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At least 1,297 records · Page 72Linked to original sources
USGS1968

Lead isotopes and the origin of granulite and eclogite inclusions in deep-seated pipes

The isotopic composition of lead and the concentrations of lead, uranium, and thorium in Delegate basic pipes from Australia and in South African kimberlite pipes have been determined. The observed 238 U/ 204 Pb and observed 232 Th/ 238 U of eclogite inclusions in the pipes range from 2.9 to 18.7 and from 3.5 to 5.9, respectively. This result as well as the isotopic composition of lead suggests that the upper mantle is chemically heterogeneous with regard to the trace elements. Pyrochemically extracted leads from eclogite inclusions in the Delegate basic pipes and in a South African kimberlite pipe appear to be different in isotopic compositions from leads extracted from the host rock (matrix). These data are consistent with the hypothesis that the eclogitic inclusions in deep-seated pipes are of “accidental” origin and represent upper mantle materials caught up in the host materials during their intrusion. Lead extracted from a two-pyroxene granulite inclusion in one of the Delegate pipes has an isotopic composition indistinguishable from lead in the host rock. This observation is consistent either with a “cognate” origin for the granulite inclusion or with a modified “accidental” origin in which the isotopic composition of the original lead in the inclusion has been contaminated by lead from the host magma. Other evidence would indicate that an “accidental” origin be preferred.

Earth and Planetary Science Letters↗
USGS1968

The geochronology of foraminiferal ooze deposits in the "Southern Ocean"

Many cores raised from the Drake Passage are characterized by alternating zones of foraminiferal ooze and sandysilt. Cores raised from the East Pacific Rise are foraminiferal ooze or alternating siliceous and carbonate ooze. The uranium and thorium concentrations and isotopic ratios in foraminifers separated from these cores were measured by alpha-spectroscopy. 230 Th in foraminiferal tests is found to be out of equilibrium with its parent ( 234 U), enabling the “excess 230 Th” method to be used in estimating sedimentation rates. The uranium concentration of the tests is constant, suggesting constant uranium concentrations in the sea and thus constant production of 230 Th during periods of foraminiferal production. 232 Th concentration is 2.1 ± 0.7 ppm in the Drake Passage and below the level of detection in tests from the East Pacific Rise. Within each core the 232 Th is constant, varying closely with distance from the source. Based on the degree of disequilibrium of 230 Th, correlations between cores can be made. Age estimates of the foraminiferal zones indicate deposition occurred between 8 000 and 120 000 and between 190 000 and 280 000 years before present. Reported dates of high stands of the sea fall between the ages estimated for the foraminiferal zones, suggesting these zones were deposited during northern hemisphere glacial maxima.

Earth and Planetary Science Letters↗
USGS1968

Effects of selective fusion on the thermal history of the earth's mantle

A comparative study on the thermal history of the earth's mantle was made by numerical solutions of the heat equation including and excluding selective fusion of silicates. Selective fusion was approximated by melting in a multicomponent system and redistribution of radioactive elements. Effects of selective fusion on the thermal models are (1) lowering (by several hundred degrees centigrade) and stabilizing the internal temperature distribution, and (2) increasing the surface heat-flow. It was found that models with selective fusion gave results more compatible with observations of both present temperature and surface heat-flow. The results therefore suggest continuous differentiation of the earth's mantle throughout geologic time, and support the hypothesis that the earth's atmosphere, oceans, and crust have been accumulated throughout the earth's history by degassing and selective fusion of the mantle.

Earth and Planetary Science Letters↗
USGS1968

Effects of selective fusion on the thermal history of the Moon, Mars, and Venus

A comparative study on the thermal history of the Moon, Mars, and Venus was made by numerical solutions of the heat equation including and excluding selective fusion of silicates. Selective fusion was approximated by melting in a multicomponent system and redistribution of radioactive elements. Effects on selective fusion on the thermal models are (1) lowering (by several hundred degrees centigrade) and stabilizing the internal temperature distribution, and (2) increasing the surface heat-flow.

Earth and Planetary Science Letters↗
USGS1969

Heat flow measurements on the southeast coast of Australia

Three boreholes have been drilled for the Australian National University near the southeast coast of New South Wales, Australia. The heat flows found are 1.1, 1.0, and 1.3 μcal/cm 2 sec. The errors resulting from the proximity of the sea and a lake, surface temperature change, conductivity structure and water flow have been examined. The radioactive heat production in some of the intrusive rocks of the area have also been measured. The heat flows are much lower than the values of about 2.0 found elsewhere in south eastern Australia. The lower values appear to be part of a distinct heat flow province in eastern Australia.

