Hf-W chronology of the accretion and early evolution of asteroids and terrestrial planets

The ¹⁸²Hf-¹⁸²W systematics of meteoritic and planetary samples provide firm constraints on the chronology of the accretion and earliest evolution of asteroids and terrestrial planets and lead to the following succession and duration of events in the earliest solar system. Formation of Ca,Al-rich inclusions (CAIs) at 4568.3 ± 0.7 Ma was followed by the accretion and differentiation of the parent bodies of some magmatic iron meteorites within less than ∼1 Myr. Chondrules from H chondrites formed 1.7 ± 0.7 Myr after CAIs, about contemporaneously with chondrules from L and LL chondrites as shown by their ²⁶Al-²⁶Mg ages. Some magmatism on the parent bodies of angrites, eucrites, and mesosiderites started as soon as ∼3 Myr after CAI formation and may have continued until ∼10 Myr. A similar timescale is obtained for the high-temperature metamorphic evolution of the H chondrite parent body. Thermal modeling combined with these age constraints reveals that the different thermal histories of meteorite parent bodies primarily reflect their initial abundance of ²⁶Al, which is determined by their accretion age. Impact-related processes were important in the subsequent evolution of asteroids but do not appear to have induced large-scale melting. For instance, Hf-W ages for eucrite metals postdate CAI formation by ∼20 Myr and may reflect impact-triggered thermal metamorphism in the crust of the eucrite parent body. Likewise, the Hf-W systematics of some non-magmatic iron meteorites were modified by impact-related processes but the timing of this event(s) remains poorly constrained.The strong fractionation of lithophile Hf from siderophile W during core formation makes the Hf-W system an ideal chronometer for this major differentiation event. However, for larger planets such as the terrestrial planets the calculated Hf-W ages are particularly sensitive to the occurrence of large impacts, the degree to which impactor cores re-equilibrated with the target mantle during large collisions, and changes in the metal-silicate partition coefficients of W due to changing fO₂ in differentiating planetary bodies. Calculated core formation ages for Mars range from 0 to 20 Myr after CAI formation and currently cannot distinguish between scenarios where Mars formed by runaway growth and where its formation was more protracted. Tungsten model ages for core formation in Earth range from ∼30 Myr to >100 Myr after CAIs and hence do not provide a unique age for the formation of Earth. However, the identical ¹⁸²W/¹⁸⁴W ratios of the lunar and terrestrial mantles provide powerful evidence that the Moon-forming giant impact and the final stage of Earth’s core formation occurred after extinction of ¹⁸²Hf (i.e., more than ∼50 Myr after CAIs), unless the Hf/W ratios of the bulk silicate Moon and Earth are identical to within less than ∼10%. Furthermore, the identical ¹⁸²W/¹⁸⁴W of the lunar and terrestrial mantles is difficult to explain unless either the Moon consists predominantly of terrestrial material or the W in the proto-lunar magma disk isotopically equilibrated with the Earth’s mantle.Hafnium-tungsten chronometry also provides constraints on the duration of magma ocean solidification in terrestrial planets. Variations in the ¹⁸²W/¹⁸⁴W ratios of martian meteorites reflect an early differentiation of the martian mantle during the effective lifetime of ¹⁸²Hf. In contrast, no ¹⁸²W variations exist in the lunar mantle, demonstrating magma ocean solidification later than ∼60 Myr, in agreement with ¹⁴⁷Sm-¹⁴³Nd ages for ferroan anorthosites. The Moon-forming giant impact most likely erased any evidence of a prior differentiation of Earth’s mantle, consistent with a ¹⁴⁶Sm-¹⁴²Nd age of 50-200 Myr for the earliest differentiation of Earth’s mantle. However, the Hf-W chronology of the formation of Earth’s core and the Moon-forming impact is difficult to reconcile with the preservation of ¹⁴⁶Sm-¹⁴²Nd evidence for an early (<30 Myr after CAIs) differentiation of a chondritic Earth’s mantle. Instead, the combined ¹⁸²W-¹⁴²Nd evidence suggests that bulk Earth may have superchondritic Sm/Nd and Hf/W ratios, in which case formation of its core must have terminated more than ∼42 Myr after formation of CAIs, consistent with the Hf-W age for the formation of the Moon.     

