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Osmium (Os), strontium (Sr), neodymium (Nd) and lead (Pb) isotopes have been measured on a suite of aphyric basalts from 12 islands of the Austral–Cook island archipelago, an area which exhibits a range in Pb isotope compositions that encompasses almost the entire range displayed by ocean island basalts (OIB). Although the samples have Os concentrations (1.69–34.80 ppt) at the lower end of the range measured for OIB, they display a range of initial 187Os/188Os ratios (between 0.1279 and 0.1594) similar to that defined by olivine–phyric, Os-rich OIB. Positive Os–Nd, Os–Pb and negative Os–Sr isotope correlations are documented, indicating that the isotopic arrays represent various proportions of mixing between a HIMU-type end-member represented by Mangaia and Tubuai islands and characterized by radiogenic Os and Pb isotopic compositions, and an end-member represented by Rarotonga island which is characterized by unradiogenic Os and intermediate Sr, Nd and Pb isotopic compositions. The HIMU signature of the mantle component involved in Tubuai–Mangaia mantle sources requires long-term enrichments of U and Th relative to Pb and Re relative to Os, without associated increase in Rb/Sr, that are consistent with recycled oceanic crust. The end-member represented by Rarotonga basalts shows Os, Sr, Nd, and Pb isotopic signatures similar to those presumed for the ‘bulk silicate earth’ (BSE), which cannot be obtained by mixing the four mantle components (DMM, HIMU and EM1 and 2) generally used to circumscribe the Sr–Nd–Pb isotopic data of OIB. The primitive-like isotopic characters of this end-member might be ascribed to the presence of undepleted material from a lower segment of the mantle in the source of the Austral–Cook island basalts (and more specifically Rarotonga basalts); however, such a hypothesis is challenged by both the absence of a primordial 3He signature and the non-primitive Ce/Pb and Nb/U values for the Austral–Cook island basalts. Alternatively, assuming that the primitive-like isotopic composition of the Rarotonga samples reflects mixing proportions between the HIMU component and a mantle component characterized by unradiogenic Os, Nd and Pb and radiogenic Sr isotopic composition relative to BSE, the involvement of recycled, old subcontinental lithosphere in the genesis of this component should then be considered.

The early development of Mars is of enormous interest, not just in its own right, but also because it provides unique insights into the earliest history of the Earth, a planet whose origins have been all but obliterated. Mars is not as depleted in moderately volatile elements as are other terrestrial planets. Judging by the data for Martian meteorites it has Rb/Sr ≈ 0.07 and K/U ≈ 19,000, both of which are roughly twice as high as the values for the Earth. The mantle of Mars is also twice as rich in Fe as the mantle of the Earth, the Martian core being small (∼20% by mass). This is thought to be because conditions were more oxidizing during core formation. For the same reason a number of elements that are moderately siderophile on Earth such as P, Mn, Cr and W, are more lithophile on Mars. The very different apparent behavior of high field strength (HFS) elements in Martian magmas compared to terrestrial basalts and eucrites may be related to this higher phosphorus content. The highly siderophile element abundance patterns have been interpreted as reflecting strong partitioning during core formation in a magma ocean environment with little if any late veneer. Oxygen isotope data provide evidence for the relative proportions of chondritic components that were accreted to form Mars. However, the amount of volatile element depletion predicted from these models does not match that observed — Mars would be expected to be more depleted in volatiles than the Earth. The easiest way to reconcile these data is for the Earth to have lost a fraction of its moderately volatile elements during late accretionary events, such as giant impacts. This might also explain the non-chondritic Si/Mg ratio of the silicate portion of the Earth. The lower density of Mars is consistent with this interpretation, as are isotopic data. 87Rb-87Sr, 129I-129Xe, 146Sm-142Nd, 182Hf-182W, 187Re-187Os, 235U-207Pb and 238U-206Pb isotopic data for Martian meteorites all provide evidence that Mars accreted rapidly and at an early stage differentiated into atmosphere, mantle and core. Variations in heavy xenon isotopes have proved complicated to interpret in terms of 244Pu decay and timing because of fractionation thought to be caused by hydrodynamic escape. There are, as yet, no resolvable isotopic heterogeneities identified in Martian meteorites resulting from 92Nb decay to 92Zr, consistent with the paucity of perovskite in the martian interior and its probable absence from any Martian magma ocean. Similarly the longer-lived 176Lu-176Hf system also preserves little record of early differentiation. In contrast W isotope data, Ba/W and time-integrated Re/Os ratios of Martian meteorites provide powerful evidence that the mantle retains remarkably early heterogeneities that are vestiges of core metal segregation processes that occurred within the first 20 Myr of the Solar System. Despite this evidence for rapid accretion and differentiation, there is no evidence that Mars grew more quickly than the Earth at an equivalent size. Mars appears to have just stopped growing earlier because it did not undergo late stage (>20 Myr), impacts on the scale of the Moon-forming Giant Impact that affected the Earth.

