Olivine Dissolution Rates Research Articles

Peridotite Weathering and Ni Redistribution in New Caledonian Laterite Profiles: Influence of Climate, Hydrology, and Structure

The peridotite massifs of New Caledonia are characterised by complex hydrodynamics influenced by intense inherited fracturing, uplift, and erosion. Following the formation of the erosion surfaces and alteration processes, these processes drive chemical redistribution during weathering; particularly lateritisation and saprolitisation. Magnesium, silica, and trace elements such as nickel and cobalt—released as the dissolution front advances—are redistributed through the system. New observations and interpretations reveal how lateritic paleo-land surfaces evolved, and their temporal relationship with alteration processes since the Oligocene. Considering the geometry of discontinuity networks ranging from micro-fractures to faults, the transfers occur in dual-permeability environments. Olivine dissolution rates are heterogeneously due to differential solution renewal caused by erosion and valley deepening. Differential mass transfer occurs between mobile regions of highly transmissive faults, while immobile areas correspond to the rock matrix and the secondary fracture network. The progression of alteration fronts controls the formation of boulders and the distribution of nickel across multiple scales. In the saprolite, nickel reprecipitates mostly in talc-like phases, as well as minor nontronite and goethite with partial diffusion in inherited serpentine. The current nickel distribution results from a complex interplay of climatic, hydrological and structural factors integrated into a model across different scales and times.

Potential of CO2 sequestration by olivine addition in offshore waters: A ship-based deck incubation experiment

Ocean alkalinity enhancement is considered as an effective atmospheric CO2 removal approach, but currently, little is known about the carbon sequestration potential of implementing olivine addition in offshore waters. We investigated the effect of olivine addition on the seawater carbonate system by carrying out a deck incubation experiment in the Northern Yellow Sea; the dissolution rate of olivine was calculated based on the increase in seawater alkalinity (TA), and the CO2 sequestration potential was evaluated. The results showed that the dissolution of olivine increased seawater TA and decreased partial pressure of CO2, resulting in oceanic CO2 uptake from the atmosphere through sea-air exchange; it also increased seawater pH and mitigated ocean acidification to a certain extent. The addition of 1 ‰ olivine had a more significant effect on the seawater carbonate system than 0.5 ‰ olivine addition. The average dissolution rate constant of olivine was 1.44 ± 0.15 μmol m−2 d−1. Assuming that olivine settles completely on the seabed due to gravity, the theoretically maximum amount of CO2 removed by applying 1 tonne of olivine per square meter area in the Northern Yellow Sea is only 2.0 × 10−4 t/m2. Therefore, when olivine addition is implemented in the offshore waters, it is necessary to consider reducing the olivine size, prolonging the settling time of olivine in the water column; and spreading olivine in well-mixed waters to prolong the residence time through repeated resuspension, thus increasing its potential in carbon sequestration.

Influence of surface orientation on the variability of olivine dissolution rates

Alkaline solid wastes containing olivine minerals can be suitable for CO2 mineralization due to their relatively higher reactivity and intrinsic alkalinity. However, an activation process is required to improve cation dissolution rates. Four pristine facets in an olivine single crystal were used to study their dissolution kinetics at far from thermodynamic equilibrium conditions. Vertical scanning interferometry was employed to analyse the distribution of surface dissolution rate and the evolution of etch pits. The dissolution rate exhibited an anisotropic distribution, by up to two orders of magnitude among facets. The rate law differed for each plane, with (010) face following zero-order, (120) plane exhibiting first-order, and (110) and (111) planes showing second-order reactions. Etch pits also displayed significant anisotropy, with spindle-shaped on (010) plane, droplet-shaped on (110) plane, and funnel-shaped on (111) plane. Strip-like etch pits only formed along certain scratches on (120) plane. Initially, the etch pits deepened, but after 200 h of reaction time, they grew in size. Raman and TEM analysis confirmed atomic-scale rearrangement of the dissolved surface, providing insights into etch pit morphology evolution. This work unveiled the anisotropic dissolution mechanism of olivine surfaces, enabling predictions of long-term reaction rates under diverse geochemical conditions.

