High temperature hydrothermal alteration and amphibole formation in Gakkel Ridge abyssal peridotites

International audienceAlteration mineral assemblages in abyssal peridotites offer insights into the temperature and pressure conditions during hydrothermal fluid circulation in the oceanic lithosphere. Abundant hydrothermal venting has been documented along the ultraslow spreading Gakkel Ridge and peridotites have been extensively sampled from the ridge, yet these peridotites have not been systematically evaluated for the nature and extent of their alteration. Here, we present an analysis of the alteration of 40 Gakkel peridotites from 19 dredges spanning ~600 km of the ridge within the Sparsely Magmatic and Eastern Volcanic Zones. We use a petrographic-based alteration scoring system to assign a rank to each sample of 1 (unaltered) to 5 (altered) in 0.5 step intervals, based on the alteration scale of Birner et al. (2016). Gakkel peridotites cover the full range of alteration, from essentially unaltered to completely serpentinized, but are generally moderately to highly altered (average score of 3.5). The extent of alteration is independent of peridotite lithology and is not systematically different between the two zones. Serpentine is present in all samples, magnetite occurs in all samples with alteration scores >2.5, and 27% of samples contain carbonate veins that cross-cut all other minerals. In 20% of samples, all with alteration scores >2.5, tremolite + talc occurs as rims around pyroxene, while chlorite occurs as haloes around spinel. A gabbro-veined lherzolite also contains pargasite replacing pyroxene, with later overprinting by tremolite-talc intergrowths. The abundance of tremolite, chlorite, and talc in Gakkel samples is much lower than reported for talc-tremolite-chlorite schists from oceanic core complexes, where fluid flow through the lower crust leads to open-system behavior. At Gakkel, our petrological observations suggest that alteration was nearer to closed-system conditions, which can be explained by the absence of crust along some sections of the ridge. Pseudosection modeling with Perple_X, which assumes closed-system behavior, indicates that pyroxene reacts to form tremolite + chlorite + talc at temperatures >500 °C, corresponding to fluid flow to depths >20 km for the Gakkel Ridge. The sample with pargasitic amphibole requires temperatures >750 °C to form. Overall, our results suggest that hydrothermal alteration of Gakkel peridotites, and more generally of the lithospheric mantle in regions that lack significant crust, results in different alteration mineral abundances and relationships than in regions where the mafic crust is involved in alteration