scholarly journals Microstructural evidence for convection in high-silica granite

Geology ◽  
2021 ◽  
Author(s):  
Brendan Dyck ◽  
Marian Holness
Keyword(s):  
Boundary Energy ◽  
Coincident Site Lattice ◽  
Element Distribution ◽  
Orientation Relationships ◽  
Quartz Crystals ◽  
Apparent Lack ◽  
Key Issues ◽  
High Silica ◽  
Modal Layering ◽  
Melt Segregation

High-silica (>70 wt% SiO2) granites (HSGs) are critical carriers of tin, copper, and other melt-incompatible elements, yet much remains unknown about the mechanisms responsible for their formation. One of the key issues is the apparent lack of evidence for crystal-melt segregation (e.g., modal layering), without which little can be inferred about the dynamics (or lack thereof) of crystallizing HSGs. We examined the crystallographic orientation relationships of clustered quartz crystals from the 300-m-thick Bobbejaankop sill, Bushveld Complex, South Africa. We report an inward increase in the number density and size of quartz clusters toward the central horizon of the sill, coinciding with a significant increase in concentrations of tin, copper, and tungsten. The majority of crystal pairs within each cluster exhibit coincident-site lattice orientation relationships, representing low grain-boundary energy configurations. These clusters must have formed by synneusis in a magmatic environment where crystals could have moved freely, rotating into low-energy orientations on contact. We argue that this not only demonstrates that 100-m-scale crystal-poor and liquid-rich regions can be present in bodies of HSG, but also that such bodies can undergo long-lived convection during crystallization, driven by downwards movement of crystal-rich plumes at the roof, without significant crystal-melt segregation. This dynamic behavior provides a mechanism to homogenize major-element distribution across HSGs and to concentrate highly incompatible and economic elements into central mineralized horizons.

Distribution of Grain Boundary Planes at Coincident Site Lattice Misorientations

2004 ◽  
Vol 819 ◽  
Author(s):  
Gregory S. Rohrer ◽  
Bassem S. El-Dasher ◽  
Herbert M. Miller ◽  
Anthony D. Rollett ◽  
David M. Saylor
Keyword(s):  
Grain Boundary ◽  
Habit Plane ◽  
Boundary Energy ◽  
Coincident Site Lattice ◽  
Orientation Relationships ◽  
Low Surface Energy ◽  
Site Density ◽  
Grain Boundary Plane ◽  
Boundary Orientation ◽  
Grain Surface

AbstractThe grain boundary plane distributions in MgO, SrTiO3, MgAl2O4, and Al are compared at lattice misorientations with a coincident site density of greater than or equal to 1/9. In most situations, the most frequently adopted grain boundary orientation is a habit plane of low index and low surface energy that depends on the particular material. Cases where the most common boundary orientation is a plane of high planar coincident site density instead of a characteristic habit plane are rare. In fact, in most cases, the distributions of grain boundary planes at misorientations with high lattice coincidence are not substantially different from the distributions at other, more general misorientations. The results indicate that a model for grain boundary energy and structure based on grain surface relationships is more appropriate than the widely accepted models based on lattice orientation relationships.

scholarly journals Transient rhyolite melt extraction to produce a shallow granitic pluton

2021 ◽  
Vol 7 (21) ◽  
pp. eabf0604
Author(s):  
Allen J. Schaen ◽  
Blair Schoene ◽  
Josef Dufek ◽  
Brad S. Singer ◽  
Michael P. Eddy ◽  
...  
Keyword(s):  
Time Scales ◽  
Explosive Eruptions ◽  
Upper Crust ◽  
Saturation Model ◽  
Granitic Pluton ◽  
Melt Extraction ◽  
High Silica ◽  
Melt Segregation ◽  
Zircon Saturation

Rhyolitic melt that fuels explosive eruptions often originates in the upper crust via extraction from crystal-rich sources, implying an evolutionary link between volcanism and residual plutonism. However, the time scales over which these systems evolve are mainly understood through erupted deposits, limiting confirmation of this connection. Exhumed plutons that preserve a record of high-silica melt segregation provide a critical subvolcanic perspective on rhyolite generation, permitting comparison between time scales of long-term assembly and transient melt extraction events. Here, U-Pb zircon petrochronology and 40Ar/39Ar thermochronology constrain silicic melt segregation and residual cumulate formation in a ~7 to 6 Ma, shallow (3 to 7 km depth) Andean pluton. Thermo-petrological simulations linked to a zircon saturation model map spatiotemporal melt flux distributions. Our findings suggest that ~50 km3 of rhyolitic melt was extracted in ~130 ka, transient pluton assembly that indicates the thermal viability of advanced magma differentiation in the upper crust.

