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Geology topics

G. Brent Dalrymple

Publications and source records attributed to G. Brent Dalrymple.

At least 37 records · Page 2Linked to original sources
USGS1975

K-Ar ages of Pleistocene rhyolitic volcanism in the Coso Range, California

Rhyolite domes and rhyolite and basalt flows near Coso Hot Springs in the Coso Range, California, give K-Ar ages that range from about 0.04 ± 0.02 m.y. to 0.96 ± 0.19 m.y. Most of the units appear to be between 0.05 and 0.15 m.y. old. The total volume of erupted rhyolitic magma is about 2.4 km 3 . The age, volume, and composition of these volcanic rocks indicate that the Coso volcanic field is a potential geothermal resource area.

California↗
USGS1974

40Ar/39Ar age spectra of some undisturbed terrestrial samples

40 Ar/ 39 Ar age spectra and 40 Ar/ 36 Ar vs 39 Ar/ 36 Ar isochrons were determined by incremental heating for 11 terrestrial rocks and minerals whose geology indicates that they represent essentially undisturbed systems. The samples include muscovite, biotite, hornblende, sanidine, plagioclase, dacite, diabase and basalt and range in age from 40 to 1700 m.y. For each sample, the 40 Ar/ 39 Ar ratios, corrected for atmospheric and neutron-generated argon isotopes, are the same for most of the gas fractions released and the age spectra, which show pronounced plateaus, thus are consistent with models previously proposed for undisturbed samples. Plateau ages and isochron ages calculated using plateau age fractions are concordant and appear to be meaningful estimates of the crystallization and cooling ages of these samples. Seemingly anomalous age spectrum points can be attributed entirely to small amounts of previously unrecognized argon loss and to gas fractions that contain too small (less than 2 per cent) a proportion of the 39 Ar released to be geologically significant. The use of a quantitative abscissa for age spectrum diagrams is recommended so that the size of each gas fraction is readily apparent. Increments containing less than about 4–5 per cent of the total 39 Ar released should be interpreted cautiously. Both the age spectrum and isochron methods of data reduction for incremental heating experiments are worthwhile, as each gives slightly different but complementary information about the sample from the same basic data. Use of a least-squares fit that allows for correlated errors is recommended for 40 Ar/ 36 Ar vs 39 Ar/ 36 Ar isochrons. The results indicate that the 40 Ar/ 39 Ar incremental heating technique can be used to distinguish disturbed from undisturbed rock and mineral systems and will be a valuable geochronological tool in geologically complex terranes.

Geochimica et Cosmochimica Acta↗
USGS1973

Age of Kōko Seamount, Emperor Seamount chain

K— Ar ages obtained by the conventional isotope-dilution and the 40 Ar/ 39 Ar techniques on two sanidine trachytes, four basalts, and a phonolite dredged from the top of Kōko Seamount, 300 km north of the Hawaiian-Emperor bend, show that the seamount is 46.4 ± 1.1 my old. These data indicate that the volcanoes in the Hawaiian-Emperor chain continue to increase in age to the west and north beyond Midway Atoll, as predicted by the melting-spot hypothesis for the origin of the chain, and that the rate of volcanic migration along the chain was nonlinear between the time of formation of the island of Hawaii and Kōko Seamount.

Earth and Planetary Science Letters↗
USGS1973

Potassium-argon ages and paleomagnetism of the Waianae and Koolau Volcanic Series, Oahu, Hawaii

