Basaltic Parent Research Articles

The geochemical evolution of syncollisional magmatism and the implications for significant magmatic-hydrothermal lead–zinc mineralization (Gangdese, Tibet)

In addition to well-known subduction processes, the collision of two continents also generates abundant ore deposits, as in the case of the Tibetan Plateau, which is the youngest and most spectacular collisional belt on Earth. During the building history of the Gangdese magmatic belt, several magmatic flare-up events developed, however, significant magmatic-hydrothermal lead–zinc mineralization dominantly accompanied the magmatism during the syncollisional period (~65–41Ma). Based on integrated geochemical and isotopic data, we provide insights into the genesis and evolution of syncollisional magmas, and their implications for significant magmatic-hydrothermal lead–zinc mineralization. The Sr–Nd isotopic compositions of most syncollisional igneous rocks (87Sr/86Sr=0.7034–0.7123; εNd(t)=−9.0 to +1.8) indicate a mixing origin between mantle-derived basaltic magmas and ancient crustal melts, and fractional crystallization is a fundamental mechanism by which syncollisional magmas evolve towards intermediate to silicic compositions. Most lead–zinc mineralization-related plutons are high silica (76.14% wt.% SiO2 on average), high oxygen fugacity (average ΔFMQ +2.5) granites with highly evolved chemical signatures [average Eun/Eun*=0.33, high Rb/Sr (average=3.9)], and they represent the final products from primary magmas. Due to the contribution of ancient crustal melts to the genesis of mineralization-related parent magmas, the spatial distribution of Pb–Zn deposits within the northern Gangdese magmatic belt is controlled by the lithospheric architecture. In compressional environments, magmas have low evacuation efficiency and long magma chamber lifespan, which is favorable for basaltic parents evolved to high silica granites through sufficient fractional crystallization. This scenario contributes to our understanding of the significant magmatic-hydrothermal lead–zinc mineralization that occurred in the syncollisional period.

Keterkaitan parameter nilai pH, C-organik, N-Total, C/N dan alofan pada beberapa Andisols di Jawa Barat

The correlation of chemical parameters and soil mineralogy one to another in Andisols were interesting to be studied, to increase the understanding of soil reactions, nutrient availability and soil mineral content. Andisols from three locations and derived from three different volcanic eruptions namely G. Tangkuban Parahu, G. Patuha and G. Tilu, with andesite, andesite-basalt and basalt parent materials respectively, were examined the correlation of several soil parameters. The values of pH, organic carbon, total nitrogen, C/N and allophane content were obtained from the soil analysis of every horizon of the soil profiles at each site. Correlation analyses were used to see the relationship of the parameters. The results showed a negative correlation between pH and organic carbon (r = -590 *). The soil pH values were positively correlated with the amount of allophane (r = 0.687 *). The pH values were correlated positively with imogolite content (r = 0.356 *). The pH values were negatively correlated with organic carbon (r = -0.590 *). The organic carbon content was negatively correlated with depth (r = - 0.582 *). The organic carbon content was negatively correlated with allophane (r = 0.707 *). Total nitrogen values were negatively correlated with increasing depth (r = -0.531 *).Keywords: Mt. Tangkuban Parahu, Mt. Patuha, Mt. Tilu, andesit, andesit-basalan, alophane

The influence of weathering and soil organic matter on Zn isotopes in soils

Zinc is an essential micronutrient that is ultimately released during mineral weathering. In soils, organic matter plays a key role in influencing Zn partitioning and therefore on Zn biogeochemical cycling. Soil organic matter is partitioned between carbon that is more readily available for decomposition by microorganisms, and more stable carbon transiently preserved from decomposition. The role of the stable pool of soil organic matter on Zn biogeochemical cycling remains poorly understood. The pool of stable carbon is controlled by combination with mineral constituents or is material that is intrinsically resistant to decomposition. The Zn stable isotopes are fractionated by interactions between Zn and soil mineral and organic constituents. This study reports the Zn isotope composition of five Icelandic soil profiles derived from the same parent basalt and characterized by contrasting degrees of weathering and organic matter content (δ66Zn=+0.10±0.05 to +0.35±0.02‰), the distribution of reactive mineral constituents available to form associations with soil organic matter, and the amount of stable organic carbon. Throughout these soils, the δ66Zn isotope variations are little influenced by mineral constituents, but rather by soil organic matter content. These data suggest that a combination of organic matter accumulation and Zn loss by leaching is required to explain the observed decrease in Zn concentration in soils and lighter soil δ66Zn with increasing organic carbon content. These results suggest that the presence of stable organic carbon in soils provides a pool of light Zn, attributed to the Zn isotope signature of organic matter partially preserved from decomposition. Crucially, this stable organic carbon pool may also contribute to the formation of the light Zn isotope sink reported in organic-rich marine sediments, a key output required to explain the oceanic mass balance of Zn isotopes.

