Basaltic Lithologies Research Articles

Holocene environmental and climate evolution of central west Patagonia as reconstructed from lacustrine sediments of Meseta Chile Chico (46.5° S, Chile)

Abstract. Holocene environmental changes in Patagonia were mostly shaped by fluctuating ice cover recession. Consequently, environmental reconstructions are largely based on discontinuous moraine chronologies from valley deposits. Here, we present a 3 m long continuous sediment record recovered from Laguna Meseta (LME), a lake located on Meseta Chile Chico. Its altitude and location relative to the North Patagonian Icefield provide a unique opportunity to reconstruct the glacial history and related environmental dynamics. Our radiocarbon chronology constrains sedimentation to the last ∼10 000 years and provides a minimum age for postglacial ice-free lacustrine conditions, due to a westward retreat of the ice cap. Lacustrine productivity reached its maximum at the start of the lake phase and decreased afterwards. Between 5500 and 4600 cal yr BP, a major shift towards allochthonous sediment accumulation occurred, caused by an abrupt increase in clastic deposition from basaltic lithologies of the Meseta Chile Chico. This episode correlates with the precipitation-driven Mid-Holocene glacier advance of Patagonian glaciers and suggests that conditions were colder and/or wetter on Meseta Chile Chico at that time. After 4600 cal yr BP, these conditions continued to supply LME with clastic sediments until a stepped decrease around 900 cal yr BP. Thereupon, lacustrine productivity distinctly increased and stabilized around 300 cal yr BP. Our findings indicate that changes in sedimentation on Meseta Chile Chico were mainly controlled by regional variability in the precipitation. Furthermore, strong correlation between our records and available proxies for oscillations of the Southern Hemisphere Westerly Winds suggest a pronounced climatic control by this prominent wind system for central west Patagonia during the last 10 000 years.

Significance of geomorphic indicators for identification of groundwater recharge potential in the Khapri watershed of Deccan Volcanic Province (DVP), district Dangs, western India

The district Dangs of western India, marks the extension of Western Ghats in Gujarat state. It is characterized by less transmissible basaltic lithology of Deccan Volcanic Province (DVP), rugged trappean highlands and moderate to very steeply sloping surfaces. Despite receiving high average annual rainfall (2000 mm) the terrain framework produces high runoff than infiltration (recharge), leading to groundwater crisis. In such terrains, where lithology is homogenous, the natural groundwater recharge is significantly influenced by geomorphic indicators (landforms, drainage characteristics and slope). Therefore, identification of groundwater recharge potential (here after referred as GWRPOT) is crucial for the sustainable development of groundwater resource. In the present study GWRPOT is identified on the basis of geomorphic indicators (Gi) in sub-watersheds of Khapri, district Dangs of DVP. To generate the database on geomorphic indicators the IRS-LISS-III (24 m) satellite data, Survey of India topographic maps (1:50,000), and SRTM-DEM are used. The ranks are assigned to geomorphic indicators of each sub-watershed by considering their aerial coverage, influence on GWRPOT and field studies. The summation of average ranks of each geomorphic indicator viz., landform (Gc), drainage characteristics (Dc) and slope (Sc) is used to compute Gi value. On the basis of Gi, the GWRPOT is identified and categorized as very poor (SW-5), poor (SW-3, SW-2, SW-10), moderate (SW-7, SW-4), good (SW-11), very good (SW-1, SW-8, SW-9) and excellent (SW-6). These results are validated with seasonal groundwater level fluctuations in 33 dug-wells, which shows positive correlation between Gi and groundwater fluctuation (R2 = 0.77). Further, the sub-watershed prioritization is carried out to highlight the need of improving the GWRPOT and thereby sustainability.

High-pressure phase transition and equation of state of hydrous Al-bearing silica

