Search

The widespread High Arctic Large Igneous Province (HALIP) exhibits prolonged melting over more than 50 Myr, an observation that is difficult to reconcile with the classic view that large igneous provinces (LIPs) originate from melting in plume heads. Hence, the suggested plume‐related origin and classification of HALIP as a LIP have been questioned. Here, we use numerical models that include melting and melt migration to investigate a rising plume interacting with lithosphere of variable thickness, that is, a basin‐to‐craton setting applicable to the Arctic. Models reveal that melt migration introduces significant spatial and temporal variations in melt volumes and pulses of melt production, including protracted melting for at least about 30–40 Myr, because of the dynamic feedback between migrating melt and local lithosphere thinning. For HALIP, plume material deflected from underneath the Greenland craton can re‐activate melting zones below the previously plume‐influenced Sverdrup Basin after a melt‐free period of about 10–15 Myr, even though the plume is already ∼500 km away. Hence, actively melting zones do not necessarily represent the location of the deeper plume stem at a given time, especially for secondary pulses. Additional processes such as (minor) plume flux variations or local lithospheric extension may alter the timing and volume of HALIP pulses, but are to first order not required to reproduce the long‐lived and multi‐pulse magmatism of HALIP. Since melting zones are always plume‐fed, we would expect HALIP magmatism to exhibit plume‐related trace element signatures throughout time, potentially shifting from mostly tholeiitic toward more alkalic compositions. Plain Language Summary: Typically, the arrival of a large mantle upwelling ("mantle plume") is expected to cause catastrophic large‐scale volcanism that lasts a few million years. However, a massive past volcanic event now distributed onshore and offshore across the Arctic (the High Arctic Large Igneous Province—HALIP) defies this definition. This wide‐spread magmatism exhibits dates spanning more than 50 Myr, with several pulses of activity. Based on this prolonged magmatism, it has been questioned whether all of it can be attributed to a mantle plume, despite the geochemistry of basalts indicating a plume source. Here, we show that a plume can cause prolonged and multi‐pulse magmatism if it interacts with an increase in lithosphere thickness. Once the plume moves below the thicker lithosphere, hot plume material is channeled along the base of the lithosphere toward the adjacent thinner part, where it can reactivate previous melting regions. At this time, the active plume can be about 500 km away from the melting region, hence plume‐related melt cannot be used as a proxy for the plume position at the given time. Based on the models, we suggest that the prolonged HALIP magmatism was caused by a plume interacting with the edge of a craton. Key Points: Mantle plumes interacting with changes in lithosphere thickness at craton edges can cause prolonged melting with pulses in the same regionRejuvenated melting happens underneath previously melt‐affected thinned lithosphere several hundred km downstream of the plume stemThe timing and duration of rejuvenated melting in models correspond to and therefore may explain observations of magmatic pulses from High Arctic Large Igneous Province [ABSTRACT FROM AUTHOR]

Granitic magma bodies form in the ephemeral part of magma mush systems and are emplaced by a variety of mechanisms in different tectonic settings. This study investigates how granitic magma emplacement processes and tectonomagmatic interactions assert control over the architecture of mush state pluton‐scale magma transport pathways. The 1.45 Ga shallow‐crustal Götemar pluton is a 4.5 km diameter circular pluton that consists of three granite units: a coarse‐grained red granite, a medium‐grained pale to red granite, and fine‐grained pale microgranite sheets. We employed geological mapping supported by Anisotropy of Magnetic Susceptibility (AMS) to examine the magmatic and regional tectonic controls on late‐stage magma transport in the Götemar granitic magma mush system. Multiple parallel arcuate subhorizontal microgranite and medium‐grained granite sheets (from 0.1 to 10s of meters thick) were mapped within the pluton. The arcuate sheets pinch out from the northern part of the pluton toward the SE inferring magma propagation direction. A dominant set of vertical granitic sheets within the granite body strikes NW‐SE. The AMS fabrics are contact‐parallel in the main medium‐grained granite body and indicate inflation. Within the microgranite sheets, the AMS fabrics are parallel to the sheet strike and support a sheet propagation direction to the SE. The Götemar pluton displays a clear link between arcuate (concentric) magma‐transporting sheets and concentric strain‐partitioning related to the intrusion of medium‐grained granite magma. The vertical magma sheet orientations are consistent with an NE‐SW extensional stress field that is associated with the extensional back‐arc stress regime of the contemporary Hallandian Orogen. Plain Language Summary: The eruptive products of volcanoes are thought to be stored in pockets of melt in crystal‐dominated magmatic systems called crystal mushes prior to volcanic eruptions. An understanding of where magma is stored and how it is transported in mush systems is important in order to predict the eruptive behavior of the volcanic system. This contribution investigates the magma transport pathways in the Götemar granite in Sweden and its relationship to local magmatic deformation and regional deformation related to the Hallandian mountain building event. We show that magma is transported in vertical sheets parallel to the front of the Hallandian Orogen and laterally in sub‐horizontal arcuate sheets that reflect the circular shape of the granite pluton. Our study highlights the importance of understanding the shape and the formation of the magmatic granite body for deciphering the melt transport in the magma mush system under volcanoes. Key Points: Subhorizontal magma transport in a granitic magma mush controlled by magma emplacement structuresVertical magma sheets in granitic magma mush controlled by the regional stress fieldMagma transported in concentric magma fingers in the circular Götemar granite pluton [ABSTRACT FROM AUTHOR]

