Ash Alteration Research Articles

Chemolithotrophic biosynthesis of organic carbon associated with volcanic ash in the Mariana Trough, Pacific Ocean

Volcanic ash is a major component of marine sediment, but its effect on the deep-sea carbon cycle remains enigmatic. Here, we analyzed mineralogical compositions and glycerol dialkyl glycerol tetraether (GDGT) membrane lipids in submarine tuffs from the Mariana Trough, demonstrating a fraction of organic carbon associated with volcanic ash is produced in situ. This likely derives from chemolithotrophic communities supported by alteration of volcanic material. Tuff GDGTs are characterized by enrichment of branched GDGTs, as in chemolithotrophic communities. Scanning electron microscope, Raman spectrum and nano secondary ion mass spectrometry analysis demonstrates organic carbon exists around secondary heamatite veins in the altered mafic minerals, linking mineral alteration to chemolithotrophic biosynthesis. We estimate organic carbon production of between 0.7 − 3.7 × 1011 g if all the chemical energy produced by ash alteration was fully utilized by microorganisms. Therefore, the chemolithotrophic ecosystem maintained by ash alteration likely contributes considerably to organic carbon production in the seafloor.

Geochemical and molecular characteristics of ferromanganese deposits and surrounding sediments in the Mariana Trench: An Implication for the geochemical Mn cycle in sedimentary environments of the trench zone

Trench systems are some of the most important geodynamic settings on Earth and have a substantial influence on the global metal cycle. Trench environments are affected not only by episodic deposition of volcanic ash, but also exhibit a continuous water–rock chemical exchange. Ferromanganese nodules, which are archives for the marine Mn cycle, were first discovered in the southern Mariana Trench during China’s major 10,000-meter hadal trench scientific expedition in 2016. Nevertheless, their geochemical characteristics and formation mechanism in sedimentary environments of the trench zone remain enigmatic. In this study, these ferromanganese nodules and surrounding sediments were examined by geochemical and microbial methods. Bulk geochemistry indicated that trench nodules are characterized by high Mn/Fe ratios and nodule textures indicate a rapid growth rate, with Mn mainly from two sources: volcanic ash alteration and fluids discharge from the trench seabed. High-resolution in situ geochemical analysis categorized microlayers of an individual nodule into three types. Type I is the interior part that has a high Mn/Fe ratio (>20) and high growth rate (52.19–3571.73 mm/Myr), and formed by the influence of fluid discharge activities around this region. Type II is laminae from the exterior part that have a hydrogenetic origin with lower Mn/Fe ratio (0.01–2.50) and lower growth rate (4.54–9.86 mm/Myr). Type III is laminae from the exterior part that have a diagenetic origin with moderately high Mn/Fe ratio (2.5–15.0) and moderately high growth rate (12.40–18.93 mm/Myr). 16S rRNA gene sequence analysis revealed that the bacterial community structures in the nodule-bearing sediment samples, dominated by Shewanella and Colwellia, were substantially different from those of reference sediment samples. For the first time, we propose a conceptual model for a geochemical Mn cycle and nodule formation in trench sedimentary environments using both geochemical and microbial data.

Impact of iron release by volcanic ash alteration on carbon cycling in sediments of the northern Hikurangi margin

