Sambhudas Chaudhuri

Rare-earths in size fractions and sedimentary rocks of Pennsylvanian-Permian age from the mid-continent of the U.S.A.

Abstract The REE (rare-earth) contents of sixty-three Lower Permian Sand and gravel-size fractions consist mostly of quartz or chert so their REE content is low (7.9–40.6 ppm) although heavy minerals may contribute a large fraction of the REE content. Unexpectedly, silt-size fractions have REE contents (74–355 ppm) that are usually lower but similar to their

Rare earth distributions in clay minerals and in the clay-sized fraction of the Lower Permian Havensville and Eskridge shales of Kansas and Oklahoma

The REE (rare earth element) content of a wide variety of clay mineral groups have been analyzed using radiochemical neutron activation and have been found to be quite variable in absolute REE content (range of ∑REE = 5.4–1732) and less variable in relative REE content (range of chondritenormalized La/Lu = 0.9–16.5). The variable REE content of the clay mineral groups is probably determined by the REE content of the source rock from which the clay mineral was derived and not from the separate minerals in the rock. The clay-sized fractions of the Havensville and Eskridge shales of Kansas and Oklahoma have similar relative REE distributions and identical negative Eu anomaly size as the composite of NAS (N. American shales), but an absolute REE content (range of ∑REE = 46–348) that may differ significantly from the composite of NAS. The clay-sized fraction of samples from any given outcrop did not vary much in absolute or relative REE content, but samples from northern Oklahoma, probably composed of continental to near-shore marine sediments, have higher absolute REE contents and higher La/Lu ratios than samples of marine deposits in Kansas (e.g. mean ∑REE in Oklahoma = 248; mean ∑REE in Kansas = 69–116). The differencess in the REE content between samples in Oklahoma and Kansas may be caused by chemical weathering processes in the source area, exchange reactions in the environment of deposition, or diagenesis and do not appear to be a result of the different clay minerals. Most samples have Eu anomalies relative to chondrites (range of Eu/Sm ratios of samples = 0.035–1.17; chondrites = 0.35). Some montmorillonites and kaolinites are anomalous in Eu relative to the NAS (range of Eu/Sm ratios of samples = 0.056–0.21; NAS = 0.22). These anomalies may be inherited from source rocks with Eu anomalies originally produced by igneous processes, or they may be produced by chemical weathering processes in the source area.

The distribution of rare-earth elements in deeply buried Gulf Coast sediments

Abstract Analyses of rare-earth elements (REE) in Gulf Coast sediments indicate that the absolute REE content of the bulk sample is nearly constant between 174 and 165 ppm at depths of 1.81 and 3.67 km, respectively; but it increases perceptibly to 212 ppm at a depth of 4.77 km. Similarly, the absolute REE contents in the clay-sized fractions range between 175 and 205 ppm at depths of 1.81 and 4.42 km but they increase moderately to 272 ppm at a depth of 4.77 km. The ratios of the light to heavy rare-earths (LREE/HREE) of the bulk samples and their clay-sized fractions are nearly constant between 7.2 to 8.4 for depths between 1.81 and 4.42 km, whereas the ratio decreases to about 5.5 and 6.6 for sediments at 4.77 km. Both the absolute REE content and the LREE/HREE ratio of the sediments most likely reflect the character of the provenance(s) of the sediments, and they are influenced only to a minor extent by diagenesis. The Eu/Sm ratios of all samples cluster very tightly around the value of 0.21, similarly to that of the North American Shale composite. A nearly constant Eu/Sm ratio among samples at all depths may have been inherited primarily at the provenance(s) of these Gulf Coast sediments.

