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Magde, LS, Detrick RS, Kent GM, Harding AJ, Orcutt JA, Mutter JC, Buhl P.  1995.  Crustal and Upper-Mantle Contribution to the Axial Gravity-Anomaly at the Southern East Pacific Rise. Journal of Geophysical Research-Solid Earth. 100:3747-3766.   10.1029/94jb02869   AbstractWebsite

This paper reassesses the crustal and upper mantle contribution to the axial gravity anomaly and isostatic topography observed at two segments (14 degrees S and 17 degrees S) of the southern East Pacific Rise (SEPR) in order to determine what constraints these data place on the amount of melt present in the underlying mantle. Gravity effects due to seafloor topography and relief on the Moho (assuming a constant crustal thickness and density) overpredict the amplitude of the gravity high at the EPR by 8-10 mGal. About 70% of this mantle Bouguer anomaly (MBA) low (6-7 mGal) can be explained by a region of partial melt and elevated temperatures in the mid-to-lower crust beneath the rise axis. Compositional density reductions in the mantle due to melt extraction are shown to make a negligible contribution to the amplitude of the observed MBA. Temperature-related mantle density variations predicted by a simple, plate-driven, passive flow model with no melt retention can adequately account for the mantle contribution to the observed MRA within the experimental uncertainty (+/- 1 mGal). However, the retention of a small amount of melt (less than or equal to 1-2% at 14 degrees S;less than or equal to 4% at 17 degrees S) in a broad region (tens of kilometers wide) of upwelling mantle is also consistent with the observed gravity data given the uncertainty in crustal thermal models. The anomalous height of the narrow, topographic high at the EPR provides the strongest evidence for the existence of significant melt fractions in the underlying mantle. It is consistent with the presence of a narrow (similar to 10 km wide) partial melt conduit that extends to depths of 50-70 km with melt concentrations up to 2% higher than the surrounding mantle. Along-axis variations in mantle melt fraction that might potentially indicate focused upwelling are only marginally resolvable in the gravity data due to uncertainties,in crustal thermal models. The good correlation between along-axis variations in depth, and changes in axial volume and gravity, argue against the mantle melt conduit as being the major source of this along-axis variation. Instead, this variability can be adequately explained by a combination of along-axis changes in crustal thermal structure and/or alone-axis crustal thickness changes of a few hundred meters.

Michael, PJ, Forsyth DW, Blackman DK, Fox PJ, Hanan BB, Harding AJ, Macdonald KC, Neumann GA, Orcutt JA, Tolstoy M, Weiland CM.  1994.  Mantle Control of a Dynamically Evolving Spreading Center - Mid-Atlantic Ridge 31-34-Degrees-S. Earth and Planetary Science Letters. 121:451-468.   10.1016/0012-821x(94)90083-3   AbstractWebsite

A segment of the slow-spreading Mid-Atlantic Ridge (MAR) at 33-degrees-S changes dramatically as its center is approached. Towards the center of the segment, the axis shoals from 3900 to 2400 m and a deep median valley nearly disappears. There is a prominent bullseye gravity low centered over the shallow summit, indicating thicker crust or lower density mantle or both. Incompatible element and radiogenic isotope ratios in MORB increase, creating a 'spike high' centered on the summit of the segment. The basalts' enrichment is confined to this robust ridge segment alone and is geochemically unlike the nearby hotspots at Tristan da Cunha, Gough and Discovery Islands. The average extent of mantle melting for the entire segment, as determined from mid-ocean ridge basalt (MORB) major element chemistry, is slightly greater than for adjacent segments. The segment has lengthened to 100 km by ridge propagation at both ends during the past 3.5 m.y., and is presently the longest and shallowest segment in the region. Although the ridge crest anomalies of this ridge segment strongly resemble those caused by the interaction of mid-ocean ridges with mantle hotspots, the geochemical and geophysical evidence suggests that they may instead be related to interaction of the ridge with a passively embedded chemical heterogeneity in the mantle.

Mutter, JC, Barth GA, Buhl P, Detrick RS, Orcutt J, Harding A.  1988.  Magma Distribution Across Ridge-Axis Discontinuities on the East Pacific Rise from Multichannel Seismic Images. Nature. 336:156-158.   10.1038/336156a0   AbstractWebsite

Detailed studies of the morphology of the East Pacific Rise axis have shown that its linearity is disrupted by many small but distinct non-transform offsets, including overlapping spreading centres (OSCs) and deviations from axial linearity (devals), which display variable geochemical signals1–9. Using multichannel seismic reflection profiling, we have mapped the distribution of a bright, shallow reflector that Detrick et al. 10 have associated with an axial magma chamber. We have found that it is neither continuous across the 9°03' OSC6, nor separated into two parallel bodies2, and that its lateral offset does not conform to that of the topographic offset associated with the 9° 17' deval. These observations provide important insight into a causative relationship between morphological and petrological segmentation in this region of the East Pacific Rise, and we speculate that the discontinuities may be the morphological response to fluctuations in the spatial pattern of magma delivery.

Mutter, JC, Carbotte SM, Su WS, Xu LQ, Buhl P, Detrick RS, Kent GM, Orcutt JA, Harding AJ.  1995.  Seismic Images of Active Magma Systems Beneath the East Pacific Rise Between 17-Degrees-05' and 17-Degrees-35'S. Science. 268:391-395.   10.1126/science.268.5209.391   AbstractWebsite

Seismic reflection data from the East Pacific Rise between 17 degrees 05' and 17 degrees 35'5 image a magma lens that varies regularly in depth and width as ridge morphology changes, confirming the notion that axial morphology can be used to infer ridge magmatic state. However, at 17 degrees 26'S, where the ridge is locally shallow and broad, the magma lens is markedly shallower and wider than predicted from regional trends. In this area, submersible dives reveal recent volcanic eruptions. These observations indicate that it is where the width and depth of the magma chamber differ from regional trends, indicating an enhanced magmatic budget, that is diagnostic of current magmatism.