Altered Basalts Research Articles

Controls on dolomitization in extensional basins: An example from the Derbyshire Platform, U.K.

ABSTRACTFault-controlled dolomitization has been documented in Lower Carboniferous (Viséan) platform carbonates at various localities in the Pennine Basin and North Wales. The largest of these dolomite bodies (approx. 60 km2) occurs on the Derbyshire Platform, on the southern margin of the Pennine Basin. This study tests the hypothesis that dolomitization occurred at this locality during deposition, platform drowning, and the earliest stages of burial, coincident with the transition from a late syn-rift to post-rift regime. It also assesses the importance of syn-rift volcanism on dolomitization. Planar, fabric-retentive dolomite with single-phase (i.e., low temperature) fluid inclusions occurs along NW–SE and E–W oriented faults, and in platform margin facies and in proximity to the Masson Hill Volcanic Complex. Oxygen isotope data are consistent with dolomitization from seawater, but slightly depleted δ13C values reflect mixing with magmatic fluids. Volcanic activity is likely to have produced a thermal drive for fluid circulation on the platform margin, and post-depositional alteration of basalts by CO2-rich fluids could have led to alteration of olivine and release of magnesium to convecting seawater. Consequently, the large volume of dolostone on the southern margin of the Derbyshire Platform is attributed to the increased geothermal gradient and a localized increase in the Mg/Ca ratio of dolomitizing fluids at this locality, compared to elsewhere in the Pennine Basin. The results suggest that syn-rift carbonate platforms in volcanically active areas of rift basins have a greater potential for dolomitization from seawater than non-volcanic platforms in the same basin.

Evidence of Localized Failure Along Altered Basaltic Blocks in Tectonic Mélange at the Updip Limit of the Seismogenic Zone: Implications for the Shallow Slow Earthquake Source

AbstractField studies have led to several interpretations on the mechanics behind slow earthquake phenomena downdip of the seismogenic zone. To date, field studies have not examined the shallow subduction interface which may also host slow earthquake phenomena. We examine a subduction mélange exhumed from conditions representing the source of shallow slow earthquake phenomena. The mélange consists of a shale matrix containing rigid blocks, including basalt which is altered along the margins. Cataclasite‐bearing faults attest to localized faulting along the altered margins of basaltic blocks, concurrent with distributed shear in the shale matrix. These cataclasite‐bearing faults link individual blocks. Microstructures show mutually crosscutting tensile and shear veins, consistent with failure having occurred at, or near, lithostatic pore fluid pressures. We model the stress concentrations around the altered margins of basaltic blocks during distributed shear and show that frictional failure of the altered basalt is predicted to occur at lower imposed strain rates than frictional failure of the shale, favoring fault development along block margins. Calculations of critical nucleation lengths for the blocks show they would fail dynamically at hydrostatic pore fluid pressures, producing microearthquakes. At near‐lithostatic pore fluid pressures, block lengths are below the critical nucleation length for dynamic failure and may produce slow earthquake phenomena. Mixing of velocity‐weakening blocks into a viscously flowing, velocity‐strengthening matrix may serve as a common mechanism for slow earthquake phenomena updip and downdip of the seismogenic zone.

Iron isotopic and chemical tracing of basalt alteration and hematite spherule formation in Hawaii: A prospective study for Mars

