Interactions of magmas and highly reduced fluids during intraplate volcanism, Mt Carmel, Israel: Implications for mantle redox states and global carbon cycles

Creators: Griffin, W.L.¹; Bindi, L.²; Cámara, F.³; Ma, C.⁴; Gain, S.E.M.^{1, 5}; Saunders, M.⁶; Alard, O.^{1, 7}; Huang, J.-X.¹; Shaw, J.⁶; Meredith, C.⁸; Toledo, V.⁹; O'Reilly, S.Y.¹

1. Macquarie University
2. University of Florence
3. University of Milan
4. California Institute of Technology
5. Fortescue Metals Group, Perth, WA 6004, Australia
6. University of Western Australia
7. Australian National University
8. Pennsylvania State University
9. Shefa Gems, Netanyzna 4210602, Israel

Abstract

Oxygen fugacity (ƒO₂) controls the speciation of COH fluids in Earth’s mantle; a major question is whether the sublithospheric mantle is metal-saturated, maintaining ƒO₂ near the Iron-Wüstite (IW) buffer reaction. If so, then COH fluids from this source will be dominated by CH₄ + H₂, rather than the more oxidized CO₂-H₂O fluids commonly considered in petrological studies. A key to this question is found in rare but widespread examples of natural mineral assemblages that require unusually low ƒO₂. We summarize an investigation of super-reduced mineral assemblages in corundum xenocrysts from Late Cretaceous alkali-basalt volcanoes on Mt Carmel, northern Israel and related Plio-Pleistocene alluvial deposits. P-T estimates indicate that the corundum xenocrysts crystallized in the uppermost mantle. The well-documented geological controls on the origin of these deposits, and radiometric dating of the super-reduced phases, ensure the “naturalness” of the controversial assemblages and make these mineral parageneses a benchmark for evaluation of related occurrences worldwide.

The tuffs contain a “basalt-megacryst” mineral suite (zircon, sapphire, ilmenite, spinel). The megacryst chemistry and the geochronology of the zircons indicate that the megacrysts crystallized from broadly syenitic melts that differentiated at subcrustal levels (P ca 1 GPa) within a thick gabbroic underplate built up from Permian through Pliocene time and perhaps into the Pleistocene. Reaction of mantle-derived CH₄-H₂ fluids with these syenitic melts led to the separation of immiscible Fe⁰ and Fe-Ti oxide melts near fO₂ = IW. Trace-element distributions suggest the syenitic melts then separated into immiscible Si-Al-Na-K-rich and FeO-rich oxide melts; the latter were enriched in HFSE, REE, P and Zr as in other natural and synthetic examples of melt-melt immiscibility.

In a model magma chamber the FeO-rich melts would sink, leaving the Si-Al-Na-K melts in an upper zone, both still fluxed by CH₄-H₂ fluids. At fO₂ of ΔIW-6 to -7 the removal of immiscible Fe-Ti-Si-C silicide melts from the FeO-rich melt would leave a desilicated Ca-Al-Si oxide melt that crystallized high-Ti corundum hibonite cumulates with inclusions requiring fO₂ from ΔIW + 2 to ΔIW-9, while the less-reduced conjugate silicate melts in the upper levels crystallized low-Ti corundum. Aggregates of skeletal, strongly Ti-zoned corundum crystals. reflect rapid crystallization from very reduced melt-fluid mixtures, probably in fluid-escape channels. Explosive eruptions sampled individual magma chambers at different depths and with different initial compositions, fluid mixtures and fluid dynamics to produce Mt Carmel’s mineralogical diversity.

A review of similar occurrences worldwide suggests that the Mt Carmel assemblages reflect a fundamental process – the rise of CH₄-H₂ fluids into the upper mantle -- that accompanies mantle-derived magmatism in many tectonic settings. The interaction of these fluids with lithospheric mantle rocks and melts can lead to extreme fractionation via the separation of immiscible Fe-Ti-Si-C melts and residual desilicated melts. The oxidation of CH₄-H₂ fluids in the lithospheric mantle may be the ultimate source of metasomatic fluids dominated by CO₂ + H₂O, and of many diamonds. More attention should be paid to the role of methane and other reduced fluids in mantle petrology, and their relevance to metasomatic processes and global carbon cycles.

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Acknowledgement

We are very grateful to: John Ward for education on alluvial systems, including insightful guidance in the field; Reli Wald for education on the geological and geophysical history of the region; Eitan Sass for discussions on the geology of Mt Carmel; Hongkun Dai for education on basalt chemistry; Norm Pearson for his innovative and dedicated leadership of the laboratories of the Geochemical Analysis Unit; Yi-Jen Lai for assistance in the lab; Manal Bebbington for many, many sample preparations; Mike DeWit and Dave Apter for discussions on the Mt Carmel volcanism; Paul Asimow and Dan Harlow for insightful comments on several previous papers.

Prof. H. He (IGG-CAS, Beijing) contributed noble-gas analyses, Vlad Malkovets some kimberlite concentrates, Dima Kamenetsky the initial FE-SEM imaging, Ananuer Halumalati the analyses of trapped gasses and Anita Andrew the C-isotope analyses of carbonates. Sally-Ann Hodgekiss and Montgarri Castillo-Oliver patiently and calmly brought order from chaos to make the final graphics for the “crazily long paper”. We especially thank Dr. M. Santosh for his encouragement and editorial handling, and Tim Horscroft for the invitation to present this review.

Funding

The research was funded in part by MIUR-PRIN2017, project “TEOREM: Deciphering geological processes using Terrestrial and Extraterrestrial ORE Minerals”, prot. 2017AK8C32 (PI: Luca Bindi). Fernando Cámara gratefully acknowledges funding from the Italian Ministry of University and Research (MIUR) through the project “Dipartimenti di Eccellenza 2023–2027”. SEM, EBSD and EPMA analyses were carried out at the Caltech GPS Division Analytical Facility, which is supported, in part, by NSF Grants EAR-0318518 and DMR-0080065. The authors acknowledge the facilities and the scientific and technical assistance of Microscopy Australia at the Centre for Microscopy, Characterisation & Analysis, The University of Western Australia, a facility funded by the University, State and Commonwealth Governments. This study also used instrumentation funded by ARC LIEF and DEST 567 Systemic Infrastructure Grants, Macquarie University and industry. This is contribution 1761 from the ARC Centre of Excellence for Core to Crust Fluid Systems (www.ccfs.mq.edu.au) and 1530 from the GEMOC Key Centre (www.gemoc.mq.edu.au).

This research did not receive any specific grant from funding agencies in the public, commercial or not-for-profit sectors. Shefa Gems Ltd. provided resources required for field work, conference presentations and laboratory visits by WLG, SYOR, SEMG and JXH.

Conflict of Interest

The authors declare the following financial interests/personal relationships which may be considered as potential competing interests: Author Vered Toledo is no longer employed by Shefa Gems, and has no financial, personal or other connection with the company. She therefore declares no competing interests. All other authors declare that they have no competing interests.

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