Abstract The rare earth element (REE) composition of bioapatite has long been used as a proxy for ancient seawater chemistry and paleomarine environmental reconstruction, based on the assumption of preservation of a hydrogenous (seawater-derived) REE signal. Recent work, however, has begun to question the provenance of REEs in conodonts, emphasizing the importance of REEs released by the lithogenous fraction of the sediment and subsequently adsorbed onto conodont apatite in the burial environment. Here, we investigate patterns of REE and trace-element abundance in conodonts and their host sediments from the Early to Late Ordovician Huanghuachang and Chenjiahe sections of Hubei Province, South China. Several lines of evidence indicate that REEs in the conodont samples were acquired mainly from clay minerals in the host sediment during burial diagenesis: (1) REEs in conodonts show a strong positive correlation to Th and other lithogenic elements; (2) conodonts and whole-rock samples show general patterns of REE and trace-element enrichment that are highly similar to each other and bear no resemblance to seawater elemental concentrations; (3) similar patterns are observed in Triassic conodonts and whole-rock samples; and (4) Y/Ho ratios in conodonts are mostly 90% of REEs from lithogenous sources. Conodonts show pronounced middle rare earth element (MREE) enrichment, a pattern that is unambiguously of diagenetic origin owing to its association with lower Y/Ho ratios. With increasing MREE enrichment of conodont samples, U concentrations and La N /Yb N ratios shift from high to low, and Mn concentrations from low to high. These patterns suggest that conodont diagenesis was initiated at shallow burial depths under suboxic conditions (i.e., in the zone of Mn(IV) and Fe(III) reduction) but continued at greater burial depths, with most acquisition of secondary REEs at later diagenetic stages. Our findings indicate that (1) conodont apatite frequently does not preserve a recognizable hydrogenous REE signal, and (2) the results of many earlier studies in which REEs in bioapatite were used as a paleoseawater proxy may need re-evaluation.
Complete dissolution is essential to obtain accurate analytical results for geological samples. Felsic rocks are known to be very difficult to dissolve because of the presence of refractory minerals such as zircon. In this study, we undertook a systematic evaluation of the effect of the HF/HNO3 ratio, digestion time, digestion temperature, digested test portion mass and the presence of insoluble fluorides on analytical results for the felsic rock GSP-2 using high-pressure HF and HF/HNO3 digestion. Digestion in mixtures of HF and HNO3 acids is a commonly used method of dissolution for geological samples. However, our results clearly indicate that adding HNO3 inhibited the digestion capabilities of HF for refractory minerals such as zircon. It took 8–12 hr for Zr to be completely recovered in GSP-2 at 190 °C, whereas it needed about 36 and 72 hr at 160 and 140 °C, respectively. White precipitates were observed in the final solution for test portion mass > 100 mg, irrespective of which of the five different digestion solutions was used (1 ml HF, 2 ml HF, 1 ml HF + 0.5 ml HNO3, 1 ml HF + 1 ml HNO3 and 1.5 ml HF + 1.5 ml HNO3). Environmental scanning electron microscopy showed that these precipitates were mainly composed of AlF3. Instead of further HCl, HNO3 or HClO4 attack, we propose that using ultra-fine samples and a small sample size is a good way to avoid the formation of insoluble residues (e.g., fluorides). To further investigate the precision and accuracy of the proposed method (using HF alone as the digestion solution during the first acid attack step), a suite of silicate rock reference materials was analysed. Most of the results were found to be in reasonable agreement with the reference values, with a relative error of < 10%.
Une dissolution complete est essentielle pour obtenir des resultats precis au cours de l’analyse des echantillons geologiques. Les roches felsiques sont connues pour etre tres difficile a dissoudre a cause de la presence de mineraux refractaires tels que le zircon. Dans cette etude, nous avons entrepris une evaluation systematique de l’effet du rapport HF/HNO3, du temps de digestion, de la temperature de digestion, de la quantite d’echantillon digeres et de la presence de fluorures insolubles sur les resultats d’analyse pour la roche felsique SGP-2 en utilisant la methode de digestion a haute pression utilisant HF et HF/HNO3. La digestion dans un melange d’acides HF et HNO3 est une methode de dissolution couramment utilisee pour les echantillons geologiques. Cependant, nos resultats indiquent clairement que l’ajout de HNO3 inhibe les capacites de digestion de l’HF pour les mineraux refractaires tels que le zircon. Il a fallu entre 8 et 12 hr pour completement recuperees le Zr de SGP-2 a 190 °C, alors qu’il en faut environ 36 hr et 72 hr respectivement a 160 °C et 140 °C. Des precipites blanc ont ete observes dans la solution finale lorsque les quantites d’echantillon etaient superieures a 100 mg, et ceci independamment de la solution de digestion utilisee (1 ml HF, 2 ml HF, 1 ml HF + 0.5 ml HNO3, HF 1 ml + 1 ml HNO3 et 1.5 ml HF + 1.5 ml HNO3). La microscopie electronique a balayage environnementale a montre que ces precipites sont principalement composes d’AlF3. Au lieu de continuer les attaques utilisant HCl, HNO3 ou HClO4, nous proposons l’utilisation d’une petite quantite d’echantillons ultrafins pour eviter la formation de residus insolubles (par exemple, de fluorures). Pour approfondir la precision et l’exactitude de la methode proposee (en utilisant HF comme seule solution de digestion pendant la premiere etape d’attaque acide) une suite de roches silicatees de reference a ete analysee. La plupart des resultats s’accordent raisonnablement avec les valeurs de reference, avec une erreur relative de moins de 10%.
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