Alan M. Stueber

Rubidium, strontium and the isotopic composition of strontium in ultramafic nodule minerals and host basalts

Abstract The concentrations of rubidium and strontium and the isotopic composition of strontium have been determined in minerals separated from ultramaflc nodules occurring in late Tertiary and Quaternary basalts of wide geographic distribution. Clinopyroxene, orthopyroxene and olivine from each of three Iherzolite nodules show a relatively wide range of 87Sr/86Sr disequilibrium and none of the minerals is in isotopic equilibrium with its host basalt. In two cases there is a correlation between 87Sr/86Sr and 87Rb/86Sr ratios of the nodule minerals, indicating apparent isochron relationships which may represent relict mantle events. Clinopyroxene and olivine from each of two wehrlite nodules are not in isotopic equilibrium, although the magnitude of the disequilibrium is smaller than that observed in the Iherzolite nodules. None of these ultramafic nodules can be a crystal cumulate from its host basalt, and it is doubtful that any type of genetic relationship exists. The strontium isotopic disequilibrium between nodule minerals seems to be a primary feature inherited from past mantle histories.

Rb-Sr ages of Precambrian dolerite and alkaline dikes, southeast Mysore state, India

Abstract Rb-Sr isochron ages have been determined for two suites of Precambrian dikes in the Bidadi-Harohalli area of southeast Mysore State. Whole-rock samples of unmetamorphosed dolerites yield an age of 2420±246 (2σ) m.y., which is a minimum value for the intruded Peninsular Gneiss and Closepet Granite. The dolerite magma originated in the mantle, as indicated by the initial 87 Sr/ 86 Sr ratio of 0.7012±0.0010 (2σ). A suite of alkaline dikes, also referred to as felsite and feldspar porphyry dikes, has an age of 832±40 (2σ) m.y., which correlates with the intrusion of the Chamundi Hill Granite and the feldspar porphyry dikes near Srirangapatnam. One of the alkaline dikes has a K-Ar age of 810±25 m.y., indicating an absence of subsequent thermal events in the area.

Geochemistry of Strontium in the Scioto River Drainage Basin, Ohio

Ground water that emanates from carbonate bedrock in the Scioto River drainage basin is characterized by 87 Sr/ 86 Sr ratios in the range of 0.708 to 0.709; usually high Sr/Ca ratios in this water identify celestite lenses within the carbonate bedrock as the dominant source of strontium. Ground water from clastic bedrock, principally shale, has 87 Sr/ 86 Sr ratios that vary from about 0.710 to about 0.713 and shows low Sr/Ca ratios. Thus, there are two basic ground-water types that emanate from bedrock within the basin. They can be identified by these two parameters. Most ground water that has been in contact only with glacial till, which covers the northern two-thirds of the basin, has carbonate-type 87 Sr/ 86 Sr ratios as well as high Sr/Ca ratios. Celestite is apparently present in the till throughout much of the Scioto basin. Ground water that contains celestite-derived strontium, whether from the carbonate bedrock or the till, has so great a strontium content as to control completely the 87 Sr/ 86 Sr ratios of surface water northwest of the glacial boundary. This fact limits the usefulness of the 87 Sr/ 86 Sr parameter as a tracer in water studies within the basin.

Rb-Sr Ages Of Precambrian Mafic Dikes, Bighorn Mountains, Wyoming

Rb-Sr isochron data from whole-rock samples of dolerite and metadolerite indicate emplacement of dikes 2,826 ± 58 m.y. ago, during formation of the Bighorn gneiss and granitic complexes. Whole-rock samples of other dolerite dikes and mineral separates mark another intrusive event at 2,200 ± 35 m.y. B.P., well after the regional metamorphism. Two episodes of mafic dike emplacement are consistent with field relations but are not in agreement with recent K-Ar age determinations.

Rb-Sr isochron age of the Precambrian basement complex, Bighorn Mountains, Wyoming

Rb-Sr analyses of 13 whole-rock samples of gneiss, quartz monzonite, and quartz diorite from the Precambrian basement complex in the Bighorn Mountains provide an isochron age of 2,849 ± 60 m.y. and an initial 87 Sr/ 86 Sr ratio of 0.7016 ± 0.0005. This age is interpreted as the time of regional metamorphism and metasomatism which produced the gneiss and the granitic rocks; independent isochron ages for both groups of rocks are not significantly different. The 2,850-m.y. age for the metamorphic event is consistent with other radiometric ages from the Bighorns and can be correlated with ages reported from several other mountain ranges in Wyoming.