The VEI-7 Millennium eruption, Changbaishan-Tianchi volcano, China/DPRK: New field, petrological, and chemical constraints on stratigraphy, volcanology, and magma dynamics

Field relations, petrography, bulk and micro-scale chemistry reveal that the most recent history of hazardous Changbaishan-Tianchi volcano should be revised with important implications for volcanic hazard in NE Asia. Currently, the two most recent large eruptions are identified separately as a VEI 5 trachytic Baguamiao eruption (BGM) and the much heralded VEI 7, late 946CE (Common Era) “Millennium” eruption (ME) of comendite. However, we find that the former is part of the latter based on the following evidence: (1) trachytic fallout of the BGM lies directly on the comendite tephra of the ME without any indication of depositional hiatus; (2) abundant mingled trachyte-comendite pumice in the tephra deposits; (3) similar chemistry of mingled pumice and its components to those in the BGM and ME products; (4) correlation of bimodal glass shard compositions in the distal ‘B-Tm’ ash from the Japan Sea with comendite and trachyte glass from the BGM and ME products. Based on the above evidence, we suggest that the great Millennium eruption of 946CE should be revised finally to include the BGM trachyte as its final stage. Furthermore, deposits attributed to two other trachytic eruptions in 1668 and/or 1702CE (also called Baguamiao by some authors), and 1903CE referred to in historic accounts were also examined. Our field observations, petrography, bulk and micro-scale chemistry combined with previously published Ra/Th ages indicate that all these trachytes are either primary deposits of the ME or its reworked deposits. Thus our findings do not support two separate post-ME eruptions and require that volcanic hazard assessment at Changbaishan volcano include this new interpretation.Recently published geochronological data integrated with our new petrochemical and volcanological framework informs the magma dynamics leading to the ME. The ME comendite, derived from a parental trachyte similar to the BGM started accumulating at shallow levels around 12ka to 8ka. Around 4 to 1.6ka the BGM trachyte sensu strictu separated from its basaltic parent and started accumulating and crystallizing separately from the ME comendite. Just prior to the ME eruption, mingling of trace amounts of a third more mafic hybrid is implicated by the composition of mafic glass selvedges. The strong evidence for mixing of all these endmembers in the eruption products suggests that recharge mixing and overturn of this predominantly comendite-trachyte system occurred during the ~1ka ME. The common occurrence of comendite and trachyte in the last 100ka (also maybe 1Ma) at Changbaishan-Tianchi suggests that conditions for trachyte-comendite magma interactions are prevalent in the magma system of Changbaishan and maybe crucial in catastrophic eruptions there.

Soil Catena Properties of Daher Al- Jabal in South Syria

Soil catena concept is a sequence of soils extends across relief positions and is developed from similar parent material. This study highlighted on the important aspects and properties of soil catena of Daher El-Jabal in Jabal Al-Arab mountainous area South eastern of Syria, by implementing pedologic study in 2010-2012. Six soil profiles have been studied along pedo-genetic transect in order to highlight the soil catena prevailing properties. The results reveal that the soil has formed from igneous basaltic parent casts, related to Neogen era, where reliefs had the key role in the developing of soil solum. Consequently, Entisols were dominated on eroded summits, Inceptisols on back slops and mountain flanks, Mollisols on depressions. Both water erosion of soil surface and leaching inside soil solum processes were responsible for variation of soil texture, as such soils showed evident of changing in particles size distribution as well as in clay content. Cation exchange capacity (CEC) was less than moderate with domination of Magnesium cation. Soil trace elements were poor to somewhat poor. Soil pH values in general were low; which reflect the pedo-genic character of igneous parent material in which soil drifted from. In some cases, where soil body subjected to continuous leaching of soil bases, in particular calcium cation; soil profiles became totally freed from calcium carbonates. Accordingly soil problems related to downing of soil reaction (pH) are more expected to be increasing by time. This is main reason for some physical diseases, which beginning arise on pomes fruits, particularly bitter pit.INTERNATIONAL JOURNAL OF ENVIRONMENT Volume-6, Issue-1, Dec-Feb 2016/17, page: 87-107

40Ar/39Ar geochronology, elemental and Sr-Nd-Pb isotope geochemistry of the Neogene bimodal volcanism in the Yükselen area, NW Konya (Central Anatolia, Turkey)

