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Major Elements Models, Taos Plateau Volcanic Field - Assignment | GEOL 567, Assignments of Geology

Material Type: Assignment; Professor: McMillan; Class: Global Geochemical Systems; Subject: GEOLOGY; University: New Mexico State University-Main Campus; Term: Spring 2005;

Typology: Assignments

Pre 2010

Uploaded on 08/09/2009

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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.
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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 SiO 2 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 SiO 2 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.
  1. Your data file contains two analyses of crustal melts in addition to the lavas. These are experimentally-derived compositions from Kilbourne Hole gneisses, so they do represent actual melt compositions. If fractional crystallization alone fails to reproduce the lava trends, try assimilation (Average Primitive Basalt + crustal melt) or AFC (adding crustal melt while fractionating crystals).
  2. Pearce element ratios were designed to discriminate between closed-system and open- system processes. Apply this to the Taos Plateau lavas. Do you get the same results from the Pearce element ratios as from your spreadsheet calculations? Your paper should contain these sections:
  • Abstract. Remember that an abstract is NOT a summary. Rather, the abstract is the condensed version of the entire paper, and achieves all of these purposes: introduce the topic, present the data, present the models, interpret the results. A data should be rich in data, so there should be numbers everywhere at first glance!
  • Introduction. Pull the reader in to your paper. Why is this work important?
  • Data presentation. In this section, describe the geochemical relationships of the major element compositions of basalt, andesite, and dacite. Don’t interpret yet--just describe! Let me know that you understand the data set. Write this after you have done the modeling so that you know what aspects of the data are the most important to your interpretations.
  • Modeling. Present the methods and results of your mass balance and Pearce element models.
  • Interpretations/Conclusions. Here, state your final answer--did this magmatic system evolve by fractional crystallization alone or by assimilation/AFC?
  • References Criteria for this project ____ Abstract introduces the topic, presents data, presents the models and their results, and provides interpretations. (3) ____ Introduction evokes interest in the paper and pulls the reader into the topic. (2) ____ Variation diagrams for all major elements are presented. (2) ____ Relationships within the major element data are described. (3) ____ Spreadsheet file(s) for mass balance calculations is unique and accompanies the paper. (5) ____ Mass balance models (spreadsheet) for fractional crystallization are presented in detail. (6) ____ Pearce element ratio models are presented. (6) ____ Interpretations are based on data and model results. (6) ____ Data presentation, model results, and interpretations are all discussed separately. (2) ____ Data presentation, model results, and interpretations are discussed at depth. (6) ____ Paper is organized. (3) ____ Author uses standard edited English. (3) ____ Paper has been proofread and edited. Please use a spell checker! (3)