Determining the cooling history of in situ lower oceanic crust—Atlantis Bank, SW Indian Ridge

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 (f850jC) drilled in Hole 735B is estimated at 11.93F0.14 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). 40 Ar/ 39 Ar analyses of biotite give plateau ages between 11 and 12 Ma (mean 11.42F0.21 Ma), implying cooling rates of >800jC/m.y. over the first 500,00 years to temperatures below f330–400jC. Fission-track ages on zircon (mean 9.35F1.2 Ma) and apatite reveal less rapid cooling to 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 f19.5 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. D 2004 Elsevier B.V. All rights reserved.