Bimodal volcanic suites occur in both orogenic and anorogenic geotectonic settings. Although their formation can be attributed to either fractional crystallization from basaltic parents to felsic derivatives or partial melting of different sources, the origin of bimodal suites is still unclear. By reporting mineral chemistry, 40Ar/39Ar geochronology, elemental and Sr-Nd-Pb isotope geochemistry data, this study aims to investigate the genesis of bimodal basalt-dacite association from the Yükselen area located on the northern end of the Sulutas Volcanic Complex (Konya, Central Anatolia).The Yükselen area volcanic rocks are represented by basaltic lava flows, and dacitic dome with enclaves and pyroclastics. Basaltic flows and pyroclastic rocks are interlayered with the Neogene fluvio-lacustrine sedimentary units, while dacitic rocks cut the pre-Neogene basement in the area. A biotite separation from dacites yielded 40Ar/39Ar plateau age of 16.11 ± 0.18 Ma. On the other hand, a whole rock sample from basalts gave two plateau ages of 16.45 ± 0.76 Ma and 22.37 ± 0.65 Ma for the first steps and next steps, respectively. The investigated basalts are sodic alkaline, and characterized by ocean island basalt (OIB)-like anorogenic geochemical signatures. However, dacites are calc-alkaline and metaluminous, and carry geochemical signatures of orogenic adakites. Sr-Nd-Pb isotopic systematics suggest that the basalts were derived from an asthenospheric mantle source enriched by recycled crustal rocks. The dacites show more enriched Sr and Pb ratios and more depleted Nd ones relative to the basalts, which at the first glance might be attributed to crustal contamination of the associated basalts. However, trace element features of the dacites rule out cogenetic relationship between the two rock types, and point to an origin by melting of lower crust. On the other hand, enclaves share several elemental and isotopic characteristics with the dacites, and appear to be fragments of sub-volcanic intrusions closely related to the dacitic host magma. Based on the obtained geochemical data combined with the published geological and geophysical data, the investigated bimodal volcanic activity can be explained by slab break-off process in the convergence system between the African and Anatolian plates.

The alkaline-carbonatite complex of Jacupiranga (Brazil): Magma genesis and mode of emplacement

A comprehensive study including new field, petrological and geochemical data is reported on the Jacupiranga alkaline-carbonatite complex (133–131Ma) which, together with other alkaline complexes, occurs in southern Brazil and is coeval with the Paraná CFB province. It consists of a shallow intrusion (ca. 65km2) in the Precambrian crystalline basement, and can be subdivided in two main diachronous plutonic bodies: an older dunite-gabbro-syenite in the NW and a younger clinopyroxenite-ijolite (s.l.) in the SE, later injected by a carbonatitic core (ca. 1km2). An integrated petrogenetic model, based on bulk rock major and trace element analyses, mineral chemistry and Sr-Nd-Pb-C isotopic data, suggests that the two silicate intrusions generated from different mantle-derived magmas that evolved at shallow level (2–3km depth) in two zoned cup-shaped plutonic bodies growing incrementally from independent feeding systems. The first intrusion was generated by OIB-like alkaline to mildly alkaline parental basalts that initially led to the formation of a dunitic adcumulate core, discontinuously surrounded by gabbroic cumulates, in turn injected by subanular syenite intrusive and phonolite dykes. Nephelinitic (±melilite) melts – likely generated deep in the lithosphere at ≥3GPa – were the parental magmas of the second intrusion and gave rise to large coarse-grained clinopyroxenite ad- to meso-cumulates, in turn surrounded, and partially cut, by semi-annular fine-layered melteigite-ijolite-urtite ortho-cumulates. The available isotopic data do not evidence genetic links between carbonatites and the associated silicate intrusions, thus favouring an independent source from the mantle. Moreover, it may be suggested that, unlike gabbro-syenites and carbonatites, mostly generated from lithospheric mantle sources, the parental magmas of the ijolite-clinopyroxenite intrusion also record the influence of sublithospheric (plume-related?) geochemical components.

The Smoothness and Shapes of Chondrite-normalized Rare Earth Element Patterns in Basalts

