Ferropicrites as evidence for lithological heterogeneity in the mantle source of continental flood basalts

Jennings, Eleanor S. (2016) Ferropicrites as evidence for lithological heterogeneity in the mantle source of continental flood basalts. PhD thesis, University of Cambridge.

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Abstract

The sublithospheric mantle must be chemically heterogeneous as a consequence of billions of years worth of continuous subduction. Evidence for heterogeneity is strong in ocean island settings, where variability in isotopic, major and trace element composition is frequently attributed to variability in the mantle source. In continental fl ood basalt (CFB) provinces, signals of mantle-derived hetero- geneity are obscured by fractionated and contaminated nature of CFB magmas. However, outcrops of primitive magmas that more closely represent primary mantle melts are found in some CFB provinces. This study focuses on picrites and ferropicrites from the Paraná-Etendeka CFB province, and also ex- amines ferropicrites from the Karoo CFB province, which are found as dykes and thin fl ows. Ferropi- crites are Fe-rich, Al-poor magmas equally primitive and Mg-rich as the peridotite-derived picrites, but cannot have formed from peridotite melting. The sample sets are investigated by supplementing published whole-rock and mineral analyses with new mineral chemistry and melt inclusion data. Incompatible trace element compositions of their olivine-hosted melt inclusions are very homogeneous, suggesting that their primary fractional mantle melts underwent extensive mixing prior to the onset of crystallisation. Compared with primitive melt inclusions from other settings, inclusions in these and other CFB provinces are well-mixed. The crystallisation temperatures of olivine phenocrysts in these samples were determined by Al-in-olivine thermometry, with maximum crystallisation temperatures of ∼ 1500 °C identifi ed in some Etendeka picrites. These require a very high mantle potential temperature (TP ); the ferropicrites crystallised at somewhat lower temperatures. XANES analyses confi rmed that the spinel Fe 3+ /FeT were within the thermometer’s calibrated range. Ferropicrite has been suggested to originate from high pressure, low fraction melting of mantle pyroxenite. The Etendeka ferropicrite geochemistry is examined with the aid of a new thermodynamic model in order to interrogate its mantle source lithology and melting conditions. Modelling indicates that both the major and trace element composition of ferropicrite is indeed more compatible with garnet clinopyroxenite melting than peridotite melting, and that elevated TP plume conditions are required in its formation. By comparison, picrite major element chemistry is consistent with a high TP depleted peridotite melt. If a depleted peridotite and hybrid pyroxenite source mineralogy are used for picrite and Etendeka ferropicrite, respectively, then they both represent ∼ 10–20% mantle melting.

Item Type: Thesis (PhD)
Uncontrolled Keywords: IA70; weekly list
Subjects: 05 - Petrology - Igneous, Metamorphic and Volcanic Studies
Divisions: 05 - Petrology - Igneous, Metamorphic and Volcanic Studies
Depositing User: Sarah Humbert
Date Deposited: 22 Jan 2016 12:52
Last Modified: 03 Feb 2017 10:56
URI: http://eprints.esc.cam.ac.uk/id/eprint/3576

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