According to their chemical composition, rich in volatile compounds, comets are thought to be primitive materials. They may provide prime samples for the study of nucleosynthetic components of the solar system and of the processes occurring during the formation of the outer planets. Their origin is largely a matter of conjecture. Chromium isotopic measurements in carbonaceous chondrites illustrate how the non-volatile part of cometary material can be investigated both for isotopic heterogeneity and for the extinct nuclide 53Mn. Questions like the possible presence of 26Al as a heat source can also be addressed by these measurements.

Abstract—A set of iron meteorites was investigated for Re‐Os isotopes and provides a well‐defined isochron age of 4.624 ± 0.017 Ba and an initial 187Os/188Os ratio of 0.095636 ± 0.00009. Actual ages may be a few percent lower due to uncertainty on the 187Re decay constant. Within the small sample number presented here, there is no evidence for age differences between classes. A more detailed study of the meteorite Kodaïkanal shows that the metal reservoir of this meteorite formed approximately at the same time as the other iron meteorites, in contrast with the silicate inclusions which display a formation age of 3.67 Ba with all other chronometers. A collisional origin is in agreement with the data on this meteorite. Major secondary events do not affect the Re‐Os system at the bulk rock scale in metal as it is the sole significant reservoir of these two elements. A Re‐Os and Rb‐Sr investigation of meteoritic troilite exhibits disturbed chronometric systems, which we attribute to the terrestrial history of the meteorite.

The 87Rb–87Sr isochrons for H-, E- and LL-, chondrites are undistinguishable from each other. The joint isochron yields an age of 4,498 ± 15 Myr if the rubidium decay constant λ 87Rb = 1.42×10−11yr−1, and a strontium initial ratio 87Rb/87Sr = 0.69885±0.00010. For the age to agree with U–Th–Pb data, λ 87Rb would have to be changed to 1.402±0.008×10−11yr−1

Differentiated meteorites have numerous similarities with igneous rocks from the Earth and the Moon. They are of primordial importance in our understanding of magmatic processes in the early Solar System. The howardites constitute a particular class of differentiated meteorite as they are mechanical mixtures1–7(polymict breccia) of components from two other classes—eucrites and diogenites. The eucrites are representative of magmatic liquids and the diogenites are the corresponding cumulates3. This relationship is supported by oxygen isotopic data8. The 87Rb–87Sr chronology of eucrites9,10shows that these objects result from a very early igneous activity near the surface of their parent body (<150 Myr after formation of the Solar System). As for howardites, within the same meteorite Kapoeta, basaltic clasts have been dated by the internal Rb–Sr isochron at 4,400 Myr and around 3,500 Myr (refs 11, 12). 39Ar–40Ar dating of these clasts provides evidence for their initial crystallisation 4,500 Myr ago, and for their independent history until the final lithification of Kapoeta, probably as late as 2,200 Myr ago13. Due to this complexity their initial age of crystallisation, and in particular, their 87Sr/86Sr initial ratio is not precisely known. We report here our investigation of the Binda howardite which seems to have had a simpler history; it is the only one which is a monomict breccia. The component representing the cumulate fraction seems to be in equilibrium with that which represents the eucritic melt3. Therefore one might expect to date the howardites formation by dating this meteorite, assuming that they all formed in the same parent body.