Responses of globally important phytoplankton species to olivine dissolution products and implications for carbon dioxide removal via ocean alkalinity enhancement

Abstract. Anthropogenic greenhouse gas emissions are leading to global temperature increases, ocean acidification, and significant ecosystem impacts. Given current emissions trajectories, the IPCC's reports indicate that rapid abatement of CO2 emissions and development of carbon dioxide removal (CDR) strategies are needed to address legacy and difficult-to-abate emissions sources. These CDR methods must efficiently and safely sequester gigatons of atmospheric CO2. Coastal enhanced weathering (CEW) via the addition of the common mineral olivine to coastal waters is one promising approach to enhance ocean alkalinity for large-scale CDR. As olivine weathers, it releases several biologically active dissolution products, including alkalinity, trace metals, and the nutrient silicate. Released trace metals can serve as micronutrients but may also be toxic at high concentrations to marine biota, including phytoplankton, which lie at the base of marine food webs. We grew six species representing several globally important phytoplankton species under elevated concentrations of olivine dissolution products via a synthetic olivine leachate (OL) based on olivine's elemental composition. We monitored their physiological and biogeochemical responses, which allowed us to determine physiological impacts and thresholds at elevated olivine leachate concentrations, in addition to individual effects of specific constituents. We found both positive and neutral responses but no evident toxic effects for two silicifying diatoms, a calcifying coccolithophore, and three cyanobacteria. In both single and competitive co-cultures, silicifiers and calcifiers benefited from olivine dissolution products like iron and silicate or enhanced alkalinity, respectively. The non-N2-fixing picocyanobacterium could use synthetic olivine-derived iron for growth, while N2-fixing cyanobacteria could not. However, other trace metals like nickel and cobalt supported cyanobacterial growth across both groups. Growth benefits to phytoplankton groups in situ will depend on species-specific responses and ambient concentrations of other required nutrients. Results suggest olivine dissolution products appear unlikely to cause negative physiological effects for any of the phytoplankton examined, even at high concentrations, and may support growth of particular taxa under some conditions. Future studies can shed light on long-term eco-evolutionary responses to olivine exposure and on the potential effects that marine microbes may in turn have on olivine dissolution rates and regional biogeochemistry.

Enhanced olivine dissolution in seawater through continuous grain collisions

Carbon dioxide removal (CDR) technologies at a gigaton scale need to be developed and implemented within the next decades to keep global warming below 1.5 °C. Coastal enhanced silicate weathering is one of the proposed CDR techniques that aims to accelerate the natural process of CO2-sequestration during marine chemical weathering of silicate minerals. To this end, finely ground rock containing olivine (MgxFe2−xSiO4) could be dispersed in dynamic coastal environments, where local biotic and abiotic factors potentially enhance the weathering process. However, accurate predictions of the olivine dissolution rate and the associated CO2 sequestration under in situ conditions are currently lacking and ecosystem impacts remain to be assessed. Previously, it has been hypothesized that in situ grain collisions, induced by bed load transport due to currents and waves, could accelerate the in situ chemical weathering of olivine particles. To examine this, we investigated the effects of continuous grain tumbling on olivine dissolution in natural seawater. A 70-day experiment was conducted in which forsterite olivine sand was continuously tumbled in filtered seawater at different rotation speeds, and dissolution rates were measured on a weekly basis. Results showed that continuously tumbled olivine dissolved 8 to 19 times faster compared to stagnant (no rotation) conditions. Olivine dissolution was complete and stoichiometric (except for Ni release), air-seawater CO2 exchange was not significantly rate limiting, and minimal particle fragmentation and secondary mineral formation were observed. Hence, we infer that olivine weathering was mainly enhanced via advective pore water flushing, which limits saturation effects at the grain scale. Overall, this study provides evidence that ambient physical stresses in coastal environments could enhance marine silicate weathering, which has implications for both the natural silicon cycle as well as the use of enhanced coastal weathering of silicates as a CDR technique.