Abstract Stishovite, a rutile-structured polymorph of SiO2, is a main component of subducted basaltic lithologies in the lower mantle. At mid lower-mantle depths, a second-order ferroelastic transition to orthorhombic CaCl2-type (post-stishovite) structure occurs, causing extensive elastic shear softening. Previous studies showed that Al incorporation can decrease the transition pressure, while it is still debated whether H has a similar effect. Here we report the equations of state, structural evolution, and phase transformation of Si0.948Al0.052O1.983H0.018 (Al5) stishovite and Si0.886Al0.114O1.980H0.074 (Al11) post-stishovite samples using diamond-anvil cells in combination with synchrotron X-ray diffraction and Raman spectroscopy. The Al5 sample transformed to the orthorhombic polymorph upon compression to 16 GPa, displaying a drop of ~12% in its bulk modulus across the transformation. The Al11 sample did not undergo any phase transition in the pressure range investigated. Single-crystal structural refinements and Raman spectroscopy measurements on the Al5 sample show that the soft optic mode B1g is decoupled from the tetragonal-to-orthorhombic structural transformation and shows a plateau in the stability field of post-stishovite, between 20 and 30 GPa. This observation indicates that the transformation is not pseudo-proper ferroelastic as in SiO2 stishovite and that existing Landau expansions are likely not applicable to H-rich Al-bearing silica samples. Using the equation of state parameters of orthorhombic Al5 and Al11 and literature data on SiO2 post-stishovite we then discuss the possibility of non-ideal mixing along the SiO2-AlOOH join.

Melting of basaltic lithologies in the Earth's lower mantle

As basaltic rocks formed at the base of the oceanic floor are transported back to the Earth's mantle along subduction zones, they undergo transitions and introduce compositional and thermal heterogeneities in the deeper parts of the mantle. Studying the melting phase relations of basaltic lithologies at elevated pressures and temperatures provides insights into what potentially happens at different depths in the lower mantle now and throughout the past billion years of active plate tectonics. Using laser heated – diamond anvil cell experiments combined with in situ X-Ray Diffraction measurements at synchrotron sources, we revisit the crystallization and melting properties of natural basaltic samples at 60–100 GPa and up to 4000 K. Diffraction patterns highlight the major phases: bridgmanite and Ca-perovskite, followed by crystallization of Si-rich phases (mainly stishovite) and Calcium Ferrite (CF-type) Na and Al-rich phase. Recovered samples were prepared using focused ion beam techniques for detailed chemical analyses of the extracted thin sections by electron microscopies in order to resolve sub-micron features and understand the chemical partitioning of elements induced by melting at high pressure and temperature conditions. We confirm that the liquidus phase is Ca-perovskite, which segregates during melting and is recovered as rings that encapsulate a melt pool throughout the studied pressure range. The melt pocket shows a concentric structure consisting of an alumino-silicate envelope surrounding an Fe-rich silicate part. At the center of samples, an Fe-O-S metal pond is often observed. We associate the observation of segregation of liquid phases to capillary forces. The differentiation of melt pockets into three melts is tentatively attributed to Marangoni effects, i.e. temperature-induced surface tension gradients in the samples. Central metal ponds are indirectly best interpreted as related to the disproportionation reaction of Fe2+ into Fe3+ and Fe(0) in bridgmanite whereas the two silicate-melt pools could be associated to the formation of two immiscible liquids upon melting of basalts. On the basis of these observations, we propose that melting of basaltic lithologies at lower mantle pressures could lead to important chemical differentiation mostly characterized by Fe enrichment at increasing depth.

Early silicic magmatism on a differentiated asteroid

Unlike the other terrestrial planets, Earth has a substantial silica-rich continental crust with a bulk andesitic composition. A small number of meteorites with andesitic bulk compositions have been identified that are thought to be the products of partial melting of chondritic protoliths, a mode of petrogenesis distinct from that of Earth’s continental crust. Here we show, using geochemical analyses, that unlike other known andesitic meteorites, Erg Chech 002 has strongly fractionated and low abundances of the highly siderophile elements and mineralogy consistent with origin from a melt. The meteorite’s bulk composition, which is similar to terrestrial andesites, cannot be explained by partial melting of basaltic lithologies and instead requires a metal-free chondritic source. We argue that Erg Chech 002 probably formed by ~15–25% melting of the mantle of an alkali-undepleted differentiated asteroid. Our findings suggest that extensive silicate differentiation after metal–silicate equilibration of chondritic parent bodies was already occurring within the first 2.25 million years of Solar System history and that andesitic crust formation does not necessarily require plate tectonics.