We present geochemical data collected from volcanic ash-bearing sediments on the upper slope of the northern Hikurangi margin during the RV SONNE SO247 expedition in 2016. Gravity coring and seafloor drilling with the MARUM-MeBo200 allowed for collection of sediments down to 105 meters below seafloor (mbsf). Release of dissolved Sr2+ with isotopic composition enriched in 86Sr (87Sr/86Sr minimum = 0.708461 at 83.5 mbsf) is indicative of ash alteration. This reaction releases other cations in the 30-70 mbsf depth interval as reflected by maxima in pore-water Ca2+ and Ba2+ concentrations. In addition, we posit that Fe(III) in volcanogenic glass serves as an electron acceptor for methane oxidation, a reaction that releases Fe2+ measured in the pore fluids to a maximum concentration of 184 μM. Several lines of evidence support our proposed coupling of ash alteration with Fe-mediated anaerobic oxidation of methane (Fe-AOM) beneath the sulfate-methane transition (SMT), which lies at ∼7 mbsf at this site. In the ∼30-70 mbsf interval, we observe a concurrent increase in Fe2+ and a depletion of CH4 with a well-defined decrease in δ13C-CH4 values indicative of microbial fractionation of carbon. The negative excursions in δ13C values of both DIC and CH4 are similar to that observed by sulfate-driven AOM at low SO42− concentrations, and can only be explained by the microbially-mediated carbon isotope equilibration between CH4 and DIC. Mass balance considerations reveal that the iron cycled through the coupled ash alteration and AOM reactions is consumed as authigenic Fe-bearing minerals. This iron sink term derived from the mass balance is consistent with the amount of iron present as carbonate minerals, as estimated from sequential extraction analyses. Using a numerical modeling approach we estimate the rate of Fe-AOM to be on the order of 0.4 μmol cm−2 yr−1, which accounts for ∼12% of total CH4 removal in the sediments. Although not without uncertainties, the results presented reveal that Fe-AOM in ash-bearing sediments is significantly lower than the sulfate-driven CH4 consumption, which at this site is 3.0 μmol cm−2 yr−1. We highlight that Fe(III) in ash can potentially serve as an electron acceptor for methane oxidation in sulfate-depleted settings. This is relevant to our understanding of C-Fe cycling in the methanic zone that typically underlies the SMT and could be important in supporting the deep biosphere.

Sedimentation Controls on Methane‐Hydrate Dynamics Across Glacial/Interglacial Stages: An Example From International Ocean Discovery Program Site U1517, Hikurangi Margin

AbstractDissolved chloride concentrations higher than seawater were observed over a broad depth range in pore water profiles from International Ocean Discovery Program Site U1517 on the Hikurangi Margin. This Cl maximum is not associated with an 87Sr/86Sr anomaly, indicating that it is not caused by hydration reactions during ash alteration. We use a numerical modeling approach to examine possible causes for recent gas hydrate formation that can result in the observed Cl high. Our approach considers sedimentation, sea level, and bottom water temperature (BWT) changes due to glaciation as drivers for the downward migration of the base of gas hydrate stability and gas hydrate formation. The modeling results reveal that lowering of sea level during glaciation can allow methane hydrate dissociation followed by postglacial hydrate formation as sea level rises. However, BWT cooling of 2 °C during glaciation followed by warming during deglaciation would mostly counteract the impacts of sea level change. Bottom water cooling during glaciation is expected in this region and many locations worldwide. As a result, our simulations do not support the previous hypotheses of large‐scale gas hydrate dissociation due to sea level drop during glaciation, which have been proposed as triggers for widespread gas release and slope failure. Such a mechanism is only possible where BWT remains constant or increases during glaciation. Our simulations indicate that sedimentation constitutes the largest factor driving recent methane hydrate formation at Site U1517, and we suggest that sedimentation may play a larger role in gas hydrate dynamics along margins than previously recognized.

Fluid flow and water–rock interaction across the active Nankai Trough subduction zone forearc revealed by boron isotope geochemistry