Abstract The customary practice for displaying REE abundances is to normalize them to chondritic abundances and then to plot these normalized abundances in order of atomic number, Z , although the 3 + ionic radius, r REE , is proposed here as a preferable independent variable. In basalts, the resulting CI-normalized REE patterns usually appear smooth (excepting Eu), such that they may be fitted to polynomials in r REE with three to five terms, depending on analytical precision. The polynomials can be rearranged into an orthogonal form: ln([REE]/[REE] CI )=λ 0 +λ 1 f1orth +λ 2 f2orth +… where f1orth , f2orth , etc. are themselves polynomials of r REE , chosen such that the coefficients λ 0 , λ 1 , λ 2 , etc. are not correlated with each other. The terms have a simple, intuitive meaning: λ 0 is the average of the logarithms of the CI-normalized REE abundances; the term in f1orth describes the linear slope of the pattern; that in f2orth describes the quadratic curvature, etc. For most basalts, fits using only three terms (λ 0 , λ 1 , and λ 2 ) capture REE patterns to better than ±5%. The λ n , called the ‘shape coefficients’, can be used to compare the shapes of CI-normalized REE patterns quantitatively, allowing large numbers of data to be assessed, revealing trends not evident from studies of single localities. Especially instructive are λ 2 vs λ 1 diagrams. The usefulness of this approach is demonstrated using the REE patterns of common types of basalts from (mainly) oceanic settings: ocean floor basalts (OFB), ocean island basalts (OIB), and some convergent margin basalts. It is shown that the global population of OFB is characterized by a narrow dispersion of λ 0 at a given MgO content, but with large variations of λ 1 and λ 2 . Convergent margin basalts have much greater variation of λ 0 at a given [MgO], but most plot in the same area of the λ 2 vs λ 1 diagram. OIB are well separated from the OFB global array on this diagram, with Hawaiian shield basalts occupying a unique area. Because REE mineral/melt partition coefficients are also smooth functions of r REE , many mass-balance equations for petrogenetic processes that relate observed concentrations to initial concentrations, [REE] o , such as batch or fractional melting, or crystallization, may be fitted to the same orthogonal polynomials: ln([REE]/[REE] o )=ψ 0 +ψ 1 f1orth +ψ 2 f2orth +… . The orthogonality ensures that all λ n and ψ n terms of the same order n sum independently of the terms of the other orders, such that λ n = λn0 + ψ n , where λn0 is the shape coefficient of the source or parent magma. On λ 2 vs λ 1 diagrams, this approach can be used to relate the shapes of patterns in parental basalts to the shapes of the patterns of their sources, or differentiated basalts to their parental melts, by means of ‘petrogenetic process vectors’ consisting of the ψ 1 and ψ 2 terms, which plot as vectors on the λ 2 vs λ 1 diagrams. For example, the difference between OIB and the global array of OFB can be shown to be due to garnet in the sources of OIB. The global array of OFB requires a remarkably constant degree of partial melting ( F ) of a source with constant λ 0 to produce their parental magmas, or a compensating correlation between F and source λ 0 . Assuming a constant source, with previously suggested depleted mantle compositions, F is ∼19%, with the standard deviation of the population being only 2%. Hawaiian shield tholeiites may be products of 1–2% melting at substantially higher pressures, perhaps straddling the garnet-to-spinel transition, of a source with REE patterns near the median of the REE patterns of OFB sources. Other OIB are the result of lower degrees of melting, usually of more light REE-enriched sources.

Soil types influence predictions of soil carbon stock recovery in tropical secondary forests

Tropical forests are major sinks of terrestrial carbon (C) both above- and below-ground. As a consequence their destruction and degradation is considered the second largest anthropogenic source of carbon dioxide to the atmosphere. Also contributing to the changing dynamics of the global carbon cycle is the widespread and significant expansion of secondary forest. Secondary forests that colonise abandoned agricultural lands can potentially recover above-ground C stocks to historical levels in a few decades. However, the dynamics of below-ground C stored as soil C stocks are unaccounted for in several tropical regions. Similarly, although parent materials are known to differ in chemical and physical properties, little is known about the relationships of soil C stocks with environmental predictors and whether they interact with soil types during natural forest regeneration. We investigated whether soil organic carbon (SOC) stocks change with secondary forest age in two contrasting soil types (derived from either basalt or granite). Soil and vegetation parameters were analysed to determine the best predictors of SOC stock changes in secondary forests. SOC stocks from 24 secondary forests (up to 69years since pasture abandonment) were compared with those from active pastures and mature forests. We found that clay-rich soils (originating from basalt parent material) store higher amounts of SOC, although these stocks remain unchanged as secondary forests matured. In contrast, SOC stocks in granite soils tend to be lower in young secondary forests and increase rapidly to levels comparable to mature forests. Moreover, our analysis indicated that soil pH and woody plant diversity are strong candidates as predictors of SOC stock variations, yet it appears this is within the context of soil type. Our results support the contention that models predicting SOC stocks during forest succession should not rely only on secondary forest age. Instead, predictions of SOC stocks can be improved with the inclusion of basic information on vegetation cover and soil type (especially soil texture).

Geochemistry of metamorphosed pillow basalts of the Chara Zone, NE Kazakhstan

Geochemical study of the metamorphosed pillow lavas of the Chara Zone revealed that their protoliths have N-MORB compositions. Elevated concentrations of K, Rb, Cs, and lower Ca may be related not to blueschist metamorphism, but rather they may be inherited from underwater alteration of parental basalts. Comparative analysis of the compositions of massive glaucophanites and vein rocks demonstrated the relative mobility of Sr, U, P, Ba, Rb, K, Cs, Ca, and LREEs, at least on the local scale. Their mobility has been provided by fluids circulating in the subduction zone, where the fluids have been generated through slab dehydratation processes. These studies of metamorphosed pillow lavas of the Chara Zone displayed only local alterations in the bulk composition of their protoliths, which evidences a low intensity of fluid flows: this allowed preservation of the geochemistry of oceanic basalts.

