Fresh Rock Surfaces Research Articles

Widespread clay authigenesis and highly congruent silicate weathering in the Marinoan aftermath

The termination of the Cryogenian period marks a transition from an extreme ice- to a greenhouse climate, likely initiated by increased atmospheric CO2, however uncertainties persist on continental weathering regarding this climate transition. Here we present paired lithium isotope data from carbonate (δ7Licarb) and carbonate-hosted silicate (δ7Lisili) components of the Marinoan cap carbonates to constrain C and Si cycles in the deglacial period. We find that the silicate component of cap carbonates is dominated by marine authigenic clays with minor contributions from terrestrially detrital materials and oxides. Built from δ7Lisili data, large variations of seawater δ7Li suggest a heterogeneous marine Li reservoir due to a stratified ocean in the deglacial period. Overall, widespread clay authigenesis and low seawater δ7Li values indicate active carbon-silicon cycling and congruent silicate weathering due to warm climate and exposure of fresh rock surface in the Marinoan aftermath, which promotes the deposition of cap carbonates and stabilization of Earth's surface environment.

Lichenometric Dating and Its Limitations and Problems: A Guide for Practitioners

Lichenometry, a method for dating rock surface exposure mainly in high latitudes and mountain environments, is based on estimates of lichen growth rates, but over the last 70 years it has been severely criticised. Its chief limitation is its questionable reliability due to three main problems: the species belonging to the Rhizocarpon subgenus, most often used by lichenometrists, are hard to identify; growth studies have highlighted the intrinsic variability of growth both seasonally and annually, with species sensitively responding to a wide range of environmental factors; and the same sensitive dependence also applies to the time taken for colonization on fresh rock surfaces. These problems cast doubt on many absolute dating studies and critics have suggested that, at best, the technique should only be used for relative dating. This paper provides guidance on identification procedures and suggests alternative dating methods based on lichen size-frequency distributions and cross-dating with other lichen species, thus avoiding reliance on a single species or support from other methods. With appropriate development, it is hoped that these approaches can provide a way forward that allows the technique to contribute more reliably to the dating of rock surfaces in regions where there are few other dating options.

The Diverse Indigenous Bacterial Community in the Rudna Mine Does Not Cause Dissolution of Copper from Kupferschiefer in Oxic Conditions

Blasting and fracking of rock in mines exposes fresh rock surfaces to the local water and microbial communities. This may lead to leaching of metals from the rock by chemical or biological means and can cause acidification of the water system in the mine, i.e., acid rock drainage (ARD). Failure to prevent leakage of metal contaminated mine water may be harmful for the environment, especially to the local groundwater. In the Rudna mine, Poland, an in situ bioleaching pilot test at approximately 1 km depth was performed in the H2020 BIOMOre project (Grant Agreement #642456). After the leaching stage, different methods for irreversible inhibition of acidophilic iron oxidizing microorganisms used for reoxidation of reduced iron in the leaching solution were tested and were shown to be effective. However, the potential of the natural mine water microbial communities to cause leaching of copper or acidification of the mine waters has not been tested. In this study, we set up a microcosm experiment simulating the exposure of freshly fractionated Kupferschiefer sandstone or black schist to two different chloride-rich water types in the Rudna mine. The pH of the microcosms water was measured over time. At the end of an 18-week incubation, the bacterial community was examined by high throughput sequencing and qPCR, and the presence of copper tolerant heterotrophic bacteria was tested by cultivation. The dissolution of copper into the chloride rich microcosm water was measured. The pH in the microcosms did not decrease over the time of incubation. The sandstone increased the number of bacteria in the microcosms with one or over two orders of magnitude compared to the original water. The bacterial communities in the two tested mine waters were diverse and similar despite the difference in salinity. The bacterial diversity was high but changed in the less saline water during the incubation. There was a high content of sulphate reducing bacteria in the original mine waters and in the microcosms, and their number increased during the incubation. No acidophilic iron oxidizers were detected, but in the microcosms containing the less saline water low numbers of Cu tolerant bacteria were detected. Copper to a concentration of up to 939 mg L−1 was leached from the rock also in the microbe-free negative controls, which was up to 2.4 times that leached in the biotic microcosms, indicating that the leaching was also abiotic, not only caused by bacteria.