Stratigraphical evidence for the terrestrial age of australites

Australites in an excellent state of preservation are common (up to 1 specimen per 300 m²) in lag gravels flooring corridors between seif dunes in the Motpena and Myrtle Springs areas of the Lake Torrens plain, South Australia. A study of the Quaternary stratigraphy of the region indicates that late Wisconsinan relict dunes (Lake Torrens Formation) are the most likely source of the australites. Radiocarbon dating indicates that the Lake Torrens Formation was deposited between about 24,000 and 16,000 years B.P. As the excellent preservation of most of the australites indicates that they have undergone negligible transport since their infall, it is concluded that the australites fell into the dune field sometime between about 24,000 and 16,000 years B.P.     

New evidence of scroll-bar formation on the Beatton River

In recent work, two hypotheses were proposed to explain the formation of distinct scroll bars and subsequent floodplain ridges on the Beatton River. These hypotheses are reassessed here in the light of recent field observations, which suggest that an initial scroll bar may form around a stranded dead tree that acts as a sedimentation nucleus on a point-bar platform.     

Metal-silicate partitioning and constraints on core composition and oxygen fugacity during Earth accretion

We present the results of new partitioning experiments between metal and silicate melts for a series of elements normally regarded as refractory lithophile and moderately siderophile and volatile. These include Si, Ti, Ni, Cr, Mn, Ga, Nb, Ta, Cu and Zn. Our new data obtained at 3.6 and 7.7 GPa and between 2123 and 2473 K are combined with literature data to parameterize the individual effects of oxygen fugacity, temperature, pressure and composition on partitioning. We find that Ni, Cu and Zn become less siderophile with increasing temperature. In contrast, Mn, Cr, Si, Ta, Nb, Ga and Ti become more siderophile with increasing temperature, with the highly charged cations (Nb, Ta, Si and Ti) being the most sensitive to variations of temperature. We also find that Ni, Cr, Nb, Ta and Ga become less siderophile with increasing pressure, while Mn becomes more siderophile with increasing pressure. Pressure effects on the partitioning of Si, Ti, Cu and Zn appear to be negligible, as are the effects of silicate melt composition on the partitioning of divalent cations. From the derived parameterization, we predict that the silicate Earth abundances of the elements mentioned above are best explained if core formation in a magma ocean took place under increasing conditions of oxygen fugacity, starting from moderately reduced conditions and finishing at the current mantle-core equilibrium value.     

Siderophile geochemistry of ureilites: A record of early stages of planetesimal core formation

New bulk-compositional data, including trace siderophile elements such as Ir, Os, Au, and Ni, are presented for 25 ureilites. Without exception, ureilites have siderophile abundances too high to plausibly have formed as cumulates. Ureilites undoubtedly underwent a variety of “smelting,” by which C was oxidized to CO gas while olivine FeO was reduced to Fe-metal. However, pressure-buffered equilibrium smelting is not a plausible model for engendering the wide range (75-96 mol%) of mafic-silicate core mg among ureilites. The smelting reaction produces too much CO gas. Even supposing a disequilibrium process with the smelt-gas leaking out of the mantle, none of the ureilites, least of all the ureilite with the most “reduced” (highest) olivine-core mg (ALH84136), has the high Fe-metal abundance predicted by the smelted-cores model. In principle, the Fe-metal generated by smelting could have been subsequently lost, but siderophile data show that ureilites never underwent efficient depletion of Fe-metal. Ureilites display strong correlations among siderophile ratios such as Au/Ir, Ni/Ir, Co/Ir, As/Ir, Se/Ir, and Sb/Ir. Ureilite siderophile depletion patterns loosely resemble siderophile fractionations, presumably nebular in origin, among carbonaceous chondrites. However, Zn, for an element of moderate volatility, is anomalously high in ureilites. A tight correlation between Au and Ni extrapolates to the low-Ni/Au side of the compositional range of carbonaceous chondrites. From this mismatch, mild but nonetheless significant depletions of refractory siderophile elements such as Ir and Os, and moderate depletions of strongly siderophile, weakly chalcophile elements such as Ni and Au, we infer that the ureilite siderophile fractionations are largely the result of a non-nebular process, i.e., removal of S-rich metallic melt, possibly with minor entrainment of Fe-metal. Several lines of trace-element evidence indicate that melt porosity during ureilite anatexis was at least moderate. The ureilite pattern of very mild depletions of extremely siderophile elements, but much deeper depletions of moderately siderophile, chalcophile elements, suggests that asteroidal core formation probably occurs in two discrete stages. In general, separation of a considerable proportion (several wt%) of S-rich metallic melt probably occurs long before, and at a far lower temperature than, separation of the remaining S-poor Fe-metal. Apart from the Fe-metal itself, only extremely siderophile elements wait until the second stage to sequester mainly into the core.     

