1. INTRODUCTIONThe processes that influence differentiation in magma chambers and the rate at which the associated melt-crystal phases separate have important ramifications for volcanic-plutonic connections among silicic igneous rocks. Related to volcanic-plutonic connections among silicic igneous rocks is the identification of cumulate signature in plutons. The subtlety of crystal accumulation signals in silicic igneous rocks has led to their interpretation as representing a true melt composition and being genetically separated from volcanic rocks (Coleman et al. , 2004, Glazner et al. , 2004). While the petrological signature of the cumulate nature of silicic magmas is subtle, it is discernible nonetheless (Bachmann et al. , 2007, Deering & Bachmann, 2010, Gelman et al. , 2014). Knowledge of how melt loss occurs at melt fractions relevant to silicic magma chambers and the associated textural and chemical indicators can facilitate identification of cumulates in plutons. Furthermore, the rate at which the associated melt-crystal phases separate have important ramifications for volcanic hazards. Here, we investigate the Spirit Mountain Batholith (SMB) for chemical and textural evidence of crystallization-differentiation and phase separation by repacking-driven compaction (grain reorganizations).The paper is organized such that we first introduce the geologic setting of the region and of the SMB in particular and provide evidence from previous studies supporting melt loss in the deeper parts of the SMB. Then, results of geochemical analyses are provided, including acquisition of major, minor, and trace elements of bulk rock SMB samples and results from plagioclase composition analyses. Subsequently, textural analyses of selected SMB samples are presented. We identify a near linear unmixing trend in major, minor, and trace element geochemistry defined by samples within a ca. 3 km transect at the base of the exposed batholith and pooled leucogranites near the top of the batholith. The plagioclase compositions suggest that the samples crystallized from the same parental magma and that the magma was less mafic than their bulk rock compositions. We then introduce a trace element model that allows melt and crystal to be lost to estimate relative melt loss (cumulate) or crystal loss (silicic cap) in the SMB. The benefit of this model is that it doesn’t assume a particular separation mechanism; however, it is limited in that it doesn’t provide the range of crystallinities over which melt is lost and doesn’t allow calculation of trapped melt fractions. To accomplish this, we use an unmixing model that treats the analyzed samples as combinations between two different endmembers: melt and crystal at a certain crystallinity. The trapped melt fraction profile is then compared to results from a model of mush compaction based on a crystal repacking rheology to provide order of magnitude timescale estimates for the growth of the silicic cap (melt accumulation layer).

The terrestrial planetary bodies display a wide variety of surface expressions and histories of volcanic and tectonic, and magnetic activity, even those planets with apparently similar dominant modes of heat transport (e.g., conductive on Mercury, the Moon, and Mars). Each body also experienced differentiation in its earliest evolution, which may have led to density-stabilized layering in its mantle and a heterogenous distribution of heat-producing elements. We explore the hypothesis that mantle structure exerts an important control on the occurrence and timing of geological processes such as volcanism and tectonism. We investigate numerically the behavior of an idealized model of a planetary body where heat-producing elements are assumed to be sequestered in a stabilized layer at the top or bottom of the mantle. We find that the mantle structure alters patterns of heat flow at the boundaries of major heat reservoirs: the mantle and core. This modulates the way in which heat production influences geological processes. In the model, mantle structure is a dominant control on the relative timing of fundamental processes such as volcanism, magnetic field generation, and expansion/contraction, the record of which may be observable on planetary body surfaces. We suggest that Mercury exhibits characteristics of shallow sequestration of heat producing elements and that Mars exhibits characteristics of deep sequestration.

