Henri E. Gaudette

Geochemistry of the rare-earth elements in hypersaline and dilute acidic natural terrestrial waters: Complexation behavior and middle rare-earth element enrichments

Rare-earth element (REE) speciation was modelled in acid (2.9 ≤ pH ≤ 3.5), hypersaline groundwaters from Australia and from the Palo Duro Basin in Texas, USA, using a combined specific ion interaction (Pitzer model) and ion pairing model. The free metal ion species (i.e. Ln3+) is the dominant form of dissolved REEs in these systems, accounting for 40–70% of the dissolved metal in groundwater from Lake Tyrrell (Victoria, Australia), 50–90% of the REEs in groundwater from Lake Gilmore (Western Australia), and always > 90% of each REE in the Texas groundwaters. (Lakes Tyrrell and Gilmore are actually dry lakes that act as groundwater discharge zones.) The abundance of the free metal ion species increases in these waters with increasing ionic strength and with decreasing pH. The free metal ion species is followed in abundance by REE-sulfate and REE-chloride complexes that account for: 20–50% and 10–15%, respectively, of the dissolved REEs in the groundwaters from Lake Tyrrell; 15–30% and 5–20%, respectively, in the groundwaters from Lake Gilmore; and < 3% and < 7%, respectively, in the Palo Duro Basin groundwaters. The groundwaters of the Lake Tyrrell system and the Palo Duro Basin are enriched in the middle REEs (MREEs) compared to both the light REEs (LREEs) and the heavy REEs (HREEs). (Only Nd, Sm, and Dy were determined in the Lake Gilmore groundwaters and, consequently, it is unclear whether these acid groundwaters are also enriched in the MREEs.) Previous investigators suggested that Fe-rich organic flocs, REE-phosphate complexation, and solid-liquid exchange reactions between terrestrial waters and MREE-enriched surface FeMn coatings, suspended particulates, or secondary mineral phases within aquifer materials, may promote the development of MREE enrichments in natural waters. We propose that organic colloids and REE-phosphate complexes are insignificant in acidic natural waters in regards to MREE enrichments. Instead, solid-liquid exchange reactions or dissolution of surface coatings, suspended particulates, and/or secondary phases as well as sulfate complexation, more likely controls the development of MREE enrichments in acidic natural terrestrial waters.

Calcification of cyanobacterial mats in Solar Lake, Sinai

Pore-water samples were obtained from the shallow-water part of Solar Lake (Sinai) where luxurious cyanobacterial mats grow. These samples were analyzed for Ca/sup 2 +/, Mg/sup 2 +/, Sr/sup 2 +/, Cl/sup -/, SO/sub 4//sup 2 -/, and titration alkalinity (TA) to determine the role of cyanobacterial growth and degradation on the calcification of the mats. The data are modeled thermodynamically to predict mineral-pore-water equilibria. The data support earlier bacterial and sedimentological studies suggesting that the degradation of the cyanobacterial mat via sulfate reduction is of major importance in the calcification process. 34 references, 1 figure, 2 tables.

Geology and age of the Parguaza rapakivi granite, Venezuela

The 1,550-m.y.-old Paraguaza granite of the northwestern Guayana Shield in Venezuela represents one of the larger, apparently anorogenic rapakivi intrusive rocks of the world. The massive rapakivi granite intrudes foliated granitic rocks and associated volcanic rocks of trans-Amazonian age in a structural setting transcurrent to the general northeast-southwest trend of the older basement rocks of the Guayana Shield. Age relations and the geochemistry of the rapakivi suggest an anatectic origin from tensional effects developed by internal distortions within a continental mass. The extensive 1,550-m.y.-old Parguaza intrusion in Venezuela is correlated with 1,550-m.y.-old basement rocks underlying the Amazon Basin in Brazil and suggests a widespread “Parguazan” event 1,500 to 1,600 m.y. ago which affected a large part of the northwestern and southern Guayana Shield, extending as far south as the Guapore craton of Brazil. The Parguazan event therefore marks an important Proterozoic episode in the tectonic evolution of the widespread Precambrian Shield area of northern South America.

Rare-earth element distribution in Lake Tyrrell groundwaters, Victoria, Australia

Abstract The rare-earth element (REE) geochemistry of the saline to hypersaline, acidic brines of Lake Tyrrell, Victoria, Australia, were analyzed in order to determine the relative importance of concentration and modification processes in the lake system. Several types of brines were identified on the basis of different REE patterns and concentrations. Regional groundwater flowing into Lake Tyrrell yields the highest concentrations of REE, while a transitional water group is identified on the basis of lower REE concentrations. A third water group, the reflux brine, is characterized by extremely low REE concentrations. The REE patterns suggest that REE are introduced to the Lake Tyrrell system by the regional groundwater which has concentrated REE from the aquifer material in its acidic waters. Eventually, REE are removed from the evolving water mass as higher levels of pH are reached. The data of the study strongly suggest that REE are scavenged from solution by Fe-oxyhydroxide phases which precipitate as the pH of the system rises. Water-rock interactions probably control pH in the lake system and, therefore, ultimately control the concentration of REE in solution in this system. Our results are directly correlated with studies of hydrothermal vents and geothermal solutions which have shown the concentration of REE in solution to be dominantly controlled by pH.