Early core formation in asteroids and late accretion of chondrite parent bodies: Evidence from Hf-W in CAIs, metal-rich chondrites, and iron meteorites

The ¹⁸²Hf-¹⁸²W isotopic systematics of Ca-Al-rich inclusions (CAIs), metal-rich chondrites, and iron meteorites were investigated to constrain the relative timing of accretion of their parent asteroids. A regression of the Hf-W data for two bulk CAIs, various fragments of a single CAI, and carbonaceous chondrites constrains the ¹⁸²Hf/¹⁸⁰Hf and ε_W at the time of CAI formation to (1.07 ± 0.10) × 10⁻⁴ and −3.47 ± 0.20, respectively. All magmatic iron meteorites examined here have initial ε_W values that are similar to or slightly lower than the initial value of CAIs. These low ε_W values may in part reflect ¹⁸²W-burnout caused by the prolonged cosmic ray exposure of iron meteorites, but this effect is estimated to be less than ∼0.3 ε units for an exposure age of 600 Ma. The W isotope data, after correction for cosmic ray induced effects, indicate that core formation in the parent asteroids of the magmatic iron meteorites occurred less than ∼1.5 Myr after formation of CAIs. The nonmagmatic IAB-IIICD irons and the metal-rich CB chondrites have more radiogenic W isotope compositions, indicating formation several Myr after the oldest metal cores had segregated in some asteroids.Chondrule formation ∼2-5 Myr after CAIs, as constrained by published Pb-Pb and Al-Mg ages, postdates core formation in planetesimals, and indicates that chondrites do not represent the precursor material from which asteroids accreted and then differentiated. Chondrites instead derive from asteroids that accreted late, either farther from the Sun than the parent bodies of magmatic iron meteorites or by reaccretion of debris produced during collisional disruption of older asteroids. Alternatively, chondrites may represent material from the outermost layers of differentiated asteroids. The early thermal and chemical evolution of asteroids appears to be controlled by the decay of ²⁶Al, which was sufficiently abundant (initial ²⁶Al/²⁷Al >1.4 × 10⁻⁵) to rapidly melt early-formed planetesimals but could not raise the temperatures in the late-formed chondrite parent asteroids high enough to cause differentiation. The preservation of the primitive appearance of chondrites thus at least partially reflects their late formation rather than their early and primitive origin.     

Experimental study of platinum solubility in silicate melt to 14 GPa and 2273 K: Implications for accretion and core formation in Earth