Compositional changes, dehydration reactions and fluid flow in subducted sediments influence seismogenesis and arc magmatism in subduction zones. To identify fluid flow and water–rock interaction processes in the western Nankai Trough subduction zone (SW Japan) we analyzed boron concentration and boron isotope composition (δ11B) of pore fluids sampled across the subduction zone forearc from depths of up to ∼922m below seafloor during four Integrated Ocean Drilling Program (IODP) Expeditions. The major structural regimes that were sampled by coring include: (1) sedimentary inputs, (2) the frontal thrust zone, (3) the megasplay fault zone, and (4) the forearc basin. From mass balance consideration we find that consumption of boron (B) by ash alteration and desorption of B from the solid phase, mediated by organic matter degradation, produces a net decrease in B concentrations with depth down to ∼120μM and variable δ11B values in the range of ∼+20‰ and +49‰. Interstitial water in sediments on the incoming oceanic plate are influenced by more efficient mobilization of exchangeable B from the solid phase due to higher temperatures and alteration of the oceanic crust that acts as a sink for 10B. At the tip of the megasplay fault zone, elevated B concentration and B isotopic composition suggest that underthrust coarse-grained slope sediments provide a pathway for fluids out of the upper (<2km) accretionary prism. Silt and sand layers in the underthrust section of the downgoing plate favor fluid escape in seaward direction from depths equivalent to the temperature range of 60–150°C. At both locations the δ11B signature evolves during updip migration through re-adsorption. Mass balance considerations suggest a shallower fluid source depth compared to pore fluids sampled previously near the décollement zone along the central portion of the Nankai margin.

Crustal fluid and ash alteration impacts on the biosphere of Shikoku Basin sediments, Nankai Trough, Japan.

We present data from sediment cores collected from IODP Site C0012 in the Shikoku Basin. Our site lies at the Nankai Trough, just prior to subduction of the 19 Ma Philippine Sea plate. Our data indicate that the sedimentary package is undergoing multiple routes of electron transport and that these differing pathways for oxidant supply generate a complex array of metabolic routes and microbial communities involved in carbon cycling. Numerical simulations matched to pore water data document that Ca(2+) and Cl(1-) are largely supplied via diffusion from a high-salinity (44.5 psu) basement fluid, which supports the presence of halophile Archean communities within the deep sedimentary package that are not observed in shallow sediments. Sulfate supply from basement supports anaerobic oxidation of methane (AOM) at a rate of ~0.2 pmol cm(-3) day(-1) at ~400 mbsf. We also note the disappearance of δ-Proteobacteria at 434 mbsf, coincident with the maximum in methane concentration, and their reappearance at 463 mbsf, coinciding with the observed deeper increase in sulfate concentration toward the basement. We did not, however, find ANME representatives in any of the samples analyzed (from 340 to 463 mbsf). The lack of ANME may be due to an overshadowing effect from the more dominant archaeal phylotypes or may indicate involvement of unknown groups of archaea in AOM (i.e., unclassified Euryarchaeota). In addition to the supply of sulfate from a basement aquifer, the deep biosphere at this site is also influenced by an elevated supply of reactive iron (up to 143 μmol g(-1)) and manganese (up to 20 μmol g(-1)). The effect of these metal oxides on the sulfur cycle is inferred from an accompanying sulfur isotope fractionation much smaller than expected from traditional sulfate-reducing pathways. The detection of the manganese- and iron-reducer γ-Proteobacteria Alteromonas at 367 mbsf is consistent with these geochemical inferences.

Altered marine tephra deposits as potential slope failure planes?

Weathering of tephra results in increasing proportions of mechanically weak, authigenic clay minerals (smectite). This suggests that altered tephra represent inherent weak layers in slope sediment sequences, and these may facilitate slope failure in submarine and other aquatic environments. In drained direct shear experiments, tephra in different alteration stages were compared to common sand–clay mixtures for geotechnical reference. Attention is drawn to the influence of particle shape on shear strength. The results revealed volcanic ash to have (1) a high strength end-member at low alteration stages due to particle roughness and angularity and (2) a low strength end-member after complete diagenetic alteration, both under static conditions. This would suggest that strongly altered volcanic ash layers could potentially be responsible for slope failures. However, a review of ODP and IODP Expedition reports shows that advanced ash alteration mostly occurs at depths below (>800 mbsf) those commonly observed for slope failure initiation (<400 mbsf). This, in turn, suggests that volcanic ash alteration does not play an important role in the initiation of slope failure.

