Multiple episodes of serpentinite alteration revealed by progressive leaching experiments

Abstract

Serpentinized mantle rocks typically experience multiple alteration events, but to date, identifying distinct serpentinization episodes remains difficult because whole rock geochemical and isotopic analyses reflect the cumulative sum of fluid-rock interactions. Here we unravel the alteration history of serpentinized mantle rocks by undertaking multi-stage progressive leaching experiments to isolate distinct geochemical and ⁸⁷Sr/⁸⁶Sr signatures hosted within serpentinized peridotites from the Troodos Mantle Sequence and Limassol Forest Complex, Cyprus that have different geological histories. Whole rock powders underwent an initial 10 % acetic acid solution leaching step before a subsequent 3 M HCl treatment. The remaining residue was then completely dissolved and analysed following a standard HF and HNO₃ digestion. For comparison an untreated sample of whole rock powder was also completely digested and analysed. In the Troodos Mantle Sequence, an initial serpentinization event with relatively primitive ⁸⁷Sr/⁸⁶Sr ratios (0.705 to 0.706) is recorded by the rock residues and is consistent with alteration by fluids sourced from the dehydrating downgoing Cyprus slab. A more radiogenic ⁸⁷Sr/⁸⁶Sr (0.7086–88) signal, consistent with Cyprus Messinian seawater and evaporites, is mobilised by the initial leaching step. These contrasting signatures in the mantle rocks are similar to the Sr-isotopic compositions of Troodos Mantle Sequence groundwaters. Although previous field observations within the Limassol Forest Complex indicate some serpentinization by Cretaceous seawater, evidence from progressive leaching experiments of pervasive serpentinization on the Cretaceous seafloor is not forthcoming. Our results show that most samples from the Limassol Forest Complex yield a radiogenic ⁸⁷Sr/⁸⁶Sr signal (0.7087), most consistent with mid-Miocene Messinian seawater and broadly coeval with significant mid-Miocene uplift of the Limassol Forest Complex. These results demonstrate that progressive leaching approaches can reveal distinct alteration episodes with contrasting ⁸⁷Sr/⁸⁶Sr ratios and geochemical signatures that contribute to the overall integrated bulk rock signal.

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