In-situ Rb-Sr dating of celadonite from altered upper oceanic crust using laser ablation ICP-MS/MS

Abstract

Interaction of seawater with oceanic lavas at low temperatures (10's of °C) away from mid-ocean ridges has been suggested to play a significant role in setting the major element composition of seawater. One of the abundant, void-filling, secondary minerals that forms during this process is the K-rich phyllosilicate celadonite. Here, we apply a recently developed in-situ Rb-Sr dating method, using laser ablation tandem quadrupole inductively coupled mass spectrometry (LA-ICP-MS/MS), to determine the ages of celadonites in upper ocean crust lavas. We present an analytical method and data reduction scheme for in-situ Rb-Sr dating using methyl-fluoride (CH3F) as a collision reaction gas. The 87Sr/86Sr ratios were quantified by standard sample bracketing using NIST SRM612. The 87Rb/86Sr ratios were calibrated using the MPI-DING standards ATHO-G, T1-G and StHs6/80-G. We analysed veins and amygdales in 32 samples recovered by seafloor drilling of Cretaceous and Cenozoic aged oceanic crust. The data shows that ~80% of celadonite formed within the first 20 Myr after the crust accreted, with no systematic variation in the duration of celadonite formation with crustal age, sedimentation history or depth in the crust. There is no resolvable difference in the ages of celadonite precipitated in veins and amygdales suggesting infilling of both interconnected and more isolated pores within the same time interval. The fraction of celadonite formed over any time interval after crustal accretion closely matches the fraction of hydrothermal heat loss suggesting that heat flow fraction provides a good proxy for chemical exchange in these systems. Overall, we conclude that alteration conditions within the first 20 Myr after crustal accretion must largely control the bulk chemical exchange between seawater and the upper oceanic crust.