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Oxygen, magnesium, and silicon isotopic abundances in Vigarano 1623‐5 were studied using secondary ion mass spectrometry to investigate correlations between isotopic and petrologic properties of this unique forsterite‐bearing FUN inclusion. Vigarano 1623‐5 displays large, correlated mass‐dependent fractionation effects, tightly linked to mineralogy within distinct petrologic units of the inclusion. The pyroxene‐rich and melilite‐rich interior parts of the inclusion display highly mass‐fractionated isotopic compositions of oxygen, magnesium, and silicon, consistent with Rayleigh distillation during evaporation of a melt with initial oxygen composition close to a solar composition. However, the chemical composition, enriched in magnesium and silicon, suggests a precursor already fractionated by prior melt evaporation. A discontinuous igneous rim was produced by a flash‐melting event followed by isotopic exchange in the rim melilite with planetary‐like oxygen, mechanical fragmentation, and reassembly with an accretionary rim of heterogeneous materials. Al‐rich minerals in 1623‐5 show evidence for having crystallized with live 26Al but at less than the "canonical" level of most CV calcium‐aluminum‐rich inclusions. However, well‐defined 26Al‐26Mg isochrons are not found and temporal implications are ambiguous. [ABSTRACT FROM AUTHOR]

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.

Magnesium isotope ratios are known to vary in solar system objects due to the effects of 26 Al decay to 26 Mg and mass-dependent fractionation, but anomalies of nucleosynthetic origin must also be considered. In order to infer the amount of enhancement of 26 Mg/ 24 Mg due to 26 Al decay or to resolve small nucleogenetic anomalies, the exact relationship between 26 Mg/ 24 Mg and 25 Mg/ 24 Mg ratios due to mass-dependent fractionation, the mass-fractionation “law”, must be accurately known so that the 25 Mg/ 24 Mg ratio can be used to correct the 26 Mg/ 24 Mg ratio for mass fractionation. Mass-dependent fractionation in mass spectrometers is reasonably well characterized, but not necessarily fully understood. It follows a simple power fractionation law, sometimes referred to as the “exponential law”. In contrast, mass fractionation in nature, in particular that due to high temperature evaporation that likely caused the relatively large effects observed in calcium-, aluminum-rich inclusions (CAIs), is reasonably well understood, but mass-fractionation laws for magnesium have not been explored in detail. The magnesium isotopic compositions of CAI-like evaporation residues produced in a vacuum furnace indicate that the slope on a log 25 Mg/ 24 Mg vs. log 26 Mg/ 24 Mg plot is ∼0.5128, and different from those predicted by any of the commonly used mass-fractionation laws. Evaporation experiments on forsterite-rich bulk compositions give exactly the same slope, indicating that the measured mass-fractionation law for evaporation of magnesium is applicable to a wide range of bulk compositions. We discuss mass-fractionation laws and the implications of the measured fractionation behavior of magnesium isotopes for 26 Al– 26 Mg chronology. [ABSTRACT FROM AUTHOR]

Abstract: Laboratory experiments were used to determine the thermal (Soret) isotopic fractionation of lithium and potassium in a basalt melt, which adds elements with ionic charge +1 to the list of elements for which thermal isotopic fractionations in silicate liquids have been previously reported (i.e., Ca, Mg, Fe, Si, O, Sr, Hf, and U). The new experiments were run at a moderate pressure of about 1.5GPa in a piston cylinder apparatus in order to avoid gas bubbles once the sample was melted. The samples were displaced slightly below the hot spot of the piston cylinder assembly graphite furnace so that there would be a temperature difference of about 125°C across the samples while molten. The thermal isotopic fractionation factor Ω (per mil fractionation per 100°C per one atomic mass unit difference) was found to be 6.0 for lithium isotopes and 1.1 for potassium isotopes. The isotopic fractionation in both cases resulted in the heavy isotopes becoming enriched at the cold end. The expanded data set of thermal isotopic fractionation in silicate liquids is used to evaluate the degree to which recently proposed parameterizations are able to reproduce the experimental data. [Copyright &y& Elsevier]

