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Magmatic processes in the continental crust such as crustal convection, melt ascent, magma emplacement, and batholith formation are not well understood. We solve the conservation equations for mass, momentum, and energy for two‐phase flow of melt and solid in 2D, for a thick continental crust heated from below by one or several heat pulses. A simplified binary melting model is incorporated. We systematically vary (a) the retention number, characterizing melt mobility, (b) the intensity of heat pulses applied at the bottom, and (c) the density of the solidified evolved rock. Two characteristic modes are identified: (a) in the “batholith emplacement mode,” segregation is sufficiently strong allowing melts to separate from the convective flow. This melt freezes to form buoyant SiO2‐rich layers. (b) In the “convective recycling mode,” melts are formed in the lower crust, rise together with the hot rock with little segregation, freeze at shallow depth but are partly recycled back to the lower crust where they remelt. Phase‐change‐driven convection dominates. Mode (a) is favored by high heat input, multiple heat pulses, high melt mobility, and low density of the evolved rock. Mode (b) is favored by less intense heating, less melt mobility, and denser evolved rocks. A scaling law is derived based on the thermal, melt, and compositional Rayleigh numbers and the retention number. The Altiplano‐Puna low‐velocity zone (LVZ) could represent the batholith emplacement mode with buoyant and voluminous magmas causing intense volcanism. The Tibetan LVZ is not associated with intense volcanism and might represent the convective recycling mode. Key Points Two‐phase flow models of crustal magmatic systems identify two modes: batholith emplacement versus convective recycling of evolved rock High melt mobility, multiple heating pulses, and low density of solidified evolved rock favor batholith emplacement The Altiplano‐Puna low‐velocity zone (LVZ) is in the batholith emplacement mode and the Tibetan LVZ is in the convective recycling mode

The Azufre volcano (21°47′S; 68°14′W) is emplaced above the western boundary of the Altiplano Puna Magma Body. The youngest lava of the Azufre volcano (50–331 ka) was selected for detailed studies because of the relevant petrological information that could emerge to understand its reservoir as a potential heat source of the neighboring Cerro Pabellón geothermal system. The studied lava corresponds to an andesite-dacite (61–63 SiO2 wt%) with phenocrysts of plagioclase, amphibole (Group 1), biotite, pyroxenes, quartz, and olivine. The lava also contains aphanitic enclaves (58–60 SiO2 wt%), whose groundmass have the same mineralogy of lava samples groundmass consisting of amphibole microphenocryts (Group 2) and microlites of plagioclase, pyroxenes, and Fe–Ti oxides. Disequilibrium textures are commonly observed in the studied samples such as partially resorbed plagioclase phenocrysts, amphibole breakdown, and reverse zoning in pyroxene and plagioclase phenocrysts. Thermobarometry calculations indicated pressures of ~ 2 kbar for Group 1 amphiboles, temperatures of 905–1097 °C for Mg-rich pyroxene phenocrysts and Ca-rich plagioclase (An ≥ 66), and near-solidus temperatures of 712–788 °C for Group 1 amphibole-plagioclase pairs. Group 1 amphiboles also indicate crystallization from an evolved liquid (63–79 wt% of SiO2). Oxidation conditions of QFM + 0.9–2.5 log units were recorded in amphibole and Fe–Ti oxides. Rhyolite-MELTS models reproduce the composition of the high-temperature phases from a melt composition (andesitic enclave) at similar P–T-fO2. The arrival of new hot andesite magma into a crystal-rich shallow reservoir, thermally and compositionally zoned, would have triggered the studied eruption. Diffusion models in Fe–Ti oxides microlites indicated cooling temperatures of 742–866 °C during the sub-aerial emplacement of the lava.