Hematite spherules from Mauna Kea volcano (Hawaii) resemble Martian hematite spherules at Meridiani Planum in morphology, mineralogy, and geochemistry, and they both formed in association with acid-sulfate alteration of basalt. To gain insights into the processes involved in the formation of these spherules, we have measured the Fe isotopic compositions of unaltered basalts and tephras, as well as a variety of weathering/alteration products including altered basalts, hematite-rich breccia and individual hematite spherules, jarosite-bearing tephras, palagonitic tephras, calcined tephras (high-T dry oxidation) and steam-vent tephras.Compared to other alteration processes that induce little Fe mobility, acid-sulfate alteration produces large Fe isotopic variations (δ56Fe values ranging from −0.15 to +0.94‰). We used rock slabs from a variably altered basaltic rock to constrain the Fe isotope fractionation factor during acid-sulfate alteration of basaltic rocks. We found that the process followed a Rayleigh distillation with an instantaneous fractionation factor of −0.24‰ between the fluid and the residue for δ56Fe, resulting in a heavy Fe isotope enrichment in the alteration residue. However, the constant Mn/Fe ratios of the slabs suggest that acid-sulfate leaching does not significantly fractionate Mn/Fe ratios in basalts, which is consistent with the mobilization of iron as Fe(II).We also measured the Fe isotopic compositions and Mn/Fe ratios (using NanoSIMS) of individual hematite spherules from a spherule-rich breccia HWMK745R. The spherules show heavy Fe isotopic enrichments and low Mn/Fe ratios compared to unaltered samples, which is similar to the fractionations encountered in iron formations. A Rayleigh fractionation model suggests that both Fe isotopic compositions and Mn/Fe ratios of the spherules are consistent with ∼80% Fe oxidation and precipitation from an acid-sulfate fluid. Therefore, direct Fe precipitation from a fluid can readily explain the hematite spherule formation, and a more complex scenario involving jarosite precipitation followed by conversion to hematite is not needed. Combined thermodynamic and kinetic constraints also suggest that precipitation of jarosite would be hindered. The study presents a roadmap for studying future returned Martian samples in the laboratory.

Burial dolomitization, the genesis of dolomite in the Dapu Formation (Upper Carboniferous), Guixinan area, Youjiang basin, Southwest China: petrologic and geochemical evidence

The genesis of dolomite is an intensively debated and extensively studied problem in the field of carbonate sedimentology. This study focuses on the dolomite of the Dapu Formation in the Guixinan area, with the aim of deciphering the mechanism of dolomitization in the Dapu Formation. Three types of sedimentary environment, including the open marine of inner ramp, the restricted marine of inner ramp, and the shoal of inner ramp, have been identified. Three types of dolostone (DI, DII and DIII) identified in the study area with massive structure had close relationship with fault structures, and contacted with limestone in curved diffuse way on the outcrop. Both of the above points indicated diagenetic dolomite. Three types of dolomite (D1, D2 and D3) displayed similar dark—light purple red luminescence, indicating that the characteristics of their diagenetic fluids were similar. The planar-s to nonplanar dolomite crystals indicated that the temperature was more than 50 °C. The mineral assemblage including D2, D3, and silicate minerals (feldspar, tremolite and actinolite) suggested contact metamorphism and tht the diagenetic fluid was rich in silicon and magnesium. The dolomitization process was not influenced by the leaching of meteoric water in surface or shallow buried environment and hydrothermal fluids, based on the corrected REE characteristics. The range of the δ18O of dolostone samples showed that the burial dolomitization had taken place in high-temperature environment. The lower 87Sr/86Sr ratio of the dolostone samples than the coeval seawater meant that the diagenetic fluid was derived from the alteration of mid-ocean ridge basalt. Based on the geologic setting, it was speculated that the burial dolomitization was driven by the hydro-tectonic processes in the passive margin burial diagenesis hydrologic realm. In brief, magnesium came from the low-temperature alteration of ocean basalts, the thermohaline and compactional flow supplied hydrodynamic condition, and the synsedimentary fault zones-supplied channels.

Low-temperature oceanic crust alteration and the isotopic budgets of potassium and magnesium in seawater

