Stable Isotopes of Platinum: Measurement and Application to Constraining the Formation and Differentiation of Earth

<p>A wide range of novel, non-traditional, stable isotope systems have been developed over the last decade, largely as a result of the advent of multiple-collector inductively coupled plasma mass spectrometry (MC-ICPMS), and continue to provide valuable new insights in the earth, environmental and planetary sciences. The platinum (Pt) stable isotope system represents a potentially powerful but, as yet, unexplored addition to this suite of stable isotope tracers. Pt has six naturally occurring isotopes, and can exist in a range of oxidation states. The geochemical behaviour of Pt coupled with the relatively large mass difference (ca. 2%) between the abundant heavy and light isotopes and its variable oxidation states leads to potential applications in tracing a range of natural processes. In particular, the strong elemental partitioning of Pt between metals and silicates makes the Pt stable isotope system uniquely suited to tracing processes of Earth’s accretion and differentiation. This study aims to develop new techniques for measurement of Pt stable isotopes in geological samples, and to apply these to terrestrial and meteorite samples to attempt to resolve outstanding questions relating to Earth’s early evolution. A technique was developed for measurement of Pt stable isotope ratios using multiple collector inductively coupled plasma mass spectrometry (MCICPMS), employing a ¹⁹⁶Pt–¹⁹⁸Pt double-spike to correct for instrumental mass fractionation. Results are reported in terms of δ¹⁹⁸Pt, which represents the per mil difference in the ¹⁹⁸Pt/¹⁹⁴Pt ratio from the IRMM-010 Pt isotope standard. A range of analytical tests were conducted and show that this approach has a reproducibility of ca. ±0.04 %∘ on δ¹⁹⁸Pt (i.e., ±0.01%∘ amu⁻¹) for Pt solution standards, and is insensitive to minor amounts of matrix that may be retained after chemical purification of Pt. Measurements of Pt solution standards conducted using two different MC-ICPMS instruments showed resolvable variations, which suggest that natural fractionations exist that exceed the reproducibility of the technique. Techniques were also developed for dissolution of natural samples and chemical separation of Pt. Geological standards were digested using a nickel sulphide fire assay technique, which pre-concentrates the highly siderophile elements in a NiS bead that is readily dissolved in acid. This was followed by chemical separation of Pt from digested samples using anion exchange chemical techniques. Elution curves were constructed for a range of synthetic rock matrices. These tests show that Pt separation is achieved with >90% Pt yield and ca. 95% purity. Analytical tests show that this level of Pt separation is sufficient for accurate determination of Pt stable isotope ratios by double-spike MC-ICPMS. These techniques were then applied to 11 international geological standard reference materials representing mantle peridotites, igneous samples, and Pt ore materials. The reproducibility in natural samples was determined by processing multiple replicate digestions of a standard reference material, and was shown to be ca. ±0.08%∘ (2 sd). Pt stable isotope data for the full set of reference materials have a range of δ¹⁹⁸Pt values with offsets of up to 0.40%∘ from the IRMM-010 standard, which are readily resolved with this technique. Mantle samples yielded the lightest (most negative) isotopic compositions of the terrestrial standards, with igneous and Pt ore samples defining a continuous trend towards zero, which is consistent with the IRMM-010 standard being derived from a Pt ore. These results demonstrate the potential of the Pt isotope system as a tracer in geochemical systems. The techniques developed above were then applied to investigate an outstanding problem relating to Earth’s accretion and differentiation. Highly siderophile elements (HSE) are strongly partitioned into the cores of terrestrial planets during core formation, and the abundances of HSE in Earth’s mantle compared with primitive meteorites have provided key constraints on models of Earth’s early evolution. Two leading models to explain the HSE abundances in the silicate Earth involve either a late-veneer of chondritic material that was added after core formation or core formation in a deep magma ocean. The platinum (Pt) stable isotope system represents a novel tool for investigating these processes. Using the techniques developed above, Pt stable isotope ratios were measured in a range of meteorite samples, including enstatite, ordinary and carbonaceous chondrites, primitive achondrites, achondrites and iron meteorites, as well as additional terrestrial mantle xenolith samples. Our data set reveals that the Pt stable isotopic composition of Earth’s mantle overlaps with all of the chondrite groups. Primitive achondrite and ureilite samples revealed the heaviest compositions of all meteorite groups. These data suggest that metal–silicate differentiation produces an isotopic fractionation for Pt, with heavy isotopes being preferentially retained in the silicate phase. Thus, Earth’s mantle is expected to have been significantly enriched in the heavy isotopes of Pt during core formation, even if metal–silicate differentiation took place in a magma ocean. The absence of a large fractionation between chondrites, representing the composition of the undifferentiated Earth, and the mantle suggests that the signature of core formation in the mantle has been subsequently overprinted. Considering the overlap between the Pt stable isotopic compositions of the mantle and chondrites, the most likely means for overprinting the composition of the mantle is by addition of a chondritic late-veneer. Mixing calculations show that addition of 0.5% of Earth’s mass by a late-veneer of chondritic material would be sufficient to overprint highly fractionated Pt stable isotope signatures resulting from core-formation.</p>