Darien Florez1,2, Christian Huber1, Susana Hoyos2, Matej Pec2, E.M. Parmentier1, James A. D. Connolly3, Greg Hirth11Department of Earth, Environmental and Planetary Sciences, Brown University, Providence, RI, USA2Department of the Earth, Atmospheric and Planetary Sciences, Massachusetts Institute of Technology, Cambridge, MA, USA3Department of Earth Sciences, ETH Zurich, Zürich, SwitzerlandCorresponding author: Darien Florez ([email protected])Key Points:Continuum model fits repacking experiments data of Hoyos et al.(2022) despite their stochastic nature.At intermediate melt fractions, mechanical repacking of particles may contribute significantly to the resistance of mushes to compaction.Particle-particle friction, rather than hydrodynamic effects, dominates viscous resistance associated with mechanical repacking.AbstractBefore large volumes of crystal poor rhyolites are mobilized as melt, they are extracted through the reduction of pore space within their corresponding crystal matrix (compaction). Petrological and mechanical models suggest that a significant fraction of this process occurs at intermediate melt fractions (ca. 0.3 – 0.6). The timescales associated with such extraction processes have important ramifications for volcanic hazards. However, it remains unclear how melt is redistributed at the grain-scale and whether using continuum scale models for compaction is suitable to estimate extraction timescales at these melt fractions. To explore these issues, we develop and apply a two-phase continuum model of compaction to two suites of analog phase separation experiments – one conducted at low and the other at high temperatures, T, and pressures, P. We characterize the ability of the crystal matrix to resist porosity change using parameterizations of granular phenomena and find that repacking explains both datasets well. Furthermore, repacking may explain the difference in compaction rates inferred from high T + P experiments and measured in previous deformation experiments. When upscaling results to magmatic systems at intermediate melt fractions, repacking may provide an efficient mechanism to redistribute melt. Finally, outside nearly instantaneous force chain disruption events occasionally recorded in the low T + P experiments, melt loss is continuous, and two-phase dynamics can be solved at the continuum scale with an effective matrix viscosity. Further work, however, must be done to develop a framework to parameterize the effect of particle size and shape distributions on compaction.

Particle-fluid separation by settling is an ubiquitous process in Earth and Planetary Sciences. The rate of growth and the initial structure of cumulate layers in magma oceans or, over smaller scales, crustal magmatic systems depend on the crystal settling dynamics in melt-rich environments. The settling velocity of particles is controlled by a balance between buoyancy

Compositional heterogeneities within Europa’s ice shell likely impact the dynamics and habitability of the ice and subsurface ocean, but the total inventory and distribution of impurities within the shell is unknown. In sea ice on Earth, the thermochemical environment at the ice-ocean interface governs impurity entrainment into the ice. Here, we simulate Europa’s ice-ocean interface and bound the impurity load (1.053-14.72 g/kg (parts per thousand weight percent, or ppt) bulk ice shell salinity) and bulk salinity profile of the ice shell. We derive constitutive equations that predict ice composition as a function of the ice shell thermal gradient and ocean composition. We show that evolving solidification rates of the ocean and hydrologic features within the shell produce compositional variations (ice bulk salinities of 5-50% of the ocean salinity) that can affect the material properties of the ice. As the shell thickens, less salt is entrained at the ice-ocean interface, which implies Europa’s ice shell is compositionally homogeneous below ~ 1 km. Conversely, the solidification of water filled fractures or lenses introduces substantial compositional variations within the ice shell, creating gradients in mechanical and thermal properties within the ice shell that could help initiate and sustain geological activity. Our results suggest that ocean materials entrained within Europa’s ice shell affect the formation of geologic terrain and that these structures could be confirmed by planned spacecraft observations.

Constraining the volatile budget of the lunar interior has important ramifications for models of Moon formation. While many early and previous measurements of samples acquired from the Luna and Apollo missions suggested the lunar interior is depleted in highly volatile elements like H, a number of high-precision analytical studies over the past decade have argued that it may be more enriched in water than previously thought. Here, we integrate recent remotely sensed near-infrared reflectance measurements of several Dark-Mantle-Deposits (DMDs), interpreted to represent pyroclastic deposits, and physics-based eruption models to better constrain the pre-eruptive water content of lunar volcanic glasses. We model the trajectory and water loss of pyroclasts from eruption to deposition, coupling eruption dynamics with a volatile diffusion model for each pyroclast. Modeled pyroclast sizes and final water contents are then used to predict spectral reflectance properties for comparison with the observed orbital near-infrared data. We develop an inversion scheme based on the Markov-Chain Monte-Carlo (MCMC) method to retrieve constraints between governing parameters such as the initial volatile content of the melt and the pyroclast size distribution (which influences the remotely measured water absorption strengths). The MCMC inversion allows us to estimate the primordial (pre-eruption) water content for different DMDs and test whether their source is volatile-rich. Our results suggest that the parts of the lunar interior sampled by the source material of the DMDs investigated in this study range in water content from 400 to 800 ppm.