We determined the solubility limit of Pt in molten haplo-basalt (1 atm anorthite-diopside eutectic composition) in piston-cylinder and multi-anvil experiments at pressures between 0.5 and 14 GPa and temperatures from 1698 to 2223 K. Experiments were internally buffered at ∼IW + 1. Pt concentrations in quenched-glass samples were measured by laser-ablation inductively coupled-plasma mass spectrometry (LA-ICPMS). This technique allows detection of small-scale heterogeneities in the run products while supplying three-dimensional information about the distribution of Pt in the glass samples. Analytical variations in ¹⁹⁵Pt indicate that all experiments contain Pt nanonuggets after quenching. Averages of multiple, time-integrated spot analyses (corresponding to bulk analyses) typically have large standard deviations, and calculated Pt solubilities in silicate melt exhibit no statistically significant covariance with temperature or pressure. In contrast, averages of minimum ¹⁹⁵Pt signal levels show less inter-spot variation, and solubility shows significant covariance with pressure and temperature. We interpret these results to mean that nanonuggets are not quench particles, that is, they were not dissolved in the silicate melt, but were part of the equilibrium metal assemblage at run conditions. We assume that the average of minimum measured Pt abundances in multiple probe spots is representative of the actual solubility. The metal/silicate partition coefficients (D^met/sil) is the inverse of solubility, and we parameterize D^met/sil in the data set by multivariate regression. The statistically robust regression shows that increasing both pressure and temperature causes D^met/silto decrease, that is, Pt becomes more soluble in silicate melt. D^met/sil decreases by less than an order of magnitude at constant temperature from 1 to 14 GPa, whereas isobaric increase in temperature produces a more dramatic effect, with D^met/sil decreasing by more than one order of magnitude between 1623 and 2223 K. The Pt abundance in the Earth’s mantle requires that D^met/sil is ∼1000 assuming core-mantle equilibration. Geochemical models for core formation in Earth based on moderately and slightly siderophile elements are generally consistent with equilibrium metal segregation at conditions generally in the range of 20-60 GPa and 2000-4000 K. Model extrapolations to these conditions show that the Pt abundance of the mantle can only be matched if oxygen fugacity is high (∼IW) and if Pt mixes ideally in molten iron, both very unlikely conditions. For more realistic values of oxygen fugacity (∼IW − 2) and experimentally-based constraints on non-ideal mixing, models show that D^met/sil would be several orders of magnitude too high even at the most favorable conditions of pressure and temperature. These results suggest that the mantle Pt budget, and by implication other highly siderophile elements, was added by late addition of a ‘late veneer’ phase to the accreting proto-Earth.     

Late orogenic crustal extension in the northern Menderes massif (western Turkey): evidence for metamorphic core complex formation

The Simav metamorphic core complex of the northern Menderes massif, western Turkey, consists of a plutonic (Tertiary) and metamorphic (Precambrian) core (footwall) separated from an allochthonous cover sequence (hanging wall) by a low-angle, ductile-to-brittle, extensional fault zone (i.e. detachment fault). The core rocks below the detachment fault are converted into mylonites with a thickness of a few hundred metres. Two main deformation events have affected the core rocks. The first deformational event (D₁) was developed within the Precambrian metamorphic rocks. The second event (D₂), associated with the Tertiary crustal extension, includes two distinct stages. Stage one is the formation of a variably developed ductile (mylonitic) deformation (D_2d) in metamorphic and granitic core rocks under greenschist facies conditions. The majority of the mylonites in the study area have foliations that strike NNW to NNE and dip SW to SE. Stretched quartz and feldspar grains define the mineral lineation trending SW-NE direction and plunging gently to SW. The kinematic indicators indicate a top-to-NE sense of shear. Stage two formation of brittle deformation (D_2b) that affected all core and cover rocks. D_2b involves the development of cataclasites and high-angle normal faults. An overall top towards the north sense of shear for the ductile (mylonitic) fabrics in the core rocks is consistent with the N-S regional extension in western Turkey.     

Metal-silicate partitioning of cesium: Implications for core formation

Here we present the first set of metal-silicate partitioning data for Cs, which we use to examine whether the primitive mantle depletion of Cs can be attributed to core segregation. Our experiments independently varied pressure from 5 to 15 GPa, temperature from 1900 to 2400 °C, metallic sulfur content from pure Fe to pure FeS, silicate melt polymerization, expressed as a ratio of non-bridging oxygens to tetrahedrally coordinated cations (nbo/t) from 1.26 to 3.1, and fO₂ from two to four log units below the iron-wüstite buffer. The most important controls on the partitioning behavior of alkalis were the metallic sulfur content, expressed as X_S, and the nbo/t of the silicate liquid. Normalization of X_S to 0.5 yielded the following expressions for D-values as a function of nbo/t: log D_Na = −2.0 + 0.44 × (nbo/t), log D_K = −2.4 + 0.67 × ( nbo/t), and log D_Cs = −3.2 + 1.17 × (nbo/t). Normalization of nbo/t to 2.7 resulted in the following equations for D-values as a function of S content: log D_Na = −4.1 + 6.4 × X_S, log D_K = −7.7 + 13.9 × X_S, and log D_Cs = −12.1 + 23.3 × X_S.There appears to be a negative pressure effect up to 15 GPa, but it should be noted that this trend was not present before normalization, and is based on only two measurements. There is a positive trend in cesium’s metal-silicate partition coefficient with increasing temperature. D_Cs exhibits the largest change and increased by a factor of three over 500 °C. The effect of oxygen fugacity has not been precisely determined but in general, lowering fO₂ by two log units resulted in a rise in all D-values of approximately an order of magnitude. In general, the sensitivity of partition coefficients to changing parameters increased with atomic number.The highest D-value for Cs observed in this study is 0.345, which was obtained at nbo/t of 2.7 and a metal phase of pure FeS. This metallic composition has far more S than has been suggested for any credible core-forming metal. We therefore conclude that the depletion of Cs in Earth’s mantle is either caused by radically different behavior of Cs at pressures higher than 15 GPa or is not related to core formation. Even so, we have shown that a planet with a sufficient S inventory may incorporate significant amounts of alkali elements into its core.     

