Igneous Rocks and Minerals: Processes (3 & 4)

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What are the two important processes that influence the crystallization of minerals in magma?

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1

What are the two important processes that influence the crystallization of minerals in magma?

temperature and cooling rate

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2

How does magma transition into a solidified igneous rock?

cooling and solidification of the molten material

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3

Describe the subsurface architecture of a volcano.

Magma originates from the mantle and rises buoyantly through less dense crust until it reaches a level of neutral buoyancy or a permeability barrier. It forms a magma chamber and can propagate through cracks known as dykes. Some dykes form sills or feed volcanic eruptions.

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4

What are sills in the context of volcanic activity?

thin sub-horizontal sheets formed from dikes that stall within the crust.

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5

What are dikes in the context of volcanic activity?

melt-filled cracks that propagate towards the surface, potentially feeding volcanic eruptions.

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6

Define ‘Magma’

mixture of liquid melt, solid crystals, and fluid bubbles

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7

define “melt”

specifically refers to the silicate liquid within the magma

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8

What is equilibrium crystallization?

Process where a silicate melt transforms into a crystalline solid in a closed system, with continuous chemical exchange between crystals & the melt

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9

Describe fractional crystallization.

Process in which a silicate melt transforms into a crystalline solid in an open system. Crystals are removed as soon as they form, leading to a decrease in melt volume & changes in system chemistry

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10

What is closed-system crystallization?

Process involving no volumetric change & limited chemical exchange. Crystals may sink to the floor of the system, & no mass exchange occurs with the exterior.

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11

What does Bowen's reaction series describe?

the order in which different igneous minerals start to crystallize from magma as it cools, based on their respective crystallization temperatures

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12

What is the primary characteristic of the minerals in the discontinuous reaction sequence?

Varying structures as temperature decreases. Minerals crystallizing at higher temperatures have simpler structures, while those crystallizing at lower temperatures have more complex

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13

Explain the continuous series in terms of feldspar crystallization.

These minerals crystallize across a wide temperature and compostion range, with Ca-rich plagioclase crystallizing at higher temperatures & Na-rich plagioclase at lower temperatures.

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14

How does Bowen's reaction series affect igneous rock mineralogy?

Determins the order in which minerals crystallise as magma cools.

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15

What is nucleation in the context of crystal growth?

Initial formation of crystals from a silicate melt as it cools. It involves the aggregation of ions into stable crystal nuclei.

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16

How does undercooling affect nucleation and growth?

Higher undercooling results in increased nucleation rates and finer crystal growth, while lower undercooling leads to slower nucleation but coarser crystal growth.

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17

What is undercooling?

the difference between the liquidus temperature and the actual temperature, affects nucleation and growth rates

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18

What texture is associated with coarse-grained igneous rocks, and how does it form?

Plutonic texture, formed in slow-cooling environments where nucleation rates are low but crystal growth rates are high, resulting in larger crystals.

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19

Describe the texture of fine-grained igneous rocks and how it forms

volcanic texture, formed in rapidly cooling environments where nucleation rates are high, but crystal growth rates are low, leading to small crystals.

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20

What happens in extremely fast cooling environments, and what texture results?

Both nucleation and growth are suppressed, preventing crystal formation. This leads to the formation of glassy textures, where no crystals are present.

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21

How does cation diffusion in silicate melts change with their silica content?

slowest in highly polymerized silica-rich melts and fastest in silica-poor melts with shorter polymerized silica chains

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22

How does the structure of liquid diopside differ from liquid silica and crystalline silica?

It has shorter polymer chains, positivly charged cations as network modifiers, and it breaks up the cilica polymer chains, allowing faster cation diffusion.

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23

How does silica content affect the formation of basalt glass?

Low silica content requires an extremely fast cooling rate to form due to the rapid cation diffusion

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24

How does the formation of basalt glass differ in subaerial eruptions compared to subaqueous eruptions?

Subaerial eruption: basalt glass forms when erupted magma quickly cools upon contact with air.

Subaqueous eruptions: pillow lavas form as the magma comes into contact with water & cools rapidly.

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25

What is the significance of the texture of pillow lavas in subaqueous basaltic eruptions?

Pillow lavas exhibit a glassy rind on the edges due to rapid cooling in contact with water, while the interior structure reflects the cooling rate it experiences.

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26

What characterizes a porphyritic texture in igneous rocks?

Larger euhedral to subhedral phenocrysts within a finer-grained groundmass, indicating two stages of cooling and crystallization.

