“Rare earth element geochemistry of scleractinian coral skeleton during meteoric diagenesis: a sequence through neomorphism of aragonite to calcite” by Webb et al., Sedimentology, 56, 1433–1463: Discussion

摘要

In a recent article, Webb et al. (2009) examined the behaviour of the rare earth elements (REEs) in scleractinian coral during diagenesis from original aragonite to secondary calcite and showed that the mobilization of REEs in the coral skeleton was minimal. In their argument against mobilization of the REEs during carbonate diagenesis they used a paper that I co-authored in 2006 (Johannesson et al., 2006). Specifically, Webb et al. (2009) stated: ‘‘...and Johannesson et al. (2006) recently showed that some groundwaters have REY patterns that are similar to those of sea water (sic) and inferred that ancient sea water-like trace element patterns in limestones and dolostones might represent diagenetic patterns rather than the original fluids in which the limestones originally precipitated’’. Here, REY indicates the REEs and yttrium. Later on in the Implications portion of their manuscript, Webb et al. (2009) write: ‘‘Hence, although terrestrial weathering and groundwater processes are capable of producing fluids with REY_(SN) patterns (where SN indicates shale-normalized) similar, in some ways, to those of sea water (Johannesson et al., 2006), it is very unlikely that the extremely low concentrations of REEs in such fluids could adequately contaminate an existing limestone so as to impose a radically different REE pattern than that which it already contained’’. However, despite these implications, we did not discuss the effects of diagenetic fluids on REE concentrations and shale-normalized REE patterns of carbonate rocks, either modern or ancient. In fact, the word ‘diagenesis’ does not even appear in Johannesson et al. (2006), and the word ‘diagenetic’ is used only once in our Introduction when reviewing previous studies that examined preservation of REE patterns in palaeo-sea water proxies (i.e. biogenic skeletal carbonates and phosphates, hydrogenous Fe–Mn minerals, abiotic carbonates, phosphates and silicates). Specifically, we stated that: ‘‘These proxies fail for a number of reasons including post-diagenetic mobilization of REEs, fractionation of REEs during precipitation/ uptake, and/or because they exhibit exceedingly low REE concentrations that are easily contaminated by exogenous sources (Palmer, 1985; Elderfield & Pagett, 1986; Palmer & Elderfield, 1986; Sholkovitz & Shen, 1995; Bau et al., 1996; Reynard et al., 1999; Shields & Stille, 2001)’’. Consequently, it is important to emphasize that, although it is possible to infer from Webb et al. (2009) that we argued that diagenetic fluids with: ‘‘…extremely low concentrations of REEs… could…contaminate an existing limestone so as to impose a radically different REE pattern than that which it already contained’’, this contention was never put forth by Johannesson et al. (2006).