Diamond ascent by rift-driven disruption of cratonic mantle keels

Diamonds are erupted at Earth’s surface in volatile-rich magmas called kimberlites1,2,3. These enigmatic magmas, originating from depths exceeding 150 kilometres in Earth’s mantle1, occur in stable cratons and in pulses broadly synchronous with supercontinent cyclicity4. Whether their mobilization is driven by mantle plumes5 or mechanical weakening of cratonic lithosphere4,6 remains unclear. Here we show that most kimberlites spanning the past billion years erupted approximately 25 million years after the onset of continental fragmentation, suggesting an association with rifting processes. Our dynamic models show that physically steep lithosphere-asthenosphere boundaries formed during terminal rifting (necking) generate convective instabilities in the asthenosphere that slowly migrate many hundreds of kilometres inboard of the rift, causing destabilization of cratonic mantle keel tens of kilometres thick. Displaced lithosphere is replaced by hot, upwelling asthenosphere in the return flow, causing partial melting of carbonated mantle and variable assimilation of lithospheric material. The resulting small-volume kimberlite magmas ascend rapidly and adiabatically, exsolving amounts of carbon dioxide (CO2) that are consistent with independent constraints7. Our model reconciles diagnostic kimberlite features including association with cratons and geochemical characteristics that implicate a common asthenospheric mantle source contaminated by cratonic lithosphere8. Together, these results provide a quantitative and mechanistic link between kimberlite episodicity and supercontinent cycles via progressive disruption of cratonic keels.

Stable Isotopes of Platinum: Measurement and Application to Constraining the Formation and Differentiation of Earth

