Magmatic Processes

Cashman, Katharine V.; Bergantz, George W.

doi:10.1002/rog.1991.29.s2.500

Cited by 4 publications

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References 475 publications

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“…Magmas derived from continental crust tend to be more enriched in incompatible elements, including both large-ion lithophiles (those with a large ionic radius to ionic charge ratio) such as rubidium (Rb), strontium (Sr), and barium (Ba) and high field strength elements (those with a high ionic charge and small ionic radius) such as yttrium (Y), zirconium (Zr), and niobium (Nb) (White 2015, 98-101). The composition of a melt is thus partially a product of the magma source, but factors such as temperature, pressure, mixing, gravimetric sorting, and the convection dynamics of the magma chamber all further influence elemental fractionation (Cashman and Bergantz 1991;Bergantz 1995;Cashman, Sparks, and Blundy 2017). During an eruption, rapid cooling of silicic lava into obsidian flows or the sintering of rhyolitic ash into glassy pyroclasts may prevent mineralization of a portion of the melt, thereby preserving its major and trace element composition at the time of the eruption in volcanic glass (R. E. Hughes and Smith 1993;Glascock 2002;Rust and Cashman 2007;Gardner et al 2019).…”

Section: What Is An Obsidian "Source"?mentioning

confidence: 99%

The geology, geochemistry, and geographic distribution of southern Idaho obsidian

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2023

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The Snake River Plain Volcanic Province of Southern Idaho is host to an abundance of pyroclastic obsidian deposits that constitute the principal lithic raw material utilized by Native Americans of the region throughout the Precontact Period. Archaeologists have long looked to obsidian as a source of information on past patterns of raw material selection, landscape use, and mobility through geochemical analysis of obsidian artifacts and lithic raw materials. Formation of the Snake River Plain through cataclysmic "super-eruptions" of the Yellowstone hotspot, however, presents one of the most complex and poorly understood obsidian source-scapes of North America. To clarify the geographic distribution of Southern Idaho obsidian source groups, the INL CRMO has compiled a comprehensive reference library of geologic obsidian from 138 source locales distributed across the southern half of the state. In this report, we define 28 geochemically distinct obsidian source groups that occur at these deposits through Xray Fluorescence spectrometry and contextualize the geographic distribution of each source group relative to the history of silicic magmatism on the Snake River Plain. This research facilitates ongoing investigations at the INL CRMO into patterns of obsidian procurement, conveyance, and exchange in Southern Idaho throughout the Precontact Period.

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Section: What Is An Obsidian "Source"?mentioning

confidence: 99%

The geology, geochemistry, and geographic distribution of southern Idaho obsidian

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2023

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“…Isotopic and geochemical discontinuities indicate that not all facies are the result of the chemical evolution of a unique blob of magma as a closed system (Cashman & Bergantz 1991). Too large isotopic differences are commonly interpreted as the result of mixing between different source regions.…”

Section: Intermediate Magma Chambers and Magma Mixingmentioning

confidence: 99%

“…The continental crust first differentiates from the mantle. A second process, internal to the crust, leads to crustal differentiation and granite formation (review in Cashman & Bergantz 1991).…”

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Granitic batholiths: from pervasive and continuous melting in the lower crust to discontinuous and spaced plutonism in the upper crust

