Basaltic Parent Research Articles

40Ar/39Ar Geochronology of Subaerial Ascension Island and a Re-evaluation of the Temporal Progression of Basaltic to Rhyolitic Volcanism

Ar/Ar geochronology of basaltic to rhyolitic lavas, domes, and pyroclastic deposits from Ascension Island indicates that the maximum age of subaerially exposed samples is 1094 ka. Thirty-eight Ar/Ar ages, coupled with new and existing geochemical data, constrain the eruptive histories of the four distinct mafic magma types (high Zr/Nb, low Zr/Nb, intermediate Zr/Nb, and Dark Slope Crater) and document temporal variations in magma sources. Lavas from the eastern felsic complex, previously assumed to be as old as or slightly younger than the 602^1094 ka Middleton Ridge complex, are as young as 52 ka. Basaltic to benmoreitic scoria cones and associated flows of the intermediate Zr/Nb magma type have been inferred to be the most recent eruptive products, yet their eruptive histories extend back to 705 ka. These intermediate Zr/ Nb magmas are likely to be parental to the abundant trachytic to rhyolitic lavas and domes, which contradicts previous interpretations that call upon a high Zr/Nb parental basalt. Two distinct fractionation trends are observed in the trace element variations of Ascension trachytes and rhyolites. Ar/Ar ages of the samples defining the two trends suggest that ilmenite fractionation dominated the Nb budget and thus controlled Zr/Nb ratios in early (4931ka) Ascension evolved magmas, whereas zircon^titanite fractionation was predominant in younger felsic magmas.The eruptive sequence and compositions of the subaerial lavas and domes at Ascension Island are unique in comparison with other ocean island volcanoes because of its on-axis location and eruptions of highSiO2 trachyte and rhyolite.

Magnesium retention on the soil exchange complex controlling Mg isotope variations in soils, soil solutions and vegetation in volcanic soils, Iceland

Understanding the biogeochemical cycle of magnesium (Mg) is not only crucial for terrestrial ecology, as this element is a key nutrient for plants, but also for quantifying chemical weathering fluxes of Mg and associated atmospheric CO2 consumption, requiring distinction of biotic from abiotic contributions to Mg fluxes exported to the hydrosphere. Here, Mg isotope compositions are reported for parent basalt, bulk soils, clay fractions, exchangeable Mg, seasonal soil solutions, and vegetation for five types of volcanic soils in Iceland in order to improve the understanding of sources and processes controlling Mg supply to vegetation and export to the hydrosphere. Bulk soils (δ26Mg=−0.40±0.11‰) are isotopically similar to the parent basalt (δ26Mg=−0.31‰), whereas clay fractions (δ26Mg=−0.62±0.12‰), exchangeable Mg (δ26Mg=−0.75±0.14‰), and soil solutions (δ26Mg=−0.89±0.16‰) are all isotopically lighter than the basalt. These compositions can be explained by a combination of mixing and isotope fractionation processes on the soil exchange complex. Successive adsorption–desorption of heavy Mg isotopes leads to the preferential loss of heavy Mg from the soil profile, leaving soils with light Mg isotope compositions relative to the parent basalt. Additionally, external contributions from sea spray and organic matter decomposition result in a mixture of Mg sources on the soil exchange complex. Vegetation preferentially takes up heavy Mg from the soil exchange complex (Δ26Mgplant-exch=+0.50±0.09‰), and changes in δ26Mg in vegetation reflect changes in bioavailable Mg sources in soils. This study highlights the major role of Mg retention on the soil exchange complex amongst the factors controlling Mg isotope variations in soils and soil solutions, and demonstrates that Mg isotopes provide a valuable tool for monitoring biotic and abiotic contributions of Mg that is bioavailable for plants and is exported to the hydrosphere.

TTG-type plutonic rocks formed in a modern arc batholith by hydrous fractionation in the lower arc crust

We present the geochemistry and intrusion pressures of granitoids from the Kohistan batholith, which represents, together with the intruded volcanic and sedimentary units, the middle and upper arc crust of the Kohistan paleo-island arc. Based on Al-in-hornblende barometry, the batholith records intrusion pressures from ~0.2 GPa in the north (where the volcano-sedimentary cover is intruded) to max. ~0.9 GPa in the southeast. The Al-in-hornblende barometry demonstrates that the Kohistan batholith represents a complete cross section across an arc batholith, reaching from the top at ~8–9 km depth (north) to its bottom at 25–35 km (south-central to southeast). Despite the complete outcropping and accessibility of the entire batholith, there is no observable compositional stratification across the batholith. The geochemical characteristics of the granitoids define three groups. Group 1 is characterized by strongly enriched incompatible elements and unfractionated middle rare earth elements (MREE)/heavy rare earth element patterns (HREE); Group 2 has enriched incompatible element concentrations similar to Group 1 but strongly fractionated MREE/HREE. Group 3 is characterized by only a limited incompatible element enrichment and unfractionated MREE/HREE. The origin of the different groups can be modeled through a relatively hydrous (Group 1 and 2) and of a less hydrous (Group 3) fractional crystallization line from a primitive basaltic parent at different pressures. Appropriate mafic/ultramafic cumulates that explain the chemical characteristics of each group are preserved at the base of the arc. The Kohistan batholith strengthens the conclusion that hydrous fractionation is the most important mechanism to form volumetrically significant amounts of granitoids in arcs. The Kohistan Group 2 granitoids have essentially identical trace element characteristics as Archean tonalite–trondhjemite–granodiorite (TTG) suites. Based on these observations, it is most likely that similar to the Group 2 rocks in the Kohistan arc, TTG gneisses were to a large part formed by hydrous high-pressure differentiation of primitive arc magmas in subduction zones.

