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The 19th-century scientific community grappled at length with the question of the age of the Earth, a subject for which a definitive answer did not arrive until the refinement of […]

A procedure was developed that allows precise determination of Fe isotopic composition. Purification of Fe was achieved by ion chromatography on AG1-X8 strongly basic anion-exchange resin. No isotopic fractionation is associated with column chemistry within 0.02 [per thousand]/amu at 2[sigma]. The isotopic composition was measured with a Micromass IsoProbe multicollection inductively coupled plasma hexapole mass spectrometer. The Fe isotopic composition of the Orgueil CI1 carbonaceous chondrite, which best approximates the solar composition, is indistinguishible from that of IRMM-014 (-0.005 [+ or -] 0.017 [per thousand]/amu). The IRMM-014 reference material is therefore used for normalization of the isotopic ratios. The protocol for analyzing mass-dependent variations is validated by measuring geostandards (IF-G, DTS-2, BCR-2, AGV-2) and heavily fractionated Fe left after vacuum evaporation of molten wustite (FeO) and solar (MgO-[Al.sub.2][O.sub.3]-Si[O.sub.2]-CaO-FeO in chondritic proportions) compositions. It is shown that the isotopic composition of ICe during evaporation of FeO follows a Rayleigh distillation with a fractionation factor [alpha] equal to [([m.sub.1]/[m.sub.2]).sup.1/2], where [m.sub.1] and [m.sub.2] are the masses of the considered isotopes. This agrees with earlier measurements and theoretical expectations. The isotopic composition of Fe left after vacuum evaporation of solar composition also follows a Rayleigh distillation but with a fractionation factor (1.013 22 [+ or -] 0.000 67 for the [sup.56]Fe/[sup.54]Fe ratio) that is lower than the square root of the masses (1.018 35). The protocol for analyzing mass-independent variations is validated by measuring terrestrial rocks that are not expected to show departure from mass-dependent fractionation. After internal normalization of the [sup.57]Fe/[sup.54]Fe ratio, the isotopic composition of Fe can be measured accurately with precisions of 0.2[epsilon] and 0.5[epsilon] at 2[sigma] for [sup.56]Fe/[sup.54]Fe and [sup.58]Fe/[sup.54]Fe ratios, respectively ([epsilon] refers to relative variations in parts per 10 000). For [sup.58]Fe, this precision is an order of magnitude better than what had been achieved before. The method is applied to rocks that could potentially exhibit mass-independent effects, meteorites and Archaean terrestrial samples. The isotopic composition of a 3.8-Ga-old banded iron formation from Isua (IF-G, Greenland), and quartz--pyroxene rocks from Akilia and Innersuartuut (GR91-26 and SM/GR/171770, Greenland) are normal within uncertainties. Similarly, the Orgueil (CI1), Allende (CV3.2), Eagle Station (ESPAL), Brenham (MGPAL), and Old Woman (IIAB) meteorites do not show any mass-independent effect.

The [sup.87.Sr]/[sup.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 [sup.87.Sr]/[sup.86.Sr] ratio at 121 to 124 million years ago (Ma), 110 to 115 Ma, and 89 to 91 Ma. The [sup.87.Sr]/[sup.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-[sup.87S][Sr/.sup.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., The strontium isotopic composition of seawater exhibits both large and small fluctuations throughout the Phanerozoic; these are attributed to changes in the riverine Sr flux controlled by orogenic uplifting and [...]

B. Lynn Ingram et al. (1) model a general decrease in mid-Cretaceous strontium (Sr) isotope ratios that occurred as a result of an increase in oceanic crustal production. Their model [...]

Research conducted on a multicomponent silicate melt in the CaO-Al2O3-SiO2 system reveals that diffusive coupling is needed for convection promotion. The density-affecting components show a possibility of coupling by off-diagonal terms in the diffusion matrix. These studies also indicate that convection occurs regardless of the proportion between density and depth.

It was recently shown that relatively large (compared to analytical precision) steady state thermal isotope fractionations are produced in silicate melts whenever temperature differences are maintained for a sufficiently long [...]