Knowledge of temperature and pressure, however qualitative, has been central to our views of geology since at least the early 19th century. In 1822, for example, Charles Daubeny presented what may be the very first “Geological Thermometer,” comparing temperatures of various geologic processes (Torrens 2006). Daubeny (1835) may even have been the first to measure the temperature of a lava flow, by laying a thermometer on the top of a flow at Vesuvius—albeit several months following the eruption, after intervening rain (his estimate was 390°F). In any case, pressure ( P ) and temperature ( T ) estimation lie at the heart of fundamental questions: How hot is Earth, and at what rate has the planet cooled. Are volcanoes the products of thermally driven mantle plumes? Where are magmas stored, and how are they transported to the surface—and how do storage and transport relate to plate tectonics? Well-calibrated thermometers and barometers are essential tools if we are to fully appreciate the driving forces and inner workings of volcanic systems.

This chapter presents methods to estimate the P-T conditions of volcanic and other igneous processes. The coverage includes a review of existing geothermometers and geobarometers, and a presentation of approximately 30 new models, including a new plagioclase-liquid hygrometer. Our emphasis is on experimentally calibrated “thermobarometers,” based on analytic expressions using P or T as dependent variables. For numerical reasons (touched on below) such expressions will always provide the most accurate means of P-T estimation, and are also most easily employed. Analytical expressions also allow error to be ascertained; in the absence of estimates of error, P-T estimates are nearly meaningless. This chapter is intended to complement the chapters by Anderson et al. (2008), who cover granitic systems, and by Blundy and Cashman (2008) and Hansteen and Klugel (2008), who consider additional methods …

Thermometers and Barometers for Volcanic Systems

The composition of the primitive mantle derived here shows that Earth was assembled from material that shows many of the same chemical fractionation processes as chondritic meteorites. These processes occurred at the initial stage of the solar system formation, under conditions thought to be present in the solar nebula. But the stable isotope record excludes chondritic meteorites as the ‘building blocks’ of Earth. Meteorites formed in local environments separated from that part of the inner solar system where much of the material forming the terrestrial planets was sourced.

https://booksite.elsevier.com/brochures/treatiseongeochemistry/contents/sample2.pdf

Cosmochemical Estimates of Mantle Composition

The Cretaceous-Paleogene boundary similar to 65.5 million years ago marks one of the three largest mass extinctions in the past 500 million years. The extinction event coincided with a large asteroid impact at Chicxulub, Mexico, and occurred within the time of Deccan flood basalt volcanism in India. Here, we synthesize records of the global stratigraphy across this boundary to assess the proposed causes of the mass extinction. Notably, a single ejecta-rich deposit compositionally linked to the Chicxulub impact is globally distributed at the Cretaceous-Paleogene boundary. The temporal match between the ejecta layer and the onset of the extinctions and the agreement of ecological patterns in the fossil record with modeled environmental perturbations (for example, darkness and cooling) lead us to conclude that the Chicxulub impact triggered the mass extinction.

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The Chicxulub Asteroid Impact and Mass Extinction at the Cretaceous-Paleogene Boundary

Whenever a mineral or mineral assemblage comes into contact with a fluid with which it is out of equilibrium, reequilibration will tend to take place to reduce the free energy of the whole system (i.e., of the solid + fluid). Such fluid-solid interactions span a very wide range of possible reactions, and are responsible for most of the mineral assemblages we see in the Earth’s crust. However, before discussing mineral replacement reactions, we will put them into the broader context of fluid-solid interactions by considering some examples of such reequilibration.

In the simplest case, we could consider the situation where a mineral, thermodynamically stable under some speciflc temperature and pressure conditions, comes into contact with pure water, such as quartz within its stability field (e.g., at T = 100 °C and 1 atmosphere pressure). Clearly, quartz will tend to dissolve until, at equilibrium, the aqueous silica solution, which at neutral pH is H4SiO4(aq), becomes saturated with respect to quartz. The reaction for this equilibration can be written:

\batchmode \documentclass[fleqn,10pt,legalpaper]{article} \usepackage{amssymb} \usepackage{amsfonts} \usepackage{amsmath} \pagestyle{empty} \begin{document} \[SiO\_{2\ (qtz)}\ +\ H\_{2}O\ {\leftrightarrow}\ H\_{4}SiO\_{4(aq)}\] \end{document}(1) 

