Isotopic composition of strontium in volcanic rocks from oahu

Analysis of several well-documented specimens from each of the three volcanic series on Oahu gives the following mean ratios of Sr(87) to Sr(86): the Waianae series, 0.7030 +/- 0.00010 (sigma); the Koolau series, 0.70385+/- 0.00009 (sigma); and the Honolulu series, 0.7029 ++/- 0.00006 ( sigma). The mean ratio of Sr(87) to Sr(86) of the Koolau series specimens is significantly higher than the means of the other two series. With one exception, significant differences in Sr(87)/ Sr(86) within a series were not found, even though some large compositional differences existed.     

Abundance of alkali metals, alkaline and rare earths, and strontium-87/ strontium-86 ratios in lunar samples

The variation in trace element abundance patterns indicates that lunar igneous rocks are the product of extensive igneous fractionation. Variations in the Sr(87)/ Sr(86) ratio indicate that these rocks crystallized 3.5+/-0.3 x 10(9) years ago.     

Rubidium-strontium age and elemental and isotopic abundances of some trace elements in lunar samples

Data on six lunar crystalline rocks give an apparent Rb-Sr isochron age of 4.42 +/- 0.24 x 10(9) years (95 percent confidence limits) and initial (87)Sr/(86)Sr ratio similar to that in a basaltic achondrite. Relationships between K, Rb, Sr, and Ba and depletion of Eu in these samples point to plagioclase separation from the melts that produced these rocks. The abundance of (157)Gd in the three lunar samples is similar to terrestrial abundance within < 0.2 percent, thus setting a limit of < 6 x 10(15) neutrons per square centimeter for the integrated thermal neutron flux difference between lunar and terrestrial materials.     

Chondrites: initial strontium-87/strontium-86 ratios and the early history of the solar system

A sodium-poor, calcium-rich inclusion in the carbonaceous chondrite Allende had a (87)Sr/(86)Sr ratio at the time of its formation of 0.69880, as low a value as that found in any other meteorite. The higher (87)Sr/(86)Sr ratios found in ordinary chondrites indicate that their formation or isotopic equilibration occurred tens of millions of years later.     

Strontium isotopic composition of mid-cretaceous seawater

The (87)Sr/(86)Sr ratio in fish teeth separated from mid-Cretaceous marl and black shale from the northeastern Apennines and Venetian Alps (Italy) define three periods of low (87)Sr/(86)Sr ratio at 121 to 124 million years ago (Ma), 110 to 115 Ma, and 89 to 91 Ma. The (87)Sr/(86)Sr excursions correspond to oceanic anoxic events represented by the Livello Selli, Livello 113, Livello Urbino, and Livello Bonarelli black shale marker beds and probably reflect an increase in the low-(87)Sr/(86)Sr hydrothermal strontium flux associated with the emplacement of the Ontong-Java and Kerguelen plateaus (120 to 110 Ma) and the Caribbean Plateau (89 to 91 Ma). The modeled flux is consistent with the volumes and eruption rates of the oceanic plateaus but is far smaller than expected from the proposed Cretaceous crustal production rates of 50 to 100 percent greater than modern.     

Seawater strontium isotopic variations from 2.5 million years ago to the present

Measurements of marine carbonate samples indicate that during the past 2.5 million years the (87)Sr/(86)Sr ratio of seawater has increased by 14 x 10(-5). The high average rate of increase of (87)Sr/(86)Sr indicates that continental weathering rates were exceptionally high. Nonuniformity in the rate of increase suggests that weathering rates fluctuated by as much as +/-30 percent of present-day values. Some of the observed shifts in weathering rates are contemporaneous with climatic changes inferred from records of oxygen isotopes and carbonate preservation in deep sea sediments.     

