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The Sources and Origin of Magmas
Published in O.A. Bogatikov, R.F. Fursenko, G.V. Lazareva, E.A. Miloradovskaya, A. Ya, R.E. Sorkina, Magmatism and Geodynamics Terrestrial Magmatism Throughout the Earth’s History, 2020
E.V. Sharkov, O.A. Bogatikov, V.I. Kovalenko
Theoretically, strongly depleted peridotites that underwent advanced melting at the early stages of the Earth’s history could be preserved in the mantle. Their existence can be postulated from the unusual character of early Precambrian magmatism, with the widespread occurrence of komatiitic and particularly boninite-like melts. Such peridotites have to be identical to harzburgites of subtype IIB in the content of major oxides and ILE, but will be distinguished by their extremely low concentrations of radiogenic Sr and non-radiogenic Nd (εSr<–40–50, εNd> 20–25). Rocks with these characteristics (which conventionally could be referred to as the IIBB variety) have not yet been recognized in nature. Petrochemically, they are consistent with xenoliths from unusual diamond-bearing dunite–harzburgites and are deficient in clinopyroxene garnet peridotites from kimberlites (Sobolev, 1974; Sobolev et al, 1975; Pokhilenko, 1989; Laz’ko, 1988c). However, in terms of their geochemistry all these rocks are enriched in ILE (Shimizu and Richardson, 1987).
Minerals, rocks, discontinuities and rock mass
Published in Ömer Aydan, Rock Mechanics and Rock Engineering, 2019
Peridotite is a dense, coarse-grained rock, consisting mostly of the minerals olivine and pyroxene. Peridotite is ultramafic and ultrabasic, as the rock contains less than 45% silica. Peridotite is the dominant rock of the upper mantle of the Earth.
The Earth: Surface, Structure and Age
Published in F.G.H. Blyth, M. H. de Freitas, A Geology for Engineers, 2017
F.G.H. Blyth, M. H. de Freitas
These values correspond to those derived from elasticity tests in the laboratory on the igneous rocks granite, basalt, and peridotite respectively. Peridotite is a rock whose mineralogy is formed at pressures and temperatures similar to those expected in the upper mantle. Thus the fastest waves, P and S, travel for the greater part of their course in material of peridotite composition, in the upper part of the mantle just below the Moho. Above the Moho is the basaltic crust, in which the P* and S* waves travel. The granitic layer, which forms the upper part of the continental crust, transmits the Pg and Sg vibrations. The granitic layer itself is mainly covered by sedimentary rocks, in which velocities of transmission are lower, from about 2 to 4kms−1. The thicknesses of the crustal layers varies considerably in different situations. The average thickness of the crust in a continental area is about 30 km, but beneath a mountain mass it may thicken to 40 km or more as discussed below. In an oceanic area the crust is thinner, 5 to 10 km, and is composed of basalt with a thin sedimentary cover and no granitic layer. This distinction between continental crust and oceanic crust is referred to again on p. 10. The study of earthquake waves has demonstrated that the Earth consists of concentric shells of different density, the lightest being the outer lithosphere. This contains the oceanic and continental crust which rests upon the heavier rock at the top of the upper mantle, whose character is in part revealed by the vertical and horizontal movements of the lithosphere. These movements require the presence of a weaker layer at depth; the asthenosphere. To explain the vertical movements of the lithosphere the theory of isostacy was proposed: horizontal displacements required the theory of continental drift for their explanation. The new theory of Plate Tectonics unifies both these concepts.
Anthophyllite asbestos from Staten Island, New York: Longitudinal fiber splitting
Published in Archives of Environmental & Occupational Health, 2022
The Staten Island serpentinite is a lens-shaped body covering 55 km2 in the central to north area of the Island. The serpentinite was derived from peridotite.10 The peridotite protolith was composed mostly of olivine and pyroxene, with accessary chromite. During the Taconic Orogeny, beginning about 450 million years ago, the peridotite was thrust by obduction along Cameron’s Line, a suture extending down to the mantle and stretching from Newfoundland to Alabama, bringing the peridotite up from the mantle to the depth of the greenschist facies.10 In the process, the peridotite hydrated such as to convert most of the olivine and orthopyroxene to serpentine and variable concentrations of accessory minerals including cummingtonite and anthophyllite (including anthophyllite asbestos; the name used in Federal Asbestos Regulations). The serpentinite now occurs as pods in bedrock along the zone of Cameron’s Line.10 In Staten Island the serpentinite is composed primarily of lizardite and chrysotile.11 Residual or relict olivine is variably present and sometimes abundant, along with some antigorite, primarily in fault zones, and accessory minerals including relict peridotite minerals.
An updated catalogue of New Zealand’s mantle peridotite and serpentinite
Published in New Zealand Journal of Geology and Geophysics, 2020
New Zealand has a remarkable distribution of Permian to Holocene exhumed mantle material in the form of peridotite or its hydrated equivalent, serpentinite (Figure 1A; Table 1). Peridotite is classified by the International Union of Geological Sciences as a rock composed of less than 10% modal quartz-feldspar-feldspathoid and greater than 40 modal % olivine, along with variable amounts of orthopyroxene and clinopyroxene (Figures 2 and 3A). In addition to these three mineral phases, mantle peridotite commonly contains an aluminium-rich phase that is plagioclase at < c. 10 kbar, spinel at c. 10–20 kbar, or garnet at higher-P (Klemme and O'Neill 2000; Borghini et al. 2010) with these phase stabilities pushed to greater P in depleted peridotites (Klemme 2004a). Despite the rarity of mantle peridotite exposed at Earth’s surface, occurrences are globally significant because this rock-type makes up the major proportion of the continental and oceanic lithosphere (e.g. Griffin et al. 2009) (Figure 1B). The bulk composition of peridotite over large areas influences the melting potential of the mantle lithosphere and also its strength and buoyancy, with individual samples capable of providing insight into the age of lithosphere roots (e.g. Pearson et al. 2014).