REMEMBER: CA = CA0 / x, k = 0
This lab provides an insight, firstly into the ‘fingerprinting’ identification techniques used to identify the tectonic environment in which igneous rock form, and, secondly, into the fractionation process involved in continental growth during continental rifting.
EXERCISE 1
The following table contains a set of major and trace element abundances and abundance ratios characteristic of volcanic rocks representative of all the main tectonic environments. Use the plotting sheets provided to estimate the fractionation level (low, moderate, high) and environment of formation (MORB, oceanic arc, continental margin arc, rift, alkalic, shoshonitic, etc) of each analysis. Some of the analyses may give ambiguous answers - which ones and why?
d = REE, depleted; f = REE, flat pattern; e = REE, enriched. Y* = 3Y/((Ti/100)+Zr+3Y)
LREE Cr TiO2 Zr Ti/Zr Y Y* Ni Al203 Nb Ba K2O SiO2
Anomolous Somali basin site 236 37-2 118-120 Bryan
et al
1) d 215
.57 41
83 20 44
20 14.20
1 9
.03 55
N-type 3-18 Frey et al
2) d 470
.73 45
97 25 46
200 16.2
3 4
.01 50
T-type Cent Cord GTJ422 Diabase Gp Millward et
al.
3) f 437
1.08 55 118
18 31 140
14.4 5
53 .18
50.5
E-Type 2-10 Frey et al
4) e 250
1.51 100 91
25 28 95
15.80 20
200 .51
51.0
Continental flood basalt US11 Thompson et al.
5) e 230
1.42 143 60
20 21 101 15.44
14 394 1.00
50.4
Alkalic 1845 St. Georges Church Addis Ababa Thompson
et al.
6) e 33
2.38 224 64
28 18 49
16.74 45
360 1.24 46.4
Boninite 1403-34 Mariana Dietrich et al
7) d 821
.21 39
32 30 65 272
10.60 2
2 .001
55.6
Oceanic arc Eua E7
8) d 75
.73 37
118 20 42
24 20.6
2 27
.30 51.6
Oceanic arc Macauley 10380
9) e 75
.95 37
154 13 30
37 15.67
1 145 .40
49.2
Oceanic arc Lau 374
10) e 31
.96 100 58
29 36 17
17.33 3
116 1.00
55.3
Shoshonite arc Mba volcanics Fiji 68-63
11) e 3
.54 67
48 13 28
6 17.04
1 677 4.00
50.5
Back arc Casma Fm Peru Atherton et al
12) f 320 .48
26 111 14 43
96 14.67
2 269 .22
48.2
Oceanic MORB, Korombasanga
13) d 111 2.45
225 57 31 20
81 16.77
7 233 1.16
48.0
Korombasanga 382
14) e 737
.7 34
124 13 34
187 14.11
3 197
.63 48.4
Korombasanga 372, arc basalt
15) e 100
.84 32 158
15 35 35
18.32 1
131 .62
48.2
Peruvian Andes J3-5 Noble
16) e 250 1.39
225 37 35
25 79 16.8
12 768
1.8 52.8
Chilean Andes #3 Siegers and Anders
17) e 85
.8 180
27 7
8 30 16.1
1 720
2.7 61.5
Chilean Andes #18 Siegers and Anders
18) e 500
.8 170
28 5
6 195 15.9
1 300 1.1
54.7
Chilean Andes #22 Siegers and Anders
19) e 230 .7
200 21
5 5 210
15.4
1 490 2.3
60.4
LREE Cr TiO2 Zr Ti/Zr Y Y* Ni Al203 Nb Ba K2O SiO2
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EXERCISE 2
The most important process operative in the ‘mantle to crust’ part of the Earth system cycle is that of mineralogical/chemical fractionation. For example, quartz arenaceous sedimentary rocks, representing tropical beaches of nearly pure quartz, form via a long complicated process of fractionation of ultramafic mantle source material - material that itself contains no quartz. The following exercise illustrates a ‘normative’ method of testing the validity of a supposed fractionation relationship between typical volcanic rocks extruded in a rifting environment.
The following table present the weight % proportions of normative minerals in two basaltic rocks (S40, S51) from the Afar rift in north east Africa. The basalts are thought to be related through fractional crystallization (Barberi, F. et al., 1975. A transitional Basalt - Pantellerite sequence of fractional crystallization, the Boina Centre (Afar Rift, Ethiopia), Journal of Petrology, 16, 22-56). You do not need to know how to carry out a normative calculation; but you should know what is meant by ‘normative mineral’, ‘basalt’, ‘fractional crystallization’, and ‘cumulate’, and even the location of the Afar.)