The orientation relationships of nanobelt-like Si2Hf precipitates in an Al–Si–Mg–Hf alloy

2016 ◽  
Vol 49 (4) ◽  
pp. 1223-1230 ◽  
Author(s):  
Xueli Wang ◽  
Huilan Huang ◽  
Xinfu Gu ◽  
Yanjun Li ◽  
Zhihong Jia ◽  
...  
Keyword(s):  
Electron Microscopy ◽  
Heat Treatment ◽  
Coincident Site Lattice ◽  
Orthorhombic Structure ◽  
Orientation Relationships ◽  
Lattice Distribution ◽  
Site Lattice ◽  
Transmission Electron ◽  
Habit Planes ◽  
Al Matrix

The orientation relationships (ORs) between the Al matrix and Si2Hf precipitates with an orthorhombic structure in an Al–Si–Mg–Hf alloy after heat treatment at 833 K for 20 h were investigated by transmission electron microscopy and electron diffraction. Four ORs are identified as (100)Al||(010)p, (0\overline {1}1)Al||(101)pand [011]Al||[\overline {1}01]p; (11\overline {1})Al||(010)pand [011]Al||[\overline {1}01]p; (12\overline {1})Al||(010)p, (101)Al||(100)pand [1\overline {11}]Al||[001]p; (\overline {11}1)Al||(010)pand [112]Al||[\overline {1}01]p. The habit planes of these four ORs are rationalized by the fraction of good atomic matching sites at the interface. In addition, the formation of Si2Hf precipitates with a nanobelt-like morphology is interpreted on the basis of the near-coincident site lattice distribution.

Exsolved magmatic fluid and its role in the formation of comb-layered quartz at the Cretaceous Logtung W-Mo deposit, Yukon Territory, Canada

1996 ◽  
Vol 87 (1-2) ◽  
pp. 291-303 ◽  
Author(s):  
Jacob B. Lowenstern ◽  
W. David Sinclair
Keyword(s):  
Volcanic Rocks ◽  
Hydrothermal Solution ◽  
Aqueous Fluid ◽  
Yukon Territory ◽  
Magmatic Fluid ◽  
Type I ◽  
Type Ii ◽  
Quartz Crystals ◽  
High Silica ◽  
Solidification Texture

ABSTRACT:Comb-layered quartz is a type of unidirectional solidification texture found at the roofs of shallow silicic intrusions that are often associated spatially with Mo and W mineralisation. The texture consists of multiple layers of euhedral, prismatic quartz crystals (Type I) that have grown on subplanar aplite substrates. The layers are separated by porphyritic aplite containing equant phenocrysts of quartz (Type II), which resemble quartz typical of volcanic rocks and porphyry intrusions. At Logtung, Type I quartz within comb layers is zoned with respect to a number of trace elements, including Al and K. Concentrations of these elements as well as Mn, Ti, Ge, Rb and H are anomalous and much higher than found in Type II quartz from Logtung or in igneous quartz reported elsewhere. The two populations appear to have formed under different conditions. The Type II quartz phenocrysts almost certainly grew from a high-silica melt between 600 and 800°C (as β-quartz); in contrast, the morphology of Type I quartz is consistent with precipitation from a hydrothermal solution, possibly as α-quartz grown below 600°C. The bulk compositions of comb-layered rocks, as well as the aplite interlayers, are consistent with the hypothesis that these textures did not precipitate solely from a crystallising silicate melt. Instead, Type I quartz may have grown from pockets of exsolved magmatic fluid located between the magma and its crystallised border. The Type II quartz represents pre-existing phenocrysts in the underlying magma; this magma was quenched to aplite during fracturing/degassing events. Renewed and repeated formation and disruption of the pockets of exsolved aqueous fluid accounts for the rhythmic banding of the rocks.

scholarly journals Floating sandstones off El Hierro (Canary Islands, Spain): the peculiar case of the October 2011 eruption

2011 ◽  
Vol 3 (2) ◽  
pp. 975-999 ◽  
Author(s):  
V. R. Troll ◽  
A. Klügel ◽  
M.-A. Longpré ◽  
S. Burchardt ◽  
F. M. Deegan ◽  
...  
Keyword(s):  
Trace Element ◽  
Volcanic Rock ◽  
Canary Islands ◽  
Sedimentary Rocks ◽  
Silica Content ◽  
Quartz Crystals ◽  
El Hierro ◽  
High Silica Content ◽  
High Silica ◽  
Peculiar Case

