Elemental Abundance Data Research Articles

Estimating erosion rates and exposure ages with 36Cl produced by neutron activation

The isotopic composition of water-soluble Cl, extracted from mineral grains in granitic rocks, appears to reflect the relative age of debris-flow fan surfaces and the erosion rate of bedrock outcrops. Our data indicate that some granitic landforms are stable landscape elements eroding on the order of meters to tens of meters per million years. Samples collected from debris-flow fan surfaces of three distinct relative ages show the expected increase of model age with time but the 36Cl-exposure ages for boulders on any particular surface vary by a factor of more than five.The method we used for extracting Cl was designed to isolate the 35Cl (n, γ) 36Cl production pathway and to simplify the chemical preparation of samples. Such isolation from fluid inclusions allows direct interpretation of measured 36Cl/Cl ratios but requires a model to predict the thermal-neutron production and absorption profile below the rock surface. Using such a model, we find good agreement between measured and predicted 36Cl/Cl for eight samples collected below the ground surface as well as consistency between several groups of samples collected near one another. Using elemental abundance data for 129 granitic rocks, we modeled 36Cl/Cl supported by radiogenic neutrons and find that it varies from 4 to 125 × 10 −15 (mean = 21 ± 18, 1 σ). However, from the surface to a depth of 10 m, thermal neutrons resulting from muon interactions have the potential to generate more 36Cl than do radiogenic neutrons. The production of 36Cl in the subsurface has important implications for surface-exposure dating of young, low-altitude samples collected from moraines and debris-flow or alluvial fans, especially at ages where erosion of the boulder surface is significant or where the source area for boulders is at altitudes much higher than the landform to be dated.

Noble gas state in the mantle

Noble gas elemental and isotopic data on mantle‐derived materials such as mid‐ocean ridge basalt (MORB), hotspot volcanics, diamonds, and mantle xenoliths published up to August 1993 are reviewed critically. Characteristic features of the mantle‐derived materials, which are important in evaluating the significance of the data, are discussed. We choose MORB glasses, hotspot volcanics, mantle xenoliths, and diamonds as the sources to infer the noble gas state in the mantle: MORB and hotspot volcanics to represent the modern depleted and the less degassed mantle, and diamonds to represent the ancient mantle. Because of various fractionation processes, the elemental abundance data obtained from the mantle‐derived materials is unlikely to constrain the noble gas composition of the mantle, except for the systematic enrichment of the heavier noble gases relative to air. On the other hand, apart from the secondary addition of radiogenic components such as 4He, 21Ne, 40Ar, 129Xe, and 131–136Xe, the noble gas isotopic compositions deduced from the mantle‐derived materials can provide useful information as to the values in the mantle; a best estimate of the mantle noble gas state is presented.

The evolution of Mauna Kea Volcano, Hawaii: Petrogenesis of tholeiitic and alkalic basalts

Mauna Kea Volcano has three exposed rock units. Submarine shield‐building tholeiites form the oldest unit. Subaerial, interbedded tholeiitic and alkalic basalts form the intermediate age unit (70–240 Ka), and they are partially covered by evolved alkalic lavas, hawaiites and mugearites (4–66 Ka). In contrast to other Hawaiian volcanoes, such as Haleakala and Kauai, lavas from Mauna Kea do not define systematic temporal variations in Pb, Sr or Nd isotopic ratios. However with decreasing age the tholeiitic basalts are increasingly enriched in incompatible elements; therefore the shield and postshield tholeiites were derived from compositionally distinct parental magmas. Submarine shield lavas from the east rift contain forsterite‐rich olivine (up to Fo90.5) providing evidence for MgO‐rich (14.4 to 17%) magmas. Postshield tholeiitic and alkalic basalts with similar isotopic ratios may have been derived from the same source composition by different degrees of partial melting. If a compositionally and isotopically homogeneous source and a batch melting model are assumed, inversion of incompatible element abundance data for the postshield basalts requires low degrees (<2%) of melting of a garnet Iherzolite source which had near‐chondritic abundances of heavy rare‐earth elements (REE) but less than chondritic abundances of highly incompatible elements such as Ba, Nb and light REE. As the volcano migrated away from the hotspot, eruption rates decreased enabling high Fe‐Ti basalts to form by fractional crystallization in shallow crustal magma chambers. The associated phenocryst‐rich, high‐MgO postshield lavas (picrites and ankaramites) are products of phenocryst accumulation. Eventually basaltic eruptions ceased, and the youngest Mauna Kea lavas are exclusively hawaiites and mugearites which formed from alkalic basalt parental magmas by clinopyroxene‐dominated fractionation at lower crustal pressures.

