Experimental investigation of the mechanism of the reaction: 1 tremolite+11 dolomite ⇌ 8 forsterite+13 calcite+9 CO2+1H2O

Stoichiometric mixtures of tremolite and dolomite were heated to 50° C above equilibrium temperatures to form forsterite and calcite. The pressure of the CO₂-H₂O fluid was 5 Kb and \(X_{{\text{CO}}_{\text{2}} }\) varied from 0.1 to 0.6. The extent of the conversion was determined by the amount of CO₂ produced. The resulting mixtures of unreacted tremolite and dolomite and of newly-formed forsterite and calcite were examined with a scanning electron microscope. All tremolite and dolomite grains showed obvious signs of dissolution. At fluid compositions with \(X_{{\text{CO}}_{\text{2}} }\) less than about 0.4, the forsterite and calcite crystals are randomly distributed throughout the charges, indicating that surfaces of the reactants are not a controlling factor with respect to the sites of nucleation of the products. A change is observed when \(X_{{\text{CO}}_{\text{2}} }\) is greater than about 0.4; the forsterite and calcite crystals now nucleate and grow at the surface of the dolomite grains, thus indicating a change in mechanism at medium CO₂ concentrations. As the reaction progresses, the dolomite grains become more and more surrounded by forsterite and calcite, finally forming armoured relics of dolomite. Under experimental conditions this characteristic texture can only be formed if the CO₂-concentration is greater than about 40 mole %. These findings make it possible to estimate the CO₂-concentration from the texture of the dolomite+tremolite+forsterite+calcite assemblage. The results suggest a dissolution-precipitation mechanism for the reaction investigated. In a simplified form it consists of the following 4 steps:

1. Dissolution of the reactants tremolite and dolomite.
2. Diffusion of the dissolved constituents in the fluid.
3. Heterogeneous nucleation of the product minerals.
4. Growth of forsterite and calcite from the fluid.

Two possible explanations are discussed for the development of the amoured texture at \(X_{{\text{CO}}_{\text{2}} }\) above 0.4. The first is based upon the assumption that dolomite has a lower rate of dissolution than tremolite at high \(X_{{\text{CO}}_{\text{2}} }\) values resulting in preferential calcite and forsterite nucleation and growth on the dolomite surface. An alternative explanation is the formation of a raised CO₂ concentration around the dolomite grains at high \(X_{{\text{CO}}_{\text{2}} }\) values, leading to product precipitation on the dolomite crystals.     

X-ray diffraction study of the orientational order/disorder transition in NaNO3: Evidence for order parameter coupling

The orientational ordering transition R \(R\bar 3m - R\bar 3c\) in NaNO₃ near 552 K has been investigated using x-ray diffraction techniques. NaNO₃ is a model system for CaCO₃ and other minerals with orientational disorder of triangular molecules in a simple NaCl-type matrix. The temperature evolution of the integrated intensities of the superlattice reflection \(\bar 1\) 23 and the fundamental reflection 110 are discussed in terms of Landau theory of two coupled order parameters. It is shown that the known phenomenological critical exponent (Poon and Salje 1988) and the anomalous thermal expansion at T > T _tr (Reeder et al. 1988) can be understood as the result of a Z point instability which mainly describes the NO ₃ ^- disorder, and a second order parameter linked with the spontaneous strain of this phase transition.     

Tephra fallout hazard assessment at the Campi Flegrei caldera (Italy)

Tephra fallout associated with renewal of volcanism at the Campi Flegrei caldera is a serious threat to the Neapolitan area. In order to assess the hazards related with tephra loading, we have considered three different eruption scenarios representative of past activity: a high-magnitude event similar to the 4.1 ka Agnano-Monte Spina eruption, a medium-magnitude event, similar to the ∼3.8 ka Astroni 6 eruption, and a low-magnitude event similar to the Averno 2 eruption. The fallout deposits were reconstructed using the HAZMAP computational model, which is based on a semi-analytical solution of the two-dimensional advection–diffusion–sedimentation equation for volcanic tephra. The input parameters into the model, such as total erupted mass, eruption column height, and bulk grain-size and components distribution, were obtained by best-fitting field data. We carried out tens of thousands simulations using a statistical set of wind profiles, obtained from NOAA re-analysis. Probability maps, relative to the considered scenarios, were constructed for several tephra loads, such as 200, 300 and 400 kg/m². These provide a hazard assessment for roof collapses due to tephra loading that can be used for risk mitigation plans in the area.     

Computational reproducibility in geoscientific papers: Insights from a series of studies with geoscientists and a reproduction study

Reproducibility is a cornerstone of science and thus for geographic research as well. However, studies in other disciplines such as biology have shown that published work is rarely reproducible. To assess the state of reproducibility, specifically computational reproducibility (i.e. rerunning the analysis of a paper using the original code), in geographic research, we asked geoscientists about this topic using three methods: a survey (n = 146), interviews (n = 9), and a focus group (n = 5). We asked participants about their understanding of open reproducible research (ORR), how much it is practiced, and what obstacles hinder ORR. We found that participants had different understandings of ORR and that there are several obstacles for authors and readers (e.g. effort, lack of openness). Then, in order to complement the subjective feedback from the participants, we tried to reproduce the results of papers that use spatial statistics to address problems in the geosciences. We selected 41 open access papers from Copernicus and Journal of Statistical Software and executed the R code. In doing so, we identified several technical issues and specific issues with the reproduced figures depicting the results. Based on these findings, we propose guidelines for authors to overcome the issues around reproducibility in the computational geosciences.     

