USGS Science⌕ Search
Home/Archive/USGS

USGS · 70011675

The isotopic composition of lead in potassium feldspars from some 1.0-b.y. old North American igneous rocks

R. E. Zartman·G.J. Wasserburg
View original source ↗Export metadata ↓

Abstract

The isotopic composition of lead and the uranium, thorium and lead concentrations in potassium feldspars are determined for more than 30 1.0-b.y. old North American igneous rocks. Samples representing a broad spectrum in petrographic type and mode of occurrence were chosen; an effort was made to include only rocks having well-documented ages from 950 to 1140 m.y. and showing minimal evidence of subsequent metamorphism. Most samples, including those from extensive terranes of contemporaneous age, have limited lead isotope variations ( Pb 206 Pb 204 = 16.74–17.38 "> Pb206Pb204= 16.74–17.38 ; Pb 207 Pb 204 = 15.39–15.59 "> Pb207Pb204= 15.39–15.59 ; Pb 208 Pb 204 = 36.38–37.10 "> Pb208Pb204= 36.38–37.10 ), which yield model ages close to the radiometric ages. Granite, pegmatite, and rhyolite from within the Grenville province of Canada and age-equivalent rocks of New York, Virginia, Texas, and Colorado and granophyric units associated with the Duluth Gabbro Complex of Minnesota are among the materials yielding this main isotopic spectrum. Several samples were encountered which had isotopic compositions very different from the above group. Lead showing a marked deficiency in radiogenic isotopes was found in two granitic bodies associated with older Labrador Trough rocks from Quebec, in a rapakivi granite from southern Nevada, and in a small granite stock from Mellen, Wisconsin. These occurrences all involve small intrusions of granite which lie near considerably older areas of basement rock. Model ages calculated from the Pb 206 Pb 204 "> Pb206Pb204 ratio are older than the age of the intrusions and approach the age of the host basement rock. Several possible interpretations are offered to explain the isotopic behavior encountered in this study. In particular, a “vertically differentiated crust” model is proposed which will account for both the main spectrum and the anomalous lead. The significance of lead isotopic studies in understanding crustal structure in continental regions is discussed.

Explore related subjects

Geochimica et Cosmochimica ActaArticle

Keep this discovery

Explore connections, maps & timelines

BibTeXRIS

R. E. Zartman, G.J. Wasserburg. 1969. The isotopic composition of lead in potassium feldspars from some 1.0-b.y. old North American igneous rocks. https://doi.org/10.1016/0016-7037(69)90104-5

Cite the original work for its findings. Save a collection to share your selection of sources.

KEEP EXPLORING

Related USGS reports

USGS2026

Organic and isotopic indicators for sorting of sedimentary organic matter along a marginal submarine canyon

Submarine canyons are incised features of many continental margins that can have significant influence on the hydrodynamic distribution of sediments and organic matter eroded and deposited from the continents. Baltimore Canyon, on the mid-Atlantic margin of the United States, contains a complex set of sedimentary processes that simultaneously create unique benthic habitats and control the deposition of organic matter. Along the canyon axis, loci of net erosion, net deposition, and intense winnowing each host diverse faunal assemblages and varying mixtures of sedimentary organic matter derived both from production in the overlying water column and from mobilized sediments. Bioavailable components of this deposited organic matter sustain benthic communities, while recalcitrant components can contribute to long-term carbon burial in the deep sea. However, commonly employed bulk geochemical analyses provide little information about the relative bioavailability or depositional history of sedimentary organic matter. Here we employ a range of organic and isotopic analyses to explore in more detail how canyon-specific sediment dynamics determine the sorting of organic matter from shelf to open ocean. In combination with bulk geochemical characteristics, we subjected surface sediments from water depths of ∼200–1200 m in Baltimore Canyon to a sequential extraction procedure, isolating nonpolar and polar lipid classes, an acid-soluble fraction, and an acid-insoluble fraction. Each class was analyzed for carbon and nitrogen quantities and stable isotope ratios, and radiocarbon content where possible, along with compound-specific carbon and nitrogen isotope analysis of individual amino acids in the acid-hydrolysed fraction. We find different organic matter sources and depositional history recorded in the properties of younger, bioavailable organic matter components (polar lipids, amino acids) in comparison to the older, more recalcitrant components (nonpolar lipids, acid-insoluble fraction). These differences in source and bioavailability of organic matter vary along the canyon, correlating with grain size and erosion/deposition dynamics, and may help shape the benthic faunal assemblages. Additionally, our results suggest that determining the relative concentrations of acid-soluble and acid-insoluble organic matter may provide an easily accessible method to improve our understanding of the nutritional quality of sediments for benthic fauna than more commonly used bulk carbon or nitrogen concentrations.