Abstract. The eruption that started off the south coast of El Hierro, Canary Islands, in October 2011 has emitted intriguing eruption products found floating in the sea. These specimens appeared as floating volcanic "bombs" that have in the meantime been termed "restingolites" (after the close-by village of La Restinga) and exhibit cores of white and porous pumice-like material. Currently the nature and origin of these "floating stones" is vigorously debated among researchers, with important implications for the interpretation of the hazard potential of the ongoing eruption. The "restingolites" have been proposed to be either (i) juvenile high-silica magma (e.g. rhyolite), (ii) remelted magmatic material (trachyte), (iii) altered volcanic rock, or (iv) reheated hyaloclastites or zeolite from the submarine slopes of El Hierro. Here, we provide evidence that supports yet a different conclusion. We have collected and analysed the structure and composition of samples and compared the results to previous work on similar rocks found in the archipelago. Based on their high silica content, the lack of igneous trace element signatures, and the presence of remnant quartz crystals, jasper fragments and carbonate relicts, we conclude that "restingolites" are in fact xenoliths from pre-island sedimentary rocks that were picked up and heated by the ascending magma causing them to partially melt and vesiculate. They hence represent messengers from depth that help us to understand the interaction between ascending magma and crustal lithologies in the Canary Islands as well as in similar Atlantic islands that rest on sediment/covered ocean crust (e.g. Cape Verdes, Azores). The occurrence of these "restingolites" does therefore not indicate the presence of an explosive high-silica magma that is involved in the ongoing eruption.

Geochemical evolution of copper-bearing granitic rocks of Guichon Creek Batholith, British Columbia, Canada

1976 ◽  
Vol 13 (2) ◽  
pp. 199-209 ◽  
Author(s):  
M. A. Olade
Keyword(s):  
British Columbia ◽  
Porphyry Copper ◽  
Source Region ◽  
Element Distribution ◽  
Granitic Rocks ◽  
Geochemical Evolution ◽  
Trace Element Distribution ◽  
Apparent Lack ◽  
South Central ◽  
Normal Differentiation

The Guichon Creek batholith, located in south-central British Columbia, is host to numerous porphyry copper deposits that are considered as being closely related to the pluton. Variations in major and trace element distribution are consistent with a model of crystallization-fractionation of a K-poor calc-alkaline magma of intermediate composition. Relatively low Rb and Rb/Sr values, and high K/Rb ratios are consistent with Sr isotopic ratios that suggest a subcrustal source region for the generation of the magma, probably by partial melting of subducted oceanic crust at relatively shallow depths.Cu, like other femic elements, (Zn, Mn, V, Ni, Co, Ti) generally decreases with increasing fractionation, which reflects normal differentiation trends. The apparent lack of positive correlation between Cu contents of rocks and ore potential of intrusive units may be explicable if mineralization is regarded as an independent by-product of magma generation, rather than the result of differentiation processes. Close relationships between trace metal values and degree of fractionation emphasize the need for assigning different background values to each intrusive unit, during geochemical exploration.

The central Kenya peralkaline province: a unique assemblage of magmatic systems

2009 ◽  
Vol 73 (1) ◽  
pp. 1-16 ◽  
Author(s):  
R. Macdonald ◽  
B. Bagiński
Keyword(s):  
Fractional Crystallization ◽  
Magma Mixing ◽  
Experimental Studies ◽  
Liquid Immiscibility ◽  
Element Distribution ◽  
Low Viscosity ◽  
South Central ◽  
High Silica ◽  
Silicate Liquid Immiscibility ◽  
Central Kenya

The review focuses on the evolution of five contiguous peralkaline salic complexes in the south-central Kenya Rift Valley, stressing new developments of general significance to peralkaline magmatism. The complexes have evolved dominantly by combinations of fractional crystallization and magma mixing; volatile-melt interactions, remobilization of plutonic rocks and crystal mushes, and carbonate-silicate liquid immiscibility have been additional petrogenetic processes. Geochemical and experimental studies have shown that pantelleritic magmas can be generated by fractional crystallization of trachyte and high-silica rhyolite. Melts of comenditic composition were also formed by fractionation of trachyte but also locally by partial meltingof syenites. Studies of apparent partition coefficients have provided some of the first data on element distribution between phenocrysts and peralkaline silicic melts. Compositional zonation has been ubiquitous in the complexes, probably a result of the very low viscosity of the magmas.