Microporosity development in shallow marine sediments from the Nankai Trough

Abstract I characterized microporosity by performing low pressure nitrogen adsorption measurements on 13 shallow marine mudstone samples from the Nankai Trough offshore Japan. The samples were from two reference Sites on the incoming Philippine Sea Plate, and one Site above the accretionary prism. I determined pore size distributions using the Barrett–Joyner–Hallenda (BJH) model, and merged these with existing mercury injection capillary pressure (MICP) measurements to construct a full distribution covering micro- to macropores. I found that overall pore sizes decrease with consolidation, and that microporosity content (pores + , Ca 2 + , Sr, Si) show inconsistent relationships with microporosity development and cannot confirm or deny the role of ash alteration in this process. The strongest correlation observed at the three Sites was between microporosity volume and clay mineral fraction. This suggests that microporosity content is determined mainly by detrital clay abundance and development of clay as an ash alteration product, with some contribution from amorphous silica cement precipitated in the zones of anomalous high porosity.

Repeated occurrences of methanogenic zones, diagenetic dolomite formation and linked silicate alteration in southern Bering Sea sediments (Bowers Ridge, IODP Exp. 323 Site U1341)

Diagenetic precipitates, such as dolomite, and the chemistry of residual deeply buried porewater often represent the only traces of past biogeochemical activity in marine sediments. A 600m thick sedimentary section, recently drilled at Integrated Ocean Drilling Program (IODP) Site U1341 on Bowers Ridge (southern Bering Sea), provides insight into such a 4.3Ma old paleo-diagenetic archive. Hard-lithified calcite–dolomite layers, and laminae of disseminated carbonate, were recovered in diatom-rich sediments over a depth range of 400m. Carbon isotope values of the diagenetic carbonates between −16.6 and −14.4‰ (VPDB) and strontium isotope ratios of dolomites close to past seawater values suggest carbonate precipitation induced by the production of dissolved inorganic carbon (DIC) during elevated rates of organic carbon mineralization, primarily via sulfate reduction, at shallow sediment depth below the paleo-seafloor. Diagenetic carbonates at 280–440m below seafloor were likely also produced by the intermittent onset of sulfate reduction coupled to the anaerobic oxidation of methane (AOM) at sulfate–methane transition zones (SMTZ). These microbially mediated processes do not occur in the sediment at this site at present but were likely connected to the presence of a methanogenic zone at 2.58–2.51Ma. A minimum in sulfate concentrations in modern porewaters and low sedimentary Ba/Al ratios resulting from former sulfate depletion are reminiscent of the presence of this large methanogenic zone. The minimum in sulfate concentrations is reflected in a minimum in magnesium concentrations, less radiogenic strontium and isotopically light calcium in the porewater. It is proposed that magnesium was removed from the porewater during carbonate precipitation and volcanic ash alteration which occurred in the former methanogenic zone and also released strontium with a less radiogenic isotope ratio and isotopically light calcium into the porewater. The isotopic composition of porewater calcium was also influenced by ammonium–calcium exchange on clay minerals and carbonate recrystallization. Our study elucidates the response of porewater element concentrations and isotopic profiles interlinked with the formation of diagenetic carbonates to changes in the deposition of organic carbon in sediments of deeper water sites (>2000m water depth) over prolonged timescales. It shows that variations in biogeochemical processes in response to changes in oceanographic conditions and a dynamic subseafloor biogeochemical zonation have to also be taken into account at these deep water sites for a global assessment of organic carbon burial fluxes and remineralization.

Fluid-Sediment Interactions in the Nankai Subduction Zone (NanTroSEIZE): Pore Fluid and Mineralogical Records

The Nankai Trough Seismogenic Zone Experiment (IODP), SW Japan, aimed at unraveling hydrologic processes at subduction megathrusts and in situ rock and fluid properties to understand the fault zone behavior during earthquake nucleation and rupture propagation. At different sites, characterization of the sediments at high resolution allows tracing fluid–rock reactions that are either associated to early transformations (hydration or dehydration of clay minerals) during the accretion phase or to mineral recrystallization processes linked to deformation phases in gouge zones. In the sedimentary strata at the front of the accretionary prism changes in clay composition are inferred from chemical trends observed in pore fluids, which further indicate the influence of volcanic ash alteration (zeolite formation). Along the megasplay fault, recrystallization processes (e.g. pyrite) are observed in foliated gouge zones at the contact with non-foliated silty clay. The data highlight the importance of small-scale observations for process and reactive phases such as clays in such variable fluid-sediment reactions.