Sources of continental crust: neodymium isotope evidence from the sierra nevada and peninsular ranges

Granitic rocks from batholiths of the Sierra Nevada and Peninsular Ranges exhibit initial (143)Nd/(144)Nd ratios that vary over a large range and correlate with (87)Sr/(86)Sr ratios. The data suggest that the batholiths represent mixtures of materials derived from (i) chemically depleted mantle identical to the source of island arcs and (ii) old continental crust, probably sediments or metasediments with a provenance age of approximately 1.6 x 10(9) years. These conclusions are consistent with a model for continental growth whereby new crustal additions are repeatedly extracted from the same limited volume of the upper mantle, which has consequently become depleted in elements that are enriched in the crust. There is little evidence that hydrothermally altered, subducted oceanic crust is a primary source of the magmas.     

Strontium isotopes in deep-sea sediments

Recent deep-sea sediments, leached of carbonate, have Sr(87) Sr(86) ratios ranging from 0.7044 to 0.7394. Strontium in the detrital sediment has not equilibrated isotopically with seawater strontium. Amounts of strontium-bearing authigenic material in the clay-mineral assemblage are not great enough to mask strontium-isotope ratios inherited from source areas.     

Orphan strontium-87 in abyssal peridotites: daddy was a granite

The (87)Sr/(86)Sr ratios in some bulk abyssal and alpine peridotites are too high to be binary mixtures of depleted mantle and seawater components. The apparent excess, or "orphan," (87)Sr appears to be separated from its radioactive parent. Such observations were widely held to be analytical artifacts. Study of several occurrences of orphan (87)Sr shows that the orphan component in abyssal peridotite is located in the alteration products of olivine and enstatite in the peridotite. The orphan (87)Sr is most likely introduced by infiltration of low-temperature (<200 degrees C) seawater bearing suspended detrital particulates. These particulates include grains of detrital clay that are partly derived from continental (that is, granitic) sources and thus are highly radiogenic. Orphan (87)Sr and other radiogenic isotopes may provide a tracer for low-temperature seawater penetrating into the oceanic crust.     

Strontium isotopic composition of estuarine sediments as paleosalinity-paleoclimate indicator

The strontium isotopic composition of biogenic precipitates that occur in estuarine sediments can be used as proxy indicator of paleosalinity and for assessing precipitation and river discharge rates over thousands of years. In the San Francisco Bay estuary, river water with low (87)Sr/(86)Sr ratio (average, 0.7065) and low Sr concentration (0.13 parts per million) mixes with seawater with a higher (87)Sr/(86)Sr ratio (0.7092) and Sr concentration (7.9 parts per million). The predicted mixing relation between salinity and Sr isotopic composition is confirmed by measurements of modern estuarine surface waters. A paleosalinity record obtained from foraminifera for the ancestral San Francisco Bay during oxygen isotope substage 5e of the last interglacial reflects a global rise and fall of sea level, and short time-scale variations related to fluctuations in discharge rates of the Sacramento and San Joaquin rivers.     

Sphene: uranium-lead ages

Uranium-lead ages were measured on 14 samples of sphene from rocks aged from 1000 to 2750 x 10(6) years. All samples gave concordant or nearly concordant ages, the maximum difference between the Pb(206)-U(238) and Pb(207)-Pb(206) ages being 10 percent. Sphene has more concordant ages than has the coexisting zircon in each of seven rocks in which they were compared. Sphene sometimes has greater ages than does coexisting biotite, although in two metamorphic rocks, in which metamorphism was sufficiently intense to cause redistribution of radiogenic strontium-87 between various mineral phases, sphene dates the time of metamorphism rather than of original crystallization of the rocks.     

High-resolution stratigraphy with strontium isotopes

The isotopic ratio of strontium-87 to strontium-86 shows no detectable variation in present-day ocean water but changes slowly over millions of years. The strontium contained in carbonate shells of marine organisms records the ratio of strontium-87 to strontium-86 of the oceans at the time that the shells form. Sedimentary rocks composed of accumulated fossil carbonate shells can be dated and correlated with the use of high precision measurements of the ratio of strontium-87 to strontium-86 with a resolution that is similar to that of other techniques used in age correlation. This method may prove valuable for many geological, paleontological, paleooceanographic, and geochemical problems.     