Low-temperature (<100°C) alteration of oceanic crust plays an important role in determining the chemical composition of the oceans. Although a major sink of seawater potassium, little is known about the effects of low-temperature basalt alteration on the potassium isotopic composition of seawater (δ41K∼0‰), which is ∼0.50‰ enriched relative to bulk silicate Earth (BSE, δ41K= -0.54‰). Here, we present a suite of isotopic systems (δ41K, δ26Mg, δ7Li, 87Sr/86Sr) and major/minor elements in bulk rock, veins and mineral separates from the upper volcanic section of Cretaceous (Troodos ophiolite) and Jurassic (Ocean Drilling Program Hole 801C) oceanic crust. We use these data to estimate the K isotopic fractionation associated with low-temperature oceanic crust alteration and provide new constraints on the role of this process in the global geochemical cycles of Mg and K in seawater. We find that hydrothermally altered basalts from the Troodos ophiolite and ODP Hole 801C, most of which are enriched in K relative to the unaltered glass compositions, have δ41K values both higher and lower than BSE, ranging from +0.01‰ to -1.07‰ (n=83) and +0.04‰ to -0.88‰ (n=17), respectively. Average δ41K values of bulk-rock samples from Troodos and Hole 801C are indistinguishable from each other at ∼-0.50‰, indicating that low-temperature basalt alteration is a sink of 39K from seawater, and explaining, in part, why seawater has a higher 41K/39K than BSE. In contrast to K, average δ26Mg values for both Troodos (∼0.00‰) and Hole 801C (∼0.20‰) indicate that altered oceanic crust (AOC) is a sink of 26Mg from seawater, likely contributing to the light δ26Mg composition of seawater (∼-0.8‰) relative to BSE (∼-0.2‰). We observe isotopically heavy δ26Mg values in basalt samples characterized by small to no changes in bulk Mg content, consistent with extensive isotopic exchange of Mg between seawater and oceanic crust during low-temperature oceanic crust alteration. Finally, we find that variability in δ7Li and δ41K across three sites in the Troodos ophiolite can be explained by different styles of alteration that appear to be related to the timing of sedimentation and its effects on chemical and isotopic exchange between seawater and oceanic crust.

Frictional Strengths of Subduction Thrust Rocks in the Region of Shallow Slow Earthquakes

AbstractEarthquakes are expected to nucleate within velocity‐weakening materials; however, at the updip limit of the subduction seismogenic zone, the principal lithologies exhibit velocity‐strengthening behavior. At an exhumed analogue for present‐day subduction at Nankai (the Mugi Mélange, Japan) two examples of paleoseismic features occur within altered basalts, suggesting that they may be velocity‐weakening. We shear altered basalt and shale matrix from the mélange in the triaxial saw cut configuration at in situ conditions of deformation (Pc = 120 MPa, T = 150 °C) for two pore fluid factors (λ = 0.36, 0.7). The shale matrix exhibits a coefficient of friction between 0.4 and 0.5, and velocity‐strengthening behavior. Altered basalt exhibits Byerlee friction and velocity‐weakening behavior. In most experiments deformation was partitioned into Riedel shears, which have a lower percentage of clasts, clast size distributions with higher fractal dimensions, and shape factors indicating rounder clasts compared to the matrix between Riedel shears. We hypothesize that earthquakes preferentially nucleate within altered basalt at the updip limit of the seismogenic zone. More complex forms of deformation (e.g., shallow very low frequency earthquakes) may occur by mixing velocity‐weakening altered basalt into the velocity‐strengthening shale matrix. In subduction zones where the matrix is composed of shale, this complex behavior is limited to shallow depths (T &lt; ~250 °C), above the updip transition to velocity‐weakening behavior for the matrix. Altered basalt is a ubiquitous subducting material, and future studies on its behavior through a range of subduction zone conditions are required for a full understanding of the mechanical behavior of subduction zones.

Evolution of the Global Carbon Cycle and Climate Regulation on Earth

AbstractThe existence of stabilizing feedbacks within Earth's climate system is generally thought to be necessary for the persistence of liquid water and life. Over the course of Earth's history, Earth's atmospheric composition appears to have adjusted to the gradual increase in solar luminosity, resulting in persistently habitable surface temperatures. With limited exceptions, the Earth system has been observed to recover rapidly from pulsed climatic perturbations. Carbon dioxide (CO2) regulation via negative feedbacks within the coupled global carbon‐silica cycles are classically viewed as the main processes giving rise to climate stability on Earth. Here we review the long‐term global carbon cycle budget, and how the processes modulating Earth's climate system have evolved over time. Specifically, we focus on the relative roles that shifts in carbon sources and sinks have played in driving long‐term changes in atmospheric pCO2. We make the case that marine processes are an important component of the canonical silicate weathering feedback, and have played a much more important role in pCO2 regulation than traditionally imagined. Notably, geochemical evidence indicate that the weathering of marine sediments and off‐axis basalt alteration act as major carbon sinks. However, this sink was potentially dampened during Earth's early history when oceans had higher levels of dissolved silicon (Si), iron (Fe), and magnesium (Mg), and instead likely fostered more extensive carbon recycling within the ocean‐atmosphere system via reverse weathering—that in turn acted to elevate ocean‐atmosphere CO2 levels.

