Melt Percolation, Melt-Rock Reaction and Oxygen Fugacity in Supra-Subduction Zone Mantle and Lower Crust from the Leka Ophiolite Complex, Norway

Samples of peridotites and pyroxenites from the mantle and lower crustal sections of the Leka Ophiolite Complex (LOC; Norway) are examined to investigate the effects of melt-rock reaction and oxygen fugacity variations in the sub-arc oceanic lithosphere. The LOC is considered to represent supra-subduction zone (SSZ) oceanic lithosphere, but also preserves evidence of pre-SSZ magmatic processes. Here we combine field and microstructural observations with mineral chemical and structural analyses of different minerals from the major lithologies of the LOC. Wehrlite and websterite bodies in both the mantle and lower crust contain clinopyroxene likely formed at a pre-SSZ stage, characterised by high Al, high Cr, low Mg crystal cores. These clinopyroxenes also exhibit low Al, low Cr, high Mg outer rims and intracrystalline dissolution surfaces, indicative of reactive melt percolation during intrusion and disruption of these lithologies by later, SSZ-related, dunite-forming magmas. Chromian-spinel compositional variations correlate with lithology; dunite-chromitite Cr-spinels are characterised by relatively uniform and high TiO2 and Al2O3, indicating formation by melt-rock reaction associated with SSZ processes. Harzburgite Cr-spinel compositions are more variable but preserve a relatively high Al2O3, low TiO2 endmember that may reflect crystallisation in a pre-SSZ oceanic spreading centre setting. An important finding of this study is that the LOC potentially preserves the petrological signature of a transition between oceanic spreading centre processes and subsequent supra-subduction zone magmatism. Single crystal Cr-spinel Fe3+/ΣFe ratios calculated on the basis of stoichiometry (from electron microprobe [EPMA] and crystal structural [X-ray diffraction; XRD] measurements) correlate variably with those calculated by point-source (single crystal) Mössbauer spectroscopy. Average sample EPMA Fe3+/ΣFe ratios overestimate or underestimate the Mössbauer-derived values for harzburgites, and always overestimate the Mössbauer Fe3+/ΣFe ratios for dunites and chromitites. The highest Fe3+/ΣFe ratios, irrespective of method of measurement, are therefore generally associated with dunites and chromitites, and yield calculated log(fO2)FMQ values of up to ~+1.8. While this lends support to the formation of the dunites and chromitites during SSZ-related melt percolation in the lower part of the LOC, it also suggests that these melts were not highly oxidised, compared to typical arc basalts (fO2FMQ of >+2). This may in turn reflect the early (forearc) stage of subduction zone activity preserved by the LOC and implies that some of the arc tholeiitic and boninitic lava compositions preserved in the upper portion of the ophiolite are not genetically related to the mantle and lower crustal rocks, against which they exhibit tectonic contacts. Our new data also have implications for the use of ophiolite chromitites as recorders of mantle oxidation state through time; a global comparison suggests that the Fe3+/ΣFe signatures of ophiolite chromitites are likely to have more to do with local environmental petrogenetic conditions in sub-arc systems than large length-scale mantle chemical evolution.

Subduction initiation-induced rapid emplacement of garnet-bearing peridotites at a nascent forearc: Petrological and Os-Li isotopic evidence from the Purang ophiolite, Tibet

