313 research outputs found
Effect of melt/mantle interactions on MORB chemistry at the easternmost Southwest Indian Ridge (61 to 67°E)
No full textThe easternmost part of the Southwest Indian Ridge (61°-67°E) is an end-member of the global ridge system in terms of very low magma supply. As such, it is a good laboratory to investigate the effect of melt/mantle interactions on the composition of erupted basalts: for a given volume of erupted basaltic melt, the volume of reacted mantle is potentially greater than at more magmatically robust ridges. We analyzed major, trace element and isotopic compositions in three groups of rocks: plagioclase-bearing ultramafic and gabbroic rocks dredged in nearly amagmatic spreading corridors; basalts from the sparse volcanic cover of these corridors (“ultramafic seafloor basalts”); and basalts dredged from the intervening, more volcanically active domains (“volcanic seafloor basalts”). Ultramafic seafloor basalts have significantly lower CaO and Al2O3 contents at a given MgO than most volcanic seafloor basalts. We propose that both types of basalts are derived from similar parental melts, but that the ultramafic seafloor basalts are more affected by reactions between these parent melts and the mantle rocks in the lithosphere below the ridge. We infer that these reactions occur in the walls of conduits that allow the aggregated melts extracted from the melting mantle to rise through the axial lithosphere and to the eruption sites. The principal effect of these reactions is to enrich the asthenospheric melts in MgO through olivine dissolution. This effect is not expected to be as noticeable, but could still play a role in basalt petrogenesis at more magmatic regions of the global slow-spreading MOR system
A 3-D seismic tomographic study of spreading structures and smooth seafloor generated by detachment faulting – the ultra-slow spreading Southwest Indian Ridge at 64˚30’E
No full textAt ultra-slow spreading ridges, with full spreading rates less than ∼20 mm/yr, spreading is accommodated both by highly spatially and temporally segmented magmatism, and tectonic extension along large-scale detachment faults that exhume ultramafic material to the seafloor. In the most magma-poor regions, detachment faulting alternates in polarity over time, producing a “flip-flopping” effect of subsequent detachment dips. The resulting seafloor in these regions displays a morphology termed “smooth seafloor” comprising elongate, broad ridges with peridotite/serpentinite lithologies. We conducted tomographic travel-time inversion of a 3-D wide-angle seismic data set acquired over a region of smooth seafloor around 64°30′E along the Southwest Indian Ridge (SISMOSMOOTH; Cruise MD199), to produce a seismic velocity volume through the crustal section and into the uppermost mantle. We observe patterns of velocity anomalies that correspond with variations in the bathymetry arising from the mode of spreading and are interpreted as changes in the degree of alteration with depth resulting from spatial and temporal variations in fluid-rock interaction, controlled by faulting and tectonic damage processes. The detachment faults do not show simple planar structures at depth but instead mirror the shapes of the bathymetric ridges that they exhume. Magmatic input is overall highly limited, but there is one region on the lower part of an exhumed detachment footwall where a thickness of volcanic material is observed that suggests a component of syn-tectonic volcanism, which could contribute to detachment abandonment
Seismicity and accretion processes along the Mid-Atlantic Ridge south of the Azores using data from the MARCHE autonomous hydrophone array
No full textThe seismicity of the South Atlantic Ocean has been recorded by the MARCHE network of 4 autonomous underwater hydrophones (AUH) moored within the SOFAR channel on the flanks of the Mid-Atlantic Ridge (MAR). The instruments were deployed south of the Azores Plateau between 32° and 39°N from July 2005 to August 2008. The low attenuation properties of the SOFAR channel for earthquake T-wave propagation result in a detection threshold reduction from a magnitude completeness level (Mc) of ~4.3 for MAR events recorded by the land-based seismic networks to Mc=2.1 using this hydrophone array. A spatio-temporal analysis has been performed among the 5600 events recorded inside the MARCHE array. Most events are distributed along the ridge between lat. 39°N on the Azores Platform and the Rainbow (36°N) segment. In the hydrophone catalogue, acoustic magnitude (Source Level, SL) is used as a measure of earthquake size. The source level above which the data set is complete is SLc=205 dB. We look for seismic swarms using the cluster software of the SEISAN package. The criterion used are a minimum SL of 210 to detect a possible mainshock, and a radius of 30 km and a time window of 40 days after this mainshock (Cevatoglu, 2010, Goslin et al., 2012). 7 swarms with more than 15 events are identified using this approach between 32°et 39°N of latitude. The maximum number of earthquake in a swarm is 57 events. This result differs from the study of Simao et al. (2010) as we processed a further year of data and selected sequences with fewer events. Looking at the distribution of the SL as a function of time after the mainshock, we discuss the possible mechanism of these earthquakes : tectonic events with a "mainshock-aftershock" distribution fitting a modified Omori law or volcanic events showing more constant SL values. We also present the geophysical setting of these 7 swarms, using gravity, bathymetry, and available local geological data. This study illustrates the potential of hydrophone data to monitor segment-scale ridges processes in the vicinity of the Lucky Strike seafloor observatory (lat. 37°20'N), the Azores node of the EMSO (European Multidiciplinary Subsea Observatory) system
multiporos, a modified porous convection model implementing thermal cracking and magma intrusions
No full textThis is a Matlab based 2D porous-elastic convection code modified from Fan et al. (2021), available at https://figshare.com/s/3c506b1aeafd28dff5ee.
We made magma injections available to emplace at any Tr (temperature of host rock upon melt emplacement).
Supplementary for: Chen, J., Olive, J.-A., & Cannat, M. (2022). Thermal regime of slow and ultraslow spreading ridges controlled by melt supply and modes of emplacement. Journal of Geophysical Research: Solid Earth, 127, e2021JB023715. https://doi.org/10.1029/2021JB023715</p
(Table spinel) Average compositions and analyses range for spinel from the MARK area at ODP Site 153-920D
No full text(Table spinel) Average compositions and analyses range for spinel from the MARK area at ODP Site 153-920
(Table ortho) Average compositions and analyses range for orthopyroxene from the MARK area at DSDP Site 45-395A
No full text(Table ortho) Average compositions and analyses range for orthopyroxene from the MARK area at DSDP Site 45-395
(Table olivin) Average compositions and analyses range for olivines from the MARK area at DSDP Site 45-395A
No full text(Table olivin) Average compositions and analyses range for olivines from the MARK area at DSDP Site 45-395
(Table olivin) Average compositions and analyses range for olivines from the MARK area at ODP Site 153-920B
No full text(Table olivin) Average compositions and analyses range for olivines from the MARK area at ODP Site 153-920
(Table ortho) Average compositions and analyses range for orthopyroxene from the MARK area at ODP Site 153-920B
No full text(Table ortho) Average compositions and analyses range for orthopyroxene from the MARK area at ODP Site 153-920
(Table 4) Chemical composition of selected primary phases in ODP Hole 109-670A samples
No full text(Table 4) Chemical composition of selected primary phases in ODP Hole 109-670A sample
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