Implications of basement rock alteration in the Nankai Trough, Japan for subduction megathrust slip behavior

The Nankai Trough is an exceptionally well-studied convergent margin known to host damaging megathrust earthquakes as well as various forms of slow fault slip. Outcrop studies of exhumed analogues for the modern subduction thrust, as well as seismic reflection images of the active subduction zone, suggest that the basaltic basement participates in plate-boundary shearing at depths of a few km and greater. We obtained altered basaltic upper basement from the modern Nankai Trough seaward of the trench, recovered by drilling on IODP Expedition 333. We performed laboratory friction experiments on bare surfaces and gouge powders of this material, in addition to and in combination with other materials for comparison. The altered basalt exhibits predominantly velocity-strengthening frictional behavior, indicating a tendency for stable slip. For bare surface experiments, few instances of velocity-weakening friction occur and are restricted to slip rates of <10−6 m/s. Thin sections and XRD analyses of the starting material indicate that the velocity-strengthening behavior is likely associated with the presence of clay minerals, mostly Mg-smectite, which coat the rims of larger, stronger grains. Based our friction data, we suggest that in addition to creep the altered Nankai basalts may also allow the possibility of slow slip events for two reasons: (1) velocity-weakening friction may be expected at low slip rates, but velocity-strengthening at higher rates will damp any potential slip instabilities; and (2) based on a critical stiffness criterion for accelerating (stable) slip, SSEs are capable of nucleating in the altered Nankai basalt despite velocity-strengthening friction. In light of these observations, and given the depth at which altered basement appears to be incorporated along the plate interface, we raise the possibility that some shallow slow slip events in the Nankai accretionary prism could originate from shearing in altered basalt.

Basalt weathering as the key to understand the past human use of hematite-based pigments in southernmost Patagonia

The present work aims at understanding the composition and source of the red rock art pigments used by hunter-gatherer groups during the late Holocene in southern Patagonia (southernmost South America), by combining micro X-ray fluorescence (μXRF) and Raman spectroscopy. In the first place, μXRF geochemical data do not reveal any significant compositional contrast between anthropogenic pigments and altered basalts but, instead, only different degrees of basalt alteration. Thus, past human populations inhabiting the Pali Aike volcanic field might have locally found the raw material to obtain red pigments from highly weathered basalt outcrops and/or at less than 20 cm depth, where basalt-derived saprolite is widespread. On the other hand, Raman spectroscopy allows distinguishing different hematite populations based on the width and position of the 226 cm−1 band, being the former a proxy of hematite crystallinity. Natural hematite, derived from basalt alteration, shows a relatively low crystallinity and a wide spectrum of Raman parameters, whereas anthropogenic hematite from red rock art pigments shows higher crystallinity and more clustered Raman parameters, indicating possible heating processes of the latter. In addition, the low crystallinity recorded in hematite from an archaeological grindstone suggests that it was used to grind the raw material, a step prior to heating.

A re-assessment of the nitrogen geochemical behavior in upper oceanic crust from Hole 504B: Implications for subduction budget in Central America

The geochemical behavior of N during seawater-oceanic crust alteration remains poorly constrained. Yet, it is a central parameter to assess the flux of N to subduction zones. Most studies proposed that hydrothermally altered basaltic rocks are enriched in N relative to fresh basalts. However, published data from DSDP/ODP Hole 504B, a reference site for the composition of the oceanic crust, suggest that seawater alteration leads to the N depletion of the upper ocean crust. To better address this issue, we analyzed N concentration and isotope composition of 21 altered basalts from the lavas and sheeted dikes sampled by Hole 504B. These new analyses show significant N enrichment (up to 14.1 ppm) relative to fresh degassed MORB (∼1 ppm). The differences observed between earlier and modern data are interpreted as resulting from analytical artifact due to the earlier use of a molybdenum crucible for N extraction. Furthermore, our new data show a progressive decrease of N concentration with depth, from 14.1 to 1.4 ppm. Nitrogen isotope compositions display a large range, with δ15N values from −0.9 to +7.3‰, and most likely reflect multiple stages of alteration with fluids of various compositions. In contrast to N concentration, δ15N values do not show a global depth trend but oscillate around a mean value of 3.0 ± 2.2‰ (1SD). The N concentration shows a positive correlation with bulk rock δ18O values, suggesting that N behavior during alteration process is mainly controlled by temperature. We propose that N speciation in the hydrothermal fluid is dominated by NH3/NH4 at low temperature (<200°C) but is transformed to N2, associated with H2, at higher temperature (>200°C). These new data are used to re-evaluate the global flux of N input into Central American subduction zone, showing that the upper basaltic crust represent about 20% of the total N buried in subduction zone. A comparison with previous results obtained on N degassed in volcanic arc illustrates that, in “warm” subduction zone like Central America, up to 50% of the subducted N may be transferred to the deep mantle. This contrasts with “cold” subduction environments, where >80% of the N inputs escape sub-arc slab devolatilization and supports that the geothermal gradient plays a major role in determining the N fate in subduction zones.

