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Results for “Earth and Planetary Science Letters”

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USGS1999

Metal emissions from Kilauea, and a suggested revision of the estimated worldwide metal output by quiescent degassing of volcanoes

Measurements of a large suite of metals (Pb, Cd, Cu, Zn and several others) and sulfur at Kilauea volcano over an extended period of time has yielded a detailed record of the atmospheric injection of ordinarily-rare metals from this quiescently degassing volcano, representative of an important type. We have combined the Kilauea data with data of recent studies by others (emissions from volcanoes in the Indonesian arc; the large Laki eruption of two centuries ago; Etna: estimates of total volcanic emissions of sulfur) to form the basis for a new working estimate of the rate of worldwide injection of metals to the atmosphere by volcanoes. The new estimate is that volcanoes inject a substantially smaller mass of ordinarily-rare metals into the atmosphere than was stated in a widely cited previous estimate [J.O. Nriagu, A global assessment of natural sources of atmospheric trace metals, Nature 338 (1989) 47-49]. Our estimate, which is an upper limit, is an annual injection mass of about 10,000 tons of the metals considered, versus the earlier estimate of about 23,000 tons. Also, the proportions of the metals are substantially different in our new estimate.

Earth and Planetary Science Letters↗
USGS1999

Actual timing of neodymium isotopic variations recorded by Fe-Mn crusts in the western North Atlantic

Hydrogenetic ferromanganese (Fe-Mn) crusts from the western North Atlantic record variations in the Nd and Pb isotopic composition of Cenozoic deep water preserved during their growth. The timing and cause of the most striking change have been the subject of debate. Some have proposed that the shift took place after 4 Ma in response to the closure of the Panama gateway. Others have argued that the major change in isotope composition occurred as early as 8 Ma. This study presents high-resolution Nd isotope records for crusts previously dated using 10Be/9Be chronology. These data confirm that the shifts in Nd occurred after 4 Ma, consistent with a likely relationship with the closure of the Central American Isthmus and intensification of Northern Hemisphere Glaciation, and in accordance with changes seen in other physical and chemical records. These results illustrate the need for both a robust chronological framework and high-resolution records before a reliable paleoceanographic interpretation can be made of the variations recorded by Fe-Mn crusts.

Earth and Planetary Science Letters↗
USGS1999

Osmium isotope variations in the oceans recorded by Fe-Mn crusts

This study presents osmium (Os) isotope data for recent growth surfaces of hydrogenetic ferromanganese (Fe-Mn) crusts from the Pacific, Atlantic and Indian Oceans. In general, these data indicate a relatively uniform Os isotopic composition for modern seawater, but suggest that North Atlantic seawater is slightly more radiogenic than that of the Pacific and Indian Oceans. The systematic difference in the Os isotopic composition between the major oceans probably reflects a greater input of old continental material with a high Re/Os ratio in the North Atlantic Ocean, consistent with the distribution of Nd and Pb isotopes. This spatial variation in the Os isotope composition in seawater is consistent with a residence time for Os of between 2 and 60 kyr. Indian Ocean samples show no evidence of a local source of radiogenic Os, which suggests that the present-day riverine input from the Himalaya-Tibet region is not a major source for Os. Recently formed Fe-Mn crusts from the TAG hydrothermal field in the North Atlantic yield an Os isotopic composition close to that of modern seawater, which indicates that, in this area, the input of unradiogenic Os from the hydrothermal alteration of oceanic crust is small. However, some samples from the deep Pacific (???4 km) possess a remarkably unradiogenic Os isotope composition (187Os/186Os ratios as low as 4.3). The compositional control of Os incorporation into the crusts and mixing relationships suggest that this unradiogenic composition is most likely due to the direct incorporation of micrometeoritic or abyssal peridotite particles, rather than indicating the presence of an unradiogenic deep-water mass. Moreover, this unradiogenic signal appears to be temporary, and local, and has had little apparent effect on the overall evolution of seawater. These results confirm that input of continental material through erosion is the dominant source of Os in seawater, but it is not clear whether global Os variations are due to the input of mantle or meteoritic material, or simply indicate that the continental source itself is not uniform.

Earth and Planetary Science Letters↗
USGS1999

Age and thermal history of the Geysers plutonic complex (felsite unit), Geysers geothermal field, California: A 40Ar/39Ar and U-Pb study

Sixty-nine ion microprobe spot analyses of zircons from four granite samples from the plutonic complex that underlies the Geysers geothermal field yield 207Pb/206Pb vs. 238U/206Pb concordia ages ranging from 1.13 ?? 0.04 Ma to 1.25 ?? 0.04 (1??) Ma. The weighted mean of the U/Pb model ages is 1.18 ?? 0.03 Ma. The U-Pb ages coincide closely with 40Ar/39Ar age spectrum plateau and 'terminal' ages from coexisting K-feldspars and with the eruption ages of overlying volcanic rocks. The data indicate that the granite crystallized at 1.18 Ma and had cooled below 350??C by ~0.9-1.0 Ma. Interpretation of the feldspar 40Ar/39Ar age data using multi-diffusion domain theory indicates that post-emplacement rapid cooling was succeeded either by slower cooling from 350??to 300??C between 1.0 and 0.4 Ma or transitory reheating to 300-350??C at about 0.4-0.6 Ma. Subsequent rapid cooling to below 260??C between 0.4 and 0.2 Ma is in agreement with previous proposals that vapor-dominated conditions were initiated within the hydrothermal system at this time. Heat flow calculations constrained with K-feldspar thermal histories and the present elevated regional heat flow anomaly demonstrate that appreciable heat input from sources external to the known Geysers plutonic complex is required to maintain the geothermal system. This requirement is satisfied by either a large, underlying, convecting magma chamber (now solidified) emplaced at 1.2 Ma or episodic intrusion of smaller bodies from 1.2 to 0.6 Ma.

