Water in nominally anhydrous minerals

507 Pages·2006·9.246 MB·English

by  Hans Keppler, J R Smyth

REVIEWS MINERALOGY
IN
GEOCHEMISTRY
AND
Volume 62 2006
WATER IN NOMINALLY
ANHYDROUS MINERALS
EDITORS
Hans Keppler Joseph R. Smyth
Universität Bayreuth University Colorado
Bayreuth, Germany Boulder, Colorado
COVER PHOTOGRAPH: Thin section of a garnet lherzolite mantle xenolith from
Pali-Aike, Patagonia. The almost colorless grains are olivine, orthopyroxene is
brownish-green, clinopyroxene bright green and garnet is red. Grain size is about 1
mm. Photograph courtesy of Sylvie Demouchy.
Series Editor: Jodi J. Rosso
GEOCHEMICAL SOCIETY
MINERALOGICAL SOCIETY OF AMERICA
SHORT COURSE SERIES DEDICATION
Dr. William C. Luth has had a long and distinguished career in research,
education and in the government. He was a leader in experimental petrology
and in training graduate students at Stanford University. His eff orts at
Sandia National Laboratory and at the Department of Energy’s headquarters
resulted in the initiation and long-term support of many of the cu(cid:308) ing edge
research projects whose results form the foundations of these short courses.
Bill’s broad interest in understanding fundamental geochemical processes and
their applications to national problems is a continuous thread through both
his university and government career. He retired in 1996, but his eff orts to
foster excellent basic research, and to promote the development of advanced
analytical capabilities gave a unique focus to the basic research portfolio in
Geosciences at the Department of Energy. He has been, and continues to be,
a friend and mentor to many of us. It is appropriate to celebrate his career in
education and government service with this series of courses.
Reviews in Mineralogy and Geochemistry, Volume 62
Water in Nominally Anhydrous Minerals
ISSN 1529-6466
ISBN 0-939950-74-X
Copyright 2006
THE MINERALOGICAL SOCIETY OF AMERICA
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Mineralogical Society of America as to the royalty due to the Society.
WATER in NOMINALLY
ANHYDROUS MINERALS
62 Reviews in Mineralogy and Geochemistry 62
FROM THE SERIES EDITOR
The review chapters in this volume were the basis for a four day short course on Water in
Nominally Anhydrous Minerals held in Verbania, Lago Maggiore, Italy (October 1-4, 2006).
The editors Hans Keppler and Joe Smyth have done an excellent job organizing this volume
and the associated short course. Meeting deadlines (often ahead of schedule!) and keeping track
of so many authors can be a thankless job at times but I truly appreciate all their hard work.
Hans’ “friendly reminder” e-mails certainly kept us all on task and his eye for detail (small and
large) made my job much more enjoyable! I extend my sincere thanks to him for his efforts!
Any supplemental material and errata (if any) can be found at the MSA website
www.minsocam.org.
Jodi J. Rosso, Series Editor
West Richland, Washington
August 2006
PREFACE
Earth is a water planet. Oceans of liquid water dominate the surface processes of the planet.
On the surface, water controls weathering as well as transport and deposition of sediments.
Liquid water is necessary for life. In the interior, water fl uxes melting and controls the solid-
state viscosity of the convecting mantle and so controls volcanism and tectonics. Oceans cover
more than 70% of the surface but make up only about 0.025% of the planet’s mass. Hydrogen
is the most abundant element in the cosmos, but in the bulk Earth, it is one of the most poorly
constrained chemical compositional variables.
Almost all of the nominally anhydrous minerals that compose the Earth’s crust and mantle
can incorporate measurable amounts of hydrogen. Because these are minerals that contain
oxygen as the principal anion, the major incorporation mechanism is as hydroxyl, OH−, and
the chemical component is equivalent to water, HO. Although the hydrogen proton can be
2
considered a monovalent cation, it does not occupy same structural position as a typical cation
in a mineral structure, but rather forms a hydrogen bond with the oxygens on the edge of the
coordination polyhedron. The amount incorporated is thus quite sensitive to pressure and the
amount of H that can be incorporated in these phases generally increases with pressure and
sometimes with temperature. Hydrogen solubility in nominally anhydrous minerals is thus
much more sensitive to temperature and pressure than that of other elements.