FUN (Fractionation and Unidentified Nuclear) calcium-, aluminum-rich inclusions (CAIs) have large mass-dependent fractionations of silicon, magnesium, and oxygen isotopes (up to δ29Si ∼15‰ and δ25Mg ∼40‰), and mass-independent isotopic anomalies in many elements. To test the proposition that the mass-fractionation effects of all three isotopic systems in FUN CAIs were the result of evaporation of at least partially molten precursors, we conducted a series of experiments in which two magnesium- and silicon-rich melts (FUN1 with 53.4wt% MgO and 41.3% SiO2, and FUN2 with 32.7% MgO and 38.7% SiO2, and Al2O3 and CaO in solar proportions) were evaporated into vacuum at 1900°C for various lengths of time. The chemical and isotopic compositions of the evaporation residues were measured and compared to two of the most highly mass-fractionated FUN CAIs, Vigarano 1623-5 and Allende C1. The isotopic composition of the evaporation residues was also used to determine the kinetic isotopic fractionation factors α 25,24 =0.98372±0.00041 for 25Mg/24Mg and α 17,16 =0.9883±0.0006 for 17O/16O for residues containing >15wt% MgO, and α 25,24 =0.98567±0.00046 and α 17,16 ∼0.994 for residues containing <15wt% MgO. The 29Si/28Si fractionation factor α 29,28 =0.9899±0.0004 was found to fit the data from the entire set of residues. Simple linear correlations were found for δ29Si, δ25Mg, and δ17O as a function of the fraction of magnesium or silicon remaining in the residues. The fact that the isotopic fractionations of magnesium, silicon and oxygen of C1 are in the same proportions as in the experimental evaporation residues suggests that the evaporation played a major role in the chemical evolution of this FUN inclusion. In the case of Vigarano 1623-5, the magnesium and oxygen isotopic fractionations are consistent with the experimental data, but fractionation of silicon isotopes relative to that of magnesium in 1623-5 is about a third less than in the experimental residues. Assuming that Allende C1 and Vigarano 1623-5 are evaporation residues that were produced in much the same way as our experimental residues (i.e., evaporation of completely molten droplets), the chemical compositions of their precursors were calculated using bulk chemical and isotopic compositions of C1 and 1623-5 together with the experimentally determined kinetic fractionation factors α 25,24 and α 29,28. It was found that the present chemical and isotopic compositions of C1 can be explained by evaporation of a precursor with a bulk composition close to that of a condensate from a solar composition gas. In the case of Vigarano 1623-5, however, the calculated precursor is significantly enriched in magnesium and depleted in silicon compared to plausible condensates from a solar composition gas. Among the possible reasons for such misfit could be uncertainties in bulk chemical and isotopic compositions measured in Vigarano 1623-5, or evaporation at lower temperatures from partially rather than completely molten precursors which could have different evaporation kinetics and isotopic fractionation factors. [ABSTRACT FROM AUTHOR]

Magnesium and silicon isotopic profiles across melilite grains in two type B1 and two type B2 calcium-aluminum-rich inclusions (CAIs) reveal differing but constant enrichments in heavy isotopes everywhere except ≤1000 μm from the CAI margins. There is no close correlation in the B1s or the B2s between isotopic composition and åkermanite content of the melilite, a measure of progressive igneous crystallization, yet such a correlation might be expected in a type B2: without a melilite mantle (as in B1s) to seal the interior off and prevent further evaporation, the melt would have maintained communication with the external gas. These observations indicate a model in which B1s and B2s solidified under differing conditions. The B2s solidified under lower hydrogen pressures ( [ABSTRACT FROM AUTHOR]

