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1. Post-melting oxidation of highly primitive basalts from the southern Andes

   https://doi.org/10.1016/j.gca.2020.01.042  

   Tassara, S. ( January 2020 , Geochimica et cosmochimica acta)

   null (Ed.)

   The oxygen fugacity (fO2) of the Earth’s upper mantle and its melting products is an important parameter in the geochemical evolution of arc magmas and their connection with the continental crustal construction and growth. Several works have focused on the fO2 of peridotite xenoliths, primitive melts in relatively young arc settings, and mid-ocean ridge basalts (MORB) but few studies have attempted to examine the early redox history of primitive magmas in mature arcs. Hence, our understanding of the nature and evolution of fO2 during the subduction cycle remains limited. Here, we investigate the basaltic tephra from the Los Hornitos monogenetic cones in central-southern Chile, which are among the most primitive materials reported in the Southern Andes (olivine Mg#
   92.5, and Ni
   5000 mgg1). These features offer a unique opportunity to explore the fO2 conditions below the Andean arc by studying olivine phenocrysts and their contained crystal and melt inclusions. We integrated EPMA, LA-ICP-MS, and m-XANES analyses to constrain the redox conditions recorded in the basaltic tephra by three different and self-reliant methods. First, we determined the fO2 based on the olivine-spinel equilibrium, yielding average values DFMQ + 1.3 ± 0.4 (1r). Second, we constrained the fO2 conditions of melt inclusions using Fe m-XANES data and the redox dependent olivine-melt vanadium partitioning. After correcting for post-entrapment crystallization and diffusive iron loss, the Fe m-XANES data indicate that the melt inclusions were trapped in average at DFMQ +2.5 ± 0.5 (1r). Results using the olivine-melt vanadium partitioning oxybarometer in melt inclusions are in agreement with Fe m-XANES data, yielding average DFMQ values of +2.6 ± 0.3 (1r). In order to test the potential effects of other postentrapment modifications of the melt inclusions that could have affected the fO2 prior to eruption, we assessed the residence time of these magmas using Mg-Fe interdiffusion modelling in olivine. The short residence times (<200 days) compared to vanadium re-equilibration models strongly suggest that the melt inclusions preserve the prevailing fO2 conditions during their entrapment. Correlations between melt inclusions major element composition and their fO2 determined by Fe m-XANES, as well as V/Sc modelling reveal a case of post-melting oxidation of the LHC magmas. We argue that primitive arc magmas behave as an open system with respect to fO2 during their early geochemical evolution. Our data indicate a complex fO2 early history of primitive melts in the southern Andes and provide a cautionary note on the direct extrapolation of primitive melts fO2 values to that of their mantle source. 

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2. The Shallow Magmatic Plumbing System of the Deccan Traps, Evidence from Plagioclase Megacrysts and Their Host Lavas

   https://doi.org/10.1093/petrology/egac075  

   Marzoli, Andrea ; Renne, Paul R ; Andreasen, Rasmus ; Spiess, Richard ; Chiaradia, Massimo ; Ruth, Dawn C ; Tholt, Andrew J ; Pande, Kanchan ; Costa, Fidel ( September 2022 , Journal of Petrology)