Fractionation of yttrium and holmium during basaltic soil weathering

The anomalously low affinity of yttrium (Y) for iron (Fe) (oxyhydr)oxides relative to lanthanides with similar ionic radius (e.g., Ho) has been demonstrated in experiments with isolated Fe minerals and in a variety of marine systems that contain high concentrations of solid phase Fe. However, it has not previously been demonstrated to occur during soil genesis, despite the common observation that many soils become enriched in Fe over time. We hypothesized that Y would become progressively depleted in soils relative to Ho with increased weathering. Since, trivalent Y has an anomalously low Misono softness relative to other trivalent ions included in the rare earth element and yttrium group (REY3+), we also investigated whether soil REY fractionation reflects variation in Misono softness. To test this, we measured trends in total REY concentrations for Hawaiian soils derived from basaltic parent materials aged 0.3–4100ky, and measured REYs released from the same samples during short-time (3h) dissolution experiments conducted as part of a previous investigation linking dissolution with surface charge properties (Chorover et al., 2004). The chondrite-normalized Y/Ho ratios in the parent Hawaiian basalt (Chond[Y/Ho]=0.998) and continental dust (Chond[Y/Ho]=0.994) inputs are remarkably similar, and thus we can interpret deviations from Chond[Y/Ho]∼1.0 to result from soil biogeochemical processes and not source mixing. Between 0.3 and 20ky, the Chond[Y/Ho] ratio of the subsurface soils decreased from 0.96±0.07(2σ) to 0.71±0.05, and then remained unchanged across the rest of the weathering sequence. In contrast, the Chond[Y/Ho] ratio of the surface soils decreased from 0.99±0.07 to 0.76±0.05 at 150ky and then, most likely due to continued dust inputs, increased to 1.04±0.07 in the oldest soils. Analysis of the short-time dissolution experiments revealed preferential release of Y relative to Ho (and also La relative Pr) at intermediate pH where aqueous REY concentrations are governed by proton competition for adsorption sites. Proton-competition-control over REY release is bounded at high pH by the onset of colloidal dispersion—represented by the point of minimum dissolution (p.m.d.) of Al—and at low pH by the soil’s point of zero net charge (p.z.n.c.) and/or when proton-promoted dissolution of REY-containing solids, including Fe-(oxyhydr)oxides, control REY release. Results of our dissolution experiments suggest that complexation of REYs by dissolved organic matter (DOM) does not drive Y–Ho fractionation during pedogenesis, but rather may suppress it. Synthesis of these field and laboratory experiments suggests the Y/Ho ratio decreases early in soil development (<20ky) when weathering rates are high and competitive proton adsorption affects REY fractionation. Given that Fe-(oxyhydr)oxide sorbents exhibit greater affinity for Ho relative to Y, their prevalent neo-formation during incipient pedogenesis likely plays a central role in Y–Ho fractionation in these soils. Persistence of low Chond[Y/Ho] ratios in the subsurface soils even at 4100ky suggests Y–Ho fractionation continues, albeit at a slower rate, as weathering proceeds.

Geochemical characteristics of Kanigorgeh ferruginous bauxite horizon, West-Azarbaidjan province, NW Iran