Experimental petrochemistry of some highly siderophile elements at high temperatures, and some implications for core formation and the mantle''s early history

The highly siderophile elements (HSE's: Ru, Rh, Pd, Re, Os, Ir, Pt and Au) and those elements with distribution coefficients between Fe-rich metal and silicate phases which exceed 10⁴. The large magnitude of these distribution coefficients makes them exceedingly difficult to measure experimentally. We describe a new experimental campaign aimed at obtaining reliable values of D_M^{mets/sil melt} for selected HSE's indirectly, by measuring the solubilities of the pure metals (or simple HSE alloys) in haplobasaltic melts as a function of oxygen fugacity.

Preliminary results for Pd, Au, Ir and Re indicate that the HSE's may dissolve in silicate melts in unusually low valence states, e.g., 2+ for Ir and 1+ for the others. These unusual valence states may be important in understanding the geochemical properties of the HSE's. Inferred values of D_M^{met/sil melt} from the solubility data at 1400°C and IW −1 are 10⁷ for Pd and Au, and 10⁹−10¹² for Ir. Metal/silicate partition coefficients are thus confirmed to be very large, and are also different for the different HSE's.

A review of the abundance of the HSE's in the Earth's upper mantle shows that they are all present at 0.8% of chondritic, i.e. they have the same relative abundance, and the ratios of their concentrations are chondritic (e.g., Re/Os). Both the low degree of depletion (compared to the high values of D_M^{met/sil melt}) and the chondritic relative abundances support the idea that the mantle's HSE's were added in a “late veneer” after the cessation of core formation. Sulfur is even more depleted in the mantle relative to CI chondrites than the HSE's: this implies a late veneer which was depleted in volatile elements, and which was added to a mantle stripped of S. Since considerable S dissolves in silicate melt, this further implies that core formation in the Earth either occurred under P−T conditions below the solicate solidus, or, if the process occurred over a range of temperatures in a cooling Earth, then the process continued down to conditions below the silicate solidus.

The chondritic relative abundances of the HSE's in the upper mantle argue for a chemically unstratified primitive mantle, unless the late veneer was mixed only into the upper mantle.     

Integrated marine and terrestrial evidence for abrupt Congo River palaeodischarge fluctuations during the last deglaciation

We present a high‐resolution reconstruction of tropical palaeoenvironmental changes for the last deglacial transition (18 to 9 cal. kyr BP) based on integrated oceanic and terrestrial proxies from a Congo fan core. Pollen, grass cuticle, Pediastrum and dinoflagellate cyst fluxes, sedimentation rates and planktonic foraminiferal δ¹⁸O ratios, u₃₇^K′ sea‐surface temperature and alkane/alkenone ratio data highlight a series of abrupt changes in Congo River palaeodischarge. A major discharge pulse is registered at around 13.0 cal. kyr BP which we attribute to latitudinal migration of the Intertropical Convergence Zone (ITCZ) during deglaciation. The data indicate abrupt and short‐lived changes in the equatorial precipitation regime within a system of monsoonal dynamics forced by precessional cycles. The phases of enhanced Congo discharge stimulated river‐induced upwelling and enhanced productivity in the adjacent ocean. Copyright © 2001 John Wiley & Sons, Ltd.     

