TY - JOUR

T1 - Zircon melt inclusions in mafic and felsic rocks of the Bushveld Complex

T2 - Constraints for zircon crystallization temperatures and partition coefficients

AU - Gudelius, D.

AU - Zeh, A.

AU - Almeev, Renat R.

AU - Wilson, A. H.

AU - Fischer, L. A.

AU - Schmitt, A. K.

N1 - Funding Information: DG and AZ thank the Deutsche Forschungsgemeinschaft (DFG grant ZE 424/12-1), and Linda Marko, Richard Albert Roper and Axel Gerdes (Frankfurt Isotope and Element Research Center (FIERCE), Goethe-University Frankfurt, Germany) for support with sample preparation and LA-ICP-MS analyses. RA thanks DFG for support of the experimental program (DFG grant KO1723/20). We are grateful to Robert Balzer, Stefan Linsler, Florian Pohl and Christian Singer for the help with experiments. All authors are also indebted to late Joe Aphanane (University of the Witwatersrand) for zircon separation and preparation, David Schiller (Salzburg University, Austria) for support during thermodynamic modelling as well as to Samancor Chrome and Bokoni Platinum Mine Proprietary Ltd. for support during sampling. This work has greatly benefited from detailed reviews by Emilie Bruand, Jill VanTongeren and N. Alex Zirakparvar.

PY - 2020/11/15

Y1 - 2020/11/15

N2 - Melt inclusions in zircon represent time capsules, which provide deep insights into igneous rock formation including timing, physicochemical conditions, and the compositional evolution of cogenetic magmas with respect to major, trace, and volatile elements. However, their full potential as petrogenetic indicators, in particular their usability and consistency as geothermometers, is poorly investigated. Therefore, we present new mineralogical and chemical data for recrystallized and homogenized melt inclusions and host zircon from different mafic and felsic rocks of the Bushveld Complex (BC), South Africa. Samples include rutile-bearing cumulate rocks of the Marginal and Critical zones, as well as rutile-free, magnetite-ilmenite-titanite-bearing diorites and granites of the Upper Zone, Rashoop Granophyre and Lebowa Granite Suite. All melt inclusions, irrespective of rock type, have rhyolitic compositions with SiO2 contents ranging from 65 to 78 wt.%, and H2O from 1.6 to 4.0 wt.%, whereas trace element contents differ systematically between rock types. In rutile-bearing mafic rocks, melt inclusions commonly show higher Ti contents (>800 ppm), higher Th/U ratios (up to 38), and lower REE contents (ƩREE < 150 ppm) compared to those in rutile-free mafic and felsic rocks (Ti < 800 ppm; Th/U < 5; ƩREE > 150 ppm). Liquidus temperatures of melt inclusions obtained from normative Qz-Ab-Or and rhyolite-MELTS modelling indicate melt entrapment mostly at 930–850 °C (at 200 MPa), tailing down in some samples to 700 °C. For rutile-bearing rocks, these temperatures overlap with those obtained by TiO2-in-melt and Ti-in-zircon geothermometry. For all rutile-free mafic and felsic rocks, reduced TiO2 activities of aTiO2 ∼ 0.3 are required for Ti-in-zircon geothermometry, and TiO2rutile ∼ 30 wt.% for TiO2 saturation geothermometry, to match temperatures from other geothermometers. Furthermore, partition coefficients obtained from melt inclusion - host zircon pairs are within error identical for mafic and felsic rocks and also reveal no systematic dependency on melt inclusion size, composition and entrapment temperature. The results of this study demonstrate that melt inclusions in zircon are a powerful tool for geothermometry and to constrain magma compositions, H2O contents, and TiO2 activities, which are critical for the understanding of magmatic processes shaping Earth's crust.

AB - Melt inclusions in zircon represent time capsules, which provide deep insights into igneous rock formation including timing, physicochemical conditions, and the compositional evolution of cogenetic magmas with respect to major, trace, and volatile elements. However, their full potential as petrogenetic indicators, in particular their usability and consistency as geothermometers, is poorly investigated. Therefore, we present new mineralogical and chemical data for recrystallized and homogenized melt inclusions and host zircon from different mafic and felsic rocks of the Bushveld Complex (BC), South Africa. Samples include rutile-bearing cumulate rocks of the Marginal and Critical zones, as well as rutile-free, magnetite-ilmenite-titanite-bearing diorites and granites of the Upper Zone, Rashoop Granophyre and Lebowa Granite Suite. All melt inclusions, irrespective of rock type, have rhyolitic compositions with SiO2 contents ranging from 65 to 78 wt.%, and H2O from 1.6 to 4.0 wt.%, whereas trace element contents differ systematically between rock types. In rutile-bearing mafic rocks, melt inclusions commonly show higher Ti contents (>800 ppm), higher Th/U ratios (up to 38), and lower REE contents (ƩREE < 150 ppm) compared to those in rutile-free mafic and felsic rocks (Ti < 800 ppm; Th/U < 5; ƩREE > 150 ppm). Liquidus temperatures of melt inclusions obtained from normative Qz-Ab-Or and rhyolite-MELTS modelling indicate melt entrapment mostly at 930–850 °C (at 200 MPa), tailing down in some samples to 700 °C. For rutile-bearing rocks, these temperatures overlap with those obtained by TiO2-in-melt and Ti-in-zircon geothermometry. For all rutile-free mafic and felsic rocks, reduced TiO2 activities of aTiO2 ∼ 0.3 are required for Ti-in-zircon geothermometry, and TiO2rutile ∼ 30 wt.% for TiO2 saturation geothermometry, to match temperatures from other geothermometers. Furthermore, partition coefficients obtained from melt inclusion - host zircon pairs are within error identical for mafic and felsic rocks and also reveal no systematic dependency on melt inclusion size, composition and entrapment temperature. The results of this study demonstrate that melt inclusions in zircon are a powerful tool for geothermometry and to constrain magma compositions, H2O contents, and TiO2 activities, which are critical for the understanding of magmatic processes shaping Earth's crust.

KW - Bushveld Complex

KW - Melt inclusions

KW - Partition coefficients

KW - Thermometry

KW - Zircon

UR - http://www.scopus.com/inward/record.url?scp=85090857891&partnerID=8YFLogxK

U2 - 10.1016/j.gca.2020.08.027

DO - 10.1016/j.gca.2020.08.027

M3 - Article

AN - SCOPUS:85090857891

VL - 289

SP - 158

EP - 181

JO - Geochimica et Cosmochimica Acta

JF - Geochimica et Cosmochimica Acta

SN - 0016-7037

ER -