Ferruginous bauxite horizon of Kanigorgeh is located ~ 20 km northeast of Bukan, West- Azarbaidjan province, northwest Iran. The horizon is a part of the Irano-Himalayan karst bauxite belt that was developed in a form of 8 discontinuous stratified layers and lenses varying in thicknesses (5-17 m) with extending over 3.2 km along the contact of Permian carbonates, Triassic dolomites. In this study, parental affinity and controlling factors of elements distribution in the bauxite ores are surveyed by applying analytical mineralogy, mass and volume changes calculations (method of isocon), elemental ratios, and correlation coefficients. Mineralogical analyses reveal that diaspore, hematite, and kaolinite are the major minerals in the ores with lesser and variable amounts of boehmite, goethite, muscovite-illite, rutile, and montmorillonite. In contrast to the presence of diasporic-boehmitic mineralogical composition in ores, the geochemical data (i.e., ratios of Pb/Y, Ga/Pb, Zr/Pb, and Cr/Ni), testify to gibbsitic composition for the original aluminum hydroxides. Microscopic studies and geochemical characteristics of major and trace elements indicate that the ores were formed authigenically by the alteration and weathering of basaltic parent rocks. Mass change calculations suggests that enrichment of many elements in the ores resulted by losing Si, Ca, K, Na, Mg, and P during weathering of plagioclase, K-feldspar, ferromagnesians, and apatite. In addition, variable amounts of Co, Sr, and Ba were lost during bauxitization. Geochemical considerations prove that distribution of major, minor, trace, and rare earth elements in the studied ores were principally controlled by factors such as cation exchanges, adsorption, increasing of pH in weathering solutions due to buffering by carbonate bedrocks, scavenging by Fe-oxides and hydroxides, isomorphic substitutions, co-precipitation, and differences in stability of primary minerals.

Selected soil properties as indicators of soil water regime in the Cathedral Peak VI catchment of KwaZulu-Natal, South Africa

The morphological features and physicochemical properties of 14 soil profiles, representing the major soil types, were studied in the Cathedral Peak VI catchment of KwaZulu-Natal, South Africa. The soils are characterised by a high organic carbon content in the topsoil, red and yellow freely drained subsoils, and some signs of water saturation in the deep subsoil and saprolite. The lack of variability in the morphology of the soils is probably a result of deep weathering of the chemically homogeneous basaltic parent material. Shelving controls the development of return flow on the slopes, feeding into the two prominent wetlands. The freely drained nature of the subsoil horizons are reflected in the red colour and low pH values of these horizons. The iron and manganese redoximorphic features, occurring in the lower horizons, are indicative of periodic short periods of water saturation.

Redistributions of 137Cs and soil components on cultivated hill slopes with hedgerows as conservation measures

137Cs tracing technique combined with soil composition characterization was used to investigate the mechanism and effectiveness of conservation measures for cultivated hill slopes of strongly weathered acidic Acrisol. The results of analysis of 66 soil samples taken across six plots showed much lower losses of 137Cs, soil organic carbon (SOC), and nutrients in the four treatments of alley cropping with hedgerows compared to the two control treatments of bare land and without conservation measures. No-tillage cultivation combined with vetiver grass hedgerows was most effective in securing high levels of 137Cs, SOC, N, P, K, and CEC while reducing considerably the contents of dissolved cations and anions. Within the plots of alley cropping, 137Cs, SOC, nutrients, dissolved ions, and sand were accumulated in the areas immediately upslope of the hedgerows, whereas the clay content and bulk density were highest downslope of the hedgerows. SOC inversely correlates with clay in concurrence with observations for the soils derived from basaltic parent materials, in which clay minerals are predominated by kaolinite. The correlations of 137Cs with dissolved Ca, Mg, and P indicate that in the highly leaching Acrisol 137Cs may partly occur in the soil solution that is susceptible to leaching and loss. The interrelationships among soil components were interpreted as being driven by the redistribution of soil aggregates which were fractionated in situ according to their sizes and specific densities by erosion forces combined with tillage and conservation measures (hedgerows).

Distribution of Organic Matter in the Particle Size Fractions of Lateritic Soil (Plinthosol)

Abstract The distribution of organic matter in the genetic horizons of lateritic soil within a 100-cm profile to the basaltic parent rock is almost except for horizon Ap. Assuming that the sum of organic matter in 100 cm of the soil profile is 100%, 25.7% of these compounds occur in horizon Ap, whereas in the remaining horizons this value varies within 18-19.2%. In all size fractions, except the clay fraction in diameter of &lt;0.002 mm, the content of organic matter decreases to a certain depth, and increases again in the deepest horizon located directly on the solid basaltic rock. The clay fraction displays an opposite trend; the content of organic matter in them increases with depth. In the horizon at the depth of 60-80 cm, the clay fraction &lt;0.002 mm accumulates half of the total sum of organic compounds of all the remaining fractions. Such distribution of organic matter in soil and among its particle size probably results from the character of the basaltic weathered debris, as well as climate and vegetation covering the studied area.