Earth and Planetary Science Letters↗
USGS1969

Uranium, thorium, and lead systematics in Granite Mountains, Wyoming

Uranium, thorium and lead concentrations and isotopic compositions were determined on total rocks and a feldspar sample from widely separated parts of the Granite Mountains in central Wyoming. Linear relations defined by 206 Pb/ 204 Pb − 207 Pb/ 204 Pb and 208 Pb/ 204 Pb − 232 Th/ 204 Pb for the total rock samples define 2.8 billion-year isochrons. In contrast, 238 U/ 206 Pb ages are anomalously old by a factor of at least four. The low 238 U/ 204 Pb values, coupled with the radiogenic 206 Pb/ 204 Pb and radiogenic 207 Pb/ 204 Pb ratios, indicate that contents of uranium in near-surface rocks would have had to have been considerably greater than those presently observed to have generated the radiogenic lead. It is possible that more than 10 11 kg of uranium has been removed from the Granite Mountains, and the most feasible interpretation is that most of this uranium was leached from near-surface rocks at some time during the Cenozoic, thus providing a major source for the uranium deposits in the central Wyoming basins.

Wyoming↗
USGS1969

Melting relations in the Fe-rich portion of the system FeFeS at 30 kb pressure

The melting relations of FeFeS mixtures covering the composition range from Fe to Fe 67 S 33 have been determined at 30 kb pressure. The phase relations are similar to those at low pressure. The eutectic has a composition of Fe 72.9 S 27.1 and a temperature of 990°C. Solubility of S in Fe at elevated temperatures at 30 kb is of the same order of magnitude as at low pressure. Sulfur may have significantly lowered the melting point of iron in the upper mantle during the period of coalescence of metal prior to core formation in the primitive earth.

Earth and Planetary Science Letters↗
USGS1969

Some triple-filament lead isotope ratio measurements and an absolute growth curve for single-stage leads

Triple-filament analyses of three standard lead samples are used to calibrate a mass spectrometer in an absolute sense. The bias we measure is 0.0155 percent per mass unit, and the precision (for 95% confidence limits) is ±0.13% or less for all ratios relative to 204 Pb. Although its precision is not quite so good as that of the lead-tetramethyl method in the analysis of large samples, the triple-filament method is less complex and is an attractive alternative for smaller sample sizes down to 500 μg. Triple-filament data are presented for six possibly single-stage lead ores and one feldspar. These new data for ores are combined with corrected tetramethyl data for stratiform lead deposits to compute absolute parameters for a universal single-stage lead isotope growth curve. Absolute isotopic ratios for primeval lead have been determined by Oversby and because all the previous data for both meteorites and lead ores were similarly fractionated, the absolute value of 238 U 204 Pb= 9.09 ± 0.06 for stratiform leads is little different from the value 8.99 ± 0.05 originally computed by Ostic, Russell and Stanton. Absolute values for lead isotope ratios for all interlaboratory standard samples presently available from the literature are tabulated.

Earth and Planetary Science Letters↗
USGS1969

40Ar/36Ar analyses of historic lava flows

The ratio 40 Ar/ 36 Ar was measured for 26 subaerial historic lava flows. Approximately one-third of the samples had 40 Ar/ 36 Ar ratios either higher or lower than the atmospheric value of 295.5 at the 95% confidence level. Excess radiogenic 40 Ar in five flows ranged from about 1 × 10 −13 to 1.5 × 10 −12 mol/g. Possible excess 36 Ar in three flows was on the order of 10 −16 to 10 −15 mol/g. Upper 95% confidence limits for excess 40 Ar in samples with normal 40 Ar/ 36 Ar ratios are generally less than 3 × 10 −13 mol/g. The origin of the excess 36 Ar is unknown but it may be due either to the incorporation of primitive argon that has been stored in the mantle in very low potassium environments or to enrichment in 36 Ar as atmospheric argon diffuses into the rocks after they cool.

Earth and Planetary Science Letters↗
USGS1969

Temporal variation of alkaline earth element/chlorinity ratios in the sargasso sea

An open ocean hydrographic station located 14 miles SE of the Bermuda Islands was sampled at two week intervals through a vertical profile of 2600 meters and over the period June 1966 to March 1967. 428 samples were analyzed for Ca, Mg, Sr and chlorinity. Large temporal variations in element/chlorinity ratios were observed throughout the water column.

