Volatile abundances and oxygen isotopes in basaltic to dacitic lavas on mid-ocean ridges: The role of assimilation at spreading centers

Most geochemical variability in MOR basalts is consistent with low- to moderate-pressure fractional crystallization of various mantle-derived parental melts. However, our geochemical data from MOR high-silica glasses, including new volatile and oxygen isotope data, suggest that assimilation of altered crustal material plays a significant role in the petrogenesis of dacites and may be important in the formation of basaltic lavas at MOR in general. MOR high-silica andesites and dacites from diverse areas show remarkably similar major element trends, incompatible trace element enrichments, and isotopic signatures suggesting similar processes control their chemistry. In particular, very high Cl and elevated H2O concentrations and relatively light oxygen isotope ratios (~5.8‰ vs. expected values of ~6.8‰) in fresh dacite glasses can be explained by contamination of magmas from a component of ocean crust altered by hydrothermal fluids. Crystallization of silicate phases and Fe-oxides causes an increase in δ18O in residual magma, but assimilation of material initially altered at high temperatures results in lower δ18O values. The observed geochemical signatures can be explained by extreme fractional crystallization of a MOR basalt parent combined with partial melting and assimilation (AFC) of amphibole-bearing altered oceanic crust. The MOR dacitic lavas do not appear to be simply the extrusive equivalent of oceanic plagiogranites. The combination of partial melting and assimilation produces a distinct geochemical signature that includes higher incompatible trace element abundances and distinct trace element ratios relative to those observed in plagiogranites. © 2011 Elsevier B.V

Reconciling geochemical and geophysical observations of magma supply and melt distribution at the 9N overlapping spreading center, East Pacific Rise

Early studies of mid-ocean ridge discontinuities, such as transform faults and overlapping spreading centers, suggested a lower magma supply compared to ridge segment centers. This is reflected in bathymetrically deeper ridge axes, decreased hydrothermal activity, and the eruption of more evolved lava compositions. While many signatures of lower magma supply are observed at the 9N overlapping spreading center on the East Pacific Rise, geophysical studies indicate extensive sub-surface melt in the region, suggesting that the present magmatic system is not diminished. Here major and trace element concentrations of erupted lavas are used to better understand magma supply at a large second-order ridge discontinuity. We show that the wide range of lava compositions erupted at the 9N overlapping spreading center is generally consistent with early petrologic models of ridge propagation and require variable degrees of fractional crystallization, extensive magma mixing, and in some instances crustal assimilation. Moderately evolved ferrobasalts and FeTi basalts erupted at the OSC indicate that crustal residence times are long enough for significant crystallization of all magmas within the region, but the presence of dacitic lavas reflects periods of even lower magma supply, where melt replenishment is subordinate to cooling and crystallization. The geophysical observations of extensive melt within the shallow crust are reconciled with the geochemistry of the lavas, if melts are supplied intermittently to the propagating ridge over relatively short timescales. © 2012. American Geophysical Union. All Rights Reserved

Seafloor photo-geology and sonar terrain modeling at the 9°N overlapping spreading center, East Pacific Rise

A fundamental goal in the study of mid-ocean ridges is to understand the relationship between the distribution of melt at depth and seafloor features. Building on geophysical information on subsurface melt at the 9°N overlapping spreading center on the East Pacific Rise, we use terrain modeling (DSL-120A side scan and bathymetry), photo-geology (Jason II and WHOI TowCam), and geochemical data to explore this relationship. Terrain modeling identified four distinct geomorphic provinces with common seafloor characteristics that correspond well to changes in subsurface melt distribution. Visual observations were used to interpret terrain modeling results and to establish a relative seafloor age scale, calibrated with radiometric age dates, to identify areas of recent volcanism. On the east limb, recent eruptions in the north are localized over the margins of the 4 km wide asymmetric melt sill, forming a prominent off-axis pillow ridge. Along the southern east limb, recent eruptions occur along a neovolcanic ridge that hugs the overlap basin and lies several kilometers west of the plunging melt sill. Our results suggest that long-term southward migration of the east limb occurs through a series of diking events with a net southward propagation direction. Examining sites of recent eruptions in the context of geophysical data on melt distribution in the crust and upper mantle suggests melt may follow complex paths from depth to the surface. Overall, our findings emphasize the value of integrating information obtained from photo-geology, terrain modeling, lava geochemistry and petrography, and geophysics to constrain the nature of melt delivery at mid-ocean ridges. Key Points Terrain modeling and photogeology show links between eruptions and crustal melt Eruptions above 4-km wide melt sill occur only above sill's margins Terrain modeling found four provinces that differ from classic tectonic view of OSC © 2013 The Authors. Geochemistry, Geophysics, Geosystems published by Wiley Periodicals, Inc. on behalf of American Geophysical Union