Because the mass of rock in the mantle is so large relative to ocean mass, the amount that is
incorporated the nominally anhydrous phases of the interior may constitute the largest reservoir
of water in the planet. Understanding the behavior and chemistry of hydrogen in minerals at
the atomic scale is thus central to understanding the geology of the planet. There have been
signifi cant recent advances in the detection, measurement, and location of H in the nominally
anhydrous silicate and oxide minerals that compose the planet. There have also been advances
in experimental methods for measurement of H diffusion and the effects of H on the phase
1529-6466/06/0062-0000$05.00 DOI: 10.2138/rmg.2006.62.0
Water in Nominally Anhydrous Minerals ‒ Preface
boundaries and physical properties whereby the presence of H in the interior may be inferred
from seismic or other geophysical studies. It is the objective of this volume to consolidate
these advances with reviews of recent research in the geochemistry and mineral physics of
hydrogen in the principal mineral phases of the Earth’s crust and mantle.
The chapters
We begin with a review of analytical methods for measuring and calibrating water contents
in nominally anhydrous minerals by George Rossman. While infrared spectroscopy is still the
most sensitive and most convenient method for detecting water in minerals, it is not intrinsically
quantitative but requires calibration by some other, independent analytical method, such as
nuclear reaction analysis, hydrogen manometry, or SIMS. A particular advantage of infrared
spectroscopy, however, is the fact that it does not only probe the concentration, but also the
structure of hydrous species in a mineral and in many cases the precise location of a proton in a
mineral structure can be worked out based on infrared spectra alone. The methods and principles
behind this are reviewed by Eugen Libowitzky and Anton Beran, with many illustrative
examples. Compared to infrared spectroscopy, NMR is much less used in studying hydrogen in
minerals, mostly due to its lower sensitivity, the requirement of samples free of paramagnetic
ions such as Fe2+ and because of the more complicated instrumentation required for NMR
measurements. However, NMR could be very useful under some circumstances. It could detect
any hydrogen species in a sample, including such species as H that would be invisible with
2
infrared. Potential applications of NMR to the study of hydrogen in minerals are reviewed by
Simon Kohn. While structural models of “water” in minerals have already been deduced from
infrared spectra several decades ago, in recent years atomistic modeling has become a powerful
tool for predicting potential sites for hydrogen in minerals. The review by Kate Wright gives an
overview over both quantum mechanical methods and classical methods based on interatomic
potentials. Joseph Smyth then summarizes the crystal chemistry of hydrogen in high-pressure
silicate and oxide minerals. As a general rule, the incorporation of hydrogen is not controlled
by the size of potential sites in the crystal lattice; rather, the protons will preferentially attach to
oxygen atoms that are electrostatically underbonded, such as the non-silicate oxygen atoms in
some high-pressure phases. Moreover, heterovalent substitutions, e.g., the substitution of Al3+
for Si4+, can have a major effect on the incorporation of hydrogen.
Data on water in natural minerals from crust and mantle are compiled and discussed
in three reviews by Elisabeth Johnson, Henrik Skogby and by Anton Beran and Eugen
Libowitzky. Among the major mantle minerals, clinopyroxenes usually retain the highest
water contents, followed by orthopyroxenes and olivine, while the water contents in garnets are
generally low. Most of these water contents need to be considered as minimum values, as many
of the mantle xenoliths may have lost water during ascent. However, there are some cases where
the correlation between the water contents and other geochemical parameters suggest that the
measured water concentrations refl ect the true original water content in the mantle.
The basic thermodynamics as well as experimental data on water solubility and partition-
ing are reviewed by Hans Keppler and Nathalie Bolfan Casanova. Water solubility in minerals
depends in a complicated way on pressure, temperature, water fugacity and bulk composition.
