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Atoms to volcanoes - 33739 > Basaltic magmas and mafic minerals > Flashcards

Basaltic magmas and mafic minerals Flashcards

1
Q

What does distribution of earths subaerial volcanoes give an insight into?

A

tectonic settings of magma generation

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2
Q

How are subaerial volcanoes usually found?

A

in long linear chains linked most commonly to convergent plate margins and subduction

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3
Q

What area is most important for magma generation?

A

Mid-ocean ridges

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4
Q

What are isolated sub-aerial volcanoes linked to?

A

Hotspots and mantle plumes (very hot magma melt surface) lead to creation of island arcs (Hawaii)

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5
Q

What is volcanism a manifestation of?

A

magma reaching the surface

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6
Q

Where will you find volcanic arcs?

A

destructive convergent plate margins

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7
Q

Where will you find rift valleys/ mid ocean ridges?

A

Divergent/ constructive plate boundaries

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8
Q

What are mantle plumes associated with?

A

intraplate volcanism (hotspots)

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9
Q

What leads to the generation of primary magmas?

A

partial melting

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10
Q

What does mantle melting lead to the production of in the crust?

A

igneous rocks

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11
Q

What is the mantle rock?

A

Lherzolite – the type of peridotite within
the upper mantle

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12
Q

What will peridotite be like?

A

mixture of minerals
Assemblage:
olivine, orthopyroxene, clinopyroxene
garnet (spinel low pressure, even low pressure plagioclase)

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13
Q

What can a rock be defined by?

A

bulk chemistry

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14
Q

How is bulk chemistry often defined?

A

in terms of wt% of oxides of major elements

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15
Q

Why does the mantle undergo partial melting? (general not examples)

A

this will be due how the geotherm and solidus interact

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16
Q

What is the solidus and how does it relate partial melting?

A

the solidus is the line which define if melting will occur if crossed due to higher temp or pressure

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17
Q

What will cause partial melting at mid-ocean ridges?

A

decompression melting

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18
Q

What is decompression melting?

A

hot mantle upwelling in response to divergence which reduces pressure

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19
Q

What will cause partial melting of mantle at mantle plumes?

A

hot mantles ascending to lower pressures

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20
Q

What is flux melting and how does lead to partial melting at subduction zones?

A

water added lowering the melting point

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21
Q

Will the mantle undergo full melting?

A

no only a small proportion (20-30%)

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22
Q

What does the proportion of rock melt depend on?

A

extent to which conditions exceed the solidus

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23
Q

How does partial melting affect chemical composition?

A

melt formed will not be same composition as mantle lherzolite

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24
Q

What does partially mantle peridotite produce?

A

broadly basaltic composition

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25
Q

What do we call melts sourced directly from the mantle?

A

primary magmas

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26
Q

What is the density of mantle peridotite?

A

3300kg/m*3

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27
Q

What is the density of basaltic magma?

A

2700kg/m*3

28
Q

What does the difference between densities of mantle peridotite and basaltic magma give basaltic liquids?

A

buoyancy- ascend upward to pressure gradient

29
Q

What will buoyancy do to primary magmas?

A

drive it to crustal levels where they lose heat and start to crystallise

30
Q

What can basalts that lie at the low-silica range of volcanic rocks also be referred to as?

A

basic magmas or mafic magmas due to common occurrence Fe- and Mg
or
Primitive magmas (close to primary composition)

31
Q

What is the counterpart to primitive magmas?

A

evolved - rich silica and alkali rich

32
Q

What are evolved magmas also referred to as?

A

acidic magmas
or
felsic (common occurrence in feldspar and quartz)

33
Q

What happens after magmas ascend from the mantle?

A

they may stall and completely solidify forming intrusive igneous rocks
or
erupt on surface forming extrusive igneous rocks

34
Q

What happens if basaltic magma cool and completely crystallise without further modifiction?

A

will form rocks of the same composition

35
Q

What will the cooling rate of basaltic magma control?

A

coarsely or finely crystalline

36
Q

What will fine grain rock formed from cooling basaltic lava be called?

A

basalt

37
Q

What will coarse grain rock formed from cooling basaltic lava be called?

A

gabbro

38
Q

What are the common minerals in basalt and gabbro?

A

Olivine
Pyroxene
Plagioclase feldspar

39
Q

What is the structure of olivine?

A

isolated silicate tetrahedra with 2 cations per tetrahedra balancing charge

40
Q

What are the 2 end members for the solid solution of olivine?

A

Forsterite - Mg
Fayalite- Fe

41
Q

What important general principle for the chemical evolution of magmas can be highlighted by depletion of minerals in liquid?

A

composition of crystals growing in liquid aren’t the same as composition of liquid
thus liquid composition evolves through crystallisation

42
Q

What is the colour property olivine?

A

Olive green / greenish-yellow in hand

43
Q

What will crystals be like in olivine?

A

granular masses of blocky to rounded crystals

44
Q

What will cleavage be like in olivine?

A

no good cleavage (but often has curved fractures in thin section)

45
Q

What is olivine prone to at lower temp?

A

alteration- often visible in thin sections

46
Q

What are the optical properties of olivine in thin section?

A

Colourless - ppl
High relief
Equant blocky grain (6 sided) rounded/ sub-rounded
No cleavage but curved cracks
Mid-high second order birefringence

47
Q

What is the structure of pyroxenes?

A

single-chain silicate tetrahedra (joined by shared Si) cation in between balancing charge

48
Q

What 2 distinct series can pyroxenes be split into based upon crystal structure?

A

Orthopyroxene (orthorhombic)
Clinopyroxene (monoclinic)

49
Q

What are the properties of pyroxene crystals?

A

Dark green to black colour in hand specimen
Short stubby prisms square to eight side in cross section

50
Q

What are the optical properties of pyroxenes in thin section for ppl?

A

Colourless to pale pink (opx), colourless to pale brown/green (cpx)
Moderate relief
Often 1 good cleavage visible 2 in end section

51
Q

What are the optical properties of pyroxenes in thin section xpl for orthopyroxene?

A

First order birefringence (yellow grey)
Straight extinction relative to cleavage

52
Q

What are the optical properties of pyroxenes in thin section xpl for Clinopyroxene?

A

Up to second order birefringence
Inclined extinction relative to cleavage

53
Q

What is plagioclase feldspar like in hand specimen?

A

white blocky crystals

54
Q

What is plagioclase feldspar like in hand specimen?

A

low relief
colourless
low first order birefringence

55
Q

What is plagioclase feldspar like in xpl?

A

Very distinctive lamellar twinning
(black/grey/white stripes)

56
Q

What can the assemblage of a rock be thought of as?

A

related “family” of minerals which reflect bulk chemistry of magma crystallised from

57
Q

How many minerals will igneous rocks by predominantly comprised of?

A

3 or 4

58
Q

What is the amphibole mineral group like?

A

common in intermediate igneous as well as evolved

59
Q

What is an example of an amphibole?

A

Hornblende

60
Q

What is the structure of hornblende like?

A

Double chain silicate
large ionic site

61
Q

What does the large ionic site of of hornblende mean it can accomodate?

A

hydroxyl (OH) ions

62
Q

What will hornblende be like in hand specimen?

A

crystals prismatic and dark coloured
(black/ dark brown)
Lozenge shape end section

63
Q

What is hornblende like in thin section ppl?

A

brown or green colour
strong pleochroism
moderate relief

64
Q

What is hornblende like in thin section xpl?

A

Up to low second-order birefringence
Well-developed cleavage

65
Q
A

Atoms to volcanoes - 33739 (16 decks)

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