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.     

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.     

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.     

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.     

Early and Late Alkali Igneous Pulses and a High-3He Plume Origin for the Deccan Flood Basalts

Several alkalic igneous complexes of nephelinite-carbonatite affinities occur in extensional zones around a region of high heat flow and positive gravity anomaly within the continental flood basalt (CFB) province of Deccan, India. Biotites from two of the complexes yield (40)Ar/(39)Ar dates of 68.53 +/- 0.16 and 68.57 +/- 0.08 million years. Biotite from a third complex, which intrudes the flood basalts, yields an (40)Ar/(39)Ar date of 64.96 +/- 0.1 1 million years. The complexes thus represent early and late magmatism with respect to the main pulse of CFB volcanism 65 million years ago. Rocks from the older complexes show a (3)He/(4)He ratio of 14.0 times the air ratio, an initial (87)Sr/(86)Sr ratio of 0.70483, and other geochemical characteristics similar to ocean island basalts; the later alkalic pulse shows isotopic evidence of crustal contamination. The data document 3.5 million years of incubation of a primitive, high-(3)He mantle plume before the rapid eruption of the Deccan CFB.     

Mantle plumes and entrainment: isotopic evidence

Many oceanic island basalts show sublinear subparallel arrays in Sr-Nd-Pb isotopic space. The depleted upper mantle is rarely a mixing end-member of these arrays, as would be expected if mantle plumes originated at a 670-kilometer boundary layer and entrained upper mantle during ascent. Instead, the arrays are fan-shaped and appear to converge on a volume in isotopic space characterized by low (87)Sr/(86)Sr and high (143)Nd/(144)Nd, (206)Pb/(204)Pb, and (3)He/(4)He ratios. This new isotopic component may be the lower mantle, entrained into plumes originating from the core-mantle boundary layer.     

Radiogenic Strontium-87 as an Index of Geologic Processes

The abundance of radiogenic Sr(87) relative to Sr(86) at the time of crystallization has been determined for 45 rocks. The total range in the ratio Sr(87)/Sr(86) is less than 2 percent. Ratios for recent lavas range from 0.702 to 0.711. Oceanic basalts are closely grouped at 0.703, whereas ratios for continental volcanic rocks spread from 0.702 to 0.711. Among the volcanic rocks, ranging from basalt to rhyolite, no correlation was found between original ratio and rock type. Older mafic and felsic rocks that include both plutonic and extrusive types also cover this same range in original Sr(87)/Sr(86) ratios; however, there is a definite trend with geologic time. Precambrian rocks give values as low as 0.700. The data indicate that Sr(87)/Sr(86) of the weathering crust has changed 1.1 percent in 3000 million years, while the ratio in the mantle has changed no more than 0.5 percent.     

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.     

Time differences in the formation of meteorites as determined from the ratio of lead-207 to lead-206

Measurements of the lead isotopic composition and the uranium, thorium, and lead concentrations in meteorites were made in order to obtain more precise radiometric ages of these members of the solar system. The newly determined value of the lead isotopic composition of Canyon Diablo troilite is as follows: (206)Pb/(204)Pb = 9.307, (207)Pb/(2O4)Pb = 10.294, and (208)Pb/(204)Pb = 29.476. The leads of Angra dos Reis, Sioux County, and Nuevo Laredo achondrites are very radiogenic, the (206)Pb/(204)Pb values are about 200, and the uranium-thorium-lead systems are nearly concordant. The ages of the meteorites as calculated from a single-stage (207)Pb/(206)Pb isochron based on the newly determined primordial lead value and the newly reported (235)U and (838)U decay constants, are 4.528 x 10(9) years for Sioux County and Nuevo Laredo and 4.555 x 10(9) years for Angra dos Reis. When calculated with the uranium decay constants used by Patterson, these ages are 4.593 x 10(9) years and 4.620 x 10(9) years, respectively, and are therefore 40 to 70 x 10(6) years older than the 4.55 x 10(9) years age Patterson reported. The age difference of 27 x 10(6) years between Angra dos Reis and the other two meteorites is compatible with the difference between the initial (87)Sr/(86)Sr ratio of Angra dos Reis and that of seven basaltic achondrites observed by Papanastassiou and Wasserburg. The time difference is also comparable to that determined by (129)1-(129)Xe chronology. The ages of ordinary chondrites (H5 and L6) range from 4.52 to 4.57 x 10(9) years, and, here too, time differences in the formation of the parent bodies or later metamorphic events are indicated. Carbonaceous chondrites(C2 and C3) appear to contain younger lead components.     