Measurement of the Diurnal Variation of the OH Radical Concentration and Analysis of the Data by Modelling

Daily variations of the hydroxyl radical concentration have been measured during a campaign at the Weybourne Atmospheric Observatory (WAO) in June 1995. These measurements are compared with box model calculations, based on a slightly modified, second generation Regional Acid Deposition Model (RADM2). Results from eight days of the comparison are presented. A detailed analysis and discussion of the different source and sink terms is given for two days: Julian Day (JD) 170 (19 June, and 178 (27 June). In both cases excellent agreement between the measurements and the calculation is obtained, indicating that the model describes the OH chemistry sufficiently well. Furthermore, the analysis of these days demonstrate that JD 170 is dominated by the NOx catalysed OH production, whereas JD 178 is influenced by OH formation via ozone photolysis.     

Beginning of melting in the granite system Qz-Or-Ab-An-H2O

The beginning of melting in the system Qz-Or-Ab-An-H₂ O was experimentally reversed in the pressure range kbar using starting materials made up of mixtures of quartz and synthetic feldspars. With increasing pressure the melting temperature decreases from 690° C at 2 kbar to 630° C at 17 kbar in the An-free alkalifeldspar granite system Qz-Or-Ab-H₂O. In the granite system Qz-Or-Ab-An-H₂O the increase of the solidus temperature with increasing An-content is only very small. In comparison to the alkalifeldspar granite system the solidus temperature increases by 3° C (7° C) if albite is replaced by plagioclase An 20 (An 40). The difference between the solidus temperatures of the alkalifeldspar granite system and of quartz — anorthite — sanidine assemblages (system Qz-Or-An-H₂O) is approximately 50° C. With increasing water pressures plagioclase and plagioclase-alkalifeldspar assemblages become unstable and are replaced by zoisite+kyanite+quartz and zoisite+muscovite-paragonite_ss +quartz, respectively. The pressure stability limits of these assemblages are found to lie between 6 and 16 kbar at 600° C. At high water pressures (10–18 kbar) zoisite — muscovite — quartz assemblages are stable up to 700 and 720° C. The solidus curve of this assemblage is 10–20° C above the beginning of melting of sanidine — zoisite — muscovite — quartz mixtures. The amount of water necessary to produce sufficient amounts of melt to change a metamorphic rock into a magmatic looking one is only small. In case of layered migmatites it is shown that 1 % of water (or even less) is sufficient to transform portions of a gneiss into (magmatic looking) leucosomes. High grade metamorphic rocks were probably relatively dry, and anatectic magmas of granitic or granodioritic composition are usually not saturated with water.     

Thermal history of the upper mantle beneath a young back-arc extensional zone: ultramafic xenoliths from San Luis Potosí, Central Mexico

At the San Luis Potosí (SLP) volcanic field (Central Mexico), Quaternary basanites and tuff breccias have sampled a suite of ultramafic xenoliths, predominately spinel lherzolites, spinel-olivine websterites, spinel pyroxenites, and hornblende-rich pyroxenites. Spinel lherzolites from the La Ventura maars have protogranular to equigranular textures, those from the Santo Domingo maars are strongly sheared. Both spinel-lherzolite types show similar whole-rock major and trace-element abundances. They are fertile to slightly depleted with mineralogical and geochemical heterogeneities induced by partial melting processes. Pyroxenites with either magmatic or metamorphic textures are high-pressure cumulates. Hornblende-rich pyroxenites are genetically linked to the host basanites. Most of the protogranular spinel lherzolites contain veinlets of glass along grain boundaries. These glasses are chemically homogeneous and have trachybasaltic to trachyandesitic compositions. Mg- and Fe²⁺-partitioning between olivine and glass suggests chemical equilibrium between the melts represented by the glasses and the spinel-lherzolite mineral assemblage at about 1,000°C and 10 to 15 kbar. The melts are interpreted to be of upper mantle origin. They may have been formed by in-situ partial melting in the presence of volatiles or represent percolating melts chemically buffered by the spinel-lherzolite mineral assemblage at uppermost mantle conditions. Mineral chemistry in all rock types of the whole xenolith suite reveals distinct disequilibrium features reflecting partial re-equilibration stages towards lower temperatures estimated to be from 1,050°C to 850°C at 9 to 15 kbar. The presence of similar zoning and exsolution features mainly documented in pyroxenes along with similar maximum and minimum temperatures requires all sampled xenoliths to have undergone the same temperature regime within the upper mantle. The sheared spinel lherzolites from the Sto. Domingo field are interpreted as formerly protogranular material which was sheared during uplift and cooling. The estimated mantle temperatures are higher than those predicted by low heat-flow measurements at the SLP fild, indicating that surface heat flow has not equilibrated to elevated temperatures at depth. This strongly supports a young perturbation event beneath the SLP area and connects the onset of uplift and cooling of the SLP-mantle segment with the back-arc extensional regime of the Quaternary volcanic cycle of the Transmexican Volcanic Belt.