Diffusive isotopic contamination of mafic magma by coexisting silicic liquid in the muskox intrusion

Shifts in (87)Sr/(86)Sr and (143)Nd/(144)Nd ratios measured in cumulates from the upper levels of the Muskox mafic intrusion indicate that isotopic and bulk chemical exchange were decoupled across a mafic-silicic liquid interface during crystallization of the intrusion. Modeling of diffusive exchange between liquid layers demonstrates that isotopic compositions of silicate liquids in layered magma chambers may be strongly affected by this process on time scales of 10(3) to 10(4) years. Diffusive contamination can be used to place constraints on the physical processes and time scales of magmatic systems.     

Gas bubbles in fossil amber as possible indicators of the major gas composition of ancient air

Gases trapped in Miocene to Upper Cretaceous amber were released by gently crushing the amber under vacuum and were analyzed by quadrupole mass spectrometry. After discounting the possibility that the major gases N(2), O(2), and CO(2) underwent appreciable diffusion and diagenetic exchange with their surroundings or reaction with the amber, it has been concluded that in primary bubbles (gas released during initial breakage) these gases represent mainly original ancient air modified by the aerobic respiration of microorganisms. Values of N(2)/(CO(2) + O(2)) for each time period give consistent results despite varying O(2)/CO(2) ratios that presumably were due to varying degrees of respiration. This allows calculation of original oxygen concentrations, which, on the basis of these preliminary results, appear to have changed from greater than 30 percent O(2) during one part of the Late Cretaceous (between 75 and 95 million years ago) to 21 percent during the Eocene-Oligocene and for present-day samples, with possibly lower values during the Oligocene-Early Miocene. Variable O(2) levels over time in general confirm theoretical isotope-mass balance calculations and suggest that the atmosphere has evolved over Phanerozoic time.     

Large groundwater strontium flux to the oceans from the Bengal Basin and the marine strontium isotope record

Strontium concentration and isotopic data for subsurface flowing groundwaters of the Ganges-Brahmaputra (G-B) delta in the Bengal Basin demonstrate that this is a potentially significant source of strontium to the oceans, equal in magnitude to the dissolved strontium concentration carried to the oceans by the G-B river waters. The strontium concentrations of groundwaters are higher by a factor of about 10 than typical G-B river waters and they have similar 87Sr/86Sr ratio to the river waters. These new data suggest that the present contribution of the G-B system to the rise in 87Sr/86Sr ratio in seawater is higher by at least a factor of 2 to 5 than the average over the past 40 million years.     

Age determinations and isotopic abundance measurements on lunar samples

A K-Ar age of 2300 x 10(6) years has been determined for a sample of type A crystalline rock (57,34). The presence of an anomalously large quantity of 4 degrees Ar, in a sample of type C breccia (65,35) precluded the calculation of its K-Ar age. Both of the rock types are characterized by low Rb/Sr ratios and consequently low (87)Sr/(86)Sr values. The U-Th-Pb results for a sample of type D fines (84,33) yield a (207)Pb/(206)Pb age of 4760 x 10(6) years, but ages based on U-Pb and Th-Pb ratios are anomalously high. Isotopic compositions of Li, K, Rb, Sr, U, and Th are very close to the accepted values for terrestrial materials.     

