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doi:10.1016/j.epsl.2004.02.014 
Copyright © 2004 Elsevier B.V. All rights reserved.
Determining the cooling history of in situ lower oceanic crust—Atlantis Bank, SW Indian Ridge
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Barbara E. John
, a, , 
, David A. Fosterb, John M. Murphya, Michael J. Cheadlea, A. Graham Bainesa, C. Mark Fanningc and Peter Copelandd
Received 3 July 2003;
Abstract
The cooling history and therefore thermal structure of oceanic lithosphere in slow-spreading environments is, to date, poorly constrained. Application of thermochronometric techniques to rocks from the very slow spreading SW Indian Ridge provide for the first time a direct measure of the age and thermal history of in situ lower oceanic crust. Crystallization of felsic veins (
850°C) drilled in Hole 735B is estimated at 11.93±0.14 Ma, based on U–Pb analyses of zircon by ion probe. This crystallization age is older than the ‘crustal age’ from remanence inferred from both sea surface and near-bottom magnetic anomaly data gathered over Hole 735B which indicate magnetization between major normal polarity chrons C5n.2n and C5An.1n (10.949–11.935 Ma). 40Ar/39Ar analyses of biotite give plateau ages between 11 and 12 Ma (mean 11.42±0.21 Ma), implying cooling rates of >800°C/m.y. over the first 500,00 years to temperatures below 330–400°C. Fission-track ages on zircon (mean 9.35±1.2 Ma) and apatite reveal less rapid cooling to <110°C by 7 Ma, some 4–5 m.y. off axis.
Comprehensive thermochronometric data from the structurally intact block of gabbro between 700 and 1100 m below sea floor suggest that crust traversed by ODP Hole 735B mimics conductive cooling over the temperature range 900–330°C, characteristic of a 2-D plate-cooling model for oceanic lithosphere. In contrast, lower temperature chronometers (fission track on zircon, titanite, and apatite; T≤280°C) are not consistent with these predictions and record anomalously high temperatures for crust >700 m below sea floor at 8–10 Ma (i.e. 2–4 m.y. off axis). We offer two hypotheses for this thermal anomaly:
- (i) Off-axis (or asymmetric) magmatism that caused anomalous reheating of the crust preserved in Hole 735B. This postulated magmatic event might be a consequence of the transtension, which affected the Atlantis II transform from 19.5 to 7.5 Ma.
- (ii) Late detachment faulting, which led to significant crustal denudation (2.5–3 km removed), further from the ridge axis than conventionally thought.
Author Keywords: Ocean Drilling Program Site 735B; cooling; lower oceanic crust; thermochronometry