<p>A wide range of novel, non-traditional, stable isotope systems have been developed over the last decade, largely as a result of the advent of multiple-collector inductively coupled plasma mass spectrometry (MC-ICPMS), and continue to provide valuable new insights in the earth, environmental and planetary sciences. The platinum (Pt) stable isotope system represents a potentially powerful but, as yet, unexplored addition to this suite of stable isotope tracers. Pt has six naturally occurring isotopes, and can exist in a range of oxidation states. The geochemical behaviour of Pt coupled with the relatively large mass difference (ca. 2%) between the abundant heavy and light isotopes and its variable oxidation states leads to potential applications in tracing a range of natural processes. In particular, the strong elemental partitioning of Pt between metals and silicates makes the Pt stable isotope system uniquely suited to tracing processes of Earth’s accretion and differentiation. This study aims to develop new techniques for measurement of Pt stable isotopes in geological samples, and to apply these to terrestrial and meteorite samples to attempt to resolve outstanding questions relating to Earth’s early evolution. A technique was developed for measurement of Pt stable isotope ratios using multiple collector inductively coupled plasma mass spectrometry (MCICPMS), employing a ¹⁹⁶Pt–¹⁹⁸Pt double-spike to correct for instrumental mass fractionation. Results are reported in terms of δ¹⁹⁸Pt, which represents the per mil difference in the ¹⁹⁸Pt/¹⁹⁴Pt ratio from the IRMM-010 Pt isotope standard. A range of analytical tests were conducted and show that this approach has a reproducibility of ca. ±0.04 %∘ on δ¹⁹⁸Pt (i.e., ±0.01%∘ amu⁻¹) for Pt solution standards, and is insensitive to minor amounts of matrix that may be retained after chemical purification of Pt. Measurements of Pt solution standards conducted using two different MC-ICPMS instruments showed resolvable variations, which suggest that natural fractionations exist that exceed the reproducibility of the technique. Techniques were also developed for dissolution of natural samples and chemical separation of Pt. Geological standards were digested using a nickel sulphide fire assay technique, which pre-concentrates the highly siderophile elements in a NiS bead that is readily dissolved in acid. This was followed by chemical separation of Pt from digested samples using anion exchange chemical techniques. Elution curves were constructed for a range of synthetic rock matrices. These tests show that Pt separation is achieved with >90% Pt yield and ca. 95% purity. Analytical tests show that this level of Pt separation is sufficient for accurate determination of Pt stable isotope ratios by double-spike MC-ICPMS. These techniques were then applied to 11 international geological standard reference materials representing mantle peridotites, igneous samples, and Pt ore materials. The reproducibility in natural samples was determined by processing multiple replicate digestions of a standard reference material, and was shown to be ca. ±0.08%∘ (2 sd). Pt stable isotope data for the full set of reference materials have a range of δ¹⁹⁸Pt values with offsets of up to 0.40%∘ from the IRMM-010 standard, which are readily resolved with this technique. Mantle samples yielded the lightest (most negative) isotopic compositions of the terrestrial standards, with igneous and Pt ore samples defining a continuous trend towards zero, which is consistent with the IRMM-010 standard being derived from a Pt ore. These results demonstrate the potential of the Pt isotope system as a tracer in geochemical systems. The techniques developed above were then applied to investigate an outstanding problem relating to Earth’s accretion and differentiation. Highly siderophile elements (HSE) are strongly partitioned into the cores of terrestrial planets during core formation, and the abundances of HSE in Earth’s mantle compared with primitive meteorites have provided key constraints on models of Earth’s early evolution. Two leading models to explain the HSE abundances in the silicate Earth involve either a late-veneer of chondritic material that was added after core formation or core formation in a deep magma ocean. The platinum (Pt) stable isotope system represents a novel tool for investigating these processes. Using the techniques developed above, Pt stable isotope ratios were measured in a range of meteorite samples, including enstatite, ordinary and carbonaceous chondrites, primitive achondrites, achondrites and iron meteorites, as well as additional terrestrial mantle xenolith samples. Our data set reveals that the Pt stable isotopic composition of Earth’s mantle overlaps with all of the chondrite groups. Primitive achondrite and ureilite samples revealed the heaviest compositions of all meteorite groups. These data suggest that metal–silicate differentiation produces an isotopic fractionation for Pt, with heavy isotopes being preferentially retained in the silicate phase. Thus, Earth’s mantle is expected to have been significantly enriched in the heavy isotopes of Pt during core formation, even if metal–silicate differentiation took place in a magma ocean. The absence of a large fractionation between chondrites, representing the composition of the undifferentiated Earth, and the mantle suggests that the signature of core formation in the mantle has been subsequently overprinted. Considering the overlap between the Pt stable isotopic compositions of the mantle and chondrites, the most likely means for overprinting the composition of the mantle is by addition of a chondritic late-veneer. Mixing calculations show that addition of 0.5% of Earth’s mass by a late-veneer of chondritic material would be sufficient to overprint highly fractionated Pt stable isotope signatures resulting from core-formation.</p>

Breaking of Henry’s law for sulfide liquid–basaltic melt partitioning of Pt and Pd

Platinum group elements are invaluable tracers for planetary accretion and differentiation and the formation of PGE sulfide deposits. Previous laboratory determinations of the sulfide liquid–basaltic melt partition coefficients of PGE ($${D}_{PGE}^{SL/SM}$$ D P G E S L / S M ) yielded values of 102–109, and values of >105 have been accepted by the geochemical and cosmochemical society. Here we perform measurements of $${D}_{Pt,\,Pd}^{SL/SM}$$ D P t , P d S L / S M at 1 GPa and 1,400 °C, and find that $${D}_{Pt,\,Pd}^{SL/SM}$$ D P t , P d S L / S M increase respectively from 3,500 to 3.5 × 105 and 1,800 to 7 × 105, as the Pt and Pd concentration in the sulfide liquid increases from 60 to 21,000 ppm and 26 to 7,000 ppm, respectively, implying non-Henrian behavior of the Pt and Pd partitioning. The use of $${D}_{Pt,\,Pd}^{SL/SM}$$ D P t , P d S L / S M values of 2,000–6,000 well explains the Pt and Pd systematics of Earth’s mantle peridotites and mid-ocean ridge basalts. Our findings suggest that the behavior of PGE needs to be reevaluated when using them to trace planetary magmatic processes.