Igneous Petrology EPSC423, Francis 2013
Lab 9: MORB
1. Adiabatic Melting and the Major Element Composition of MORB.
Mid-Ocean Ridge basalts (MORB) represent the dominant form of
magmatism on the Earth, comprising ~60%of the Earth’s crust. Despite
its global extent, the major element composition of MORB is remarkably
similar everywhere. The composition of MORB is best approximated by
15-20% experimental partial melts at 10 - 15 kbs. Such experiments are
carried out isobarically, whereas MORB is thought to be produced by the
decompression of adiabatically upwelling mantle. In order to appreciate
the significance of this difference, in this lab you will use AlphaMelts to
carry out a series of melting calculations on the estimated mantle source
of MORB (DMM). First, partially melt DMM at 10kbs and plot the
series of equilibrium and aggregate fractional melts from 0 to 20%
melting in cation plots of Mg versus Fe, and Al versus Si. Next, perform
4 aggregate fractional partial melting calculations adiabatically, in
which rising DMM crosses the solidus at 10, 15, 20, and 25 Kbs 1,
respectively, and continues rise and melt until a depth of 5 kbs. Plot the
evolving aggregate melt composition with increasing degree of partial
melting for the 20kb model in the Al versus Si and Mg versus Fe
diagrams constructed previously.
Compare the calculated melt compositions to that of primitive MORB
and comment and describe the model that most closely approaches it.
SiO2
TiO2
Al2O3
Cr2O3
MgO
FeO
MnO
CaO
K2O
Na2O
P2O5
PUM
44.90
0.20
4.44
0.38
37.71
8.03
0.13
3.54
0.029
0.36
0.021
DMM
44.71
0.13
3.98
0.57
38.73
8.18
0.13
3.17
0.006
0.28
0.019
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Primitive MORB
40.51
0.90
16.75
0.07
9.74
8.05
0.14
12.50
0.065
2.18
0.095
Igneous Petrology EPSC423, Francis 2013
2. The Trace Element Signature of MORB
The chemical variations in MORB are subtle, yet their origins continue to be a
matter of active debate. There is typically little variation in the compatible trace
element composition of MORB, but a large variation in the incompatible trace
element composition. Two end-members can be defined; E and N MORB, the
former being enriched in incompatible trace elements compared to the latter. These
two MORB types are virtually indistinguishable from a major element or
compatible trace element point of view, but have distinct incompatible trace element
contents that can not be related by crystal fractionation.
In this lab you will evaluate the relative abilities of variable degrees of partial
melting to explain the differences between E & N MORB. In the attached table
you have been given selected trace element data for E & N MORB, model values
for both depleted (DMM) and primitive upper mantle. Plot two spider diagrams in
which the trace elements are ordered in terms of increasing bulk Kd, calculated
using the mineral Kd's and the source mode given in the table.
In the first spider diagram, normalise the trace element compositions of E &
N MORB to the composition of the primitive mantle (PUM).
In second spider diagram normalise the trace elements of E and N MORB,
to the composition of a depleted mantle source (DMM).
Using the equations for non-modal melting, calculate the trace element compositions
of both the aggregate fractional liquids, and the corresponding residues left behind
for the cases of 1, 2, 5, 10, and 20 % partial melting at a pressure of 10Kbs (ie. in
the spinel lherzolite stability field). Use the melt mode in the attached table to
calculate the factor Po that is required in the relevant equations. Plot your 5
calculated liquid compositions in the two spider diagrams above and, after some
reflection, comment on the implications of the results of your calculations for
whether the relationship between N & E MORB can be explained by differing
degrees of partial melting of a common spinel lherzolite mantle source.
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Igneous Petrology EPSC423, Francis 2013
Data and Formulae:
All values in ppm by weight
Primitive
Mantle
Nb
La
Sm
Zr
Y
Yb
Depleted
MORB Mantle
0.66
0.64
0.44
11.4
5.0
0.42
0.15
0.20
0.24
5.00
3.33
0.37
Kd's:
Nb
La
Sm
Zr
Y
Yb
E-MORB
N-MORB
11.3
10.0
4.0
118.0
24.0
2.3
4.3
2.8
73.0
23.0
3.4
3.3
Oliv
Opx
Cpx
0.001
0.005
0.005
0.01
0.01
0.01
0.005
0.01
0.01
0.05
0.4
0.05
0.01
0.05
0.20
0.25
0.5
0.40
Source Mode at 10Kbs:
olivine 60%, opx 25%, cpx 15%
Melt mode at 10 Kbs:
olivine 0.0%, opx 30%, cpx 70%
Equilibrium partial melt:
Ciliq = Cio / (Dio + F × (1-Pi))
Aggregate fractional melt:
Cialiq = Cio × (1 - (1- (Pi × F) / Dio)1/Pi) / F
Bulk Kd:
Dio
= Xα × Kiα + Xβ × Kiβ + ……….
Melt Kd:
Pi
= Pα × Kiα + Pβ × Kiβ + ……
References:
Wood, B.J., and Fraser, D.G.; 1978: Elementary Thermodynamics for Geologists. Chapter
6, 195-228.
Workman, R.K. and Hart, S.R.; 2005: Major and trace element composition of the depleted
MORB mantle (DMM). Earth and Planetary Science Letters 231, 53-72.
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