Nickel isotope fractionation during metal-silicate differentiation of planetesimals: Experimental petrology and ab initio calculations

Metal-silicate fractionation of nickel isotopes has been experimentally quantified at 1623 K, with oxygen fugacities varying from 10−8.2 to 10−9.9 atm and for run durations from 0.5 to 1 h. Both kinetic and equilibrium fractionations have been studied. A wire loop set-up was used in which the metal reservoir is a pure nickel wire holding a silicate melt droplet of anorthite-diopside eutectic composition. During the course of the experiment, diffusion of nickel from the wire to the silicate occurred. The timescale to reach chemical equilibrium was fO2 dependent and decreased from 17 to 1 hour, as conditions became more reducing. The isotopic composition of each reservoir was determined by Multicollector-Inductively Coupled Plasma-Mass Spectrometry (MC-ICPMS) after Ni purification. The isotopic composition was found to be constant in the metallic wire, which therefore behaved as an infinite reservoir. On the contrary, strong kinetic fractionation was observed in the silicate melt (δNi down to −0.98‰.amu−1 relative to the standard). Isotopic equilibrium was typically reached after 24 hours. For equilibrated samples at 1623 K, no metal-silicate fractionation was observed within uncertainty (2SD), with ΔNiMetal-Silicate = 0.02 ± 0.04‰.amu−1. Theoretical calculations of metal-silicate isotope fractionation at equilibrium were also performed on different metal-silicate systems. These calculations confirm (1) the absence of fractionation at high temperature and (2) a weak temperature dependence for Ni isotopic fractionation for the metal-olivine and metal-pyroxene pairs with the metal being slightly lighter isotopically. Our experimental data were finally compared with natural samples. Some mesosiderites (stony-iron meteorites) show a ΔNiMetal-Silicate close to experimental values at equilibrium, whereas others exhibit positive metal-silicate fractionation that could reflect kinetic processes. Conversely, pallasites display a strong negative metal-silicate fractionation. This most likely results from kinetic processes with Ni diffusion from the silicate to the metal phase due to a change of Ni partition coefficient during cooling. In this respect we note that in these pallasites, iron isotopes show metal-silicate fractionation that is opposite direction to Ni, supporting the idea of kinetic isotope fractionation, associated with Fe-Ni interdiffusion

On the iron isotope composition of Mars and volatile depletion in the terrestrial planets

Iron is the most abundant multivalent element in planetary reservoirs, meaning its isotope composition (expressed as δ57Fe) may record signatures of processes that occurred during the formation and subsequent differentiation of the terrestrial planets. Chondritic meteorites, putative constituents of the planets and remnants of undifferentiated inner solar system bodies, have δ57Fe ≈ 0‰; an isotopic signature shared with the Martian Shergottite–Nakhlite–Chassignite (SNC) suite of meteorites. The silicate Earth and Moon, as represented by basaltic rocks, are distinctly heavier, δ57Fe≈+0.1‰. However, some authors have recently argued, on the basis of iron isotope measurements of abyssal peridotites, that the composition of the Earth’s mantle is δ57Fe = +0.04 ± 0.04‰, indistinguishable from the mean Martian value. To provide a more robust estimate for Mars, we present new high-precision iron isotope data on 17 SNC meteorites and 5 mineral separates. We find that the iron isotope compositions of Martian meteorites reflect igneous processes, with nakhlites and evolved shergottites displaying heavier δ57Fe(+0.05 ± 0.03‰), whereas MgO-rich rocks are lighter (δ57Fe≈−0.01 ±0.02‰). These systematics are controlled by the fractionation of olivine and pyroxene, attested to by the lighter isotope composition of pyroxene compared to whole rock nakhlites. Extrapolation of the δ57Fe SNC liquid line of descent to a putative Martian mantle yields a δ57Fe value lighter than its terrestrial counterpart, but indistinguishable from chondrites. Iron isotopes in planetary basalts of the inner solar system correlate positively with Fe/Mn and silicon isotopes. While Mars and IV-Vesta are undepleted in iron and accordingly have chondritic δ57Fe, the Earth experienced volatile depletion at low (1300 K) temperatures, likely at an early stage in the solar nebula, whereas additional post-nebular Fe loss is possible for the Moon and angrites

Hydrothermally-induced changes in mineralogy and magnetic properties of oxidized A-type granites

