Texturally controlled oxygen isotope analyses of serpentine phases record the multistage hydration history during abyssal serpentinization

Serpentinization of ultramafic mantle rocks is one of the main reactions leading to a significant water incorporation into the oceanic lithosphere. The multiphase hydration is yet poorly constrained, in terms of sequence of events, their chemical and isotopic compositions, and the reaction conditions (temperature, fluid composition and the water/rock ratio). We present here the first study on Iberia and Newfoundland passive margins samples (oceanic drill core samples, ODP, from Site 1070 and Site 1277) that closely correlates petrographic observation, in situ oxygen isotopic data and major and trace elements mobility in serpentine phase.

The serpentine minerals lizardite and chrysotile are the main hydrous phases formed during the serpentinization reaction. These minerals crystallize in specific textures, depending on the primary minerals being replaced: (i) serpentine in mesh texture after the alteration of olivine, and (ii) serpentine as bastite pseudomorphing pyroxenes. As the textural control is the key to detect multistage fluid uptake during progressive hydration, we used Secondary Ion Mass Spectrometry (SIMS) to achieve a good spatial resolution of ∼20 µm for in situ oxygen isotope measurements. The trace elements analyses were analyzed with laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) and a spot size of ∼38 µm. Newfoundland samples show less variability in the oxygen isotope composition than Iberia samples and have values higher than the mantle composition, suggesting low temperature of serpentinization (110-200°C). One of the Iberia samples shows similar variability of the oxygen isotope composition but lower values than 𝛿¹⁸O_mantle (temperature around 140-200°C). The second sample has the highest variability: (i) homogeneous mesh rim texture with 𝛿¹⁸O = 4.4 ± 0.8 ‰ (2𝜎), (ii) wide compositional range for mesh centers with 𝛿¹⁸O = 4.0 - 7.7 ‰, and (iii) bastite with large isotopic variation from 5.5 to 13.5 ‰. These values suggest a large hydration temperature range from 60-200°C within a single sample.

Texturally controlled rare-earth element (REE) analyses of the serpentine minerals reveal inherited, typical melt depletion patterns of the protolith, for both localities. The trace element composition of serpentine in the different textural domains displays typical signatures related to the precursor minerals (olivine vs. pyroxene), particularly in terms of Ni/Cr ratio. Positive anomalies of fluid-mobile elements (e.g. B, Sr, As) confirm hydration of the mantle rocks during oceanic serpentinization.