Melt inclusions as indicators of parental magma diversity on the northern East Pacific Rise

Abstract

The axial plumbing system is recognized as the site of much of the chemical processing that takes place at mid-ocean ridges. However, the extent of that processing and the characteristics of the original parental components are difficult to characterize using the lavas that erupted at the surface alone. To better understand the relative roles of different magmatic processes in the formation of axial and off-axis magma systems, we have studied plagioclase and olivine-hosted melt inclusions from two axial segments, 10°30′N and 11°20′N, and four seamounts between 5°47′N and 9°17′N of the northern East Pacific Rise (EPR). These data are a potential source of information on the characteristics of precursor magmas. By comparing the compositions of the inclusions with those of the host lava suite, one can discern the nature of the intervening processes. The melt inclusions found in these axial and seamount lavas were rehomogenized and analyzed by electron and ion microprobe for major and trace elements. The lava groups and their inclusions overlap and contain very similar N-MORB compatible and incompatible element concentrations, with the exception of one N-MORB host lava which contains both N- and E-MORB inclusions. Inclusion compositions are co-linear with and generally fall at the primitive end of the host lava suites. Inclusion diversity, i.e. the chemical variation in melt inclusion compositions within a single sample, decreases with decreasing MgO, probably representing contemporaneous mixing and fractionation. Seamount lavas differ from axial lavas in that they are more crystal-rich and they contain a greater number of inclusions that are generally more primitive. In addition, they exhibit a larger compositional range in both the incompatible major and trace elements. Axial and seamount lavas have comparable crystal assemblages and chemistry, melt inclusion diversity and range of compositions. All these characteristics suggest that axial and seamount magmas are formed in the same mantle-melting regime. We believe that axial and seamount magmas are created from a single parent magma array derived by the melting of a relatively homogeneous source. The parent magmas undergo an initial stage of fractionation and mixing, then, the axial and seamount magma paths diverge. Axial lavas continue through the axial magma chamber (AMC), becoming more fractionated and having more phenocrysts removed (possibly in the upper melt lens). Seamount magmas travel to their volcanoes without further significant fractionation and crystal separation. The East Pacific Rise differs from slow- and moderate-spreading rate ridges (AMAR, Juan de Fuca (JdF), Gorda and SEIR) in its lack of phyric lavas, scarcity of inclusions, lack of chemical variation between melt inclusions from the same crystal, lack of overall compositional variability and absence of E-MORB inclusions in the majority of the N-MORB host lavas, suggesting that EPR must have a fundamentally different axial plumbing system characterized by more extensive fractionation and mixing in the upper mantle and lowermost crust.