Bowen's reaction series A2 Flashcards
Types of pyroxene
Augite and Diopside
Types of olivine
Forsterite and Fayalite
what is the most common type of amphibole
Hornblende
2 sections Bowens reaction series
discontinuous
continuous
discontinuous series minerals/rocks
high temp olivine UM
pyroxene
M
amphibole
biotite I low temp
Continuous series minerals/rocks
High temp Ca rich (UM)
(plagioclase feldspar)
low temp Na rich (I)
whole Bowen’s reaction series
olivine Ca rich
pyroxene
amphibole
(hornblende)
biotite Na rich
orthoclase
Muscovite
Quartz
As we move down temp and silica content decrease
Discontinuous reaction series explanation
Left hand branch
crystallisation of minerals rich in Mg and Fe
in M magma olivine first to form
> 1500 degrees
as temp decreases:
pyroxene—>amphibole—>biotite
slow cooling allows high temp minerals to react with remaining magma to form low temp minerals
if fast cooling e.g. eruption
olivine may be preserved
if one reaction is incomplete may see a reaction rim of old mineral
continuous reaction series explanation
right hand branch
crystallisation of P feldspar from Anorthite (Ca rich) to albite (Na rich)
in-between there are intermediate versions of P feldspar (mix of Ca and Na)
Ca rich only really seen in UM/M rocks
Na rich only really seen in IM/ maybe S rocks
Eutectic systems
melt looks/is homogenous (ratio of comp determines which crystal forms first)
temp decreases
crystallisation simultaneous on cont and discont
when crystals form they are immiscible (non homo) from 2 solid planes
2 solid planes are known as eutectic system
+ can be seen in eutectic phase diagram
eutectic point
point at which all 3 phases exist
liquid, solid and mix
liquidus
Phase boundary
everything above this line is melt
below some crystal and some melt- partial melt
2 different types of Eutectic phase diagrams
binary e.g. diopside-Anorthite
and
solid solution e.g. anorthite-Albite
and Forsterite-Fayalite
solidus
phase boundary
shows temp when the rock first begins to melt
anything below = solid
differences between binary and solid solution eutectic diagrams
(how to read them)
binary:
discont series
looks like 2 hills
To see which crystals precipitate in different comps you go up from comp and see which hill
to consider og melt composition you have to go across at the temperature and down to the comp when u hit the liquidus of the relevant crystal
solid solution:
cont series
onle 1 region
when reading this diagram you must go horizontally across the blob
e.g. if you were trying to figure out what comp crystals would first form you would drop down at the melt till you hit the liquidus
move along till you hit the solidus and then move down to the comp
Zone crystals
when crystals form at high temp it will be mafic
magma cools more layers added to crystals
outer layers must be less mafic
e,g, with plagioclase feldspar
more Ca rich inside and Na rich outside
drawn as squares inside squares
forsterite vs fayalite
both olivine
Zoomed in look at Bowens reaction series
high temp Mg rich-Forsterite
e.g. 1500
Low temp Fe rich-Fayalite
e.g. 1400
as temp decreases Fe subs for Mg
Minerals at low temps of Bowens reaction series
2 series converge at low temp
final group of minerals don’t react with remaining liquid
they are Silicic
orthoclase–> muscovite–>Quartz
as temp decreases
Why do large igneous intrusions display a range of diff rock types
magma may not be homogenous
diff cooling rates
stoping or assimilation of C rock (changes comp)
as magma cooled injection of new magma from below (change comp)
How can magmas mix +why don’t they usually?
don’t usually as:
diff densities, diff temps, diff viscosities
can mix if there is a strong mixing mechanism = strong convection current required
How can magma become contaminated with diff rock
stoping and assimilation of C rock
change comp
begin with xenolith, melts, changes comp
assimilation changes bulk comp e.g. M magma + s country rock = inter magma
3 types of magma differentiation
fractional crystallisation
gravity settling
filter pressing
fractional crystallisation
diff mineral crystals form at diff temps
e.g. olivine/pyroxene form at higher temp
these incorp a lot of Fe and Mg
remaining Magma is more silicic
becomes increasingly more silicic
Gravity settling
solid nearly always denser than liquid
crystals form = denser than melt
first crystals form are Mafic = most dense
gravity causes minerals to settle at bottom of magma chamber
forms cumulate layers with mafic layers at bottom
if they stay in melt instead then they cont bowens reaction series
Filter pressing
during crystallisation there is a point when liquid + crystals together = slushy mess
mass of overlying crystals squeezes liquid out
forms layer above = more silicic
chem analysis of rocks
rocks contain many diff chemicals e.g.
SiO2
MgO
FeO
Al2O3
etc.
proportions are dependent on S vs I vs M vs UM
e.g. less silicic= less SiO2
more mafic = more MgO (also more FeO but less diff)
Examples of where magma differentiation occurred
Palisade sill
Skaergaard intrusion
Hekla volcano
Bushveld igneous complex
palisade sill description and explanation
see diagram in booklet
C rock = sandstone
sandstone-metaquartizite-basalt-dolerite-gabbro-dolerite-olivine-basalt-metaquartzite-sandstone
metaquartzite = baked margin
basalt= cooled margin
dolerite= fractionation results comp lower in M minerals + richer in S
Gabbro= coarse cooled slowest
olivine- first crystals to form as high MP (fractionation), denser, sink, gravity settling
Skaergaard intrusion description and explanation
see diagram in booklet
upper border rock-
mirrors zoned layers but thinner as early formed minerals stuck to roof of M chamber
contains section of S rock (formed from last part of melt which was highly S due to fractionation)
upper zone layer
mid zoned layer
lower zoned layer
l———-v———-l
layered series shoes rhyming layers
crystal settling interrupted by large scale convection
gravity settling
filter pressing > expulsion of L > convection mixes > repeat > each time more evolved (more s)
each layer has plagio on top of pyroxene and olivine
Marginal border zone- chilled margin + contaminated by C rock (Gneiss)+ alternating hot fluids
Helka volcano description + explanation
see booklet for diagram
Expected to be mafic as Iceland + D plate margin
yet is inter/ Rhyolite
longer between eruptions= more S
fractionation: mafic min crystalise at higher temps > gravity settling > silicic at top > eruption get progressively more mafic
Bushveld igneous complex
description
South Africa
layered Igneous intrusion
400KM * 800Km
vol= 1000000 Km^3
varied thickness up to 8 km
formed 2 million years ago
contains rich deps- largest reserve of platinum
mineral dep of platinum group elements
e.g. Pt Pd Ag Cu
rare in crust as mostly react with Fe and in outer core
the one that don’t react with Fe react with sulphur
react with Fe= siderophile (outer core)
react with S= chalcophile (crust)
