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G. Brent Dalrymple

Publications and source records attributed to G. Brent Dalrymple.

At least 19 recordsLinked to original sources
USGS2012

40AR/39Ar ages of six Apollo 15 impact melt rocks by laser step heating

We have obtained 15 high resolution (21–51 step) 40 Ar/ 39 age spectra on six Apollo 15 impact melt rocks of different compositions using a continuous laser system on submilligram subsamples and on single crystal plagioclase clasts. Four of the six samples gave reproducible age spectra with well-defined intermediate temperature plateaus over 48% or more of the 39 Ar released; the plateaus are interpreted as crystallization ages. Samples 15304,7,69, 15294,6,21, and 15314,26,156 gave virtually identical plateau ages whose weighted mean is 3,870±6 Ma. These three melt rocks differ in composition and likely formed in three separate impact events. Sample 15356,9 gave replicate plateau ages that average 3,836±12 Ma and date a fourth and younger impact event. The age spectra for samples 15308,9 and 15414,3,36 increase with increasing increment temperature and may have been formed in or affected by impacts at about 2,700 Ma and 3,870 Ma, respectively. So far there continues to be no convincing evidence in the lunar record for impact melts older than about 3.9 Ga.

Geophysical Research Letters↗
USGS2012

Radiometric and paleomagnetic evidence for the Emperor reversed polarity event at 0.46 ± 0.05 M.Y. in basalt lava flows from the eastern Snake River Plain, Idaho

K-Ar and paleomagnetic data from cores through a sequence of basalt flows in the eastern Snake River Plain provide evidence for a brief (0.005 to 0.01 m.y.) reversal of the geomagnetic field 0.46 ± 0.05 m.y. ago. This reversed polarity event has also been found in sea-floor magnetic anomalies and in sediment cores and is probably the Emperor event of Ryan [1972].

Idaho, Oregon↗
USGS2012

Revised geomagnetic polarity time scale for the interval 0–5 m.y. B.P.

A change in the constants used in K‐Ar dating and a significant increase in new data have made a recompilation and recomputation of data used to define the Late Cenozoic K‐Ar polarity time scale highly desirable at this time. All available data in the range 0–5 m.y. have been recalculated using the refined constants, with 354 data points in this time interval now meeting the minimum criteria for acceptability. Recalculation of the major polarity epoch boundaries has yielded ages of 0.73 m.y. for the Brunhes‐Matuyama, 2.48 m.y. for the Matuyama‐Gauss, and 3.40 m.y. for the Gauss‐Gilbert boundaries. A revised polarity time scale has been constructed based on available K‐Ar data and information obtained from marine magnetic anomalies and deep‐sea sedimentary cores.

Journal of Geophysical Research B: Solid Earth↗
USGS2012

Late Cenozoic volcanism, geochronology, and structure of the Coso Range, Inyo County, California

The Coso Range lies at the west edge of the Great Basin, adjacent to the southern part of the Sierra Nevada. A basement complex of pre‐Cenozoic plutonic and metamorphic rocks is partly buried by ∼35 km 3 of late Cenozoic volcanic rocks that were erupted during two periods, as defined by K‐Ar dating: (1) 4.0–2.5 m.y., ∼31 km 3 of basalt, rhyodacite, dacite, andesite, and rhyolite, in descending order of abundance, and (2) ≤1.1 m.y., nearly equal amounts of basalt and rhyolite, most of the rhyolite being ≤0.3 m.y. old. Vents for the volcanic rocks of the younger period are localized on and near a horst of basement rocks within a concavity defined by the distribution of vents of the older period. The alignment of many vents and the presence of a considerable number of roughly north‐trending normal faults of late Cenozoic age reflect basin and range tectonics dominated by roughly east‐west lithospheric extension. Fumaroles, intermittently active thermal springs, and associated altered rocks occur within and immediately east of the central part of the field of Quaternary rhyolite, in an area characterized by various geophysical anomalies that are evidently related to an active hot‐water geothermal system. This system apparently is heated by a reservoir of silicic magma at ≥8‐km depth, itself produced and sustained through partial melting of crustal rocks by thermal energy contained in mantle‐derived basaltic magma that intrudes the crust in response to lithospheric extension.

California↗
USGS2012

Paleomagnetism and K-Ar ages of volcanic rocks from Long Valley caldera, California

Paleomagnetic measurements and K‐Ar age determinations on volcanic rocks from Long Valley caldera, California, have enabled further refinement of eruptive activity within this large silicic volcanic center. K‐Ar age determinations show that postcaldera volcanic eruptions began 0.73 m.y. ago and continued periodically until about 50,000 years ago. The eruptions were not temporally random but tended to occur in distinct episodes separated by periods of quiescence. Volcanism in the western half of the caldera was particularly intense between 0.15 and 0.50 m.y. ago, when many units ranging in composition from basalt to rhyolite were erupted. An average of the virtual geomagnetic poles for 33 units from the caldera yields a paleomagnetic pole at 89.7°N, 138.4°E, (α 95 = 5.1°), which is indistinguishable from the earth's rotational axis. The ancient geomagnetic field dispersion about this mean pole is 16.0°, with upper and lower limits of 19.3° and 13.6°, respectively.

California↗
USGS2000

First-principles calibration of 38Ar tracers :implications for the ages of 40Ar/39Ar fluence monitors

We describe the analytical procedure for first-principles calibrations of 38Ar tracers in the U.S. Geological Survey (USGS) Laboratory in Menlo Park, California. The ages of fluence monitors determined in the USGS laboratory and those reported in the literature differ by nearly 2 percent. However, the radiogenic-40Ar content of the primary mineral standard on which these ages are based has never been verified by first-principles measurement in other laboratories.

