Magmas Under Pressure

Advances in High-Pressure Experiments on Structure and Properties of Melts

Yoshio Kono, Chrystèle Sanloup

• Publisher: Elsevier
• Year: 2018
• Print ISBN: 9780128113011
• eBook ISBN: 9780128112748
• Topic: Physical Sciences
• Subtopic: Geology & Earth Sciences

Magmas under Pressure: Advances in High-Pressure Experiments on Structure and Properties of Melts summarizes recent advances in experimental technologies for studying magmas at high pressures. In the past decade, new developments in high-pressure experiments, particularly with synchrotron X-ray techniques, have advanced the study of magmas under pressure. These new experiments have revealed significant changes of structure and physical properties of magmas under pressure, which significantly improves our understanding of the behavior of magmas in the earth's interior. This book is an important reference, not only in the earth and planetary sciences, but also in other scientific fields, such as physics, chemistry, material sciences, engineering and in industrial applications, such as glass formation and metallurgical processing. - Includes research and examples of high-pressure technologies for studying the structure and properties of magma - Summarizes the current knowledge on the structure and properties of high-pressure magma - Highlights the importance of magma in understanding the evolution of the earth's interior

Part 1

Magmas in the Earth's Interior

Outline

1. Chapter 1. Primary Melt Compositions in the Earth's Mantle
2. Chapter 2. Carbon-Bearing Magmas in the Earth's Deep Interior
3. Chapter 3. The Influence of Pressure on the Properties and Origins of Hydrous Silicate Liquids in Earth's Interior
4. Chapter 4. Melting in the Earth's Deep Interior

Chapter 1

Primary Melt Compositions in the Earth's Mantle

Stephen F. Foley, and Zsanett Pintér Macquarie University, Sydney, NSW, Australia

Abstract

Primary melts are the melt compositions in equilibrium with their source at the time of extraction from that source. Many mantle-derived melts are derived chiefly from peridotite, but have additional components in the source that originate as recycled crust and sediments, or as ultramafic rocks formed by solidification of migrating mantle melts. Most melts originate between 40 and 120 km depth, with exceptions up to 250 km, particularly around and beneath cratons, where fluxed by volatile components. Melts of dry peridotite are varieties of basalt with increasing MgO and alkalies toward high pressures: they range from picrite and komatiite at high degrees of melting to basanite and nephelinite at small melt fractions. More alkaline compositions, including melilitite, kimberlite, lamprophyre, and carbonatite, require H₂O and CO₂ in the source. Water promotes melts richer in silica than basalt, whereas CO₂ causes SiO₂-poor melts. At high pressures, primary melts may be transitional between silicate and carbonate melt compositions. Volatile components depress the melting point of peridotite by 150–400°C relative to dry melting conditions, and melting in oxidized conditions occurs at lower temperatures than in reduced conditions or with H₂O alone. Incipient melts are widespread, universal precursors to more voluminous melt production and cause modification and evolution of the lithosphere.

Keywords

CO₂; H₂O; Mantle; Primary melts; Redox; Volatile components

Chapter Outline

1. 1. Introduction
2. 2. Melting of Mantle Peridotite
   1. 2.1 Melting of Peridotite Without Volatile Components
   2. 2.2 Melting of Peridotite in the Presence of Water
   3. 2.3 Melting in the Presence of Carbon Dioxide
   4. 2.4 Melting of Peridotite With H₂O and CO₂
   5. 2.5 Melting in Reducing Conditions
3. 3. Storage and Availability of Volatile Components in the Mantle
   1. 3.1 Nominally Anhydrous Minerals
   2. 3.2 Hydrous Phases
   3. 3.3 Carbon-Bearing Phases
4. 4. Melting of Rocks Other Than Peridotite in the Mantle
   1. 4.1 Recycling of Ocean Crust Basalt/Gabbro
   2. 4.2 Ultramafic Cumulates From the Ocean Crust and Arcs
   3. 4.3 Serpentinites: Hydrothermally Altered Ultramafic Rocks
   4. 4.4 Clastic Sedimentary Material
   5. 4.5 Carbonate Sedimentary Material
   6. 4.6 Ultramafic Rocks Originating From Solidification of Transient Incipient Melts
5. 5. Geodynamic Variations in the Sources and Compositions of Melts
   1. 5.1 Oceanic Lithosphere
   2. 5.2 Continental Lithospheric Mantle
   3. 5.3 Cratonic Lithosphere
   4. 5.4 Subduction Zones
   5. 5.5 Shallow Convergent Zones
6. Acknowledgments
7. References

1. Introduction

The term primary magma has a long history that predates the advent of high-pressure equipment to study the melting of mantle rocks. At the time of the introduction of the end-loaded piston–cylinder apparatus (Boyd and England, 1958), which initiated abundant experimental studies of mantle melting, Turner and Verhoogen (1960) lamented that the term primary magma was “frequently used but seldom defined.” This haziness remained through the 1960s, with imprecise definitions such as “partial melting of the mantle, yielding primary magmas” (O'Hara, 1965), and “liquids formed by processes of partial melting or complete melting of mantle rock” (Green and Ringwood, 1967), which “moved to the surface without further modification” (Thompson, 1974). Presnall's (1979) simple formulation would be acceptable for almost all petrologists: “primary magma will refer to a magma as it exists immediately after separation from its source region.” Here, we refine this by explicitly including a requirement expedient for experimental petrologists, and which they have followed implicitly for years (Wyllie, 1987)—namely, equilibrium: a primary melt is the melt composition in equilibrium with its source at the time of extraction from that source. This is distinct from a parental melt, which is the starting point for an observed crystal fractionation series, but need not be primary, and from a primitive melt, which may have been modified since extraction from its source. Note that the emphasis of these definitions is on the melts' unmodified, pristine state rather than on the identity of the mantle rock that melts: most experimental studies assume the mantle to be a simple, four-phase peridotite consisting of olivine, orthopyroxene, clinopyroxene, and an aluminous phase: garnet, spinel, or plagioclase depending on pressure.

Partial melting on the modern Earth is almost entirely restricted to a magmatic zone between 40 and about 250 km depth. Most melting occurs in the uppermost 140 km, ranging from decompression melting beneath midocean ridges to picrites in large igneous provinces. Melting deeper than 140 km occurs only where promoted by volatile components. The scope of this chapter is restricted to melting of the mantle with and without volatile components: we do not consider melting deeper than 250 km (see Chapters 2 and 4), major melting on the hot early Earth, or details of melting in subduction zones (Chapter 3).

First H₂O, and later CO₂, were added to peridotite melting studies, whereas reduced conditions in which H₂O + CH₄ fluids are stable remain little studied. Volatiles have two important effects: firstly a strong depression of melting temperatures, which results in a broad incipient melting regime (Green and Falloon, 1998), ...

Table of contents

• Cover image
• Title page
• Table of Contents
• Copyright
• List of Contributors
• Preface
• Part 1. Magmas in the Earth's Interior
• Part 2. Advances in Experimental Studies of Melts at High Pressures
• Part 3. Current Knowledge on Structure and Properties of Magmas Under Pressure
• Author Index
• Subject Index