The influence of volcanic ash alteration on the REE composition of marine pore waters

Two volcanic ash layer dominated marine gravity cores were recovered during research cruise RV Sonne 173/3 along the Pacific margin offshore Nicaragua. Thick ash layers dominate the recovered sediments. Pore water samples of these – one incoming plate and one slope site – sediment cores have been analysed for light rare earth elements, alkalinity, sulphate, phosphate, ammonia, calcium, and manganese contents. The data provide a systematic look at changes in REE during diagenesis proceeding from open ocean sediments to highly reducing near-shore sediments. The pore water patterns of all the dissolved species mentioned above suggest that volcanic glass alteration constitutes a major source for REE release in marine sediments when low rates of anaerobic degradation of organic matter, like typical for incoming plate/open ocean sediments, prevail. Under these conditions the release of REE results in fluxes of at least 0.04 nmol La cm − 2 yr − 1 and 0.09 nmol Ce cm − 2 yr − 1 .

Controls on calcium isotope fractionation in sedimentary porewaters

The calcium isotopic composition of porewaters and authigenic carbonates in the anoxic sediments of a convergent continental margin drilled during Ocean Drilling Program (ODP) provides first insight into the different processes that control Ca geochemistry in clastic marine, organic-rich sedimentary environments. In 4 sites drilled during Leg 204 at Hydrate Ridge (Cascadia Margin, offshore Oregon/USA), sulfate is consumed during anaerobic oxidation of methane and of organic matter via sulfate reduction within the upper meters of the sedimentary section. These reactions promote the precipitation of authigenic carbonates through the generation of bicarbonate, which is reflected in a pronounced decrease in calcium concentration. Although Ca isotope fractionation is observed during carbonate precipitation, Ca concentration in the pore fluids from ODP Leg 204 is decoupled from Ca isotopy, which seems to be mainly controlled by the release of light Ca isotopes that completely overprint the carbonate formation effect. Different processes, such as the release of organically bound Ca, ion exchange and ion pair formation may be responsible for the released light Ca. Deeper within the sedimentary section, additional processes such as ash alteration influence the Ca isotopic composition of the porewater. Two sites, drilled into the deeper core of the accretionary prism, reveal the nature of fluids which have reacted with the oceanic basement. These deep fluids are characterized by relatively high Ca concentrations and low δ 44/40Ca ratios.

Volcanogenic sediment–seawater interactions and the geochemistry of pore waters

Four volcanic ash-bearing marine sediment cores and one ash-free reference core were examined during research cruise RV Meteor 54/2 offshore Nicaragua and Costa Rica to investigate the chemical composition of pore waters related to volcanic ash alteration. Sediments were composed of terrigenous matter derived from the adjacent continent and contained several distinct ash layers. Biogenic opal and carbonate were only minor components. The terrigenous fraction was mainly composed of smectite and other clay minerals while the pore water composition was strongly affected by the anaerobic degradation of particulate organic matter via microbial sulphate reduction. The alteration of volcanic matter showed only a minor effect on major element concentrations in pore waters. This is in contrast to prior studies based on long sediment cores taken during the DSDP, where deep sediments always showed distinct signs of volcanic ash alteration. The missing signal of ash alteration is probably caused by low reaction rates and the high background concentration of major dissolved ions in the seawater-derived pore fluids. Dissolved silica concentrations were, however, significantly enriched in ash-bearing cores and showed no relation to the low but variable contents of biogenic opal. Hence, the data suggest that silica concentrations were enhanced by ash dissolution. Thus, the dissolved silica profile measured in one of the sediment cores was used to derive the in-situ dissolution rate of volcanic glass particles in marine sediments. A non-steady state model was run over a period of 43 kyr applying a constant pH of 7.30 and a dissolved Al concentration of 0.05 μM. The kinetic constant ( A A) was varied systematically to fit the model to the measured dissolved silica-depth profile. The best fit to the data was obtained applying A A = 1.3 × 10 −U9 mol of Si cm − 2 s − 1 . This in-situ rate of ash dissolution at the seafloor is three orders of magnitude smaller than the rate of ash dissolution determined in previous laboratory experiments. Our results therefore imply that field investigations are necessary to accurately predict natural dissolution rates of volcanic glasses in marine sediments.

