Lead Isotope Study of the Laacher See (Germany) Tephra: Implications for Magmatic Evolution and Crust-Mantle Interaction in an Intraplate Setting

Abstract:

The Laacher See volcano (VEI = 6, age = 12,930 ± 40 yr, Baldini et al., 2018) erupted from a compositionally zoned magma chamber (Schmincke et al., 1999) and yielded 6.3 km³ of phonolitic tephra that is divided into three units: Lower (LLST; top of the magma chamber), Middle (MLST) and Upper (ULST) Laacher See tephra (Bogaard and Schmincke, 1985). The tephra sequence represents a continuous differentiation trend from the parent basanite through the mafic ULST to the highly evolved LLST. Here we report Pb isotope compositions of selected Laacher See tephra samples from each unit with a focus on the magmatic evolution and role of crustal contamination.

A steady increase in the ²⁰⁶Pb/²⁰⁴Pb ratios is observed from the ULST (18.984) to LLST (18.993). The LLST sample (LLST 1002) representing the top of the magma chamber with a highly radiogenic ⁸⁷Sr/⁸⁶Sr (0.7122; Wörner et al., 1985) exhibits the most radiogenic ²⁰⁶Pb/²⁰⁴Pb ratio (18.993) in the entire sequence and shows a close affinity to the Devonian country rock suggestive of crustal contamination. It is possible that as the differentiation proceeded, the lead content in the magma decreased due to sulphide fractionation making it more susceptible to assimilation from the surrounding country rock.

Recent work on the generation of basanite and other alkaline melts (Pilet et al., 2008) has suggested that partial melting of metasomatized lithosphere may be a source of such melts. In the ²⁰⁶Pb/²⁰⁴Pb-²⁰⁷Pb/²⁰⁴Pb plot, the LST samples lie in the field of the ‘metasomatized sub-continental lithospheric (SCLM) mantle xenoliths’ (Witt-Eickschen et al., 2003) overlapped by the field of ‘lower crustal granulites’ (Rudnick and Goldstein, 1990). The ²⁰⁸Pb/²⁰⁴Pb ratios of the LST are somewhat more radiogenic than the SCLM and crustal granulite field. The most straightforward interpretation of these data and those published from East Eifel (Wörner et al., 1986) would be that the mantle source of the LST parent (basanitic) melt is the metasomatized SCLM. In this scenario, thermal perturbation of the melting of the metasomatic veins could have been provided by the rising Eifel plume (Ritter et al., 2001). The current study also has implications for the link between the LSE and the Younger Dryas cold event. This is reported in a separate abstract (Seo et al., 2020, this meeting).