Solar system Nd isotope heterogeneity: Insights into nucleosynthetic components and protoplanetary disk evolution

Abstract

High-precision Nd isotope measurements of a diverse set of solar system materials including bulk chondrites and achondrites reveal that their Nd isotope composition is governed by several distinct nucleosynthetic components. The full spectrum of non-radiogenic, mass-independent Nd isotope compositions of solar system materials is best explained by heterogeneous distribution of at least three nucleosynthetic components - the classical s-process component, pure p-process component and an anomalous, previously unidentified s-/r-process component. The ${}^{142}$Nd/${}^{144}$Nd variations in solar system reservoirs specifically fall into three distinct trends - those that result from variations in the s-process component, those resulting from variations in the pure p-process component, and those resulting from coupled s-process and p-process variations. The $\mu$${}^{148}$Nd value, a proxy for s-process variations, as well as $\mu$${}^{142}$Nd that has been corrected for s-process heterogeneity to reflect p-process variations, broadly show an inverse correlation with $\epsilon$${}^{54}$Cr. The linearity in $\mu$${}^{148}$Nd - $\epsilon$${}^{54}$Cr space for inner solar system bodies, CI chondrite and Allende-type CAIs possibly suggests the thermally labile nature of some s-process carrier grains unlike the mainstream refractory s-process SiC grains. The p-process carrier for Nd is inferred to be a refractory phase enriched in inner solar system materials through thermal processing. The bulk meteorite regression lines that specifically correspond to s- and p-process heterogeneity, largely define $\mu$${}^{142}$Nd intercepts indistinguishable from terrestrial composition within analytical uncertainty, ruling out resolvable radiogenic $\mu$${}^{142}$Nd excess on Earth that cannot otherwise be accounted for by nucleosynthetic heterogeneity.

Introduction

The short-lived ${}^{146}$Sm - ${}^{142}$Nd decay system is a powerful tool for studying early episodes of silicate differentiation in terrestrial planets (e.g.: Caro et al., 2006, Debaille et al., 2007, Brandon et al., 2009). A conventional assumption that underpinned the interpretation of ${}^{142}$Nd/${}^{144}$Nd variations is that primitive chondrites represent proxies of the material from which inner solar system planets formed (Boyet and Carlson, 2005, Bouvier et al., 2008). The $\sim$ 20 ppm excess in the ${}^{142}$Nd/${}^{144}$Nd ratio of modern terrestrial mantle relative to chondrites was interpreted as evidence for an early global silicate differentiation during the lifetime of ${}^{146}$Sm (Boyet and Carlson, 2005). However, it was demonstrated that chondrites show significant variability in their ${}^{142}$Nd/${}^{144}$Nd ratios that do not correlate with Sm/Nd ratios suggesting that this variability could reflect nucleosynthetic heterogeneity (Andreasen and Sharma, 2006, Carlson et al., 2007, Gannoun et al., 2011). Enstatite chondrites have ${}^{142}$Nd/${}^{144}$Nd ratios marginally lower than modern terrestrial mantle ($\sim$ −10 ppm), whereas ordinary and carbonaceous chondrites have ${}^{142}$Nd/${}^{144}$Nd ratios lower than that of modern terrestrial mantle by up $\sim$ 20 to 40 ppm. Isotope anomalies resulting from variable incorporation of nucleosynthetic components are increasingly known for a wide range of elements in solar system materials (e.g.: Trinquier et al., 2009, Burkhardt et al., 2011, Paton et al., 2013, Akram et al., 2015). Though earlier studies recognized that ${}^{142}$Nd/${}^{144}$Nd variations could be the result of nucleosynthetic heterogeneity, at least in carbonaceous chondrites, poor analytical precision available at the time on stable Nd isotopes precluded identification of the exact nucleosynthetic components involved. The apparent ${}^{142}$Nd/${}^{144}$Nd excess for modern terrestrial mantle relative to ordinary chondrites continued to be interpreted as radiogenic in origin (e.g.: Rizo et al., 2012, Debaille et al., 2013). A number of high-precision studies have recently showed that ${}^{142}$Nd/${}^{144}$Nd variations in primitive meteorites correlate with their ${}^{148}$Nd/${}^{144}$Nd and ${}^{150}$Nd/${}^{144}$Nd ratios demonstrating that the elevated, non-chondritic ${}^{142}$Nd/${}^{144}$Nd on Earth is a result of nucleosynthetic s-process heterogeneity, and not the result of an early global differentiation event that left accessible Earth’s mantle with a superchondritic Sm/Nd ratio (Burkhardt et al., 2016, Bouvier and Boyet, 2016, Fukai and Yokoyama, 2017).

Though most bulk solar system materials fall on s-process heterogeneity trends in Nd isotope diagrams, carbonaceous chondrites remain a notable exception in diagrams involving ${}^{142}$Nd. Burkhardt et al. (2016) attributed the departure of Allende carbonaceous chondrite from the s-process and terrestrial mixing line to admixture of calcium-aluminium-rich inclusions (CAIs). Various studies have demonstrated that at least some of Allende-CAIs are characterized by deficits in ${}^{148}$Nd/${}^{144}$Nd and ${}^{150}$Nd/${}^{144}$Nd, as well as in ${}^{142}$Nd/${}^{144}$Nd (Brennecka et al., 2013, Burkhardt et al., 2016, Bouvier and Boyet, 2016). Notably, Allende also contains CAIs that are indistinguishable from modern terrestrial mantle composition (Bouvier and Boyet, 2016). S-process excesses or r-process deficits relative to terrestrial composition typically result in deficits in ${}^{145}$Nd/${}^{144}$Nd, ${}^{148}$Nd/${}^{144}$Nd and ${}^{150}$Nd/${}^{144}$Nd and correlated excesses in ${}^{142}$Nd/${}^{144}$Nd. This is because ${}^{142}$Nd is primarily an s-process nuclide while ${}^{145}$Nd, ${}^{148}$Nd and ${}^{150}$Nd are either pure r-process nuclides or receive a major contribution from r-process (Arlandini et al., 1999, Bisterzo et al., 2011). The peculiar isotope anomaly pattern in Allende CAIs testifies to possible p-process ${}^{142}$Nd deficits overprinting s-process excesses/r-process deficits as ${}^{142}$Nd receives $\sim$ 4% contribution from p-process according to nucleosynthetic models (Arlandini et al., 1999). The p-process deficit in Allende-CAIs is also reflected in the fact that they carry an $\sim$ 250 ppm deficit in ${}^{144}$Sm, a pure p-process isotope (Brennecka et al., 2013, Burkhardt et al., 2016). Bulk carbonaceous chondrites have also been shown to carry deficits in ${}^{144}$Sm of $\sim$ 100 ppm (Andreasen and Sharma, 2006, Carlson et al., 2007). This points to the involvement of multiple nucleosynthetic components in defining the full spectrum of Nd isotope variations in solar system materials.

In this study, we take advantage of the MC-ICPMS Nd isotope analytical protocol reported in Saji et al. (2016) and present high-precision Nd isotope data for a suite of chondrites from different groups as well as an achondrite and a martian meteorite. We aim to better define the bulk silicate Earth Nd-isotope composition in the context of diverse heterogeneity trends in Nd-isotope space. Given the distinct nucleosynthetic components involved in defining the Nd-isotope composition of solar system materials, the results are used to place constraints on the existing models for origin of planetary-scale isotope variations.