We simulated entrainment of carbonates (calcite, dolomite) in silicate impact melts by 1‐bar laser melting of silicate–carbonate composite targets, using sandstone, basalt, calcite marble, limestone, dolomite marble, and iron meteorite as starting materials. We demonstrate that carbonate assimilation by silicate melts of variable composition is extremely fast (seconds to minutes), resulting in contamination of silicate melts with carbonate‐derived CaO and MgO and release of CO2 at the silicate melt–carbonate interface. We identify several processes, i.e., (1) decomposition of carbonates releases CO2 and produces residual oxides (CaO, MgO); (2) incorporation of residual oxides from proximally dissociating carbonates into silicate melts; (3) rapid back‐reactions between residual CaO and CO2 produce idiomorphic calcite crystallites and porous carbonate quench products; (4) high‐temperature reactions between Ca‐contaminated silicate melts and carbonates yield typical skarn minerals and residual oxide melts; (5) mixing and mingling between Ca‐ or Ca,Mg‐contaminated and Ca‐ or Ca,Mg‐normal silicate melts; (6) precipitation of Ca‐ or Ca,Mg‐rich silicates from contaminated silicate melts upon quenching. Our experiments reproduce many textural and compositional features of typical impact melts originating from silicate–carbonate targets. They reinforce hypotheses that thermal decomposition of carbonates, rapid back‐reactions between decomposition products, and incorporation of residual oxides into silicate impact melts are prevailing processes during impact melting of mixed silicate–carbonate targets. However, by comparing our results with previous studies and thermodynamic considerations on the phase diagrams of calcite and quartz, we envisage that carbonate impact melts are readily produced during adiabatic decompression from high shock pressure, but subsequently decompose due to heat influx from coexisting silicate impact melts or hot breccia components. Under certain circumstances, postshock conditions may favor production and conservation of carbonate impact melts. We conclude that the response of mixed carbonate–silicate targets to impact might involve melting and decomposition of carbonates, the dominant response being governed by a complex variety of factors.

Past orbital parameters of the Moon are difficult to reconstruct from geological records because relevant data sets of tidal strata are scarce or incomplete. The sole Archean data point is from the Moodies Group (ca 3.22 Ga) of the Barberton Greenstone Belt, South Africa. From the time‐series analysis of tidal bundles from a well‐exposed subaqueous sand wave of this unit, Eriksson and Simpson (Geology, 28, 831) suggested that the Moon’s anomalistic month at 3.2 Ga was closer to 20 days than the present 27.5 days. This is in apparent accordance with models of orbital mechanics which place the Archean Moon in a closer orbit with a shorter period, resulting in stronger tidal action. Although this study’s detailed geological mapping and section measuring of the site confirmed that the sandstone bed in question is likely a migrating dune, the presence of angular mud clasts, channel‐margin slumps, laterally aggrading channel fills and bidirectional paleocurrents in overlying and underlying beds suggests that this bedform was likely located in a nearshore channel near lower‐intertidal flats and subtidal estuarine bars; it thus carries risk of incomplete preservation. Repeated measurements of foreset thicknesses along the published traverse, measured perpendicular to bedding, failed to show consistent spectral peaks. Larger data sets acquired along traverses measured parallel to bedding along the 20.5 m wide exposure are affected by minor faulting, uneven outcrop weathering, changing illumination, weather, observer bias and show a low reproducibility. The most robust measurements herein confirm the periodicity peak of approximately 14 in the original data of Eriksson and Simpson (Geology, 28, 831). Because laminae may have been eroded, the measurements may represent a lower bound of about 28 lunar days per synodic month. This estimate agrees well with Earth–Moon dynamic models which consider the conservation of angular momentum and place the Archaean Moon in a lower orbit around a faster‐spinning Earth.

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<p>Fluid-rock interaction is one of the most important factors regarding the evolution of the Earth’s crust, as it is strongly affecting its petrophysical properties and enabling chemical transport. Therefore, its impact on the Earth’s crustal chemical reservoirs and geodynamic processes can be significant. Fluid-mediated mineral reactions are dependent on the availability of fluids and their capability to percolate through the rock and interact with the minerals, often through pre-existing fluid pathways.</p> <p>The Kråkenes Gabbro is a mafic enclave, embedded in the felsic gneisses of the Western Gneiss Region in Norway. Although the whole region reached (ultra-)high pressure metamorphic conditions, the gabbro remained in a metastable state and preserved its igneous textures and magmatic minerals. The dry and low permeability gabbro is cut by a N-S-trending fracture network of mode-I cracks, which opened during exhumation. These fractures served as fluid pathways for an aqueous fluid to infiltrate the rock and trigger mineral reactions. Along these fractures the dry gabbro is “hydrated” under amphibolite-facies conditions. The resulting amphibolite reaction front is sharp on outcrop scale and propagates on dm-scale into the gabbro. A complete profile of rock spanning 32 cm in length was taken perpendicular to the vein, including sample material from the vein, the alteration zone, and the mostly pristine gabbroic wall rock.</p> <p>The gabbro-amphibolite-transition is displayed by the development of a hydrous mineral assemblage, accompanied with a densification and therefore porosity formation. The main cause of this is a drop in the abundance of plagioclase during the amphibolitization. Thermodynamic analysis using Thermolab were done to predict the amphibolite mineral assemblage from the original bulk rock composition of the gabbro. The calculations reveal that mainly H<sub>2</sub>O is added to the system and minor further element transport is needed. Furthermore, we observe that even the most reacted amphibolite still contains unaffected gabbroic mineral relicts and the main chemical reactions during amphibolitization are limited to a few minerals. The incoming fluid is consumed as soon as the hydrous phases of the amphibolite are formed. As amphibolitization favors porosity formation, a free fluid phase remains in the pore space as soon as the gabbro at the reactive surface of the affected minerals is completely transformed. The fluid progresses through the newly formed pore space and advances as a sharp the amphibolitization front.</p> <p>In order to test our hypothesis, we formulate a reactive flow model based on local equilibrium thermodynamics, mass balance and Darcy flow, that simulates the hydration of the dry gabbro to amphibolite including the porosity and fluid pressure evolution. Results confirm the formation of a sharp reaction front and the decrease in porosity during the hydration as a potential physical explanation for the observations without the further need for kinetically delayed reactions. We conclude that the metastability of gabbro is mostly controlled by the availability of fluid to the rock.</p>