169-6 REE-Y Composition of Different Microfacies Indicates Preservation of Highly Variable Paleoredox Information in Cambrian Phosphorites of Central Asia

Session: Phantastic Phosphorus and Its Historical Biogeochemistry

Presenting Author:

Elizabeth Huang

Authors:

Huang, Elizabeth Shin-En¹, Newell, Caroline², Brenner, Dana C.³, Viete, Daniel Ricardo⁴, Smith, Emily Frances⁵, Sanders, Cecilia Brooke⁶

(1) Geological, Environmental, and Planetary Sciences, University of Maryland, College Park, College Park, MD, USA, (2) Earth and Planetary Sciences, Johns Hopkins University, Baltimore, MD, USA, (3) Earth and Planetary Sciences, Johns Hopkins University, Baltimore, MD, USA, (4) Earth and Planetary Sciences, Johns Hopkins University, Baltimore, MD, USA, (5) Earth and Planetary Sciences, Johns Hopkins University, Baltimore, MD, USA, (6) Geological, Environmental, and Planetary Sciences, University of Maryland, College Park, College Park, MD, USA,

Abstract:

In marine sediments, spatial variations in oxygen create variations in the distribution of redox-sensitive ions, including many transition metals and Rare Earth Elements and Yttrium (REE-Y). These variations can be significant over fine (submillimeter) spatial scales and may provide insight into the conditions for sedimentary processes such as phosphate mineralization (phosphogenesis). However, redox conditions continue to evolve after deposition, altering the trace element chemistries of phosphatic sediments and cements. Ca-phosphate minerals precipitating from fluids readily incorporate redox-sensitive metals and REE-Y as they form, but it is unclear to what extent primary and earliest diagenetic processes control the distribution and textural associations of those elements compared to later diagenetic processes. Here we report detailed petrography and REE-Y compositions of early Cambrian fossiliferous phosphorites from the Bayangol and Salaagol formations of southwestern Mongolia. Petrographic observations strongly indicate organic matter-associated authigenic and early diagenetic phosphogenesis within fossil skeletons and restricted pore spaces rather than wholesale diagenetic replacement of a non-phosphatic primary phase. Spatial distributions of apatite and carbonate cements are highly variable across and within bioclasts, suggesting unique parageneses and non-uniform phosphatization. REE-Y compositions of texturally diverse apatite cements were measured using laser ablation inductively coupled plasma mass spectrometry, revealing broad preservation of depositional and early diagenetic signals. La_N/Yb_N ratios in most samples are above seawater values, indicating adsorption as the primary mechanism of diagenetic REE-Y uptake, and La_N/Sm_N ratios are consistently greater than values associated with late diagenetic uptake. Strong middle REE (MREE) enrichment can therefore be explained by uptake from MREE-enriched porewaters during early diagenesis rather than preferential substitution of MREEs into apatite during recrystallization. While MREE enrichment in porewaters results from dissolution of Fe-Mn oxyhydroxides in reducing conditions, true negative Ce anomalies in almost all samples suggest prolonged interaction with seawater, suggesting phosphogenesis occurred near the sediment water interface and, likely, across a relatively shallow redoxcline. Y/Ho values decrease with increasing Sm_N/Yb_N, indicating the progressive release of particulate-bound Ho and MREEs in anoxic porewaters. Further, REE-Y patterns are strongly controlled by locality and only weakly controlled by sample type. Localities in close spatial proximity to each other (meter-scale) show high variability, suggesting differences in local depositional or early diagenetic conditions produced the observed variability in coeval sediments rather than late diagenetic processes. 

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