Delaware, New Jersey↗
USGS2025

Bayesian calibration of the 40K decay scheme with implications for 40K-based geochronology

The K/Ar and 40 Ar/ 39 Ar geochronometers are based on the naturally occurring radionuclide 40 K. Their precision and accuracy are limited by uncertainties on the 40 K decay constants and, in the case of the 40 Ar/ 39 Ar geochronometer, the isotopic composition of neutron fluence monitors. To address these limitations, we introduce a Bayesian calibration of the 40 K decay scheme. We formulate robust priors for all model parameters including partial 40 K decay constants, 238 U and 235 U decay constants, and age offset parameters to account for phenomena that can perturb apparent U-Pb and 40 Ar/ 39 Ar ages. We then harness a set of complementary 40 Ar/ 39 Ar, 238 U/ 206 Pb, and 235 U/ 207 Pb data from well- characterized geological samples with ages from 1.919 ka to 2000 Ma to derive Bayesian estimates of the 40 K decay constants. Posterior values for the partial 40 K decay constants are λ β - "> λ β - = (4.9252 ± "> ± 0.0054) × "> × 10 −10 yr −1 , λ β + "> λ β + = (5.6658 ± "> ± 0.1543) × "> × 10 −15 yr −1 , λ EC ∗ "> λ EC0 = (5.7404 ± "> ± 0.0053) × "> × 10 −11 yr −1 , and λ EC 0 "> λ EC0 = (4.9060 ± "> ± 0.2942) × "> × 10 −13 yr −1 (uncertainties reported at the 68 % (1 σ "> σ ) credible interval). These combine to a total 40 K decay constant λ tot "> λ tot = (5.5042 ± "> ± 0.0054) × "> × 10 −10 yr −1 . Model estimates of the 238 U and 235 U decay constants are statistically indistinguishable from those reported by Jaffey et al. (1971) . Posterior values of the 40 K decay constants and the 40 Ar*/ 40 K isotopic composition of Fish Canyon sanidine (FCs) define a K/Ar FCs age of 28.183 ± "> ± 0.017 Ma (1 σ "> σ ). Significantly, Bayesian calibrated 40 Ar/ 39 Ar ages align with astronomically tuned ages throughout the Cenozoic and with 238 U/ 206 Pb and 235 U/ 207 Pb ages in the Mesozoic, Paleozoic, and Proterozoic, as well as having comparable precision to the 238 U/ 206 Pb method. Thus, Bayesian calibration of the 40 K decay scheme and the K/Ar age of FCs reconciles the 40 Ar/ 39 Ar, U-Pb, and astronomical chronometers.

Geochimica et Cosmochimica Acta↗
USGS2024

Characterizing sulfur redox state and geochemical implications in deep-time using mineral chemistry network analysis

Sulfur (S) is a central element in global biogeochemical cycling and Earth’s redox evolution. Minerals that contain S are an important record of local environmental conditions at the time of their formation based on chemical speciation and redox. However, the oxidation state of S for hundreds of different S-containing minerals and thousands of S-containing mineral localities is unknown, largely sulfides and sulfosalts, and the redox state alone does not fully capture mineral chemistry diversity, thus limiting understanding of S redox evolution. Here, we use mineral chemistry network analysis and the weighted Mineral Element Electronegativity Coefficient of Variation (wMEE CV ) metric to investigate the element interactions and localities of S-containing minerals from the Mineral Evolution Database (MED) to infer the redox state of S in minerals where the redox state is unknown (S U ). Louvain community detection of the S mineral chemistry redox network reveals that there are three main network communities that are separated by redox state. The S 6+ community includes minerals that contain the S 6+ redox state and a small number of S 4+ and S 2+ minerals, the S 2− community includes S 2− -containing minerals, and the S U community includes minerals in which the redox state of S is unknown. The wMEE CV values of the S U community closely overlap with the wMEE CV values of the S 2− community, and do not overlap with the wMEE CV values of the S 6+ community, indicating the S U community minerals contain predominately reduced S. Assuming that S U community minerals contain reduced S, as supported by their network chemical associations and wMEE CV values, then reduced S-containing minerals make up approximately 81 % of S-containing mineral localities in the S mineral chemistry network, even though the majority of all mineral localities (S-containing and non-S-containing) are oxygen (O)-containing minerals. Additionally, reduced S-containing minerals make up the majority (∼75 %) of all non-O containing mineral localities in the MED, representing the importance of reduced S as an electron source and substrate in the evolution of microbial metabolic networks. The range wMEE CV values of S 6+ community minerals expands through time due primarily to formation of chemically diverse sulfate minerals, coinciding with crustal oxidation from the late Proterozoic to Phanerozoic and the expansion of the marine sulfate reservoir. The intersection of shared constituent elements among reduced and oxidized S in the mineral chemistry network represents redox convergence of weathered S in the geosphere that was crucial in the formation of natural resource deposits and the evolution of biogeochemical cycles.

Geochimica et Cosmochimica Acta↗