For example, water solubility in the same mineral can increase or decrease with temperature,
depending on the pressure of the experiments. Nevertheless, the pressure and temperature
dependence of water solubility can be described by a rather simple thermodynamic formalism
and for most minerals of the upper mantle, the relevant thermodynamic parameters are known.
The highest water solubilities are reached in the minerals wadsleyite and ringwoodite stable
in the transition zone, while the minerals of the lower mantle are probably mostly dry. The
rather limited experimental data on water partitioning between silicate melts and minerals are
reviewed by Simon Kohn and Kevin Grant. One important observation here is that comparing
vi
Water in Nominally Anhydrous Minerals ‒ Preface
the compatibility of hydrogen with that of some rare earth element is misleading, as such cor-
relations are always limited to a small range of pressure and temperature for a given mineral.
The stabilities of hydrous phases in the peridotite mantle and in subducted slabs are
reviewed by Daniel Frost and by Tatsuhiko Kawamoto. While most of the water in the mantle
is certainly stored in the nominally anhydrous minerals, hydrous phases can be important
storage sites of water in certain environments. Amphibole and phlogopite require a signifi cant
metasomatic enrichment of Na and K in order to be stabilized in the upper mantle, but serpentine
may be an important carrier of water in cold subducted slabs.
The diffusion of hydrogen in minerals is reviewed by Jannick Ingrin and Marc Blanchard.
An important general observation here is that natural minerals usually do not loose hydrogen as
water, but as H generated by redox reaction of OH with Fe2+. Moreover, diffusion coeffi cients
2
of different mantle minerals can vary by orders of magnitude, often with signifi cant anisotropy.
While some minerals in a mantle xenolith may therefore have lost virtually all of their water
during ascent, other minerals may still preserve the original water content and in general, the
apparent partition coeffi cients of water between the minerals of the same xenolith can be
totally out of equilibrium. Accordingly, it would be highly desirable to directly deduce the
water content in the mantle from geophysical data. One strategy, based on seismic velocities
and therefore ultimately on the effect of water on the equation of state of minerals, is outlined
by Steve Jacobsen. The dissolution of water in minerals usually increases the number of cation
vacancies, yielding reduced bulk and shear moduli and seismic velocities. Particularly, the
effect on shear velocities is strong and probably larger than the effect expected from local
temperature variations. Accordingly, the v / v ratio could be a sensitive indicator of mantle
S P
hydration. A more general approach towards remote sensing of hydrogen in the Earth’s mantle,
including effects of seismic anisotropy due to lattice preferred orientation and the use of
electrical conductivity data is presented by Shun-ichiro Karato.
Probably the most important effect of water on geodynamics is related to the fact that even
traces of water dramatically reduce the mechanical strength of rocks during deformation. The
physics behind this effect is discussed by David Kohlstedt. Interestingly, it appears that the main
mechanism behind “hydrolytic weakening” is related to the effect of water on the concentration
and mobility of Si vacancies, rather than to the protons themselves.
Water may have major effects on the location of mantle discontinuities, as reviewed by Eiji
Ohtani and Konstantin Litasov. Most of these effects can be rationalized as being due to the
expansion of the stability fi elds of those phases (e.g., wadsleyite) that preferentially incorporate
water. Together with other geophysical data, the changes in the depths of discontinuities are a
promising tool for the remote sensing of water contents in the mantle.
The global effects of water on the evolution of our planet are reviewed in the last two
chapters by Bernard Marty, Reika Yokochi and Klaus Regenauer-Lieb. By combining hydrogen
und nitrogen isotope data, Marty and Yokochi demonstrate convincingly that most of the Earth´s
water very likely originated from a chondritic source. Water may have had a profound effect
on the early evolution of our planet, since a water-rich dense atmosphere could have favored
melting by a thermal blanketing effect. However, Marty and Yokochi also show very clearly that
it is pretty much impossible to derive reliable estimates of the Earth´s present-day water content
from cosmochemical arguments, since many factors affecting the loss of water during and after
accretion are poorly constrained or not constrained at all.