Fe-rich Sepiolite from the Basalt Fault Gouge in the South of Pohang, Korea

The black fault gouge having 5-10 cm width occurs at the center of the fault fracture zone developed at the early Miocene basaltic rocks that is located at the Geumkwang-ri area in Donghae-myeon, Pohang city. The fault gouge was analyzed with XRD, FTIR, DTA/TGA, SEM, TEM, XRF, EPMA. Analyses reveal that the fault gouge is Fe-rich sepiolite having high iron content. Alteration minerals observed in the fault fracture zone are mainly smectite. The significant amount of smectite also observed in the basaltic parent rocks. The occurrence of constituent minerals indicates that the Fe-rich sepiolite was crystallized by faulting and hydrothermal alteration after the consolidation of basaltic rock at the deep place.

Productivity and Suitability Evaluation of Orange (Citrus reticulata)-Growing Soils of Hot and Semi-Arid Region of Rajasthan (AESR 5.2)

Fourteen pedons were evaluated for their productivity and suitability on the basis of variation in physiography, parent materials, soils and productivity of orange in Jhalawar district of Rajasthan. The pedons were developed by sandstone and shale (P1-P6) and basalt parent materials (P7–14) in Vindhyan and Malwa region, respectively and classified as Vertisols (P3, P5, P7-P13) and Inceptisols (P1, P2, P4, P6 and P14). In general, sand silt and clay contents ranged from 3.4 to 48.1, 27.8 to 71.0 and 18.6 to 60.9% in different horizons, respectively. The soils were calcareous and moderately alkaline to strongly alkaline in reaction and sub-soil sodicity is just arising mostly in soils of Malwa region (P3, P9 and P12). The organic carbon ranged from 0.36 to 9.13 g kg−1 and cation exchange capacity from 22.5 to 62.5 cmol(p+)kg−1 soil. The soils were low to optimum in available N and K and low to excess in available P. The Riquier productivity index appears to be more reliable as compared to Storie index and these ratings are not in reliance with the Sys and proposed suitability criteria and yield of orange realized. On computation of the existing limitations, a new criterion has been proposed (against Sys suitability criteria for citrus species) which indicated that P2, P3, P7, P9, P10, P11 and P12 were moderately suitable, P1, P4-P6, P8, P13 and P14 were suitable for orange production.

High-K andesite petrogenesis and crustal evolution: Evidence from mafic and ultramafic xenoliths, Egmont Volcano (Mt. Taranaki) and comparisons with Ruapehu Volcano, North Island, New Zealand

This study uses the geochemistry and petrology of xenoliths to constrain the evolutionary pathways of host magmas at two adjacent andesitic volcanoes in New Zealand’s North Island. Egmont (Mt. Taranaki) is located on the west coast of the North Island and Ruapehu lies 140km to the east at the southern end of the Taupo Volcanic Zone, the principal locus of subduction-related magmatism in New Zealand. Xenoliths are common in the eruptives of both volcanoes but the xenoliths suites are petrographically and geochemically different. Ruapehu xenoliths are predominantly pyroxene–plagioclase granulites derived from Mesozoic meta-greywacke basement and the underlying oceanic crust. The xenolith population of Egmont Volcano is more complex. It includes sedimentary, metamorphic and plutonic rocks from the underlying basement but is dominated by coarse grained, mafic and ultramafic igneous rocks. Gabbroic xenoliths (Group 1) are composed of plagioclase, clinopyroxene and amphibole whereas ultramafic xenoliths are dominated by amphibole (Group 2) or pyroxene (Group 3) or, in very rare cases, olivine (Group 4). In Group 1 xenoliths plagioclase and clinopyroxene and in some cases amphibole show cumulate textures. Amphibole also occurs as intercumulate poikilitic crystals or as blebs or laminae replacing pyroxene. Some Group 2 xenoliths have cumulate textures but near monomineralic amphibole xenoliths are coarse grained with bladed or comb textures. Pyroxene in Group 3 xenoliths has a polygonal granoblastic texture that is commonly overprinted by veining and amphibole replacement.Group 1 and most Group 2 xenoliths have major, trace element and Sr, Nd and Pb isotope compositions indicating affinity with the host volcanic rocks. Geochemical variation can be modelled by assimilation fractional crystallisation (AFC) and fractional crystallisation (FC) of basaltic parents assuming an assimilant with the composition of average crystalline basement and Group 1 xenoliths have compositions approximating the solid material extracted during these processes. Some Group 2 xenoliths have relatively unevolved Sr and Nd isotopic compositions and they are interpreted to have derived as crystal cumulates from a more primitive parental basalt or through metasomatic alteration of other xenolith types by isotopically less evolved fluids or melts. Some Group 3 xenoliths could have originally been pyroxene cumulates but the granoblastic textures of others are more consistent with an origin as restites generated during anatexis of amphibolite. Group 4 xenoliths have textures similar to those of mantle-derived peridotite xenoliths found in intraplate basalts.The geochemical variation is consistent with a system fed by mantle-derived magmas that underplated and intruded the lower crust. At this level AFC and FC and crustal anatexis generated cumulates and pyroxene restite represented by the mafic and ultramafic xenoliths. The magmas segregating at these deep levels moved higher into the crust where a complex dispersed magma storage and plumbing system formed. Here magmas evolved further through AFC and FC with the formation of cumulates and crystal mushes that are represented by some Group 1 and Group 2 xenoliths. Xenoliths were further modified by interaction with host magmas or by alteration at the side walls of magma storages and conduits.

