In situ analysis of major and trace elements of anhydrous minerals by LA-ICP-MS without applying an internal standard

http://www.geo.umass.edu/petrology/Giordano%20et%20al_EPSL_2008_visc.pdf

Viscosity of magmatic liquids: A model

A pyrope-quartzite originally described by Vialon (1966) from the Dora Maira massif was resampled and reinvestigated. Garnet (up to 25 cm in size), phengite, kyanite, talc and rutile are in textural equilibrium in an undeformed matrix of polygonal quartz. The garnet is a pyrope-almandine solid solution with 90 to 98 mol % Mg end-member. It contains inclusions of coesite which has partially inverted to quartz, resulting in a typical radial cracking of the host garnet around the inclusions. Several lines of evidence show that coesite crystallised under nearly static pressure conditions and that the whole matrix has once been coesite. The formidable pressures of formation implied (≧28 kbar) are independently indicated by i) the coexistence of nearly pure pyrope with free silica and talc, ii) the coexistence of jadeite with kyanite, iii) the high Si content of phengite. Water activity must have been low. The stability of talc-phengite and the presence of rare glaucophane inclusions in pyrope point to low formation temperatures (about 700 °C) and to a probable Alpine age for the assemblage. This is evidence that low temperature gradients, how essentially transient they are, may nevertheless persist to considerable depths. Moreover, the upper crustal (evaporite-related?) origin of the quartzite and its interbedding within a continental unit implies that continental crust may also be subducted to depths of 90 km or more. The return back to the surface is problematic; the retrograde assemblages observed show that it must be tectonic. If the rocks remain at depth, new perspectives open for the genesis of intermediate to acidic magmas. Eventually, the role of continental crust in geodynamics may have to be reconsidered.

Coesite and pure pyrope in high-grade blueschists of the Western Alps: a first record and some consequences

Most minerals of Earth's upper mantle contain small amounts of hydrogen, structurally bound as hydroxyl (OH). The OH concentration in each mineral species is variable, in some cases reflecting the geological environment of mineral formation. Of the major mantle minerals, pyroxenes are the most hydrous, typically containing ∼200 to 500 parts per million H_2O by weight, and probably dominate the water budget and hydrogen geochemistry of mantle rocks that do not contain a hydrous phase. Garnets and olivines commonly contain ∼1 to 50 parts per million. Nominally anhydrous minerals constitute a significant reservoir for mantle hydrogen, possibly accommodating all water in the depleted mantle and providing a possible mechanism to recycle water from Earth's surface into the deep mantle.

https://science.sciencemag.org/content/sci/255/5050/1391.full.pdf

Water in Earth's Mantle: The Role of Nominally Anhydrous Minerals

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

Several of the supposedly anhydrous major minerals of the upper mantle have been shown to regularly contain small amounts of hydrogen. The concentrations measured in the most important minerals obtained from mantle xenoliths are, expressed in ppm H 2 O, 100-1300 for clinopyroxene, 60-650 for orthopyroxene, 0- 140 for olivine and 1-200 for garnet. Hydrogen is normally structurally incorporated as hydroxyl ions, and in many cases the hydrogen ions seem to act as charge compensators associated with point defects, such as metal vacancies or substitution by mono- or trivalent cations. The determination of the exact amount of hydrogen stored in these nominally anhydrous upper mantle minerals is a key-step toward quantification of the water content of the mantle, as well as understanding of its internal water cycle. For instance, a concentration of 100 ppm H 2 O homogeneously distributed within the upper mantle above 410 km depth is approximately equivalent to a 100 m water layer at the Earth9s surface. However, the relatively fast kinetics of dehydrogenation with concomitant oxidation of iron within these minerals, implies that hydrogen as well as Fe 3+ concentrations in equilibrium with mantle conditions might be different from those measured from recovered xenolith samples. High-pressure experimental measurements of hydrogen solubility as a function of PH 2 O show a trend similar to the hydrogen contents of natural samples, with hydrogen saturation levels that decrease following the mineral series: diopside > enstatite > olivine > pyrope. Except pyrope, these minerals may incorporate more than 1000 ppm H 2 O. Based on recent data of water solubility, stability and partitioning, we suggest that an entire upper mantle saturated in hydrogen is highly unprobable and that the maximum average amount of hydrogen stored in the nominally anhydrous minerals of the upper mantle is around 600 ppm H 2 O. Despite the important progress achieved during the last years, our knowledge of the concentration of hydrogen stored as point defects in the mantle above 410 km is still too poorly constrained. The importance of nominally anhydrous minerals for the water budget of the upper mantle is now well established but still awaits complete quantification.

