Petrology — Spring 2006
Name __________________________
Basalts lab
Due Tuesday 5/9
Problem 9-5: The Early Paleozoic Stowe Formation in Vermont is part of a discontinuous chain
of metabasalts and associated ophiolitic rocks that apparently mark a major suture of the
Appalachian system. It is of interest to determine if the volcanics were created as part of a
volcanic arc, or are oceanic slices. The geochemistry of two samples are listed below:
SiO2
TiO2
Al2O3
FeO
MnO
MgO
CaO
Na2O
K2O
P2O5
Zr (ppm)
Y (ppm)
Nb (ppm)
Cr (ppm)
Sample #1
47.61
1.04
16.20
10.30
0.06
8.86
14.00
1.52
0.28
0.04
74
23
11
556
Sample #2
49.95
1.07
15.49
9.14
0.08
6.52
13.41
3.75
0.41
0.09
78
28
11
290
Plot the results on a:
a) Ti/100 – Zr – Y x 3 diagram (All in ppm. Note 1% = 10,000 ppm)
**In order to calculate the part (a) diagram, you have to convert Wt. % TiO2 to ppm Ti. This is
done to a relatively accurate approximation by dividing the wt. % TiO2 by the molecular weight
of TiO2 (79.88 g/mol) which yields moles of TiO2. This is equivalent to the number of moles of
Ti, so you can then multiply this by the atomic weight of Ti (47.88) to give the wt. % Ti. Then
multiply by 10,000 to convert to ppm.**
b) TiO2 – MnO x 10 – P2O5 x 10 diagram
c) Cr-Y diagram
Are the meta-volcanics of the Stowe Formation part of an island arc sequence on the basis of
these data?
Name __________________________
Petrology — Spring 2006
Name __________________________
Petrology — Spring 2006
Name __________________________
Petrology — Spring 2006
Name __________________________
Petrology — Spring 2006
Problem 13-1: Given the plots of K2O, P2O5, and La/Sm versus latitude for basalts from three
oceanic plateaus — the Azores (38-40°N), Iceland (63.6-66°N), and Jan Mayen (71-72°N) —
briefly describe and interpret the results.
Name __________________________
Petrology — Spring 2006
What other plot(s) of chemical analyses would be useful to determine whether basalts are derived
from enriched or depleted mantle?