Geochemistry and Petrogenesis of Silicic Magmas in the Intra-Oceanic Kermadec Arc

The geochemistry of pyroclasts sampled from four volcanoes along the Kermadec arc in the SW Pacific is used to investigate the genesis of silicic magmas in a young (<2 Myr), archetypical intra-oceanic arc setting. Raoul, Macauley and Raoul SW volcanoes in the northern Kermadec arc, and Healy volcano in the southern Kermadec arc have all recently erupted dacitic to rhyolitic crystal-poor pumice. In addition to whole-rock analyses, we present a detailed study of mineral and glass chemistries to highlight the complex structure of the Kermadec magmatic systems. Major and trace element bulk-rock compositions mostly fall into relatively narrow compositional ranges, forming discrete groups by eruption for Raoul, and varying with relative crystal contents for Healy. In contrast, pumices from Macauley cover a wide range of compositions, between 66 and 72·5 wt % SiO2. At all four volcanoes the trace element patterns of pumice are subparallel to both those of previously erupted basalts and/or whole mafic blebs found both as discrete pyroclasts and as inclusions within pumices. Pb and Sr isotopic compositions have limited ranges within single volcanoes, but vary considerably along the arc, being more radiogenic in the southern volcanoes. Distinctive crystal populations and zonation patterns in pumices, mafic blebs and plutonic xenoliths indicate that many crystals did not grow in the evolved magmas, but are instead mixed from other sources including gabbros and hydrothermally altered tonalites. Such open-system mixing is ubiquitous at the four volcanoes. Oxygen isotope compositions of both phenocrysts (silicic origin) and xenocrysts or antecrysts (mafic origin) are typical for mantle-derived melts. Whole-rock, glass and mineral chemistries are consistent with evolved magmas being generated at each volcano through ∼70–80% crystal fractionation of a basaltic parent. Our results are not consistent with silicic magma generation via crustal anatexis, as previously suggested for these Kermadec arc volcanoes. Although crystallization is the dominant process driving melt evolution in the Kermadec volcanoes, we show that the magmatic systems are open to contributions from both newly arriving melts and wholly crystalline plutonic bodies. Such processes occur in variable proportions between magma batches, and are largely reflected in small-scale chemical variations between eruption units.

Magmatic history of Dabbahu, a composite volcano in the Afar Rift, Ethiopia

Dabbahu is a composite volcano at the north end of the active Manda-Hararo segment of the Afar Rift in northern Ethiopia. We present 93 new whole-rock analyses, mineral analyses from 65 samples, and 9 new 40Ar39Ar dates for rocks ranging in composition from mildly alkaline basalt through trachyandesite to peralkaline rhyolite (comendite and pantellerite) erupted from Dabbahu. These data, supplemented by a new geological map, are used to provide insights into the evolution of the volcano. We show that Dabbahu has been active for over 67 k.y., but an apparent hiatus occurred between the eruption of comendite (29 ka) and pantellerite (ca. 8 ka) lavas. Mineral data for olivine, clinopyroxene, plagioclase, and alkali feldspar show a remarkably extensive range of solid solution across the rock suite consistent with protracted fractionation from basalt to rhyolite. The parental basalt is compositionally similar to recent rift-related basalts in the Manda-Hararo rift, with low initial H2O contents (<1 wt%). Closed-system fractionation increased H2O contents of residual liquids sufficiently for some rhyolites to erupt both explosively and effusively. The diverse magma types were erupted from a relatively closely spaced network of vents and fissures. Field evidence indicates that magmas were not erupted in a simple fractionation sequence. Some mixing occurred between magmas of less-evolved compositions and more-evolved compositions shortly prior to, or during, eruption. The differentiation of basalt to rhyolite must have occurred on time scales that were relatively short compared to the lifetime of the volcano, probably due to the small volumes of basalt intruded into the crust and consequently enhanced cooling and crystallization rates. A network of stacked sills or closely spaced dikes in the shallow to midcrust represents the most plausible configuration of the subvolcanic plumbing system. Input of new magma batches into such a system may serve as a key eruption trigger at Dabbahu.

Petrology and geochemistry of igneous inclusions in recent Merapi deposits: a window into the sub-volcanic plumbing system

Recent basaltic-andesite lavas from Merapi volcano contain abundant and varied igneous inclusions suggesting a complex sub-volcanic magmatic system for Merapi volcano. In order to better understand the processes occurring beneath Merapi, we have studied this suite of inclusions by petrography, geochemistry and geobarometric calculations. The inclusions may be classified into four main suites: (1) highly crystalline basaltic-andesite inclusions, (2) co-magmatic enclaves, (3) plutonic crystalline inclusions and (4) amphibole megacrysts. Highly crystalline basaltic-andesite inclusions and co-magmatic enclaves typically display liquid–liquid relationships with their host rocks, indicating mixing and mingling of distinct magmas. Co-magmatic enclaves are basaltic in composition and occasionally display chilled margins, whereas highly crystalline basaltic-andesite inclusions usually lack chilling. Plutonic inclusions have variable grain sizes and occasionally possess crystal layering with a spectrum of compositions spanning from gabbro to diorite. Plagioclase, pyroxene and amphibole are the dominant phases present in both the inclusions and the host lavas. Mineral compositions of the inclusions largely overlap with compositions of minerals in recent and historic basaltic-andesites and the enclaves they contain, indicating a cognate or ‘antelithic’ nature for most of the plutonic inclusions. Many of the plutonic inclusions plot together with the host basaltic-andesites along fractional crystallisation trends from parental basalt to andesite compositions. Results for mineral geobarometry on the inclusions suggest a crystallisation history for the plutonic inclusions and the recent and historic Merapi magmas that spans the full depth of the crust, indicating a multi-chamber magma system with high amounts of semi-molten crystalline mush. There, crystallisation, crystal accumulation, magma mixing and mafic recharge take place. Comparison of the barometric results with whole rock Sr, Nd, and Pb isotope data for the inclusions suggests input of crustal material as magma ascends from depth, with a significant late addition of sedimentary material from the uppermost crust. The type of multi-chamber plumbing system envisaged contains large portions of crystal mush and provides ample opportunity to recycle the magmatic crystalline roots as well as interact with the surrounding host lithologies.