Authigenic Mineral Texture in Submarine 1979 Basalt Drill Core, Surtsey Volcano, Iceland

AbstractMicrometer‐scale maps of authigenic microstructures in submarine basaltic tuff from a 1979 Surtsey volcano, Iceland, drill core acquired 15 years after eruptions terminated describe the initial alteration of oceanic basalt in a low‐temperature hydrothermal system. An integrative investigative approach uses synchrotron source X‐ray microdiffraction, microfluoresence, micro‐computed tomography, and scanning transmission electron microscopy coupled with Raman spectroscopy to create finely resolved spatial frameworks that record a continuum of alteration in glass and olivine. Microanalytical maps of vesicular and fractured lapilli in specimens from 157.1‐, 137.9‐, and 102.6‐m depths and borehole temperatures of 83, 93.9, and 141.3 °C measured in 1980, respectively, describe the production of nanocrystalline clay mineral, zeolites, and Al‐tobermorite in diverse microenvironments. Irregular alteration fronts at 157.1‐m depth resemble microchannels associated with biological activity in older basalts. By contrast, linear microstructures with little resemblance to previously described alteration features have nanocrystalline clay mineral (nontronite) and zeolite (amicite) texture. The crystallographic preferred orientation rotates around an axis parallel to the linear feature. Raman spectra indicating degraded and poorly ordered carbonaceous matter of possible biological origin are associated with nanocrystalline clay mineral in a crystallographically oriented linear microstructure in altered olivine at 102.6 m and with subcircular nanoscale cavities in altered glass at 137.9‐m depth. Although evidence for biotic processes is inconclusive, the integrated analyses describe the complex organization of previously unrecognized mineral texture in very young basalt. They provide a foundational mineralogical reference for longitudinal, time‐lapse characterizations of palagonitized basalt in oceanic environments.

Reactive transport modeling of mineral carbonation in unaltered and altered basalts during CO2 sequestration

Basalt formation is a new promising CO2 sink for providing secure long-term carbon storage via trapped CO2 in mineral phases. When water enters into the crystal structures of the divalent cation-bearing unaltered basalt, the rock is altered and serpentinized. The serpentinization of the unaltered basalt occurs widely around the world. To evaluate the mineral carbonation efficiency of unaltered and altered basalt over a longer time frame, a predictive modeling frame is established in TOUGHREACT based on data from core-scale static batch experiments and the field-scale Carbfix project. Results indicate that unaltered basalt has higher carbonation efficiency than altered basalt both in the carbonation rate and extent due to serpentine kinetic limitations for aqueous phase CO2 injection, and the carbonates mineralization extent and rate increase with the content of olivine minerals in basalt. Furthermore, under these conditions, the longer CO2 migration distance from the injection site enhances the chance for mineral carbonation in the serpentinized basalt. In addition, rapid aqueous phase CO2 injection may limit mineral carbonation and cause porosity reduction near wellbores in the basaltic reservoir, and this effect is more pronounced in case of unaltered basalt than in the altered basalt. Compared to the injection rate, the impact of reservoir conductivity is found to be significant on mineral carbonation in both the unaltered basalt and altered basalt; higher conductivity is more advantageous for CO2 mineralization under certain circumstances. In combination, the injectivity rate should be carefully assessed at sites based on the reservoir conductivity and whether the basalt is altered or unaltered and the extent of alteration.