Bacterial use of siderophores increases olivine dissolution rates by nearly an order of magnitude

Bacterial use of siderophores increases olivine dissolution rates by nearly an order of magnitude

Aqueous carbonation of peridotites for carbon utilisation: a critical review.

Peridotite and serpentinites can be used to sequester CO2 emissions through mineral carbonation. Olivine dissolution rate is directly proportional with temperature, presence of CO2, surface area of mineral particles and presence of ligands and is inversely proportional to pH. Olivine dissolution is better under air flow and increases seven times when rock-inhibiting fungus (Knufia petricola) is used. Olivine dissolution retards as silica layers form during reaction. Sonication, acoustic and concurrent grinding using various grinding medias have been used to artificially break these silica layers and achieve high magnesium extraction. Wet grinding using 50 wt.% ethanol enhanced CO2 uptake of dunite 6.9 times and CO2 uptake of harzburgite by 4.5 times. The best economical process is single-stage concurrent grinding at 130 bar, 185 °C, 15 wt.% solids and 50 wt.% grinding media (zirconia) using 0.64 M NaHCO3. Ratio of grinding media to feed should not be less than 3:1. Yield increases with temperature, pressure, time of reaction, pH and rpm and using additives and grinding media and reducing particle size. This review aims to investigate the progress from 1970s to 2021 on aqueous mineral carbonation of olivine and its naturally available rocks (harzburgite and dunite). This paper comprehensively reviews all aspects of olivine carbonation including olivine dissolution kinetics, effects of grinding and concurrent grinding, thermal activation of olivine feedstock (dunites and harzburgites) as well as chemistry of olivine mineral carbonation. The effects of different reaction parameters on the carbonation yield, role of mineral carbonation accelerators and costs of mineral carbonation process are discussed.

Dynamics of coupled olivine dissolution and serpentine precipitation revealed by hydrothermal flow-through experiments at 260 °C–300 °C

Serpentinization is an important process that influences rock rheology, global fluid circulation, and microbiological activity in the deep sea, and is induced by hydrothermal fluid flow. Elemental olivine dissolution and serpentine precipitation govern the overall dynamics of serpentinization, and its prediction and modeling require an understanding of the reaction mechanism including the rate-limiting process. In this study, we examined the initial stage of coupled olivine dissolution and serpentine precipitation at 260 °C–300 °C and 50 MPa using a hydrothermal flow-through experimental apparatus. Saline water (0.5 m NaCl) was continuously injected into olivine powder at a constant flow rate (3.3 × 10−5 L s−1). Petrologic observation of solid products after experimental runs revealed that serpentine is produced from olivine. The hydration flux of serpentinization in this experiment is similar to estimates made using closed-system experiments, indicating that fluid flow does not affect the hydration rate at the flow rates adopted. Moreover, mass-balance calculations based on outlet fluid compositions during experiments suggest that both the overall serpentine precipitation and olivine dissolution rates decrease over time, and the serpentine precipitation rate is lower than the olivine dissolution rate during serpentinization. Although a comparison of speciated solutions observed in this experiment with results obtained by kinetic reaction modeling did not constrain the relationship between serpentine precipitation and olivine dissolution, a comparison with other experimental data and kinetic modeling results indicates that serpentine precipitation occurs more slowly than olivine dissolution at 170 °C. These results suggest that the relationship between serpentine precipitation and olivine dissolution kinetics varies with temperature and time, with such dynamic behavior governing serpentinization processes in the oceanic crust.

The Dissolution of Olivine Added to Soil at 4°C: Implications for Enhanced Weathering in Cold Regions

Crushed olivine was added to a soil core to mimic enhanced weathering, and water was continually dripped through for ~6 months. Our experiments were conducted at 4°C, and are compared to previously run identical experiments at 19°C. Olivine dissolution rates in both experiments start out similar, likely due to fines and sharp crystal corners. However, after >100 days of reaction, the dissolution rate at 4°C was two orders of magnitude lower than at 19°C. The accumulation of heavy metals, such as Ni and Cd, was low in both experiments, but soil retention of these elements was proportionally higher at higher temperatures, likely due to enhanced sorption and formation of clays. Overall, this study suggests that olivine dissolution rates in experiments that mimic natural settings are orders of magnitude slower than in normal laboratory experiments, and that enhanced weathering may be a considerably less efficient method of carbon dioxide removal at low climatic temperatures. Both of these conclusions have implications for the application of enhanced weathering as a CO2 removal method.