Earth and Planetary Science Letters↗
USGS1970

UThPb age of Apollo 12 rock 12013

A UThPb isotopic study of three chips from lunar rock 12013 indicates that parental material of the intrusion breccia formed quite early in the moon's history, possibly 3.9 to 4.3 by ago. The UThPb characteristics of the rock are distinctly different from those of other Apollo 12 igneous rocks and suggest a different origin..

Earth and Planetary Science Letters↗
USGS1980

Calculation of uncertainties of U-Pb isotope data

Equations are derived for the estimation of errors and error correlations for various types of U-Pb isotope data, taking into account ion-beam instabilities, run-to-run variability in mass-discrimination, uncertainties in Pb and U concentrations, and uncertainties in initial-Pb and blank-Pb amount and isotopic composition. Equations are also given for the calculation of concordia intercept errors. ?? 1980.

Earth and Planetary Science Letters↗
USGS1978

Strontium isotopes in basalts from the Pacific Ocean basin

Basalts being erupted in the Pacific Ocean Basin vary in Sr isotopic composition in a simple geographic pattern. 87 Sr/ 86 Sr increases away from the East Pacific Rise to very high values for islands in the south-central Pacific. The 87 Sr/ 86 Sr variations are almost certainly related to past segregation of Rb, K, and other large cations. The segregation process was probably incipient partial melting which resulted in various mantle zones being enriched or depleted.

Earth and Planetary Science Letters↗
USGS1991

Geochronology and petrogenesis of MORB from the Juan de Fuca and Gorda ridges by 238U230Th disequilibrium

A highly precise mass spectrometric method of analysis was used to determine 238 U 234 U 230 Th 232 Th in axial and off-axis basalt glasses from Juan de Fuca (JDF) and Gorda ridges. Initial 230 Th activity excesses in the axial samples range from 3 to 38%, but generally lie within a narrow range of 12 to 15%. Secondary alteration effects were evaluated using δ 234 U and appear to be negligible; hence the 230 Th excesses are magmatic in origin. Direct dating of MORB was accomplished by measuring the decrease in excess 230 Th in off-axis samples. 238 U 230 Th ages progressively increase with distance from axis. Uncertainties in age range from 10 to 25 ka for U Th ages of 50 to 200 ka. The full spreading rate based on U Th ages for Endeavour segment of JDF is 5.9 ± 1.2 cm/yr, with asymmetry in spreading between the Pacific ( 4.0 ± 0.6 cm/yr) and JDF ( 1.9 ± 0.6 cm/yr) plates. For northern Gorda ridge, the half spreading rate for the JDF plate is found to be 3.0 ± 0.4 cm/yr. These rates are in agreement with paleomagnetic spreading rates and topographic constraints. This suggests that assumptions used to determine ages, including constancy of initial 230 Th/ 232 Th ratio over time, are generally valid for the areas studied. Samples located near the axis of spreading are typically younger than predicted by these spreading rates, which most likely reflects recent volcanism within a 1–3 km wide zone of crustal accretion. Initial 230 /Th/ 232 Th ratios and 230 Th activity excesses were also used to examine the recent Th/U evolution and extent of melting of mantle sources beneath these ridges. A negative anomaly in 230 Th/ 232 Th for Axial seamount lavas provides the first geochemical evidence of a mantle plume source for Axial seamount and the Cobb-Eickelberg seamount chain and indicates recent depletion of other JDF segment sources. Large 230 Th activity excesses for lavas from northern Gorda ridge and Endeavour segment indicate formation from a lower degree of partial melting than other segments. An inverse correlation between 230 Th excess and 230 Th/ 232 Th for each ridge indicates that these lower degree melts formed from slightly less depleted sources than higher degree melts. Uniformity in 230 Th excess for other segments suggests similarity in processes of melt formation and mixing beneath most of the JDF-Gorda ridge area. The average initial 230 Th/ 232 Th activity ratio of 1.31 for the JDF-Gorda ridge area is in agreement with the predicted value of 1.32 from the Th Sr isotope mantle array.

Earth and Planetary Science Letters↗
USGS2023

Global seismic networks operated by the U.S. Geological Survey

The U.S. Geological Survey (USGS) Global Seismographic Network (GSN) Program operates two thirds of the GSN, a network of state‐of‐the‐art, digital seismological and geophysical sensors with digital telecommunications. This network serves as a multiuse scientific facility and a valuable resource for research, education, and monitoring. The other one third of the GSN is funded by the National Science Foundation (NSF), and the operations of this component are overseen by EarthScope. This collaboration between the USGS, EarthScope, and NSF has allowed for the development and operations of the GSN to be a truly multiuse network that provides near real‐time open access data, facilitating fundamental discoveries by the Earth science community, supporting the earthquake hazards mission of the USGS, benefitting tsunami monitoring by the National Oceanic and Atmospheric Administration, and contributing to nuclear test monitoring and treaty verification. In this article, we describe the installation and evolution of the seismic networks operated by the USGS that ultimately led to the USGS portion of the GSN (100 stations under network codes IU, IC, and CU) as they are today and envision technological advances and opportunities to further improve the utility of the network in the future. This article focuses on the USGS‐operated component of the GSN; a companion article on the GSN stations funded by the NSF and operated by the Cecil and Ida Green Institute of Geophysics and Planetary Physics, Scripps Institution of Oceanography, University of California at San Diego by Davis et al. (2023) appears in this volume.

Seismological Research Letters↗