Application of Strontium Isotopes to Origin of Smackover Brines and Diagenetic Phases, Southern Arkansas: ABSTRACT

The abundance of the isotope 87Sr is variable in nature, as it is the radiogenic product of 87Rb decay. The relative amount of this Sr isotope that is dissolved in a brine, as expressed by the 87Sr/86Sr ratio, might be used as a tracer of the origin and subsequent history of the brine, including its diagenetic effects in petroleum reservoirs. Strontium isotopic analyses of 40 brines from oil fields in southern Arkansas have been conducted to investigate the sources of the dissolved Sr, the pathways of brine migration, and the relationship between the brines and diagenetic phases in the Jurassic upper Smackover Formation. The 87Sr/86Sr ratios of 33 brines from the upper Smackover lime-grainstone range from 0 7071 to 0.7101; seven brines from formations stratigraphically above the Smackover range from 0.7090 to 0.7112. Thus the Sr in these brines is variably more radiogenic than Jurassic sea water End_Page 553------------------------------ (~0.7070). The brines are from 15 oil fields within an area of approximately 2,000 mi2 (5,200 km2); the isotopic variability shows no geographic pattern in the study area. The variation in 87Sr/86Sr ratios of brines occurs to a lesser degree within individual oil fields. For example, over a distance of about 10 mi (16 km) at Walker Creek, the isotopic ratios of 11 Smackover brines range from 0.7080 in the east to 0.7086 in the west. The observation that Smackover brines are variably more radiogenic than Jurassic seawater is important because it indicates that a significant proportion of the Sr dissolved in these brines has been acquired from a source material that has not formed wholly by precipitation from Jurassic seawater. That is, some radiogenic Sr must have been added to the brines from a detrital source material. The nature and distribution pattern of the 87Sr/86Sr ratios indicate the acquisition of variable amounts of radiogenic Sr on a local basis. If the Smackover brines originated in the Louann Salt, with 87Sr/86Sr equivalent to that of Jurassic seawater, their present isotopic compositions may be the result of varying degrees of subsequent interaction with d trital sediments or they may have been produced by mixing in variable proportions with solutions containing more radiogenic Sr. Potential sources of the radiogenic Sr are the Norphlet Formation and the lower Smackover argillaceous lime-mudstone, both of which lie stratigraphically between the Louann salt and the upper Smackover, as well as the Bossier shale which interfingers with the upper Smackover in the North Louisiana salt basin. Anhydrites from the Werner and Buckner formations and from northern Louisiana salt domes, which constitute additional potential sources of brine Sr, yield 87Sr/86Sr ratios equivalent to those of Jurassic seawater. Diagenetic phases of the upper Smackover, such as post compaction calcspar cement and baroque dolomite, have 87Sr/86Sr ratios more radiogenic than Jurassic seawater, suggesting their subsurface precipitation in Sr isotopic equilibrium with Smackover brines. However, ooids and oncolites from the upper Smackover lime-grainstone yield 87Sr/86Sr ratios indicating isotopic equilibrium with Jurassic seawater. End_of_Article - Last_Page 554------------

Evolution of formation waters in the Permian Basin, United States: Late Permian evaporated seawater to Neogene meteoric water

Understanding subsurface waters is important for hydrocarbon exploration and development. Waters from the Permian Basin were collected and analyzed for stable isotopes, ionic concentrations, and strontium isotopes to determine their origin. Three main geochemical groups of waters are present. Group 1 has high δ18O and δD values with high total dissolved solids (TDS; 100–240 g/L), including high concentrations of Na+, Cl−, and Br−.Group 1 waters contain distinctly less sodium (Na+) than chlorine (Cl−) on a molar basis, similar to modern seawater. Group 2 and 3 waters have low δ18O and δD values. Group 2 waters have relatively low TDS (4–75 g/L). Group 3 waters have high TDS (170–225 g/L), Na+ and Cl− in approximately equal molar amounts, and low Br− concentrations. Group 1 waters are interpreted as forming from highly evaporated seawater during precipitation of uppermost Permian salts. Because of their high density, those waters displaced preexisting formation waters throughout the Permian Basin during the latest Permian. Waters in groups 2 and 3 came mainly from precipitation in the mountains of southeast New Mexico. Those mountains formed during Neogene tectonic uplifts. Group 3 waters acquired their Na+ and Cl− by dissolution of upper Permian salt. Many group 2 and 3 waters acquired their Ca2+ and SO42− by dissolution of Permian anhydrite or gypsum. Some waters contain a mixture of these groups. Understanding the origin of subsurface waters helps predict subsurface salinity, aquifer drive, sulfate reduction, chemical reactions during water injection, and chemistry of diagenetic waters.