Duration of off-axis, low-temperature, hydrothermal alteration of the upper oceanic crust lavas based on Rb/Sr age distributions of celadonite samples from the Troodos Ophiolite and DSDP/ODP drill cores

The lavas of the ocean crust interact with seawater-like fluids at low temperatures (10's of ° C) in off-axis hydrothermal systems which chemically modifies both seawater and rock, but the duration of chemical exchange is incompletely understood. Previous K-Ar dating of celadonite suggested that alteration starts soon after crust accretion and continues for at least 50 Myrs [1], however Rb-Sr dating suggests a shorter duration [2]. The duration of alteration depends, at least in part, on the evolution of permeability, related to the sedimentation history and crustal structure (e.g. sheet flows have lower permeability than pillows). To better understand the duration of alteration, and the controls, we have dated a large suite of celadonite samples from three different ~100 km² regions in the lavas of the Troodos ophiolite as well as samples from DSDP/ODP drill cores 417A, 417D, 418A, 556, 558, 564 by Rb/Sr using a recently developed triple quadrupole mass spectrometry method that allows rapid analysis and hence large data sets [3]. Age distributions from the Troodos ophiolite, and preliminary ages from the drill cores, confirm that most celadonites formed within ~10 Myrs after crust accretion in all sample suites. Both, published and new Rb/Sr ages of celadonites from sites 417A and 417D, which have a similar lithological make up but significantly different sedimentation histories, show that there is no evidence that alteration continued beyond ~15 Myrs after crust formation, inconsistent with sedimentation history controlling the duration of celadonite formation here. However, in the Troodos ophiolite there is a marked difference in the celadonite ages between a region with a relatively high proportion of sheet flows and an earlier initiation of sedimentation, compared to a more pillow dominated region that underwent later sedimentation.The former area shows a longer duration of celadonite formation suggesting that the permeability structure of the lavas could be key to explaining observed regional variability and should be considered in attempts to quantify chemical exchange between seawater and basalt at off-axis, low-temperature, hydrothermal systems. [1] Gallahan & Duncan, JGR, 99, 3147-3161; [2] Booij et al. 1995, Chem.Geol., 126, 155-167; [3] Bolea-Fernandez et al. (2016), JAAS, 31, 303-310.