How the rock-inhabiting fungus K. petricola A95 enhances olivine dissolution through attachment

Free-living and mycorrhizal fungi are able to enhance the weathering of rock and other solid substrates. Deciphering the exact mechanisms of these natural processes requires their experimental simulation. Moreover, by performing these simulations with genetically amenable rock-weathering fungi, one can knock-out certain fungal traits and consequently identify their weathering-relevant function. Here, the effect of the rock-inhabiting fungus, Knufia petricola A95, on the dissolution kinetics of an Fe-bearing olivine (Mg1.86Fe0.19SiO4) is investigated at 25 °C and pH 6 using reproducible batch and mixed flow experiments. The availability of a melanin-deficient mutant (ΔKppks) of K. petricola A95, which produces more extracellular polymeric substances (EPS) than the wild type (WT), enables the comparative study of the role of melanin and EPS in olivine dissolution. In abiotic dissolution experiments, the olivine dissolution rate decreased considerably over time at pH 6 but not at pH 3.5. This inhibition of abiotic olivine dissolution at pH 6 was most likely caused by the in-situ oxidation of ferrous Fe and/or the precipitation of ferric hydroxides at the olivine surface. In corresponding biotic experiments at pH 6, both the wild type K. petricola and its melanin-deficient mutant ΔKppks solubilised and bound significant amounts of Fe released by olivine dissolution. Fe oxidation and precipitation were thus prevented and olivine dissolution proceeded faster than in the abiotic experiments. By sequestering Fe directly at the olivine surface, the attached wild type K. petricola cells were particularly efficient at preventing the oxidation of Fe at the mineral surface: the slowdown of olivine dissolution almost completely disappeared. The attachment capacity of these wild type cells is most likely mediated by wild type-specific EPS. Our presented experimental systems allow the oxidation of mineral-released Fe and include a rock-inhabiting fungus, thus simulating chemical, physical and biological conditions that set dissolution rates in a way that is relevant to natural ecosystems.

Olivine dissolution and hydrous Mg carbonate and silicate precipitation in the presence of microbial consortium of photo-autotrophic and heterotrophic bacteria

Olivine is an important mineral that controls the sequestration of atmospheric CO2 in the form of secondary carbonate minerals during chemical and biological weathering of mafic rocks on Earth. Despite significant efforts in characterization of olivine reactivity and coupled secondary mineral precipitation both in abiotic and biotic systems, little is known on olivine behavior in the presence of bacterial consortia, which are the dominant forms of microbial life impacting mineral reactivity in natural settings. To address this gap, we studied the interaction of olivine with a bacterial consortium composed of typical freshwater cyanobacterium Synechococcus sp. and heterotrophic aerobic Pseudomonas reactans, isolated from a CO2 storage site in Icelandic basalts. We quantified the impact of this consortium on the dissolution rate of olivine and we characterized the precipitation of secondary mineral phases while monitoring various biological (number of cells, bacterial biomass) and physicochemical (pH, Si, Mg, Ca, alkalinity, dissolved organic and inorganic carbon) parameters of the medium over a period of ∼21 days. Heterotrophic bacteria and their organic exometabolites enhanced the release of Mg and Si and produced leaching features (etch pits) at the olivine surface whereas cyanobacterial photosynthesis raised the pH and favored precipitation of hydrous Mg carbonates and silicates in the vicinity of the cells. During microbially-induced transformation of aquatic carbon, the latter was sequestered in the form of cyanobacterial biomass (about 66%) and their soluble organic exometabolites (11%), and stored as secondary Mg carbonates (23%). Overall, the impact of bacterial consortium is higher than that of individual species and may represent important and understudied biotically-controlled mechanism of CO2 sequestration in natural waters.