Natural hydrogen is known to be generated in the crust by water/rock interactions, especially the oxidation of iron-rich rock or radiolysis. However, other sources, especially deeper ones, exist. In the context of subduction, the dehydration of the slab, the destabilization of the NH4, and the hydration of the mantle wedge above the subducting lithosphere may generate H2. We present here a compilation of the known gases in the central part of the Pacific subduction and the results of a first field acquisition dedicated to H2 measurements in Bolivia between La Paz and South Lipez. Various zones have been studied: the emerging thrust faults of the western borders of the Eastern Cordillera, the Sajama area that corresponds to the western volcanic zone near the Chile border northward from the Uyuni Salar, and finally, the Altiplano-Puna Volcanic Complex in South Lipez. Soil gas measurement within and around the Salar itself was not fully conclusive. North of the Uyuni Salar, the gases are very rich in CO2, enriched in N2 and poor in H2. On the opposite, southward, all the samples contain some H2; the major gas is nitrogen, which may overpass 90% after air correction, and the CO2 content is very limited. On the western border of the Cordillera, the δC13 isotope varies between −5 and −13‰, and it is not surprisingly compatible with volcanic gas, as well as with asthenospheric CO2. The methane content is close to 0, and only a few points reach 1%. The isotopes (−1‰) indicate an abiotic origin, and it is thus related to deep H2 presence. The high steam flow in the geothermal area of South Lipez combined with the H2 content in the water results in at least 1 ton of H2 currently released per day from each well and may deserve an evaluation of its economic value. The nitrogen content, as in other subduction or paleo-subduction areas, questions the slab alteration.

The Cerro Bitiche Andesitic Field (CBAF) is one of the two largest mafic volcanic fields in northern Puna (22–24° S) and is spatially and temporally associated with ignimbrites erupted from some central Andean Altiplano-Puna Volcanic Complex calderas. The CBAF comprises seven scoria cones and widespread high-K calcalkaline lava flows that cover an area of 200 km 2 . Although all erupted rocks have a relatively narrow chemical range (56–62 % SiO 2 , 3–6 % MgO), there is a broad diversity of mineral compositions and textures. The least evolved lavas (∼58–61 % SiO 2 ) are high-Mg andesites with scarce (<10 %) microphenocrysts of either olivine or orthopyroxene. The small compositional range and low phenocryst content indicate evolution controlled by low percentages (<10 %) of fractional crystallization of olivine and clinopyroxene of magmas similar to the least evolved rocks from the field, accompanied by assimilation during rapid ascent through the crust. Evolved andesites (∼62 wt% SiO 2 ), on the other hand, are porphyritic rocks with plagioclase + orthopyroxene + biotite and ubiquitous phenocryst disequilibrium textures. These magmas were likely stored in crustal reservoirs, where they experienced convection caused by mafic magma underplating, magma mixing, and/or assimilation. Trace element and mineral compositions of CBAF lavas provide evidence for complex evolution of distinct magma batches.

This research analyzes the extent and frequency of forest fires in the humid puna ecosystem from 2013 to 2021, identifying the affected areas and determining the causes in order to improve fire management in the department of Ayacucho (Andean region). from Peru). The methodology combines cartographic analysis, satellite images and semi-structured interviews to identify the causes, consequences and mitigation alternatives of the fires. The results show that the areas affected by the fires during the 9 years are very differentiated, with 2020 being the year with the largest burned area, reaching 2,836 ha, which represents 14.89% of the humid puna of the study area. In addition, the most frequently burned areas are repeated between 7 and 9 times in different years, with an average of 182 ha burned recurrently. The causes of the fires are clearly of anthropic origin caused by the (i) opening of new fields or burning of stubble; (ii) ichu burning for pasture regrowth and (iii) for cultural reasons. It is concluded that the combination of cartographic analysis methodology, satellite images and semi-structured interviews provide information that allows understanding the dynamics of the territory and improve the management and implementation of territorial policies in the mitigation of fires. Esta investigación analiza la extensión y frecuencia de los incendios forestales en el ecosistema de la puna húmeda desde el año 2013 al 2021, identificando las áreas afectadas y determinando las causas con la finalidad de mejorar el manejo de fuego en el departamento de Ayacucho (región andina de Perú). La metodología combina análisis cartográfico, imágenes satelitales y entrevistas semiestructuradas para identificar las causas, consecuencias y alternativas de mitigación de los incendios. Los resultados muestran que las áreas afectadas por los incendios durante los 9 años son muy diferenciadas, siendo el 2020 el año con mayor área incendiada llegando a 2,836 ha lo que representa el 14.89% de la puna húmeda del área de estudio. Además, las áreas incendiadas con mayor frecuencia se repiten entre 7 y 9 veces en diferentes años, con un promedio de ha quemadas de forma reincidente de 182 ha. Las causas de los incendios son netamente de origen antrópico ocasionados por la (i) apertura de nuevas chacras o quema de rastrojo; (ii) quema del ichu para el rebrote de pastos y (iii) por razones culturales. Se concluye que la combinación de metodología de análisis cartográfico, imágenes satelitales y las entrevistas semiestructuras proporcionan información que permite entender las dinámicas del territorio y mejorar el manejo e implantación de políticas territoriales en la mitigación de los incendios.