Garnet-bearing peridotites commonly occur in the deeper parts of mature or thickened oceanic lithosphere, and are rarely exhumed and emplaced onto the seafloor. The Purang ophiolitic peridotites in south Tibet contain rare symplectite pseudomorphs after garnet, offering a unique window into the still poorly understood evolution of the deep oceanic lithosphere. Here, integrated petrologic and Os-Li isotopic data are used to constrain the evolution and dynamics of emplacement for these garnet peridotite protoliths. The Purang peridotites show wide variations of chemical compositions (spinel Cr#: 0.2−0.8) and Os model ages (up to 2.0 Ga), thus representing a piece of heterogeneous oceanic mantle lithosphere. Dunite channels show two distinctive groups of Cr# of spinels and Os-isotope compositions, with the low- to medium-Cr# (0.2−0.6) and high-Cr# (0.7−0.8) dunites reflecting the reaction of host lherzolites/harzburgites with percolating mid-ocean ridge basalt−like and boninitic melts, respectively. This confirms recent subduction initiation-related melt percolation in the Purang peridotites. Coexisting olivines and pyroxenes in the peridotites show systematic Li elemental and isotopic disequilibrium, suggesting fast cooling of the peridotites to Li closure temperature shortly after the melt percolations, likely during exhumation of the peridotites onto the seafloor. This supports a close link between subduction initiation and tectonic emplacement of the Purang peridotites. Combined with other geological evidence, we suggest the Purang peridotites may originate from the deep part of old, thick oceanic lithosphere of the Neo-Tethys. This thick oceanic lithosphere was progressively weakened and thinned likely during widespread plume-lithosphere interaction, triggering the transformation of garnet peridotite protoliths to spinel peridotites. Subsequently, initiation of a new subduction zone along the lithospheric weakness caused rapid ascent and emplacement of the Purang peridotites at a nascent forearc.

Using rock magnetics to resolve composite magmatic state fabrics: a case study from the Younger Giant Dyke Complex, SW Greenland

<p>Understanding the geometry of magma chambers plays a critical role in determining the igneous petrogenic processes that occur as intrusions cool. Quantitative fabric analysis methods, such as anisotropy of magnetic susceptibility (AMS), are routinely used to measure magma flow dynamics and determine the mechanism of magma transport and emplacement. However, magma mushes typically experience multiple flow events; e.g. emplacement, convection, and interstitial melt percolation. There is thus a need to develop a more a sophisticated approach to unravelling complex rock fabrics that record more than one magmatic state process. This study uses novel rock magnetic datasets to untangle the evolution of the 1163 Ma Younger Giant Dyke Complex (YGDC) of SW Greenland, a multi-sheeted troctolite dyke system that attains widths up to 800 m and encloses several evolved and/or modally layered ovoid pods.</p><p>Field results identify that ovoid pods occur in the thickest dyke segments. Several pods are defined by gently inward dipping modal layers and/or a parallel mineral foliations, and in-phase AMS magnetic foliations lie parallel to the observed field fabrics. Critically, imbricated plagioclase crystals record a magma transport direction toward the center of each pod, and this observation is substantiated by in-phase AMS lineations that plunge down dip of the foliation and shallow toward the center of each pod. These observations are interpreted to show gravitational settling under a convective flow regime.</p><p>In addition, 66% of out-of-phase AMS fabrics are non-parallel with in-phase AMS results. Out-of-phase AMS is a relatively new technique and is thought to reflect anisotropy controlled by a restrictive group of ferromagnetic minerals such as single domain magnetite and pyrrhotite. Out-of-phase lineations in layered pods are relatively steeply inclined and do not shallow towards the center, we therefore hypothesize that these lineations record a late stage filter-pressing process within the crystal mush. To test this hypothesis, anisotropy of anhysteretic remanent magnetism (AARM) data were collected from 15 samples. Results show that the AARM and out-of-phase AMS tensor axes are parallel, indicating that the sub-fabric detected by out-of-phase AMS is normal and most likely controlled by single domain magnetite.</p><p>Our results show that the application of rock magnetic techniques is effective in unravelling magma convection fabrics from later melt migration fabrics in mushy magmas.</p>

Syn-deformational melt percolation through a high-pressure orthogneiss and the exhumation of a subducted continental wedge (Orlica-Śnieżnik Dome, NE Bohemian Massif)

