Corundum (sapphire) and zircon relationships, Lava Plains gem fields, NE Australia: Integrated mineralogy, geochemistry, age determination, genesis and geographical typing

AbstractGem minerals at Lava Plains, northeast Queensland, offer further insights into mantle-crustal gemformation under young basalt fields. Combined mineralogy, U-Pb age determination, oxygen isotope and petrological data on megacrysts and meta-aluminosilicate xenoliths establish a geochemical evolution in sapphire, zircon formation between 5 to 2 Ma. Sapphire megacrysts with magmatic signatures (Fe/Mg ∼100–1000, Ga/Mg 3–18) grew with ∼3 Ma micro-zircons of both mantle (δ18O 4.5–5.6%) and crustal (δ18O 9.5–10.1‰) affinities. Zircon megacrysts (3±1 Ma) show mantle and crustal characteristics, but most grew at crustal temperatures (600–800°C). Xenolith studies suggest hydrous silicate melts and fluids initiated from amphibolized mantle infiltrated into kyanite+sapphire granulitic crust (800°C, 0.7 GPa). This metasomatized the sapphire (Fe/Mg ∼50–120, Ga/Mg ∼3–11), left relict metastable sillimanite-corundum-quartz and produced minerals enriched in high field strength, large ion lithophile and rare earth elements. The gem suite suggests a syenitic parentage before its basaltic transport. Geographical trace-element typing of the sapphire megacrysts against other eastern Australian sapphires suggests a phonolitic involvement.

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Experimental determination of the partitioning behavior of rare earth and high field strength elements between pargasitic amphibole and natural silicate melts

Geochimica et Cosmochimica Acta ◽

10.1016/s0016-7037(99)00379-8 ◽

2000 ◽

Vol 64 (6) ◽

pp. 1103-1120 ◽

Cited By ~ 37

Author(s):

Mark Hilyard ◽

Roger L. Nielsen ◽

James S. Beard ◽

Alberto Patinõ-Douce ◽

James Blencoe

Keyword(s):

Rare Earth ◽

Field Strength ◽

Experimental Determination ◽

High Field ◽

Silicate Melts ◽

High Field Strength ◽

Natural Silicate ◽

Partitioning Behavior

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Evolution of deep mantle sourced carbonated melt in the mantle lithosphere

10.5194/egusphere-egu2020-13055 ◽

2020 ◽

Author(s):

Guoliang Zhang

Keyword(s):

Rare Earth ◽

Field Strength ◽

Rare Earth Elements ◽

Lithospheric Mantle ◽

High Field ◽

Volcanic Rocks ◽

Mantle Lithosphere ◽

High Field Strength ◽

Alkali Basalts ◽

Deep Mantle

Deep sourced magmas play a key role in distribution of carbon in the Earth&#8217;s system. Oceanic hotspots rooted in deep mantle usually produce CO2-rich magmas. However, the association of CO2 with the origin of these magmas remains unclear. Here we report geochemical analyses of a suite of volcanic rocks from the Caroline Seamount Chain formed by the deep-rooted Caroline hotspot in the western Pacific. The most primitive magmas have depletion of SiO2 and high field strength elements and enrichment of rare earth elements that are in concert with mantle-derived primary carbonated melts. The carbonated melts show compositional variations that indicate reactive evolution within the overlying mantle lithosphere and obtained depleted components from the lithospheric mantle. The carbonated melts were de-carbonated and modified to oceanic alkali basalts by precipitation of perovskite, apatite and ilmenite that significantly decreased the concentrations of rare earth elements and high field strength elements. These magmas experienced a stage of non-reactive fractional crystallization after the reactive evolution was completed. Thus, the carbonated melts would experience two stages, reactive and un-reactive, of evolution during their transport through in thick oceanic lithospheric mantle. We suggest that the mantle lithosphere plays a key role in de-carbonation and conversion of deep-sourced carbonated melts to alkali basalts. This work was financially supported by the National Natural Science Foundation of China (91858206, 41876040).

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