Os-isotopic Compositions of Peridotite Xenoliths from the Oceanic Mantle: Implications for the Age of Isotopic Domains in the Oceanic Mantle.

Abstract:

Os-isotopic compositions of abyssal peridotites and peridotite xenoliths from oceanic hotpots that sample the convecting mantle extend to relatively unradiogenic compositions. However, they do not preserve a record of early-formed (Hadean and Archean) depleted mantle domains, either by earlier cycles of ridge-related depletion, continent extraction, or subcontinental lithospheric mantle erosion. The lack of preservation of early-formed (Hadean and Archean) depleted Os-isotopic compositions is consistent with the lack of preservation of Hadean ¹⁴²Nd/¹⁴⁴Nd variability in the modern convecting mantle, but is in stark contrast to the existence of early-formed (early-Hadean), heterogeneous ¹²⁹Xe/¹³⁰Xe isotopic anomalies in the modern mantle.

Mukhopadhay (Nature, 2012) suggested that the erasure of ¹⁴²Nd/¹⁴⁴Nd, but not ¹²⁹Xe/¹³⁰Xe, anomalies from the convecting mantle may be due to the small magnitude of the ¹⁴²Nd/¹⁴⁴Nd anomalies (< 0.004% variability has been observed in terrestrial Archean terraines) compared to ¹²⁹Xe/¹³⁰Xe anomalies (>10% variability observed) because the smaller ¹⁴²Nd/¹⁴⁴Nd anomalies would have been more easily erased than the larger magnitude ¹²⁹Xe/¹³⁰Xe anomalies. This model does not work for the Re-Os system because the magnitude of Os-isotopic heterogeneities in the mantle are large, yet early-formed Os-isotopic signatures have been erased. For example, the ¹⁸⁷Os/¹⁸⁸Os of early-formed depleted mantle at 4.55 Ga was ~0.095, which is >25% lower than the modern mantle. Given that there were substantial amounts of mantle partial melting throughout the Hadean and Archean, it is hard to understand how some refractory residues retaining some vestige of these low, early-formed Os isotopic compositions have not have been preserved in the modern mantle. Therefore, the lack of preservation of early-formed, large magnitude ¹⁸⁷Os/¹⁸⁸Os excursions in the modern convecting mantle suggests that the preservation of early geochemical heterogeneities was not necessarily a function of the original geochemical anomaly. We explore alternative solutions to the paradox of preservation of Hadean ¹²⁹Xe/¹³⁰Xe anomalies, but the lack of Hadean ¹⁴²Nd/¹⁴⁴Nd and ¹⁸⁷Os/¹⁸⁸Os signatures, in the modern oceanic mantle.