Estimation of rock Fe content based on hyperspectral indices

Information on the Fe content of bare rocks is needed for implementing geochemical processes and identifying mines. However, the influence of Fe content on the spectra of bare rocks has not been thoroughly analyzed in previous studies. The Saur Mountain region within the Hoboksar of the Russell Hill depression was selected as the study area. Specifically, we analyzed six hyperspectral indices related to rock Fe content based on laboratory measurements (Dataset I) and field measurements (Dataset II). In situ field measurements were acquired to verify the laboratory measurements. Fe content of the rock samples collected from different fresh and weathered rock surfaces were divided into six levels to reveal the spatial distributions of Fe content of these samples. In addition, we clearly displayed wavelengths with obvious characteristics by analyzing the spectra of these samples. The results of this work indicated that Fe content estimation models based on the fresh rock surface measurements in the laboratory can be applied to in situ field or satellite-based measurements of Fe content of the weathered rock surfaces. It is not the best way to use only the single wavelengths reflectance at all absorption wavelengths or the depth of these absorption features to estimate Fe content. Based on sample data analysis, the comparison with other indices revealed that the performance of the modified normalized difference index is the best indicator for estimating rock Fe content, with R2 values of 0.45 and 0.40 corresponding to datasets I and II, respectively. Hence, the modified normalized difference index (the wavelengths of 2220, 2290, and 2370 nm) identified in this study could contribute considerably to improve the identification accuracy of rock Fe content in the bare rock areas. The method proposed in this study can obviously provide an efficient solution for large-scale rock Fe content measurements in the field.

Characteristics and mechanism of hydrocarbon alteration of faded sandstone in the uranium-bearing Luohe Formation, Pengyang area, southwestern Ordos Basin

Investigating hydrocarbon alteration characteristics and the alteration mechanism of faded sandstone in uranium-bearing red beds is important for understanding the relationship between hydrocarbon-bearing fluids and uranium mineralization. In this study, a series of experiments were conducted on the faded alteration sandstone of the Cretaceous Luohe Formation in Pengyang area of the southwestern Ordos Basin. The results show that the faded alteration of the Luohe Formation is consistent with temporal and spatial pyritization and uranium mineralization and that the alteration degree can be divided into three alteration levels: gray green to gray, yellow to gray white, and reddish brown to light red. Moreover, hydrocarbon alteration is observed in the faded sandstones with good permeability in the Luohe Formation, appearing as oil traces on fresh rock surface, diffuse fluorescence in the cement, hydrocarbon inclusions around fractures, and organic components in pores. In addition, the content of hydrocarbon increases with increasing faded alteration degree. The following results are obtained: (1) The most typical mineralogical response characteristics of hydrocarbon alteration are the decreasing change of trivalent iron oxide and the enrichment of pyrite and uranium minerals with a symbiotic relationship. (2) The hydrocarbon alteration intensity is generally weakened from bottom to bottom and enhanced from southwest to northeast. Except for in the northwest, which is controlled by fault migration, hydrocarbon micro leakage is the main mechanism. (3) Hydrocarbon alteration as a reduction mechanism is mainly affected by the sedimentary environment of the Luohe Formation and the oxidation conditions of the surface fluid, scale of fault, permeability of deep rock, and intensity of hydrocarbon generation and emission from deep source rocks.

Ground vibration generated by hydraulic rock breakers and its impact on the safety of nearby earthfill dam in a graphite mine

A detailed impact analysis on the effects of non-explosive chemicals and hydraulic rock breakers at a graphite mine in India close to an irrigation dam was carried out in this study. The dam was 70 m from the mining lease boundary. When a single hydraulic rock breaker worked on fractured rock mass, the magnitude of ground vibration recorded at 10 m on the same working bench was 2.37 mm s −1 but when it worked on fresh rock surface the ground vibration at 9 m on the same working bench was 4.67 mm s −1 . Beyond 35 m on the same bench, no ground vibrations were recorded. Ground vibrations generated by three jack hammer drilling machines operating simultaneously on the same working bench were less than 0.5 mm s −1 when measured at 7.2 m distance. The combined effect on the magnitudes of ground vibrations by two hydraulic rock breakers operating simultaneously on the same bench 6 m apart was found to be negligible. The ground vibrations generated by hydraulic rock breakers and other mining activities were found to be lower than required to cause any structural damage or threat to the stability of the dam.