Strontium isotopic evolution of oil-field waters from carbonate reservoir rocks in Bindley field, central Kansas, U.S.A

Abstract Oil-field waters produced from Mississippian carbonate reservoir rocks in Bindley field, Kansas had an average salinity of about 42.8 mg/l. They were enriched in Ca, Sr, Na, K, Rb, and Li and depleted in Mg relative to sea water at the same level of either Cl or Br concentration. The average abundances of different elements are as follows: Na—13,460 mg/l, K—325 mg/1, Rb—0.9 mg/l, Li—12 mg/l, Ca—1,515 mg/l, Sr—42 mg/l, Mg—430 mg/l, Cl—23,000 mg/l, SO4—2,630 mg/l, Br—32 mg/l. The 87 Sr 86 Sr values of the waters ranged between 0.7221 and 0.7230, whereas the values of the host carbonate rocks were between 0.7090 and 0.7093. The very high 87 Sr 86 Sr values of the waters probably occurred as a result of their reaction with alkali feldspar minerals in buried Precambrian crystalline rocks to the east of the Bindley field. The study demonstrates that despite their residence in carbonate reservoir rocks oil-field waters can retain sufficiently distinct isotopic memory that may provide important dues about mineral-water interactions in the chemical evolutionary history of the waters. The study further indicates that very late diagenetic reactions in many carbonate rocks are of minor extent. Difference in the Sr isotopic data among the oil-field waters suggests that the Sr isotopic data can be used as a reliable guide in recognising existence of separate pools in an oil field.

Rare earth elements in Silurian pelitic schists from N.W. Maine

Abstract Thirty-two samples of a series of metamorphosed Silurian (?) pelitic schists in the greenschist and amphibolite facies from N.W. Maine have been analyzed for their rare-earth element (REE) content. The REE contents of these samples do not change as a function of metamorphic grade. Two different metasedimentary formations have been sampled, and they differ significantly in their light REE content. The absolute and relative distribution of the REE in the Bangeley Formation are quite similar to the composites of N. American, European and Russian shales that have been determined thus far (e.g. median La/Lu ratio of the Bangeley Formation normalized to chondrites = 8.7 ± 3.0). Samples from the Perry Mt. Formation show large depletions in the light REE compared to the Bangeley Formation and previously analyzed shale and metamorphosed shale samples, but heavy REE concentrations that are quite similar to the other samples (e.g. median La/Lu ratio of the Perry Mt. normalized to chondrites = 1.2 ± 0.6). These differences in light REE content between the Perry Mt. Formation and other sedimentary rocks are probably due to differences in the original clay mineral compositions as modified by weathering and/or depositional environments.

Strontium isotopic compositions and potassium and rubidium contents of formation waters in sedimentary basins: Clues to the origin of the solutes

Abstract Strontium isotopic compositions of formation waters in sedimentary basins relate primarily to Sr from dissolution-replacement of alkali feldspar minerals, dissolution-recrystallization-precipitation of marine carbonate and sulfate minerals, and illitization. Formation waters in sedimentary basins are often depleted in K but enriched in Rb relative to evaporated seawater and have K Rb ratios commonly between 200 and 2000. The K and Rb characteristics of formation waters suggest that deep burial conversion of smectite to illite has a major influence on the evolutionary history of these solutes.

Strontium isotopic composition of several oilfield brines from Kansas and Colorado

Abstract The 87 Sr 86 Sr ratios of several oilfield brines associated with Paleozoic dolomites, cherty dolomites and sandstones from Kansas and Colorado range from 0.7113 to 0.7341. Two brines in the Mississippian dolomites from Colorado were found to contain the most radiogenic strontium. The 87 Sr 86 Sr ratios of brines in a small oilfield in eastern Kansas are constant, and the ratios may suggest that in this field there may be only one oil pool. The isotopic composition of strontium in subsurface waters could be useful in determining hydrologic continuity among reservoirs and for obtaining additional information on the origin and migration of these fluids.