Characterization of bauxite deposits from Kachchh Area, Gujarat

The bauxites deposits of Kachchh area in Gujarat are investigated to characterize them based on mineralogical and petrographic studies. The major bauxitic mineral in these occurrences is gibbsite, with minor concentration of boehmite and diaspore. Apart from the bauxitic minerals, the other associate minerals are kaolin, calcite, alunite and the iron ore minerals such as hematite and goethite and titanium rich anatase. The iron ore minerals (hematite and goethite) are 10-50microns in size and are disseminated throughout the oolitic and pisolitic bauxitic minerals. At places the goethite exhibits colloform texture. The preservation of basaltic texture in some of the samples indicate that the insitu nature of these bauxites, which are formed by the alteration of calcic plagioclase from the parent basalt. Although, the basalt occurs as the main parent rock for these bauxites, the presence of calcite in some of the samples represent the possibility of having a limestone parent rock at least in some of the bauxite occurrences.

Metabasic rocks in the Varied Group of the Moldanubian Zone, southern Bohemia - their petrology, geochemical character and possible petrogenesis

Metabasic rocks form an important constituent of the Chýnov and Ceský Krumlov units belonging to the Varied Group (Moldanubian Zone, south Bohemia). The amphibolites are dominated by amphibolite-facies mineral assemblages of mainly tschermakitic amphibole and plagioclase. Hornblendes show compositional variation with Si ~ 6.5 apfu, Mg/(Mg + Fe) ~ 0.5 and (Na + K)A ~ 0.5 apfu. Garnet with clinopyroxene are subordinate and occur in a few samples only. No relics of previous greenschistor granulite-facies assemblages have been observed, most likely due to the relatively simple metamorphic history. The petrology indicates rather close correlation of the Chýnov and Ceský Krumlov units. The similarities include presence of dolomite in carbonate bodies, graphite schists, rocks with marialitic scapolite, locally also Ti-andradite (± magnetite, epidote) oxidic assemblages and thin layers of Mn-rich garnet-quartz rocks. However, there is a major difference in the oxidation state. Most Chýnov amphibolites have Fe2O3/FeO = 0.70–1.00 and their protolith probably experienced an early incipient oxidation. Great deal of the parental basalts thus could have been effusive. The Ceský Krumlov amphibolites have Fe2O3/FeO ≤ 0.4, perhaps because they show much closer association with graphite schists that could have been responsible for the reduction of the adjacent rock units. The dataset is dominated by EMORB-like tholeiite basalts interpreted as having been derived by Early Palaeozoic melting of a strongly depleted mantle source (e50 Nd 0 = +8.6 to +9.4; TD Nd M = 0.43–0.50 Ga). This argues stoutly against Precambrian age of the Varied Group in south Bohemia. The composition of the remaining samples reflects contamination by upper continental crust (e50 Nd 0 = +3.1 to +1.3, progressive enrichment in Th, development of a significant negative Nb, and lesser P and Ti anomalies on the NMORB-normalized spiderplots). A much smaller group of amphibolites is characterised by steep REE patterns (LaN/YbN = 5.5–11) and high contents of HFSE (Nb, Ta, Zr and P). It is of a clear OIB affinity, with parental alkali basalt (Nb/Y = 0.7–1.6) generated by a low degree of partial melting of a deep, garnet-bearing asthenospheric mantle source (e50 Nd 0 = +4.5 to +6.1; TD Nd M = 0.75–0.83 Ga). Metamorphosed doleritic/gabbroic dykes cutting the Palaeoproterozoic Světlik orthogneiss show rather unradiogenic Nd isotopic composition (e50 Nd 0 = +0.1 and –3.6; TD Nd M = 1.34 and 2.03 Ga). This precludes closed-system crystallization from depleted mantle derived melts in Phanerozoic times. The exact age and nature of their parental magma remain enigmatic but any genetic link with the amphibolites in the structurally overlying Ceský Krumlov Varied Unit seems ruled out. Overall, the most likely tectonic setting of the magmatism was attenuated lithosphere, subjected to an Early Palaeozoic extension, leading eventually to fragmentation of the northern Gondwana margin. The minor OIB component preserved as alkali basalts as well as some contribution to the EMOR-like basaltic magmas was probably added by a rising mantle plume.