The apollo 16 lunar samples: petrographic and chemical description

The preliminary characterization of the rocks and soils returned from the Apollo 16 site has substantiated the inference that the lunar terra are commonly underlain by plagioclase-rich or anorthositic rocks. No evidence has been found for volcanic rocks underlying the regolith in the Apollo 16 region. In their place, we have found anorthositic rocks that are thoroughly modified by crushing and partial melting. The textural and chemical variations in these rocks provide some evidence for the existence of anorthositic complexes that have differentiated on a scale of tens to hundreds of meters. The occurrence of deep-seated or plutonic rocks in place of volcanic or pyroclastic materials at this site suggests that the inference from physiographic evidence that the latter materials are widespread in terra regions may be incorrect. Several additional, more specific conclusions derived from this preliminary examination are: 1) The combination of data from the Descartes region with data from the orbital x-ray fluorescence experiment indicates that some backside, highland regions are underlain by materials that consist of more than 80 percent plagioclase. 2) The soil or upper regolith between North Ray and South Ray has not been completely homogenized since the time of formation of these craters. 3) The chemistry of the soil indicates that rocks rich in potassium, uranium, and thorium, similar to those that prevail at the Fra Mauro site, are relatively abundant (10 to 20 percent) in the Descartes region. 4) The K/U ratio of the lunar crust is similar to that of the KREEP basalts. 5) The carbon content of the premare lunar crust is even lower than that of the mare volcanic rocks.     

High-3He Plume Origin and Temporal-Spatial Evolution of the Siberian Flood Basalts

An olivine nephelinite from the lower part of a thick alkalic ultrabasic and mafic sequence of volcanic rocks of the northeastern part of the Siberian flood basalt province (SFBP) yielded a (40)Ar/(39)Ar plateau age of 253.3 +/- 2.6 million years, distinctly older than the main tholeiitic pulse of the SFBP at 250.0 million years. Olivine phenocrysts of this rock showed (3)He/(4)He ratios up to 12.7 times the atmospheric ratio; these values suggest a lower mantle plume origin. The neodymium and strontium isotopes, rare earth element concentration patterns, and cerium/lead ratios of the associated rocks were also consistent with their derivation from a near-chondritic, primitive plume. Geochemical data from the 250-million-year-old volcanic rocks higher up in the sequence indicate interaction of this high-(3)He SFBP plume with a suboceanic-type upper mantle beneath Siberia.     

Ages, irradiation history, and chemical composition of lunar rocks from the sea of tranquillity

The (87)Rb-(87)Sr internal isochrons for five rocks yield an age of 3.65 +/-0.05 x 10(9) years which presumably dates the formation of the Sea of Tranquillity. Potassium-argon ages are consistent with this result. The soil has a model age of 4.5 x10(9) years, which is best regarded as the time of initial differentiation of the lunar crust. A peculiar rock fragment from the soil gave a model age of 4.44 x 10(9) years. Relative abundances of alkalis do not suggest differential volatilization. The irradiation history of lunar rocks is inferred from isotopic measurements of gadolinium, vanadium, and cosmogenic rare gases. Spallation xenon spectra exhibit a high and variable (131)Xe/(126)Xe ratio. No evidence for (129)I was found. The isotopic composition of solar-wind xenon is distinct from that of the atmosphere and of the average for carbonaceous chondrites, but the krypton composition appears similar to average carbonaceous chondrite krypton.     

Electron-microprobe analyses of phases in lunar samples

In fine (type A) and coarse-grained (type B) Apollo 11 lunar volcanic rocks clinopyroxenes are extremely inhomogeneous. Ferrosilite-rich areas in type B rocks have decomposed to submicron vermicular intergrowths of clinopyroxene-fayalite-cristobalite(?). Plagioclase has normal zoning with K(2)O up to 0.5 percent in rims. Ilmenites are relatively homogeneous with low mgo(0.1 to 2 percent) and high zro(2) (up to 0.26 percent). Metal phase in troilite has <0.02 percent nickel. The breccias (type C) and fines (type D) containing 0.09 to 10.52 percent Ti0(2.) Rare metal fragments with meteorite-like compositions occur in breccias and fines. Gross similarities between euctites and Apollo 11 volcanic rocks indiacate similar evolutionary environments, but detailed mineralogical differences suggest either separate origins or if eucrites are lunar, chemical inhomogeneities on the lunar surface.