In the last chapter, Klaus Regenauer-Lieb investigates the effect of water on the style of
global tectonics. He demonstrates that plate tectonics as we know it is only possible if the water
content of the mantle is above a threshold value. The different tectonic style observed on Mars
and Venus may therefore be directly related to differences in mantle water content. Earth is the
water planet – not just because of its oceans, but also because of its tectonic evolution.
vii
Water in Nominally Anhydrous Minerals ‒ Preface
Acknowledgments
This volume and the accompanying short course in Verbania were made possible by
generous support from the Mineralogical Society of America, the Geochemical Society,
the United State Department of Energy, the German Mineralogical Society and Bayerisches
Geoinstitut. The Verbania short course is the fi rst MSA/GS short course ever held in Italy. We
are very grateful for the generosity and the international spirit of the supporting institutions,
which made this project possible. The preparation of the short course benefi ted enormously
from the permanent advice by Alex Speer. Finally, we would like to thank Jodi Rosso for the
effi cient and professional handling of the manuscript and for her patience with authors and
editors who ignore deadlines.
August 2006
Hans Keppler Joseph R. Smyth
Bayreuth, Germany Boulder, Colorado, USA
viii
WATER in NOMINALLY
ANHYDROUS MINERALS
62 Reviews in Mineralogy and Geochemistry 62
FROM THE SERIES EDITOR
The review chapters in this volume were the basis for a four day short course on Water in
Nominally Anhydrous Minerals held in Verbania, Lago Maggiore, Italy (October 1-4, 2006).
The editors Hans Keppler and Joe Smyth have done an excellent job organizing this volume
and the associated short course. Meeting deadlines (often ahead of schedule!) and keeping track
of so many authors can be a thankless job at times but I truly appreciate all their hard work.
Hans’ “friendly reminder” e-mails certainly kept us all on task and his eye for detail (small and
large) made my job much more enjoyable! I extend my sincere thanks to him for his efforts!
Any supplemental material and errata (if any) can be found at the MSA website
www.minsocam.org.
Jodi J. Rosso, Series Editor
West Richland, Washington
August 2006
PREFACE
Earth is a water planet. Oceans of liquid water dominate the surface processes of the planet.
On the surface, water controls weathering as well as transport and deposition of sediments.
Liquid water is necessary for life. In the interior, water fl uxes melting and controls the solid-
state viscosity of the convecting mantle and so controls volcanism and tectonics. Oceans cover
more than 70% of the surface but make up only about 0.025% of the planet’s mass. Hydrogen
is the most abundant element in the cosmos, but in the bulk Earth, it is one of the most poorly
constrained chemical compositional variables.
Almost all of the nominally anhydrous minerals that compose the Earth’s crust and mantle
can incorporate measurable amounts of hydrogen. Because these are minerals that contain
oxygen as the principal anion, the major incorporation mechanism is as hydroxyl, OH−, and
the chemical component is equivalent to water, HO. Although the hydrogen proton can be
2
considered a monovalent cation, it does not occupy same structural position as a typical cation
in a mineral structure, but rather forms a hydrogen bond with the oxygens on the edge of the
coordination polyhedron. The amount incorporated is thus quite sensitive to pressure and the
amount of H that can be incorporated in these phases generally increases with pressure and
sometimes with temperature. Hydrogen solubility in nominally anhydrous minerals is thus
much more sensitive to temperature and pressure than that of other elements.
Because the mass of rock in the mantle is so large relative to ocean mass, the amount that is
incorporated the nominally anhydrous phases of the interior may constitute the largest reservoir
of water in the planet. Understanding the behavior and chemistry of hydrogen in minerals at
the atomic scale is thus central to understanding the geology of the planet. There have been
signifi cant recent advances in the detection, measurement, and location of H in the nominally
anhydrous silicate and oxide minerals that compose the planet. There have also been advances
in experimental methods for measurement of H diffusion and the effects of H on the phase
1529-6466/06/0062-0000$05.00 DOI: 10.2138/rmg.2006.62.0
Water in Nominally Anhydrous Minerals ‒ Preface
boundaries and physical properties whereby the presence of H in the interior may be inferred
from seismic or other geophysical studies. It is the objective of this volume to consolidate
these advances with reviews of recent research in the geochemistry and mineral physics of
hydrogen in the principal mineral phases of the Earth’s crust and mantle.