Relações solo-paisagem em topossequência de origem basáltica

Variações nos atributos do solo dependem da posição do solo na paisagem e processos de drenagem, erosão e deposição. Este estudo objetivou avaliar os atributos físicos e químicos do solo, em uma topossequência de origem basáltica, na região de Batatais (SP). A área possui relevo aplanado e altitude oscilando entre 740 m e 610 m, em região dominada por basaltos. Foi estabelecido caminhamento de 3.000 m, a partir do espigão da vertente, no seu declive mais suave. As superfícies geomórficas foram identificadas e delimitadas conforme critérios topográficos e estratigráficos, com base em intensas investigações detalhadas de campo. Foram coletadas amostras laterais aos perfis modais representativos das diversas superfícies geomórficas (S.G.) da topossequência (S.G. I = topo; S.G. II = meia encosta e sopé de transporte; S.G. III = ombro e sopé de deposição), totalizando 142 amostras. Além disto, foram abertas trincheiras, nos segmentos de vertente inseridos nas superfícies geomórficas mapeadas. As amostras coletadas foram analisadas quanto à densidade do solo, textura, bases trocáveis (Ca2+, K+ e Mg2+), soma de bases, capacidade de troca catiônica, saturação por bases, pH (água e KCl), SiO2, Al2O3, Fe2O3 (ataque por H2SO4), óxidos de Fe livres extraídos com ditionito-citrato-bicarbonato e Fe mal cristalizado extraído com oxalato de amônio. Os resultados revelaram que os solos oriundos de basalto apresentaram atributos físicos e químicos com comportamento dependente das formas do relevo. Com o uso de técnicas estatísticas multivariadas, foi possível distinguir três diferentes ambientes, que equivalem às três superfícies geomórficas.

Long‐term carbon storage through retention of dissolved aromatic acids by reactive particles in soil

AbstractSoils retain large quantities of carbon, thereby slowing its return to the atmosphere. The mechanisms governing organic carbon sequestration in soil remain poorly understood, yet are integral to understanding soil‐climate feedbacks. We evaluated the biochemistry of dissolved and solid organic carbon in potential source and sink horizons across a chronosequence of volcanic soils in Hawai'i. The soils are derived from similar basaltic parent material on gently sloping volcanic shield surfaces, support the same vegetation assemblage, and yet exhibit strong shifts in soil mineralogy and soil carbon content as a function of volcanic substrate age. Solid‐state13carbon nuclear magnetic resonance spectra indicate that the most persistent mineral‐bound carbon is comprised of partially oxidized aromatic compounds with strong chemical resemblance to dissolved organic matter derived from plant litter. A molecular mixing model indicates that protein, lipid, carbohydrate, and char content decreased whereas oxidized lignin and carboxyl/carbonyl content increased with increasing short‐range order mineral content. When solutions rich in dissolved organic matter were passed through Bw‐horizon mineral cores, aromatic compounds were preferentially sorbed with the greatest retention occurring in horizons containing the greatest amount of short‐range ordered minerals. These minerals are reactive metastable nanocrystals that are most common in volcanic soils, but exist in smaller amounts in nearly all major soil classes. Our results indicate that long‐term carbon storage in short‐range ordered minerals occurs via chemical retention with dissolved aromatic acids derived from plant litter and carried along preferential flow‐paths to deeper B horizons.

Thermomechanical maturation of the continental crust and its effects on the late Eocene–early Oligocene volcanic record of the Sierra Madre del Sur Province, southern Mexico

We interpret the voluminous late Eocene–early Oligocene volcanic successions of the north-central Sierra Madre del Sur as the eruptive manifestation of a progressive thermomechanical maturation of the crust, driven by sustained igneous activity that affected the region since the early Eocene. Widespread Eocene magmatism and injection of mantle-derived melts into the crust beneath the Michoacán-Puebla area promoted the development of a hot zone extending to upper crustal levels, and the formation of a mature intracrustal magmatic system. Within this context, the intermediate siliceous compositions of the Tilzapotla, Muñeca, and Goleta explosive centres were generated through fractional crystallization, crustal contamination, and anatexis. In particular, decreasing bulk-rock Sr and Eu concentrations and Nd isotopes with increasing silica in the Tilzapotla and Muñeca suites document an evolution through low-pressure fractional crystallization of plagioclase-dominated assemblages, simultaneous with the assimilation of middle–upper crustal materials. In contrast, marked Eu, Sr, and Ba depletions coupled with high and variable Rb/Nd at constant 143Nd/144Nd in the Goleta rhyolites suggest their derivation from partial melting of biotite-bearing quartz-feldspathic lithologies. Ascent of the thermal anomaly induced by magma emplacement and accumulation at shallow depths shifted the brittle–ductile crustal transition close to the surface, and produced an ignimbrite flare-up through caldera-forming eruptions. A different petrogenetic–volcanologic scenario developed in north-western Oaxaca, where less profuse early–middle Eocene igneous activity and an ancient lower crustal basement made up of refractory granulitic lithologies inhibited the expansion of the hot zone to shallow levels, and constrained magmatic evolution at depth. Here, composite and monogenetic volcanoes with intermediate compositions were produced through high-pressure fractional crystallization and crustal contamination. Specifically, increasing La/Yb and Sm/Yb with increasing silica in the Oaxaca suite, and negative correlations of Nd isotopes with SiO2 at low Rb/Nd, suggest garnet fractionation from parental basalts, coupled with the assimilation of Rb-depleted lower crustal materials.

