Incremental Growth of Layered Mafic-Ultramafic Intrusions Through Melt Replenishment Into a Crystal Mush Zone Traced by Fe-Hf Isotope Systematics

Oliver Nebel,Nicholas J Gardiner,Timothy J Ivanic,Richard L Langford,Paolo A Sossi,Antoine Bénard,Richard J Arculus

doi:10.3389/feart.2020.00002

Abstract

Layered mafic-ultramafic intrusions are among the largest igneous bodies on Earth, and represent aggregations of large volumes of mantle- and some crustal-derived melts. Melts are emplaced over time-intervals of less than one million years, predominantly through multiple pulses of injections into pre-existing melt-crystal slurries. The dynamic interaction of physical processes, including density-driven separation and mixing of different components, within a solidifying magma chamber leads to such extreme chemical diversity between cumulate rock units, that no unified model currently explains all aspects of the genesis of these intrusions. Here we present whole-rock stable Fe isotope data (expressed in ‰ variations as 57Fe relative to IRMM-014) for samples of drill core taken from the stratified paleo-magma chamber of the Upper Zone of the late-Archean Windimurra Igneous Complex, Western Australia. Variations from near chondritic (57Fe ~ +0 ‰) to heavy (57Fe~ +0.2 ‰) values show a co-variation with initial radiogenic Hf isotope data that is unique to the Windimurra Upper Zone. The systematic isotopic variations from the roof to the base of the Upper Zone are best explained by an intricate sequence of events that included fractional crystallisation and physical mixing. We propose that melt freshly sourced from the mantle was injected into and inflated a pre-existing crystal-melt mush comprising the upper Middle Zone. Re-establishment of crystal layering after replenishment introduced a chemical stratification with the formation of what became the Upper Zone and a crystal-interstitial melt ratio decreasing from roof to base. Basal, vanadiferous magnetitite horizons crystallised through enhanced fO2 during liquid replenishment. Variable degrees of perturbation and chaotic stirring of crystals with imperfect mixing of new and old components was followed by rapid crystal settling and subsequent cumulate stratification. Such melt rejuvenation, proposed here to be the cause for a newly established Upper Zone, leaves no unique petrologic fingerprint but can explain not only the observed coupled Fe-Hf isotope systematics but also mineral disequilibria and cryptic layering in layered intrusions.

Highlights

Layered mafic-ultramafic intrusions (LMI) are among the largest igneous bodies on Earth (Cawthorn, 1996; Ernst and Buchan, 2001) and have been subject to detailed investigations for decades, yet their petrogenesis remains a matter of ongoing debate
Due to the cumulate nature of the layered series, and often unknown mineral-melt ratios, bulk rock compositions are often deemed to be of limited use for modeling crystal fractionation as they do not relate to a simple parental liquid composition and cannot straightforwardly be related to a liquid line of descent (Morse, 1976)
There is no observed correlation between Fe isotope compositions and major element abundances (Ca-Al-Fe-Mg-Si-Ti), rock type or associated modal mineral abundance

Summary

Introduction

Layered mafic-ultramafic intrusions (LMI) are among the largest igneous bodies on Earth (Cawthorn, 1996; Ernst and Buchan, 2001) and have been subject to detailed investigations for decades, yet their petrogenesis remains a matter of ongoing debate. The stratiform packages, or “zones,” are characterized by rhythmic or intermittent modal layering of mineral abundances, which relate to complex magma chamber processes These processes can include, but are not limited to: (i) physical crystalliquid separation through gravity settling (Naslund et al, 1991); (ii) compaction and expulsion, or filter pressing (Berger et al, 2017), often in conjunction with (iii) convection, including double-diffusive convection (Cawthorn and McCarthy, 1980; McBirney, 1995; Bons et al, 2015); (iv) melt recharge and mixing (Kruger, 2005; Yuan et al, 2017), or not (Tegner et al, 2006); (v) liquid immiscibility (VanTongeren and Mathez, 2012; Charlier et al, 2013; Vukmanovic et al, 2018) and/or accompanied by (vi) crustal contamination. From the sum of these complex observations, it is clear that most minerals are not in equilibrium with each other, both compositionally (Dunham and Wadsworth, 1978; Czamanske and Loferski, 1996) and isotopically (Chen et al, 2018), rendering parental liquid calculations from mineral compositions specious

Results

Discussion

Conclusion