¹Christopher W. Dale, ²Thomas S. Kruijer, ¹Kevin W. Burton
Earth and Planetary Science Letters (in Press) Link to Article [http://dx.doi.org/10.1016/j.epsl.2016.11.001]
¹Department of Earth Sciences, Durham University, Durham, DH1 3LE, UK
²Institut für Planetologie, University of Münster, Wilhelm-Klemm-Str. 10, 48149 Münster, Germany
Copyright Elsevier

The higher-than-expected concentrations of highly siderophile elements (HSE) in Earth’s mantle most likely indicate that Earth received a small amount of late accreted mass after core formation had ceased, known as the ‘late veneer’. Small 182W excesses in the Moon and in some Archaean rocks – such as the source of 3.8 billion-year-old Isua magmatics – also appear consistent with the late veneer hypothesis, with a lower proportion received. However, 182W anomalies can also relate to other processes, including early mantle differentiation. To better assess the origin of these W isotope anomalies – and specifically whether they relate to the late veneer – we have determined the HSE abundances and 182W compositions of a suite of mafic to ultramafic rocks from Isua, from which we estimate HSE abundances in the source mantle and ultimately constrain the 182W composition of the pre-late veneer mantle.

Our data suggest that the Isua source mantle had HSE abundances at around 50–65% of the present-day mantle, consistent with partial, but not complete, isolation from the late veneer. These data also indicate that at least part of the late veneer had been added and mixed into the mantle at the time the Isua source formed, prior to 3.8 Ga. For the same Isua samples we obtained a 13±4 ppm13±4 ppm182W excess, compared to the modern terrestrial mantle, in excellent agreement with previous data. Using combined 182W and HSE data we show that the Moon, Isua, and the present-day bulk silicate Earth (BSE) produce a well-defined co-variation between 182W composition and the mass fraction of late-accreted mass, as inferred from HSE abundances. This co-variation is consistent with the calculated effects of various late accretion compositions on the HSE and 182W signatures of Earth’s mantle. The empirical relationship, therefore, implies that the Moon, Isua source and BSE received increasing proportions of late-accreted mass, supporting the idea of disproportional late accretion to the Earth and Moon, and consistent with the interpretation that the lunar 182W value of 27±4 ppm27±4 ppm represents the composition of Earth’s mantle before the late veneer was added. In this case, the Isua source can represent ambient mantle after the giant moon-forming impact, into which only a part of Earth’s full late veneer was mixed, rather than an isotopically distinct mantle domain produced by early differentiation, which would probably require survival through the giant Moon-forming impact.

¹Maximilien J. Verdier-Paoletti, ²Yves Marrocchi, ²Guillaume Avice, ¹Mathieu Roskosz, ²Andrey Gurenko, ^1,3Matthieu Gounelle
Earth and Planetary Science Letters (in Press) Link to Article [http://dx.doi.org/10.1016/j.epsl.2016.10.055]
¹IMPMC, MNHN, UPMC, UMR CNRS 7590, 61 rue Buffon, 75005 Paris, France
²CRPG, CNRS, Université de Lorraine, UMR 7358, Vandoeuvre les Nancy, F-54501, France
³Institut Universitaire de France, Maison des Universités, 103 bd. Saint-Michel, 75005 Paris, France
Copyright Elsevier

We report a systematic oxygen isotopic survey of Ca-carbonates in nine different CM chondrites characterized by different degrees of alteration, from the least altered known to date (Paris, 2.7–2.8) to the most altered (ALH 88045, CM1). Our data define a continuous trend that crosses the Terrestrial Fractionation Line (TFL), with a general relationship that is indistinguishable within errors from the trend defined by both matrix phyllosilicates and bulk O-isotopic compositions of CM chondrites. This bulk-matrix-carbonate (BMC) trend does not correspond to a mass-dependent fractionation (i.e., slope 0.52) as it would be expected during fluid circulation along a temperature gradient. It is instead a direct proxy of the degree of O-isotopic equilibration between 17,18O-rich fluids and 16O-rich anhydrous minerals. Our O-isotopic survey revealed that, for a given CM, no carbonate is in O-isotopic equilibrium with its respective surrounding matrix. This precludes direct calculation of the temperature of carbonate precipitation. However, the O-isotopic compositions of alteration water in different CMs (inferred from isotopic mass-balance calculation and direct measurements) define another trend (CMW for CM Water), parallel to BMC but with a different intercept. The distance between the BMC and CMW trends is directly related to the temperature of CM alteration and corresponds to average carbonates and serpentine formation temperatures of 110 °C and 75 °C, respectively. However, carbonate O-isotopic variations around the BMC trend indicate that they formed at various temperatures ranging between 50 and 300 °C, with 50% of the carbonates studied here showing precipitation temperature higher than 100 °C. The average Δ17O and the average carbonate precipitation temperature per chondrite are correlated, revealing that all CMs underwent similar maximum temperature peaks, but that altered CMs experienced protracted carbonate precipitation event(s) at lower temperatures than the least altered CMs. Our data suggest that the Δ17O value of Ca-carbonates could be a reliable proxy of the degree of alteration experienced by CM chondrites.