Samples
S40 S51
Normative %
or
3.5
6.5
ab
20.5
24.5
an
29.2
29.8
cpx
23.5
11.7
opx
7.4
8.7
ol
7.0
3.1
mt
3.5
8.2
il
3.7
5.4
ap
0.7
1.2
Total
100
100
Ce
38.9
53.2
FeOt/MgO wt% 1.27
2.17
TiO2 wt%
1.92 2.84 (TiO2 is greater than FeO/MgO, as is typical of rift basalts)
Assuming that Ce is totally incompatible (k = 0) with any of the fractionated minerals, calculate the proportion of liquid represented by sample S51 relative to sample S40 for a constant amount of Ce (CCe = cce0 / x,). Then calculate the relative proportion of the minerals in the normalized fraction of S51. Compare the composition of S40, the source basalt, with the supposed fractionated basalt S51. Is the fractional crystallization model plausible? Does it need to be modified?
Samples
S40
S51 es38.9/ces53.2 (= 0.7312)
cumulates
| or | 3.5 | 6.5 | 4.75 = | -1.25 | (Larger -ve) | |
| ab | 20.5 | 24.5 | 17.91 = | 2.59 | ||
| an | 29.2 | 29.8 | 21.78 = | 7.42 | ||
| cpx | 23.5 | 11.7 | 8.55 = | 14.95 | ||
| opx | 7.4 | 8.7 | 6.36 = | 0.82 | ||
| ol | 7.0 | 3.1 | 2.27 = | 4.73 | ||
| mt | 3.5 | 8.2 | 6.0 = | -2.5 | (Larger -ve) | |
| il | 3.7 | 5.4 | 3.95 = | -0.25 | (Small -ve) | |
| ap | 0.7 | 1.2 | 0.88 = | -0.18 | (Small -ve) | |
| other | 1.0 | 0.9 | 0.65 = | 0.35 | ||
| Total | 100% | 100% | 73% = | 27% | ||
| Ce | 38.9 | 53.2 | 38.9 = | 0 |
Source basalt would have to be richer in K2O, TiO2, Fe2O3, and P2O5
In the following table, sample G495 is another relatively primitive basalt from the same region:
Samples
G495 S51
Normative %
or
4.7 6.5
ab
22.8 24.5
an
23.5 29.8
cpx
19.4 11.7
opx
2.7 8.7
ol
15.1 3.1
mt
4.6 8.2
il
4.4 5.4
ap
1.1 1.2
other
1.7 0.9
Total
100% 100%
Ce
35.9 53.2
FeOt/MgO wt%
1.12 2.2
TiO2 wt%
2.3 2.84 (again, G495 is
high in TiO2 relative to FeOt/MgO, typical of rift basalts)
Is it feasible that basalt S51 was derived from basalt G495 by fractional crystallization? What other process may have played a role in linking G495 to S51?
Samples
G495
S51 ce35.9/ce53.2 (= 0.6748)
Cumulates
| or | 4.7 | 6.5 | 4.39 = | 0.31 | ||
| ab | 22.8 | 24.5 | 16.53 = | 6.27 | ||
| an | 23.5 | 29.8 | 20.11 = | 3.39 | ||
| cpx | 19.4 | 11.7 | 7.90 = | 11.50 | ||
| opx | 2.7 | 8.7 | 5.87 = | -3.17 | (Large -ve) | |
| ol | 15.1 | 3.1 | 2.09 = | 13.01 | ||
| mt | 4.6 | 8.2 | 5.53 = | -0.93 | (Small -ve) | |
| il | 4.4 | 5.4 | 3.64 = | 0.76 | ||
| ap | 1.1 | 1.2 | 0.81 = | 0.29 | ||
| other | 1.7 | 0.9 | 0.61 = | 1.09 | ||
| Total | 100% | 100% | 67.5% = | 32.5% | ||
| Ce | 35.9 | 53.2 | 35.9 0 |
Negative Opx would mean that orthopyroxene would have had to have been added to S51, which is unlikely. Alternatively, we could consider that S51 contains a fraction of SiO2 that has been added through the assimilation of continental crust. In this case the -3.17 opx could be considered the equivalent of -3.17 olivine (ol) and -3.17 Quartz (qtz), and the fractionated solid inventory would have olivine as 13.01 - 3.17 = 9.84, and the residual liquid an assimilated addition of 3.17 SiO2.
Notes: molecular weights of major elements and normative minerals, and %SiO2 content of normative minerals.
CaO = 56.08; MgO = 40.31; FeO = 71.85; SiO2 =
60.09; Al2O3 = 101.96; Na2O = 61.97
CaMgSi2O6 = 216.51; %SiO2 = 55.5%.
CaFeSi2O6 = 248.11; %SiO2 = 48.44.
Mg2SiO4 = 140.71; %SiO2
= 42.7
Fe2SiO4 =
203.79; %SiO2 = 29.48; SiO2-80% Fo = 40.6
CaAl2Si2O8 = 278.22; %SiO2 = 43.78.
NaAlSi3O8 = 262.22; %SiO2 = 68.75;
SiO2-50% Ab = 56.27; SiO2-30% Ab = 51.27; SiO2-20% Ab = 48.77