GEOL 567 Name ____________________________
Global Geochemical Systems
Problem Set One
Major Element Models, Taos Plateau Volcanic Field, NM
50 points
due 11 Feb 05
You are supplied with major element analyses for 24 Plio-Pleistocene lavas from the
Taos Plateau volcanic field in northern New Mexico (McMillan and Dungan, 1988; Dungan et
al., 1986). The data are in an EXCEL file that is available on my website. You also have it on
CD.
The three major rock types (basalt, andesite, and dacite) are comagmatic, based on their
close relationships in space and time, and their similar major and trace element characteristics.
Phenocryst assemblages are as follows:
basalt olivine, Cr-spinel, + plagioclase
andesite olivine + plagioclase + augite
dacite plagioclase + augite + orthopyroxene
Your goal in this assignment is to determine whether this magmatic system evolved by
fractional crystallization or whether assimilation played a role. Use two modeling methods:
mass balance (using a spreadsheet) and Pearce element ratios. Your final project must contain
the following items:
* variation diagrams for these rocks with SiO2 or MgO as the abscissa
* an electronic copy of your spreadsheet (supplied on disk or through email).
Please be sure to work up your own spreadsheet, so that no two files are identical!
* diagrams used for Pearce element analyses
* a technical paper, complete with abstract, that presents the data, your modeling,
and interprets the results. More on this later in this assignment.
You are free to explore major element modeling as you please, but here are some steps
you can follow if you so desire.
1) Plot variation diagrams for these rocks with SiO2 or MgO as the abscissa. Use a different
symbol for each rock type.
2) Using a spreadsheet, calculate the results of fractional crystallization. Average Primitive
Basalt is a good parental composition for your calculations. Use the mineral-melt distribution
coefficients given in class.
