ocean ridge basalts from the axial and West Valleys of the Endeavour Segment, Juan de F uca Ridge. This area was selected for study because of the demonstrated clo se association of enriched (E-MORB) lavas and incompatible element enriched depleted (N-MORB) lavas. Rehomogenized melt inclusions from E-MORB, T-MORB , and N-MORB lavas have been analyzed by electron and ion microprobe for ma jor and trace elements, The depleted and enriched lavas, as well as their m elt inclusions, have very similar compatible element concentrations (major elements, Sr, Ni and Cr). Inclusion compositions are more primitive than. v et collinear with, the host lava suites. In contrast, the minor and trace e lement characteristics of melt inclusions from depleted and enriched lavas are different both in range and absolute concentration. N-MORB lavas contai n both depleted and enriched melt inclusions, and therefore exhibit the lar gest compositional range (K2O: 0.01 to 0.4 oxide wt%, P2O5: <0.01 to 0.2 ox ide wt%, La-N: 7 to 35, Yb-N: 1 to 13, and Ti/Zr: <100 to 1300). E-MORB lav as contain only enriched inclusions, and are therefore relatively homogeneo us (K2O: 0.32 to 0.9 oxide wt%, P2O5: 0.02 to 0.15 oxide wt%, La-N: 11 to 6 0, Yb-N: 4 to 21, and Ti/Zr: similar to 100). In addition, the most primiti ve E-32 inclusions are similar in composition to the most enriched inclusio ns from the depleted hosts. Major element data for melt inclusions from bot h N-MORB and E-MORB lavas suggest that the magmas lie on a low pressure cot ectic, consistent with a petrogenesis including fractional crystallization. However, the minor and trace element compositions in melt inclusions vary independently of the major element composition implying an alternative hist ory. When fractionation-corrected, inclusion compositions correlate with th eir host glass composition. Hence, the degree of enrichment of the lavas is a function of the composition of aggregated melts, not of processing in th e upper mantle or lower crust. Based on this fact. the lava suites are not produced from a single parent magma, but from a suite of primary magmas. Th e chemistry of the molt inclusions from the enriched lavas is consistent wi th a derivation from variable percentages of partial melting within the spi nel stability field by a process of open system (continuous or critical) me lting assuming a depleted Iherzolite source veined with clinopyroxenite. Th e low percentage melts are dominantly enriched melts of the clinopyroxenite . In contrast, the depleted lavas were created by melting of a harzburgite source, possibly fluxed with a fluid enriched in K, Ba and the LREE. Such a source was likely melted up to or past the point at which all of its clino pyroxene wets consumed. This set of characteristics is consistent with a sc enario by which diverse melts produced at different depths travel through t he melting regime to the base of the crust without homogenizing en route. T he homogeneous major element characteristics are created in the lower crust by fractional crystallization and reaction with lower crustal gabbros. Therefore, the degree of decoupling between major and trace element charact eristics of the melt inclusions (and lavas) is dictated by the reaction rat e of the melts with the materials in the conduit walls, as well as the resi dence times and flux rate, in the upper mantle and lower crust.