The equilibrium solubility constant for reaction (1) is given by

\batchmode \documentclass[fleqn,10pt,legalpaper]{article} \usepackage{amssymb} \usepackage{amsfonts} \usepackage{amsmath} \pagestyle{empty} \begin{document} \[\mathit{K}\_{\mathit{sp}}\ (\mathit{qtz})\ =\ \frac{\mathit{a}(H\_{4}SiO\_{4})\_{\mathit{aq}}}{\mathit{a}(SiO\_{2})\_{\mathit{qtz}}\ {\cdot}\ \mathit{a}(H\_{2}O)}\ =\ \mathit{a}(H\_{4}SiO\_{4})\_{\mathit{aq}}\] \end{document} 

where a ( i ) aq stands for the activity of the parenthetical aqueous species. For the case of pure quartz and pure water where the activity is 1, K sp ( qtz ) ~1.2 × 10−3 at 100 °C and 1 atmosphere pressure. However, if under these conditions the solid silica phase in contact with pure water was cristobalite (the high temperature polymorph of SiO2), the resulting aqueous solution, saturated with respect to cristobalite, would be supersaturated with respect to quartz, since the less stable phase is more soluble. The value of K sp for cristobalite at 100 °C and 1 atmosphere pressure is ~5.1 × 10−3. Thus the thermodynamics would indicate that quartz …

Mineral Replacement Reactions

Natural zircon crystals often show complex secondary textures that cut across primary growth zones. In zircon showing structural damage caused by self-irradiation, such textures are the result of a diffusion-reaction process in which a hydrous species diffuses inwards and “catalyzes” structural recovery. Nanoscale pores develop, solvent elements such as Ca, Al and Fe are gained, and radiogenic Pb is lost. In both aqueous fluids and melts, replacement of zircon with undamaged structure by a coupled dissolution-reprecipitation process can produce similar textures. The reacted domains usually have lower trace element contents and may contain micrometer-sized pores and inclusions of uranium, thorium and/or yttrium phases, originally in solid solution. Both processes have considerable implications for zircon geochronology.

Re-equilibration of Zircon in Aqueous Fluids and Melts

Crystallization experiments were performed at 200MPa in the temperature range 1150–950 C at oxygen fugacities corresponding to the quartz–fayalite–magnetite (QFM) and MnO–Mn3O4 buffers to assess the role of water and fO2 on phase relations and differentiation trends in mid-ocean ridge basalt (MORB) systems. Starting from a primitive (MgO 9 8 wt %) and an evolved MORB (MgO 6 49 wt %), crystallization paths with four different water contents (0 35–4 7 wt % H2O) have been investigated. In primitive MORB, olivine is the liquidus phase followed by plagioclase þ clinopyroxene. Amphibole is present only at water-saturated conditions below 1000 C, but not all fluid-saturated runs contain amphibole. Magnetite and orthopyroxene are not stable at low fO2 (QFM buffer). Residual liquids obtained at low fO2 show a tholeiitic differentiation trend. The crystallization of magnetite at high fO2 (MnO–Mn3O4 buffer) results in a decrease of melt FeO */ MgO ratio, causing a calc-alkaline differentiation trend. Because the magnetite crystallization temperature is nearly independent of the H2O content, in contrast to silicate minerals, the calc-alkaline differentiation trend is more pronounced at high water contents. Residual melts at 950 C in a primitive MORB system have compositions approaching those of oceanic plagiogranites in terms of SiO2 and K2O, but have Ca/Na ratios and FeO * contents that are too high compared with the natural rocks, implying that fractionation processes are necessary to reach typical compositions of natural oceanic plagiogranites.

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An Experimental Investigation of the Influence of Water and Oxygen Fugacity on Differentiation of MORB at 200 MPa

Small amounts of felsic, evolved plutonic rocks, often called oceanic plagiogranites, always occur as veins or small stocks within the gabbroic section of the oceanic crust. Four major models are under debate to explain the formation of these rocks: (1) late-stage differentiation of a parental MORB melt, (2) partial melting of gabbroic rocks, (3) immiscibility in an evolved tholeiitic liquid, and (4) assimilation and partial melting of previously altered dikes. Recent experimental data in hydrous MORB-type systems are used to evaluate the petrogenesis of oceanic plagiogranites within the deep oceanic crust. Experiments show that TiO2 is a key parameter for the discrimination between different processes: TiO2 is relatively low in melts generated by anatexis of gabbros which is a consequence of the low TiO2 contents of the protolith, due to the depleted nature of typical cumulate gabbros formed in the oceanic crust. On the other hand, TiO2 is relatively high in those melts generated by MORB differentiation or liquid immiscibility. Since the TiO2 content of many oceanic plagiogranites is far below that expected in case of a generation by simple MORB differentiation or immiscibility, these rocks may be regarded as products of anatexis. This may indicate that partial melting processes triggered by water-rich fluids are more common in the deep oceanic crust than believed up to now. At slow-spreading ridges, seawater may be transported via high-temperature shear zones deeply into the crust and thus made available for melting processes.