Ninetyeast Ridge (Indian Ocean): A 5000 km record of a Dupal mantle plume

Data and observation from Drifting Program Leg 121 and plate-tectonic reconstructions indicate that the Ninetyeast Ridge (Indian Ocean) was derived from the interaction of a deep-seated Dupal hotspot and a nearby spreading-ridge axis. The 5000-km-long ridge, from lat 34°S to Rat 10°N, was drilled at three sites during Leg 121. About 178 m of basalt, >38 to >80 Ma, were recovered from a total penetration of ∼310 m. Shipboard petrographic and geochemical studies showed that each site has distinctive characteristics. Most of the cored lavas have a tholeiitic basalt composition. Incompatible-element abundanes and ratios show systematic trends, consistent with an origin for the Ninetyeast Ridge lavas by mixing between a depleted component-Indian Ocean mid-ocean ridge basalt-and an enriched component-oceanic-island basalt similar to that observed in the youngest alkalic basalts from the Kerguelen archipelago. Preliminary shore-based trace element abundance and isotopic data are compatible with this hypothesis, although Pb isotopes indicate the involvement of another component. The long-lasting and more or less continuous activity of the Kerguelen-Heard plume (ca. 115 Ma), now located under Heard Island, south of the Southeast Indian Ridge, provides evidence that the source of the Dupal anomaly is deep seated.

Trace element abundances in megacrysts and their host basalts: Constraints on partition coefficients and megacryst genesis

In an effort to obtain information about mineral/melt trace element partitioning during the high pressure petrogenesis of basic rocks, we determined rare earth and other trace element abundances in megacrysts of clinopyroxene, orthopyroxene, amphibole, mica, anorthoclase, apatite and zircon, and in their host basalts. In general, the ranges of mineral/melt partition coefficients established from experimental partitioning studies and phenocryst/matrix measurements overlap with the ranges of megacryst/host abundance ratios. Our data for Hf, Sc, Ta and Th partitioning represent some of the only estimates available. Consideration of phase equilibria, major element partitioning and isotopic ratios indicate that most of the pyroxene and amphibole megacrysts may have been in equilibrium with their host magmas at high pressures (mostly 10–25 kb). In contrast, it is unlikely that mica, anorthoclase, apatite and zircon megacrysts formed in equilibrium with their host basalts; instead, we conclude that they were precipitated from more evolved magmas and have been mixed into their present host magmas. Consequently, the trace element abundance ratios for megacryst/host should not be interpreted as partition coefficients, but only as guides for understanding trace element partitioning during high pressure petrogenesis. With this caveat, we conclude that the megacryst/ host trace element abundance data indicate that mineral/melt partition coefficients in basaltic systems during high pressure fractionation are not drastically different from partition coefficients valid for low pressure fractionation.