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27

How does a poikilitic texture form, and what are the terms used to describe the enclosed and enclosing minerals?

Forms when one mineral is completely enclosed by another. The enclosed grains are called chadacrysts, and the surrounding grains are called oikocrysts.

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28

What is a glomeroporphyritic texture, and what is its characteristic feature?

Occurs when phenocrysts are clustered & stuck together in a clot, with grouping of phenocrysts in a fine-grained groundmass.

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29

Describe the ophitic and sub-ophitic textures, and what do they indicate about crystallization order?

They are poikilitic textures that feature lath-like plagioclase chadacrysts enclosed by anhedral augite clinopyroxene oikocrysts, indicating the sequence of crystallization.

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30

How are the chemical compositions of rocks typically expressed, and why are oxides reported instead of elements?

As major & minor element oxides, as cations are always bonded to oxygen.

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31

What is the significance of major elements in igneous rocks?

Elements with concentrations greater than 1 wt.% determine the formation of rock-forming mineral phases

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32

How can minor elements affect mineral phases in igneous rocks?

Elements with concentrations of 0.1 to 1 wt.% and can substitute into mineral phases, altering their characteristics.

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33

What does the Total Alkalis vs. Silica (TAS) diagram allow us to do in terms of classifying igneous rocks?

Classification of igneous rocks by plotting their bulk silcia content against the sodium and potasssium oxides

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34

How are volcanic rocks from different compositions typically shown on variation diagrams?

With compositional variability, with silica (SiO2) being commonly used on the x-axis due to its abundance and fractionation indicator.

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35

How does the placement of rocks on variation diagrams provide insights into their relation and formation?

A straight line connecting basalts, intermediate compositions, & rhyolites on a variation diagram suggests relation via fractional crystallization.

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36

What are the main uses of variation diagrams and their significance in geological studies?

understand chemical variations in volcanic rocks, allowing comparisons between lava flows from the same volcano & between different volcanoes

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37

What is the purpose of using a binary phase diagram, such as the MgO-SiO2 diagram, when studying liquid lines of descent?

help illustrate the crystallization process and how the composition of a magma changes as it crystallizes

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38

How does the composition of a magma evolve during olivine crystallization?

MgO is removed from the melt and partitioned into olivine, causing the liquid composition to become slightly more SiO2-rich.

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39

What happens to the melt composition as it continues to crystallize olivine and cool down?

evolves down the liquidus, moving towards a more evolved composition with lower MgO and higher SiO2

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40

How does fractional crystallization influence the magma's composition during olivine crystallization?

Removal of crystallized minerals. In this case, if olivine has been removed as it forms, the melt composition will evolve along the olivine liquidus.

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41

What mineral starts to crystallize when the melt composition becomes too evolved for further olivine crystallization?

When the melt composition becomes too evolved for olivine, plagioclase begins to crystallize due to its different composition and characteristics.

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42

What happens to the melt composition as it continues to crystallize olivine and cool down?

Evolves down the liquidus, moving towards a more evolved composition with lower MgO and higher SiO2.

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43

How does plagioclase crystallization affect the melt composition?

leads the melt composition to evolve directly away from the plagioclase composition, resulting in a kinked liquid line of descent

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44

How can you determine the relative proportions of olivine and plagioclase in a crystallizing assemblage when both minerals are crystallizing?

By projecting the trajectories of the liquid line of descent & the Lever Rule, the relative proportions of the two crystallizing phases can be determined.

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45

What happens when a third mineral, like augite, is added to the crystallization process?

The liquid line of descent may intersect tie lines between various mineral compositions, indicating different possible mineral assemblages

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46

How does the position of the liquid line of descent indicate the crystallizing mineral assemblage?

Position relative to tie lines reveals the possible mineral assemblages, helping to determine which minerals are crystallizing & in what proportions.

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47

What are primary melts, and how do they relate to fractional crystallization?

Produced by melting the mantle and undergo little or no fractional crystallization before eruption

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48

How can you distinguish primitive melts from other compositions in a liquid line of descent?

compositions with the highest MgO and lowest SiO2 contents along a liquid line of descent

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49

What does it mean for a melt to be considered parental to another melt in terms of chemical compositions?

parental to a more evolved melt if their compositions are related by fractional crystallization

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50

How do peridotites primarily differ from pyroxenites in terms of mineral composition?

Peridotites are mostly composed of olivine, orthopyroxene, and clinopyroxene, while pyroxenites are dominated by the two pyroxenes

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51

What is the main mineral present in most mantle rocks, and how much of it is typically found in peridotites?