The age, origin, and tectonics of the Grão Pará Group and associated rocks, Serra dos Carajás, Brazil: Archean continental volcanism and rifting

The Grao Para Group, which is host to the large Serra dos Carajas Fe deposits, is located in the eastern part of the Central Brazil Shield, SSW of Belem, Brazil. It is composed of a lower sequence of metavolcanic rocks, a middle sequence of iron formation (Carajas Formation) and an upper sequence of mixed metavolcanic and metasedimentary rocks. The Grao Para Group is overlain by an unnamed sedimentary (dominantly sandstone) unit, and unconformably overlies the older Xingu Complex. The lower metavolcanic sequence of the Grao Para, in the region of the Serra Norte Fe deposits, is bimodal, with dominant metabasalts, metabasaltic andesites, and metatrachyandesites, and subordinate (10–15%) metarhyolitic tuffs and flows. Most of the metavolcanic rocks are at low to medium metamorphic grade, with more intense metamorphism and deformation along the northern contact with the Xingu Complex. The thickness of the metavolcanic section is not known, but may be ∼4–6 km. The mafic metavolcanic rocks are enriched in Rb, Ba and K with respect to oceanic basalts and basalts of greenstone belts, and are relatively depleted in Th, Sr, Ta, Ti and P. The mafic rocks also show an enrichment in LREEs with flat HREE patterns. The HREEs are correlated with Mg number. U-Pb zircon analyses of the metarhyolites from the Grao Para Group yield an age of 2758±39 Ma. Rb-Sr wholerock analyses of the mafic rocks yield an isochron with an age of 2687±54 Ma, with an initial ratio of 0.7057±0.0010. The results indicate a late Archean age of volcanism in the Grao Para Group, much older than previously thought. Rb-Sr results from the metarhyolites have been partially reset, yielding an age 2497±62 Ma. Sm-Nd analyses from the Grao Para metavolcanic rocks were too scattered to yield an isochron, but ϵNd values relative to CHUR at 2758 Ma indicate values similar to depleted mantle at this time, although with some scatter. The trace element data, and the Sr and Nd isotopic results from the mafic rocks, are consistent with contamination of mantle-derived melts by significant, but variable, amounts of diverse older continental crust. The stratigraphy, geochemical and isotopic compositions show that the Grao Para Group originated on extending, older continental crust during the Archean. An age terrane boundary must exist between this Archean continental terrane and the apparently ensimatic early Proterozoic greenstone belts of the northern Amazonian craton. Similarities to other sequences (e.g., the Fortescue Group, Australia) suggest that similar tectonic settings were widespread at this time. Many major iron formations were deposited within these environments, indicating significant iron formation development on sialic crust in the late Archean. U-Ph zircon analyses from the Serra dos Carajas Granite, which intrudes the unnamed sedimentary unit overlying the Grao Para Group, yield an age of 1820 ± 49 Ma, in agreement with previous age determinations. This age is similar to the ages for the Uatuma Supergroup volcanic and shallow plutonic rocks, which occur throughout the Amazonian Craton.

Seasonal metal remobilization in the sediments of Great Bay, New Hampshire

Abstract Concentrations of dissolved iron, manganese, molybdenum, copper, and organic carbon (DOC) were measured in the pore waters from surficial sediments of a temperate estuary to delineate seasonal metal remobilization from 1978 through 1980. Iron and DOC data were collected for 31 months and covaried inversely and exponentially. Iron dissolution occurred during the spring and during periods of active bioturbation with concentrations as high as 18 mg 1 −1 . Iron values were low during winter due to oxidation to ferric oxides. The lack of active bioturbation during the summer of 1978 allowed for the nearly complete removal of iron as a monosulfide precipitate. However, bioturbation resumed during the summer of 1979 and 1980 and dissolved iron concentrations as high as 10 mg 1 −1 were observed at those times. The iron and DOC data were a qualitative measure of bioturbation activity. Dissolved manganese, molybdenum, and copper data were collected for 18 months during 1978 and 1979. All three metals displayed spring maxima covariate with iron, suggesting that they behaved chemically like iron and/or were associated with iron- or manganese-rich phases during this time of the year. In general, manganese and molybdenum varied temporally with iron while copper concentrations mimicked iron variations only during the spring.