Comparing headwater stream thermal sensitivity across two distinct regions in Northern California

AbstractThermal regimes in headwater streams are critical for freshwater ecological condition and habitat resilience to disturbance, and to inform sustainable forest management. However, stream temperatures vary depending on characteristics of the stream, catchment, or region. To improve our knowledge of stream thermal regimes, we collected stream and air temperature data along eight headwater streams in two regions in Northern California. Five streams were in the Coast Range, which is characterized by permeable sandstone lithology, rain dominated precipitation regime, and dense coast redwood forests. Three streams were in the Cascade Range, which is characterized by fractured and resistant basalt lithology, snow dominated precipitation, and low to moderate density pine forests. We instrumented each stream with 12 stream temperature and four air temperature sensors during summer 2018. We compared stream thermal regimes and thermal sensitivity—slope of the linear regression between daily stream and air temperature—within and between study regions. Mean daily stream temperatures were ~4.7°C warmer in the Coast Range but were less variable (SD = 0.7°C) compared to the Cascade Range (SD = 2.3°C). Median thermal sensitivity was 0.33°C °C−1 in the Coast Range and 0.23°C °C−1 in the Cascade Range. We posit that the volcanic lithology underlying the Cascade streams likely supported discrete groundwater discharge locations of cold snowmelt water, which dampened thermal sensitivity. At locations of apparent groundwater discharge in these streams, median stream temperatures rapidly decreased by 2.0–7.0°C relative to locations 70–90 m upstream. In contrast, thin friable soils in the Coast Range likely contributed warmer, rain dominated baseflow from shallow subsurface sources, which strongly co‐varied with air temperature and generally warmed downstream (up to 2.1°C km−1). Our study revealed distinct longitudinal thermal regimes in streams with contrasting lithology, precipitation regimes, and stand densities suggesting that streams in these different regions may respond differentially to forest disturbances or climate change.

Experiments and thermodynamic modelling on the blueschists in the Longmu Co‐Shuanghu Suture Zone, North Tibet: Estimation of the metamorphic conditions and implications for garnet stability in the subduction zone

The pressure–temperature (P‐T) path of blueschist is usually applied to understanding the evolution of the subducted oceanic crust. Garnet plays a key role in calculating the metamorphic conditions, but sometimes it is absent in the blueschists. Nevertheless, the high‐P experimental simulation may serve as a useful tool to constrain the P‐T conditions. In the middle of the Longmu Co‐Shuanghu Suture, North Tibet, blueschist blocks embedded in the marble were discovered, where garnet is abundant in the core but absent in the marble‐contacting margin. These two kinds of blueschist contain similar mineral assemblage (glaucophane, epidote, and phengite) and show comparable basaltic character. Pseudosection modelling was applied to the garnet‐bearing blueschist that calculated the peak metamorphic condition of ca. 1.9 GPa and 530°C. As for the garnet‐absent blueschist, experiments (1.2–2.3 GPa and 500–800°C) were performed on the designed mixture of 93 wt% basalt and 7 wt% calcite. The results show that the total Fe content in the glaucophane and barroisite is positively correlated with the pressure but not with temperature, which could be regarded as a geobarometer. Accordingly, the peak condition was constrained at 2.3 GPa and 500–600°C. Therefore, the equivalent metamorphic conditions of the blueschists indicate that the Palaeo‐Tethys oceanic crust subducted into at least 70 km. Furthermore, both the experiments and the T‐X (CaCO3) pseudosection, constructed based on basaltic lithology, support that the absence of garnet in the blueschist is predominantly caused by the mixing of calcite. In summary, this study provides a valid attempt in exploring metamorphic conditions by experiment simulation, and evaluates the profound influence of the interaction between the basaltic crust and overlying carbonate‐bearing sediments in the subduction zone.

Dynamic analyses of high-position long-runout landslide in Shuicheng, Guizhou, China: perspectives from overloading erosion effect

On July 23, 2019, a large-scale, high-position landslide was triggered by heavy rainfall at Pingdi Village, Jichang Town, Shuicheng County, Guizhou, China. Based on a field investigation, multi-temporal remote sensing images, and topographic maps, the elevation of the crown of the landslide and the front edge was approximately 1700 m and 1233 m, respectively. The height difference of the landslide was 467 m, and the horizontal distance was approximately 1332 m with a main sliding direction of NE20°. Its volume was up to 1.81 million m3. The landslide buried 21 houses, leading to the death of 51 people. The landslide first exited from the upper part of the steep slope in the Emeishan Formation with a basaltic lithology, which accumulated continuously at the back of a previous residual landslide and triggered the slope instability under the exit. The erosion volume due to the “overloading effect” was up to 1.44 million m3, and the landslide then transferred to a long-runout double-channeled debris flow. The landslide then converted to diffused flow and finally accumulated to the east side of the Jichang reservoir because the terrain was wide and the slope angle decreased gradually. Based on the above investigation, the entire movement process of the Jichang landslide was retrieved using dynamic numerical simulation technology. In contrast to the previous erosion model, the overloading erosion effect was proven, which added loads and drove the deposit below to move downward together.