Hydrothermal alteration of the Ediacaran Volyn-Brest volcanics on the western margin of the East European Craton

The Ediacaran continental flood basalts and associated tuffs were studied to identify and quantify alteration processes by means of XRD and chemistry, supplemented by Mössbauer and FTIR spectroscopies, petrography, oxygen and iron isotopes, K-Ar dating, and organic geochemistry. Two superimposed alteration processes were identified: the Ediacaran hydrothermal alteration, induced by meteoric waters, heated and put in motion by the cooling basalt, and the Caledonian and/or Variscan potassic alteration. The degree of basalt alteration was quantified using as an index the sum of primary minerals in the bulk rock. The sequence of minerals dissolved and crystallized during the hydrothermal alteration was established. The alteration resulted in the loss of Ca (dissolution of plagioclases), compensated by the gain of water and Mg (crystallization of clays), and proceeded from the edges of the basalt flows in an oxidizing environment, evidenced by the increasing amount of hematite and Fe3+/Fe2+ ratio of the bulk rock. Cyanobacteria were active in the hydrothermal system, most probably responsible for the measured negative δ56Fe values and more reducing conditions at the stage of intense alteration. Chlorophaeite (palagonite), following quartz as the earliest petrographically identifiable basalt alteration product was found to vary systematically from fully isotropic to birefringent. The chlorophaeite was identified as a mixture of Fe-montmorillonite and Fe-saponite, identical with griffithite and oxysmectites, probably preceded by a finer-grained ferrosaponite at the isotropic stage. REE content of chlorophaeite indicates basaltic volcanic glass (sideromelane) as the major source of material. REE in clays are contained mostly in the dioctahedral smectite, while in the bulk rock mostly in phosphates. The smectite characteristics and Mg enrichment are indicative of the hydrothermal basalt alteration process, which perhaps was dominant also on Mars.Both mineral and chemical composition of tuffs vary continuously from basaltic to felsic, the latter close to the measured rhyodacite composition, dominated by quartz and feldspars. The basaltic tuffs resemble the most altered basalts but contain also abundant albite and chlorite, indicative of higher alteration temperatures, up to 220 °C. Tuff composition indicates stronger component of felsic volcanism in the trap formation than evidenced by the preserved bodies of effusive rocks.

Mineralogy of Cobalt-Rich Ferromanganese Crusts from the Perth Abyssal Plain (E Indian Ocean)

Mineralogy of phosphatized and zeolitized hydrogenous cobalt-rich ferromanganese crusts from Dirck Hartog Ridge (DHR), the Perth Abyssal Plain (PAP), formed on an altered basaltic substrate, is described. Detail studies of crusts were conducted using optical transmitted light microscopy, X-ray Powder Diffraction (XRD) and Energy Dispersive X-ray Fluorescence (EDXRF), Differential Thermal Analysis (DTA) and Electron Probe Microanalysis (EPMA). The major Fe-Mn mineral phases that form DHR crusts are low-crystalline vernadite, asbolane and a feroxyhyte-ferrihydrite mixture. Accessory minerals are Ca-hydroxyapatite, zeolites (Na-phillipsite, chabazite, heulandite-clinoptilolite), glauconite and several clay minerals (Fe-smectite, nontronite, celadonite) are identified in the basalt-crust border zone. The highest Ni, Cu and Co contents are observed in asbolane and Mn-(Fe) vernadite. There is significant enrichment of Ti in feroxyhyte−ferrihydrite and vernadite. The highest rare earth element (REE) content is measured in the phosphate minerals, less in phyllosilicates and Na-phillipsite. The geochemical composition of minerals in the DHR crusts supports the formation of crusts by initial alteration, phosphatization and zeolitization of the substrate basalts followed by oscillatory Fe-Mn oxyhydroxides precipitation of hydrogenous vernadite (oxic conditions) and diagenous asbolane (suboxic conditions).

