Active near-surface mobilisation of slab-derived geochemical signatures by hyperalkaline waters in brecciated serpentinites

Imaging of boron in altered mantle rocks illuminates progressive serpentinisation episodes

“…An interpretation of multiple temporally distinct alteration events with contrasting fluid sources yielding serpentine of progressively lower boron concentrations is in agreement with the outlined geological history of the Troodos Mantle Sequence and interpretations of previous stable oxygen and hydrogen isotope analyses (Evans et al, 2021). The initial high boron (∼80 μg/g) pervasive serpentinisation most likely results from fluid liberated by dehydration of ocean crust and sediments from the subducting Cyprus slab (e.g., Robertson, 1998;Evans et al, 2021Evans et al, , 2024, similar to high B concentration fluids from Mariana forearc serpentinite mud volcanoes that are interpreted to be upwelling slab-derived fluids (∼40 μg/g; Benton et al, 2001;Mottl et al, 2004).…”

“…The low boron fluid end member attributed to the second and third Troodos serpentinisation events is consistent with alteration by meteoric waters, where boron derived from the initial pervasive serpentinisation is remobilised by high pH (>9) groundwaters (Evans et al, 2024) re-precipitated serpentine with lower boron concentrations but with the boron isotope signature of the initial event (Fig. 3).…”

“…Subduction beneath Cyprus stalled due to the collision of the Eratosthenes plateau with the Cyprean trench less than 5 million years ago. This collision focused fluids released by the dehydration of the downgoing Cyprus slab beneath the Mount Olympus region, inducing serpentinisation hydration reactions and isostatic uplift (Robertson, 1998;Evans et al, 2021Evans et al, , 2024.…”

“…The Troodos mantle peridotites have been uplifted and exposed since the Pleistocene (Poole and Robertson, 1991), a process enhanced by the incursion of meteoric waters. Oxygen and hydrogen isotope analyses of serpentinised Troodos rocks have yielded various interpretations, including exchange with Cretaceous seawater at the spreading ridge, exchange at 200 to 300 °C with waters sourced from the Cyprus slab, or exchange at less than 50 °C with meteoric water (Magaritz and Taylor, 1974;Sheppard, 1980;Nuriel et al, 2009;Evans et al, 2021Evans et al, , 2024.…”

“…The high lithium abundance in some serpentine veins within the element map (Fig. 2d) likely reflects precipitation from Li-enriched meteoric-derived hyperalkaline groundwaters (Evans et al, 2024).…”