The Rosebank Field, NE Atlantic: Volcanic characterisation of an inter‐lava hydrocarbon discovery

AbstractThe Rosebank Field is located in the Faroe‐Shetland Basin and hosts hydrocarbons within siliciclastic sediments interlayered with volcanic packages of the Late Paleocene to Early Eocene aged Flett Formation. Within this study the volcanic sequences are investigated based on an integrated appraisal of available drill cuttings, sidewall cores, core and wireline logs including image log and geochemical logs from eight wells supported by 3D seismic data. The Rosebank lower (RLV), middle (RMV) and upper (RUV) volcanic sequences are inter‐layered with Colsay Member (C1–C4) fluvial to shallow marine siliciclastic intervals. A comprehensive cross‐field borehole based lithofacies interpretation is presented characterising simple, compound and ponded effusive lava flow facies along with pillow lavas, invasive lava flows, volcaniclastic sediments and complex lava–sediment interactions. Geochemical analyses of core, sidewall core, and hand‐picked cuttings spanning the field reveal separate high‐titanium (RHT) and relatively lower‐titanium (RLT) basaltic magma suites. These compositions can be identified and correlated across much of the field utilising geochemical logging data which, in combination with the geochemical analyses, reveals a two‐part stratigraphic sub‐division of each of the RLV, RMV and RUV. Geochemical logging data is also used to define a volcanic proxy (Fe/10+Ti) which utilises the elevated iron (Fe) and titanium (Ti) within all effusive and volcaniclastic basaltic lithologies to differentiate siliciclastic from volcaniclastic sediments where other logging parameters overlap. By comparing the borehole analyses with seismic data, a localised eruptive vent is interpreted within the north of the field. Finally, a cross‐field volcanic model is presented and compared with relevant global field analogues, providing a constrained spatial framework for sub‐surface modelling of inter‐volcanic sequences.

The miscibility of calcium silicate perovskite and bridgmanite: A single perovskite solid solution in hot, iron-rich regions

Calcium silicate perovskite and bridgmanite are two phases believed to coexist throughout the lower mantle, which at some temperature, at least theoretically, dissolve into each other to form a single perovskite solid solution (CaxMg1−xSiO3). This may have large seismic and geochemical implications due to the changes in density, elasticity and element partition coefficients between single and mixed phase perovskites. DFT Molecular Dynamics has been used to estimate the miscibility of bridgmanite and calcium perovskite at pressures between 25 and 125 GPa. At 125 GPa (where mixing is the greatest in our pressure range) to mix 1% of Ca-pv into bridgmanite requires a temperature of 2042 K, 5% 2588 K, 10% 2675 K and 50% 2743 K. Therefore, in a simplified lower mantle chemistry an extensive MgSiO3–CaSiO3 solid solution is not expected to occur. However, a simple model was employed to test whether the presence of other elements might influence this mutual solid solution and it was demonstrated that if sufficient concentrations (>1 at.%) of additional elements are present then miscibility may become favourable. Of the elements likely to be present at these concentrations it appears that ferrous iron promotes, whilst aluminium inhibits, a single-phase perovskite solid solution. To a lesser extent ferric iron may both increase and decrease perovskite miscibility. Modelling for realistic mantle compositions suggests that basaltic lithologies will always retain two perovskite components, whereas a single perovskite solid solution may be preferred in hot and/or iron-rich pyrolytic bulk compositions near the base of the lower mantle. Static calculations indicate perovskite miscibility may cause pyrolytic lithologies (with 12.5% CaSiO3) to possess lower density (−0.14–0.25%), Vs (−1.5–3.5%) and Vp (−0.5–1.2%), and higher VΦ (+ 0.00–0.75%) than predicted for assemblages containing two perovskites. These seismic changes, while preliminary, are similar to those observed in the LLSVPs which are also regions that are likely hotter than the surrounding mantle and thus possess conditions promoting the formation of a single perovskite phase.

