Earth Mat Flashcards - Ch 8
Solidify to produce igneous rocks in Earth’s crust are initially formed by a process called ___, which refers to the partial melting of a source rock.
anatexis
Anatexis produces:
A liquid melt fraction enriched in lower temperature constituents.
A residual rock component enriched in higher temperature, refractory elements.
In the simplest approach, rock melting within Earth’s interior is modeled as two idealized end-member processes:
equilibrium melting and disequilibrium (fractional) melting.
occurs in a closed system where chemicals are neither added nor removed from the plutonic environment.
requires that the melt remains in contact with the residual rock throughout the melting process.
Equilibrium melting
Incomplete chemical reactions may be due to
large crystal size, high magma viscosity, and/or low ion migration rates.
The net result of inequilibrium melting is the generation of
zoned crystals
Cause of Fractional (disequilibrium)melting
melts are separated from the refractory crystals, liquids and crystals do not remain in equilibrium
what happens in fractional melting of rocks containing plagioclase and olivine
The early melts are highly enriched in low melting temperature constituents, such as sodium plagioclase and iron olivine.
This leaves behind a more refractory residual solid enriched in calcium plagioclase and magnesium olivine.
Pressure is related to rock depth, whereby 10 km depth corresponds approximately to
3.3 kbars.
is the primary means by which basaltic magmas are generated at ocean spreading ridges and continental rifts.
Decompression melting of mantle peridotite
Volatiles act as a ____, which is an agent that reduces the melting temperature of a substance.
flux
As the degree of partial melting increases, the degree of enrichment of incompatible elements ________.
decreases
Light rare earth elements (LREE) are generally less incompatible with solid minerals than heavy rare earth elements (HREE).
T or F
F; more incompatible
processes involve changes in bulk magma chemistry after its initial generation.
Diversification
includes all processes where the original melt evolves into one or more melts with a different composition, without material being exchanged with an external source.
Closed-system diversification
Father of Modern petrology
Norman Bowen
The subject of study in Bowens reseach
Palisades Sill
When discontinuous and continuous reactions in magmas do not reach completion under equilibrium, chemical variations are preserved in crystals, leading to the formation of
zoned crystals and reaction rims.
show a systematic chemical variation from the core to the edge, recording incomplete continuous reactions between the crystal and surrounding melt.
Zoned crystals
occur when a new mineral forms around the edges of a crystal with a partially resorbed core.
Reaction rims
Crystallization along the walls of the magma chamber in which crystals preferentially form and adhere to the edges results in
marginal accretion.
Heat rises so that the upper margin of the magma chamber may cool relatively quickly.
As a result, __________ results from early crystallization of minerals along the ceiling or roof due to preferential heat loss.
roof accretion
___________develops as the magma chamber walls release heat to the relatively cold country rock, generating crystals that adhere to the side margins of the magma chamber.
Sidewall accretion
occurs as crystals form along the base of the magma chamber.
Floor accretion
Because they lose heat rapidly to the wall rocks, pluton margins adjacent to cool country rock commonly display finer grained ____________ compared to the coarser grained crystals of the pluton interior.
chilled margins
includes fractionation processes that occur when crystals develop with significantly different densities than the surrounding magma.
Gravitational separation
occurs when higher density, ferromagnesian crystals settle to the base of a magma chamber relative to the lower density liquid magma.
Crystal settling
can occur if early formed crystals, such as plagioclase, are less dense than the magma.
As a result, crystals may float towards the roof of a magma chamber, effectively segregating them from the remaining melt.
Crystal flotation
segregation occurs whereby liquids and crystals are segregated due to factors such as velocity, density, or temperature.
Convective flow
Cooling can be accompanied by deformation whereby crystals are compacted and rotated while liquids experience expulsion.
This variation on crystal–liquid fractionation involves a set of processes under the umbrella of
filter pressing.
In _________, one parent magma fractionates to produce two or more distinctly different daughter magmas with different compositions.
liquid fractionation
Liquid fractionation processes include
differential diffusion and liquid immiscibility.
involves the preferential diffusion of select ions within the magma in response to compositional, thermal, or density gradients as well as water content.
Differential diffusion
also called liquid–liquid fractionation, occurs when magma separates into two or more distinct immiscible liquid phases.
Liquid immiscibility,
How does carbonitite form
CO₂ exsolves as a separate fluid phase from alkalic magmas, generating a carbonate-rich fluid which crystallizes as a carbonatite.
processes such as assimilation, magma mixing, and magma mingling involve chemical changes due to interaction of magma with the surrounding country rock or with other magmas entering the system
Open-system
occurs whereby the surrounding wall rock (country rock) is intruded by and reacts chemically with the magma.
Assimilation
Forceful injection of magma commonly fractures the surrounding wall rock by a process called .
stoping
involving multiple magma injections over time, produces complex relationships and constitutes an important diversification mechanism.
Magma replenishment,
occurs when two or more dissimilar magmas coexist, displaying contact relations but retaining their distinctive individual magma characteristics.
Magma mingling
implies thorough mixing so that the individual magma components are no longer recognizable.
Magma mixing
consists of genetically related magmas that have changed composition from a common original parental magma
magma series
Igneous rock series include:
(1) alkaline (or alkalic), and (2) sub-alkaline assemblages.
have high Na₂O and K₂O concentrations compared to SiO₂ and are generally undersaturated with respect to SiO₂.
Alkaline rocks
rocks have moderate to high Na₂O, K₂O, and SiO₂ concentrations.
Calc-alkaline
have low Na₂O and K₂O but high FeO and CaO concentrations.
Tholeiites
provide a means to concisely and clearly display rock chemistry variations between samples collected in an area with an eye to establishing their origins and relationships.
