Igneous General

Question 1

Basalts in general:

Answer 1

• Fine grained – aphanitic = Fast cooling rate (eruption) = aphanitic = rate of cooling is impeding crystal growth
• Microphenocrysts of euhedral lath shaped plagioclase
• Fine grained grain mass

Question 2

How to distinguish Tholeiitic Basalts

Answer 2

• Olivine phenocrysts can be present but not abundant
• Olivine absent from groundmass
• Presence of OPX (2-pyroxenes common)
• Presence of interstitial glass
• Silica infills to vesicles
• If a dyke has glassy rims or is chilled totally to glass, it is likely to be a tholeiite

Question 3

How to distinguish Alkalic Basalts

Answer 3

• Abundant olivine phenocrysts
• Olivine in the groundmass
• Absence of OPX
• Absence of interstitial glass
• Clinopyroxene – pale pink to light green
• Interstitial zeolite
• Zeolite infills to vesicles

Question 4

Geochemical Plots and Fractional Crystallisation

Answer 4

• Geochemical plot taken from lava
• Direction of crystallisation is the direction in which MgO decreases because olivine is crystallised first
• Find out what’s being depleted from lava = what’s being crystallised out
• What’s enriched in lava = what’s being left behind
• Take elements out and elements remaining to determine which minerals are crystallising
• Normally will be Olivine first in magmas

Question 5

Geokem files for lavas

Answer 5

• Because this is the lava, if the element is depleted then it is being crystallised and is leaving the melt
o It is therefore behaving as a compatible element because it is leaving melt
o Those which become enriched are remaining and are therefore incompatible

Question 6

Cumulate Layers

Answer 6

A - The plagioclase crystals (white) accumulate in mutual contact, and an intercumulus liquid fills the interstices
B - Orthocumulate: in which the cumulus (plagioclase) crystals are enclosed in material that has crystallised from the intercumulus liquid. In this example, the intercumulus liquid crystallizes to form additional plagioclase rims plus other phases in the interstitial volume. This texture suggests that there is little or no exchange between the intercumulus liquid and the main magma chamber
C - Adcumulate: in which the cumulus (plagioclase) crystals continue to grow and displace the intercumulus liquid. This texture is taken to indicate continue open-system exchange between the intercumulus liquid and the main chamber (plus compaction of the cumulate pile) allowing components that would otherwise create additional intercumulus minerals to escape, and plagioclase fills most of the available space.
D - Heteradcumulate: in which intercumulus liquid crystallizes to additional plagioclase rims, plus other large minerals (hatched and shaded) that nucleate poorly and poikilitically envelop the plagioclases.