A new method for dating the surface exposure age of granite rock walls in the Mont Blanc massif by reflectance spectroscopy

In the high mountain rock walls of the Mont Blanc massif, changes in the granite surface colour are related to its exposure age. The light grey colour of fresh rock surfaces turns orange when is long exposed to weathering. In order to study this colour/age relationship, reflectance spectroscopy was performed on 73 samples, and Terrestrial Cosmogenic Nuclide (TCN) dating was used to obtain their surface exposure age. The standard deviation of the reflectance values was calculated for each wavelength of the visible spectrum to study the behaviour of each spectral region. The aim was to find two colour regions that showed opposite behaviour, and once they are combined, they could provide a representative index of the rock colour changes that are linked to the degree of weathering. As an adaptation of the Green Red Vegetation Index used to measure colour changes in vegetation, the GReen-Infrared GRanite Index (GRIGRI), a normalized difference between the granite 770 nm and 530 nm reflectance values, was developed. The GRIGRI value of a weathered granite surface has a close relationship with its exposure age (R 2 = 0.85). The reflectance spectra of seven samples for which TCN dating failed and two samples for which the TCN age was considered to be an outlier were used to calculate their GRIGRI value to assess the colour-based ages, that were plausible according to rock wall morphology and the TCN exposure ages of the surrounding surfaces. We propose a new method of surface dating for the rock walls of the Mont Blanc massif using reflectance spectroscopy. • Mont Blanc granite surface colour is related to the time that it has been exposed to weathering. • Reflectance spectroscopy was used to quantify granite surface colour. • TCN dating was used to estimate granite surface exposure age. • We use different approaches to explore the relationship between the granite colour and its exposure age. • The ratio between the Green and Infrared reflectance values can be used to estimate granite exposure age.

Initial colonisation by Rhizocarpon geographicum in the Tatra Mountains

Previous lichenometric dating of young geomorphological forms in the Tatra Mountains has been hindered by differences in the determination of initial colonisation of fresh rock surfaces by the Rhizocarpon geographicum lichen. In order to determine precisely the time needed for the first thalli to appear on newly exposed rock surfaces, two experimental sites were created in 2013. The results from these sites were compared with the measurements taken on boulders in debris formed by a 2013 debris flow. In 2018, the largest thalli measured within the experimental sites and the debris flow area reached a size of approximately 2 mm. Based on this, the time of initial colonisation of rock surfaces was determined to be 5 ± 1 years, which is about half the period previously described in the literature.

Microseismic Events Cause Significant pH Drops in Groundwater

AbstractEarthquakes cause rock fracturing, opening new flow pathways which can result in the mixing of previously isolated geofluids with differing geochemistries. Here, we present the first evidence that seismic events can significantly reduce groundwater pH without the requirement for fluid mixing, solely through the process of dynamic rock fracturing. At the Grimsel Test Site, Switzerland, we observe repeated, short‐lived groundwater pH drops of 1–3.5 units, while major and minor ion groundwater concentrations remain constant. Acidification coincides with reservoir drainage and induced microseismic events. In laboratory experiments, we demonstrate that fresh rock surfaces made by particle cracking interact with the in situ water molecules, likely through creation of surface silanols and silica radicals, increasing the H+ concentration and significantly lowering groundwater pH. Our findings are significant; pH exerts a fundamental control on the rate and outcome of most aqueous geochemical reactions and microseismic events are commonplace, even in seismically inactive regions.

Spectral properties of weathered and fresh rock surfaces in the Xiemisitai metallogenic belt, NW Xinjiang, China

AbstractSurfaces weathering of rocks in which mineral materials may be similar to or quite different from the minerals in the underlying parent rock completely control the reflectance spectra of the terrain. Our study of typical weathered and fresh rock samples from the Xiemisitai metallogenic belt, Western Junggar region, Xinjiang, found that weathering results in the formation of new materials that cause differences in the spectral features of fresh and weathered rock surfaces. Alterations induce variations in spectrum brightness, presence and intensity of characteristic absorption features, and spectral slope. Spectral differences between weathered and fresh rock surfaces are small for rhyolite, granite, and tuffaceous sandstone, but large for andesite, basalt, and diorite. Spectral changes in the 350–1000 nm wavelength region are attributed to alteration of iron oxides by atmospheric processes or secondary alteration of iron-rich minerals. Spectral features between 1000–2500 nm are caused by O–H vibrations, with features at 2200–2500 nm solely attributed to hydroxyl groups. The strongest Al–OH bands appear near 2200 nm, while Mg–OH bands are found near 2300 nm and 2350 nm. Results from this study can be used to better characterize and discriminate lithological units and potential mineral zones using hyperspectral and multispectral remote sensing techniques.

