Abstract
The Mariana forearc is a unique location for exploring the role serpentinization plays in the marine Si cycle by means of Si stable isotope variations. Here, active mud volcanism transports deep, serpentinized mantle wedge material to the surface and thus offers a natural window to slab dehydration processes in dependence of changing temperature and pressure with depth. Si isotopes were measured in situ by femtosecond laser ablation MC-ICPMS in serpentine within ultramafic clasts from three mud volcanoes (Yinazao, Fantangisña, and Asùt Tesoru) sampled during International Ocean Discovery Program Expedition 366. To corroborate the results, serpentinization of olivine was studied in batch experiments. The Si isotope ratios show large variations between the mud volcanoes and between individual serpentine generations within a given mud volcano. Serpentine that formed early under low water/rock ratios exhibits δ30Si of −0.41 ± 0.04‰ (1SD) similar to unaltered olivine which agrees well with experimental findings predicting no significant isotope fractionation during early serpentinization. In contrast, late serpentine veins formed under higher water/rock ratios span a wide range of Si isotope ratios that differ significantly between the individual mud volcanoes. With increasing distance to the trench, δ30Si of late veins are −0.10 ± 0.07‰, −1.94 ± 0.13‰, and −0.80 ± 0.22‰ and −0.93 ± 0.21‰. These δ30Si values are interpreted to record the isotopic composition of the fluid source, namely subducted biogenic silica and pore fluids, clays, and altered oceanic crust that dehydrate as consequence of rising pressure and temperature with depth. We show that Si isotopes of mantle wedge serpentinites can be used as a reliable new proxy for slab dehydration processes. They may be used in paleo-forearc systems to unravel oceanic sediment and silica biomineralization evolution through geological time.
Highlights
The global silicon (Si) cycle is of interest in many research areas
Late serpentine veins formed under higher water/rock ratios span a wide range of Si isotope ratios that differ significantly between the individual mud volcanoes
This can likely be attributed to the low w/r ratios characterizing the early hydration of the mantle wedge, i.e., to the fact that the amount of Si added to a given rock volume was minor and δ30Si of the primary rock dominates
Summary
The global silicon (Si) cycle is of interest in many research areas. Si is a key nutrient in the ocean controlling primary production and is closely linked to the carbon cycle in weathering reactions that control climate feedbacks (e.g. Tréguer et al, 2021). Si sources, migration pathways, and precipitation reactions during serpentinization are not fully disclosed and the role of serpentinization in the global Si cycle is still underconstrained (see recent reviews by Frings et al (2016) and Sutton et al (2018)). Serpentine has in its structural formula a higher number of Si per mole Mg (or Fe) with 2 mole Si per 3 mole Mg (2/3 mole) than olivine (0.5 mole). This shortage in Si leads to generally low aSiO2 during the isochemical hydration of olivine (Frost and Beard, 2007) and, resulting from the excess in Mg, to the formation of brucite The replacement of pyroxene by serpentine results in excess Si in the vicinity of the pyroxene grains which may allow the direct transformation of olivine into serpentine (e.g. Schwarzenbach et al, 2016)
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