Mechanisms of olivine dissolution by rock-inhabiting fungi explored using magnesium stable isotopes

To unravel the dissolution mechanisms of olivine by a rock-inhabiting fungus we determined the stable isotope ratios of Mg on solutions released in a laboratory experiment. We found that in the presence of the fungus Knufia petricola the olivine dissolution rates were about seven-fold higher (1.04 × 10−15 mol cm−2 s−1) than those in the abiotic experiments (1.43 × 10−16 mol cm−2 s−1) conducted under the same experimental condition (pH 6, 25 °C, 94 days). Measured element concentrations and Mg isotope ratios in the supernatant solutions in both the biotic and the abiotic experiment followed a dissolution trend in the initial phase of the experiment, characterized by non-stoichiometric release of Mg and Si and preferential release of 24Mg over 26Mg. In a later phase, the data indicates stoichiometric release of Mg and Si, as well as isotopically congruent Mg release. We attribute the initial non-stoichiometric phase to the rapid replacement of Mg2+ in the olivine with H+ along with simultaneous polymerization of Si tetrahedra, resulting in high dissolution rates, and the stoichiometric phase to be influenced by the accumulation of a Si-rich amorphous layer that slowed olivine dissolution. We attribute the accelerated dissolution of olivine during the biotic experiment to physical attachment of K. petricola to the Si-rich amorphous layer of olivine which potentially results in its direct exposure to protons released by the fungal cells. These additional protons can diffuse through the Si-rich amorphous layer into the crystalline olivine. Our results also indicate the ability of K. petricola to dissolve Fe precipitates in the Si-rich amorphous layer either by protonation, or by Fe(III) chelation with siderophores. Such dissolution of Fe precipitates increases the porosity of the Si-rich amorphous layer and hence enhances olivine dissolution. The acceleration of mineral dissolution in the presence of a rock-dissolving fungus further suggests that its presence in surficial CO2 sequestration plants may aid to accelerate CO2 binding.

The kinetics of siderophore-mediated olivine dissolution.

Silicate minerals represent an important reservoir of nutrients at Earth's surface and a source of alkalinity that modulates long-term geochemical cycles. Due to the slow kinetics of primary silicate mineral dissolution and the potential for nutrient immobilization by secondary mineral precipitation, the bioavailability of many silicate-bound nutrients may be limited by the ability of micro-organisms to actively scavenge these nutrients via redox alteration and/or organic ligand production. In this study, we use targeted laboratory experiments with olivine and the siderophore deferoxamine B to explore how microbial ligands affect nutrient (Fe) release and the overall rate of mineral dissolution. Our results show that olivine dissolution rates are accelerated in the presence of micromolar concentrations of deferoxamine B. Based on the non-linear decrease in rates with time and formation of a Fe3+ -ligand complex, we attribute this acceleration in dissolution rates to the removal of an oxidized surface coating that forms during the dissolution of olivine at circum-neutral pH in the presence of O2 and the absence of organic ligands. While increases in dissolution rates are observed with micromolar concentrations of siderophores, it remains unclear whether such conditions could be realized in natural environments due to the strong physiological control on microbial siderophore production. So, to contextualize our experimental results, we also developed a feedback model, which considers how microbial physiology and ligand-promoted mineral dissolution kinetics interact to control the extent of biotic enhancement of dissolution rates expected for different environments. The model predicts that physiological feedbacks severely limit the extent to which dissolution rates may be enhanced by microbial activity, though the rate of physical transport modulates this limitation.