Petrology and volatile content of magmas erupted from Tolbachik Volcano, Kamchatka, 2012–13

Abstract We report petrography, and bulk rock, mineral and glass analyses of eruptive products of the 2012–13 eruption of Tolbachik volcano, Central Kamchatka Depression, Russia. Magmas are shoshonitic in composition, with phenocrysts of olivine and plagioclase; clinopyroxene phenocrysts are scarce. Samples collected as bombs from the active vent, from liquid lava at the active lava front, and as naturally solidified “toothpaste” lava allow us to quantify changes in porosity and crystallinity that took place during 5.25 km of lava flow and during solidification. Olivine-hosted melt inclusions from rapidly-cooled, mm-size tephra have near-constant H 2 O contents (1.19 ± 0.1 wt%) over a wide range of CO 2 contents ( 2 O and 50 ppm CO 2 . The presence of small saturation, rather than shrinkage, bubbles testifies to volatile saturation at the time of entrapment. Calculated saturation pressures are 0.3 to 1.7 kbar, in agreement with the depths of earthquake swarms during November 2012 (0.6 to 7.5 km below the volcano). Melt inclusions from slowly-cooled and hot-collected lavas have H 2 O contents that are lower by an order of magnitude than tephras, despite comparable CO 2 contents. We ascribe this to diffusive H 2 O loss through olivine host crystals during cooling. The absence of shrinkage bubbles in the inclusions accounts for the lack of reduction in dissolved CO 2 (and S and Cl). Melt inclusions from tephras experienced 2 O-poor and K 2 O-rich basaltic parent.

A trachyte–syenite core within a basaltic nest: filtering of primitive injections by a multi-stage magma plumbing system (Oki-Dōzen, south-west Japan)

Oki-Dōzen (Japan) is a Late Miocene (7–5 Ma) intraplate alkalic volcano composed of a central trachytic pyroclastic complex surrounded by a ring-shaped succession of basaltic to trachybasaltic lavas and pyroclastic rocks and dispersed trachytic bodies. The central trachytic complex is in contact with a syenite that was intruded into the basement early Miocene volcano-sedimentary succession. In the centre of the system there are no alkalic basaltic rocks that are correlative of the outer flank ring. We present whole-rock major and trace element chemistry, Sr–Nd–Pb isotopic compositions and petrological data from the central trachytic volcanic complex and the intrusive syenite body, as well as from the outer ring basaltic succession. We also present and discuss a new set of zircon U/Pb ages collected from the central trachyte and syenite bodies. All the eruptive products of Oki-Dōzen, as well as the syenite, plot on a single liquid line of descent initiated from a mantle-derived alkalic basaltic parent. A younger (2.8 Ma) basaltic eruption (Uzuka basalt) has isotopic compositions that distinguish it from the rest of the system. Geochemical modelling indicates that magmatic differentiation through crystal fractionation and minor crustal assimilation occurred in crustal and shallow sub-volcanic magma reservoirs. In the central part of the system, a number of vertically spaced reservoirs acted as a filter, capturing basaltic dykes and hindering their ascent. In the outer region, dykes either reached the surface unhindered and erupted to form the basaltic/trachybasaltic succession or stalled at crustal levels and differentiated to trachyte before forming dispersed domes/flows. The central plumbing system “filter” resulted in a nest-shaped volcano, with a trachytic core surrounded by basaltic products, and stopped direct injection of basaltic magmas into the shallow syenitic magma reservoir, likely preventing its destabilization and explosive eruption.

Geochemistry of mafic lavas from Sivas, Turkey and the evolution of Anatolian lithosphere

Near-contemporaneous suites of mafic lavas from Sivas, Central Anatolia record different petrogenetic histories on the eastern and western sides of a major regional suture marked by the Kızılırmak River. The Sivas basaltic suite has major and trace element compositions suggesting derivation from an anhydrous peridotitic mantle source region. Basaltic trachyandesites in this group are related by up to ~65% fractional crystallization of the observed anhydrous mineral phases from a frequently-erupted basaltic parent with ~9wt.% MgO. Chondrite-normalized Tb/Yb values indicate that the basaltic magmas likely originate from melting peridotite at pressures near the spinel-garnet transition. Clinopyroxene with a range of Alvi/Aliv values suggest multiple stages of mineral growth, consistent with a mantle origin followed by fractional crystallization at shallow crustal levels. In contrast, Sivas basanites have higher abundances of key incompatible elements that suggest a source area in the subcontinental lithosphere with hydrous metasomatic mineral phases. Clinopyroxene in Sivas basanites have Alvi/Aliv values that cluster around 0.25, suggesting the magmas stalled at mid-crustal depths prior to rapid ascent and eruption. High abundances of incompatible major and trace elements in the most magnesian basanites show that the basanite lavas are not related to one another or to the basalt suite by fractionation. The thermal environment that led to formation of parental magmas for both the basalt suite and the basanites likely reflects ascent of asthenospheric material around and through torn slabs beneath Central Anatolia, providing heat to melt both ascending mantle and regionally metasomatized subcontinental lithosphere. This process is enhanced by recent trans-tensional deformation in the Anatolian plate that allows mafic melts to rise to shallow depths on both sides of the Kızılırmak River.