We investigate the structure and evolution of multiphase ice-ocean interfaces (‘mushy layers’) and the implications for the geophysics and habitability of ice-ocean worlds. Understanding the potential diversity of these multiphase layers across solar system bodies provides insight into the potential rates and mechanisms of heat and solute transport between their respective oceans and ice shells - which remain largely unconstrained. Additionally, variations in mushy layer properties may drive diverse geophysical processes unique to individual bodies or that may vary regionally on an individual icy world. We explore mushy layer evolution by analytically solving for the thickness of a simplified ice-ocean mushy layer system. We investigate two dynamic regimes, one driven by molecular diffusion and one driven by convection of brine within the mushy layer. We analyze the impact of gravity, thermal gradient, and ocean composition on the thickness of mushy layers. Additionally, a perturbation analysis is carried out to investigate the existence of mushy layer steady states. We show that stable mushy layers exist when ice shells are thickening, suggesting that mushy layers are likely persistent and common features of growing ice shells and accretionary regions of ice-ocean worlds.

Non-ice impurities within the ice shells of ocean worlds (e.g., Europa, Enceladus, Titan) are believed to play a fundamental role in their geophysics and habitability and may become a surface expression of subsurface ocean properties. Heterogeneous entrainment and distribution of impurities within planetary ice shells have been proposed as mechanisms that can drive ice shell overturn, generate diverse geological features, and facilitate ocean-surface material transport critical for maintaining a habitable subsurface ocean. However, current models of ice shell composition suggest that impurity rejection at the ice-ocean interface of thick contemporary ice shells will be exceptionally efficient, resulting in relatively pure, homogeneous ice. As such, additional mechanisms capable of facilitating enhanced and heterogeneous impurity entrainment are needed to reconcile the observed physicochemical diversity of planetary ice shells. Here we investigate the potential for hydrologic features within planetary ice shells (sills and basal fractures), and the unique freezing geometries they promote, to provide such a mechanism. By simulating the two-dimensional thermal and physicochemical evolution of these hydrological features as they solidify, we demonstrate that bottom-up solidification at sill floors and horizontal solidification at fracture walls generate distinct ice compositions and provide mechanisms for both enhanced and heterogeneous impurity entrainment. We compare our results with magmatic and metallurgic analogs that exhibit similar micro- and macroscale chemical zonation patterns during solidification. Our results suggest variations in ice-ocean/brine interface geometry could play a fundamental role in introducing compositional heterogeneities into planetary ice shells and cryoconcentrating impurities in (re)frozen hydrologic features.

We propose a new approach to the solution of the wave propagation and full waveform inversions (FWIs) based on a recent advance in deep learning called Physics-Informed Neural Networks (PINNs). In this study, we present an algorithm for PINNs applied to the acoustic wave equation and test the model with both forward wave propagation and FWIs case studies. These synthetic case studies are designed to explore the ability of PINNs to handle varying degrees of structural complexity using both teleseismic plane waves and seismic point sources. PINNs’ meshless formalism allows for a flexible implementation of the wave equation and different types of boundary conditions. For instance, our models demonstrate that PINN automatically satisfies absorbing boundary conditions, a serious computational challenge for common wave propagation solvers. Furthermore, a priori knowledge of the subsurface structure can be seamlessly encoded in PINNs’ formulation. We find that the current state-of-the-art PINNs provide good results for the forward model, even though spectral element or finite difference methods are more efficient and accurate. More importantly, our results demonstrate that PINNs yield excellent results for inversions on all cases considered and with limited computational complexity. Using PINNs as a geophysical inversion solver offers exciting perspectives, not only for the full waveform seismic inversions, but also when dealing with other geophysical datasets (e.g., magnetotellurics, gravity) as well as joint inversions because of its robust framework and simple implementation.