Thin crust as evidence for depleted mantle supporting the Marion Rise

Abstract

The global ridge system is dominated by oceanic rises reflecting large variations in axial depth associated with mantle hotspots. The little studied Marion Rise is as large as the Icelandic, considering length and depth, but has an axial rift rather than a high nearly its entire length. Uniquely, along the SW Indian Ridge systematic sampling allows direct examination of crustal architecture over its full length. Unlike Iceland, peridotites are extensively exposed high over the rise. This shows for the 1 st time that the crust is generally thin, and often missing over a rifted rise. Thus the rise must be largely an isostatic response to ancient melting events that created low-density depleted mantle beneath the ridge rather than thickened crust and/or a large thermal anomaly. The likely origin for the depleted mantle is that emplaced into the African asthenosphere during the Karoo and Madagascar flood basalt events. Following Morgan1 , common wisdom equates oceanic rises to a hot fertile mantle plume producing a flow of plume-derived mantle to the ridge and down the sub-axial asthenospheric channel, resulting in elevated mantle temperature, ridge topography, and thickened igneous crust (the mantle wedge hypothesis, e.g.:2 ). Studies of the Reykjanes Ridge support this, with seismic crust thickening from ~6 km near the Gibbs FZ for 1600 km to ~18 km at the Reykjanes Peninsula e.g.: 3 . The Icelandic Rise, also has a long axial high consistent with such robust magmatism 4 (Table 1). The current consensus is the igneous crust thickens from east to west up the Marion Rise, and indeed up all rises (e.g.: 5 6 ). Basalt sodium contents, like other rises, decrease systematically towards Marion, and are interpreted to represent higher degrees of mantle melting and thicker crust 7 . Dick et al. 8 , however, point out that such correlations can be due to variable mantle temperature, or an increasingly depleted mantle source composition, and do not require thicker crust. The Marion Rise, in particular, has numerous large-offset transforms that would block sub-axial asthenospheric flow (e.g.: 9 ), and its deep rift valley indicates anemic rather than robust magmatism 4 . Niu and O'Hara 10 , though assuming thick crust over rises, suggest the global correlation of basalt chemistry and ridge depth is best explained by mantle composition variations, with many rises supported by depleted chemically buoyant mantle as first proposed by O'hara 11 for the Iceland Rise and Presnall and Helsley 12 for the Azores.2 14 . c) Maximum and minimum elevations are measured from shallowest point on axis, or midpoint of Iceland plateau to the rift valley axis point where average depth stabilizes or blocking transform. d) The 200-km average represents average elevation about the highest or lowest point. e) Iceland average plateau height; crest of Azores Rise, SWIR @ 36°15Ed) MAR @ Charlie Gibbs FZ; axial valley floor, MAR @ 24°N, axial valley floor @61°45'E. e) Depth anomaly calculated for the 200-km averages to eliminate local topographic e...

Major and trace elements of abyssal peridotites: evidence for melt refertilization beneath the ultraslow-spreading Southwest Indian Ridge (53° E segment)

“…Ancient melting may be a widespread feature of the oceanic upper mantle, especially at slow-ultraslow-spreading ridges where the mantle history may be more complicated (Meyzen et al 2003;Seyler et al 2004;Liu et al 2008;Suhr et al 2008). If the mantle beneath the 53°E ridge segment was indeed depleted by previous melting events as revealed by the occurrence of both fertile and depleted mantle rock in a restricted area, this may agree with the scenario proposed by Zhou and Dick (2013) who envisaged that the asthenosphere beneath the SWIR was drawn up from beneath southern Africa, Madagascar, and Antarctica, and thus may represent the depleted source mantle for the Karoo, Madagascar, and Ferrar flood basalt events.…”

“…The deep rift valley lacks an axial high and has a water depth of 3.9-5 km along the axial ridge Zhou and Dick (2013). The colour scales give depth in metres.…”

“…All the sample stations were located south of the spreading centre except for D9 and TVG10, which were located in the northern wall of the central rift valley. The rock assemblages of these stations were analysed by Zhou and Dick (2013) who defined two plutonic terranes (dashed white lines in Figure 1b) according to the massive exposure of the peridotites and gabbros. The western plutonic terrane ( Figure 1b) is floored predominantly by mantle peridotites with scarce crustal rocks, suggesting that the crustal thickness there is less than 1-2 km .…”

Re-Os isotopic constraints on the evolution of the Bangong-Nujiang Tethyan oceanic mantle, Central Tibet

“…The Ligurian Tethys appears to provide an example of the stranding of buoyant old SCLM remnants within the oceanic domain as a consequence of rifting and listric faulting at continental margins . Thin or missing crust above abyssal peridotites also supports the conclusion that the depleted mantle in the Marion Rise is probably SCLM emplaced into asthenosphere, representing fragments of the ancient Gondwana continental mantle (Zhou and Dick, 2013).…”

Drilling into deep-seated hard rocks of the oceanic plate formed at the Mid-Ocean Ridge: results and future perspectives: Deep-seated Hard Rock Drilling

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