Exploring the Moon's surface for remnants of the lunar mantle 1. Dunite xenoliths in mare basalts. A crustal or mantle origin?

AbstractRemotely sensed observations from recent missions (e.g., GRAIL, Kaguya, Chandrayaan‐1) have been interpreted as indicating that the deep crust and upper mantle are close to or at the lunar surface in many large impact basins (e.g., Crisium, Apollo, Moscoviense). If this is correct, the capability of either impact or volcanic processes to transport mantle lithologies to the lunar surface should be enhanced in these regions. Somewhat problematic to these observations and interpretations is that examples of mantle lithologies in the lunar sample collection (Apollo Program, Luna Program, lunar meteorites) are at best ambiguous. Dunite xenoliths in high‐Ti mare basalt 74275 are one of these ambiguous examples. In this high‐Ti mare basalt, olivine occurs in three generations: olivine associated with dunite xenoliths, olivine megacrysts, and olivine microphenocrysts. The dunite xenoliths are anhedral in shape and are generally greater than 800 μm in diameter. The interior of the xenoliths are fairly homogeneous with regard to many divalent cations. For example, the Mg# (Mg/Mg + Fe × 100) ranges from 82 to 83 in their interiors and decreases from 82 to 68 over the 10–30 μm wide outer rim. Titanium and phosphorus X‐ray maps of the xenolith illustrate that these slow diffusing elements preserve primary cumulate zoning textures. These textures indicate that the xenoliths consist of many individual olivine grains approximately 150–200 μm in diameter with low Ti, Al, and P cores. These highly incompatible elements are enriched in the outer Fe‐rich rims of the xenoliths and slightly enriched in the rims of the individual olivine grains. Highly compatible elements in olivine such as Ni exhibit a decrease in the rim surrounding the xenolith, an increase in the incompatible element depleted cores of the individual olivine grains, and a slight decrease in the “interior rims” of the individual olivine grains. Inferred melt composition, liquid lines of descent, and zoning profiles enable the reconstruction of the petrogenesis of the dunite xenoliths. Preservation of primary magmatic zoning (Ti, P, Al) and lack of textures similar to high‐pressure mineral assemblages exhibited by the Mg‐suite (Shearer et al. 2015) indicate that these xenoliths do not represent deep crustal or shallow mantle lithologies. Further, they are chemically and mineralogically distinct from Mg‐suite dunites identified from the Apollo 17 site. More likely, they represent olivine cumulates that crystallized from a low‐Ti mare basalt at intermediate to shallow crustal levels. The parent basalt to the dunite xenolith lithology was more primitive than low‐Ti basalts thus far returned from the Moon. Furthermore, this parental magma and its more evolved daughter magmas are not represented in the basalt sample suite returned from the Taurus‐Littrow Valley by the Apollo 17 mission. The dunite xenolith records several episodes of crystallization and re‐equilibration. During the last episode of re‐equilibration, the dunite cumulate was sampled by the 74275 high‐Ti basalt and transported over a period of 30–70 days to the lunar surface.

The ‘orogenic andesite’ puzzle of C. E. Tilley: 1—Is the volatile phase the missing piece?