How do secondary iron enrichments form within basaltic eucrites? An experimental approach

AbstractSome basaltic eucrites and basaltic lithologies in howardites derived from the asteroid 4 Vesta exhibit unusual secondary veinlet textures consisting mostly of fayalitic olivine and Fe‐enrichments within pyroxenes. Recent studies discussed the formation of these Fe‐rich phases either by interaction with a vapor and/or liquid phase (metasomatism), or by a high‐temperature melting process. We therefore performed a series of heating and hydrothermal experiments with liquids of different compositions on natural pyroxene crystals (augite and orthopyroxene) to evaluate these contrasting hypotheses. The results of the heating experiments show that incongruent melting of pyroxenes at about 1070 °C causes textures that are very similar to those observed in the meteorites. We conclude that a part of the natural secondary veins might be explained by heating processes at similar temperatures. The hydrothermal experiments with aqueous liquids of different Fe‐enriched compositions clearly indicate ion exchange reactions resulting in partial Fe‐enrichments of the pyroxene. Interestingly, these Fe‐enrichments occurred independent of the Fe content of the liquid, which can be explained by an internal origin of Fe from the pyroxenes. In one hydrothermal experiment of augite with Fe‐oxalate solution, deposition of fayalitic olivine was observed. From our experimental observations, we conclude that aqueous liquids are plausible candidates for explaining the deposition of Fe‐enrichments and fayalitic olivine inside the fractures of pyroxene. However, we cannot rule out a high‐temperature melting process slightly above the peritectic point of pyroxene to explain a fraction of observed secondary Fe‐enrichments.

Reproduction of melting behavior for vitrified hillforts based on amphibolite, granite, and basalt lithologies

European Bronze and Iron Age vitrified hillforts have been known since the 1700s, but archaeological interpretations regarding their function and use are still debated. We carried out a series of experiments to constrain conditions that led to the vitrification of the inner wall rocks in the hillfort at Broborg, Sweden. Potential source rocks were collected locally and heat treated in the laboratory, varying maximum temperature, cooling rate, and starting particle size. Crystalline and amorphous phases were quantified using X-ray diffraction both in situ, during heating and cooling, and ex situ, after heating and quenching. Textures, phases, and glass compositions obtained were compared with those for rock samples from the vitrified part of the wall, as well as with equilibrium crystallization calculations. ‘Dark glass’ and its associated minerals formed from amphibolite or dolerite rocks melted at 1000–1200 °C under reducing atmosphere then slow cooled. ‘Clear glass’ formed from non-equilibrium partial melting of feldspar in granitoid rocks. This study aids archaeological forensic investigation of vitrified hillforts and interpretation of source rock material by mapping mineralogical changes and glass production under various heating conditions.

Early indicators of pedogenesis at Harrat Khaybar volcano, Saudi Arabia

The interactive effects of paleoclimates and current conditions on pedogenesis in soils developed on volcanic parent materials (VPMs) is still poorly understood, particularly for soils in dry environments. Eight representative profiles located at Harrat Khaybar, Saudi Arabia and representing different landforms and positions including alluvial plain, alluvial fans, active slopes and lava field were investigated to better understand the effect of climate and basalt lithology on pedogenesis. The morphological and physicochemical soil properties and clay mineralogy composition were evaluated to identify the primary diagnostic horizons and therefore the pedogenic pathways that created the studied soils. All investigated profiles showed the influence of slope processes, resulting in heterogeneous soil profiles. The investigated soils can be described as polygenetic due to evidence of andisolization and the presence of salic and calcic diagnostic horizons. Andisolization implies development under a sub-humid paleoclimate during initial pedogenesis, while calcic and salic diagnostic horizons, are typical of the current arid conditions. As a result, about 63% of the investigated soils were classified as Torrands/Vitric Andosols and 37% as Salids/Solonchaks based on Soil Taxonomy and World Reference Base, respectively. Values of Alo, Feo, and Sio ranged from 0.20 to 0.68%, 0.24 to 3.27%, and 0.23 to 1.03%, respectively, implying the presence of poorly-crystalline fine components (i.e., allophane and ferrihydrite). Vermiculite, kaolinite, and interstratified vermiculite-chlorite were abundant, with illite, palygorskite, and Ca-zeolites as minor crystalline minerals. A combination of paleo/present-climate influenced pedogenesis leading to andisolization and the coexistence of poorly/crystalline minerals and formation of salic and calcic diagnostic horizons. This study demonstrates that andic properties can persist in dry-hot environments, highlighting the critical role of paleo/modern climate interactions as an influential factor driving the pedogenic pathways of VPMs-derived soils.