Abstract: Laboratory experiments are used to document isotopic fractionation of magnesium by chemical diffusion in a silicate melt and the results compared to the magnesium isotopic composition across contacts between igneous rocks of different composition in natural settings. The natural samples are from transects from felsic to mafic rocks at Vinal Cove in the Vinalhaven Intrusive Complex, Maine and from the Aztec Wash pluton in Nevada. Two laboratory diffusion couples made by juxtaposing melts made from powders of the felsic and mafic compositions sampled at Vinal Cove were annealed at about 1500°C for 22.5 and 10h, respectively. The transport of magnesium in the diffusion couples resulted in easily measured magnesium isotopic fractionations at the interface (δ 26Mg∼1.5‰). These isotopic fractionations provide a distinctive isotopic “fingerprint” that we use to determine whether chemical gradients in natural settings where melts of different composition were juxtaposed were due to chemical diffusion. The magnesium isotopic fractionation along one profile at Vinal Cove is exactly what one would expect based on the fractionations found in the laboratory experiments. This is an important result in that it shows that the isotope fractionation by chemical diffusion found in highly controlled laboratory experiments can be found in a natural setting. This correspondence implies that chemical diffusion was the dominant process responsible for the transport of magnesium across this particular contact at Vinal Cove. A second Vinal Cove profile has a very similar gradient in magnesium concentration but with significantly less magnesium isotopic fractionation than expected. This suggests that mass transport at this location was only partly by diffusion and that some other mass transport mechanism such as mechanical mixing must have also played a role. The magnesium isotopic composition of samples from Aztec Wash shows no resolvable isotopic fractionation across the contact between the mafic and felsic rocks. The different degrees of magnesium isotopic fractionation associated with otherwise similar composition gradients in natural settings show that kinetic isotope fractionations provide a key discriminator for establishing whether or not molecular diffusion was the process responsible for an observed elemental gradient. In the one case of a contact at Vinal Cove where we are confident that the magnesium elemental and isotopic gradients were produced by diffusion, we deduced a cooling rate of about 1.5°C per day. [Copyright &y& Elsevier]

- Evaporation rates of K2O, Na2O, and FeO from chondrule-like liquids and the associated potassium isotopic fractionation of the evaporation residues were measured to help understand the processes and conditions that affected the chemical and isotopic compositions of olivine-rich type IA and type IIA chondrules from Semarkona. Both types of chondrules show evidence of having been significantly or totally molten. However, these chondrules do not have large or systematic potassium isotopic fractionation of the sort found in the laboratory evaporation experiments. The experimental results reported here provide new data regarding the evaporation kinetics of sodium and potassium from a chondrule-like melt and the potassium isotopic fractionation of evaporation residues run under various conditions ranging from high vacuum to pressures of one bar of H2+CO2, or H2, or helium. The lack of systematic isotopic fractionation of potassium in the type IIA and type IA chondrules compared with what is found in the vacuum and one-bar evaporation residues is interpreted as indicating that they evolved in a partially closed system where the residence time of the surrounding gas was sufficiently long for it to have become saturated in the evaporating species and for isotopic equilibration between the gas and the melt. A diffusion couple experiment juxtaposing chondrule-like melts with different potassium concentrations showed that the diffusivity of potassium is sufficiently fast at liquidus temperatures (DK > 2 × 10−4cm2 s−1 at 1650 °C) that diffusion-limited evaporation cannot explain why, despite their having been molten, the type IIA and type IA chondrules show no systematic potassium isotopic fractionation. [ABSTRACT FROM AUTHOR]

Experiments exposing Type B calcium-, aluminum-rich inclusion (CAI)-like melts at high temperatures to high vacuum or reducing hydrogen-rich gas mixtures were used to determine the rates and consequences of elemental and isotopic fractionation by evaporation. Silicon and magnesium were found to evaporate much faster than calcium and aluminum, and the resulting residual liquid trajectories in composition space are reproduced via a thermodynamic model for the saturation vapor pressure of the evaporating species. Isotopic fractionations associated with evaporation were measured for magnesium. The resulting relationship between fraction of magnesium lost and enrichment of the residue in the heavy isotopes of magnesium follows a Rayleigh fractionation curve with a fractionation factor that is close to, but not exactly, the theoretically expected value. The rate of evaporation is found to be a strong function of temperature, oxygen fugacity, and melt composition, which can be understood and modeled in terms of the dependence of the saturation vapor pressures on these variables. The relationship between evaporation rate, which we measure, and calculated saturation vapor involves empirical evaporation coefficients that we find to be significantly less than one (∼0.1). Analytical and numerical models are used to characterize how diffusion in both the melt and in the surrounding gas affects evaporation rates and the degree of chemical and isotopic fractionation. The experimental data and theoretical considerations are combined to give a parameterization of the rates and consequences of evaporation of Type B CAI-like liquids, which is then used to translate the measured isotopic fractionation of Type B CAIs into constraints on their thermal history. Cooling rates of the order of 10°C per hour are indicated. [Copyright &y& Elsevier]

Examines the role of diffusive coupling in promoting convection in multicomponent silicate melt. Role of multicomponent convection in the convective evolution and differentiation of terrestrial magmas.