Geoscience Reference
In-Depth Information
Exercises
1.
Table 2.2
shows the major element compositions of the glass (
=
gl) and mineral phases
plag) in a basaltic lava from the
Réunion Island hot spot (Indian Ocean). Calculate the composition of a bulk lava made
of this glass with 15 wt% olivine and 10 wt% clinopyroxene phenocrysts.
2. Using the same data, calculate the major element composition of a wehrlite cumulate
(80 wt% olivine and 20 wt% clinopyroxene), of a gabbroic cumulate (10 wt% olivine,
40 wt% clinopyroxene, 50 wt% plagioclase).
3. Using the same data, we consider how much cumulus olivine may change whole-rock
compositions. Keeping the same interstitial glass and assuming variable proportions
of olivine phenocryst from 0 to 40 percent, calculate the major element contents of the
whole rock and turn the MgO and FeO weight percents into moles per 100 grams of
rock using the following atomic weights: 55.847 for Fe, 24.305 for Mg, and 16.00 for
O. It is known that, at equilibrium, (FeO/MgO)
ol
/(FeO/MgO)
gl
=
(olivine
=
ol, clinopyroxene
=
cpx, and plagioclase
=
0.3 and this condi-
tion can be drawn as a line in the same diagram. Plot the molar FeO/MgO ratio of
the olivine vs. this ratio in the whole rock for olivine with different forsterite con-
tents (e.g. different FeO/MgO ratios). Can you suggest a method to estimate whether
olivine megacrysts present in a lava are phenocrysts in equilibrium with their host
glass or xenocrysts? This method should not involve the analysis of the glass itself.
4.
Table 2.3
shows the partial analysis (wt% of oxide) of a deep-sea sediment composed
of quartz, clay, and carbonates. Calculate the abundance of each mineral phase in this
sample.
5. Let us evaluate the concentration of the siderophile elements Fe and Ni and the pro-
portion of other elements in the Earth's core. We assume that the Earth is made of a
silicate mantle (we neglect the crustal contribution), which is often referred to as the
Bulk Silicate Earth, and a metallic core. The core makes one third of the total mass of
the Earth. We further assume that the Earth is made of ordinary chondrites and we will
Table 2.2
Concentration of major elements (in weight percent of oxide) in the different
phases of a basaltic lava from the Réunion Island (Indian Ocean)
Glass
Olivine
Clinopyroxene
Plagioclase
SiO
2
48.81
38.79
46.63
52.49
TiO
2
2.68
0.047
3.8
0.153
Al
2
O
3
14.40
0.026
6.27
29.05
FeO
11.07
20.02
8.25
0.893
MnO
0.17
0.269
1.03
0.027
MgO
6.65
40.56
13.5
0.095
CaO
11.65
0.297
19.64
12.27
Na
2
O
2.74
0.04
0.59
4.33
K
2
O
0.79
0.024
0.05
0.372