Geochemistry of Major Constituents, Boron and Boron Isotopes in Pore Waters from ODP Site 1202, Okinawa Trough

geochemical results show that major constituents and boron content varied largely in pore fluids and possibly were affected by sulfate reduction, recrystallization of biogenic carbonate or silica, ash alteration, and organic matter degradation. Mixing of fluids along high-porosity sandy layers or fracture zones also changes the pore water chemical compositions significantly. The down-core distribution of B and δ 11 B in the pore waters are sensitive tracers for assessing fluid migration and water/sediment interaction at various depths. Pore water B content at Site 1202 falls in a range between 0.25 and 1.16 mM compared to that of 0.42 mM in seawater. The δ 11 B values, however, vary considerably from ~32.7 to 50.9 0 00 relative to the seawater value of 39.5 0 00. The δ 11 B vs. 1/B plot indicates that sedimentary B released from clays is the most important source to pore waters, resulting in elevated B with low δ 11 B. Other processes including precipitation of calcium carbonate, fluid advection through high-porosity permeable sandy horizons, interaction with terrigenous sediments and/or ash alteration may also modify the B and δ 11 B distributions.

Pore water anomalies of submarine gas-hydrate zones as tool to assess hydrate abundance and distribution in the subsurface: What have we learned in the past decade?

The most significant new contributions to the study of natural gas hydrates in the past decade have come from findings of the Ocean Drilling Program (ODP), notably legs 112, 141, 146 and 164, and to a lesser extent legs 131, 160, 170 and 186. Leg 164, in particular, was dedicated to gas-hydrate drilling in the Blake Ridge gas-hydrate field in the West Atlantic in an unprecedented multidisciplinary research effort [Paull et al., 1996. Proc. Ocean Drill. Program, Initial Rep. 164, 623 pp.; Paull C.K., Matsumoto, R., Wallace, P.J., Dillon, W.P. (Eds.), 2000a. Proc. Ocean Drill. Program Sci. Results 164, Ocean Drilling Program, College Station, TX, 459 pp.]. Most important for the progress of hydrochemical studies related to gas hydrates has been the growing awareness of the significance of diffusion-modulated advective processes shaping the chemical and isotopic pore water profiles in hydrate zones. This started with qualitative evidence for advective flow from drilling the (hydrate-bearing) Peru and (hydrate-free) Barbados active margins, continued with the Nankai Trough accretionary prism, the Japan Trench Slope and the Cascadia and Costa Rica active margins and culminated in the quantitative advection-diffusion model of Egeberg and Dickens [Chem. Geol. 153 (1999) 53.] for the passive margin setting of the Blake Ridge. Advective flow regimes are different at active and passive margins, as there is a tendency for the flow at active margins to be focussed along landward-dipping thrust planes and faults in the wedge of imbricated thrust sheets that finds expression in a step-pattern of the pore water profiles. For passive margins, but also for some active margin sites, advection is from sources below the drilled section, either through subvertical faults (passive margins) or from the décollement zone (active margins). We have learned that the well-known coupled pore water anomalies that are ascribed to hydrate dissociation—downward chlorinity decrease combined with δ 18O increase [Hesse and Harrison, 1981, Earth Planet. Sci. Lett. 55 (1981) 453.]—need not occur together in the presence of hydrates because the isotope effect may be overprinted by the effects of other reactions such as volcanic ash alteration or by the advection of low-δ 18O fluids. However, if the anomalies show up, hydrates are present almost invariably (with the exception of advected low-Cl −/high-δ 18O waters). Coming to terms with the effects of advection and diffusion has allowed successful modeling of the simpler hydrate-affected pore water profiles at passive margins. Instrumental for modeling are chlorine isotopes, which provide an effective tool to assess advection rates. The Egeberg and Dickens model allows estimation of hydrate concentration and distribution in the subsurface because it separates the effects of advection, diffusion and hydrate dissociation but critically depends on samples taken under in situ pressure and temperature conditions. Modeling the more complex pore water profiles of active margins is a challenge for the future. Compared to geophysical methods to estimate hydrate concentration, the geochemical method gives minimum amounts.