Limits to lichenometry

Lichenometry is a straightforward and inexpensive method for dating Holocene rock surfaces. The rationale is that the diameter of the largest lichen scales with the age of the originally fresh rock surface that it colonised. The success of the method depends on finding the largest lichen diameters, a suitable lichen-growth model, and a robust calibration curve. Recent critique of the method motivates us to revisit the accuracy and uncertainties of lichenometry. Specifically, we test how well lichenometry is capable of resolving the ages of different lobes of large active rock glaciers in the Kyrgyz Tien Shan. We use a bootstrapped quantile regression to calibrate local growth curves of Xanthoria elegans, Aspicilia tianshanica, and Rhizocarpon geographicum, and report a nonlinear decrease in dating accuracy with increasing lichen diameter. A Bayesian type of an analysis of variance demonstrates that our calibration allows discriminating credibly between rock-glacier lobes of different ages despite the uncertainties tied to sample size and correctly identifying the largest lichen thalli. Our results also show that calibration error grows with lichen size, so that the separability of rock-glacier lobes of different ages decreases, while the tendency to assign coeval ages increases. The abundant young (<200 yr) specimen of fast-growing X. elegans are in contrast with the fewer, slow-growing, but older (200–1500 yr) R. geographicum and A. tianshanica, and record either a regional reactivation of lobes in the past 200 years, or simply a censoring effect of lichen mortality during early phases of colonisation. The high variance of lichen sizes captures the activity of rock-glacier lobes, which is difficult to explain by regional climatic cooling or earthquake triggers alone. Therefore, we caution against inferring palaeoclimatic conditions from the topographic position of rock-glacier lobes. We conclude that lichenometry works better as a tool for establishing a relative, rather than an absolute, chronology of rock-glacier lobes in the northern Tien Shan.

Effects and quantification of acid runoff from sulfide-bearing rock deposited during construction of Highway E18, Norway

The Highway E18 between the cities of Grimstad and Kristiansand, southern Norway, constructed in the period 2006–2009, cuts through sulfide-bearing rock. The geology of this area is dominated by slowly-weathering gneiss and granites, and oxidation of fresh rock surfaces can result in acidification of surface water. Sulfide-containing rock waste from excavations during construction work was therefore deposited in three waste rock deposits off-site. The deposits consist of 630,000–2,360,000metric tons of waste rock material. Shell sand and limestone gravel were added in layers in adequate amounts to mitigate initial acid runoff in one of the deposits. The shell sand addition was not adequate in the two others. The pH in the effluents from these two was reduced from 4.9–6.5 to 4.0–4.6, and Al concentrations increased from below 0.4mg/L to 10–20mg/L. Stream concentrations of trace metals increased by a factor of 25–400, highest for Ni, and then in decreasing order for Co, Mn, Cd, Zn and Cu. Concentrations of As, Cr and Fe remained unchanged. Ratios of Co/Ni and Cd/Zn indicate that the metal sources for these pair of metals are sphalerite and pyrite, respectively. Based on surveys and established critical limits for Al, surface waters downstream became toxic to fish and invertebrates. The sulfur release rates were remarkably stable in the monitoring period at all three sites. Annual sulfur release was 0.1–0.4% of the total amount of sulfur in the deposit, indicating release periods of 250–800years. Precipitates of Al-hydroxysulfates, well-known from mining sites, were found at the base of the deposits, in streams and also along the ocean shore-line. The effects of added neutralization agents in the deposits and in treatment areas downstream gradually decreased, as indicated by reduced stream pH over time. Active measures are needed to avoid harmful ecological effects in the future.

