LA-ICP-MS measurement of U-Pb isotopes in magnetite of the Münchberg Massif: evidence for the serpentinization age of mantle material within the Prasinit-Phyllite Serie

The Münchberg Massif consists of four metamorphic nappes, which show different metamorphic grades in inverse order. The lowermost nappe, the Prasinit-Phyllite-Serie, is of lowest metamorphic grade (greenschist facies) containing mainly phyllites and greenschists with intercalated serpentinite bodies. Serpentinization, a water-rock reaction, can generate molecular hydrogen (H₂), a potential energy carrier. These serpentinites, however, have received limited attention.

Hydrogen is a molecule with very low diameter, which might escape after serpentinization. Understanding the timing and conditions of serpentinization is thus essential for evaluating the preservation and potential of natural hydrogen accumulations. As part of the Prasinit-Phyllite-Serie, serpentinization could have happened at different stages: in the mantle during subduction, during obduction, or during the different stages of the uplift of the Franconian Line. To relate the serpentinization to any of these stages, the age of the newly formed minerals, such as serpentinite or magnetite, is of importance. Therefore, magnetite and some serpentine were analysed by Laser Ablation-Inductively Coupled Plasma-Mass Spectrometry (LA-ICP-MS) for their U, Th and Pb isotopic composition to constrain the timing of serpentinization.

Although the content of U, Th and Pb was low, the results of these measurements show a defined age range, which lies in the range of the ages of the Prasinit-Phyllite-Series and the Randamphibolite, which are supposed to be part of the Rheic Ocean (Koglin et al. 2018). Such ages older than the main stage of the Variscan Orogeny also indicate a serpentinization and hydrogen formation in the mantle beneath the Rheic Ocean.