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Abstract This article is dedicated to the occurrence, relevance, and structure of minerals whose formation involves high pressure. This includes minerals that occur in the interior of the Earth as well as minerals that are found in shock-metamorphized meteorites and terrestrial impactites. I discuss the chemical and physical reasons that render the definition of high-pressure minerals meaningful, in distinction from minerals that occur under surface-near conditions on Earth or at high temperatures in space or on Earth. Pressure-induced structural transformations in rock-forming minerals define the basic divisions of Earth's mantle in the upper mantle, transition zone, and lower mantle. Moreover, the solubility of minor chemical components in these minerals and the occurrence of accessory phases are influential in mixing and segregating chemical elements in Earth as an evolving planet. Brief descriptions of the currently known high-pressure minerals are presented. Over the past 10 years more high-pressure minerals have been discovered than during the previous 50 years, based on the list of minerals accepted by the IMA. The previously unexpected richness in distinct high-pressure mineral species allows for assessment of differentiation processes in the deep Earth.
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Olivine—The Little Green Science Machine
In some ways, olivine has driven the evolution of the Solar System and likely beyond. As one of the earliest-crystallizing silicate minerals, olivine controls the initial chemical evolution of planet-wide magma oceans and individual lava flows alike. In solid aggregate form, it controls and records deformation of the mantle and smaller-scale intrusive complexes. The components of its crystal structure are mobile at high temperatures and their migration can be used to explore the timing of magmatic events. During chemical weathering, these olivine crystals capture carbon dioxide from the atmosphere as secondary minerals are formed. All of these processes take place not only on Earth, but also on other planetary bodies, making olivine ideally suited to shed light on both primordial planet-building processes and current-day volcanism and surface processes.
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- PAR ID:
- 10515987
- Publisher / Repository:
- geoscienceworld
- Date Published:
- Journal Name:
- Elements
- Volume:
- 19
- Issue:
- 3
- ISSN:
- 1811-5209/23/0019-0138$2.50
- Page Range / eLocation ID:
- 138 to 143
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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Abstract Seismic anisotropy arises in the upper mantle due to the alignment of olivine crystal lattices and is often used to interpret mantle flow direction. Experiments on the evolution of olivine crystal‐preferred orientation (CPO) have found that the texture that develops is dependent on many factors, including water content, differential stress, preexisting CPO, and deformation kinematics. To evaluate the role of these factors in naturally deformed samples, we present microstructural transects across three shear zones in the Josephine Peridotite. Samples from these shear zones exhibit a mixture of A‐type textures, which have been associated with dry conditions and primary activation of the olivine [100](010) slip system, and of E‐type textures, which have been associated with wetter conditions and primary activation of the [100](001) slip system. CPOs with characteristics of both A‐type and E‐type textures are also present. CPO type does not evolve systematically as a function of either strain or water content. We used a micromechanical model to evaluate the roles of preexisting texture and kinematics on olivine CPO evolution. We find that the preexisting texture controls CPO evolution at strains up to 5 during simple shear. Kinematics involving a combination of simple shear and pure shear can explain the olivine CPOs at higher strain. Hence, preexisting CPOs and deformation kinematics should be considered in the interpretation of CPOs measured in naturally deformed rocks and of large‐scale patterns in upper‐mantle seismic anisotropy.more » « less
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The sulfur isotope composition of volcanic rocks in arcs can be difficult to constrain because significant fractionation can occur during degassing. Mafic and ultramafic cumulates represent the least degassed part of the magmatic arc system, thereby offering an opportunity to investigate undegassed sulfur in arcs. Recent work on high pressure metamorphic rocks has suggested that subducted materials can retain their original isotopic composition to sub-arc depths. In particular, extreme negative δ34S values can be retained in subducted sediments. The purpose of this project is to investigate to what extent these deep subduction zone processes are reflected in the sulfur isotope signature of arc magmas. In the Lesser Antilles arc, there is a gradual decrease in terrigenous sediment being subducted from south to north. An estimated ~15% subducted sediment in the south and ~2% in the north is reflected in the chemical and isotopic composition of the Lesser Antilles arc magmas. Sulfides in these magma- derived cumulates record the earliest stages of magma evolution and are a more faithful monitor of the sulfur isotopic composition of the magma source region in the mantle than erupting lavas. We hypothesize that the decrease in terrigenous sediment being subducted from the south to north will be reflected in the S isotopes in cumulate samples. Samples of mafic and ultramafic cumulates have been collected from fourteen islands across the Lesser Antilles arc. Primary rock types are olivine gabbro, amphibole gabbro, plagioclase gabbro, and olivine gabbronorite. Sulfide minerals include pyrite, chalcopyrite, and pyrrhotite, and typically occur as spherical blebs. Sulfides are found primarily as inclusions in clinopyroxene, amphibole, olivine, and plagioclase. Sulfides occur less frequently as inclusions in magnetite and within the matrix. Analyses of sulfur isotopes in cumulate sulfides are currently underway. The decrease in the amount sediment being subducted from south to north in the Lesser Antilles arc should result in δ34S values that increase from south to north (more sediment subducted = more negative δ34S values).more » « less
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When magmas erupt at the surface, they may have undergone many changes since their inception. While olivine drives some of these changes through crystallization and fractionation, it also records the magma evolution via mineral chemistry and by trapping mineral and melt inclusions. Olivine is an effective recorder of intensive parameters, such as temperature and melt composition, and provides an outstanding petrological tool for constraining dynamic processes, such as ascent, mixing, and cooling. Olivine sheds light on magmatic puzzles that involve both mafic and more evolved magmas, with protracted and complex magmatic histories that often obscure earlier and deeper processes. This contribution summarizes the current state of how olivine helps reconstruct source-to-surface magma assembly through its chemistry, inclusions, and textures.more » « less
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Olivine occurs across the galaxy, from Earth to extraterrestrial bodies including the Moon, Mars, and asteroids, to particles of comet dust and distant debris disks. The mineral is critical to our understanding of early Solar System chronology, planetary formation processes (e.g., magma ocean solidification), crustal evolution (e.g., volcanic eruptions), and surface weathering. Olivine’s ability to shed light on these processes lies in the linkage of small, physical samples and satellite-derived data. Laboratory spectra become the basis for olivine detection and compositional interpretation in remotely sensed spectra ranging from high-resolution planetary maps to single extra-solar datapoints. In turn, petrologic studies of olivine underpin the geologic interpretations of these spectral datasets. Finally, olivine chemistry records Solar System formation conditions and relative chronology. Olivine is our bridge across time and space.more » « less
- Free Publicly Accessible Full Text
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Accepted Manuscript1.0
- Journal Article:
- https://doi.org/10.2138/gselements.19.3.138
