Biogenic and abiogenic low-Mg calcite (bLMC and aLMC): Evaluation of seawater-REE composition, water masses and carbonate diagenesis

Abstract

The ΣREE and shale-normalized (PAAS) REE SN values of modern brachiopods (biogenic low-Mg calcite: bLMC) represented by several species from high- to low latitudes, from shallow- to deep waters and from warm- and cold-water environments, define three distinct average ‘seawater’ trends. The warm- and cold-water brachiopods define two indistinguishable ( p < 0.050) groups that mimic open-ocean seawater REE chemistry, exhibiting the typical LREE enrichment with a slightly positive to negative Ce anomaly followed by an otherwise invariant series. Other recent brachiopods from an essentially siliciclastic seabed environment are distinct in both ΣREE and REE SN trends from the previous two populations, showing a slight enrichment in the MREEs and an increasing trend in the HREEs. Other groups of modern brachiopods are characterized by elevated REE SN trends relative to the ‘normal’ groups as well as by complexity of the series trends. The most characteristic feature is the decrease in the HREEs in these brachiopods from areas of unusual productivity (i.e., such as upwelling currents, fluvial input and aerosol dust deposition). Preserved brachiopods from the Eocene and Silurian exhibit REE SN trends and Ce anomalies similar to that of the ‘open-ocean’ modern brachiopods, although, their enriched ΣREE concentrations suggest precipitation of bLMC influenced by extrinsic environmental conditions. Preservation of the bLMC was tested by comparing the ΣREE and REE SN trends of preserved Eocene brachiopods to those of Oligocene brachiopods that were altered in an open diagenetic system in the presence of phreatic meteoric-water. The altered bLMC is enriched by approximately one order of magnitude in both ΣREE and REE SN trends relative to that in bLMC of their preserved counterparts. Similarly, the ΣREE and REE SN of preserved Silurian brachiopod bLMC were compared to those of their enclosing altered lime mudstone, which exhibits features of partly closed system, phreatic meteoric-water diagenesis. Despite these differences in the diagenetic alteration systems and processes, the ΣREEs and REE SN trends of the bLMC of altered brachiopods and of originally mixed mineralogy lime mudstones (now diagenetic low-Mg calcite) are enriched by about one order of magnitude relative to those observed in the coeval and preserved bLMC. In contrast to the changes in ΣREE and REE SN of carbonates exposed to phreatic meteoric-water diagenesis, are the REE compositions of late burial calcite cements precipitated in diagenetically open systems from burial fluids. The ΣREE and REE SN trends of the burial cements mimic those of their host lime mudstone, with all showing slight LREE enrichment and slight HREE depletion, exhibiting a ‘chevron’ pattern of the REE SN trends. The overall enrichment or depletion of the cement REE SN trends relative to that of their respective host rock material reflects not only the openness of the diagenetic system, but also strong differences in the elemental and REE compositions of the burial fluids. Evaluation of the (Ce/Ce*) SN and La = (Pr/Pr*) SN anomalies suggests precipitation of the burial calcite cements essentially in equilibrium with their source fluids.