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.     

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.     

Evolution of the ratio of strontium-87 to strontium-86 in seawater from cretaceous to present

A detailed record of the strontium-87 to strontium-86 ratio in seawater during the last 100 million years was determined by measuring this ratio in 137 well-preserved and well-dated fossil foraminifera samples. Sample preservation was evaluated from scanning electron microscopy studies, measured strontium-calcium ratios, and pore water strontium isotope ratios. The evolution of the strontium isotopic ratio in seawater offers a means to evaluate long-term changes in the global strontium isotope mass balance. Results show that the marine strontium isotope composition can be used for correlating and dating well-preserved authigenic marine sediments throughout much of the Cenozoic to a precision of +/-1 million years. The strontium-87 to strontium-86 ratio in seawater increased sharply across the Cretaceous/Tertiary boundary, but this feature is not readily explained as strontium input from a bolide impact on land.     

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.     

Himalayan tectonics, weathering processes, and the strontium isotope record in marine limestones

The time evolution of the isotopic composition of seawater strontium (the ratio of strontium-87 to strontium-86) over the last 500 million years has the form of an asymmetric trough. The values are highest in the Cambrian and Recent (0.7091) and lowest in the Jurassic (0.7067). Superimposed on this trend are a number of smaller oscillations. Consideration of the geochemical cycle of strontium and the dynamics of weathering shows that only Himalayan-style continental collisions can influence the isotope ratio on the scale observed. The contemporary Himalayan orogeny is by far the largest since the late Precambrian Pan-African event that produced the high in the Cambrian.     

Fault zone connectivity: slip rates on faults in the san francisco bay area, california

The slip rate of a fault segment is related to the length of the fault zone of which it is part. In turn, the slip rate of a fault zone is related to its connectivity with adjoining or contiguous fault zones. The observed variation in slip rate on fault segments in the San Francisco Bay area in California is consistent with connectivity between the Hayward, Calaveras, and San Andreas fault zones. Slip rates on the southern Hayward fault taper northward from a maximum of more than 10 millimeters per year and are sensitive to the active length of the Maacama fault.     

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.     

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.     

Butler, missouri: an iron meteorite with extremely high germanium content

The Butler iron meteorite has been found to have a germanium concentration of 2000 parts per million, which is about five times higher than the highest concentration that has been measured previously in an iron meteorite. The gallium concentration is 87 parts per million, which is among the highest concentrations found in these objects. The nickel content is 16.0 percent, the second highest nickel concentration known in a meteorite displaying a Widman st?tten pattern. The high Ge/Ni ratio, as well as the association of a high nickel content with high gallium and germanium contents, make this object an exception to two geochemical generalizations regarding the iron meteorites.     

Rhenium-osmium isotope systematics of carbonaceous chondrites

Rhenium and osmium concentrations and Os isotopic compositions of eight carbonaceous chondrites, one LL3 ordinary chondrite, and two iron meteorites were determined by resonance ionization mass spectrometry. Iron meteorite (187)Re/(186)Os and (l87)Os/(l86)Os ratios plot on the previously determined iron meteorite isochron, but most chondrite data plot 1 to 2 percent above this meteorite isochron. This suggests either that irons have significantly younger Re-Os closure ages than chondrites or that chondrites were formed from precursor materials with different chemical histories from the precursors of irons. Some samples of Semarkona (LL3) and Murray (C2M) meteorites plot 4 to 6 percent above the iron meteorite isochron, well above the field delineated by other chondrites. Murray may have lost Re by aqueous leaching during its preterrestrial history. Semarkona could have experienced a similar loss of Re, but only slight aqueous alteration is evident in the meteorite. Therefore, the isotopic composition of Semarkona could reflect assembly of isotopically heterogeneous components subsequent to 4.55 billion years ago or Os isotopic heterogeneities in the primordial solar nebula.     

Isotopic abundances of actinide elements in lunar material

The abunldanzces of uranium and thorium were measured mass spectrometrically as 0.59 +/- 0.02 and 2.24 +/- 0.06 parts per million, respectively, in the fines of Apollo 11 builk sample. The ratio (235)U/(238)U was 0.007258 +/- 0.000016, in agreement with terrestrial uranium. The following upper limits were set: (236)U/(238)U相似文献