Most mantle rocks are peridotites, containing more than 40% olivine as a major mineral.

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52

How does the mineral composition of mantle olivine differ from the olivines found in basalts or gabbros?

Mantle olivines are Mg-rich and contain a smaller proportion of iron compared to Fe-rich olivines in basalts or gabbros.

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53

What is the typical composition of mantle olivines, and how is this composition expressed in terms of cations?

composition of about Fo92, where 92% of the cation sites are occupied by Mg2+ and 8% by Fe2+

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54

How does the stable aluminous mineral phase in peridotites change with pressure?

At pressures less than 0.8 GPa, the stable phase is plagioclase; between 0.8 and 2 GPa, it's spinel; and at pressures greater than 2 GPa, it's garnet.

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55

What additional mineral might be found in some peridotites, and how does it differ from biotite?

Some peridotites may contain phlogopite, a mica with a paler brown color in thin sections, which contains OH groups in its crystal lattice.

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56

What is the crystal structure of mantle spinel, and where do magnesium and aluminium cations occupy in the structure?

Mantle spinel has a cubic structure. Mg2+ cations occupy tetrahedral sites (yellow), and Al3+ cations occupy octahedral sites (blue).

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57

How does spinel appear in plane-polarized light, and how does it appear under cross-polarized light?

strongly colored and can appear nearly opaque in plane-polarized light. It will always be in extinction under cross-polarized light.

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58

What color do mantle garnets typically have in hand specimen & thin section?

typically pink or red in hand specimen and pale pink in thin section.

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59

How can you recognize mantle garnets in thin section, even if they appear colorless?

high relief in thin section, even if they appear colorless.

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60

What are the two major discontinuities shown on the depth-pressure diagram of the Earth's mantle, and what causes them?

410 km and 660 km depths. These are caused by changes in olivine's crystallographic structure

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61

What does the Mohorovicic discontinuity (Moho) mark the boundary between?

the boundary between crustal rocks and mantle rocks.

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62

How can we obtain information about the mantle's structure and composition despite its inaccessibility?

geophysical methods, examine mantle xenoliths, and conduct petrological experiments

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63

What is the density and p-wave velocity of rocks in the uppermost mantle, and how do these values match laboratory measurements?

density of about 3.4 kg/m3 and a p-wave velocity of about 8 km/s, matching laboratory measurements of peridotite

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64

How does the p-wave velocity of the upper mantle compare to the seismic velocities of pure olivine and pyroxene?

close to the seismic velocities of pure olivine and pyroxene measured in the lab at upper mantle pressures

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65

What are mantle xenoliths, and how do they provide insight into the mantle's composition and mineralogy?

pieces of mantle rock entrained into magma and brought to the surface, that show information about mantle properties that are otherwise inaccessible.

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66

how are xenoliths formed?

rock fragments enveloped in magma and transported to the surface as part of the magma

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67

How do mantle xenoliths differ from the host magma, and where are they usually derived from?

Foreign to the host magma and not chemically related to it. They are usually derived from pieces of the mantle rock entrained into the magma.

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68

How do kimberlite magmas ascend rapidly to the surface, and why are they buoyant?

due to their buoyancy, which is a result of their high concentrations of CO2 and H2O

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69

What are kimberlite magmas, and how are they different from other types of magma?

formed from a small fraction of melted mantle. They have high concentrations of CO2 and H2O, resulting in low bulk density and high buoyancy

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70

How do kimberlite magmas incorporate mantle xenoliths, and why are mantle xenoliths important?

drill through country rock, incorporating dense xenoliths of mantle and crustal rock that are important because they provide information about mantle composition and mineralogy

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71

What is the stable polymorph of carbon in the cratonic lithosphere, and what geological feature marks its stability boundary?

Diamond is the stable polymorph of carbon in the cratonic lithosphere.

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72

How do kimberlite magmas transport diamond-bearing mantle xenoliths to the surface?

by drilling through the overlying lithosphere and incorporating the xenoliths as they ascend

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73

How do mantle xenoliths in kimberlites typically appear in hand specimen?

coarse-grained and resemble fragmental sedimentary rocks

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74

why might mantle xenoliths be rare in kimberlites?

due to the chaotic nature of kimberlite magmas

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75

What are xenocrysts, and how do they appear in thin sections of mantle xenoliths?

Singular mantle crystals within mantle xenoliths. In thin sections, xenocrysts can be seen as rounded, brightly birefringent olivine crystals.

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76

What are garnet lherzolite and spinel lherzolite, and how can they coexist in a single kimberlite pipe?