Tracing euxinia by molybdenum concentrations in sediments using handheld X-ray fluorescence spectroscopy (HHXRF)

Elevated molybdenum (Mo) contents in organic-rich sediments are indicative of deposition from an anoxic and sulfide-rich (euxinic) water-column. This can be used for tracing past euxinic conditions in ancient oceans from sedimentary archives. Conventional analytical detection of elevated molybdenum levels is, however, expensive and cannot be directly performed in the field. Here, we show that handheld x-ray absorption spectroscopy (HHXRF) on both rock-powders and rock surfaces provides a powerful tool for laboratory or field-based, geochemical characterization of Mo content and the tracing of euxinic depositional conditions. We further present a sedimentary Mo database, compiled from over 300 globally distributed modern marine sites, to show that modern euxinic marine basins are characterized by molybdenum enrichment (>25ppm) relative to average crust (~1.5ppm) and river suspended matter (~3ppm). Only Mn-oxide rich deposits from oxygenated deep sea settings match such high Mo enrichments, but these are easily distinguished from Mo-rich sediments from the continental shelves. Hence, past and present organic rich sediments enriched in Mo are indicative of (at least intermittent) water-column euxinia.We analyzed the molybdenum content of powdered and homogenized samples of Holocene, Miocene, Triassic–Jurassic, Ordovician and Cambrian age by HHXRF and Inductively-Coupled-Plasma-Mass-Spectrometry (ICPMS). We further analyzed 2 certified NIST standard references (NIST-2781 with Mo: 47ppm and NIST-2702 with Mo: 10ppm) for analytical control. Analytical precision (1σ) after 30, 120, and 300seconds of measuring time was 4, 2, and 1ppm, with a respective detection limit of 11, 5, 3ppm (3σ, noise level). The data were accurate to within the given precision (1σ) after a daily calibration to samples covering a range of Mo concentrations from 0 to >30ppm.Hand-held XRF equipment also allows Mo measurements directly on fresh rock surfaces, both in the field and under laboratory conditions. Rock-samples from a Cambrian drill core closely match ICPMS obtained Mo concentrations from homogenous powders of the same rocks. Collectively, our data show that HHXRF as a quick and reliable method for the precise and accurate quantification of molybdenum concentrations in both rock powders and at fresh rock surfaces. The non-destructive nature of the HHXRF measurements is combined with low (per sample) analytical costs, minor sample preparation and easy applicability in the field.

Development of alteration rinds by oxidative weathering processes in Beacon Valley, Antarctica, and implications for Mars

Alteration of fresh rock surfaces proceeds very rapidly in most terrestrial environments so that initial stages of modification of newly exposed surfaces are quickly masked by subsequent aqueous weathering processes. The hyper-arid and hypo-thermal environment of Beacon Valley, Antarctica, is limited in terms of available liquid water and energy available for alteration, which severely slows weathering processes so that the initial stages of alteration can be studied in detail. We report on the nature of initial chemical alteration of the Ferrar Dolerite in Beacon Valley, Antarctica, using a multiplicity of approaches to characterize the process. We suggest that initial chemical alteration is primarily driven by cation migration in response to the oxidizing environment. Morphological studies of altered rock surfaces reveal evidence of small-scale leaching and dissolution patterns as well as physical erosion due to surface weakening. Within the alteration front, mineral structures are largely preserved and alteration is only indicated by discrete zones of discoloration. Mineralogical investigations expose the complexity of the alteration process; visible/near-infrared reflectance and mid-infrared emission spectroscopy reveal significant variations in mineralogical contributions that are consistent with the introduction of oxide and amorphous phases at the surfaces of the rocks, while X-ray diffraction analyses reveal no definitive changes in mineralogy or material properties. Chemical analyses reveal large-scale trends that are consistent with cation migration and leaching, while small-scale electron microprobe analyses indicate that chemical variations associated with magmatic processes are still largely preserved within the alteration rind. This work confirms the incomplete and immature chemical alteration processes at work in the McMurdo Dry Valleys. Liquid water is not a significant contributor to the alteration process at this early stage of rind development, but assists in the removal of alteration products and their local accumulation in the surrounding sediments. These results also suggest that the McMurdo Dry Valleys (and Beacon Valley, in particular) are relevant terrestrial analogs to hyper-arid and hypo-thermal alteration processes that may be dominant on the martian surface.

