The accuracy of diffusion chronometry is entirely dependent on experimental calibrations of diffusivity. Corroboration of experimental diffusivities using natural systems is thus extremely valuable. For Ti-inquartz, this would mean finding systems where (1) time scales relevant for quartz crystallization are independently well-constrained, and (2) Ti profiles have been measured in quartz, then determining diffusion time scales using the experimental diffusivities. Gualda and Pamukçu (2020) have carried out such an exercise and concluded that the Cherniak et al. (2007) diffusivities are compatible with geological observations, whereas those of Jollands et al. (2020), are not; however, in the examples that they provide, conditions 1 and/or 2 are not met. Firstly, Gualda and Pamukçu suggest that time scales derived using the new diffusivities are inconsistent with fast quartz growth during decompression. This is valid if quartz growth is indeed due to decompression, which is debatable. Blundy and Cashman (2001) used experimental phase equilibria to show that quartz should be resorbed during decompression of hydrous rhyolitic melt, and Evans et al. (2016) suggested that the Bishop Tuff quartz rims relate not to decompression but to a recharge event. In the latter case, acceptable time scales could plausibly be much longer than those associated with decompression. Secondly, considering time scales from melt inclusion faceting, it is notable that the Gualda et al. (2012) study begins with the caveat that the "kinetics of melt inclusion faceting has not been treated in detail" and that their model constitutes a "simplified treatment" only. To our knowledge, the model has not been experimentally validated. Melt inclusion faceting, while potentially a powerful tool, does not currently provide well-constrained time scales to which others can be compared. Thirdly, crystal size distributions (CSDs) apparently yield time scales in line with those determined from Ti-in-quartz profiles using the Cherniak et al. (2007) diffusivities. In this case, there is circular logic. Extracting time scales from CSDs requires a growth rate. Both Gualda et al. (2012) and Pamukçu et al. (2020) extract growth rates from Ti-inquartz diffusion profiles using the data set of Cherniak et al. (2007). CSDs in the Gualda et al. (2012) and Pamukçu et al. (2020) studies therefore do not provide an independent constraint to evaluate differing Ti-in quartz diffusivities. Fourthly, turning to Oruanui, it is indeed the case that the Ti-in-quartz profiles of Pamukçu et al. (2020), fitted using the Cherniak et al. (2007) diffusivities, give a <3 k.y. time scale. This is consistent with the time interval between eruptions determined by radiocarbon dating (e.g., Wilson et al., 2005). Applying the Jollands et al. (2020) diffusivities to the Pamukçu et al. (2020) data, after accounting for effects of beam convolution, ~50% of the time scales are over 3 k.y.. Approximately 90% are <40 k.y., the time associated with assembly of the system, although this may relate ...

&lt;p&gt;The dehydration of serpentinite during subduction and the associated formation of dehydration veins is an important process for the global water cycle and the dynamics of the subducting plate. Field observations suggest that olivine veins can form by dehydration during viscous shear deformation of serpentinite. However, this hypothesis of olivine vein formation, involving the coupling of rock deformation, dehydration reactions and fluid flow, has not been tested and quantified by hydro-mechanical-chemical (HMC) models. Here, we present a new two-dimensional HMC numerical model to test whether olivine veins can form by dehydration during viscous shearing of serpentinite. The applied numerical algorithm is based on the pseudo-transient finite difference method. We consider the simple reaction antigorite + brucite = forsterite + water. Volumetric deformation is viscoelastic and shear deformation is viscous with a shear viscosity that is an exponential function of porosity. In the initial model configuration, total and fluid pressures are homogeneous and in the antigorite stability field. Small, initial perturbations in porosity, and hence in viscosity, cause pressure perturbations during far-field simple shearing. During shearing, the fluid pressure can locally decrease and reach the thermodynamic pressure required for the dehydration reaction, so that dehydration is triggered locally. The simulations show that dehydration veins form during progressive shearing and grow in a direction parallel to the maximum principal stress. During the dehydration the porosity can increase locally from 2% (initial value) to more than 50% inside the dehydration vein. The numerical model allows quantifying the mechanisms and variables that control the evolution of porosity and fluid pressure. We show that the porosity evolution is controlled by three mechanisms: (1) volumetric deformation of the porous solid, (2) temporal variation of the solid density and (3) mass transfer during the dehydration reaction. We quantify the evolution of the fluid pressure that is controlled by five variables and processes: (1) the total pressure of the porous rock, (2) elastic effects of the total volumetric deformation, (3) the temporal variation of porosity, (4) the temporal variation of solid density and (5) mass transfer during the dehydration reaction. This model supports the observation-based hypothesis of the formation of olivine veins due to dehydration during viscous shearing of serpentinite. More generally, our HMC model provides quantitative insights into the evolution of porosity, and hence dynamic permeability, fluid pressure and mass transfer during dehydration reactions in deforming rock.&lt;/p&gt;

The rift-to-drift transition at rifted margins is an area of active investigation due to unresolved issues of the ocean-continent transition (OCT). Deep structures that characterize modern OCTs are often difficult to identify by seismic observations, while terrestrial exposures are preserved in fragments separated by tectonic discontinuities. Numerical modeling is a powerful method for contextualizing observations within rifted margin evolution. In this article, we synthesize geological observations from fossil ocean-continent transitions preserved in ophiolites, a recent seismic experiment on the Ivorian Margin of West Africa, and GeoFLAC models to characterize mantle deformation and melt production for magma-poor margins. Across varied surface heat fluxes, mantle potential temperatures, and extension rates our model results show important homologies with geological observations. We propose that the development of large shear zones in the mantle, melt infiltration, grain size reduction, and anastomosing detachment faults control the structure of OCTs. We also infer through changes in fault orientation that upwelling, melt-rich asthenosphere is an important control on the local stress environment. During the exhumation phase of rifting, continentward-dipping shear zones couple with seaward-dipping detachment faults to exhume the subcontinental and formerly asthenospheric mantle. The mantle forms into core-complex-like domes of peridotite at or near the surface. The faults that exhume these peridotite bodies are largely anastomosing and exhibit magmatic accretion in their footwalls. A combination of magmatic accretion and volcanic activity derived from the shallow melt region constructs the oceanic lithosphere in the footwalls of the out-of-sequence, continentward-dipping detachment faults in the oceanic crust and subcontinental mantle.