   Abstract We investigate the shallow plumbing system of the Deccan Traps Large Igneous Province using rock and mineral data from Giant Plagioclase Basalt (GPB) lava flows from around the entire province, but with a focus on the Saurashtra Peninsula, the Malwa Plateau, and the base and top of the Western Ghats (WG) lava pile. GPB lavas in the WG typically occur at the transition between chemically distinct basalt formations. Most GPB samples are evolved basalts, with high Fe and Ti contents, and show major and trace elements and Sr-Nd-Pb isotopic compositions generally similar to those of previously studied Deccan basalts. Major element modeling suggests that high-Fe, evolved melts typical of GPB basalts may derive from less evolved Deccan basalts by low-pressure fractional crystallization in a generally dry magmatic plumbing system. The basalts are strongly porphyritic, with 6–25% of mm- to cm-sized plagioclase megacrysts, frequently occurring as crystal clots, plus relatively rare olivine and clinopyroxene. The plagioclase crystals are mostly labradoritic, but some show bytownitic cores (general range of anorthite mol%: 78–55). A common feature is a strong Fe enrichment at the plagioclase rims, indicating interaction with an Fe-rich melt similar to that represented by the matrix compositions (FeOt up to 16–17 wt%). Plagioclase minor and trace elements and Sr isotopic compositions analyzed by laser ablation inductively coupled plasma mass spectrometry show evidence of a hybrid and magma mixing origin. In particular, several plagioclase crystals show variable 87Sr/86Sri, which only partially overlaps with the 87Sr/86Sri of the surrounding matrix. Diffusion modeling suggests residence times of decades to centuries for most plagioclase megacrysts. Notably, some plagioclase crystal clots show textural evidence of deformation as recorded by electron back-scatter diffraction analyses and chemical maps, which suggest that the plagioclase megacrysts were deformed in a crystal-rich environment in the presence of melt. We interpret the plagioclase megacrysts as remnants of a crystal mush originally formed in the shallow plumbing system of the Deccan basalts. In this environment, plagioclase acquired a zoned composition due to the arrival of chemically distinct basaltic magmas. Prior to eruption, a rapidly rising but dense Fe-rich magma was capable of disrupting the shallow level crystal mush, remobilizing part of it and carrying a cargo of buoyant plagioclase megacrysts. Our findings suggest that basaltic magmas from the Deccan Traps, and possibly from LIPs in general, are produced within complex transcrustal magmatic plumbing systems with widespread crystal mushes developed in the shallow crust. 

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3. Magmatic Response to Subduction Initiation: Part 1. Fore‐arc Basalts of the Izu‐Bonin Arc From IODP Expedition 352

   https://doi.org/10.1029/2018GC007731  

   Shervais, John W. ; Reagan, Mark ; Haugen, Emily ; Almeev, Renat R. ; Pearce, Julian A. ; Prytulak, Julie ; Ryan, Jeffrey G. ; Whattam, Scott A. ; Godard, Marguerite ; Chapman, Timothy ; et al ( January 2019 , Geochemistry, Geophysics, Geosystems)

   The Izu‐Bonin‐Mariana (IBM) fore arc preserves igneous rock assemblages that formed during subduction initiation circa 52 Ma. International Ocean Discovery Program (IODP) Expedition 352 cored four sites in the fore arc near the Ogasawara Plateau in order to document the magmatic response to subduction initiation and the physical, petrologic, and chemical stratigraphy of a nascent subduction zone. Two of these sites (U1440 and U1441) are underlain by fore‐arc basalt (FAB). FABs have mid‐ocean ridge basalt (MORB)‐like compositions, however, FAB are consistently lower in the high‐field strength elements (TiO₂, P₂O₅, Zr) and Ni compared to MORB, with Na₂O at the low end of the MORB field and FeO* at the high end. Almost all FABs are light rare earth element depleted, with low total REE, and have low ratios of highly incompatible to less incompatible elements (Ti/V, Zr/Y, Ce/Yb, and Zr/Sm) relative to MORB. Chemostratigraphic trends in Hole U1440B are consistent with the uppermost lavas forming off axis, whereas the lower lavas formed beneath a spreading center axis. Axial magma of U1440B becomes more fractionated upsection; overlying off‐axis magmas return to more primitive compositions. Melt models require a two‐stage process, with early garnet field melts extracted prior to later spinel field melts, with up to 23% melting to form the most depleted compositions. Mantle equilibration temperatures are higher than normal MORB (1,400 °C–1,480 °C) at relatively low pressures (1–2 GPa), which may reflect an influence of the Manus plume during subduction initiation. Our data support previous models of FAB origin by decompression melting but imply a source more depleted than normal MORB source mantle.

    

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4. The causes of continental arc flare ups and drivers of episodic magmatic activity in Cordilleran orogenic systems

   https://doi.org/doi.org/10.1016/j.lithos.2021.106307  

   Chapman, J. ( July 2021 , Lithos)

   null (Ed.)

