Article ID | Journal | Published Year | Pages | File Type |
---|---|---|---|---|
5784297 | Lithos | 2016 | 11 Pages |
â¢Olivine diamond inclusions have low water and trace element contents.â¢Trace-element contents in olivine advance our hypothesis for a protogenetic origin.â¢Olivine diamond inclusions preserve “micro-samples” of a dry cratonic lithosphere.â¢Olivine thermometers confirm previous estimates for the Siberian mantle.â¢Olivine water content is only one of the controlling factors of cratonic root strength.
A subject of continuing debate is how the Earth's lithospheric portion of the upper mantle has remained the thickest (>Â 200Â km) and oldest (>Â 3Â Gy) beneath cratons and is yet surrounded by a vigorously convecting asthenosphere. It is generally admitted that water is a key parameter in the strength and longevity of cratonic roots, because olivine, the main phase of the lithospheric mantle, becomes stronger if its water content decreases. Expanding upon the work presented in Novella et al. (2015) and Taylor et al. (2016), we report new water contents for additional olivine inclusions in diamonds together with the trace-element composition for all olivine inclusions, as well as for mantle xenoliths from various kimberlite pipes located on the Siberian craton. The olivine diamond inclusions from this study have systematically low-water contents (<Â 50Â ppmw H2O), moderate to high forsterite (e.g., Fo91-94) contents and low Ni, Co, and Zn ppm contents (e.g., <Â 2848, <Â 108, and <Â 47Â ppm, respectively). In contrast, olivines from Siberian craton mantle xenoliths have a wide range of water contents (6-323Â ppmw H2O) and extend to lower-Fo (91-92), Ni, Co, and Zn-rich compositions, compared to the diamond inclusions. Depleted incompatible trace-element concentrations in olivine (0.1-0.001Â ÃÂ Primitive Mantle) advance our hypothesis for the protogenetic origins for the majority of Siberian diamond inclusions. These observations are consistent with the peridotite xenoliths as representing a part of the cratonic lithosphere that has experienced melt re-fertilization, which has also transported water. The olivine diamond inclusions, on the other hand, preserve “micro-samples” of an initial, dry cratonic lithosphere, mostly resulting from melting events. These inclusions are likely sourced from the initial cratonic mantle lithosphere, which thereby, resisted delamination over time, due to its buoyancy and strength, imparted from melt and water depletion, respectively. And thus, our data provides a major argument that the kimberlite-hosted mantle xenoliths may be more metasomatized than common rocks at the base of the Siberian and other cratonic roots away from kimberlite fields.