Permian lysocline shoaling and ocean acidification along NW Pangea led to carbonate eradication and chert expansion

The Upper Carboniferous to Upper Permian deposits of the Sverdrup Basin (Arctic Canada) record a transition from carbonate to silica dominated shallow shelf ecosystems. Chert expansion started during the latest Carboniferous in distal deep-water slope environments when prolific warm-water, photozoan carbonate factories developed on the adjacent shallow shelves. A shift to heterozoan factories during the Sakmarian–Artinskian was caused by the introduction of cooler water in response to paleoceanographic changes along NW Pangea. By Kungurian time, heterozoan carbonate factories narrowed substantially while hyalosponge factories had expanded well into the mid shelves. A brief return to widespread heterozoan carbonate sedimentation occurred during the Wordian, followed by further encroachment of shallow shelves by biosiliceous factories during the Capitanian and Wuchiapingian. A major drop in relative rates of carbonate sediment accumulation with time suggests both carbonate and biosiliceous factories were under considerable stress prior to the Late Permian Extinction event. It is proposed that Sverdrup Basin waters became progressively more acidic in response to build up of atmospheric CO2 throughout the Permian which led to a gradual shoaling of the lysocline and the calcite compensation depth, which was amplified by upwelling along the northwestern margin of Pangea. Ocean acidification initiated in response to amalgamation of the Pangea supercontinent. This inhibited the silicate weathering-response through development of thick protective soil blankets. A temperature shock associated with the extinction event led to a major fall of the lysocline allowing carbonates to resume accumulation in spite of low pH conditions. The subsequent die off of terrestrial vegetation, stripping of soil cover, widespread continental erosion and repeated exposures of fresh rock surfaces associated with a major base level shift allowed silicate weathering to restart and drive oceans back to carbonate saturation.

Terrestrial Cosmogenic Nuclide Geochronology Data Reporting Standards Needed

Scientists can estimate the time at which rocks at Earth's surface became exposed (through glacial scour, faulting, sediment deposition, exhumation, etc.) in a given area using terrestrial cosmogenic nuclide (TCN) geochronology. The idea behind this technique is ingenious in its simplicity. Imagine, for example, a glacier advancing over the landscape and scouring the rocks underneath. As the glacier retreats, fresh rock surfaces become exposed to the atmosphere. Galactic cosmic rays then bombard the fresh minerals exposed at the Earth's surface, producing rare nuclides such as beryllium‐10 (10Be) and aluminum‐26 (26Al) in the process. Thus, measuring the concentration of TCNs in rocks at the Earth's surface allows scientists to estimate how long a surface has been exposed and/or the rate of surface denudation.

The volcanic acidification of glacial Lake Caviahue, Province of Neuquen, Argentina

Lake Caviahue (northern Patagonia, Argentina) is a large glacial lake acidified by volcanic fluids from Copahue volcano. The lake and the feeding rivers were sampled annually from 1997 till early 2006, including the eruptive period of 2000. Lake Caviahue waters evolved over time, with the most concentrated waters in 2000 during the eruptive period, followed by gradual dilution that was interrupted by renewed acidification in 2003–2004. Inversion of the lake water data and application of a dynamic non-steady state model for the lake provides our best quantitative estimates for the variation in element fluxes over the 9-year period. The model flux results agree well with most of the measured fluxes. The Copahue hydrothermal system had gently declining element fluxes between 1997 and mid-1999, although the lake was still becoming more concentrated. About 2–3 months before the 2000 eruption, element fluxes increased strongly, but the hydrothermal fluxes almost shutoff directly after the main eruptive events. The fluxes of several elements recovered post-2001, with an increase in element fluxes in 2003–2004; the lake became more dilute between 2004 and 2006. The intrusion of new magma into the hydrothermal system just prior to the 2000 eruption led to enhanced water rock interaction, with higher concentrations of the rock forming elements in the fluids, and the hot spring flow rate increased as a result of the higher pressure in the reservoir. The fluids became saturated in alunite and jarosite, and they were already saturated with anhydrite. Precipitation of these minerals possibly led to a decreased permeability of the hydrothermal reservoir, leading to the strongly reduced element fluxes just after the eruption. In addition, K, Al and S were retained in the newly precipitated minerals as well, further diminishing their export. The acidification in 2003–2004 may have resulted from a new small intrusion of magma or resulted from seismic activity that created new permeability and fresh rock surfaces for water rock interaction. The volcano is a significant source of toxic trace elements such as F, As, B and Li as well as a nutrient (P) for the local watershed. Monitoring of the hydrothermal fluids in the river that drains Copahue, especially the S/Cl, Mg/Cl and Mg/K values as well as the magnitude of the element fluxes would provide the best information for eruption forecasting for this volcano.