The chapters
We begin with a review of analytical methods for measuring and calibrating water contents
in nominally anhydrous minerals by George Rossman. While infrared spectroscopy is still the
most sensitive and most convenient method for detecting water in minerals, it is not intrinsically
quantitative but requires calibration by some other, independent analytical method, such as
nuclear reaction analysis, hydrogen manometry, or SIMS. A particular advantage of infrared
spectroscopy, however, is the fact that it does not only probe the concentration, but also the
structure of hydrous species in a mineral and in many cases the precise location of a proton in a
mineral structure can be worked out based on infrared spectra alone. The methods and principles
behind this are reviewed by Eugen Libowitzky and Anton Beran, with many illustrative
examples. Compared to infrared spectroscopy, NMR is much less used in studying hydrogen in
minerals, mostly due to its lower sensitivity, the requirement of samples free of paramagnetic
ions such as Fe2+ and because of the more complicated instrumentation required for NMR
measurements. However, NMR could be very useful under some circumstances. It could detect
any hydrogen species in a sample, including such species as H that would be invisible with
2
infrared. Potential applications of NMR to the study of hydrogen in minerals are reviewed by
Simon Kohn. While structural models of “water” in minerals have already been deduced from
infrared spectra several decades ago, in recent years atomistic modeling has become a powerful
tool for predicting potential sites for hydrogen in minerals. The review by Kate Wright gives an
overview over both quantum mechanical methods and classical methods based on interatomic
potentials. Joseph Smyth then summarizes the crystal chemistry of hydrogen in high-pressure
silicate and oxide minerals. As a general rule, the incorporation of hydrogen is not controlled
by the size of potential sites in the crystal lattice; rather, the protons will preferentially attach to
oxygen atoms that are electrostatically underbonded, such as the non-silicate oxygen atoms in
some high-pressure phases. Moreover, heterovalent substitutions, e.g., the substitution of Al3+
for Si4+, can have a major effect on the incorporation of hydrogen.
Data on water in natural minerals from crust and mantle are compiled and discussed
in three reviews by Elisabeth Johnson, Henrik Skogby and by Anton Beran and Eugen
Libowitzky. Among the major mantle minerals, clinopyroxenes usually retain the highest
water contents, followed by orthopyroxenes and olivine, while the water contents in garnets are
generally low. Most of these water contents need to be considered as minimum values, as many
of the mantle xenoliths may have lost water during ascent. However, there are some cases where
the correlation between the water contents and other geochemical parameters suggest that the
measured water concentrations refl ect the true original water content in the mantle.
The basic thermodynamics as well as experimental data on water solubility and partition-
ing are reviewed by Hans Keppler and Nathalie Bolfan Casanova. Water solubility in minerals
depends in a complicated way on pressure, temperature, water fugacity and bulk composition.
For example, water solubility in the same mineral can increase or decrease with temperature,
depending on the pressure of the experiments. Nevertheless, the pressure and temperature
dependence of water solubility can be described by a rather simple thermodynamic formalism
and for most minerals of the upper mantle, the relevant thermodynamic parameters are known.
The highest water solubilities are reached in the minerals wadsleyite and ringwoodite stable
in the transition zone, while the minerals of the lower mantle are probably mostly dry. The
rather limited experimental data on water partitioning between silicate melts and minerals are
reviewed by Simon Kohn and Kevin Grant. One important observation here is that comparing
vi
Water in Nominally Anhydrous Minerals ‒ Preface
the compatibility of hydrogen with that of some rare earth element is misleading, as such cor-
relations are always limited to a small range of pressure and temperature for a given mineral.
The stabilities of hydrous phases in the peridotite mantle and in subducted slabs are
reviewed by Daniel Frost and by Tatsuhiko Kawamoto. While most of the water in the mantle
is certainly stored in the nominally anhydrous minerals, hydrous phases can be important
storage sites of water in certain environments. Amphibole and phlogopite require a signifi cant
metasomatic enrichment of Na and K in order to be stabilized in the upper mantle, but serpentine
may be an important carrier of water in cold subducted slabs.