The use of strontium isotopes in determining groundwater mixing and brine fingering in a playa spring zone, Lake Tyrrell, Australia. J..Hydrol. 167, 225-239

Playas in arid and semi-arid regions can be important regional groundwater discharge zones. Located in the northwestern portion of Victoria, Australia, Lake Tyrrell is a large playa that serves as a major discharge zone of regional groundwater. The chemistry of the regional groundwater is unique, having salinities of approximately 30 g 1−1 and pH values as low as 2.8. As the regional groundwater is discharged along the western edge of the playa in the ‘spring zone’, it is evapo-concentrated into a dense brine which sinks through the playa sediments. Macumbers pioneering work on the geohydrology of this region established that this denser, ‘reflux’ brine below the playa surface has evolved from the evaporation of regional groundwaters. We have measured the 87Sr/86Sr ratio of a number of groundwater samples taken at shallow depths along the spring zone at the playa edge and compared them with the regional groundwaters and the reflux brine. Our data reveal a rather complex pattern of mixing of the various water masses associated with the playa system, most probably the result of density driven convective plumes. Two types of regional groundwater can be identified based on their 87Sr/86Sr values, with groundwater from the south being less radiogenic than that flowing from the north (i.e. 0.7108 vs. 0.7142). Vertical variations in 87Sr/86Sr in the regional groundwaters observed in wells adjacent to the spring zone can also be identified along a vertical gradient in the spring zone. Besides the hydrogeological information provided by these measurements, the strontium isotope variations allow us to evaluate the source of strontium to these waters. The regional groundwaters south of the lake are very similar in 87Sr/86Sr ratio to the reflux brines, whereas the northern regional groundwaters are comparatively enriched in 87Sr. This enhanced 87Sr signature indicates the addition of Sr along the flow path via the weathering of the aquifer material, the Miocene Parilla Sand. The southern waters have 87Sr/86Sr values closer to that of present-day seawater and/or Miocene seawater, indicating that the major source of Sr to the system is from one or both of these sources. Our results demonstrate that strontium isotopes can be extremely useful in studying the dynamics as well as the geochemistry of surface-water-groundwater interactions.

Geochemical processes affecting the acidic groundwaters of Lake Gilmore, Yilgarn Block, Western Australia: a preliminary study using neodymium, samarium, and dysprosium

Neodymium, samarium, and dysprosium concentrations were determined in groundwaters along a transect of Lake Gilmore sediments from the shoreline toward the playa center with the future intention of employing the whole suite of rare earth elements (REE). Two different water bodies were identified in the groundwater/playa system based on the neodymium, samarium, and dysprosium concentrations and associated pH and salinity values. Shallow groundwaters that discharge along the playa margins had the highest concentrations of these elements and the lowest pH and salinity. A more saline entrained brine was identified toward the middle of the playa with higher pH values and the lowest observed concentrations of neodymium, samarium, and dysprosium. Groundwaters with transitional neodymium, samarium, and dysprosium values, pH, and salinity were also recorded along the transect. The data suggest that the acidic shallow groundwaters remove REE from the surrounding granitic rocks of the Yilgarn block and introduce them into the playa where they are removed from solution as the pH of the system increases. Evaporative concentration does not appear to have a significant effect on the REE concentrations in the Lake Gilmore system. Other factors such as increasing pH and scavenging of REE by iron oxyhydroxides which precipitate as pH rises appear to exert more control on REE concentrations in the groundwaters by removing them from solution.

U-Pb Zircon Age of Precambrian Basement Gneiss of South Korea

A U-Pb zircon age of 2,150 m.y. confirms the middle Precambrian age for basement gneiss of South Korea.

Organic matter in anoxic marine pore water: oxidation effects

Abstract A series of cores were obtained from the Great Bay Estuary, New Hampshire during 1980 and 1981 in order to investigate the effects on the dissolved organic matter of exposure of anoxic marine pore water to atmospheric oxygen during laboratory handling. Sediment sections from these cores were homogenized and split into two equal parts, one of which was handled in a glove bag under a nitrogen atmosphere, and the other exposed to atmospheric oxygen during laboratory processing. Analysis of the pore water for dissolved organic carbon (DOC) indicated two effects: (1) in the top 15 cm, DOC was lost from the pore water of the oxidized subsections, probably as a consequence of co-precipitation with iron (III) oxi-hydroxides, whereas (2) deeper in the cores, the pore water from the oxidized subsections showed a net increase in DOC, possibly due to oxidation of sedimentary organic matter. In addition, structural changes in the dissolved organic matter in anoxic pore water following oxidation was indicated by high pressure liquid chromatography and ultrafiltration analysis.