Igneous Petrology (EPSC-423A) – Francis, 2013
Lab 2: Nature of Mantle Source Regions
This week’s lab focuses on the relationship between the chondritic meteorites, which
approximate the starting composition of both the Solar System and the Earth, the
Earth’s mantle, and high-Mg picritic basalts that are thought to be the primary melts
of the mantle.
Station A: - Chondritic Meteorites
Specimens: NWA 4794, NWA 4842, NWA 5316
Examine one or more of these chondritic meteorites.
Tasks:
Identify as many minerals as possible in one of the above meteorites and describe
the meteorite’s texture. What are the spherical structures and what do they
represent?
Although the detailed mineralogy of chondritic meteorites is often difficult to
determine because the presence of cryptocrystalline phases, and the development of
low temperature hydrated silicates, the anhydrous high-temperature mineralogy
can be calculated from its chemical composition using the following recipe:
Simplified Recipe for Calculating the NORMATIVE mineralogy of an ultramafic rock:
Convert weight percent oxide analyses into cation units, ignore Feo, Coo, Nio and FeS, and proceed in the
following steps to calculate the molar amounts of the following minerals:
= Feo + + Nio + Coo
1)
Metal
2)
Sulfide = FeS
3)
Graphite = C
4)
Apatite = P5+ / 3
5)
Clinopyroxene (XYT2O6) = X = (Ca2+ + Na+ + K+) - 5 x Apatite
6)
7)
Spinel (Y2+Y3+2O4) = Y3+/2 = Cr3+/2 + (Al3+ - 2Ti4+ - Na+)/2
.
Orthopyroxene* (Y2T2O6) = Y/2 = ((Mg2+ + Fe2+ + Mn2+ + Ni2+ + Co2+) - Spin - Cpx)/2
7)
Si* = Si4+ - 2 x Cpx - 2 x Opx* + Ti4+
8)
If Si* is negative:
Olivine (Y2TO4) = - Si*
Si* = 00.00
Opx = Opx* - Oliv
9)
If Si* is positive:
Qtz = Si*
Olivine = 0.0
Opx = Opx*
1
Igneous Petrology (EPSC-423A) – Francis, 2013
Perform the above calculation on the chondrite meteorite analysis in table 1.
Convert the silicate mineral molar units into oxygen units, normalize to 100 %, and
plot this chondrite meteorite in the liquidus projection of the Olivine Clinopyroxene – Quartz system.
Note: You will want to set this calculation up on a spread sheet as you will use do it for 5
different rock compositions. In order to plot your points, orthopyroxene must be recalculated
as a mixture of olivine and quartz.
Table 1: H5 Ordinary Chondritic Meteorite
Feo
Nio
Coo
FeS
SiO2
TiO2
Al2O3
Cr2O3
MgO
FeO
MnO
CaO
Na2O
K2O
P2O5
H2O
15.15
1.88
0.13
6.11
36.55
0.14
1.91
0.52
23.47
10.21
0.32
2.41
0.78
0.20
0.30
0.21
Total
100.29
Fig. 1: Olivine – Clinopyroxene – Silica liquidus projection
2
Igneous Petrology (EPSC-423A) – Francis, 2013
Station B – Fertile Upper Mantle
Specimens: AL-46 or Al-103.
These samples are xenoliths of the Earth’s mantle carried to the surface by recent
alkaline basalts of the northern Canadian Cordillera. They are relatively fertile
lherzolites, that is they have experienced very little partial melting and thus have
compositions that approach those of chondritic meteorites, if the latter’s Fe- metal
and sulphide are removed.
Tasks:
Examine one of the above xenoliths in detail.
Identify the important minerals and estimate their proportions.
Describe the texture of the lherzolite xenolith and compare it to the texture
observed for the chondritic meteorite that you described at Station A. Briefly
discuss the implications of this comparison for the crystallization and cooling
history of both the lherzolite xenolith and the chondritic meteorite.
Estimate the last temperature of equilibrium of lherzolite AL-46 from the extent of
solid solution between coexisting clinopyroxene and orthopyroxene.
