Der Oxidationszustand des lithospärischen Erdmantels unterhalb Somerset Island, Rae Kraton (Kanada) mit der Tiefe und der Einfluss von Metasomatose
Zusammenfassung der Projektergebnisse
In this project, we set out to determine the oxidation state of the lithospheric mantle root (SCLM) beneath the Rae craton, Canada and to compare these results with data for the neighboring Slave craton. This initially involved the investigation of peridotite xenoliths from the kimberlites of Somerset Island, located at the northern edge of the craton, but was expanded to include a small set of xenoliths from Pelly Bay, which lies ~500 km away in the more central portion of the craton. In order to interpret the oxidation state data, trace element concentrations in garnet and clinopyroxene were also measured to provide an assessment of possible metasomatic overprinting. The suite comprises spinel peridotites, garnet-spinel peridotites as well as garnet peridotites. In the garnet-spinel peridotites, the two phases are not spatially associated with each other, suggesting that spinel and garnet may not necessarily be in mutual equilibrium. Thermobarometric calculations yield equilibration conditions for the Somerset Island samples 830-1150°C and pressures of 3.1-4.6 GPa, while the limited number of peridotites from Pelly Bay gave temperatures of 674 to 905°C and pressures from 2.5 to 4.0 GPa. This corresponds to a depth range of ~100-150 km at Somerset Island and ~80 to 130 km at Pelly Bay. Thus our samples cover a limited depth range of ~50 km at both localities. The sample suite can be divided into four groups based upon the chondrite-normalised Rare Earth Element (REE) signatures of garnet (groups A-D). All indicate metasomatic interactions to various degrees caused by several different agents. The signatures of clinopyroxene are consistent with this picture. The Zr-Y systematics of garnet indicate that group C samples plot within the fluid-metasomatism field and their low Ti/Eu and their variable Zr/Hf point to a carbonatitic metasomatic agent. Many group D garnets plot within the field for silicate melt metasomatism and are distributed across the fields of carbonatitic and kimberlitic metasomatism in terms of their Ti/Eu. The more strongly enriched garnets belonging to groups C and D occur in samples of deeper origin. Observed variations in oxidation state span up to ~4 log units. Along with the shallowest sample from Pelly Bay, several deeper-lying spinel-bearing peridotites from Somerset Island also record low ∆logƒO2 values of ≤ FMQ-3. The peridotites from Somerset Island record ΔlogƒO2 values of FMQ–3.61 to FMQ+1.08. The ƒO2 of some garnet-spinel peridotites could be estimated from the Fe3+ content of both garnet and spinel. In most cases redox disequilibrium between garnet and spinel was apparent, with the spinel-based equilibrium yielding higher ∆logƒO2 values, implying a late oxidative event during which the garnet composition was only partially reset. Similarly to that observed for the Slave craton, we find evidence for relatively oxidised domains, however, they occur at significantly shallower depths beneath the Rae craton compared to the Slave. The observed variations in mineralogy, metasomatic signatures and oxidation state within a limited depth range of 100-150 km imply different mantle domains experienced differing degrees of depletion by partial melting prior to being juxtaposed within the cratonic root. These observations appear to be consistent with numerical models that envision a two-stage process of craton development with initial tectonic shortening followed by a subsequent phase of gravitational self-thickening. Oscillatory instabilities within the root can develop due to relative buoyancy, but can be attenuated by secular cooling that promotes stabilisation of the thickened lithosphere. Although there are similarities between the Slave and Rae SCLM, some differences in the metasomatic processes of enrichment and oxidation are apparent and these have reached to shallower depths beneath the Rae craton.
Projektbezogene Publikationen (Auswahl)
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(2019) Metasomatism and oxidation state of the lithospheric mantle beneath the Rae craton, Canada as revealed by xenoliths from Somerset Island and Pelly Bay. European Geophysical Union (EGU) annual meeting, Vienna, EGU2019-9348
Gräf C, Woodland A, Höfer H, Seitz H-M, Pearson G, Kjarsgaard B
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(2019) Metasomatism and oxidation state of the mantle root beneath the Rae craton, Canada. Goldschmidt Abstracts, 20191193
Gräf C, Sandner T, Woodland A, Höfer H, Seitz H-M, Pearson G, Kjarsgaard B
