Hugh St. C. O'Neill

Cosmochemical Estimates of Mantle Composition

The composition of the primitive mantle derived here shows that Earth was assembled from material that shows many of the same chemical fractionation processes as chondritic meteorites. These processes occurred at the initial stage of the solar system formation, under conditions thought to be present in the solar nebula. But the stable isotope record excludes chondritic meteorites as the ‘building blocks’ of Earth. Meteorites formed in local environments separated from that part of the inner solar system where much of the material forming the terrestrial planets was sourced.

The relative effects of pressure, temperature and oxygen fugacity on the solubility of sulfide in mafic magmas

Abstract The sulfur contents at sulfide saturation (SCSS) of a basaltic and a picritic melt have been measured experimentally as a function of pressure and temperature from 5 to 90 kb and 1400–1800°C, using piston-cylinder and multi-anvil solid media pressure devices. Three distinct regimes of oxygen fugacity were investigated, imposed by the use of Fe100, Fe40Ir60, and Fe20Ir80 capsules. The compositions of quenched run products, including the S contents of the silicate glasses, were determined by electron microprobe analysis. Theoretical considerations suggest that SCSS values (in ppm) can be described by an equation of the form: ln[S/ppm] SCSS = A T +B+ CP T +ln a FeS sulfide where A and B are functions of the composition of the silicate melt. This equation implies that SCSS is independent of fO2 and fS2, except insofar as these factors influence the nature of the sulfide liquid (hence aFeSsulfide). The experiments reported here confirm this. The SCSS of both the basaltic and picritic compositions are rather insensitive to temperature, but show a strong exponential decrease with increasing pressure. Consequently, a magma generated in equilibrium with residual sulfide in the mantle becomes under saturated in sulfide during adiabatic ascent. At low pressure, sulfide saturation should occur only after substantial crystallization, under closed-system conditions, or after significant modification via assimilation (e.g., of S-rich sediments).

The transition between spinel lherzolite and garnet lherzolite, and its use as a Geobarometer

AbstractThe equilibrium between spinel lherzolite and garnet lherzolite has been experimentally determined in the CaO-MgO-Al2O3-SiO2 system between 800° and 1,100° C. In confirmation of earlier work and predictions from thermodynamic data, it was found that theP-T slope of the reaction was close to zero, the equilibrium ranging from 16.1 kb at 800° C to 18.7 kb at 1,100° C (±0.3 kb).The addition of Cr2O3 to the system raised the stability field of spinel to higher pressures. It was found that the pressure at which both garnet and spinel could exist with olivine+orthopyroxene+clinopyroxene in the system CMAS −Cr2O3 could best be described by the empirical relationship:

The effect of melt composition on trace element partitioning: an experimental investigation of the activity coefficients of FeO, NiO, CoO, MoO 2 and MoO 3 in silicate melts

P = P^{\text{O}} + \alpha X_{{\text{Cr}}}^{s{\text{p}}}

Thermodynamics and kinetics of cation ordering in MgAl 2 O 4 spinel up to 1600 degrees C from in situ neutron diffraction

whereP0 is the equilibrium pressure for the univariant reaction in the Cr2O3-free system,α is a constant apparently independent of temperature with a value of 27.9 kilobars, andXCrsp is the mole fraction of chromium in spinel.Use was made of the extensive literature on Mg-Fe2+ solid solutions to quantitatively derive the effect of Fe2+ on the equilibrium. The effect of other components (Fe3+, Na) was also considered.The equilibrium can be used as a sensitive geobarometer for rocks containing the five phases ol+opx+cpx+gt+sp, and thus provides the only independent check presently available for the more widely applicable geobarometer which uses the alumina content of orthopyroxene in equilibrium with garnet.

Fingerprinting the water site in mantle olivine

The partitioning of Fe and Mg between coexisting garnet and olivine has been studied at 30 kb pressure and temperatures of 900 ° to 1,400 °C. The results of both synthesis and reversal experiments demonstrate that KD (= (Fe/Mg)gt/(Fe/Mg)OI) is strongly dependent on Fe/Mg ratio and on the calcium content of the garnet. For example, at 1,000 °C/30 kb, KD varies from about 1.2 in very iron-rich compositions to 1.9 at the magnesium end of the series. Increasing the mole fraction of calcium in the garnet from 0 to 0.3 at 1,000 ° C increases KD in magnesian compositions from 1.9 to about 2.5.The observed temperature and composition dependence of KD has been formulated into an equation suitable for geothermometry by considering the solid solution properties of the olivine and garnet phases. It was found that, within experimental error, the simplest kind of nonideal solution model (Regular Solution) fits the experimental data adequately. The use of more complex models did not markedly improve the fit to the data, so the model with the least number of variables was adopted.Multiple linear regression of the experimental data (72 points) yielded, for the exchange reaction: 3Fe2SiO4+2Mg3Al2Si3O12 olivine garnet ⇌ 2Fe2Al2Si3O12+3Mg2SiO4 garnet olivine ΔH ° (30kb) of −10,750 cal and ΔS ° of −4.26 cal deg−1 mol−1. Absolute magnitudes of interaction parameters (Wij) derived from the regression are subject to considerable uncertainty. The partition coefficient is, however, strongly dependent on the following differences between solution parameters and these differences are fairly well constrained: WFeMgol-WFeMggt≃ 800 cal WCaMggt-WCaFegt≃ 2,670 cal.The geothermometer is most sensitive in the temperature and composition regions where KDis substantially greater than 1. Thus, for example, peridotitic compositions at temperatures less than about 1,300 ° C should yield calculated temperatures within 60 °C of the true value. Iron rich compositions (at any temperature) and magnesian compositions at temperatures well above 1,300 °C could not be expected to yield accurate calculated temperatures.For a fixed KDthe influence of pressure is to raise the calculated temperature by between 3 and 6 °C per kbar.

