Mark R. Muller

Mid-Atlantic Ridge–Azores hotspot interactions: along-axis migration of a hotspot-derived event of enhanced magmatism 10 to 4 Ma ago

Abstract A recent survey of the Mid-Atlantic Ridge over the southern edge of the Azores Platform shows that two anomalously shallow regions located off-axis on both sides of the ridge are the two flanks of a single rifted volcanic plateau. Crustal thickness over this plateau is up to twice that of surrounding oceanic areas, and original axial depths were near sealevel. The lack of a coherent magnetic anomaly pattern, and the near absence of fault scarps over the plateau suggest that its formation involved outpouring of lava over large distances off-axis. This volcanic plateau formed in Miocene times during an episode of greatly enhanced ridge magmatism caused, as proposed by P.R. Vogt [Geology 7 (1979) 93–98], by the southward propagation of a melting anomaly originated within the Azores hotspot. This melting anomaly could reflect excess temperatures of ∼70°C in the mantle beneath the ridge. It propagated at rates of ∼60 mm/yr and lasted no more than a few million years at any given location along the ridge. Enhanced magmatism due to this melting anomaly played a significant role, some 10 Ma ago, in the construction of the Azores Platform.

Segmentation and melt supply at the Southwest Indian Ridge

The segmentation of mid-ocean ridge axes and its variation with spreading rate provide constraints on models of mantle upwelling and the supply of basaltic melt to the crust. We present seismic refraction results from three segments of the very slow spreading Southwest Indian Ridge. From the absence of oceanic layer 3 beneath segment boundaries, and from significant variations in maximum crustal thickness between segments, we infer that melt generation and/or delivery is focused at segment midpoints and varies between segments. Anomalously thin crust—2.0–2.5 km beneath the nontransform segment boundaries and 3.5–6.0 km beneath the segment mid-points—is caused by restricted melt generation resulting from conductive heat loss from the upwelling mantle under the spreading center.

Thin crust beneath ocean drilling program borehole 735B at the Southwest Indian Ridge

Abstract A wide-angle seismic experiment at the Atlantis II Fracture Zone, Southwest Indian Ridge, together with geochemical analyses of dredged basalt glass samples from a site conjugate to Ocean Drilling Program hole 735B has allowed determination of the thickness and the most likely lithological composition of the crust beneath hole 735B. The measured Na8 composition of 3.3 ± 0.1 corresponds to a melt thickness of 3 ± 1 km, a result consistent with rare earth element inversions which indicate a melt thickness of between 1.5 and 4.5 km. The seismic crustal thickness to the north and south of the Atlantis Platform (on which hole 735B is located) is 4 ± 1 km, and probably consists largely of magmatic material since the seismic and inferred melt thicknesses agree within experimental uncertainty. Beneath hole 735B itself, the Moho is at a depth of 5 ± 1 km beneath the seafloor. The seismic model suggests that, on average, about 1 km of upper crust has been unroofed on the Atlantis Platform. However, allowing for the inferred local unroofing of 2 km of upper crust at 735B, the base of the magmatic crust beneath this location is probably about 2 km beneath the seafloor, and is underlain by a 2–3 km thick layer of serpentinised mantle peridotite. The P-wave velocity of 6.9 km/s for the serpentinised peridotite layer corresponds to a 35 ± 10 vol% serpentine content. The Moho beneath hole 735B probably represents a serpentinisation front.

Crustal structure of the Southwest Indian Ridge at the Atlantis II Fracture Zone

The Southwest Indian Ridge is a slow spreading end-member of the mid-ocean ridge system. The deepest borehole penetrating the lower oceanic crust, Ocean Drilling Program hole 735B, lies on the eastern transverse ridge of the Atlantis II Fracture Zone at 57°E. A wide-angle seismic survey in the vicinity of the borehole reveals a crustal structure that is highly heterogeneous. To the east of Atlantis Bank, on which hole 735B is located, the crust consists of a 2–2.5 km thick high-velocity-gradient oceanic layer 2 and a 1–2 km thick low-velocity-gradient layer 3. The transform valley has a 2.5–3 km thick crust with anomalously low velocities interpreted to consist largely of highly serpentinized mantle rocks. The seismically defined crust is thickest beneath the borehole, where layer 2 is thinner and the lower crust is inferred to contain 2–3 km of partially serpentinized mantle. The seismic velocity models are consistent with gravity data which show weak residual mantle Bouguer anomalies because the regions of thinner crust have lower crustal densities. Stress variations deduced from mass balances between the transform valley floor and the adjacent transverse ridges are much larger than the likely threshold for lithospheric failure and therefore indicate that the relief is supported dynamically. The variation of crustal thickness with spreading rate defined by data from the Southwest Indian Ridge and elsewhere is consistent with models of melt generation in which the upwelling mantle is cooled by conductive heat loss at very slow spreading rates, resulting in reduced melt generation under the spreading axis. Large segment-scale variations in crustal thickness suggest subcrustal along-axis migration of melt toward segment centers.

Is the oceanic Moho a serpentinization front

Abstract Our main constraint on the volume of melt coming out of the mantle at mid-ocean ridges is the thickness of oceanic crust determined by seismic methods. The recovery of serpentinized peridotites in the rift valleys of slow-spreading ridges has led recently to a revival of the suggestion that the seismically defined lower oceanic crust may include partially serpentinized peridotite. Both seismic and geochemical data from a recent experiment on the very slow-spreading Southwest Indian Ridge indicate that, away from the fracture zone valley, a total melt thickness of ∼4 km is being generated. However, in one region, around Ocean Drilling Program (ODP) hole 735B, where ∼2 km of the upper crust is estimated to have been removed, the seismic data indicate a crustal thickness of 5 km. Here, the seismically determined Moho is interpreted as a serpentinization front. Serpentinization is facilitated if there are readily available pathways for seawater to reach the upper mantle, and if the upper mantle is cool. These conditions frequently are met at fracture zones on slow-spreading ridges, in the ocean-continent transition zone at non-volcanic rifted margins, and at the axes of extinct rifts. In all these locations, serpentinization sufficient to lower mantle velocities to normal oceanic crustal velocities does not reach a depth of more than 5 km beneath the seabed, and commonly normal mantle velocities of ∼8 km s−1 are also reached at shallow depths. Therefore, although in some anomalous locations the seismically defined crustal thickness can only be used as an upper limit on the melt supply, for normal oceanic crust, where the seismically defined crustal thickness is ∼7 km, the Moho is probably not a serpentinization front, but rather a petrological boundary between mafic rocks above and ultramafic rocks below.