Evolution and time of formation of the Hohhot metamorphic core complex,North China: new structural and geochronologic evidence

The Hohhot metamorphic core complex (MCC) is one of the typical MCCs in the North China craton. Its fault systems consist of the master Hohhot detachment zone (HHDZ) on the southern flank of the Daqing Shan, and the lowermost and uppermost northern detachment zones on the northern flank. Ductile deformation temperatures of three zones were estimated as 500 ± 50°C, 650 ± 50°C, and 400 ± 50°C, respectively, by analysis of microstructures of minerals and quartz [c] crystallographic axis fabrics using electron backscattered diffraction. These measurements suggest that previous ⁴⁰Ar/³⁹Ar ages could not represent the time of the high-temperature (>500°C) ductile deformations. Therefore, we used U-Pb zircon ages of mylonitized and non-mylonitized granitic intrusions to constrain the timing of the early high-temperature shearing. Strongly mylonitized granites and weakly mylonitized granites in the lowermost northern detachment zone yielded zircon U-Pb ages of 148 ± 1 and 140 ± 1 million years respectively. A syn-kinematic pluton in the lower plate of the MCC gave a U-Pb age of 142 ± 1 million years. These allow us to speculate on the possibility that SE-directed, early tangential, high-temperature ductile shearing probably was initiated during ca. 148–140 Ma (or ca. 142 Ma) at depth, with the thrust events occurring at shallow levels. A strongly mylonitized granitic dike and a non-mylonitized pluton in the master HHDZ yielded ages of 142 ± 1 and 132 ± 2 million years respectively. A non-mylonitized pluton intrusive into the uppermost northern detachment zone was dated at 131 ± 1 Ma. All these suggest that major extensional ductile shearing along the detachment zones took place during ca. 140–132 Ma. Using these new U-Pb ages, combined with previously published ⁴⁰Ar/³⁹Ar cooling ages that range from 127 Ma to 119 million years for the master HHDZ and supradetachment basins, we discuss and derive the time of formation process of the MCC. This is one of only a few cases of detailed study of timing for the development of an MCC from earlier deep-level shearing to later thermal uplifting (doming).     

Oceanic evidence for the mechanism of rapid northern hemisphere glaciation

The oxygen isotopic stage 5/4 boundary in deep-sea sediments marks a prominent interval of northern hemisphere ice-sheet growth that lasted about 10,000 yr. During much of this rapid ice growth, the North Atlantic Ocean from at least 40°N to 60°N maintained warm sea-surface temperatures, within 1° to 2°C of today's subpolar ocean. This oceanic warmth provided a local source of moisture for ice-sheet accretion on the adjacent continents. The unusually strong thermal gradient off the east coast of North America (an “interglacial” ocean alongside a “glacial” land mass) also should have directed low-pressure storms from warm southern latitudes north-ward toward the Laurentide Ice Sheet. In addition, minimal calving of ice into the North Atlantic occurred during most of the stage 5/4 transition, indicative of ice retention within the continents. Diminished summer and autumn insolation, a warm subpolar ocean, and minimal calving of ice are conducive to rapid and extensive episodes of northern hemisphere ice-sheet growth.     

Silicon isotopes in meteorites and planetary core formation

The silicon (Si) isotope compositions of 42 meteorite and terrestrial samples have been determined using MC-ICPMS with the aim of resolving the current debate over their compositions and the implications for core formation. No systematic δ³⁰Si differences are resolved between chondrites (δ³⁰Si = −0.49 ± 0.15‰, 2σ_SD) and achondrites (δ³⁰Si = −0.47 ± 0.11‰, 2σ_SD), although enstatite chondrites are consistently lighter (δ³⁰Si = −0.63 ± 0.07‰, 2σ_SD) in comparison to other meteorite groups. The data reported here for meteorites and terrestrial samples display an average difference Δ³⁰Si_{BSE−meteorite∗} = 0.15 ± 0.10‰, which is consistent within uncertainty with previous studies. No effect from sample heterogeneity, preparation, chemistry or mass spectrometry can be identified as responsible for the reported differences between current datasets. The heavier composition of the bulk silicate Earth is consistent with previous conclusions that Si partitioned into the metal phase during metal-silicate equilibration at the time of core formation. Fixing the temperature of core formation to the peridotite liquidus and using an appropriate metal silicate fractionation factor (ε ∼0.89), the Δ³⁰Si_{BSE−meteorite∗} value from this study indicates that the Earth core contains at least 2.5 and possibly up to 16.8 wt% Si.     