On the potential of microbeam analyses in study of the ceramics, slip and paint of Late Bronze Age White Slip II ware: An example from the Canaanite site Tel Esur

Microbeam Analyses using EPMA, pXRF, LA-ICP-MS and FT-IR were conducted in this study of Late Bronze Age (LBA) White Slip II ware (WS-ware) imports at the Canaanite site Tel Esur (Tel Esur WS). The WS-ware is typically decorated with black-brown geometric patterns painted over a white slip layer. The study of the WS-ware provides useful information and a multi-analytical database regarding the composition, ceramic technology, raw materials, origin and cultural issues. The results demonstrate that the LBA potters select raw materials suitable for production of ceramics, slip and paint. The ceramic-body of the WS-ware was made of raw material that has been an appropriate selection to produce a hard and thin-walled vessel. For accentuating the black decoration over the dark reddish-grey ceramic-body, the latter was covered with white slip layer. The black decoration was made of ferromanganese-based pigment, which allows black decoration through firing of the vessels at an oxidizing atmosphere. The raw materials for the production of the ceramics, slip and paint were selected from Cypriot red basaltic clay of weathered basalt province, white hydrothermal clay of altered basalt zone and umber ore, respectively. The Tel Esur White Slip II ware is proved analytically to be imported from Cyprus.

Fluid Chemistry of Mid-Ocean Ridge Hydrothermal Vents: A Comparison between Numerical Modeling and Vent Geochemical Data

Seawater-basalt interaction taking place at mid-ocean ridges was studied using numerical modeling to determine the compositional evolution of hydrothermal fluids and associated alteration mineralogy forming within newly emplaced crustal material. Geochemical modeling was carried out in a closed seawater-basalt system at discrete temperature intervals between 2 and 400°C at 500 bars, varying fluid/rock ratios, and secondary mineral assemblages representative of basalt alteration in natural systems. In addition to temperature, the fluid/rock ratio has a fundamental control on the resulting system chemistry. At rock-buffered conditions (low fluid/rock ratios), the mineral-solution equilibrium was characterized by high cation to proton activity ratios foraCa2+/(aH+)2andaNa+/aH+and very low dissolved Mg concentrations due to the precipitation of smectites and chlorite. A complex secondary mineral alteration assemblage dominated by Ca- and Na-bearing minerals including zeolites, calcite, epidote, prehnite, clinozoisite, and albite was predicted to form. The resulting fluid composition was alkaline and reduced relative to ambient seawater, with Eh values ranging between −0.2 and −0.6 V. In contrast, seawater-buffered conditions (high fluid/rock ratios) resulted in lower cation to proton activity ratios foraCa2+/(aH+)2andaNa+/aH+and higher dissolved Mg concentrations comparable to the value of this element in ambient seawater. A more simple mineral assemblage was predicted to form at these conditions with the predominance of Al-Si- and Mg-bearing minerals including kaolinite, quartz, and talc in addition to large amounts of anhydrite. The resulting fluid composition was mildly acidic and oxidized relative to seawater with Eh values ranging between −0.2 and 0 V. These modeling results were compared to a compilation of submarine hydrothermal vent fluid compositions from mid-ocean ridge settings and analogous basalt-dominated environments. The agreement obtained between the simulations and the compiled fluid data indicates that mid-ocean ridge hydrothermal processes can be closely reproduced by mineral-solution equilibria for a broad range of temperatures and fluid/rock ratios.

How Does Fault-Related Alteration of Basalt Affect the Potential for Geological Carbon Sequestration in Basalt Formations?

Basalt is an increasingly attractive target for CO2 storage via mineral trapping, due to its abundance of reactive divalent cations such as Ca2+, Mg2+ and Fe2+. The majority of previous studies into the potential for CO2 storage in basalts have used fresh basalt samples. However, it has generally been beyond of the scope of existing studies to look at the effects of faulted and altered basalt at depth, which might have an effect on CO2 storage estimates, especially in geologically older basalt volumes. In this study, two mugearite basalt samples were collected to assess potential for CO2 reactions: a fresh basalt, and its faulted equivalent (altered by fault-related fluids). The faulted equivalent contains less olivine and clinopyroxene compared with its fresh counterpart, due to fluids altering them to clay minerals during and after faulting. Two batch experiments were conducted in synthetic aquifer water (brine) at 50C and PCO2= 130 bar with the aim of investigating potential change in basalt composition and/or concentration of the brine caused by chemical reactions between the basalt rocks and CO2-saturated brine. After 30 days of reaction, Fe2+, Ca2+, Si4- , Al3+ and Mg2+ were leached from the fresh basalt sample showing a potential dissolution of reactive minerals such as olivine and clinopyroxene minerals from the original sample. In contrast, no changes of Fe2+ concentration were observed in experiments performed for faulted basalt. The increase of Fe2+, Ca2+, Si4- , Al3+ and Mg2+ concentration was higher in the fresh basalt experiment compared with the faulted basalt sample. This is most likely due to less reactive minerals available within the faulted sample, such as olivine and pyroxene, having been partially or completely replaced by clay minerals following faulting. Our study therefore finds that faulted and altered basalt is in general less reactive to CO2 than fresh basalt, and this may have implications for the storage of CO2 via mineral trapping in basalts in geologically old and faulted basalt volumes. If basalt volumes targeted for CO2 storage in the future contain geological faults, bulk estimates for mineral trapping of CO2 should take into account the possibility of a difference in reaction rates across the rock volume, and complete additional lab studies to quantify any difference.