Quality reference values for heavy metals in soils developed from basic rocks under tropical conditions

Soil Quality Reference Values (QRV) refer to the natural heavy metal concentrations that is not influenced or is minimally influenced by anthropogenic activities. Such values are unique of each environment, and their extrapolation to different locations becomes inadequate. This research aimed to determine the natural concentrations of metals in soils (QRV) developed on essentially basaltic lithology and tropical conditions in the south of Brazil. Seventy-two soil samples from the Forest Conservation Areas in the west of the state of Paraná, Brazil, were obtained. The extraction and dosage of Ag, As, Ba, Cd, Cr, Cu, Mo, Ni, Pb, Sb, V, and Zn were carried out employing methods USEPA 3051a and ICP-OES. The QRV was set to 75th percentile of each element, from the detection and exclusion of the outliers. The natural concentration of all heavy metals was related to the geological context of the basaltic area in Brazil. There was no influence of the pedogenetic degree of soils on the natural heavy metal concentration. The only relevant process for reducing the natural heavy metal levels was the Fe and Mn oxide solubility promoted by hydromorphic conditions. Fe oxides had a significant role in the maintenance of structural and adsorbed heavy metal forms in soils. The results may help the research and monitoring of environmental heavy metals in soils developed from basalt under tropical conditions.

Magmatism versus serpentinization—crustal structure along the 13°N segment at the Mid-Atlantic Ridge

SUMMARY A region of oceanic core complexes (OCCs) exists at 13°N on the Mid-Atlantic Ridge that is regarded as a type site. This site includes two OCCs at 13°20′N and 13°30′N, thought to be in the active and dying stages of evolution, and two together called the Ashadze Complex (centred at 13°05′N) that are considered to be relict. Here we describe the results of S-wave seismic modelling along an ∼200-km-long 2-D transect traversing, south-to-north, through both the Mercurius and Marathon fracture zones, the southern outside corner of the 13°N segment, the OCCs, the ridge axis deviation in trend centred at 13°35′N, and the youngest oceanic crust of the eastern ridge flank to the north. Our inversion model, and the corresponding Vp/Vs ratio, show that the majority of the crust beneath the 13°30′N OCC comprises metamorphosed lithologies that have been exhumed to the shallowest subseabed level, while basaltic lithologies underlie the 13°20′N OCC. The transition between these contrasting crustal structures occurs over a distance of <5 km, and extends to at least ∼2 km depth below seafloor. The northern and southern OCCs of the Ashadze Complex have contrasting structures at shallow depth, with the northern OCC having a faster S-wave velocity in the upper crust. A Vp/Vs ratio of >1.9 (and equivalent Poisson's ratio of >0.3) indicates exhumed and/or metamorphosed lithologies beneath the bathymetric depression between them and within the crust beneath the southern OCC. Between the northern and southern flanks of the Marathon fracture zone and northern flank of Mercurius fracture zone, the lower crust has a relatively low Vp/Vs ratio suggesting that the deformation associated with Marathon fracture zone, which facilitates fluid ingress, extends laterally within the lower crust. Marathon fracture zone itself is underlain by a broad zone of low S-wave velocity (∼2.0 km s−1) up to ∼20 km wide from the seabed to at least the mid-crust, that is mirrored in a high Vp/Vs ratio and lower density, particularly deeper than ∼1 km below seabed within its bathymetric footprint. Volcanic domains are highlighted by a low Vp/Vs ratio of <1.6 (and equivalent Poisson's ratio of <0.15). Our combined seismic and density models favour the localized model of OCC evolution. They also show a considerable ridge-parallel variability in the amount and distribution of magmatic versus metamorphosed crust. Our results suggest that the current focus of magmatism lies to the north of the 13°20′N OCC, where the magmatic accretion-type seabed morphology observed is mirrored in the pattern of microseismicity, suggesting that its inward-facing median-valley-wall fault may link to the 13°20′N OCC detachment surface. Magmatism and active faulting behind (to the west) the footwall breakaway of the 13°30′N OCC, and the microseismicity concentrated in a band along its southern flank, suggest a readjustment of ridge geometry along axis is underway. As part of this, a transform offset is forming that will ultimately accommodate the 13°30′N OCC in its inside corner on the eastern flank of the ridge axis to the north.