The diffusion of hydrogen in minerals is reviewed by Jannick Ingrin and Marc Blanchard.
An important general observation here is that natural minerals usually do not loose hydrogen as
water, but as H generated by redox reaction of OH with Fe2+. Moreover, diffusion coeffi cients
2
of different mantle minerals can vary by orders of magnitude, often with signifi cant anisotropy.
While some minerals in a mantle xenolith may therefore have lost virtually all of their water
during ascent, other minerals may still preserve the original water content and in general, the
apparent partition coeffi cients of water between the minerals of the same xenolith can be
totally out of equilibrium. Accordingly, it would be highly desirable to directly deduce the
water content in the mantle from geophysical data. One strategy, based on seismic velocities
and therefore ultimately on the effect of water on the equation of state of minerals, is outlined
by Steve Jacobsen. The dissolution of water in minerals usually increases the number of cation
vacancies, yielding reduced bulk and shear moduli and seismic velocities. Particularly, the
effect on shear velocities is strong and probably larger than the effect expected from local
temperature variations. Accordingly, the v / v ratio could be a sensitive indicator of mantle
S P
hydration. A more general approach towards remote sensing of hydrogen in the Earth’s mantle,
including effects of seismic anisotropy due to lattice preferred orientation and the use of
electrical conductivity data is presented by Shun-ichiro Karato.
Probably the most important effect of water on geodynamics is related to the fact that even
traces of water dramatically reduce the mechanical strength of rocks during deformation. The
physics behind this effect is discussed by David Kohlstedt. Interestingly, it appears that the main
mechanism behind “hydrolytic weakening” is related to the effect of water on the concentration
and mobility of Si vacancies, rather than to the protons themselves.
Water may have major effects on the location of mantle discontinuities, as reviewed by Eiji
Ohtani and Konstantin Litasov. Most of these effects can be rationalized as being due to the
expansion of the stability fi elds of those phases (e.g., wadsleyite) that preferentially incorporate
water. Together with other geophysical data, the changes in the depths of discontinuities are a
promising tool for the remote sensing of water contents in the mantle.
The global effects of water on the evolution of our planet are reviewed in the last two
chapters by Bernard Marty, Reika Yokochi and Klaus Regenauer-Lieb. By combining hydrogen
und nitrogen isotope data, Marty and Yokochi demonstrate convincingly that most of the Earth´s
water very likely originated from a chondritic source. Water may have had a profound effect
on the early evolution of our planet, since a water-rich dense atmosphere could have favored
melting by a thermal blanketing effect. However, Marty and Yokochi also show very clearly that
it is pretty much impossible to derive reliable estimates of the Earth´s present-day water content
from cosmochemical arguments, since many factors affecting the loss of water during and after
accretion are poorly constrained or not constrained at all.
In the last chapter, Klaus Regenauer-Lieb investigates the effect of water on the style of
global tectonics. He demonstrates that plate tectonics as we know it is only possible if the water
content of the mantle is above a threshold value. The different tectonic style observed on Mars
and Venus may therefore be directly related to differences in mantle water content. Earth is the
water planet – not just because of its oceans, but also because of its tectonic evolution.
vii
Water in Nominally Anhydrous Minerals ‒ Preface
Acknowledgments
This volume and the accompanying short course in Verbania were made possible by
generous support from the Mineralogical Society of America, the Geochemical Society,
the United State Department of Energy, the German Mineralogical Society and Bayerisches
Geoinstitut. The Verbania short course is the fi rst MSA/GS short course ever held in Italy. We
are very grateful for the generosity and the international spirit of the supporting institutions,
which made this project possible. The preparation of the short course benefi ted enormously
from the permanent advice by Alex Speer. Finally, we would like to thank Jodi Rosso for the
effi cient and professional handling of the manuscript and for her patience with authors and
editors who ignore deadlines.
August 2006
Hans Keppler Joseph R. Smyth
Bayreuth, Germany Boulder, Colorado, USA
viii