μXAFS and TEM studies of Fe(III) oxides precipitated on submarine basaltic glass from South Pacific Gyre

In the present study, we investigated secondary products formed on the surface of altered basaltic glass from the sediment-basalt interface by multiple characterizations with EPMA, μXANES, μEXAFS, TEM, and EELS. μXANES and EPMA analyses showed that secondary products with high Fe abundance were precipitated on the surface of basalt glass and that the Fe species in the secondary product was dominantly composed of Fe(III) oxides, hematite, which was also agreeable with EELS result. Characterization by TEM and μEXAFS suggested that the hematite was nanoparticulate with particle size of 10–20 nm. This is the first study showing the occurrence of nanosized hematite as secondary products in altered basaltic glass, although there are many studies on the formation of Fe(III) (hydr)oxides in basalt alteration.

Role of weathering of pillow basalt, pyroclastic input and geomorphic processes on the genesis of the Monte Cerviero upland soils (Calabria, Italy)

In this paper we present new data on weathering and soil formation processes affecting alkaline pillow basalts on the summit of Mt. Cerviero (Calabria, southern Italy). We investigated two representative soil profiles using an integrated approach, including pedological, petrographic, mineralogical and geochemical investigations. We distinguished the main features inherited from the hydrothermal alteration of the pillow basalts in a submarine environment from the chemical weathering processes under meteoric conditions. Irregular geochemical patterns and chemical index of alteration values across the soil profiles indicate a lithological discontinuity between the bedrock and the upper soil horizons, as a response to soil rejuvenation, in turn controlled by erosive processes and an allochthonous pyroclastic input. The soil profiles display poor horizonation and an incipient to intermediate degree of weathering, in line with the clay mineralogy (chlorite or hydroxy-interlayered vermiculite, illite, kaolinite and varying mixed-layers). The degeneration microtextures of clay and iron-manganese coatings in one soil profile suggest their relict genesis, with an emplacement under warm-humid conditions during the last interglacial. A Late Pleistocene to Holocene age of soil development is supported by the trachytic composition of volcanic micropumices, correlated to explosive eruptions from the Campania Province or the Aeolian Islands, in both soil profiles. Geochemical indices obtained from selective extractions of pedogenetic Al, Fe and Si pools point to poor andic properties with humus-Al-complexes prevailing over short-range order minerals. Nonetheless, the latter are consistent with the presence of volcanic glass, an optically isotropic pedogenic matrix and the Andosol-like field features of the soil profiles, even if they developed in a non-volcanic area. This should lead to a partial reassessment of the volcanic versus non-volcanic origin of certain Andosols worldwide and claims a good field work as a basis for choosing the best-suited laboratory methods to fill the gap between ordinary lab and field results.

Carbon sequestration potential of altered mafic reservoirs

Porous basaltic aquifers are currently being considered as a key geologic carbon storage host due to its widespread distribution and high reactivity. The co-injection of CO2 and groundwater into basaltic reservoirs has the potential to mineralize this gas into solid phases within years, thanks to the release of divalent cations. Many natural basalts, however, contain substantial alteration minerals. Here we explore the potential of basalt alteration minerals to provide the Ca to fix injected CO2 within calcite and/or aragonite. Preliminary results suggest that altered basaltic rocks can provide this Ca as efficiently as fresh basalts at 25 and 100 °C. Further experimental work is ongoing to confirm these findings at different temperatures and as a function of injected fluid chemistry.