MORB generation beneath the ultraslow spreading Southwest Indian Ridge (9–25°E): Major element chemistry and the importance of process versus source

Abstract

[1] We report highly variable mid-ocean ridge basalt (MORB) major element and water concentrations from a single 1050-km first-order spreading segment on the ultraslow spreading Southwest Indian Ridge, consisting of two supersegments with strikingly different spreading geometry and ridge morphology. To the east, the 630 km long orthogonal supersegment (<10°obliquity) dominantly erupts normal MORB with progressive K/Ti enrichment from east to west. To the west is the 400 km long oblique supersegment (up to 56°obliquity) with two robust volcanic centers erupting enriched MORB and three intervening amagmatic accretionary segments erupting both N-MORB and E-MORB. The systematic nature of the orthogonal supersegments' ridge morphology and MORB composition ends at 16°E, where ridge physiography, lithologic abundance, crustal structure, and basalt chemistry all change dramatically. We attribute this discontinuity and the contrasting characteristics of the supersegments to localized differences in the upper mantle thermal structure brought on by variable spreading geometry. The influence of these differences on the erupted composition of MORB appears to be more significant at ultraslow spreading rates where the overall degree of melting is lower. In contrast to the moderate and rather constant degrees of partial melting along the orthogonal supersegment, suppression of mantle melting on the oblique supersegment due to thickened lithosphere means that the bulk source is not uniformly sampled, as is the former. On the oblique supersegment, more abundant mafic lithologies melt deeper thereby dominating the more enriched aggregate melt composition. While much of the local major element heterogeneity can be explained by polybaric fractional crystallization with variable H 2 O contents, elevated K 2 O and K/Ti cannot. On the basis of the chemical and tectonic relationship of these enriched and depleted basalts, their occurrence requires a multilithology mantle source. The diversity and distribution of MORB compositions, especially here at ultraslow spreading rates, is controlled not only by the heterogeneity of the underlying mantle, but also more directly by the local thermal structure of the lithosphere (i.e., spreading geometry) and its influence on melting processes. Thus at ultraslow spreading rates, process rather than source may be the principle determiner of MORB composition.

Generation of permeability barriers during melt extraction at mid‐ocean ridges

“…In addition, stronger melt focusing than predicted by our model must be in effect to explain crustal thickness of slow ridges ( Figure 5). Melt focusing has been invoked to explain geochemical systematics and crustal variations at the SWIR 9°E-16°E area [Standish et al, 2008]. To form a permeability barrier according to our model requires a critical compaction length of at least 10 km, at the high end of published estimates Katz, 2008].…”

“…We place particular emphasis on the SWIR geometry, where geochemical enrichments perhaps indicate a contribution from deeply originating, low-F melts from beneath amagmatic segments to adjacent magmatic segments [Standish et al, 2008]. [48] In the presence of a complex flow field, or if the geometry of the ridge axis changes rapidly [Cannat et al, 2006], melts trapped in the thick TBL of ultraslow ridges at amagmatic segments may become entrained with the solid flow field spreading away from the axis and may become subsequently eroded and incorporated into the melting region of the neighboring magmatic segment (Figure 7a), serving as mantle heterogeneities and manifesting themselves at the ridge axis as chemical enrichments [Cannat et al, 2008].…”

“…Melt transport along a permeability barrier has been proposed to explain lava chemistry at the ultraslow Southwest Indian Ridge (SWIR) between 9°E and 25°E [Standish et al, 2008]. Deeply originating, lowdegree melts seem to be redistributed from beneath the thick lithosphere along amagmatic segments toward neighboring magmatic segments with thinner lithosphere, explaining the unusual enrichment in K 2 O and the high K/Ti of basalts recovered from the area [Standish et al, 2008]. Based on analysis of transform and ridge segment length along the Mid-Atlantic Ridge at 33°N-35°N, a permeability barrier has been found to be more efficient than diapiric instabilities for focusing magma at slow spreading rates (15 mm/yr half rate) .…”

Syn-eruptive degassing of a single submarine lava flow : constraints on MORB CO2 variability, vesiculation, and eruption dynamics

“…concentrations that indicate incomplete degassing (i.e., supersaturation relative to their eruption depths) [Cushman et al, 2004;Dixon et al, 1988;Le Roux et al, 2006;Simons et al, 2002;Standish et al, 2008]. Only under unique circumstances do MORB samples exhibit undersaturation of CO 2 (e.g., study of melt inclusions by Saal, et al [2002]).…”

“…Contrary to Henrian behavior, MORBs exhibit a range of volatile concentrations, indicating disequilibrium with ambient seafloor pressures (i.e. eruption depths) and commonly exhibit supersaturation of CO 2 (Figure 1) [Cushman et al, 2004;Dixon et al, 1988;Fine and Stolper, 1986;Le Roux et al, 2006;Simons et al, 2002;Standish et al, 2008]. Detailed studies at the fastspreading East Pacific Rise (EPR) indicate that lava samples are often supersaturated near the axis, but have concentrations closer to their equilibrium saturation levels off-axis [Le Roux et al, 2006].…”