1. RARE-EARTH ELEMENT AND ARSENIC GEOCHEMISTRY OF THE HYDROTHERMAL CALCITE HOSTED IN THE LACUSTRINE FORMATIONS OF THE NEWARK BASIN: IMPLICATION TO THE FLUID MIGRATION AND QUALITY OF GROUNDWATER
Larbi Rddad - Kingsborough Community College, Earth and Planetary Sciences, Department of Physical Sciences, 2001 Oriental Boulevard, Brooklyn, NY 1123
Dennis Kraemer - Department of Physics and Earth Sciences, Jacobs University Bremen, Campus Ring 1, Bremen, 28759, Germany
INTRODUCTION
The lacustrine formations of the early Mesozoic Newark basin host numerous veins filled with calcite. The origin and genesis of these veins was constrained [1, 2], whereas their geochemical characterization (Rare Earth Element (REE) and arsenic (As) has not been investigated yet.
The present study aims at analyzing the REE and As in calcite to further constrain their chemical composition and most importantly their As content. Ultimately, this study aims at verifying if these calcites are rich in arsenic that contaminates the groundwater.
RESULTS AND DISCUSSION (Conti.)
Yttrium-Holmium fractionation is commonly observed in fluorine-rich hydrothermal systems [6]. However, we couldn’t find any evidence for elevated fluorine concentrations in the fluids from which the calcites precipitated. Hence, we assume that the observed Y/Ho fractionation in the hydrothermal calcites is inherited from fluid-rock interaction with carbonate rocks. The REY concentrations and the observed fractionation patterns in the hydrothermal calcites support the proposed fluid flow and evolution model (Fig. 2). The very low total REY contents of calcite and the already published C-O stable isotope and fluid inclusion studies suggest that the original fluid was REY-depleted meteoric water. The latter descended to deeper parts of the basin where it became hot. Along its migration, these hydrothermal fluids remained poor in REY. Owing to the fact that the lacustrine formations, pyrite, and hematite are rich in arsenic, we have expected that hydrothermal fluids, that had leached them, would have been As-enriched. Calcite would have been also rich in As. However, the arsenic concentration in calcite veins is extremely low. It follows that the calcite veins are not the source of the groundwater contamination.
Fig. 1. REYSN patterns of the vein calcites found in the different Triassic fluvial and lacustrine formations.
Note the overall enrichment of MREY-HREY over LREY and the fractionation of La, Eu and Y in the calcites.
Fig. 2. Genesis model for the calcite during the Late Triassic-Early Jurassic time (modified after [7]). Inset: formation of calcite hosted in Passaic Fm (A), in Lockatong Fm (B), and in Stockton Fm (C).
GEOLOGICAL CONTEXT
The early Mesozoic Newark basin was formed during the Triassic rifting of Pangaea. The structuration of the basin into a half-graben was caused by the reactivation of the Normal NE-SW-trending faults. This basin was successively filled with fluvial/alluvial sediments (Stockton Formation), dark to gray lacustrine sediments (Lockatong Formation), red clastic sediments (Passaic Formation), and finally Early Jurassic sediments intercalated with basaltic flows [3, 4],
During the opening of the Atlantic Ocean, a post-Middle inversion occurred resulting in compressional tectonic activity. This tectonic event led to the compression and structuration of the present day Newark basin.
RESULTS AND DISCUSSION (Conti.)
The REY concentration of the hydrothermal calcite is low (28.4 to 54.0 ppm) despite the fluid-rock interaction and longer residence time of the fluid as suggested by carbon-oxygen isotope data [2] . The presence of calcite as the only major mineral phase in the veins and the moderate salinity of the fluid suggest the predominance of carbonate species (CO32-, HCO3-) in the hydrothermal fluid. Fluid inclusion and C-O stable isotope data [2] suggest that meteoric waters were the ultimate fluid source for the hydrothermal fluids from which the calcites precipitated. Hence, the original fluid can be considered depleted in REY. The REYSN pattern of the studied calcites shows a depletion in LREY and a relative enrichment in MREY and HREY (Fig. 1). LREY depletion and MREY-HREY enrichment in shale-normalized patterns of hydrothermal fluids were described by [5] and may indicate that REY fractionation was mainly controlled by sorption processes and that the fluid had an acidic pH [5]. Eu is decoupled from its REY neighbors and is slightly enriched (Eu/Eu*=1.2 – 1.5) in all calcite veins. This indicates that the hydrothermal fluids had flowed through the arkosic-rich Stockton Fm and preferentially dissolved feldspar. The latter is commonly enriched in Eu because Eu2+ can substitute Ca2+ in the feldspar mineral lattice. The overall positive La and Y anomalies observed in calcites indicate intense fluid-rock interaction between hydrothermal fluid and the lacustrine carbonates.
CONCLUSION
The REY and As analysis on calcite veins led to the following conclusions:
1- The REY concentration is low despite fluid-rock interaction and suggests that the hydrothermal fluids were modified meteoric waters.
2- These hydrothermal fluids flowed through the basal feldspar-rich sandy Stockton Fm and interacted with carbonate levels.
3- The REY fractionation was mainly controlled by sorption process and that the fluid is acidic.
4- Although fluid-rock interaction occurred between arsenic-rich lacustrine formations and the hydrothermal fluids, the latters remained poor in As.
5- Calcite veins are depleted in As and thus are not the source of As contaminating the groundwater
METHODS AND MATERIALS
Four samples of calcite were collected from the Stockton, Lockatong, and Passaic Formations. REY and As analysis was performed using the LA-ICP-MS at PCIGR with a precision that is better than 0.1.
RESULTS AND DISCUSSION
The concentrations and fractionation of REY in hydrothermal calcite are controlled by redox conditions, pH, fluid composition and the temperature of the fluid. The REY composition of the fluid is largely influenced by the intensity of water-rock interaction, the fluid-rock interaction, and the abundance of potential complexing agents like carbonate, fluorine or phosphate.
References
[1]. Herman, G.C., Journal of Structural Geology. 31 p
[2]. Rddad, L., Louissaint, P., Darling., R. (2016). GSA, abstract programs, Albany
[3] Schlische, R.W., Withjack, M.O., Olsen, P.E., American Geophysical Union, Geophysical Monograph 136, p
[4]. Olsen, P. E., Lamont (Newsletter), v. 13, p. 5-6.
[5]. Bau, M Chem. Geol. 93, p
[6]. Bau, M and Dulski, P Contrib. Mineral. Petrol. 119, p
[7]. Malinconico, M. L., N.J. Geological Survey Bulletin 77, Chapter C., p.1-38.
Acknowledgement
The authors would like to thank Herman of USGS of New Jersey, Olsen of Columbia University, Kent, and Browning of Rutgers University for providing us with the samples to complete this project.