Variation diagrams
are bivariate diagrams in which the vertical ordinate (y-axis) represents weight percents of major or minor oxide compounds such as FeO, MgO, CaO, Na₂O, K₂O, or TiO₂.
Harker diagrams
If Harker diagrams display smooth, curvilinear trends for all rock data points, then the following three inferences can be made:
The rocks are genetically related.
Major element variations reflect a liquid line of descent from a common source in which diversification caused major elements to either increase or decrease progressively with respect to variations in weight percent SiO₂.
The parent magma from which the rocks are derived has a composition near that of the sample with the least SiO₂, that is, of basaltic composition
In ____________ trace element concentrations on the abscissa are plotted relative to a standard reference for each element on the ordinate.
spider diagrams,
The standard reference of spider diagrams may include
- chondrite meteorites,
- primitive mantle,
- N-MORB
- E-MORB
- ocean island basalt
depletion of europium indicates
plagioclase fractionation from the melt, which effectively removes europium from the magma.
europium enrichment would indicate
assimilation of plagioclase components into the melt.
Upon fractionation, ____ magmas record a progressive decrease in iron and magnesium with increasing SiO₂ and alkali concentrations
calc-alkaline
The calc-alkaline series consists largely of
andesite, dacite, and rhyolite, as well as high alumina basalt (16–20% Al₂O₃)
(BADR series)
The rocks in convergent margin volcanoes are known colloquially as
BADR series for the basalts, andesites, dacites, and rhyolites that occur in them.
With increasing fractionation, tholeiitic magmas experience _at low to moderate SiO₂ concentrations
iron enrichment
Alkaline magmas and rocks are highly enriched in _______________ and are less common than calc-alkaline or tholeiitic magmas.
Na₂O and/or K₂O
Alkaline rocks can contain equilibrium quartz.
FALSE
Amphibole minerals in alkaline magmas include
riebeckite and richterite.
Pyroxene minerals found in alkaline rocks include
aegerine, aegerine-augite, and spodumene.
Alkaline rocks occur in diverse environments:
Stable cratonic interiors
Continental rifts (Bailey, 1974)
Ocean islands away from spreading centers (Sorensen, 1974)
Subduction zones (Burke et al., 2003
Tholeiitic magmas are typically produced by
extensive partial melting of mantle rocks at relatively higher temperatures and lower pressures, often from a depleted source
Alkaline magmas are more likely to form through ______________________________ particularly from an undepleted or enriched mantle source
limited partial melting (less than 10%) of the mantle at higher pressure conditions
consist of large amounts of both silicic (rhyolite-dacite) and basic (basalt) rocks, but have very few intermediate (andesite) rocks.
Bimodal suites
The tectonic environments where magma series occur in summary
Calc-alkaline magmas: Convergent margins.
Tholeiitic magmas: Ocean spreading centers, continental rifts, backarc basins, ocean islands, and hotspots.
Alkaline magmas: Hotspots, ocean islands, and rift environments.
Bimodal volcanism: Continental rifts.
is the major mechanism by which continental crust has grown since the Archean, over 2.5 billion years ago
Batholith emplacement along convergent plate boundaries
Normal zoning within a pluton is characterized by a
basic rim and a silicic core.
A ___________ is a tabular, concordant pluton that parallels country rock
sill
Sills develop through ____
the injection of magma along a plane (e.g., bedding, foliations) of weakness parallel to the layering in the country rock.
Sills commonly form in__________ magmas and are generally _________ in thickness.
low-viscosity; < 50m
is a blister-like concordant pluton characterized by a flat floor and domed roof
A laccolith
Laccoliths were first described in the _of Utah by G. K. Gilbert in the 1870s.
Henry Mountains
are dish-shaped to funnel-shaped concordant plutons that resemble a champagne glass in cross-section view
Lopoliths
The Bushveld Complex has a length of ___ and is up to_____ thick.
550 km; 8 km
The champagne-glass lopolith structure can develop due to one or more of the following mechanisms:
Meteorite impact and associated crustal melting, as has been suggested for the Sudbury Complex.
Normal faulting and crustal melting associated with rifting, as in the Koillismaa Complex.
A sill-like structure that receives upwelling magma from a conical feeder tube, which would correspond to the stem in the champagne glass.
Veins can occur in great abundance, resulting in
vein swarms.
are cylindrical plutonic dikes exposed at the surface by subsequent erosion.
They represent ancient ______________________ that has long since been removed by erosion.
Necks
conduit pipes that funneled magma upward to a volcano
Notable examples of volcanic necks include _______________ which has a central neck with radiating dike ridges, and _______ , which preserves radial columnar jointing.
Ship Rock, New Mexico,; Devil’s Tower of Wyoming
are carrot-shaped, cylindrical pipes that can extend to depths of 200 km (Figure 8.19).
develop via _____________________
Diatremes; explosive intrusions that originate deep within the mantle.
are tabular intrusions that cross-cut country rock layers.
Dikes
consist of multiple dikes that can occur in parallel, sub-parallel, radiating, concentric, or random orientations.
Dike swarms
are typically produced when vertical forces related to rising magma produce fractures that radiate outward from the central vent.
Radial dikes
are circular in map view and resemble ring dikes, except that they converge at depth in cross-section view.
Cone sheet dikes
sets consist of parallel, offset dikes that form in response to shear.
En echelon dike
sets form perpendicular to extension and are particularly common in rift environments.
Parallel dike
Steeply inclined _________________, composed of gabbro, diabase, and basalt, form layer 2B of the ocean floor at ocean spreading ridges (Figure 8.23).
sheeted dikes
Sheeted dikes form by the
cooling and contraction of magma as it is injected into extensional fractures in oceanic rift valleys.