Paleomagnetic and potassium-argon measurements on 786 oriented cores from 99 volcanic units at 18 sites in the Waianae and Koolau Ranges, Oahu, when combined with data from previous studies, show that the sub-aerial Waianae Volcano was active only from about 3.6 to 2.4 m.y. ago and the subaerial Koolau Volcano from about 2.6 to 1.8 m.y. ago. There is some evidence that Waianae Volcano was still active when Koolau Volcano emerged from the sea. The predominantly tholeiitic lower and middle members of the Waianae Volcanic Series are approximately contemporaneous and were extruded during the late Gilbert and early Gauss geomagnetic polarity epochs. They were followed within less than 0.2 m.y. by the alkalic lavas of the upper member, which were probably extruded largely during the later part of the Gauss normal polarity epoch. The Koolau Volcanic Series was extruded entirely during the early Matuyama reversed polarity epoch. Data from three thick stratigraphic sections in the Waianae and Koolau Volcanic Series indicate that stacks of lava flows as much as 470 m thick can be formed in less than 0.25 m.y. and that the maximum average period between superimposed lava flows is on the order of 103 yrs. Additional data on the hawaiite flow that led to the discovery of the Kaena reversed event indicate that this reversed flow is 2.85 ± 0.05 m.y. old. Angular dispersion of virtual geomagnetic poles (VGP) in the Hawaiian Islands appears to have decreased during the past 5 m.y. This may be caused by a decrease in dipole wobble, a decrease in the nondipole component of the Earth' magnetic field, or the accumulated effects of weathering, tectonism, and geomorphic processes in older rocks. The mean Waianae and Koolau VGPs are slightly on the side of the Earth's rotation axis away from Oahu. This supports, but does not prove, the hypothesis that the axial dipole is displaced slightly northward from the Earth's center. Three VGP “excursions” were recorded in sections of lava in the Waianae and Koolau ranges. During these excursions, the VGPs appear to have traveled away from or toward the geographic axis along great circle paths, suggesting they may be related to the dipole rather than the nondipole field and may record aborted reversals in polarity or rapid and infrequent dipole tilts.

Hawai'i↗
USGS1972

Electron microprobe evaluation of terrestrial basalts for whole-rock K-Ar dating

Four basalt samples for whole-rock K-Ar dating were analyzed with an electron microprobe to locate potassium concentrations. Highest concentrations of potassium were found in those mineral phases which were the last to crystallize. The two reliable samples had potassium concentrated in fine-grained interstitial feldspar and along grain boundaries of earlier formed plagioclase crystals. The two unreliable samples had potassium concentrated in the glassy matrix, demonstrating the ineffectiveness of basaltic glass as a retainer of radiogenic argon. In selecting basalt samples for whole-rock K-Ar dating, particular emphasis should be placed on determining the nature and condition of the fine-grained interstitial phases.

Earth and Planetary Science Letters↗
USGS1971

A test of the 40Ar/39Ar age spectrum technique on some terrestrial materials

40 Ar/ 39 Ar age spectra were determined for 10 terrestrial rock and mineral samples whose geologic history is known from independent evidence. The spectra for six mineral and whole rock samples, including biotite, feldspar, hornblende, muscovite, and granodiorite, that have experienced post-crystallization heating did not reveal the age of crystallization in any obvious way. Minima in the spectra, however, give reasonable maximum ages for reheating and high-temperature maxima can be interpreted as minimum crystallization ages. High-temperature ages of microcline and albite that have not been reheated are approximately 10% younger than the known crystallization age. Apparently there are no domains in these feldspars that have retained radiogenic 40 Ar quantitatively. Spectra from two diabase samples that contain significant quantities of excess argon might mistakenly be interpreted as spectra from reheated samples and do not give the age of emplacement. The 40 Ar/ 39 Ar age spectrum technique may be a potentially valuable tool for the study of geologic areas with complex histories, but the interpretation of age spectra from terrestrial samples seems to be more difficult than suggested by some previous studies.

Earth and Planetary Science Letters↗
USGS1971

Thermoluminescence of Apollo 12 lunar samples

Thermoluminescence (TL) glow curve and decay characteristics of Apollo 12 fines and soil samples are similar to those from Apollo 11. Interpretation of the results from the core sample is difficult because of inadequate sample, spacing, but it appears that the part of the core below about 8 cm has been undisturbed for about 10 4 years whereas the part of the core above 10 cm may have been disturbed by recent surface activity. TL in the Apollo 12 samples is about twice that in the Apollo 11 samples, suggesting a lower mean daytime surface temperature of a few degrees at the Apollo 12 site.