The changes in magnetic mineralogy due to the hydrothermal alteration of A-type granitic rocks have been thoroughly\ud investigated in samples fromthe granite of Tana (Corsica, France), and compared with other A-type granites:\ud Meruoca (NE Brazil), Bushveld (South Africa), Mount Scott (Wichita Mountains, Oklahoma, USA) and the\ud stratoid hypersolvus granites of Madagascar. The altered red-colored samples and their non-altered equivalents\ud were magnetically characterized by means of magnetic susceptibility measurements, hysteresis loops, remanent\ud coercivity spectra, and Lowrie test. It is shown that hydrothermalization in magnetite-bearing granites is related\ud to the formation of fine-grained magnetite and hematite, and to coeval depletion in the content of primary lowcoercive\ud coarse-grained magnetite. These mineralogical changes give typical rock magnetic signatures, namely\ud lower susceptibilitymagnitudes and anisotropy degrees, prolateAMS (anisotropy ofmagnetic susceptibility) fabrics\ud and increased coercivities. Optical microscopy and SEM (scanning electronic microscopy) images suggest\ud that the orientation of the secondary magnetic minerals is related to fluid-pathways and micro-fractures formed\ud during the hydrothermal event and therefore may be unrelated to magma emplacement and crystallization fabrics.\ud Changes inmagnetic mineralogy and grain-size distribution have also to be considered for any paleomagnetic\ud and iron isotope studies in granites.INSU-3F (Failles, fractures, flux) 2008 projec

Iron Isotopes

International audienceAlthough less variable in abundances than carbon or oxygen stable isotopes, iron isotopes show analytically significant mass-dependent isotopic variations in nature. The geochemical community has investigated the range of Fe isotope compositions in both high- and low-temperature geological environments. Contrary to early expectations, there are no specific biological-type iron isotope signatures. On the other hand, the largest isotopic fractionation was observed..

Silicon Isotope Geochemistry

In contrast to many other stable isotopes of the elements discussed in this book, those of silicon are not strictly speaking “Non-raditional Stable Isotopes” because they have been studied for more than 60 years. After the pioneering works of Reynolds and Verhoogen (1953) and Allenby (1954), a steady increase in silicon isotope studies of geological materials has led to a substantial corpus of data. These data were compiled by Ding et al. (1996) alongside new measurements that, collectively, included over a thousand samples of rocks, minerals, waters and biological materials. Most of these data were produced using the well established method of gas source mass spectrometry after sample decomposition and silicon purification via fluorination techniques.As for many non-traditional stable isotopes, silicon isotope research has flourished with the advent of second generation of multicollector plasma source mass spectrometers (MC–ICP–MS). These instruments eliminated the requirement of hazardous gaseous fluorine sample preparation methods while permitting improved analytical precision in both wet plasma (De La Rocha 2002) and in dry plasma (Cardinal et al. 2003). Subsequent analytical developments involving high mass resolution MC–ICP–MS combined with improved silicon purification methods (Georg et al. 2006) made this analytical technique more robust and precise enough to study even the subtle silicon isotope variations produced during high temperature geological processes (Savage et al. 2014)

In situ investigations of allanite hydrothermal alteration: examples from calc-alkaline and anorogenic granites of Corsica (southeast France)

International audienceAllanite is, with monazite, the main repository for light rare earth elements (REE) in the continental crust and can be used in U-Th-Pb geochronology. This mineral has been shown to be prone to alteration. The geochemical exchanges occurring between allanite and hydrothermal fluids were explored using backscattered scanning electron microscopy, electron microprobe, and laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS). The alteration mechanisms found show the restricted role of metamictization and correspond to the allanite to epidote transformation or to the leaching of the allanite A-crystallographic sites. Hence, the REE are mostly removed from this phase during fluid-mineral interactions, although a heavy rare earth-rich fluid may imprint its geochemical signature on the allanite altered zones. It appears that even if this REE holder presents evidence of alteration, the bulk mineral and whole-rock REE and Sm-Nd isotope signatures were not significantly affected in the cases studied. Alteration results mostly in the input of common lead, thus restricting the usefulness of allanite for U-Th-Pb dating when alteration occurs. Finally, the alteration mechanisms found here confirm the much more limited flexibility of silicates crystalline lattice as compared to phosphates

In situ investigations of allanite hydrothermal alteration: examples from calc-alkaline and anorogenic granites of Corsica (southeast France)