   Continental arcs in Cordilleran orogenic systems display episodic changes in magma production rate, alternating between flare ups (70–90 km3 km􀀀 1 Myr􀀀 1) and lulls (< 20 km3 km􀀀 1 Myr􀀀 1) on timescales of tens of millions of years. Arc segments or individual magmatic suites may have even higher rates, up several 100 s of km3 km􀀀 1 Myr􀀀 1, during flare ups. These rates are largely determined by estimating volumes of arc crust, but do not reflect melt production from the mantle. The bulk of mantle-derived magmas are recycled back into the mantle by delamination of arc roots after differentiation in the deep crust. Mantle-derived melt production rates for continental arcs are estimated to be 140–215 km3 km􀀀 1 Myr􀀀 1 during flare ups and ≤ 15 km3 km􀀀 1 Myr􀀀 1 during lulls. Melt production rates averaged over multiple magmatic cycles are consistent with independent estimates for partial melting of the mantle wedge in subduction zones, however, the rates during flare ups and lulls are both anomalously high and anomalously low, respectively. The difference in mantle-derived melt production between flare ups and lulls is larger than predicted by petrologic and numerical models that explore the range of globally observed subduction parameters (e.g., convergence rate, height of the mantle wedge). This suggests that other processes are required to increase magmatism during flare ups and suppress magmatism during lulls. There are many viable explanations, but one possibility is that crystallized melts from the asthenospheric mantle wedge are temporarily stored in the deep lithosphere during lulls and then remobilized during flare ups. Basaltic melts may stall in the mantle lithosphere in inactive parts of the arc system, like the back-arc, refertilizing the mantle lithosphere and suppressing melt delivery to the lower crust. Subsequent landward arc migration (i.e., toward the interior of the continent) may encounter such refertilized mantle lithosphere magma source regions, contributing to magmatic activity during a flare up. A review of continental arcs globally suggests that flare ups commonly coincide with landward arc migration and that this migration may start tens of millions of years before the flare up occurs. The region of magmatic activity, or arc width, can also expand significantly during a flare up. Arc migration or expansion into different mantle source regions and across lithospheric and crustal boundaries can cause temporal shifts in the radiogenic isotopic composition of magmatism. In the absence of arc migration, temporal shifts are more muted. Isotopic studies of mantle xenoliths and exposures of deep arc crust suggest that that primary, mantle-derived magmas generated during flare ups reflect substantial contributions from the subcontinental mantle lithosphere. Arc migration may be caused by a variety of mechanisms, including slab anchoring or slab folding in the mantle transition zone that could generate changes in slab dip. Episodic slab shallowing is associated with many tectonic processes in Cordilleran orogenic systems, like alternations between shortening and extension in the upper plate. Studies of arc migration may help to link irregular magmatic production in continental arcs with geodynamic models for orogenic cyclicity. 

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5. Syn‐Rift Magmatism and Sequential Melting of Fertile Lithologies in the Lithosphere and Asthenosphere

   https://doi.org/10.1029/2023JB027072  

   Mayle, M. ; Harry, D. L. ( September 2023 , Journal of Geophysical Research: Solid Earth)

   The timing and rate of decompression melting of a compositionally heterogeneous mantle during continental rifting are assessed with a new one‐dimensional geodynamic code, MELT1D. MELT1D computes pressure and temperature in the extending lithosphere and rising asthenosphere and calculates the resulting melt fraction and eruption rates for different lithologies. A series of models simulate syn‐rift melt production from (a) dry and wet depleted lherzolite similar to the mantle that underlies most mid‐ocean ridges, (b) dry and wet relatively fertile ultramafic compositions representing plume or primitive mantle material, (c) pyroxenite representing recycled ultramafic oceanic crust or magmatic metasomes, and (d) basalt representing recycled mafic crust or metasomes. The models predict sequential melting of the different compositions that is broadly consistent with basalt eruption histories in many Phanerozoic rifts. Results show a progressive transition in magma sources as the lithosphere thins, beginning with melting of wet mantle and compositionally fertile mafic components near the lithosphere‐asthenosphere boundary during the earliest stages of extension. This transitions to magmatism dominated by melting of relatively fertile ultramafic components (pyrolitic and pyroxenitic compositions) as extension progresses, and finally to melting of ambient lherzolite asthenosphere as lithosphere thinning approaches breakup. Mantle composition, pre‐rift lithosphere thickness, and mantle temperature exert the greatest controls on the timing and volumes of magmas produced from each lithology. In general, a cool or thick lithosphere has a greater capacity to sequester fertile lithologies than thin or warm lithosphere, and thus has a greater capacity to produce early syn‐rift magmas without requiring a hot mantle plume.

    

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