Earth and Planetary Science Letters↗
USGS1969

Dilemma posed by uranium-series dates on archaeologically significant bones from Valsequillo, Puebla, Mexico

In an attempt to date stone artifacts of Early Man excavated from several sites at the Valsequillo Reservoir, a few kilometers south of Puebla, Mexico, Szabo applied the uranium-series method on bone samples known to be either from the same geologic formation as the sites or in direct association with the artifacts. The geologic context of the bones was studied by Malde, and the archaeological sites were excavated by Irwin-Williams. A date determined for bone associated with an artifact (Caulapan sample M-B-6, see below) agrees with a radiocarbon date for fossil mollusks in the same bed and indicates man's presence more than 20 000 years ago. However, some of these bone dates exceed 200 000 years. Because such dates for man in North America conflict with all prior archaeological evidence here and abroad, we are confronted by a dilemna — either to defend the dates against an onslaught of archaeological thought, or to abandon the uranium method in this application as being so much wasted effort. Faced with these equally undesirable alternatives, and unable to decide where the onus fairly lies (if a choice must be made), we give the uranium-series dates as a possible stimulus for further mutual work in isotopic dating of archaeological material. A sample from the Lindenmeier archaeological site north of Fort Collins and another from a Pleistocene terrace along the Arkansas River, both in Colorado, were also dated.

Puebla↗
USGS1969

A paleomagnetic study of secular variation in New Zealand

Ancient secular variation in New Zealand was determined from paleomagnetic measurements on 22 volcanic formations with ages of less than 0.68 m.y. The angular standard deviation from the field of an axial dipole is 13.2° with 95% confidence limits between 10.9° and 16.7°. The angular standard deviation of the corresponding virtual geomagnetic poles is 19.6° with confidence limits between 16.2° and 24.7°. These values are larger than those predicted by most models for secular variation. No difference was detected between the angular secular variation in New Zealand and that at the same latitude in North America.

Earth and Planetary Science Letters↗
USGS1969

Loss of uranium from crystallized silicic volcanic rocks

Dense primarily crystallized silicic groundmass material from two welded ash-flow units and one lava flow of Tertiary age from the Western United States contain only 20 to 60 percent of the uranium present in nonhydrated glass from the rock units. These differences reflect loss of uranium from the crystallized specimens, probably as a result of groundwater leaching. It appears likely that most crystallized silicic volcanic rocks have lost comparable amounts of uranium.

Earth and Planetary Science Letters↗
USGS1969

Lead isotopes in volcanic rocks and possible ocean-floor thrusting beneath island arcs

The isotopic composition of lead in the Japanese primary basalts gradually decreases in radiogenic character in a transverse from the Pacific Ocean side to the Japan Sea side, whereas the observed 238 U 204 Pb "> 238U204Pb and 232 Th 204 Pb "> 232Th204Pb ratios in the basalts increase in the same direction. The isotopic composition of lead suggests that tholeiite (Pacific side) is generated at a shallower depth than alkali basalt (Japan Sea side) as postulated by Kuno. The inverse correlation of the 238 U 204 Pb "> 238U204Pb with the 206 Pb 204 Pb "> 206Pb204Pb ratios could be due to preferential Pb extraction with respect to uranium at shallow depths. Alternatively, these geochemical results can be interpreted in terms of the Pacific oceanic rigid plate being underthurst beneath the island arc. The isotopic variation in basalts across the Japanese island arc would result from different proportions of the plate material and the upper mantle of continental side in the partial melt.

Earth and Planetary Science Letters↗
USGS1970

Jurassic (callovian) paleotemperatures from Scotland

Isotopic paleotemperature analyses of belemnites and ammonites from the Belemnite Sands and Oxford Clay of Staffin Bay, Isle of Skye, are presented. Ammonites are depleted in 18 O compared with co-existing belemnites, which give mean temperatures of around 22°C, consistent with the Skye belemnite analysed by Urey et al. [1]. Possible reasons for the ammonite-belemnite difference are discussed.

Earth and Planetary Science Letters↗
USGS1970

Strontium isotopes in Quaternary basalts of southeastern California

Strontium isotopic variations ( 87 Sr/ 86 Sr of 0.7029 to 0.7052) in Quaternary basalts of southeastern California, USA, are not readily explained by assimilation of crustal material similar to that contained as xenoliths in the basalt. Isotopic differences between chrome diopside (0.7016) and olivine (0.7087) from a lherzolite xenolith suggest that the isotopic variations in the basalts may be inherited from the source regions or modified by the upper parts of the mantle that were traversed by the lavas.

Earth and Planetary Science Letters↗
USGS1970

Paleomagnetic secular variation study of lavas from the Massif central, France

The angular standard deviation of virtual geomagnetic poles determined from a paleomagnetic study of 31 Brunhes-age lava flows from France has been obtained. The value, referred to the axial dipole (geographic axis), is 15.2° with 95% confidence limits of 18.4° and 12.9°. This value is not significantly different from that obtained in similar studies of lavas from other midlatitude localities (Western United States and New Zealand).

Earth and Planetary Science Letters↗