Occurrence and implications of secondary olivine veinlets in lunar highland breccia Northwest Africa 11273

AbstractLunar breccias preserve the records of geologic processes on the Moon. In this study, we report the occurrence, petrography, mineralogy, and geologic significance of the observed secondary olivine veinlets in lunar feldspathic breccia meteorite Northwest Africa (NWA) 11273. Bulk‐rock composition measurements show that this meteorite is geochemically similar to other lunar highland meteorites. In NWA 11273, five clasts are observed to host veinlets that are dominated by interconnecting olivine mineral grains. The host clasts are mainly composed of mafic minerals (i.e., pyroxene and olivine) and probably sourced from a basaltic lithology. The studied olivine veinlets (~5 to 30 μm in width) are distributed within the mafic mineral host, but do not extend into the adjacent plagioclase. Chemically, these olivine veinlets are Fe‐richer (Fo41.4–51.9), compared with other olivine grains (Fo54.3–83.1) in lithic clasts and matrix of NWA 11273. By analogy with the secondary olivine veinlets observed in meteorites from asteroid Vesta (howardite–eucrite–diogenite group samples) and lunar mare samples, our study suggests that the newly observed olivine veinlets in NWA 11273 are likely formed by secondary deposition from a lunar fluid, rather than by crystallization from a high‐temperature silicate melt. Such fluid could be sulfur‐ and phosphorous‐poor and likely had an endogenic origin on the Moon. The new occurrence of secondary olivine veinlets in breccia NWA 11273 reveals that the fluid mobility and deposition could be a previously underappreciated geological process on the Moon.

The pyroclastic breccia of the Cabezo Negro de Tallante (SE Spain): The first finding of carbonatite volcanism in the Internal Domain of the Betic Cordillera

The Cabezo Negro de Tallante volcano (SE Spain) is well known in literature for the presence of abundant mantle and crustal xenoliths in basaltic lithologies. Here we report field, petrographic, mineralogical and geochemical data of a poorly investigated pyroclastic deposit (breccia) and provide evidence for the presence of a primary magmatic carbonatite component. The matrix of the pyroclastic breccia contains carbonate and silicate portions. The carbonate portion is made up of calcitic ash with micro-lapilli of fine-grained carbonate, while globules and clasts of basaltic composition represent the silicate portion. A liquid immiscibility relationship between the carbonate and silicate portions is indicated by the presence of sharp, curved menisci between them, the presence of carbonate globules in silicate glass (and vice-versa) and the overall appearance as emulsion texture with bulging and pinch-off shapes. Carbonate globules in fresh silicate glass are also present as inclusions within xenolith crystals and bimodal carbonate mineralogy, consisting of either calcite (plus Mg-calcite) and dolomite, was observed in quenched liquids within mantle xenoliths. The whole-rock composition of the carbonate portion is within the compositional range of the carbonatitic rocks, but shows low concentrations of Sr, Ba, Nb, P and LREE if compared with typical carbonatite magmas. REE profiles of the carbonate portion are parallel to the patterns of associated alkaline basalts and fall in the range of both intraplate and convergent margin carbonatites. The carbonate portion shows high δ18O compared to primary igneous carbonates, a feature common to many extrusive carbonatites and plots slightly towards higher 87Sr/86Sr for a given 143Nd/144Nd in the Nd-Sr isotopic trend described by the associated rocks (alkaline basalts and mantle xenoliths) suggesting a contribution from crustal subducted material. These results support a magmatic origin for the carbonate portion and a contemporaneous eruption of carbonatic and silicatic magmas. The CO2 produced by the transformation of dolomitic melt into Mg-calcitic melt was the principal propellant in the high-energy explosive eruption at the Cabezo Negro de Tallante volcano.

New lunar meteorite NWA 10986: A mingled impact melt breccia from the highlands—A complete cross section of the lunar crust

AbstractNorthwest Africa (NWA) 10986 is a new mingled lunar meteorite found in 2015 in Western Sahara. This impact melt breccia contains abundant impact melt glass and clasts as large as 0.75 mm. Clasts are predominantly plagioclase and pyroxene‐rich and represent both highland and basalt lithologies. Highland lithologies include troctolites, gabbronorites, anorthositic norites, and troctolitic anorthosites. Basalt lithologies include crystalline clasts with large zoned pyroxenes representing very low titanium to low titanium basalts. In situ geochemical analysis of minerals within clasts indicates that they represent ferroan anorthosite, Mg‐suite, and gabbronorite lithologies as defined by the Apollo sample collection. Clasts representing magnesian anorthosite, or “gap” lithologies, are prevalent in this meteorite. Whole rock and in situ impact glass measurements indicate low incompatible trace element concentrations. Basalt clasts also have low incompatible trace element concentrations and lack evolved KREEP mineralogy although pyroxferroite grains are present. The juxtaposition of evolved, basaltic clasts without KREEP signatures and highland lithologies suggests that these basaltic clasts may represent cryptomare. The lithologies found in NWA 10986 offer a unique and possibly a complete cross section view of the Moon sourced outside of the Procellarum KREEP Terrane.