Quantitative Analysis of EBSD Data in Rocks and other Crystalline Materials: Investigation of Strain Induced Recrystallisation and Growth of New Phases

2012 ◽  
Vol 715-716 ◽  
pp. 62-71 ◽  
Author(s):  
John Wheeler ◽  
E. Mariani ◽  
S. Piazolo ◽  
D.J. Prior ◽  
P.J. Trimby ◽  
...  
Keyword(s):  
Boundary Migration ◽  
Boundary Energy ◽  
Time Lapse ◽  
Orientation Relationships ◽  
Crystalline Materials ◽  
Ebsd Data ◽  
The Earth ◽  
Boundary Length ◽  
In Situ Experiments

Misorientation can be calculated over large datasets and a theme of this paper is the usefulness of examining the results statistically. Comparing the statistics of misorientations calculated from neighbouring pixels (or grains) with those calculated from pairs of pixels (or grains) selected at random helps to indicate deformation and recrystallisation mechanisms. Taking boundary length into account provides a link to grain boundary energy, and boundary length versus misorientation data should be used to examine how boundaries with different misorientations evolve through time. Time lapse misorientation maps indicate how orientation changes through time at particular points in a microstructure during in situ experiments. The size of areas which have changed orientation by particular amounts can be linked to boundary length and boundary migration velocities. When dealing with different phases, the statistics of angular relationships, akin to intraphase misorientation analysis, can indicate orientation relationships in the absence of prior knowledge, which is advantageous in investigating the plethora of minerals that make up the Earth.

scholarly journals Floating stones off El Hierro, Canary Islands: xenoliths of pre-island sedimentary origin in the early products of the October 2011 eruption

Solid Earth ◽  
2012 ◽  
Vol 3 (1) ◽  
pp. 97-110 ◽  
Author(s):  
V. R. Troll ◽  
A. Klügel ◽  
M.-A. Longpré ◽  
S. Burchardt ◽  
F. M. Deegan ◽  
...  
Keyword(s):  
Canary Islands ◽  
Silica Content ◽  
Sea Surface ◽  
Submarine Eruption ◽  
Plumbing System ◽  
Quartz Crystals ◽  
Crustal Recycling ◽  
El Hierro ◽  
High Silica ◽  
Thermal Overprint

Abstract. A submarine eruption started off the south coast of El Hierro, Canary Islands, on 10 October 2011 and continues at the time of this writing (February 2012). In the first days of the event, peculiar eruption products were found floating on the sea surface, drifting for long distances from the eruption site. These specimens, which have in the meantime been termed "restingolites" (after the close-by village of La Restinga), appeared as black volcanic "bombs" that exhibit cores of white and porous pumice-like material. Since their brief appearance, the nature and origin of these "floating stones" has been vigorously debated among researchers, with important implications for the interpretation of the hazard potential of the ongoing eruption. The "restingolites" have been proposed to be either (i) juvenile high-silica magma (e.g. rhyolite), (ii) remelted magmatic material (trachyte), (iii) altered volcanic rock, or (iv) reheated hyaloclastites or zeolite from the submarine slopes of El Hierro. Here, we provide evidence that supports yet a different conclusion. We have analysed the textures and compositions of representative "restingolites" and compared the results to previous work on similar rocks found in the Canary Islands. Based on their high-silica content, the lack of igneous trace element signatures, the presence of remnant quartz crystals, jasper fragments and carbonate as well as wollastonite (derived from thermal overprint of carbonate) and their relatively high oxygen isotope values, we conclude that "restingolites" are in fact xenoliths from pre-island sedimentary layers that were picked up and heated by the ascending magma, causing them to partially melt and vesiculate. As they are closely resembling pumice in appearance, but are xenolithic in origin, we refer to these rocks as "xeno-pumice". The El Hierro xeno-pumices hence represent messengers from depth that help us to understand the interaction between ascending magma and crustal lithologies beneath the Canary Islands as well as in similar Atlantic islands that rest on sediment-covered ocean crust (e.g. Cape Verdes, Azores). The occurrence of "restingolites" indicates that crustal recycling is a relevant process in ocean islands, too, but does not herald the arrival of potentially explosive high-silica magma in the active plumbing system beneath El Hierro.

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