A deep fluid source of radiogenic Sr and highly dynamic seepage conditions recorded in Miocene seep carbonates of the northern Apennines (Italy)

C Argentino,F Lugli,A Cipriani,S Conti,D Fontana

doi:10.1016/j.chemgeo.2019.05.029

Abstract

The 87Sr/86Sr, REE and MoU systematics were investigated in Miocene seep carbonates formed on accretionary ridges at the front of the northern Apennine wedge. Here we evaluate fluid sources, seepage intensity and redox conditions during carbonate precipitation. Micrite matrix and early calcite cements lining cavities are the main authigenic carbonate phases and show negligible diagenetic alteration. Their AOM-related (anaerobic oxidation of methane) origin is evident from δ13C values ranging from −38.7‰ to −25.7‰. The 87Sr/86Sr ratios in carbonates vary between 0.708659 and 0.709132. Most micrite values fall within the range of Middle Miocene seawater (Langhian, MNN5a biozone), in agreement with the biostratigraphy of the host sediments, thus reflecting precipitation of seep carbonates close to the seafloor. Highly radiogenic 87Sr/86Sr ratios, of early calcite cements and micrite from conduit-rich facies at the base of carbonate bodies, likely derived from the interaction of fluids with detrital clays during the fault-controlled upward migration through the underlying terrigenous turbidite successions. Strong Mo and U enrichments in carbonates, with MoEF and UEF up to 233.5 and 86.6 respectively, coupled with (Mo/U)EF ratios ranging between 0.7 and 9.7, indicate dynamic redox conditions, episodically sulfidic and restricted to porewaters. This interpretation is also supported by enrichments in MREE of micrites and a general absence of negative Ce anomalies. These new data provide an indirect indication of rates and temporal variability of AOM at fossil methane seeps developed on a thrust related anticline and help to constrain the complex interaction between fluid migration pathways, seepage intensity and environmental conditions as observed in modern seep analogues.

Highlights

Marine seepage accounts today for ~20 Tg per year of methane emissions (Anderson et al, 2012)
Four micritic samples collected from the basal portion of the other carbonate bodies (L2, L4, L5) are characterized by more radiogenic Sr isotope ratios with values between 0.708837 and 0.708980
We characterized a carbonate outcrop formed in a fault-related seepage system by radiogenic strontium isotopes (87Sr/86Sr), rare earth elements and Mo-U systematics to constrain fluid sources and reconstruct the seepage dynamics and redox conditions

Summary

Introduction

Marine seepage accounts today for ~20 Tg per year of methane emissions (Anderson et al, 2012). Deviation from the expected Sr isotope ratios in weakly altered carbonates possibly indicates mixing with fluids modified by fluid/rock interaction processes (Hong et al, 2018; Joseph et al, 2012; Sample et al, 1993; Tong et al, 2013; Viola et al, 2017). Such a complex situation is common at accretionary wedges. In these settings, carbonates precipitate from a mix of seawater and deeper fluids, bringing hydrocarbons toward the seafloor through different migration pathways (Joseph et al 2012; Teichert et al 2005; Torres et al 2004)

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