Earth and Planetary Science Letters↗
USGS1971

40Ar/39Ar technique of KAr dating: A comparison with the conventional technique

K-Ar ages have been determined by the 40 Ar/ 39 Ar total fusion technique on 19 terrestrial samples whose conventional K-Ar ages range from 3.4 my to nearly 1700 my. Sample materials included biotite, muscovite, sanidine, adularia, plagioclase, hornblende, actinolite, alunite, dacite, and basalt. For 18 samples there are no significant differences at the 95% confidence level between the K Ar ages obtained by these two techniques; for one sample the difference is 4.3% and is statistically significant. For the neutron doses used in these experiments (≈4 × 10 18 nvt) it appears that corrections for interfering Ca- and K-derived Ar isotopes can be made without significant loss of precision for samples with K/Ca > 1 as young as about 5 × 10 5 yr, and for samples with K/Ca < 1 as young as about 10 7 yr. For younger samples the combination of large atmospheric Ar corrections and large corrections for Ca- and K-derived Ar may make the precision of the 40 Ar/ 39 Ar technique less than that of the conventional technique unless the irradiation parameters are adjusted to minimize these corrections.

Earth and Planetary Science Letters↗
USGS1971

Potassium-argon Ages from the Pololu Volcanic Series, Kohala Volcano, Hawaii

Potassium-argon ages on five lava flows from the Pololu Volcanic Series, thought to be among the oldest rocks exposed on the island of Hawaii, indicate that the main subaerial shield-building phase of Kohala Volcano occurred about 0.7 ± 0.15 m.y. ago and that most of the island was formed within the past 0.7 m.y.

Hawaii↗
USGS1968

Paleomagnetism, potassium-argon ages, and geology of rhyolites and associated rocks of the Valles Caldera, New Mexico

Paleomagnetic and potassium-argon studies support geologic evidence that the lower member of the Bandelier Tuff was deposited 1.4 m.y. ago. The upper member erupted about 1.0 m.y. ago and was followed by caldera collapse which formed the 12- to 14-mile diameter Valles Caldera. Postcaldera activity which resulted in the eruption of rhyolite domes and pyroclastic material, has occurred at about 0.9, 0.7, 0.5, and 0.4 m.y. ago, with later undated eruptions that were estimated at about 0.1 m.y. ago. These data from the Valles Caldera are the basis for the previously published age revision of the Brunhes-Matuyama geomagnetic polarity epoch boundary from 1.0 to 0.7 m.y. ago, and they were used to define the Jaramillo normal polarity event at about 0.9 m.y. ago (Doell and Dalrymple, 1966).

New Mexico↗
USGS1968

Geology, paleomagnetism, and potassium-argon ages of basalts from Nunivak Island, Alaska

Geologic mapping, paleomagnetic stratigraphy, and potassium-argon dating were used to determine the time and volume relations of tholeiitic and alkalic basalt on Nunivak Island in the Bering Sea near the coast of Alaska. Volcanism on Nunivak Island occurred in distinct episodes separated by quiet intervals that lasted from 1.6 to 0.6 m.y. During the past 6 m.y., tholeiitic basalt was erupted during at least five such episodes, and highly undersaturated alkalic basalt was erupted during at least three episodes. The oldest volcanic rock found on Nunivak Island is an alkalic basalt, succeeded by repeated alternations of tholeiitic and alkalic basalt. During the last episode of tholeiitic volcanism, 130 cu km of basalt erupted during a well-defined interval that lasted from 0.9 to 0.3 m.y. ago. A nearly contemporaneous eruption of alkalic basalt has continued vigorously to historic times and has covered the central part of the island with small cones, flows, and tephra from explosion craters. The volume of alkalic basalt of the latest episode of volcanic activity is from 0.7 to 2.0 percent of the volume of the associated tholeiite. At least one earlier eruption of alkalic basalt occurred in close association with the eruption of tholeiite. As on Hawaii, the highly undersaturated alkalic basalts on Nunivak contain abundant ultramafic inclusions.