The rift-to-drift transition at rifted margins is an area of active investigation due to unresolved issues of the ocean-continent transition (OCT). Deep structures that characterize modern OCTs are often difficult to identify by seismic observations, while terrestrial exposures are preserved in fragments separated by tectonic discontinuities. Numerical modeling is a powerful method for contextualizing observations within rifted margin evolution. In this article, we synthesize geological observations from fossil ocean-continent transitions preserved in ophiolites, a recent seismic experiment on the Ivorian Margin of West Africa, and GeoFLAC models to characterize mantle deformation and melt production for magma-poor margins. Across varied surface heat fluxes, mantle potential temperatures, and extension rates our model results show important homologies with geological observations. We propose that the development of large shear zones in the mantle, melt infiltration, grain size reduction, and anastomosing detachment faults control the structure of OCTs. We also infer through changes in fault orientation that upwelling, melt-rich asthenosphere is an important control on the local stress environment. During the exhumation phase of rifting, continentward-dipping shear zones couple with seaward-dipping detachment faults to exhume the subcontinental and formerly asthenospheric mantle. The mantle forms into core-complex-like domes of peridotite at or near the surface. The faults that exhume these peridotite bodies are largely anastomosing and exhibit magmatic accretion in their footwalls. A combination of magmatic accretion and volcanic activity derived from the shallow melt region constructs the oceanic lithosphere in the footwalls of the out-of-sequence, continentward-dipping detachment faults in the oceanic crust and subcontinental mantle.

&lt;p&gt;The volcanic&amp;#8211;plutonic connection plays a fundamental role for magmatic systems, linking crystallising plutons, volcanic activity, volatile exsolution and ore deposits. Nonetheless, our understanding of the nature of these links is limited by the scarcity of outcrops exhibiting clear relationships between the plutonic roots that feed its volcanic counterpart. One way to better characterise the volcanic&amp;#8211;plutonic connection is to quantify the amount and rates of melt segregation within a crystallising plutonic body, and to compare the volumes and rates with recent silicic eruptions. Here we investigate the processes of interstitial melt segregation in the calc-alkaline Western Adamello (WA) pluton. The WA tonalite (WAT) is part of the southern Alps and represents an intrusive body emplaced at 2.5 kbar in ~1.2 Myr (Floess and Baumgartner, 2015; Schaltegger &lt;em&gt;et al.&lt;/em&gt;, 2019). The WAT exhibits a coarse-grained, equigranular texture and is composed of hornblende partially replaced by biotite, plagioclase, quartz, K-feldspar, apatite, zircon, and secondary epidote. K-feldspar, quartz and albite-rich&amp;#160;plagioclase (An&lt;sub&gt;25-40&lt;/sub&gt;) are late and occur as interstitial phases. Several types of igneous structures, constituting &lt;0.5 vol.% of the WA, are found, comprising: (i) hornblende and biotite accumulations (0.1&amp;#8211;30 m) with interstitial K-feldspar, quartz and albite-rich plagioclase (An&lt;sub&gt;25-40&lt;/sub&gt;) representing 25&amp;#8211;45 vol.% of the rock; (ii) plagioclase (An&lt;sub&gt;40-70&lt;/sub&gt;) accumulations with 40&amp;#8211;50 vol.% of the same interstitial assemblage; and (iii) quartz-, albite- and K-feldspar-rich domains (0.1&amp;#8211;10 m) containing WAT-derived plagioclase phenocrysts which form either zoned aplitic to pegmatitic dikes or schlieren-shaped bodies probably representing &lt;em&gt;in situ&lt;/em&gt; melt segregations. The latter are spatially associated with the accumulation zones. Hornblende, biotite, and plagioclase phenocrysts have essentially the same compositional range in accumulations and segregations. This observation indicates that deformation-driven crystal&amp;#8211;crystal and crystal&amp;#8211;melt segregation operated within the host tonalite. Quantitative modal compositions and mass balance calculations indicate that the hornblende&amp;#8211;biotite accumulations lost 60&amp;#8211;90 vol.% of their plagioclase phenocrysts and 20&amp;#8211;55 vol.% interstitial melt, whereas the plagioclase accumulations lost up to 15 vol.% melt. Such calculations place the maximum efficiency of crystal&amp;#8211;melt segregation to 40&amp;#8211;55 % in the WAT, as most of the melt remains trapped within the crystal framework. Based on phase relationships and major element modelling, it is proposed that the peritectic relationship hornblende + melt&lt;sub&gt;1&lt;/sub&gt; = biotite + quartz + melt&lt;sub&gt;2&lt;/sub&gt;&amp;#160;and the efficiency of plagioclase&amp;#8211;melt separation&amp;#160;are linked to the variable composition of the felsic dikes. Such a reaction is known from experimentally derived phase relationships of tonalite (Marxer and Ulmer, 2019) and probably plays a fundamental role linking pluton solidification and extraction of interstitial liquid.&lt;/p&gt;