Abstract
Several studies have suggested that the Earth's upper mantle is slightly enriched in light molybdenum isotopes relative to bulk Earth, defined by chondrites, but there is no consensus on the presence of this subtle but potentially notable signature. To establish better whether or not the 98Mo/95Mo of Earth's upper mantle is indeed sub-chondritic, we have analysed hand-picked glasses of depleted (i.e. chondrite normalised La/Sm
Our MORB data show that resolvably sub-chondritic Mo isotope compositions are common in the upper mantle. Moreover, an appropriately weighted average MoNIST SRM 3134 of depleted and enriched MORB, taken from this study and the literature, yields an estimated mantle value of −0.20±0.01‰, indicating that the upper mantle as a whole is sub-chondritic. Since prior work demonstrates that core formation will not create a residual silicate reservoir with a sub-chondritic MoNIST SRM 3134, we propose that this feature is a result of recycling oceanic crust with low MoNIST SRM 3134 because of Mo isotope fractionation during subduction dehydration. Such an origin is in keeping with the sub-chondritic Th/U and low Ce/Pb of the depleted mantle, features which cannot be explained by simple melt extraction. We present mass balance models of the plate tectonic cycle that quantitatively illustrate that the MoNIST SRM 3134 of the Earth's mantle can be suitably lowered by such oceanic crustal recycling. Our Mo isotope study adds to the notion that the depleted mantle has been substantially modified by geodynamic cycling of subduction-processed oceanic crust.
Our MORB data show that resolvably sub-chondritic Mo isotope compositions are common in the upper mantle. Moreover, an appropriately weighted average MoNIST SRM 3134 of depleted and enriched MORB, taken from this study and the literature, yields an estimated mantle value of −0.20±0.01‰, indicating that the upper mantle as a whole is sub-chondritic. Since prior work demonstrates that core formation will not create a residual silicate reservoir with a sub-chondritic MoNIST SRM 3134, we propose that this feature is a result of recycling oceanic crust with low MoNIST SRM 3134 because of Mo isotope fractionation during subduction dehydration. Such an origin is in keeping with the sub-chondritic Th/U and low Ce/Pb of the depleted mantle, features which cannot be explained by simple melt extraction. We present mass balance models of the plate tectonic cycle that quantitatively illustrate that the MoNIST SRM 3134 of the Earth's mantle can be suitably lowered by such oceanic crustal recycling. Our Mo isotope study adds to the notion that the depleted mantle has been substantially modified by geodynamic cycling of subduction-processed oceanic crust.
| Original language | English |
|---|---|
| Article number | 117760 |
| Number of pages | 11 |
| Journal | Earth and Planetary Science Letters |
| Volume | 595 |
| Early online date | 11 Aug 2022 |
| DOIs | |
| Publication status | Published - 1 Oct 2022 |
Bibliographical note
Funding Information:We thank Chris Coath for indispensable help with the mass spectrometer and Carolyn Taylor for her contributions to sample preparations. Insightful reviews by Andreas Stracke and an anonymous reviewer greatly helped to improve our manuscript, while we appreciated Fred Moynier's fair and quick editorial handling. Kevin Burton kindly provided splits of samples 45N and ALV 518-3-2. We acknowledge support from NERC grants NE/L007428/1 and NE/H023933/1 as well as from NERC studentship NE/I528250/1 to Kate Hibbert.
Publisher Copyright:
© 2022