Basaltic magmas and mafic minerals

Question 1

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

Answer 1

tectonic settings of magma generation

Question 2

How are subaerial volcanoes usually found?

Answer 2

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

Question 3

What area is most important for magma generation?

Answer 3

Mid-ocean ridges

Question 4

What are isolated sub-aerial volcanoes linked to?

Answer 4

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

Question 5

What is volcanism a manifestation of?

Answer 5

magma reaching the surface

Question 6

Where will you find volcanic arcs?

Answer 6

destructive convergent plate margins

Question 7

Where will you find rift valleys/ mid ocean ridges?

Answer 7

Divergent/ constructive plate boundaries

Question 8

What are mantle plumes associated with?

Answer 8

intraplate volcanism (hotspots)

Question 9

What leads to the generation of primary magmas?

Answer 9

partial melting

Question 10

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

Answer 10

igneous rocks

Question 11

What is the mantle rock?

Answer 11

Lherzolite – the type of peridotite within
the upper mantle

Question 12

What will peridotite be like?

Answer 12

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

Question 13

What can a rock be defined by?

Answer 13

bulk chemistry

Question 14

How is bulk chemistry often defined?

Answer 14

in terms of wt% of oxides of major elements

Question 15

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

Answer 15

this will be due how the geotherm and solidus interact

Question 16

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

Answer 16

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

Question 17

What will cause partial melting at mid-ocean ridges?

Answer 17

decompression melting

Question 18

What is decompression melting?

Answer 18

hot mantle upwelling in response to divergence which reduces pressure

Question 19

What will cause partial melting of mantle at mantle plumes?

Answer 19

hot mantles ascending to lower pressures

Question 20

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

Answer 20

water added lowering the melting point

Question 21

Will the mantle undergo full melting?

Answer 21

no only a small proportion (20-30%)

Question 22

What does the proportion of rock melt depend on?

Answer 22

extent to which conditions exceed the solidus

Question 23

How does partial melting affect chemical composition?

Answer 23

melt formed will not be same composition as mantle lherzolite

Question 24

What does partially mantle peridotite produce?

Answer 24

broadly basaltic composition

Question 25

What do we call melts sourced directly from the mantle?

Answer 25

primary magmas

Question 26

What is the density of mantle peridotite?

Answer 26

3300kg/m*3

Question 27

What is the density of basaltic magma?

Answer 27

2700kg/m*3

Question 28

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

Answer 28

buoyancy- ascend upward to pressure gradient

Question 29

What will buoyancy do to primary magmas?

Answer 29

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

Question 30

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

Answer 30

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

Question 31

What is the counterpart to primitive magmas?

Answer 31

evolved - rich silica and alkali rich

Question 32

What are evolved magmas also referred to as?

Answer 32

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

Question 33

What happens after magmas ascend from the mantle?

Answer 33

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

Question 34

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

Answer 34

will form rocks of the same composition

Question 35

What will the cooling rate of basaltic magma control?

Answer 35

coarsely or finely crystalline

Question 36

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

Answer 36

basalt

Question 37

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

Answer 37

gabbro

Question 38

What are the common minerals in basalt and gabbro?

Answer 38

Olivine Pyroxene Plagioclase feldspar

Question 39

What is the structure of olivine?

Answer 39

isolated silicate tetrahedra with 2 cations per tetrahedra balancing charge

Question 40

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

Answer 40

Forsterite - Mg Fayalite- Fe

Question 41

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

Answer 41

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

Question 42

What is the colour property olivine?

Answer 42

Olive green / greenish-yellow in hand

Question 43

What will crystals be like in olivine?

Answer 43

granular masses of blocky to rounded crystals

Question 44

What will cleavage be like in olivine?

Answer 44

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

Question 45

What is olivine prone to at lower temp?

Answer 45

alteration- often visible in thin sections

Question 46

What are the optical properties of olivine in thin section?

Answer 46

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

Question 47

What is the structure of pyroxenes?

Answer 47

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

Question 48

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

Answer 48

Orthopyroxene (orthorhombic) Clinopyroxene (monoclinic)

Question 49

What are the properties of pyroxene crystals?

Answer 49

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

Question 50

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

Answer 50

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

Question 51

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

Answer 51

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

Question 52

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

Answer 52

Up to second order birefringence Inclined extinction relative to cleavage

Question 53

What is plagioclase feldspar like in hand specimen?

Answer 53

white blocky crystals

Question 54

What is plagioclase feldspar like in hand specimen?

Answer 54

low relief colourless low first order birefringence

Question 55

What is plagioclase feldspar like in xpl?

Answer 55

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

Question 56

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

Answer 56

related "family" of minerals which reflect bulk chemistry of magma crystallised from

Question 57

How many minerals will igneous rocks by predominantly comprised of?

Answer 57

3 or 4

Question 58

What is the amphibole mineral group like?

Answer 58

common in intermediate igneous as well as evolved

Question 59

What is an example of an amphibole?

Answer 59

Hornblende

Question 60

What is the structure of hornblende like?

Answer 60

Double chain silicate large ionic site

Question 61

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

Answer 61

hydroxyl (OH) ions

Question 62

What will hornblende be like in hand specimen?

Answer 62

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

Question 63

What is hornblende like in thin section ppl?

Answer 63

brown or green colour strong pleochroism moderate relief

Question 64

What is hornblende like in thin section xpl?

Answer 64

Up to low second-order birefringence Well-developed cleavage