The formation of SiO 2 -rich melts within the deep oceanic crust by hydrous partial melting of gabbros

This study presents improvements of internally heated pressure vessels to realize high-pressure experiments at controlled f O 2 in low-viscosity systems such as basaltic ones. The new design is a combination of two experimental techniques: a hydrogen sensor membrane made of platinum to measure f H 2 , and therefore f O 2 , and a rapid-quench system to avoid crystallization of low-viscosity melts during quench. The experimental setup has been tested successfully at temperatures up to 1250 °C and pressures up to 500 MPa. Basaltic melts containing up to 9.38 wt% water can be quenched as bubble-free and crystal-free glasses. The improvements allow synthesis of hydrated glass or partly crystallized samples with a large volume (for further studies) and to perform routine phase-equilibrium studies in basaltic systems at geologically relevant conditions. We used the new technique to determine the effect of f O 2 on water solubility in a melt with MORB composition. The results show that there is a small but significant decrease of water solubility with decreasing f O 2 from MnO-Mn 3 O 4 to QFM buffer conditions in the pressure range 50–200 MPa. Kinetic problems in crystallization experiments in basaltic systems and the duration necessary to attain equilibrium Fe 2+ /Fe 3+ ratio in the charge are discussed.

A combined rapid-quench and H2-membrane setup for internally heated pressure vessels: Description and application for water solubility in basaltic melts

During partial melting in the earth’s mantle, the noble metals become fractionated. Os, Ir, Ru, and Rh tend to remain in the mantle residue whereas Pt, Pd, and Re behave mildly incompatible and are sequestered to the silicate melt. There is consensus that sulfide plays a role in the fractionation process; the major noble metal repository in the mantle is sulfide, and most primitive mantle melts are sulfide-saturated when they leave their mantle sources. However, with sulfide–silicate partitioning, the fractionation cannot be modeled properly. All sulfide–silicate partition coefficients are so extremely high that a silicate melt segregating from a mantle source with residual sulfide should be largely platinum-group elements free. We offer a physical alternative to sulfide–silicate chemical partitioning and provide a mechanism of generating a noble metal-rich melt from a sulfide-saturated source: Because sulfide is at least partially molten at asthenospheric temperature, it will behave physically incompatible during melt segregation, and a silicate melt segregating from a mantle residue will entrain molten residual sulfide in suspension and incorporate it in the basaltic pool melt. The noble metal abundances of a basalt then become independent of sulfide–silicate chemical partitioning. They reflect the noble metal abundances in the drained sulfide fraction as well as the total amount of sulfide entrained. Contrary to convention, we suggest that a fertile, sulfide-rich mantle source has more potential to generate a noble metal-enriched basaltic melt than a refractory mantle source depleted by previous partial melting events.

Fractionation of the noble metals by physical processes

One important application of borosilicate glass is its use as a nuclear waste form to immobilize high-level nuclear waste Understanding the corrosion mechanism of borosilicate glasses in aqueous solutions is essential to reliably predict their long-term behavior in the worst-case scenario of glass-groundwater contact in a geologic repository Traditional models evaluate the long-term corrosion process on the basis of diffusion-controlled hydration and ion exchange reactions that are followed by solid-state reconstruction of the hydrolyzed glass network Here we report textural, chemical, and 18 O and 26 Mg isotope tracer results from corrosion experiments with a borosilicate glass in an acidic aqueous solution (initial pH ≈ 0, T  = 150 °C, 6 to 336 h) that contradict such a paradigm We propose a new mechanistic model for glass corrosion under acidic conditions that is based on congruent (stoichiometric) dissolution of the glass that is spatially and temporally coupled to the precipitation of amorphous silica at an inward moving reaction front The model potentially provides a novel framework to understand apparently contradictory observations made under more moderate conditions and to evaluate the long-term aqueous durability of silicate glasses

Jasper Berndt

Papers

An Experimental Investigation of the Influence of Water and Oxygen Fugacity on Differentiation of MORB at 200 MPa

The formation of SiO 2 -rich melts within the deep oceanic crust by hydrous partial melting of gabbros

A combined rapid-quench and H2-membrane setup for internally heated pressure vessels: Description and application for water solubility in basaltic melts

Fractionation of the noble metals by physical processes

Aqueous corrosion of borosilicate glass under acidic conditions: A new corrosion mechanism