Mobility and fractionation of rare earth elements in argillaceous sediments: Implications for dating diagenesis and low-grade metamorphism

Abstract

We report Sm-Nd and Rb-Sr data for the fine fractions of Lower Paleozoic argillaceous rocks from Wales, UK, and New York, USA, spanning the range of low-grade metamorphic conditions from the diagenetic zone (zeolite facies) to the epizone (greenschist facies). In all cases, leaching of the fine fractions results in a high 14'Sm/ 144Nd (0.09-0.29) acid-soluble component and a complementary low '47Sm/ '44Nd (0.05-O. 14) residual component. The observed fractionation is an ancient feature related to diagenesis, burial, and metamorphism. The magnitude of Sm-Nd fractionation between leachates and residues, as well as the resulting Sm-Nd ages, vary as a function of grain size and metamorphic grade. Uncleaved Welsh mudrocks of the diagenetic zone yield Sm-Nd leachate-residue ages of 453-484 Ma, in agreement with their Llanvimian to Caradocian biostrati~aphic ages, whereas higher grade rocks of the anchizone and epizone yield Sm-Nd ages as young as 413 Ma. These ages are transitional between the time of deposition and the time of regional deformation related to the Acadian Orogeny at 390 Ma. Distinct convex-upward rare earth element (REE) patterns of the leachates suggest that the precipitation of early diagenetic apatite controls the trace element budget of the rock, forcing a depletion of middle REEs on the subsequently formed diagenetic phyllosihcates. The amount of organic matter present and the extent of later prograde reactions are probable modifiers of this ~~ionation process. Ordovician and Devonian elastic rocks associated with the Trenton and Onondaga limestones of New York yield single-sample and multi-sample Sm-Nd isochron ages that agree well with their biostratigraphic ages of 454 Ma and 390 Ma, respectively. The REE fractionation observed in shale leachates of the Ordovician Utica Formation is related to Ca/Mg of the bulk rock, and hence to the composition of the diagenetic carbonate cement. In all cases the Sm-Nd system remained closed subsequent to the peak of diagenesis or metamorphism, including the North American rocks that show no evidence of being isotopically reset during widespread remagneti~tion of the subjacent limestone units in the late Paleozoic.

Determination of rare earth, major and trace elements in authigenic fraction of Andaman Sea (Northeastern Indian Ocean) sediments by Inductively Coupled Plasma-Mass Spectrometry

“…Hence the MREE enrichment could be caused by two different phases, either by leaching of phosphates or conversely, the precipitation of Fe-Mn oxy-hydroxides. It is reported that the REE budget during leaching of argillaceous sediments may be controlled by dissolution of authigenic apatite [15]. Contrary to that, there is no unambiguous indication that the MREE enrichment in porewaters is controlled by authigenic apatite [16].…”

“…Authigenic apatite can be easily dissolved using dilute acetic acid [15]. As the very first chemical reagent used in this study for the extraction of the carbonate fraction, preceding the reductive leaching of Fe-Mn oxy-hydroxides, is Na acetate buffer containing acetic acid.…”

Clay mineralogy, strontium and neodymium isotope ratios in the sediments of two High Arctic catchments (Svalbard)

“…9b), implying the leachate is MREE enriched. A MREE enrichment is consistent with diagenetic apatite and Fe/Mn oxyhydroxides (Ohr et al, 1994;Tricca et al, 1999;Johannesson and Zhou, 1999;Su et al, 2017) and inconsistent with a carbonate phase, which is typically HREE (high REE) enriched (Byrne and Kim, 1990;Millero, 1992;Byrne and Sholkovitz, 1996), suggesting that Nd in the leachate is derived from the former phases. Volcanic ash also has a MREE enriched REE pattern (Tepe and Bau, 2014) and would have high εNd and low 87 Sr / 86 Sr.…”

Comparison of the Mineralogical and Chemical Composition of 2 Shales from the Western Canada Sedimentary Basin and the United States Gulf Coast

“…It appears that the environment in which I/S formed can leave a durable and diagnostic O-isotopic signature in the clay fraction. Other procedures might include the analysis of bentonite beds (Hoffman and Hower 1979), illite polytypes (Velde and Hower 1963;Hower and Mowatt 1966), trace and rare earth elements in clays (Awwiller 1994;Cullers 1994;Fagel et al 1994;Ohr et al 1994) and illite dating (Mossmann 1991;Roden et al 1993), together with facies/paleogeographic analysis. Most likely, only a combination of several of these strategies will yield unambiguous answers to these questions.…”