Differential weathering of basaltic and granitic catchments from concentration–discharge relationships

A negative feedback between silicate weathering rates and climate is hypothesized to play a central role in moderating atmospheric CO2 concentrations on geologic timescales. However, uncertainty regarding the processes that regulate the operation of the negative feedback limits our ability to interpret past variations in the ocean–atmosphere carbon cycle. In particular, the mechanisms that determine the flux of weathered material for a given climatic state are still poorly understood. Here, we quantify the processes that determine catchment-scale solute fluxes for two lithologic end-members—basalt and granite—by applying a recently developed solute production model that links weathering fluxes to both discharge and the reactivity of the weathering material. We evaluate the model against long-term monitoring of concentration–discharge relationships from basaltic and granitic catchments to determine the parameters associated with solute production in each catchment. Higher weathering rates in basaltic catchments relative to granitic catchments are driven by differing responses to increases in runoff, with basaltic catchments showing less dilution with increasing runoff. In addition, results from the solute production model suggest that thermodynamic constraints on weathering reactions could explain higher concentrations in basaltic catchments at lower runoff compared to granitic catchments. To understand how the response to changing discharge controls weathering fluxes under different climatic states, we define basalt/granite weatherability as the ratio of the basalt catchment flux to the granite catchment flux. This weatherability is runoff-dependent and increases with increasing runoff. For HCO3− and SiO2(aq) fluxes, for modern global runoff, the derived mean basalt/granite weatherability is 2.2 (1.3–3.7, 2σ) and 1.7 (1.6–2.1, 2σ), respectively. Although we cannot determine the array of individual processes resulting in differences among catchments, the relative differences in certain model parameters that represent catchment-scale weathering fluxes of granitic and basaltic lithologies are robust. Our approach provides a mechanism that links runoff with the distribution of global sub-aerial silicate lithologies to understand how the basalt/granite weatherability of the Earth’s surface may have varied on geologic timescales. The relationships between basalt/granite weatherability and runoff derived in this study could be used to parameterize the silicate weathering negative feedback to model past changes in pCO2.

Iron spin state and site distribution in FeAlO3-bearing bridgmanite

DFT at the GGA, GGA+U and hybrid functional levels were used to investigate thousands of different Al and Fe3+ configurations of MgSiO3–FeAlO3 (MS–FA) and MgSiO3–FeAlO3–Al2O3 bridgmanite at deep mantle conditions. Comparison of the different functionals and atomic charge analysis suggests that GGA, frequently used to explain high to low spin transitions observed in several Mössbauer and X-ray emission spectroscopy experiments, is hampered by spurious self-interaction errors in the exchange-correlation energy. Configurational Boltzmann averaging shows that the B site is thermally inaccessible to Fe3+ at the GGA+U and hybrid levels, and we find no evidence for a spin-pairing transition in fully (thermodynamically) equilibrated samples of bridgmanite, even at the lowermost mantle conditions. The comparison of the cation radii of Fe3+ and Mg supports a spin transition accompanied by a site exchange, but the flexibility of FeO bonds to locally adapt promotes the incorporation of iron in the irregularly coordinated A-site. The concept of ionic radii is therefore unsuitable for analysis of spin state and site exchange in bridgmanite at these conditions. Consistent with previous computational work and experimental studies with glass and gel as starting material, we find that ferric iron kinetically trapped at the B site undergoes a spin transition under lowermost mantle conditions. In bridgmanite with mole fraction of Fe3+>Al a charge-balancing amount of low spin Fe3+ will be thermodynamically stable at the B site, but because bridgmanite in peridotitic and basaltic lithologies mostly has Al/Fetotal above unity, FA with high spin Fe3+ in the A-site will be the dominant iron component. The lack of a Fe3+ spin transition in the FA-component has important implications for bridgmanite–ferropericlase partitioning of iron and magnesium and the mineral physics of the lowermost mantle.