Fig. 2:
Enstatite - Diopside Join versus Temperature
3
Igneous Petrology (EPSC-423A) – Francis, 2013
At any given temperature, the width of the miscibility gap between Cpx and
Opx decreases with increasing Fe, and therefore we have to include the effect of
Fe in the Cpx using a Pyroxene Quadrilateral (Fig. 3). Plot the composition of
the Cpx in AL-46 in the following Pyroxene quadrilateral and estimate its last
temperature of equilibrium from the contoured curves.
Fig. 3 Pyroxene Quadrilateral contoured for Temperature
Projection Scheme for Cpx in the Pyroxene Quadrilateral:
Calculate an ideal Cpx formula (XYT2O6) in which the cations sum to 4
Al(1V)
= 2 - Si
Al(VI)
= Al - Al(1V)
CrCaTs
= Cr
AlCaTs
= Al(VI) – CrCaTs – Na (CaAlAlSiO6)
Jd
= Na
Wo
= (Ca - AlCaTs – CrCaTs)/2
En
= (1-Wo-Jd) × XMg
Fs
= (1-Wo-Jd) × (1-XMg)
(CaCrAlSiO6)
(NaAlSi2O6)
4
where
XMg = Mg/(Fe+Mg)
Igneous Petrology (EPSC-423A) – Francis, 2013
The extent of solid solution of enstatite in Cpx as a function of both
temperature and pressure has now been more recently parameterized by the
following equation:
T = 23644 + (24.9+126.3×XFeCpx) × Press
13.38 + (LnKd)2 + 11.59 × XFeopx
Kd = (1-Ca*)Cpx / (1-Ca*opx) where Ca* = Ca / (1-Na) and XFe = Fe/(Fe + Mg)
Note: Pressure is in Kilobars, temperature is in Kelvin, and the pyroxene
compositions must be calculated in cations normalized to a pyroxene formula
(Σcations = 4, XYT2O6).
There is no geobarometer that works for these xenoliths - the only pressure
constraints are those for the stability field of spinel lherzolite, which range from
approximately 10 to 20 Kbs (see Fig. 4). Use a pressure of 15 kbs to calculate
the last equilibrium temperature of the Al-46. How does this temperature
compare with your graphical estimate? What difference in temperature would
be produced by an increase or decrease in pressure of 5 Kbs? Plot the
approximate position of Al-46 in Fig. 4.
Calculate the NORMATIVE mineralogy of the lherzolite you have examined
(Table 2) according to the recipe used for the chondritic meteorite and
compare the calculated normative mineralogy to your visually estimated
modal mineralogy.
Plot the calculated normative composition of the
lherzolite xenolith in the Oliv-Diop-Qtz liquidus projection you plotted for the
chondritic meteorite.
Assuming that the first melt of the mantle rocks will have a composition
approximated by the invariant point in the Oliv-Diop-Qtz system at 10Kbs,
use the lever rule to calculate the amount of picritic basalt that can be
extracted from this lherzolite until clinopyroxene is exhausted.
Plot the composition of the partial melt at 15 kbs Cpx-out that you obtained
from Alpha-Melts in last week’s Lab(1). How does its position compare with
that of the invariant point P.
Finally, use AlphaMelts to construct a P-T phase diagram for Al-46 showing
the solidus and the cpx-out curves (along with the solidus and near-solidus
phases) from1atm to 30 Kbs. Do this by repeating last week’s isobaric melting
calculation at different pressures and joining the dots for the
appearance/disappearance of phases. At what pressure does garnet become
stable on the solidus of Al-46?
5
Igneous Petrology (EPSC-423A) – Francis, 2013
Table 2: Mantle Xenoliths
Sample: AL-46
Rock: lherz
AL-46
Cpx
AL-46
Opx
NK1-7
Xeno
NK1-7
Cpx
NK1-7
Opx
LG-25A
Harz
SiO2
TiO2
Cr2O3
Al2O3
MgO
NiO
FeO
MnO
CaO
Na2O
K2O
45.27
0.10
0.38
3.56
38.75
0.27
8.32
0.14
3.20
0.21
0.01
51.94
0.39
0.61
6.10
15.14
0.00
2.85
0.10
20.67
1.56
0.00
55.07
0.08
0.31
4.26
32.93
0.00
6.26
0.13
0.63
0.08
0.00
41.25
0.03
0.48
1.57
43.55
0.31
7.06
0.10
0.72
0.00
0.02
54.10
0.08
1.77
2.36
18.08
0.06
2.34
0.08
18.68
1.61
0.04
57.44
0.04
0.54
1.36
34.99
0.10
4.42
0.11
0.93
0.18
0.03
44.61
0.01
0.41
0.81
45.98
0.33
7.52
0.12
0.64
0.00
0.01
Total
100.21
99.35
99.75
95.09
99.20
100.14
100.45
6
Igneous Petrology (EPSC-423A) – Francis-13
Station C – Deeper Mantle
Specimens: NK1-5, NK1-6, NK1-7, NK1-8, or NK1-9
Theses samples are mantle xenoliths brought to the Earth’s surface by the Nikos
kimberlite pipe of Somerset Island, Nunavut.