The RbSr systematics in acid-leached clay minerals

Abstract Hydrochloric-acid leaching of four illites, two montmorillonites and one each of mixed-layer illite-montmorillonite and vermiculite increases both the Rb Sr and 87 Sr 86 Sr ratios. Calculated RbSr dates are higher for the acid-treated samples than those for the corresponding untreated samples. The RbSr dates for three illites are 10–15% higher than and, for one illite, nearly in agreement with their commonly accepted stratigraphic ages. Leachate 87 Sr 86 Sr ratios for the illites range from 0.7124 to 0.7277, the lowest value being found for the illite with the highest percentage of expandable layers. Leaching of Sr from the clay minerals may produce only a small change in 87 Sr 86 Sr ratio of the carbonate phase during dissolution of limestones containing less than 20% clay minerals.

The Rb-Sr Whole-Rock Age of the Stearns Shale (Lower Permian), Eastern Kansas, before and after Acid Leaching Experiments

Six untreated whole-rock samples of the Stearns Shale with low Sr87/Sr86 and Rb87/Sr86 ratios define a least squares isochron of 371 ± 15 m.y., with an initial Sr87/Sr86 ratio of 0.7091 ± 0.0015, using a 50 b.y. half-life for Rb87. Treatment of the samples with 2N HC1 with constant agitation for 15 minutes produced the following results: 6 samples of insoluble residue with much higher Sr87/Sr86 and Rb87/Sr86 ratios and a Rb-free leach of low Sr87/ Sr86. Least squares analysis of the insoluble residue samples yields a 276 ± 10 m.y. isochron with initial Sr87/Sr86 of 0.7195 ± 0.0005. This age is close to the presumed age of the Lower Permian and suggests that this method may allow the accurate dating of some predominantly allogenic calcareous shales.

Mineralogy and petrology of cretaceous subsurface lamproite sills, southeastern Kansas, USA

Cores and cuttings of lamproite sills and host sedimentary country rocks in southeastern Kansas from up to 312 m depth were analyzed for major elements in whole rocks and minerals, certain trace elements in whole rocks (including the REE) and Sr isotopic composition of the whole rocks. The lamproites are ultrapotassic (K2ONa2O = 2.0–19.9), alkalic [molecular (K2ONa2O)/Al2O3 = 1.3-2.8], enriched in mantle-incompatible elements (light REE, Ba, Rb, Sr, Th, Hf, Ta) and have nearly homogeneous initial Sr isotopic compositions (0.707764-0.708114). These lamproites could have formed by variable degrees of partial melting of harzburgite country rock and cross-cutting veins composed of phlogopite, K-Ti richterite, titanite, diopside, K-Ti silicates, or K-Ba-phosphate under high H2OCO2 ratios and reducing conditions. Variability in melting of veins and wall rock and variable composition of the metasomatized veins could explain the significantly different composition of the Kansas lamproites. Least squares fractionation models preclude the derivation of the Kansas lamproites by fractional crystallization from magmas similar in composition to higher silica phlogopite-sanidine lamproites some believe to be primary lamproite melts found elsewhere. In all but one case, least squares fractionation models also preclude the derivation of magmas similar in composition to any of the Kansas lamproites from one another. A magma similar in composition to the average composition of the higher SiO2 Ecco Ranch lamproite (237.5–247.5 m depth) could, however, have marginally crystallized about 12% richterite, 12% sanidine, 7% diopside and 6% phlogopite to produce the average composition of the Guess lamproite (305–312 m depth). Lamproite from the Ecco Ranch core is internally fractionated in K2O, Al2O3, Ba, MgO, Fe2O3, Co and Cr most likely by crystal accumulation-removal of ferromagnesian minerals and sanidine. In contrast, the Guess core (305–312 m depth) has little fractionation throughout most of the sill except in several narrow zones. Lamproite in the Guess core has large enrichments in TiO2, Ba, REE, Th, Ta and Sc and depletions in MgO, Cr, Co and Rb possibly concentrated in these narrow zones during the last dregs of crystallization of this magma. The Ecco Ranch sill did not show any evidence of loss of volatiles or soluble elements into the country rock. This contrasts to the previously studied, shallow Silver City lamproite which did apparently lose H2O-rich fluid to the country rock. Perhaps a greater confining pressure and lesser amount of H2O-rich fluid prevented it from escaping.