Well-preserved clotted micrite in a diagenetically altered Jurassic seep deposit

Abstract

Geochemical analyses of ancient seep carbonates provide information on past environmental conditions and the chemical composition of seep fluids, allowing for the reconstruction of the evolution of chemosynthesis-based faunal communities in the Phanerozoic. However, diagenetic processes commonly mask the primary composition of ancient seep carbonates, impeding a reliable reconstruction of formation environments. The potential for the preservation of primary element patterns in microcrystalline carbonate, which is typically the volumetrically dominant phase in seep limestones, is poorly constrained to date. This study investigates Jurassic seep limestones from the Blue Mountains of eastern Oregon, USA. The seep limestones are mostly composed of three carbonate phases, (1) clotted microcrystalline calcite (clotted micrite), (2) banded and botryoidal, fibrous calcite cement, and (3) equant calcite cement. Mineralogically, these three carbonate phases are now calcite characterized by low magnesium/calcium (Mg/Ca) ratios. The fibrous crystal habit and flat crystal terminations indicate that the precursor mineral of the banded and botryoidal cement was originally aragonite. Cathodoluminescence imaging of banded and botryoidal cement shows partially dull to non-luminescence, suggesting that only parts of this phase experienced diagenetic alteration. However, low strontium (Sr) content and positive europium (Eu) anomalies of botryoidal cement indicate that this phase was affected by diagenetic alteration. Equant calcite exhibits the largest crystal size and the highest manganese/strontium (Mn/Sr) ratios, suggesting that this phase was the product of later-stage diagenesis, probably having precipitated during progressive burial. High Sr contents and low Mn/Sr ratios of clotted micrite indicate that this phase was less affected by diagenetic alteration and has the highest preservation potential with respect to the original seep fluid composition. Shale-normalized rare earth element and yttrium (REY) patterns of clotted micrite are close to those of modern seawater, agreeing with a primary composition of clotted micrite. In spite of its small crystal size, which is generally believed to favor recrystallization, our study suggests that clotted micrite has the potential to reliably record information on fluid composition. Given the dominance of microcrystalline carbonate in many seep limestones, this phase may prove to be a valuable archive – ideally alongside other authigenic minerals – in cases where its pristine composition can be confirmed by independent petrographical and geochemical proxies.