Tasks:
Examine theses xenoliths and identify the major mineral phases. Describe how these
xenoliths differ from the lherzolites that you described at Station B, using the hand
specimens and the thin section of NK3-1.
Calculate the last temperature of equilibrium for xenolith NK1-7 using the
compositions of its coexisting clinopyroxene and orthopyroxene (Table 2) and the
algorithm used at Station B, but assuming an initial pressure of 25 Kbs.
Estimate the last pressure of equilibrium of NK1-7 by plotting it in Figure 4 using the
above temperature and the Al2O3 content of the opx coexisting with garnet.
Fig. 4:
Pressure versus Temperature in Ultramafic Rocks
Calculate the pressure using the following parameterization of the Al content in
orthopyroxene coexisting with garnet. Redo the temperature calculation with the new
estimated pressure, and then recalculate the pressure. Compare your P-T estimates
with those made for the lherzolite xenolith at Station B. From how deep in the Earth
did NK1-1 come?
7
Igneous Petrology (EPSC-423A) – Francis-13
Pressure = (Temp × Ln(XAlOPX)-1.46×Temp+3736) / -97.1 XAlOPX = wt. fraction
Al2O3 in opx
Section D – Depleted Mantle
Specimens: XLG-25A, 10A, 14A, or 16A
The proportion of olivine versus clinopyroxene and spinel in mantle samples is
thought to reflect the degree of partial melting and amount of basalt extraction they
have experienced. The composition of the basalt extracted is controlled by the
pseudo-invariant point in the system Oliv-Diop-Qtz, (Fig. 2), and is rich in
clinopyroxene compared to the mantle source. Upon removal of this melt, the
composition of the remaining restite moves away from the composition of the melt,
towards the olivine - orthopyroxene join. XLG-25A is a depleted harzburgite that
represents the restite of a relatively high degree of partial melting.
Tasks:
Examine this harzburgite xenolith:
Compare its modal mineralogy to that of the fertile lherzolite xenolith you
described from station B.
Calculate the normative mineralogy of your sample and plot it in the Olivine Clinopyroxene - Qtz liquidus projection you have already constructed. Using
the leaver rule, estimate the amount of basalt remaining in XLG-25A and
compare this with the estimates you obtained for the fertile lherzolite at station
B.
Plot a “spider” diagram in which the Y axis is (Wt% element in Harz) / (Wt%
element in Lherz), and the X axis is the major elements in the sequence: Na,
Ti, Ca, Al, Fe, Mg, Cr.
8
Igneous Petrology (EPSC-423A) – Francis-13
Station E – Mantle Xenoliths
Specimens: AL-40, Al-102, Al-103, Al-105, Al-107, XLG-12A, PX-315, PX-501,
PX-510, DX-7 (all hand specimens)
The xenoliths at this station range from relatively fertile lherzolites to highly
depleted harzburgites. The degree of partial melting each of these samples has
experienced is ~ proportional to its oliv / pyroxene + spinel ratio.
Task:
Roughly estimate the oliv / pyrox ratio in each of the hand specimens by
comparison with the calculated norms of the fertile lherzolite and depleted
harzburgite xenoliths that you examined (and plotted) from Stations B and C.
List the samples from this station in order of decreasing abundance of basalt
component, and thus increasing degree of partial melting that they have
experienced.
9
Igneous Petrology (EPSC-423A) – Francis-13
10
Igneous Petrology (EPSC-423A) – Francis-13
11