99/02560 Sulfur-bearing coatings on fly ash from a coal-firedpower plant: composition, origin, and influence on ash alteration

99/02560 Sulfur-bearing coatings on fly ash from a coal-firedpower plant: composition, origin, and influence on ash alteration

Sulfur-bearing coatings on fly ash from a coal-fired power plant: composition, origin, and influence on ash alteration

Fly ash samples collected from two locations in the exhaust stream of a coal-fired power plant differ markedly with respect to the abundance of thin (≈0.1 μm) sulfur-rich surface coatings that are observable by scanning electron microscopy. The coatings, tentatively identified as an aluminum-potassium-sulfate phase, probably form upon reaction between condensed sulfuric acid aerosols and glass surfaces, and are preferentially concentrated on ash exposed to exhaust stream gases for longer. The coatings are highly soluble and if sufficiently abundant, can impart an acidic pH to solutions initially in contact with ash. These observations suggest that proposals for ash use and predictions of ash behavior during disposal should consider the transient, acid-generating potential of some ash fractions and the possible effects on initial ash leachability and alteration.

Clay- and zeolite-bearing Triassic sediments at Kaka Point, New Zealand: evidence of microbially influenced mineral formation from earliest diagenesis into the lowest grade of metamorphism

AbstractThe distribution, mineralogy, petrology and bulk and stable isotope chemistry of altered volcanic ash beds in the marine sediments of Mid-Triassic age (Etalian) at Kaka Point, New Zealand, are described and related to lithofacies and the geological processes controlling their development.Three varieties of altered ash occur in the Kaka Point sediments — porcellanite, claystone (bentonite) and albite-rich. Porcellanites are quartz-rich and may contain analcime and heulandite: they are restricted mainly to the on-shore facies. Claystones are rich in smectitic clay minerals and occur in both the on-shore and off-shore facies. They often contain diagenetic nodules of analcime, quartz, apatite and carbonates. The authigenic carbonates of the on-shore facies are variable in composition (sideritic, rhodochrositic, calcitic), whereas in the off-shore facies they consist only of calcite. The albite-rich lithology is very rare and is known only from the off-shore facies.The development of the porcellanite and albite-rich lithologies was restricted to slowly deposited, relatively coarse-grained ash sediments in which extensive interchange took place between the sediment's pore-waters and ambient seawater, resulting in enhanced microbial activity and high pH throughout the pore-waters of the suboxic zone beneath the water-sediment interface. The high pH increased the rate of volcanic ash hydrolysis and provided the conditions necessary for the precipitation of zeolite, feldspar and quartz. The development of smectitic claystones was associated with more rapid deposition and limited interchange between the pore-waters of the parent ash and ambient seawater. The pore-water alkalinity was generally lower and enhanced microbial activity and high pHs were restricted to patches of sediment at which quartz, analcime, apatite and carbonates formed diagenetic nodules. Modelling of the stable isotopes of the smectitic clays (δ18O, δD) and diagenetic carbonates (δ18O, δ13C) suggest that: (1) ash argillization in the on-shore facies took place in brackish water (∼25% meteoric water) at an average temperature of ∼50°C and in the off-shore facies in marine pore-waters (∼10% meteoric waters) at ∼40°C and (2) diagenetic carbonate precipitation in the near-shore facies took place at ∼30°C and in the off-shore facies at 60–80°C.The pattern of ash alteration in the marine Triassic sediments at Kaka Point is considered to represent an early stage in the development of the zeolite pattern associated with the classic area of zeolite facies metamorphism in the Taringatura and Hokonui Hills.

