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

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

May 2004

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2004e%26psl.222..145j&link_type=abstract

Earth and Planetary Science Letters, Volume 222, Issue 1, p. 145-160.

Computer Science

19

Ocean Drilling Program Site 735B, Cooling, Lower Oceanic Crust, Thermochronometry

Scientific paper

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.

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