Tracking Open-system Differentiation during Growth of Santa María Volcano, Guatemala

Prior to the ad 1902 Plinian eruption of 8 km3 of dacite and subsequent growth of the >1 km3 Santiaguito dacite dome complex, Santa María volcano grew into an 8 km3 composite cone over ∼75 kyr in four phases (at 103–72, 72, 60–46, and 35–25 ka). The 1902 eruption occurred after an ∼25 kyr period of repose in growth of the composite cone. To provide context for processes that ultimately led to the 1902 eruption, we present geochemical and isotopic (Sr, Nd, Pb, U-series) data from lavas of the composite cone for which ages are constrained by 40Ar/39Ar dating. The four cone-building phases comprise basaltic to basaltic-andesite lava (51·4–56·1% SiO2) whose major- and trace-element compositions suggest that crystallization was important in differentiation. Relative to other Central American arc volcanoes, these lavas also have large 238U excesses and high 207Pb/204Pb ratios that imply melting of a mantle wedge modified to an unusual extent by fluid from subducted crust and sediment of the Cocos plate. Major- and trace-element and isotopic variations over time imply that mafic recharge and magma mixing were prevalent during early phases of cone-building, whereas assimilation processes were more dominant during the latest stage of cone growth. Indeed, some early erupted basalts have lower 143Nd/144Nd and higher 87Sr/86Sr ratios than more SiO2-rich basaltic andesites that erupted during the final phase of cone-building. These features point to an assimilant that is not typical continental crust and instead may be more like mid-ocean ridge basalt with respect to major- and trace-element composition and Sr, Nd, Pb, and U–Th isotope ratios. Energy-constrained modeling of a parental basalt that undergoes crystal fractionation, assimilation and periodic recharge with basalt in the lower crust can reproduce lava compositions erupted during phases I–III and the early part of phase IV. Modeling further indicates that assimilation within the lower crust of partially melted garnet-amphibolite metabasalt, without basaltic recharge, may produce the youngest cone-forming lavas in phase IV. These models link the 8 km3 of cone growth over 75 kyr to the mass flux of magma into the crust. Our findings suggest an along-arc magma flux into the lower crust beneath Santa María of >20 km3 km−1 Myr−1, which is higher than anticipated in recent numerical–thermal approaches to basalt–crust interaction. Consequently, the thermal incubation period needed to produce hybrid basaltic-andesite magma may be only a few tens of thousand years.

The formation and bulk composition of modern juvenile continental crust: The Kohistan arc

The intraoceanic Kohistan arc, northern Pakistan, exposes a complete crustal section composed of infracrustal basal cumulates formed at ≤55km depth, a broadly basaltic/gabbroic lower crust, a 26km thick calc-alkaline batholith, and 4km of a volcanoclastic/sedimentary sequence. The bulk composition of the Kohistan arc crust is approximated by estimating the relative volumes of exposed rocks through detailed field observations, in particular along a representative km-wide transect across the arc, through geobarometric constrains to determine the unit thicknesses, and through satellite images to estimate their lateral extent. We separated the arc into three major units: lower, mid-, and mid- to upper crust, which contain a total of 17 subunits whose average compositions were derived from employing a total of 594 whole rock analyses. The volume-integrated compositions of each unit yield the bulk composition of the arc crust. While the details of the resulting bulk composition depend slightly on the method of integration, all models yield an andesitic bulk supra Moho composition, with an average SiO2 of 56.6–59.3wt.% and XMg of 0.51–0.55. The Kohistan arc composition is similar to global continental crust estimates, suggesting that modern style arc activity is the dominant process that formed the (preserved) continental crust. A slight deficit in high field strength and incompatible elements in the Kohistan arc with respect to the global continental crust can be mitigated by adding 6–8wt.% of (basaltic) intraplate type magmas. Our results document that infra arc processes, even in a purely oceanic environment, result in an overall andesitic crust composition in mature arcs, contrary to the widely accepted view that oceanic arcs are basaltic. Bulk crust differentiation from a basaltic parent occurs through foundering of ultramafic cumulates. Our results imply that secondary reworking processes such as continental collision are of secondary importance to explain the major element chemistry of the bulk continental crust composition.