<h3>High-pressure granitic orthogneiss of the south-eastern Orlica–Śnieżnik Dome (NE Bohemian Massif) shows relics of a shallow-dipping S1 foliation, reworked by upright F2 folds and a mostly pervasive N-S trending subvertical axial planar S2 foliation. Based on macroscopic observations, a gradual transition perpendicular to the subvertical S2 foliation from banded to schlieren and nebulitic orthogneiss was distinguished. All rock types comprise plagioclase, K-feldspar, quartz, white mica, biotite and garnet. The transition is characterized by increasing presence of interstitial phases along like-like grain boundaries and by progressive replacement of recrystallized K-feldspar grains by fine-grained myrmekite. These textural changes are characteristic for syn-deformational grain-scale melt percolation, which is in line with the observed enrichment of the rocks in incompatible elements such as REEs, Ba, Sr, and K, suggesting open-system behaviour with melt passing through the rocks. The P–T path deduced from the thermodynamic modelling indicates decompression from ~15−16 kbar and ~650–740 ºC to ~6 kbar and ~640 ºC. Melt was already present at the P–T peak conditions as indicated by the albitic composition of plagioclase in films, interstitial grains and in myrmekite. The variably re-equilibrated garnet suggests that melt content may have varied along the decompression path, involving successively both melt gain and loss. The 6–8 km wide zone of vertical foliation and migmatite textural gradients is interpreted as vertical crustal-scale channel where the grain-scale melt percolation was associated with horizontal shortening and vertical flow of partially molten crustal wedge en masse.</h3>

Effects of melt-percolation, refertilization, and deformation on upper mantle seismic anisotropy: constraints from peridotite xenoliths, Marie Byrd Land, West Antarctica

We report on the petrology, microstructure, and seismic properties of 44 peridotite xenoliths extracted from the upper mantle beneath Marie Byrd Land (MBL), West Antarctica. The aim of this work is to understand how melt-rock reaction, refertilization, and deformation affected the seismic properties (velocities, anisotropy) of the West Antarctic upper mantle, in the context of MBL tectonic evolution and West Antarctic Rift System formation. Modal compositions, mineral major element compositions, microstructures, and crystallographic preferred orientations (CPOs) provide evidence for diachronous reactive melt percolation and refertilization. Olivine shows three main CPO patterns, the A-type, axial-[010], and axial-[100] texture types. Average seismic properties of the MBL mantle lithosphere are mainly controlled by the strength of olivine crystallographic texture. Reactive melt percolation and refertilization likely modified seismic velocities and anisotropy, as is suggested by a systematic decrease in maximum P-wave and S-wave anisotropies with increasing modal abundance of pyroxene. At larger spatial scales, the seismic properties of the MBL mantle xenoliths are dominated by the anisotropy resulting from the A-type olivine CPO. Variations between individual volcanic centers, however, attest to spatial variations in the mantle structure, potentially related to 3-D deformation and the prolonged tectonic history of MBL.Supplementary material at https://doi.org/10.6084/m9.figshare.c.5315261

Thermobarometry and Geochemistry of Mantle Xenoliths from Zapolyarnaya Pipe, Upper Muna Field, Yakutia: Implications for Mantle Layering, Interaction with Plume Melts and Diamond Grade

Minerals from mantle xenoliths in the Zapolyarnaya pipe in the Upper Muna field, Russia and from mineral separates from other large diamondiferous kimberlite pipes in this field (Deimos, Novinka and Komsomolskaya-Magnitnaya) were studied with EPMA and LA-ICP-MS. All pipes contain very high proportions of sub-calcic garnets. Zapolyarnaya contains mainly dunitic xenoliths with veinlets of garnets, phlogopites and Fe-rich pyroxenes similar in composition to those from sheared peridotites. PT estimates for the clinopyroxenes trace the convective inflection of the geotherm (40–45 mW·m−2) to 8 GPa, inflected at 6 GPa and overlapping with PT estimates for ilmenites derived from protokimberlites. The Upper Muna mantle lithosphere includes dunite channels from 8 to 2 GPa, which were favorable for melt movement. The primary layering deduced from the fluctuations of CaO in garnets was smoothed by the refertilization events, which formed additional pyroxenes. Clinopyroxenes from the Novinka and Komsomolskaya-Magnitnaya pipes show a more linear geotherm and three branches in the P-Fe# plot from the lithosphere base to the Moho, suggesting several episodes of pervasive melt percolation. Clinopyroxenes from Zapolyarnaya are divided into four groups according to thermobarometry and trace element patterns, which show a stepwise increase of REE and incompatible elements. Lower pressure groups including dunitic garnets have elevated REE with peaks in Rb, Th, Nb, Sr, Zr, and U, suggesting mixing of the parental protokimberlitic melts with partially melted metasomatic veins of ancient subduction origin. At least two stages of melt percolation formed the inclined PT paths: (1) an ancient garnet semi-advective geotherm (35–45 mW·m−2) formed by volatile-rich melts during the major late Archean event of lithosphere growth; and (2) a hotter megacrystic PT path (Cpx-Ilm) formed by feeding systems for kimberlite eruptions (40–45 mW·m−2). Ilmenite PT estimates trace three separate PT trajectories, suggesting a multistage process associated with metasomatism and formation of the Cpx-Phl veinlets in dunites. Heating associated with intrusions of protokimberlite caused reactivation of the mantle metasomatites rich in H2O and alkali metals and possibly favored the growth of large megacrystalline diamonds.