Microfabric of Altered Ash Layers, ODP Leg 131, Nankai Trough

AbstractSamples of ash layers and associated background sediments from Site 808 of ODP Leg 131 in the Nankai Trough accretionary prism were analyzed for changes in mineralogy, porosity and micro-fabric associated with alteration of volcanic ash. Ash layers range from incipient stages of alteration and dissolution to complete alteration to clay minerals and clinoptilolite. Ash layers contain greater abundances of total clay minerals and lower percentages of quartz than do surrounding background hemipelagic sediments. The clay-sized fraction of ash layers is dominated by pure dioctahedral smectite, whereas the background sediments contain primarily illite and chlorite with minor amounts of smectite. Analysis of microfabric revealed dramatic changes in the distributions and abundances of grains and pores during ash alteration. The relative abundances of large pores, grains, and matrix material were quantified on digital back-scattered electron images (BSEI) of ash layer and background sediment samples. During burial, the abundant glass shards of shallow ash layers are initially altered, presumably to smectite. Subsequent dissolution of the glass leaves open, shard-shaped pores, resulting in increased porosities. With greater burial, these pores are filled with clinoptilolite. Although the presence of ash and its alteration products clearly influences sediment physical properties, there is no apparent correlation of the abundance of ash or its alteration products with the formation of thrust faults or other structures within the Nankai Trough accretionary prism.

Holocene carbonatite-nephelinite tephra deposits of Oldoinyo Lengai, Tanzania

Natrocarbonatite and nephelinite tephra have been erupted together from Oldoinyo Lengai over the past few thousand years. The oldest deposits with recognizable natrocarbonatite are thin tuff beds probably between 2000 and 5000 years in age. Tuffs and agglomerates with an age of about 1250–2000 years contain evidence strongly suggestive of natrocarbonatite ash. The volcano is mantled by nephelinite-carbonatite ash deposits, termed the Footprint Tuff, that were erupted about 600 years ago. Evidence is contradictory as to whether natrocarbonatite was discharged during the cone-building phase of eruptions, which ended about 15,000 years ago. Nephelinite-carbonatite ash erupted in 1966 contains an unidentified mineral, designated NCS, with a chemical composition of Na 4.09Ca 2.76Si 5O 14.81. It may be genetically related to natrocarbonatite magma as it has thus far been identified only in the younger tephra deposits of Oldoinyo Lengai, most of which contain evidence of natrocarbonatite. A tephra deposit termed the Footprint Tuff contains footprints thought to be preserved by the rapid recrystallization of primary natrocarbonatite. Calcite of natrocarbonatite origin forms an estimated 15–20% of the airfall tuffs, and natrocarbonatite probably equalled or exceeded the volume of nephelinite tephra at the time of eruption. Nyerereite ([Na 0.82K 0.18] 2 Ca[CO 3] 2) and gregoryite ([Na 0.78K 0.05] [Ca 0.17CO 3]) were primary minerals in the natrocarbonatite, as in modern lavas of Oldoinyo Lengai. Unlike modern lavas, the groundmass contained a substantial amount of silicate material. Noncarbonate minerals in the Footprint Tuff include nepheline, melilite, augite, wollastonite, melanite, fluorite, and NCS. The Footprint Tuff was cemented soon after deposition, very likely by trona. Gaylussite, pirssonite, or both, were probably later alteration products in the transformation of natrocarbonatite ash to form calcite. Oxygen and carbon isotopic re-equilibration were essentially decoupled in alteration of natrocarbonatite ash. δ 18O values of this calcite suggest extensive to complete oxygen exchange, but δ 13C values are in and near the range for unaltered natrocarbonatite. Carbon exchange may have been retarded in ash alteration by the high pH of pore fluid.