Professional Paper↗
USGS1997

The geology and petrology of Mauna Kea Volcano, Hawaii — A study of postshield volcanism

Mauna Kea Volcano, on the Island of Hawaii, is capped by lavas of alkalic and transitional basalt (Hamakua Volcanics) erupted between approximately 250-200 and 70-65 ka and hawaiite, mugearite, and benmoreite (Laupahoehoe Volcanics) erupted between approximately 65 and 4 ka. These lavas, which form the entire subaerial surface of the volcano, issued from numerous scattered vents and are intercalated on the upper slopes with glacial deposits. The lavas record diminishing magma-supply rate and degree of partial melting from the shield stage through the postshield stage. Much of the compositional variation apparently reflects fractionation of basaltic magma in reservoirs within and beneath the volcano.

Hawaii↗
USGS1995

Further evidence for a low U/Pb source in the moon: U-Th-Pb, Sm-Nd, and Ar-Ar, isotopic systematics of lunar meteorite Yamato-793169

The coarse-grained lunar meteorites, Yamato-793169 and Asuka-881757, represent a new type of low-Ti mare basalt. This paper reports the results of a U-Th-Pb, Sm-Nd, and Ar-Ar isotopic study of Yamato-793169 performed as part of a consortium studies of lunar basaltic meteorites. The isotopic study was carried out on a small sample (100 mg) so that only three density fractions could be separated. These fractions were leached with dilute acid in order to eliminate terrestrial Pb contamination. However, the leaching procedure did not completely remove this contamination in some fractions and also apparently caused a fractionation of elements (U, Th, Pb) due to preferential leaching effects, producing a secondary disturbance of the systematics. Furthermore, the Ar-Ar analyses indicate that the isotopic systematics in this meteorite might have been disturbed sometime later than 750 Ma. For these reasons, the ages obtained using different isotopic systems disagree with each other and a precise formation age could not be obtained for this meteorite. However, using what results there are, two reasonable interpretations can be made: (1) the Sm-Nd system yielded an age of 3.4 Ga that could be interpreted as the formation age, assuming that this system is possibly the least disturbed during the metamorphic event(s) that this meteorite experienced at least once, and that all other isotopic systems and their corresponding ages were disturbed, and (2) the U-Pb system yielded a nearly concordant age of 3.8 Ga that could be interpreted as the formation age and the Sm-Nd isotopic systematics were somehow disturbed. We prefer the second interpretation for the reasons discussed below. On a concordia diagram, the CDT (Cañon Diablo troilite)-corrected U-Pb isotopic data yield a discordia line similar to the lunar catastrophic array, indicating that the source of the meteorite formed during early lunar differentiation (∼4.4 Ga) and that the basalt was generated near 3.9 Ga. Total Ar-Ar age on plagioclase is 3.26 Ga, which seems to be too old if the formation age is 3.4 Ga, because low temperature fractions lost large amounts of radiogenic 40 Ar during the late thermal event. If we assume a formation age of 3.8 Ga, the estimated source 238 U/ 204 Pb (μ) is 21.6 ± 3.5 and ϵ Nd is 3.9 ± 0.3. These results indicate that the source of Yamato-793169 is more depleted than Apollo 12 and 15 LT basalts, but less depleted than Asuka-881757. Therefore, Yamato-793169 may represent a new type of LT- or VLT-like mare basalt that is different from Asuka-881757. The wide variety of lead and neodymium isotopic characteristics among LT and VLT mare basalts indicate that the lunar mantle was very heterogeneous with respect to trace element abundances.

Geochimica et Cosmochimica Acta↗
USGS1979

Geology, geochronology, and potential volcanic hazards in the Lava Ridge-Hells Half Acre area, eastern Snake River Plain, Idaho

The evaluation of volcanic hazards for the proposed Safety Test Reactor Facility (STF) at the Argonne National Laboratory-West (ANLW) site, Idaho National Engineering Laboratory (INEL), Idaho, involves an analysis of the geology of the Lava Ridge-Hells Half Acre area and of K-At age determinations on lava flows in cored drill holes. The ANLW site at INEL lies in a shallow topographic depression bounded on the east and south by volcanic rift zones that are the locus of past shield-type basalt volcanism and by rhyolite domes erupted along the ring fracture of an inferred rhyolite caldera. The K-At age data indicate that the ANLW site has been flooded by basalt lava flows at irregular intervals from perhaps a few thousand years to as much as 300,000-400,000 years, with an average recurrence interval between flows of approximately 80,000-100,000 years. At least five major lava flows have covered the ANLW site within the past 500,000 years.

Idaho↗
USGS1976

Identification of excess 40Ar by the 40Ar 39Ar, age spectrum technique

40 Ar/ 39 Ar incremental heating experiments on igneous plagioclase, biotite, and pyroxene that contain known amounts of excess 40 Ar indicate that saddle-shaped age spectra are diagnostic of excess 40 Ar in igneous minerals as well as in igneous rocks. The minima in the age spectra approach but do not reach the crystallization age. Neither the age spectrum diagram nor the 40 Ar/ 36 Ar versus 39 Ar/ 36 Ar isochron diagram reliably reveal the crystallization age in such samples.

Earth and Planetary Science Letters↗
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↗