Part 3: Use the Clinopyroxene Geobarometer
Download the CpxBar program
Download the CpxBar geobarometry program for Windows/Excel
from the research page of Paolo Nimis at the University of Padua in Italy. Before doing anything else, read the short "Read me!" sheet in this file.
Download the clinopyroxene data
The file Cpx Data (Excel 19kB Jan3 05)
contains compositions of the same clinopyroxenes that you downloaded crystal structures for in Part 2. After reading the "Read me!" sheet in the CpxBar spreadsheet, input the clinopyroxene compositional data into the CpxBar spreadsheet. Be sure the compositions meet the various checks in columns R-W.
Answer the following questions
Volcanic crater of Mt. Noorat in Victoria, Australia.
- Based on the compositions of these rocks (as determined by their rock names), which calibration of the clinopyroxene structural geobarometer (BA, BH, TH, or MA) is the most appropriate for these samples from Australia?
Given the alkaline compositions of these samples, probably MA.
- The "BA" calibration is the only one that doesn't require a temperature estimate. For simplicity, use this calibration to determine the relative crystallization depths for the magmas at the four localities (i.e., list the localities in order of increasing average cpx-crystallization depth).
Mt. Noorat xenoliths < Lake Bullenmerri xenoliths < Meredith kimberlites < Mt. Franklin hawaiites
- How does increasing temperature affect pressure estimates as determined by the BH, TH, and MA calibrations (all of which are T-dependent)? Experiment by typing in a few T's ranging from 1200-1300 °C to find out. Does this relationship make sense? Explain.
Increasing T causes the P estimate to decrease. This makes sense, since increasing T and decreasing P both result in expanded crystal structures.
- Do these estimated pressures make sense? Assuming a lithologic column with an average density corresponding to ~3 km/kbar, would these pressure estimates correspond to upper crust, lower crust, upper mantle, or lower mantle? Does this make sense petrologically?
All of the estimated pressures correspond to the upper mantle, the likely source of these alkalic magmas.