Oxidation state of iron in komatiitic melt inclusions indicates hot Archaean mantle

Abstract The thermodynamic theory describing the partitioning of trace elements between crystals and silicate melt implies that partition coefficients should depend on the major-element composition of the melt from two different causes, namely (1) the activity coefficient of the trace-element oxide component in the melt, and (2) the activities of all the major-element components needed to balance the trace-element substitution in the crystal (the “stoichiometric control”). Partition coefficients are also expected to vary with the composition of the crystal, and temperature and pressure. Because these variables cannot be controlled independently in direct crystal/melt partitioning studies, it has not been possible to disentangle their effects, or to determine their relative importance. In order to explore the effects of melt composition on activity coefficients of trace-element oxide components, the activity coefficients of five such components, MoO 2 , MoO 3 , FeO, NiO and CoO, were measured in 18 different melt compositions in the system CaO–MgO–Al 2 O 3 –SiO 2 plus one composition in CaO–MgO–Al 2 O 3 –SiO 2 –TiO 2 at 1400 °C, by equilibration with the metal under controlled oxygen fugacity. MoO 2 and MoO 3 are expected to have geochemical properties similar to the High Field Strength Elements (HFSEs). The activity coefficients of MoO 2 and MoO 3 vary by factors of 20 and 60, respectively, over the range of compositions investigated. Their variation is highly correlated, and mainly depends on the amount of CaO in the melt, suggesting the influence of CaMoO 3 and CaMoO 4 complexes. The analogy between Mo and HFSEs implies that melt composition can be expected to have an important influence on HFSE partition coefficients. The activity coefficients of FeO, NiO and CoO vary by a factor of two over the same range of melt compositions, but show no simple dependence on any particular major-element oxide component. However, the activity coefficients of all three components are very highly correlated with each other. This means that the effect of melt composition can be largely eliminated if the ratios of two activity coefficients are taken, as, for example, when two-element distribution coefficients are used.

Quantitative absorbance spectroscopy with unpolarized light: Part II. Experimental evaluation and development of a protocol for quantitative analysis of mineral IR spectra

The compositional variations among the chondrites inform us about cosmochemical fractionation processes during condensation and aggregation of solid matter from the solar nebula. These fractionations include: (i) variable Mg–Si–RLE ratios (RLE: refractory lithophile element), (ii) depletions in elements more volatile than Mg, (iii) a cosmochemical metal–silicate fractionation, and (iv) variations in oxidation state. Moon- to Mars-sized planetary bodies, formed by rapid accretion of chondrite-like planetesimals in local feeding zones within 106 years, may exhibit some of these chemical variations. However, the next stage of planetary accretion is the growth of the terrestrial planets from approximately 102 embryos sourced across wide heliocentric distances, involving energetic collisions, in which material may be lost from a growing planet as well as gained. While this may result in averaging out of the ‘chondritic’ fractionations, it introduces two non-chondritic chemical fractionation processes: post-nebular volatilization and preferential collisional erosion. In the latter, geochemically enriched crust formed previously is preferentially lost. That post-nebular volatilization was widespread is demonstrated by the non-chondritic Mn/Na ratio in all the small, differentiated, rocky bodies for which we have basaltic samples, including the Moon and Mars. The bulk silicate Earth (BSE) has chondritic Mn/Na, but shows several other compositional features in its pattern of depletion of volatile elements suggestive of non-chondritic fractionation. The whole-Earth Fe/Mg ratio is 2.1±0.1, significantly greater than the solar ratio of 1.9±0.1, implying net collisional erosion of approximately 10 per cent silicate relative to metal during the Earths accretion. If this collisional erosion preferentially removed differentiated crust, the assumption of chondritic ratios among all RLEs in the BSE would not be valid, with the BSE depleted in elements according to their geochemical incompatibility. In the extreme case, the Earth would only have half the chondritic abundances of the highly incompatible, heat-producing elements Th, U and K. Such an Earth model resolves several geochemical paradoxes: the depleted mantle occupies the whole mantle, is completely outgassed in 40Ar and produces the observed 4He flux through the ocean basins. But the lower radiogenic heat production exacerbates the discrepancy with heat loss.