Igneous Petrology 710
Snow Canyon Lab-Calculating the Depth of Melting of the Santa Clara and Diamond
Valley Lava Flows and Comparison with the Navajo Lake Volcanic Field
Required Programs and Data:
IgPet for Windows
Snow7.Roc.txt (the data file for Snow Canyon)
NEWALL.txt (the data file for Navajo Lake)
Fo89.xls (program that recalculates the analysis to mantle compositions).
Fe-Mg_calc-use_this 4=00.xls (program that calculates aggregate batch melting curves
for various initial pressures).
The Wang et al. paper where the technique is described in some detail.
Wang. K., Plank, T., Walker, J.D., and Smith, E.I., 2002, A mantle melting profile across the
Basin and Range, southwestern USA: Journal of Geophysical Research, v. 107, no. B1,
10.1029/2001JB000209.
1. before starting the calculations, answer the following questions:
a. Why does FeO provide an estimate of the bottom of the melting column?
b. Why does Na2O provide an estimate of the top of the melting column?
c. What are the most important assumptions in the Wang et al. technique?
d. What corrections must be made to the data to determine the chemistry of a
basaltic magma in equilibrium with its mantle source?
2. The first step is to find the most primitive rock in the volcanic field.
a. Do this by calculating FeO and Na2O at MgO at 8 wt.%
b. In the file SNOW7.roc.txt, plot only the Snow Canyon and Diamond Valley
samples. These are identified by the solid green triangle symbol.
c. If the MgO values are above 8 wt.%, then use the most primitive sample to
determine FeOprim and Na2Oprim.
d. For the most primitive sample determined in steps b or c, recalculate FeO by
using the following formula:
FeO*=FeO-0.15FeO (this step basically corrects for any Fe2O3 in the sample).
e. Recalculate the entire major element analysis for the primitive sample to
100%.
f. Use the values of FeO, MgO and Na2O for the Fo89 (the olivine addition
step).
3. Use the Fo89.xls spreadsheet and add olivine until the composition of the olivine in
your most primitive basalt is Fo89.
a. Put the FeO value in the yellow cell
b. Put the MgO value in the green cell
c. Put the Na2O value in the light yellow cell
d. Scan down the blue column (values of Fo) until you reach Fo89. The
values of MgO, FeO and Na2O at Fo89 are in this row.
4. Using the values from step 3 and the Fe-Mg_calc-use_this 4=00.xls program
determine the Po value (the pressure at the bottom of the melting column). This
program calculates the accumulated fractional melt at various Po pressures. It also
assumes a certain mantle peridotite composition. In this case the peridotite has 15%
clinopyroxene with 39.5 wt.% MgO, 8.24 % FeO, 0.29 % Na2O, and 0.012 % K2O.
The program is not very sophisticated so you need to iterate on Po to find the correct
composition. The spreadsheet was set up to explore for the correct composition and
not to invert for P. So you have to enter Po values until the FeO matches the value
from step 3. After finding Po, search the Na2O column until you find a match to the
value from step 3. This is the Pf value.
5. Plot FeO vs. Na2O to obtain the melting profile. Identify Po and Pf on the profile.
6. Based on geophysical data for this area, the crust is 35 km thick and the lithosphere
70 km thick. Draw a diagram to show where your calculated melting column plots in
relation to the crust and lithosphere.
7. Now we will compare the depth of melting calculated for Snow Canyon with another
area to the northeast at Navajo Lake (just east of Cedar City, Utah). The Navajo Lake
field contains basalt flows that vary in age from about 2 Ma to less than 20,000 years.
Although this area was recently studied by Shara Stowell, a depth of melting
calculation has never been done.
8. Using the data in NEWALL.txt calculate the melting profile for the Navajo Lake
basalt. Use only the solid green triangles and follow the steps above.
9. For Navajo Lake, the depth to the base of the lithosphere is about 75 km, and the
crust is about 30 km thick.
10. Draw a diagram showing where your calculated melting column plots in relation to
the crust and lithosphere.
11. Compare melting column depths and lengths for Navajo Lake and Snow Canyon.
Explain the differences and similarities.