Leg 209 was devoted to drilling mantle peridotites and associated gabbroic rocks along the Mid-Atlantic Ridge from 14° to 16°N. This area was identified at the 1996 Workshop on Oceanic Lithosphere and Scientific Drilling into the 21st Century (OL Workshop) as the ideal region for drilling of a strike line of short holes to sample the upper mantle in a magma-starved portion of a slow-spreading ridge (spreading rate = ~25 km/m.y.). In this area, igneous crust is locally absent and the structure and composition of the mantle can be determined at sites more than ~100 km apart along strike.
A central paradigm of Ridge Interdisciplinary Global Experiments (RIDGE) studies is the hypothesis that mantle flow, or melt extraction, or both, are focused in three dimensions toward the centers of magmatic ridge segments, at least at slow-spreading ridges such as the Mid-Atlantic Ridge. This hypothesis has essentially reached the status of accepted theory, but it has never been subject to a direct test. A strike line of oriented mantle peridotite samples extending for a significant distance within magmatic segments offers the possibility of directly testing this hypothesis. Continued dredging and submersible studies cannot provide the spatial information required to make such a test.
The primary aim of drilling was to characterize the spatial variation of mantle deformation patterns, residual peridotite composition, melt migration features, plutonic rocks, and hydrothermal alteration along axis. Hypotheses for focused solid or liquid upwelling beneath ridge segments make specific predictions regarding the spatial variation of mantle lineation or the distribution of melt migration features. These predictions were directly tested by drilling. We discovered that penetrative mantle deformation fabrics are weak at every site where mantle peridotite was sampled from 14°43'N to 15°39'N. Instead, at all of these sites, deformation was localized along high-temperature shear zones and later brittle faults. Intact blocks of peridotite with high-temperature, protogranular fabrics were preserved between these zones of localized deformation and underwent substantial tectonic rotation, perhaps as much as 90° around horizontal, ride-parallel rotation axes in some places.
At most sites, drilling recovered substantial proportions of gabbroic rocks intrusive into mantle peridotite. Some of these rocks have mineral assemblages that are probably indicative of crystallization at depths of 12 to 20 km beneath the Mid-Atlantic Ridge. Localized deformation at several of these sites occurred preferentially within contact zones between peridotite and these gabbroic intrusions. Abundant gabbroic intrusions were found close to the 15°20' Fracture Zone, at Site 1271, and far from the fracture zone at Sites 1270, 1268, and 1275. Conversely, some holes intersected very little gabbroic material; these were at Site 1272, very close to the fracture zone, and Site 1274, far from the fracture zone. Thus, there is little evidence from the results of this leg for focusing of melt distribution away from the fracture zone and toward the centers of volcanically active ridge segments.
Three new hypotheses may account for our observations:
Very different hydrothermal alteration styles were observed at different Sites. In Hole 1268A, talc was particularly abundant in metaperidotites, accompanied by a dramatic metasomatic decrease in the (Mg + Fe)/Si ratio. Elsewhere, brucite was a prominent part of the alteration assemblage in peridotites, and rocks retain high (Mg + Fe)/Si. Gabbroic intrusions appear to have an important local control on serpentinization reactions in peridotite. Carbonate alteration of peridotites in some locales seems to be correlated with a metasomatic influx of calcium and may also substantially affect the trace element budget of serpentinites in some cases.
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