A reactive-transport model for weathering rind formation on basalt

Saprolite formation rates influence many important geological and environmental issues ranging from agricultural productivity to landscape evolution. Here we investigate the chemical and physical transformations that occur during weathering by studying small-scale “saprolites” in the form of weathering rinds, which form on rock in soil or saprolite and grow in thickness without physical disturbance with time. We compare detailed observations of weathered basalt clasts from a chronosequence of alluvial terraces in Costa Rica to diffusion-reaction simulations of rind formation using the fully coupled reactive transport model CrunchFlow. The four characteristic features of the weathered basalts which were specifically used as criteria for model comparisons include (1) the mineralogy of weathering products, (2) weathering rind thickness, (3) the coincidence of plagioclase and augite reaction fronts, and (4) the thickness of the zones of mineral reaction, i.e. reaction fronts. Four model scenarios were completed with varying levels of complexity and degrees of success in matching the observations. To fit the model to all four criteria, however, it was necessary to (1) treat diffusivity using a threshold in which it increased once porosity exceeded a critical value of 9%, and (2) treat mineral surface area as a fitting factor. This latter approach was presumably necessary because the mineral-water surface area of the connected (accessible) porosity in the Costa Rica samples is much less than the total porosity ( Navarre-Sitchler et al., 2009). The model-fit surface area, here termed reacting surface area, was much smaller than the BET-measured surface area determined for powdered basaltic material. In the parent basalt, reacting surface area and diffusivity are low due to low pore connectivity, and early weathering is therefore transport controlled. However, as pore connectivity increases as a result of weathering, the reacting surface area and diffusivity also increase and weathering becomes controlled by mineral reaction kinetics. The transition point between transport and kinetic control appears to be related to a critical porosity (9%) at which pore connectivity is high enough to allow rapid transport. Based on these simulations, we argue that the rate of weathering front advance is controlled by the rate at which porosity is created in the weathering interface, and that this porosity increases because of mineral dissolution following a rate that is largely surface-reaction controlled.

Mafic magmas from Mount Baker in the northern Cascade arc, Washington: probes into mantle and crustal processes

Five mafic lava flows located on the southern flank of Mount Baker are among the most primitive in the volcanic field. A comprehensive dataset of whole rock and mineral chemistry reveals the diversity of these mafic lavas that come from distinct sources and have been variably affected by ascent through the crust. Disequilibrium textures present in all of the lavas indicate that crustal processes have affected the magmas. Despite this evidence, mantle source characteristics have been retained and three primitive endmember lava types are represented. These include (1) modified low-K tholeiitic basalt (LKOT-like), (2) typical calc-alkaline (CA) lavas, and (3) high-Mg basaltic andesite and andesite (HMBA and HMA). The Type 1 endmember, the basalt of Park Butte (49.3–50.3 wt% SiO2, Mg# 64–65), has major element chemistry similar to LKOT found elsewhere in the Cascades. Park Butte also has the lowest overall abundances of trace elements (with the exception of the HREE), indicating it is either derived from the most depleted mantle source or has undergone the largest degree of partial melting. The Type 2 endmember is represented by the basalts of Lake Shannon (50.7–52.6 wt% SiO2, Mg# 58–62) and Sulphur Creek (51.2–54.6 wt% SiO2, Mg# 56–57). These two lavas are comparable to calc-alkaline rocks found in arcs worldwide and have similar trace element patterns; however, they differ from each other in abundances of REE, indicating variation in degree of partial melting or fractionation. The Type 3 endmember is represented by the HMBA of Tarn Plateau (51.8–54.0 wt% SiO2, Mg# 68–70) and the HMA of Glacier Creek (58.3–58.7 wt% SiO2, Mg# 63–64). The strongly depleted HREE nature of these Type 3 units and their decreasing Mg# with increasing SiO2 suggests fractionation from a high-Mg basaltic parent derived from a source with residual garnet. Another basaltic andesite unit, Cathedral Crag (52.2–52.6 wt% SiO2, Mg# 55–58), is an Mg-poor differentiate of the Type 3 endmember. The calc-alkaline lavas are least enriched in a subduction component (lowest H2O, Sr/PN, and Ba/Nb), the LKOT-like lavas are intermediate (moderate Sr/PN and Ba/Nb), and the HMBA are most enriched (highest H2O, Sr/PN and Ba/Nb). The generation of the LKOT-like and calc-alkaline lavas can be successfully modeled by partial melting of a spinel lherzolite with variability in composition of slab flux and/or mantle source depletion. The HMBA lavas can be successfully modeled by partial melting of a garnet lherzolite with slab flux compositionally similar to the other lava types, or less likely by partial melting of a spinel lherzolite with a distinctly different, HREE-depleted slab flux.