Siderophile elements in Martian meteorites and implications for core formation in Mars

Noble metals, Mo, W, and 24 other elements were determined in six SNC meteorites of presumably Martian origin. Based on element correlations, representative siderophile element concentrations for the silicate mantle of Mars were inferred. From a comparison with experimentally determined metal/silicate partition coefficients of the moderately siderophile elements: Fe, Ni, Co, W, Mo, and Ga, it is concluded that equilibrium between core forming metal and silicates in Mars has occurred at high temperatures (around 2200°C) and low pressures (<1 GPa). This suggests that metal segregation occurred concurrently with rapid accretion of Mars, which is consistent with the inference from excess ¹⁸²W in Martian meteorites (Lee and Halliday, 1997). Concentrations of Ir, Os, Ru, Pt, and Au in the analyzed Martian meteorites, except ALH84001, are at a level of approximately 10⁻²–10⁻³ × CI. The comparatively high abundances of noble metals in Martian meteorites require the addition of chondritic material after core formation. The similarity in Au/La and Pt/Ca ratios between ALH84001 and the other Martian meteorites suggests crystallization of ALH84001 after complete accretion of Mars.     

Geochemical evidence for consecutive accretion of oceanic fragments: The example of the Samarka Terrane,Sikhote-Alin

The data of geochemical study of Late Triassic cherts from tectonic–sedimentary complexes from different structural levels of the Samarka Terrane are reported. It is shown that the concentration and character of distribution of the major petrogenic oxides and minor and rare-earth elements in cherts of the upper and lower structural levels differ significantly, which results from differences in the facies environments of chert deposition. All the geochemical characteristics of cherts show that their deposition proceeded in the pelagic area of sedimentation, but in different parts. The Katen Complex composing the lower structural level is the most distant from the continental margin. The closest is the Amba-Matay Complex composing the upper structural level. Based on the geochemical and biostratigraphic data and the age of accretion of paleooceanic fragments, the length of the subducted oceanic plate (>6000 km) is calculated.     

Differentiation of crusts and cores of the terrestrial planets: Lessons for the early Earth?

It now appears probable that all of the terrestrial planets underwent some form of global chemical differentiation to produce crusts, mantles, and cores of variable relative mass fractions. There is direct seismic evidence for a crust on the Moon, and indirect evidence for distinct crusts on Mars and Venus. Substantial portions of these crusts have been in place since the time that heavy bombardment of the inner solar system ceased 4 Ga ago. There is direct evidence for a sizeable core on Mars, indirect evidence for one on Mercury, and bounds on a possible small core for the Moon. Core formation is an important heat source confined to times prior to 4 Ga ago for Mercury and the Earth, but was not closely linked to crustal formation on the Moon nor, apparently, on Mars. The tectonic and volcanic histories of the surfaces of the terrestrial planets Moon, Mars, and Mercury can be used, with simple thermal history models, to restrict the earliest chemical differentiation to be shallow (outer 200–400 km) for the first two bodies and much more extensive for Mercury. Extension of these models to an Earth-size planet leads to the prediction of a hot and vigorously convecting mantle with an easily deformable crust immediately following core formation, and of the gradual development of a lithosphere and of plates with some lateral rigidity in Late Archean—Proterozoic times.     

Petrological evidence for stepwise accretion of metamorphic soles during subduction infancy (Semail ophiolite,Oman and UAE)