Although wetlands contain a disproportionately high amount of earth’s total soil carbon, many regions are still poorly mapped and with unquantified carbon stocks. The tropical Andes contain a high concentration of wetlands consisting mostly of wet meadows and peatlands, yet their total organic carbon stocks are poorly quantified, as well as the carbon fraction that wet meadows store compared to peatlands. Therefore, our goal was to quantify how soil carbon stocks vary between wet meadows and peatlands for a previously mapped Andean region, Huascarán National Park, Peru. Our secondary goal was to test a rapid peat sampling protocol to facilitate field sampling in remote areas. We sampled soil to calculate carbon stocks of four wetland types: cushion peat, graminoid peat, cushion wet meadow, and graminoid wet meadow. Soil sampling was conducted by using a stratified randomized sampling scheme. Wet meadows were sampled to the mineral boundary using a gouge auger, and we used a combination of full peat cores and a rapid peat sampling procedure to estimate peat carbon stocks. In the lab, soils were processed for bulk density and carbon content, and total carbon stock of each core was calculated. We sampled 63 wet meadows and 42 peatlands. On a per hectare basis, carbon stocks varied strongly between peatlands (avg. 1092 MgC ha -1 ) and wet meadows (avg. 30 MgC ha -1 ). Overall, wetlands in Huascarán National Park contain 24.4 Tg of carbon with peatlands storing 97% of the total wetland carbon and wet meadows accounting for 3% of the wetland carbon in the park. In addition, our results show that rapid peat sampling can be an effective method for sampling carbon stocks in peatlands. These data are important for countries developing land use and climate change policies as well as providing a rapid assessment method for wetland carbon stock monitoring programs.

The fault‐bounded Cianzo basin represents a Cenozoic depocenter between the Puna plateau and Eastern Cordillera of northern Argentina. Analysis of fold‐thrust relationships, nonmarine sedimentation, and detrital provenance at 23–24°S helps constrain the origin, interconnectedness, and subsequent uplift and exhumation of the Cianzo basin, a potential analog for intermontane hinterland basins in the Andes. Structural mapping reveals a plunging syncline within the >6000‐m‐thick, upward coarsening Cenozoic clastic succession in the shared footwall of the N‐striking, E‐directed Cianzo thrust fault and transverse, NE‐striking Hornocal fault. Growth strata within upper Miocene levels indicate syncontractional accumulation adjacent to the Hornocal fault. Measured stratigraphic sections show upsection changes from (1) paleosol‐rich, distal‐fluvial sandstones (∼400 m Paleocene‐Eocene Santa Bárbara Subgroup) to (2) braided fluvial sandstones and mudstones (∼1400 m upper Eocene‐Oligocene Casa Grande Formation) to (3) distributary fluvial megafan sandstones and conglomerates (∼3300 m upper Oligocene‐Miocene Río Grande Formation) to (4) alluvial fan conglomerates (∼1600 m upper Miocene Pisungo Formation). The 40Ar/39Ar geochronological results for interbedded tuffs indicate continuous Río Grande deposition from at least 16.34 ± 0.71 to 9.69 ± 0.05 Ma. Sandstone petrographic results define distinct upsection trends in lithic and feldspar content, potentially distinguishing the Western Cordillera magmatic arc from Puna‐Eastern Cordillera thrust‐belt sources. In addition to growth stratal relationships and 40Ar/39Ar ages, conglomerate clast compositions reflect distinct lithologic differences, constraining activation of the Cianzo thrust and coeval reverse slip on the reactivated (inverted) Hornocal fault. Finally, detrital zircon U‐Pb ages, paleocurrents, and facies patterns distinguish local from distal sources, revealing a systematic forelandward (eastward) advance of Eocene through upper Miocene fold‐thrust deformation. Key Points Paleocene‐Eocene basin fill recorded the onset of major Andean shortening Paleogene foreland basin was partitioned into small Neogene intermontane basins Cenozoic shortening in northern Argentina advanced systematically eastward