Alaska↗
USGS1968

An investigation of the source of air Ar contamination in K-Ar dating

Precision of young K Ar ages is limited by air argon contamination. A series of experiments in which the exposure of basalt and sanidine samples to air argon was controlled, shows that most of the air contamination does not arise in the laboratory. Because of this, it seems unlikely that air argon contamination can be significantly reduced by special sample handling and preparation techniques.

Earth and Planetary Science Letters↗
USGS1968

Argon-40: Excess in submarine pillow basalts from Kilauea Volcano, Hawaii

Submarine pillow basalts from Kilauea Volcano contain excess radiogenic argon-40 and give anomalously high potassium-argon ages. Glassy rims of pillows show a systematic increase in radiogenic argon-40 with depth, and a pillow from a depth of 2590 meters shows a decrease in radiogenic argon-40 inward from the pillow rim. The data indicate that the amount of excess radiogenic argon-40 is a direct function of both hydrostatic pressure and rate of cooling, and that many submarine basalts are not suitable for potassium-argon dating.

Hawaii↗
USGS1967

Potassium-argon ages of recent rhyolites of the Mono and Inyo craters, California

Twenty-two KAr ages were determined for sanidine samples from 10 rhyolite domes of the Mono and Inyo Craters to test the applicability of KAr dating to volcanic rocks of Recent age. Comparison of the results with ‘blank’ and dosed analyses shows that radiogenic 40 Ar was detected and was measured to within a factor of two or better. The estimated standard deviation of precision is 12% for analyses containing 5% or more radiogenic 40 Ar. A statistical analysis suggests that real differences in apparent age were detected between three of the analyzed domes. The ages, which range from 6400 to 10,200 years for experiments with 5% or more radiogenic 40 Ar, are, in general, consistent with ionium ages on 5 of the same samples and with glacial and 14 C evidence of age. They suggest that most of the Mono Craters volcanos are on the order of 10,000 years old or less. The results also suggest that the problem of excess 40 Ar may not be severe for KAr dating of volcanic rocks.

Earth and Planetary Science Letters↗
USGS1965

Potassium-argon age and paleomagnetism of the Bishop Tuff, California

Duplicate potassium-argon age determinations on each of three samples from widely separated localities indicate that the age of the Bishop Tuff, California, is about 0.7 million years. Two of the samples are from the basal ash fall that preceded the ash flow eruptions; one of these two samples was collected within 1 m of the contact of the Bishop Tuff with the underlying Sherwin Till. The third sample is from near the present exposed surface of the Bishop Tuff. The minimum age of the Sherwin Till (Kansan?) is thus 0.7 million years. The samples used for previously published age determinations of about 1 million years were probably contaminated with older material. Paleomagnetic results from five widely separated localities indicate that the welded part of the Bishop Tuff became magnetized when the geomagnetic field was normal and that it may have cooled in several centuries or less. The Brunhes-Matuyama polarity epoch boundary is now uncertain in the range of 0.7 to 1.0 million years.

California↗
USGS1964

Argon retention in a granitic xenolith from a pleistocene basalt, Sierra Nevada, California

THIS note reports measurements made in an attempt to determine: (1) the possible effect of xenoliths on whole-rock basalt age determinations; (2) whether or not potassium-argon age determinations on xenoliths in basalt might approximate the age of the basalt flow. The field location is in the canyon of Sawmill Creek, Inyo County, sec. 7 md 18, T. 12 S., R. 34 E., and sec. 12 and 13, T. 12 S., R. 33 E.

California↗