Mineralogy at Gusev Crater from the Mössbauer spectrometer on the Spirit Rover.

Mössbauer spectra measured on Mars by the Spirit rover during the primary mission are characterized by two ferrous iron doublets (olivine and probably pyroxene) and a ferric iron doublet (tentatively associated to nanophase ferric iron oxide). Two sextets resulting from nonstoichiometric magnetite are also present, except for a coating on the rock Mazatzal, where a hematite-like sextet is present. Greater proportions of ferric-bearing phases are associated with undisturbed soils and rock surfaces as compared to fresh rock surfaces exposed by grinding. The ubiquitous presence of olivine in soil suggests that physical rather than chemical weathering processes currently dominate at Gusev crater.

Thermal infrared analysis of weathered granitic rock compositions in the Sacaton Mountains, Arizona: Implications for petrologic classifications from thermal infrared remote‐sensing data

Critical to spectral interpretations of geologic surfaces on the Earth and Mars is an understanding of the relationship between the composition of weathered rock surfaces and whole‐rock mineralogy. In this study, thermal infrared spectroscopic and remote‐sensing analyses were used to determine the composition of weathered granitoid rock surfaces in the Sacaton Mountains, Arizona. A linear spectral deconvolution approach was employed to determine the mineralogies of naturally exposed, weathered surfaces and artificially exposed, fresh rock surfaces. Deconvolution results from fresh rock surfaces yield similar bulk mineralogy to results from point counting of rock slabs and thin sections. Deconvolution of weathered rock spectra indicate that compared to fresh samples, weathered surfaces are deficient in plagioclase feldspar and are enriched in clay minerals. The differential weathering of feldspar minerals impacts the interpretation of whole‐rock mineralogy and the spectral classification of bedrock geology, which, for plutonic rocks, is dependent on the relative proportions of different feldspars. Deconvolutions of multispectral remote‐sensing or laboratory data show a deficiency of total feldspar in the weathered rock units but lack sufficient spectral resolution for the discrimination between different feldspars. These results demonstrate that the differential breakdown and removal of primary minerals in rock surfaces due to weathering can affect the interpreted rock petrology from remote‐sensing studies. The relationships observed in our granitic samples could be applied to any rock type and may be important to consider for remote sensing of Mars.

Catalysts for Fine Chemical Synthesis: hydrolysis, oxidation, and reduction

Lake Caviahue (northern Patagonia, Argentina) is a large glacial lake acidified by volcanic fluids from Copahue volcano. The lake and the feeding rivers were sampled annually from 1997 till early 2006, including the eruptive period of 2000. Lake Caviahue waters evolved over time, with the most concentrated waters in 2000 during the eruptive period, followed by gradual dilution that was interrupted by renewed acidification in 2003–2004. Inversion of the lake water data and application of a dynamic non-steady state model for the lake provides our best quantitative estimates for the variation in element fluxes over the 9-year period. The model flux results agree well with most of the measured fluxes. The Copahue hydrothermal system had gently declining element fluxes between 1997 and mid-1999, although the lake was still becoming more concentrated. About 2–3 months before the 2000 eruption, element fluxes increased strongly, but the hydrothermal fluxes almost shutoff directly after the main eruptive events. The fluxes of several elements recovered post-2001, with an increase in element fluxes in 2003–2004; the lake became more dilute between 2004 and 2006. The intrusion of new magma into the hydrothermal system just prior to the 2000 eruption led to enhanced water rock interaction, with higher concentrations of the rock forming elements in the fluids, and the hot spring flow rate increased as a result of the higher pressure in the reservoir. The fluids became saturated in alunite and jarosite, and they were already saturated with anhydrite. Precipitation of these minerals possibly led to a decreased permeability of the hydrothermal reservoir, leading to the strongly reduced element fluxes just after the eruption. In addition, K, Al and S were retained in the newly precipitated minerals as well, further diminishing their export. The acidification in 2003–2004 may have resulted from a new small intrusion of magma or resulted from seismic activity that created new permeability and fresh rock surfaces for water rock interaction. The volcano is a significant source of toxic trace elements such as F, As, B and Li as well as a nutrient (P) for the local watershed. Monitoring of the hydrothermal fluids in the river that drains Copahue, especially the S/Cl, Mg/Cl and Mg/K values as well as the magnitude of the element fluxes would provide the best information for eruption forecasting for this volcano.