5 research outputs found
The influence of ridge geometry at the ultraslow-spreading Southwest Indian Ridge (9Âș-25ÂșE) : basalt composition sensitivity to variations in source and process
Submitted in partial fulfillment of the requirements for the degree of
Doctor of Philosophy at the Massachusetts Institute of Technology
and the Woods Hole Oceanographic Institution February 2006Between 9Âș-25Âș E on the ultraslow-spreading Southwest Indian Ridge lie two sharply
contrasting supersegments. One 630 km long supersegment erupts N-MORB that is
progressively enriched in incompatible element concentrations from east to west. The
second 400 km long supersegment contains three separate volcanic centers erupting EMORB
and connected by long amagmatic accretionary segments, where mantle is
emplaced directly to the seafloor with only scattered N-MORB and E-MORB erupted.
Rather than a major break in mantle composition at the discontinuity between the
supersegments, this sharp contrast in geometry, physiography, and chemistry reflects
âsourceâ versus âprocessâ dominated generation of basalt.
Robust along-axis correlation of ridge characteristics (i.e. morphology, upwelling rate,
lithospheric thickness), basalt chemistry, and crustal thickness (estimated from gravity)
provides a unique opportunity to compare the influence of spreading geometry and rate
on MORB generation. What had not been well established until now is the importance of
melting processes rather than source at spreading rates < 20 mm/yr. Along the
orthogonally spreading supersegment (14 mm/yr) moderate degrees of partial melting
effectively sample the bulk mantle source, while on the obliquely spreading
supersegment (7-14 mm/yr) suppression of mantle melting to low degrees means that the
bulk source is not uniformly sampled, and thus âprocessâ rather than âsourceâ dominates
melt chemistry.The main
body of work consisting of major element, trace element, and isotopic data acquisition
and interpretation (Chapter 2 & 3) was funded by H. Dickâs grant from the National
Science Foundation-OCE 9907630. National Science Foundation-OCE 0137325
supported the U-series work described in Chapter 4. The published work of Chapter 5
was funded by National Science Foundation-EAR 9804891, NSF-OCE 9416620, and
NSF-OCE 0096634
Young off-axis volcanism along the ultraslow-spreading Southwest Indian Ridge
Author Posting. © The Authors, 2010. This is the author's version of the work. It is posted here by permission of Nature Publishing Group for personal use, not for redistribution. The definitive version was published in Nature Geoscience 3 (2010): 286-292, doi:10.1038/ngeo824.Mid-ocean ridge crustal accretion occurs continuously at all spreading rates
through a combination of magmatic and tectonic processes. Fast to slow spreading
ridges are largely built by adding magma to narrowly focused neovolcanic zones. In
contrast, ultraslow spreading ridge construction significantly relies on tectonic
accretion, which is characterized by thin volcanic crust, emplacement of mantle
peridotite directly to the seafloor, and unique seafloor fabrics with variable
segmentation patterns. While advances in remote imaging have enhanced our
observational understanding of crustal accretion at all spreading rates, temporal
information is required in order to quantitatively understand mid-ocean ridge
construction. However, temporal information does not exist for ultraslow spreading
environments. Here, we utilize U-series eruption ages to investigate crustal
accretion at an ultraslow spreading ridge for the first time. Unexpectedly young
eruption ages throughout the Southwest Indian ridge rift valley indicate that
neovolcanic activity is not confined to the spreading axis, and that magmatic crustal
accretion occurs over a wider zone than at faster spreading ridges. These
observations not only suggest that crustal accretion at ultraslow spreading ridges is
distinct from faster spreading ridges, but also that the magma transport
mechanisms may differ as a function of spreading rate.This work was supported by
the following NSF grants: NSF-OCE 0137325; NSF-OCE 060383800; and NSF-OCE
062705300
MORB generation beneath the ultraslow spreading Southwest Indian Ridge (9â25°E) : major element chemistry and the importance of process versus source
Author Posting. © American Geophysical Union, 2008. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Geochemistry Geophysics Geosystems 9 (2008): Q05004, doi:10.1029/2008GC001959.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 H2O contents, elevated K2O 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.This work was
originally funded in large part by NSF grants OCE-9907630
and OCE-0526905 and more recently by OPP-0425785