Sr isotopes indicate millennial-scale formation of metal-rich layers by reactive melt percolation in an open-system layered intrusion

<p>In order to test whether the crystal mushes that form layered mafic intrusions can behave as open systems, we investigated mineral-scale textural, chemical and Sr isotopic heterogeneity in the c. 60 Ma Rum intrusion, Scotland. Within Unit 10 of the Rum intrusion, intercumulus plagioclase and clinopyroxene crystals in peridotite 1-2 cm above and below millimetric Cr-spinel seams exhibit complex optical and chemical zoning (Hepworth et al. 2017). These Cr-spinel seams are closely associated with sulphide and platinum-group element (PGE) mineralization. High precision Sr isotopic analyses (undertaken by thermal ionisation mass spectrometry) of individual intracrystal zones (sampled by micromilling) in intercumulus plagioclase and clinopyroxene from within the PGE-enriched Cr-spinel seams have revealed significant intra-crystalline heterogeneity. <sup>87</sup>Sr/<sup>86</sup>Sr heterogeneity is present between plagioclase crystals, between clinopyroxene and plagioclase, and within plagioclase crystals, throughout the studied section. The preservation of Sr isotope heterogeneities at 10-100 µm length-scales implies cooling of the melts that formed the precious metal-rich layers at rates >1 °C per year, and cooling to diffusive closure within 10s-100s of years. The combined textural observations and intra-crystal plagioclase <sup>87</sup>Sr/<sup>86</sup>Sr data also highlight the importance of repeated cycles of dissolution and recrystallization within the crystal mush, and together with recent documentation of ‘out-of-sequence’ layers in other layered intrusions (Mungall et al. 2016; Wall et al. 2018), raise the prospect that basaltic magmatic systems may undergo repeated self-intrusion during solidification.</p><p>Hepworth, L.N., O’Driscoll, B., Gertisser, R., Daly, J.S. and Emeleus, H.C. 2017. Journal of Petrology 58, 137-166; Mungall, J. E., Kamo, S. L. & McQuade, S. 2016. Nature Communications 7, 13385; Wall, C. J., Scoates, J. S., Weis, D., Friedman, R. M., Amini, M. & Meurer, W. P. 2018. Journal of Petrology 59, 153–190.</p>

Coupling between Thermo-Hydro-Chemical reactive transport and Gibbs minimisation: magma evolution in evolving multiphase porous media

<p>The formation of alkaline magmas observed worldwide requires that low degree-melts, potentially formed in the asthenosphere, were able to cross the overlying lithosphere. Fracturing in the upper, brittle part of the lithosphere may help to extract this melt to the surface. However, the mechanism of extraction in the lower, ductile part of the lithosphere is still contentious. Metasomatic enrichment of the lithospheric mantle demonstrates that such low-degree melts interact with the lithosphere, but the physical aspect of this process remains unclear. The aim of this study is to better understand the percolation of magma in a porous viscous medium at pressure (P) and temperature (T) conditions relevant for the base of the lithosphere. We study such melt percolation numerically with a Thermo-Hydro-Chemical model of reactive transport coupled with thermodynamic data obtained via Gibbs energy minimisation. We perform Gibbs energy minimisation with Matlab using the linprog algorithm. We start with a simple ternary system of Forsterite/Fayalite/Enstatite solids and melts. All variables are a function of T, P and composition of the system (C), and are computed in both the Gibbs energy minimisation and in the reactive transport code, and can therefore vary freely.</p>