Hydrogen in nominally anhydrous upper-mantle minerals concentration levels and implications

Hydrogen in nominally anhydrous upper-mantle minerals: concentration levels and implications;Hydrogen in nominally anhydrous upper-mantle minerals: concentration levels and implications

The speciation and amount of water dissolved in nominally anhydrous silicates comprising eight different mantle xenoliths has been quantified using synchrotron micro-FTIR spectroscopy. Samples studied are from six geographic localities and represent a cross-section of the major upper mantle lithologies from a variety of tectonic settings. Clinopyroxene contains between 342 and 413 ppm H2O. Orthopyroxene, olivine and garnet contain 169–201, 3–54 and 0 to <3 ppm H2O, respectively. Pyroxenes water contents and the distribution of water between ortho- and clinopyroxene is identical regardless of sample mineralogy (D
 water
 cpx/opx
  = 2.1 ± 0.1). The total water contents of each xenolith are remarkably similar (113 ± 14 ppm H2O). High-resolution spectroscopic traverses show that the concentration and speciation of hydrous defects dissolved in each phase are spatially homogeneous within individual crystals and identical in different crystals interspersed throughout the xenolith. These results suggest that the amount of water dissolved in the silicate phases is in partial equilibrium with the transporting melt. Other features indicate that xenoliths have also preserved OH signatures of equilibrium with the mantle source region: Hydroxyl stretching modes in clinopyroxene show that garnet lherzolites re-equilibrated under more reducing conditions than spinel lherzolites. The distribution of water between pyroxenes and olivine differs according to xenolith mineralogy. The distribution of water between clinopyroxene and olivine from garnet peridotites (D
 water
 cpx/oliv
 (gnt) = 22.2 ± 24.1) is a factor of four greater than mineral pairs from spinel-bearing xenoliths (D
 water
 cpx/oliv
 (sp) = 88.1 ± 47.8). Such an increase in olivine water contents at the spinel to garnet transition is likely a global phenomenon and this discontinuity could lead to a reduction of the upper mantle viscosity by 0.2–0.7 log units and a reduction of its electrical resistivity by a factor of 0.5–0.8 log units.

Water partitioning between mantle minerals from peridotite xenoliths

Coesite in subducted continental crust: P-T history deduced from an elastic model

Measurement of the rate of diffusion of hydrous species in anhydrous minerals has been a subject of growing interest for the last ten years. Hydrogen diffusion studies are of fundamental importance to estimate the rate of transfer of hydrous species in natural conditions but also to identify the nature of the reactions involved in the speciation of hydrogen in minerals. For low concentration hydrous point defects, spectroscopic techniques are usually not sufficient to fully determine the location of hydrogen in the mineral structure. In these cases, kinetics studies provide additional constraints on the number of different defects stored in the mineral and their specific reaction rates.

Numerous experimental data of hydrogen diffusion in minerals have now been published in the Earth Sciences literature. This review provides a digest to these data for a variety of minerals with a special focus on hydrogen in nominally anhydrous minerals. The review will not address the question of molecular water diffusion in minerals, melts or glasses. For a recent review of that question see for instance Doremus (2002). After a brief outline of basic diffusion concepts and methods of measurement, we summarize the main features of atomic diffusion linked to hydrogen isotope exchange and hydrogen reaction in minerals.

Atomic diffusion is defined as the transport of matter in response to a driving force, which can be a chemical potential gradient or a temperature gradient. Atomic diffusion induced by a thermal gradient is very limited in solid earth sciences whereas the occurrence of strong chemical gradients is much more frequent in minerals. Atoms move from one region to the other in order to equilibrate the chemical potentials μ and reduce the Gibbs free energy of the system. The disequilibrium induced by a chemical potential gradient ( d μ/ dx )i of a component i …

Jannick Ingrin

Papers

Hydrogen in nominally anhydrous upper-mantle minerals concentration levels and implications

Hydrogen in nominally anhydrous upper-mantle minerals: concentration levels and implications;Hydrogen in nominally anhydrous upper-mantle minerals: concentration levels and implications

Water partitioning between mantle minerals from peridotite xenoliths

Coesite in subducted continental crust: P-T history deduced from an elastic model

Diffusion of Hydrogen in Minerals