Metamorphic soles are tectonic slices welded beneath most large‐scale ophiolites. These slivers of oceanic crust metamorphosed up to granulite facies conditions are interpreted as forming during the first million years of intraoceanic subduction following heat transfer from the incipient mantle wedge towards the top of the subducting plate. This study reappraises the formation of metamorphic soles through detailed field and petrological work on three key sections from the Semail ophiolite (Oman and United Arab Emirates). Based on thermobarometry and thermodynamic modelling, it is shown that metamorphic soles do not record a continuous temperature gradient, as expected from simple heating by the upper plate or by shear heating as proposed in previous studies. The upper, high‐T metamorphic sole is subdivided in at least two units, testifying to the stepwise formation, detachment and accretion of successive slices from the down‐going slab to the mylonitic base of the ophiolite. Estimated peak pressure–temperature conditions through the metamorphic sole, from top to bottom, are 850°C and 1 GPa, 725°C and 0.8 GPa and 530°C and 0.5 GPa. These estimates appear constant within each unit but differing between units by 100–200°C and ~0.2 GPa. Despite being separated by hundreds of kilometres below the Semail ophiolite and having contrasting locations with respect to the ridge axis position, metamorphic soles show no evidence for significant petrological variations along strike. These constraints allow us to refine the tectonic–petrological model for the genesis of metamorphic soles, formed via the stepwise stacking of several homogeneous slivers of oceanic crust and its sedimentary cover. Metamorphic soles result not so much from downward heat transfer (ironing effect) as from progressive metamorphism during strain localization and cooling of the plate interface. The successive thrusts originate from rheological contrasts between the sole, initially the top of the subducting slab, and the peridotite above as the plate interface progressively cools. These findings have implications for the thickness, the scale and the coupling state at the plate interface during the early history of subduction/obduction systems.     

Evidence in pre-2.2 Ga paleosols for the early evolution of atmospheric oxygen and terrestrial biota

The loss of Fe from some pre-2.2 Ga paleosols has been considered by previous investigators as the best evidence for a reduced atmosphere prior to 2.2 Ga. I have examined the behavior of Fe in both pre- and post-2.2 Ga paleosols from depth profiles of Fe3+/Ti, Fe2+/Ti, and sigma Fe/Ti ratios, and Fe3+/Ti vs. Fe2+/Ti plots. This new approach reveals a previously unrecognized history of paleosols. Essentially all paleosols, regardless of age, retain some characteristics of soils formed under an oxic atmosphere, such as increased Fe3+/Ti ratios from their parental rocks. The minimum oxygen pressure (PO2) for the 3.0-2.2 Ga atmosphere is calculated to be about 1.5% of the present atmospheric level, which is the same as that for the post-1.9 Ga atmosphere. The loss of sigma Fe, common in paleosol sections of all ages, was not due to a reducing atmosphere, but to reductive dissolution of ferric hydroxides formed under an oxic atmosphere. This reductive dissolution of ferric hydroxides occurred either (1) after soil formation by hydrothermal fluids or (2) during and/or after soil formation by organic acids generated from the decay of terrestrial organic matter. Terrestrial biomass on the early continents may have been more extensive than previously recognized.     

Age and significance of core complex formation in a very curved orogen: Evidence from fission track studies in the South Carpathians (Romania)

Twenty-four new zircon and apatite fission track ages from the Getic and Danubian nappes in the South Carpathians are discussed in the light of a compilation of published fission track data. A total of 101 fission track ages indicates that the Getic nappes are generally characterized by Cretaceous zircon and apatite fission track ages, indicating cooling to near-surface temperatures of these units immediately following Late Cretaceous orogeny.The age distribution of the Danubian nappes, presently outcropping in the Danubian window below the Getic nappes, depends on the position with respect to the Cerna-Jiu fault. Eocene and Oligocene zircon and apatite central ages from the part of the Danubian core complex situated southeast of this fault monitor mid-Tertiary tectonic exhumation in the footwall of the Getic detachment, while zircon fission track data from northwest of this fault indicate that slow cooling started during the Latest Cretaceous. The change from extension (Getic detachment) to strike-slip dominated tectonics along the curved Cerna-Jiu fault allowed for further exhumation on the concave side of this strike-slip fault, while exhumation ceased on the convex side. The available fission track data consistently indicate that the change to fast cooling associated with tectonic denudation by core complex formation did not occur before Late Eocene times, i.e. long after the cessation of Late Cretaceous thrusting.Core complex formation in the Danubian window is related to a larger-scale scenario that is characterized by the NNW-directed translation, followed by a 90° clockwise rotation of the Tisza-Dacia “block” due to roll-back of the Carpathian embayment. This led to a complex pattern of strain partitioning within the Tisza-Dacia “block” adjacent to the western tip of the rigid Moesian platform. Our results suggest that the invasion of these southernmost parts of Tisza-Dacia started before the Late Eocene, i.e. significantly before the onset of Miocene-age rollback and associated extension in the Pannonian basin.