Vigneresse¹

2006

Transactions of the Royal Society of Edinburgh: Earth Sciences

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The generation of granitic magmas begins with melting in the lower crust, under active participation of the underlying mantle. Thermally driven, melting is a pervasive and continuous process that develops over a wide region. In contrast, the building of a granitic pluton is highly discontinuous in time and space. Several inputs of magma, sometimes with a different chemical compositions, are focused toward a region where they accumulate, forming a large pluton, often separated by some 50 km from an adjacent one. The switch from a continuous to a discontinuous process represents a fundamental point of magma generation. It gives place to the modified model m(M-SAE), in which the mantle (m) and Melting (M) are separated from the Segregation (S), Ascent (A) and Emplacement (E) modes. Discontinuities result from non-linear processes that develop during segregation and ascent of the magma. They rely on the non-linear rheology of partially molten rocks. Thresholds control the change from a solid-like to liquid-like behaviour of the magma. In between, the rheology exhibits sudden jumps between states. Because two phases continuously coexist (matrix and melt), strain is highly partitioned between them. This may induce highly discontinuous melt segregation, which needs both pure and simple shear to develop. Melt focusing is controlled by the viscosity contrast between the two phases. It gives rise to different compaction lengths depending on the region, a partially melting source or a nearly brittle crust, where it develops. Because ascent and emplacement are discontinuous in time, this allows the crust to relax, avoiding the room problem for a pluton intruding the upper crust. Intermediate magma chambers could develop with different temperature and magma composition. They could be the place of enhanced magma mixing. Finally, the stress conditions, which differ for each tectonic setting, influence the shape of the granitic body.

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“…It severely reduces the implications of results obtained from numerical models based on bulk magma chamber evolution. Most thermomechanical models of convective motion, crystallisation, cooling and chemical evolution caused by crystal-mush interactions assume a stationary process or a closed chemical system (see reviews in Cashman & Bergantz 1991;Ryan 1994). These models may remain valid, but on a much smaller scale.…”

Section: Toward a New Paradigmmentioning

confidence: 99%

A new paradigm for granite generation

Vigneresse¹

2004

Earth and Environmental Science Transactions of the Royal Socie

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Ideas about granite generation have evolved considerably during the past two decades. The present paper lists the ideas which were accepted and later modified concerning the processes acting during the four stages of granite generation: melting, melt segregation and ascent, and emplacement. The active role of the mantle constitutes a fifth stage.Fluid-assisted melting, deduced from metamorphic observations, was used to explain granite and granulite formation. Water seepage into meta-sedimentary rocks can produce granitic melt by decreasing melting temperature. CO2 released by the mantle helps to transform rocks into granulites. However, dehydration melting is now considered to be the origin of most granitic melts, as confirmed by experimental melting. Hydrous minerals are involved, beginning with muscovites, followed by biotite at higher temperatures. At even deeper conditions, hornblende dehydration melting leads to calc-alkaline magmas.Melt segregation was first attributed to compaction and gravity forces caused by the density contrast between melt and its matrix. This was found insufficient for magma segregation in the continental crust because magmas were transposed from mantle conditions (decompression melting) to crustal conditions (dehydration melting). Rheology of two-phase materials requires that melt segregation is discontinuous in time, occurring in successive bursts. Analogue and numerical models confirm the discontinuous melt segregation. Compaction and shear localisation interact non-linearly, so that melt segregates into tiny conduits. Melt segregation occurs at a low degree of melting.Global diapiric ascent and fractional crystallisation in large convective batholiths have also been shown to be inadequate and at least partly erroneous. Diapiric ascent cannot overcome the crustal brittle-ductile transition. Fracture-induced ascent influences the neutral buoyancy level at which ascent should stop but does not. Non-random orientation of magma feeders within the ambient stress field indicates that deformation controls magma ascent.Detailed gravity and structural analyses indicate that granite plutons are built from several magma injections, each of small size and with evolving chemical composition. Detailed mapping of the contact between successive magma batches documents either continuous feeding, leading to normal petrographic zoning, or over periods separated in time, commonly leading to reverse zoning. The local deformation field controls magma emplacement and influences the shape of plutons.A typical source for granite magmas involves three components from the mantle, lower and intermediate crusts. The role of the mantle in driving and controlling essential crustal processes appears necessary in providing stress and heat, as well as specific episodes of time for granite generation. These mechanisms constitute a new paradigm for granite generation.

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Magmatic Processes

Cited by 4 publications

References 475 publications

The geology, geochemistry, and geographic distribution of southern Idaho obsidian

The geology, geochemistry, and geographic distribution of southern Idaho obsidian

Granitic batholiths: from pervasive and continuous melting in the lower crust to discontinuous and spaced plutonism in the upper crust

A new paradigm for granite generation

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