Common types of mantle xenoliths, that can coexist in a single kimberlite pipe due to variations in magma formation depth.

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77

What mineral is indicated by the presence of an aluminous phase in mantle xenoliths, and what does it reveal about magma formation depth?

indicates the depth of magma formation and the mantle it passed through

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78

What are ophiolites?

Oceanic lithosphere slices emplaced onto continental lithosphere.

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79

how do ophiolites form?

Through the collision of continental plates, which can lead to the incorporation of ophiolites into orogenic sequences.

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80

What is the significance of the petrological Moho in the context of ophiolite stratigraphy?

the petrological Moho marks the transition between rocks formed by crystallization and those derived from the mantle.

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81

What is the Troodos Ophiolite, and how did it form?

intact ophiolite sequence that formed 90 million years ago. It was emplaced onto continental lithosphere after the closing the Tethyan Ocean.

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82

What are pillow lavas, and how do they differ from sheeted dike complexes?

Pillow lavas are the extrusive basalt flows at the top of the ophiolite sequence. Sheeted dike complexes underlie pillow lavas & are primarily composed of dolerite dikes.

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83

What types of rocks are found in the ultramafic cumulates beneath the sheeted dike complex in an ophiolite sequence?

Layered gabbros & ultramafic cumulates are found beneath the sheeted dikes. They form from settling crystals in intrusions at the lower crust.

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84

How do weathering and alteration affect the appearance of ophiolite peridotites, and what alteration process causes the rusty red coloration?

Weathering & alteration cause Fe in olivine to oxidize, resulting in a red coloration. Hydrothermal fluids cause serpentinization, altering olivine to serpentine & creating light green veins.

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85

How does the peridotite ternary phase diagram represent the mineral compositions in the mantle?

Uses a triangle to represent olivine, clinopyroxene, and orthopyroxene compositions, with each corner representing these mineral phases.

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86

What does the cross on the peridotite ternary phase diagram indicate?

typical mineralogy of mantle lherzolite

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87

What does the colored area on the peridotite ternary phase diagram represent?

Areas with 1 stable mineral phase + liquid

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88

What is the significance of the ternary eutectic point on the peridotite ternary phase diagram?

The lowest temperature point on the liquidus surface and represents the composition of melt produced when melting begins in a mantle rock with olivine, cpx, and opx.

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89

What is equilibrium melting, and how does it differ from equilibrium crystallization?

Continuous chemical exchange between melt and crystals, with no mass exchange during melting. It is the reverse process of equilibrium crystallization.

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90

Describe the evolution of solid composition during equilibrium melting on a ternary phase diagram.

During equilibrium melting, the solid composition evolves away from the melt along a construction line from the eutectic to the initial solid composition (X).

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91

How are harzburgites formed through equilibrium melting?

by extracting a small melt fraction from mantle lherzolite.

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92

How are dunites formed through equilibrium melting?

by extracting a large fraction of partial melt from mantle lherzolite

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93

Why is fractional melting a more accurate description of mantle melting compared to equilibrium melting?

Because primary mantle melts have lower density than the solid mantle, causing formed melt to leave the system (mass loss).

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94

Explain the process of fractional melting using a ternary phase diagram.

Extraction of melt from a solid, causing changing composition & mineralogy. Starts with a lherzolite comp X, where melting begins at the ternary eutectic. The melting reaction progresses through several stages until pure olivine is reached.

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95

How does the process of fractional melting differ from the reverse process of fractional crystallization?

Formed melt leaves the system, causing changes in solid composition, whereas in fractional crystallization, formed crystals are removed from the system, causing changes in melt composition.

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96

What is the key feature that distinguishes fertile mantle from depleted mantle?

Low composition of cpx

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97

What types of mantle rocks are classified as fertile or undepleted?

Xenoliths and orogenic peridotites are mostly classified as fertile or undepleted mantle rocks with minimal or no partial melting.

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98

Why do basalts have low MgO contents compared to mantle lherzolite?

because magnesium (Mg) tends to remain in the solid phase during melting due to the strong Mg-O bonds

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99

Why are incompatible elements enriched in basaltic melts during partial melting?

They have weaker bonds with oxygen & don’t fit as well into mantle minerals. During partial melting, they enter the melt instead of staying in the solid phase.

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100

How does the composition of primary mantle melts depend on the melting conditions?

The composition of primary mantle melts depends on pressure, temperature, & the extent of partial melting. Higher degrees of partial melting yield tholeiitic basalts, while smaller degrees yield alkali basalts.

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