Melt transport across the ductile mantle is essential for oceanic crust formation or intraplate volcanism. However, mechanisms of melt migration and associated chemical interaction between melt and solid mantle remain unclear. Here, we present a thermo‐hydro‐mechanical‐chemical (THMC) model for melt migration coupled to chemical differentiation. We consider melt migration by porosity waves and a chemical system of forsterite‐fayalite‐silica. We solve the one‐dimensional (1D) THMC model numerically using the finite difference method. Variables, such as solid and melt densities or MgO and SiO2 mass concentrations, are functions of pressure (P), temperature (T), and total silica mass fraction (CTSiO2 ${C}_{\mathrm{T}}^{{\text{SiO}}_{2}}$). These variables are pre‐computed with Gibbs energy minimization and their variations with evolving P, T, and CTSiO2 ${C}_{\mathrm{T}}^{{\text{SiO}}_{2}}$ are implemented in the THMC model. We consider P and T conditions relevant around the lithosphere‐asthenosphere boundary. Systematic 1D simulations quantify the impact of initial distributions of porosity and CTSiO2 ${C}_{\mathrm{T}}^{{\text{SiO}}_{2}}$ on the melt velocity. Larger perturbations of CTSiO2 ${C}_{\mathrm{T}}^{{\text{SiO}}_{2}}$ cause larger melt velocities. An adiabatic or conductive geotherm cause fundamentally different vertical variations of densities and concentrations, and an adiabatic geotherm generates higher melt velocities. We quantify differences between melt transport (considering incompatible tracers), major element transport and porosity evolution. Melt transport is significant in the models. We also quantify the relative importance of four porosity variation mechanisms: (a) mechanical compaction and decompaction, (b) density variation, (c) compositional variation, and (d) solid‐melt mass exchange. In the models, (de)compaction dominates the porosity variation. We further discuss preliminary results of 2D THMC simulations showing blob‐like and channel‐like porosity waves. Plain Language Summary: Melt transport across the ductile mantle is essential for oceanic crust formation or intraplate volcanism. However, melt transport processes are still incompletely understood and poorly quantified. Xenoliths (inclusions in igneous rock entrained during magma ascent) sampled by kimberlites (an igneous rock that erupted from the mantle) or intraplate basalts provide evidence that there is a reaction (metasomatism) between the rising melt and the solid mantle. However, the impact of chemical processes on melt migration remains unclear. Here, we present a new mathematical two‐phase (fluid‐solid) model, based on fundamental laws of physics and thermodynamics, which couples melt migration with chemical processes. We study melt migration around the lithosphere‐asthenosphere boundary and consider the solid (not molten) rocks as highly viscous fluids due to the high temperatures in these regions (at 80–100 km depth). We present one‐dimensional results of computer simulations and show that the variation of chemical components, such as silicon dioxide, changes the densities of the solid and melt, and can, hence, have a considerable impact on melt migration. We also present two‐dimensional simulations, which show the channelization of the rising melt. Key Points: We present a thermo‐hydro‐mechanical‐chemical model for melt migration by porosity waves coupled to thermodynamic P–T–X phase calculationsAdiabatic or conductive geotherms cause different vertical variations of densities and concentrations as well as different melt velocitiesVariations in Si concentration change solid and melt densities, trigger porosity waves and impact melt velocity, porosity and permeability [ABSTRACT FROM AUTHOR]

To investigate channelization during migration of a reactive melt, we performed a series of Darcy‐type experiments in which an alkali basalt infiltrated partially molten harzburgites and lherzolites at a confining pressure of 300 MPa, temperatures of 1200°C and 1250°C, and pore pressure gradients of ~2 to 60 MPa/mm. We compare our results to those from previously published experiments performed on wehrlites. In all experiments, irrespective of the exact mineralogy, a planar reaction layer composed of olivine + melt developed in which all of the pyroxene was consumed. Under specific conditions controlled primarily by the melt flow velocity, finger‐like channels composed of olivine + melt also developed. In wehrlites, these reaction infiltration instabilities formed at 1200°C and 1250°C at pressure gradients >25 and >5 MPa/mm, respectively. In harzburgites, channelization occurred only at 1250°C at a pressure gradient of 35 MPa/mm. In lherzolites, a planar melt‐filled vein developed at 1250°C; no finger‐like channels formed under a pressure gradient of ~25 MPa/mm. Both the finger‐like channels and the planar vein led to very efficient extraction of melt from the reservoir. Channelization established large compositional variations over short distances in the crystalized phases as well as in the local melt and greatly enhanced the abundance of the reaction product, olivine, similar to dunite channels in the Earth. The range of chemical‐mechanical responses displayed by this array of compositions provides a set of targets for reactive transport and mechanical modeling studies. Key Points: Reaction infiltration instability develops in experiments on wehrlites and harzburgitesChannelization establishes large compositional variations over short distances in the crystalized phases as well as in local meltChannelization greatly increases the modal abundance of olivine in the partially molten rock [ABSTRACT FROM AUTHOR]