Olivine dissolution rates: A critical review

The dissolution rates of olivine have been considered by a plethora of studies in part due to its potential to aid in carbon storage and the ease in obtaining pure samples for laboratory experiments. Due to the relative simplicity of its dissolution mechanism, most of these studies provide mutually consistent results such that a comparison of their rates can provide insight into the reactivity of silicate minerals as a whole. Olivine dissolution is controlled by the breaking of octahedral M2+-oxygen bonds at or near the surface, liberating adjoining SiO44− tetrahedra to the aqueous fluid. Aqueous species that adsorb to these bonds apparently accelerate their destruction. For example, the absorption of H+, H2O and, at some conditions, selected aqueous organic species will increase olivine dissolution rates. Nevertheless, other factors can slow olivine dissolution rates. Notably, olivine dissolution rates are slowed by lowering the surface area exposed to the reactive aqueous fluid, by for example the presence and/or growth on these surfaces of either microbes or secondary phases. The degree to which secondary phases decelerate rates depends on their ability to limit access of the reactive aqueous fluids to the olivine surface. It seems likely that these surface area limiting processes are very significant in natural systems, lowering olivine surface reactivity in many environments compared to rates measured on cleaned surfaces in the laboratory.A survey of the literature suggests that the major factors influencing forsteritic olivine dissolution rates are 1) pH, 2) water activity, 3) temperature, and 4) mineral-fluid interfacial surface area. Evidence suggests that the effects of aqueous inorganic and organic species are relatively limited, and may be attributed at least in part to their influence on aqueous solution pH. Moreover, the observed decrease in rates due to the presence of secondary mineral coatings and/or the presence of microbes can be attributed to their ability to decrease olivine surface area directly exposed to the reactive aqueous fluid. As the reactivity of forsterite surfaces are spatially heterogeneous, its surface area normalized rates will tend to decrease as it dissolves even in the absence of a mineral or bacterial coating. Each of these factors limits and or influences the application of forsterite dissolution to 1) enhanced weathering efforts, 2) mineral carbonation, and 3) the low temperature generation of hydrogen or hydrocarbons via the oxidation of its divalent iron.

Reaction path modelling of in-situ mineralisation of CO2 at the CarbFix site at Hellisheidi, SW-Iceland

Results from injection of 175 tonnes of CO2 into the basaltic subsurface rocks at the CarbFix site in SW-Iceland in 2012 show almost complete mineralisation of the injected carbon in less than two years (Matter et al., 2016; Snæbjörnsdóttir et al., 2017). Reaction path modelling was performed to illuminate the rate and extent of CO2-water-rock reactions during and after the injection. The modelling calculations were constrained by the compositions of fluids sampled prior to, during, and after the injection, as reported by Alfredsson et al. (2013) and Snæbjörnsdóttir et al. (2017). The pH of the injected fluid, prior to CO2 dissolution was ∼9.5, whereas the pH of the background waters in the first monitoring well prior to the injections was ∼9.4. The pH of the sampled fluids used in the modelling ranged from ∼3.7 at the injection well to as high as 8.2 in the first monitoring well. Modelling results suggest that CO2-rich water-basalt interaction is dominated by crystalline basalt dissolution along a faster, high permeability flow path, but by basaltic glass dissolution along a slower, pervasive flow path through which the bulk of the injected fluid flows. Dissolution of pre-existing calcite at the onset of the injection does not have a net effect on the carbonation, but does contribute to a rapid early pH rise during the injection, and influences which carbonate minerals precipitate. At low pH, Mg, and Fe are preferentially released from crystalline basalts due to the higher dissolution rates of olivine, and to lesser extent pyroxene, compared to plagioclase and glass (Gudbrandsson et al., 2011). This favours the formation of siderite and Fe-Mg carbonates over calcite during early mineralisation. The model suggests the formation of the following carbonate mineral sequences: siderite at pH < 5, Mg-Fe-carbonates and Ca-Mg-Fe-carbonates at pH > 5, and calcite at higher pH. Other minerals forming with the carbonates are Al- and Fe-hydroxides and chalcedony, and zeolites and smectites at elevated pH. The most efficient carbonate formation is when the pH is high enough for formation of carbonates, but not so high that zeolites and smectites start to form, which compete with carbonates over both cations and pore space. The results of reaction path modelling at the CarbFix site in SW-Iceland indicate that this “sweet spot” for mineralisation of CO2 is at pH from ∼5.2 to 6.5 in basalts at low temperature (20–50 °C).