R-process abundances near the mass 130 peak

view Abstract Citations (5) References (6) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS R-process abundances near the mass 130 peak de Laeter, J. R. ; Rosman, K. J. R. Abstract The Kaeppeler et al. (1982) reevaluation of r-process abundances in view of new neutron-capture cross section and elemental abundance data indicates that the height of the r-process peak near mass 130 is largely determined by the r-only process nuclides Te-128 and Te-130. It is presently noted that the work of Smith, De Laeter and Rosman (1977) has not been given due consideration in the assessment of the solar system's Te abundance. These data indicate a 25 percent reduction in the height of the mass 130 peak, in agreement with s-process systematics predictions. Publication: The Astrophysical Journal Pub Date: July 1983 DOI: 10.1086/161165 Bibcode: 1983ApJ...270..746D Keywords: Abundance; Nuclear Fusion; Solar System; Absorption Cross Sections; Tellurium Isotopes; Astrophysics full text sources ADS |

Trace element abundances in basalts of Nauru Basin

Leg 61 of the Deep Sea Drilling Project (DSDP) was concerned with drilling a single continuously cored multiple re-entry hole at site 462 in the Central Nauru Basin (Fig. 1). Preliminary results of this drilling, which penetrated more than 1 km beneath the sea floor, were presented earlier1,2. One major result was the discovery of a late Cretaceous off-ridge volcanic/intrusive complex of basaltic composition and great thickness (>500 m). We now present trace element abundance data for these basalts. Results of the drilling provide further support for a relatively long-lived thermal and magmatic event in the late Cretaceous resulting in voluminous and widespread magmatism in the central and western Pacific consistent with earlier suggestions3–5. The trace element data show that most of the rocks produced during this event have trace element characteristics intermediate between those of normal and transitional mid-ocean ridge basalts (N- and T-type MORB)6 and different from Hawaiian basalts7. These results indicate that basalts which are depleted in light rare earth elements (LREE) relative to the heavy REE may, in certain conditions, be erupted as voluminous intra-plate eruptions far from active ridge crests.

Implications of fission mass distributions for the astrophysical r-process

The role of nuclear fission in the astrophysical r-process is examined, and calculated fission fragment mass distributions are utilized to interpret observed elemental abundance data. A number of features of the solar system abundance curve are accounted for. An extension of the r-process to neutron numbers near 200 is required to provide a fission-source explanation of the rare-earth abundance peak. An r-process path that crosses the 184-neutron shell at A=276 (about 4 Z-units lower than presently accepted) is suggested to account for the rare-earth abundances and the high ratio of masses 136 to 148. The ''xenon anomaly'' is discussed within the general context of the treatment, and a superheavy fission precursor is shown to be unlikely. Correlations of abundance anomalies in Ap stars are pointed out, and the anomalies are classified according to cycle time.

Elemental abundance data for standard rocks as a measure of the quality of analytical methods

Elemental abundance data for ten standard rocks were used to assess the quality of the analytical methods used. Questions examined were: total number of elemental determinations performed, percentage of analyses performed by various techniques over a 25-year period, precision and accuracy of analytical methods, precision as a function of elemental concentrations, and improvements in methods over a 25-year period.

Computer-derived geochemical balances and element abundances

The classical geochemical equation or balance assumes that all sediments were ultimately derived from igneous rocks. Despite unproven assumptions and many uncertain data, the geochemical equation does provide a number of internal checks on elemental and isotopic abundance data. Utilizing high speed digital computers, a new approach to geochemical balancing has been developed. The input into the system consists of 1. (a) physical estimates (for example, volume) of six sedimentary and marine domains of the earth's crust; and 2. (b) minimum and maximum geochemical abundance data (derived from forty-four published lists) for sixtyfive elements in eight lithologic categories. Through the use of an optimization-iterative technique, the system generates as output 1. (a) total mass of weathered igneous rock, 2. (b) mass distribution of the elements within the geologic and marine domains, 3. (c) a list of those elements that could not be balanced and 4. (d) elemental abundance tables derived automatically from the optimized model. Although some aspects of the model are artificial, the computations do lead to an internally consistent estimate of abundances in the crust and major sedimentary rock types. The computations also indicate that 2 × 10 18tons of igneous rock have weathered to form the sedimentary rocks of the earth's crust and ions dissolved in the ocean. The following elements cannot be brought into balance, probably due to important volcanic contributions and/or inadequate data: boron, sulfur, chlorine, manganese, arsenic, selenium, bromine, molybdenum, iodine and lead. The input into the system is left open for future revision.