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On the iron isotope homogeneity level of the continental crust

International audienceAssessing the level of homogeneity of different planetary reservoirs is an essential step to establish a new isotopic tracer. In recent studies, we concluded that most igneous rocks from the continental crust were homogeneous to similar to 71 wt.% SiO2) that we concluded were significantly heavier than less silicic rocks from the crust on the basis of Fe isotopic analyses of seven bulk rock samples from different geological settings. Beard and Johnson [Beard, B.L., Johnson, C.M., 2006. Comment on "Heavy iron isotope composition of granites determined by high resolution MC-ICP-MS" by F. Poitrasson and R. Freydier, Chemical Geology, volume 222, pages 132-147. Chem. Geol., this issue.] challenge this finding on the basis of previously "published" data. It is shown here that this controversy stems from the confusion between whole-rock and mineral iron isotope analyses, since this isotopic tracer, like many others, is more heterogeneous at the mineral scale than the bulk rock scale. Thus, minerals cannot be used in place of bulk rock Fe isotope compositions. The controversy also arises from different levels of analytical uncertainties. If only the best Fe isotope probes of planetary reservoirs-bulk rocks - are considered, then the conclusion that igneous rocks with SiO2 > 71 wt% are significantly heavier than the mean delta Fe-57 of the continental crust [Poitrasson, F., Freydier, R., 2005. Heavy iron isotope composition of granites determined by high resolution MC-ICP-MS. Chem. Geol., 222, 132-147.] remains valid. Nonetheless, given that granites represent only a minor fraction of the continental crust, this finding does not change the bulk igneous Fe isotope composition estimate previously defined for the Earth

Does planetary differentiation really fractionate iron isotopes?

International audienceThe difference in the mean Fe isotope composition of samples from the Earth, Moon, Mars and Vesta has been recently interpreted as tracking contrasted planetary accretion mechanisms [F. Poitrasson, A.N. Halliday, D.C. Lee, S. Levasseur, N. Teutsch, Iron isotope differences between Earth, Moon, Mars and Vesta as possible records of contrasted accretion mechanisms, Earth Planet. Sci. Lett. 223 (2004) 253 266]. Using newly produced Fe isotopic data on terrestrial and lunar samples, pallasites, eucrites and Martian meteorites, Weyer et al. [S. Weyer, A.D. Anbar, G.P. Brey, C. Munker, K. Mezger, A.B. Woodland, Iron isotope fractionation during planetary differentiation, Earth Planet. Sci. Lett. 240 (2005) 251 264] reinterpreted these data as fingerprinting planetary differentiation. In particular, these authors suggested that partial melting in the terrestrial and lunar mantles produced melts isotopically heavy. It is shown here that the inference of Weyer et al. [S. Weyer, A.D. Anbar, G.P. Brey, C. Munker, K. Mezger, A.B. Woodland, Iron isotope fractionation during planetary differentiation, Earth Planet. Sci. Lett. 240 (2005) 251 264] is strongly biased by the sampling approach taken. Notably, these authors used olivine in place of the host bulk peridotites ?57Fe signatures despite this mineral has been shown to be frequently isotopically lighter than coexisting phases, and they analyzed lunar samples heavily affected chemically by the meteoritic bombardment, a process known to alter Fe isotope signatures. Their pallasite metal silicate fractionation data are also likely biased by the approach adopted to estimate the iron isotope composition of the different mineral phases. In fact, their conclusion of Fe isotopic fractionation during basalt extraction from planetary mantles is invalidated by the observation that basaltic shergottites and eucrites have ?57Fe indistinguishable from those of chondrites. Therefore, the heavier Fe isotopic composition of the Moon relative to the Earth, itself heavier than most chondrites and achondrites remains best explained by loss of light iron isotopes during the high temperature event accompanying the interplanetary impact that led to the formation of the Moon [F. Poitrasson, A.N. Halliday, D.C. Lee, S. Levasseur, N. Teutsch, Iron isotope differences between Earth, Moon, Mars and Vesta as possible records of contrasted accretion mechanisms, Earth Planet. Sci. Lett. 223 (2004) 253 266., F. Poitrasson, S. Levasseur, N. Teutsch, Significance of iron isotope mineral fractionation in pallasites and iron meteorites for the core mantle differentiation of terrestrial planets, Earth Planet. Sci. Lett. 234 (2005) 151 164]