Some 65 years ago, C. E. Tilley, in his 1950 Presidential Address to the Geological Society of London, brought into sharp focus the then perhaps-not-fully-recognised problem of the origin of what he termed the ‘orogenic andesites’—and, in particular, their commonly constituent calc-alkaline lineage of lavas and pyroclastic materials. In spite of much investigation and progress since, the problem remains not fully resolved. The two most conspicuous features of such orogenic—i.e. island arc—volcanism are its copious, explosive, degassing and, as Tilley noted, its copious development of andesites and their more felsic associates. If these rock types were derived from basalt, the principal relevant changes in major element composition are reductions in Fe, Ca and Mg, and increases in Si, Al and (Na + K). Four principal mechanisms for inducing these changes have been proposed: (1) magmatic differentiation induced by fractional crystallisation, (2) magma mixing, (3) partial melting of an ultramafic-derived basaltic parent, and (4) crustal contamination. A fifth postulate—long recognised but only occasionally seriously considered—is that loss in the volatile phase during eruption and later degassing may make a significant contribution to the overall process. On the basis that Tilley's (and others’) orogenic basalt–andesite–dacite–rhyolite series constitutes a linear arrangement on the conventional FeOt–MgO–(Na2O + K2O) diagram it is shown on numerical grounds that fractional crystallisation alone cannot account for the observed compositional trend—that, while accumulates are neither uncommon nor insubstantial in amount in modern arcs, they are olivine-rich, not magnetite-rich, and that numerical, petrological and field evidence indicates that the high oxidation/magnetite subtraction hypothesis of orogenic andesite genesis can no longer be seriously entertained. The partial melting hypothesis encounters contrary numerical evidence, closely analogous with that adduced for fractional crystallisation. Magma mixing eventually encounters the difficulty of the nature of the felsic material required for rhyolite formation, and assimilation of the preservation of primitive isotope ratios as lava evolution progresses. A fifth mechanism—volatile loss—may make a significant contribution to the relevant compositional change, perhaps in combination with one or more of the other proposed processes. Geochemical, petrological and field evidence pertaining to this is reviewed and examined, and it is postulated that indeed a combination of fractional crystallisation as the principal mechanism, volatile loss as a contributing factor, provides a plausible process. A case is made for an exploratory experimental investigation of the possibility of significant loss in a gas/vapour phase.

Geochemical and Mineralogical Studies in Recent Clastic Sediments from Upper Godavari River in Peninsular India

Abstract Detailed geochemical and clay mineralogical investigations were carried out on bed sediments from upper Godavari river of peninsular India. The results suggest that these sediments are mainly derived from weathering of basaltic rocks of Deccan volcanic province. Strong relationship between Fe2O3 and TiO2 is attributed to their co-occurrence in weathering resistant minerals and/or scavenging of Ti by Fe oxy-hydroxides during weathering of basalts. The concentrations of Na2O, K2O and MgO in these upper Godavari river sediments are relatively less compared to parent basalt. The abundance of minor elements like V, Cr, Cu and Zn shows significant scatter within the range of Deccan basalts. Vanadium and nickel shows significant correlation with Fe2O3 due to their association with iron minerals or their sequestration with iron oxy-hydroxides. Higher Zn concentrations at some locations of Godavari river may have been due to anthropogenic contamination. The REE pattern is greatly influenced by the degree of source rock weathering. Light rare earth elements (LREE) show greater fractionation compared to the heavy rare earth elements (HREE) during weathering and transportation. Strong positive Ce-anomaly in some sediment samples suggests oxidizing conditions of deposition. Chemical Index of Alteration (CIA) varies significantly from 37.01 to 60.16, indicating large spatial variability in the intensity of chemical weathering of Deccan basalts. Low CIA values are observed in samples that are enriched in CaO probably due to semi-arid climate which facilitates CaCO3 precipitation and restricts chemical weathering.

Ubiquitous occurrence of basaltic-derived paleosols in the Late Archean Fortescue Group, Western Australia

The 2.76Ga old Mount Roe Basalt paleosols (MR#1 and MR#2), recognized near the base of the 2.76–2.69Ga Fortescue Group in Western Australia, represent some of the oldest definite examples of Archean paleoweathering profiles. The loss of Fe and the absence of pedogenic carbonates in these reference paleosols have been considered as strong evidence for low oxygen and moderate carbon dioxide in the Late Archean atmosphere, respectively. However, the robustness of such interpretations suffers both from the scarcity of paleosol exposures and the superposition of post-weathering alteration over primary soil profiles. Here we report new exposures of the MR#1 paleosol as well as a number of new basalt-derived paleosol occurrences distributed in the Mount Roe Basalt and the 2.73Ga old Kylena Formations of the Fortescue Group. We show that all these paleosols, including MR#1 and MR#2, were strongly affected by post-weathering reductive alteration. Nevertheless, we discovered early lithologic units, locally preserved within the hydrothermally altered paleosols, which feature distinct chemical and mineralogic compositions. These include: (i) 13C-depleted carbonaceous-rich, diaspore–pyrophyllite boudins likely inherited from the hydrolysis and bauxitization of the parent basalt and (ii) green hard core material, mostly composed of Fe–sericite associated with authigenic sphene and containing early relics of sulfate-bearing iron-rich smectite or berthierine. We argue that smectite relics may constitute a part of the primary pedogenic mineral assemblage, while berthierine and sphene formed during diagenesis through the circulation of reductive fluids. Because the depletion of iron observed in the Fortescue paleosols is not solely due to pedogenesis, but also to post-weathering alteration, particular care has to be taken when bulk chemical profiles of iron in such paleosols are used as atmospheric paleobarometer. Recognition of the regional-scale, syn-depositional alteration of the Fortescue subaerial basalts over the north Pilbara suggests the onset of intense continental weathering associated with the uplift and emergence of the Pilbara craton during continental break-up and rifting. Such a regional onset of continental weathering may have provided large amounts of nutrients to nearby marine and/or lacustrine systems, favoring the development of microbial life in shallow waters.