Effects of Synechococcus sp. cyanobacteria inert biomass on olivine dissolution: Implications for the application of enhanced weathering methods

Chemical weathering of silicates represents an essential process of both the rock and carbon cycles. The application of this natural process on a global scale could be used to mitigate excess CO2 in the atmosphere. This concept is known as enhanced weathering where fine mineral powder is spread over the land surface for carbonation of silicates and expected CO2 sequestration. Within this context, organic matter, may promote or inhibit the processes of enhanced weathering, however this has not yet been fully quantified. Motivated by this knowledge gap, the present work studied the dissolution behaviour of olivine under inorganic conditions and in the presence of inert Synechococcus sp. biomass in batch reactor setups at various ionic strengths with a constant input of atmospheric CO2. Olivine, which represents one of the most obvious candidates for use in enhanced weathering, showed no significant statistical differences in the release of dissolved cations for all studied organic and inorganic experimental setups. For batch reactors containing inert biomass, a moderate increase in alkalinity and pH is found with respect to the inorganic conditions, which points to the buffering of H2CO3 by deprotonated functional groups on the inert biomass surface. Overall, the experimental results of this study indicate a negative combined effect of inert biomass and high ionic strengths on the olivine dissolution rates in natural aquatic systems. This suggests reduced carbonation rates on the Earth's surface and a lower potential of artificially dispersed olivine powder for CO2 sequestration.

Olivine dissolution in molten silicates: An experimental study with application to chondrule formation

AbstractMg‐rich olivine is a ubiquitous phase in type I porphyritic chondrules in various classes of chondritic meteorites. The anhedral shape of olivine grains, their size distribution, as well as their poikilitic textures within low‐Ca pyroxene suggest that olivines suffer dissolution during chondrule formation. Owing to a set of high‐temperature experiments (1450–1540 °C) we determined the kinetics of resorption of forsterite in molten silicates, using for the first time X‐ray microtomography. Results indicate that forsterite dissolution in chondrule‐like melts is a very fast process with rates that range from ~5 μm min−1 to ~22 μm min−1. Forsterite dissolution strongly depends on the melt composition, with rates decreasing with increasing the magnesium and/or the silica content of the melt. An empirical model based on forsterite saturation and viscosity of the starting melt composition successfully reproduces the forsteritic olivine dissolution rates as a function of temperature and composition for both our experiments and those of the literature. Application of our results to chondrules could explain the textures of zoned type I chondrules during their formation by gas‐melt interaction. We show that the olivine/liquid ratio on one hand and the silica entrance from the gas phase (SiOg) into the chondrule melt on the other hand, have counteracting effects on the Mg‐rich olivine dissolution behavior. Silica entrance would favor dissolution by maintaining disequilibrium between olivine and melt. Hence, this would explain the preferential dissolution of olivine as well as the preferential abundances of pyroxene at the margins of chondrules. Incipient dissolution would also occur in the silica‐poorer melt of chondrule core but should be followed by crystallization of new olivine (overgrowth and/or newly grown crystals). While explaining textures and grain size distributions of olivines, as well as the centripetal distribution of low‐Ca pyroxene in porphyritic chondrules, this scenario could also be consistent with the diverse chemical, isotopic, and thermal conditions recorded by olivines in a given chondrule.

Olivine dissolution from Indian dunite in saline water.

The rate and mechanism of olivine dissolution was studied using naturally weathered dunite FO98.21(Mg1.884Fe0.391SiO4) from an Indian source, that also contains serpentine mineral lizardite. A series of batch dissolution experiments were carried out to check the influence of temperature (30-75 ∘C), initial dunite concentration (0.5 and 20 g/L), and salinity (0-35 g/L NaCl) under fixed head space CO2 pressure (P[Formula: see text] = 1 barg) on dunite dissolution. Dissolved Mg, Si, and Fe concentrations were determined by inductive coupled plasma atomic emission spectroscopy. End-product solids were characterized by scanning electron microscopy and X-ray diffraction. Initially, rates of dissolution of Si and Mg were observed to be in stoichiometric proportion. After 8 h, the dissolution rate was observed to decline. At the end of the experiment (504 h), an amorphous silica-rich layer was observed over the dunite surface. This results in decay of the dissolution rate. The operating conditions (i.e., salinity, temperature, and mineral loading) affect the dissolution kinetics in a very complex manner because of which the observed experimental trends do not exhibit a direct trend.