1. During soil development, bacteria and fungi can be differentially affected by changes in soil biogeochemistry. Since the chemistry of parent material affects soil pH, nutrient availability, and indirectly litter quality, we hypothesize that parent material has an important influence on microbial community patterns during long-term soil development. 2. In this paper, we tested for the effect of parent material, as well as, soil and litter properties upon microbial community patterns in three c. 20 000-year-old semi-arid chronosequences developed on sedimentary and volcanic (i.e. Andesitic and Dacitic) soils in the Dry Puna of Bolivia. We evaluated microbial patterns by analysing the terminal restriction fragment length polymorphism from amplified bacterial 16S rRNA genes, and the fungal internal transcribed spacer region, and quantitative real-time polymerase chain reaction. 3. Soil and litter characteristics differed significantly between the Sedimentary and volcanic chronosequences. In particular, soil pH was alkaline in all stages of the Sedimentary chronosequence; whereas it changed from alkaline to near neutral across stages in both volcanic chronosequences. Composition of bacterial communities changed across volcanic chronosequences, and this change was associated with a reduction in soil pH and increases in litter quality, whereas no differences were found in the Sedimentary chronosequence. Fungal community composition, in contrast, did not change across any chronosequence. 4. Relative microbial abundance, expressed as the fungal:bacterial ratio, declined across stages of the Sedimentary chronosequence in association with decreases in TC and TP, whereas in the Andesitic chronosequence decreases in fungalibacterial ratios were related with increases in litter quality and declines in soil pH. 5. Synthesis. Our results show the importance of parent material in affecting bacterial and fungal communities during soil development. Further, in semi-arid chronosequences, fungal:bacterial ratios tend to decline given that soil pH in young soils is rather alkaline. Our results also are consistent with the general framework that highlights the importance of above-ground (i.e. litter quality) and below-ground (i.e. soil properties) in affecting microbial relative abundance and community composition during soil development.

A new Brachionidium from Peru is described and illustrated based on living plants. Information on distribution, ecology, etymology, and phenology is also included. The new species, B. montieliae, was discovered in one of the highest mountains of the Oxapampa-Ashaninka-Yanesha Biosphere Reserve (BIOAY), situated above the forests of the Humid Puna of the Yanachaga Chemillen National Park at 3590 m in elevation. It is similar to Brachionidium vasquezii but differs in the oblong-elliptic, denticulate, and semi-erect leaves, somewhat lanuginose abaxially; the purple lanuginose adaxially and papular abaxially, entire, acuminate lateral sepals; the purplish oblong-ovate dorsal sepal, lanuginose on the adaxial surface, papular abaxially; the purple petals with scalloped edges, acuminate to obtuse apex ending in a thin, slightly abbreviated tail; the lip with apiculate lateral angles and with a central, trapezoid verrucose callus on the disc. Brachionidium montieliae is a terrestrial species that thrives in the humid Puna forests at elevations around 3400–3600 meters. The ecosystem is characterized by a sclerophyllous forest with a diverse range of tree species. The species typically blooms between July and August. There are potential threats from annual burning practices in the region. Given the potential risks, it is imperative to undertake immediate conservation measures and promote environmental awareness to safeguard this species.

Mantle xenoliths recovered from the modern backarc region of the northern Altiplano Plateau record metasomatism by slab‐derived silicic melts, and a suite of Quaternary volcanics suggest that melting of accreted crustal material has persisted since shallow subduction in the Oligocene. Xenoliths recovered from a suite of high‐K andesitic lava flows include phlogopite‐ and calcite‐rich orthopyroxenites and harzburgites, a wehrlite, and a phlogopite schist. Xenolith hosted calcite yields δ13C and δ18O values of −2.49 to +0.77‰ VPDB and +15.8 to +16.4‰ VSMOW, respectively, reflecting inputs of subducted marine carbonates in the metasomatizing melt. Arc‐like trace element patterns and 87Sr/86Sr ratios further support a subduction influence. Major and trace element characteristics of Quaternary potassic basalts and intermediate alkaline lavas, with the presence of mantle xenoliths, contradict magmatic differentiation or mixing models to yield intermediate composition melts. Instead, we suggest that intermediate composition lavas are derived from the melting of sediments accreted to the base of the continental lithosphere during shallow subduction in the late Eocene‐Oligocene. Melting of accreted material produces silicic alkaline melts, which react with peridotite to produce alkaline basaltic melts and residual phlogopite‐orthopyroxenites. These processes explain the observed xenolith suite and local high‐K basaltic volcanism, and the intermediate lavas may represent sediment melts that ascended to the surface minimally altered by exploiting the Cusco‐Vilconata fault system. These observations inform mass transfers during shallow subduction and melting and metasomatism in the lithospheric mantle, with implications for the generation of alkaline magmatism and rheologic weakening in cordilleran regions globally.