Aqueous corrosion of olivine in the Mars meteorite Miller Range (MIL) 03346 during Antarctic weathering: Implications for water on Mars

Several nakhlites (clinopyroxenite meteorites from Mars) contain olivine phenocrysts with corrosion features identical in size, shape and distribution to the smaller etch-pits of well-characterized weathered terrestrial olivine. Miller Range (MIL) 03346 is an Antarctic nakhlite find, recovered after long exposure to Antarctic conditions. The distribution of discrete olivine etch-pits almost exclusively within a few hundred microns of allocation MIL 03346,171’s documentably exposed surface suggests that they formed by terrestrial weathering in Antarctica. The small size of olivine etch-pits in MIL 03346,171 relative to commonly much larger etch-pits in even incipiently weathered terrestrial examples suggests that the duration of its exposure to weathering conditions was short, or the weathering conditions to which it was exposed did not favor olivine corrosion (in the form of etch-pit formation), or both. Time-scales for the formation of etch-pits, estimated from experimentally determined dissolution rates of olivine over a range of pHs, are comparable to the measured terrestrial age of the meteorite and short relative to the time available for possible similar corrosion on Mars. Etch-pits of the observed size on MIL 03346 olivine phenocrysts would be relatively easy to form supraglacially under brief episodic acidic Antarctic conditions, but the terrestrial age of MIL 03346 is long enough that its olivine might have been weathered to the observed state by englacial films of alkaline Antarctic water. The paucity of similar etch-pits in olivine from the interior of MIL 03346 suggests that olivine in this Mars meteorite was exposed to even less aqueous alteration after iddingsitization during its 1.3 billion years on Mars than its exterior was subjected to during its Pleistocene–Holocene exposure to Antarctic weathering conditions.

Some Compositional and Kinetic Controls on the Bioenergetic Landscapes in Oceanic Basement.

This contribution assesses the availability of catabolic energy for microbial life during water–rock reactions in the flanks of mid-ocean ridges, where basaltic and ultramafic rocks interact with circulating seawater. In addition to equilibrium thermodynamic computations, results for kinetic reaction paths are presented. In these calculations, it is assumed that dissolution of olivine and basalt glass control the rates of hydrogen forming reactions in ultramafic and basaltic rocks, respectively. The results suggest that all ocean crust basement rocks release enough hydrogen (H2,aq) to support hydrogenotrophic life at low water-to-rock ratios. Olivine dissolution rate control imposes a stronger effect on hydrogen production than phase equilibrium controls, indicating that magnetite formation is not a requirement for production of large amounts of hydrogen in ultramafic rocks. The formation of non-tronite and celadonite are primarily responsible for the formation of the moderate amounts of hydrogen (H2,aq) expected in basaltic ridge flanks. Under conditions of large seawater fluxes required to account for the great global convective heat flow in ridge flanks, however, hydrogen production in basaltic ridge flanks is insufficient for supporting hydrogenotrophic life. It is hence proposed that the role of Fe oxidation in basaltic ridge flanks is greater than previously suggested. A standing stock of 2.4∗1028 cells may be supported by Fe oxidation in basaltic ridge flanks, equivalent of about 10% of the sedimentary deep biosphere. The size of a hydrogenotrophic biomass within the ocean crust is more difficult to estimate because the rates and processes of hydrogen release are insufficiently constrained. In any case, hydrogenotrophy in the ocean crust should be of key importance only in olivine-rich basement rocks and in sedimented ridge flanks with low time-integrated seawater fluxes.