GEOL 5310 Advanced Igneous and Metamorphic Petrology Fall 09 Igneous Lab 2: Geochemical Plots of Igneous Rocks

Transcription

1 GEOL 5310 Advanced Igneous and Metamorphic Petrology Fall 09 Igneous Lab 2: Geochemical Plots of Igneous Rocks Objective: In this lab, we will become familiar with geochemical databases associated with various tectonic environments and how to portray that data on various geochemical plots using IGPET 2001 software. Procedure: Using geochemical databases from the web and those provided, you will be asked to evaluate the data by making a variety of geochemical plots commonly applied to igneous rocks. Part 1 Working with Data from Specific Tectonic Environment 1) Acquiring a Geochemical Database. There are a number of website that have been developed recently that seek to compile geochemical data for igneous rocks in a variety of tectonomagmatic environments. One of the best of these is the GEOROC website maintained by Max Plank Institute in Mainz, Germany ( mainz.gwdg.de/georoc/entry.html). a) Click on the link to Geologic Settings b) Choose one of the eleven geologic settings and then select a specific geological location. c) You will then be asked to further specify the what type of data you are seeking with your choices depending on the breadth of the database; continue on parsing your selection until requested to make a sample selection criteria d) Choose rock type and then select volcanic rocks e) When prompted to select chemical criteria, click the All box for major elements, trace elements, and REE f) At the metadata prompt, select standard output and download g) On the next page, click on Download the Table for Excel h) Open the downloaded.cvs file in Excel and save as an Excel workbook file. 2) Preparing a Data Table for Plotting in IGPET. IGPET has some basic requirements of data tables in order to plot geochem data. We will work with the data you downloaded from the GEOROC website to prepare it for plotting in IGPET. a) There are lots of ancillary information associated with each GEOROC analysis that is unnecessary for our purposes. Of the 27 or so attribute columns leading to the actual analyses, delete everything but the sample name and the rock name. For the data, delete all columns indicating analytical methods. b) There are also lots of data that are incomplete analyses. As a first pass, delete all analyses that do not have REE data; as a second pass, delete all analyses that do not have major element analyses. If you have more than 50 analyses after these two filters, you can delete more analyses until you whittle the total down to 50, but be sure to keep a good mix of different rock names. c) Resave your Excel spreadsheet. d) IGPET requires that the first four columns be 1) Sample, 2) Jcode, 3) Kcode, and 4) Lcode. Basically delete the work Name in the first cell in your spreadsheet so it just reads Sample. Then insert 3 columns after the sample columns and label the first row cells 1

2 Jcode, Kcode, and Lcode. The code columns provide the opportunity to sort data by up to three criteria. We will sort the data by rock name using the Kcode. Asign a different number for each rock name (e.g., 1 basalt, 2 basaltic andesite, 3 andesite ). e) IGPET also is looking for standard oxides and elements in the first row. This requires that you delete any wt%, ppm, or similar tags on the composition headings. You will also need to convert all of your Fe analyses to FeO(total); to convert Fe2O3 to FeO, multiply by f) After the major element data, insert two columns. For these calculate the mg# and An% (both in mol%) from your major element data. Remember mg# = MgO/(FeOt+MgO)*100 and An% =CaO/(CaO+Na2O+K2O)*100. (Don t forget to divide your wt% values by the molecular weights of each oxide to get results that are in mol%) g) When you table is cleaned up, you should save it in Excel, then save it again as a Unicode text file, which is compatible with IGPET. 3) Making X Y (Harker) Plots in IGPET. We will now load our table into IGPET and make a variety of petrologically useful plots from this data. We will start with simple X Y Harker plots. a) Open IGPET b) Pull down the File menu and select Open File ; find the text file you saved of your GEOROC data and open it. c) To check on the data file, select File Operation from the File menu and click on Modify Data. The first four columns should again be 1) Sample, 2) Jcode, 3) Kcode, and 4) Lcode with your rock type attributes distinguished in the K code column. If OK, cancel. If not OK, go back to your Excel table and modify. d) From the Plot menu select XY. You will then be prompted to select among various compositional parameters for the X axis. We will use the standard parameter of SiO2. Then you will be prompted to select a Y axis parameter. Initially choose MgO. An SiO2 Mgo diagram should appear. (If the diagram doesn t show the lower part of the diagram, click Position from the Edit menu and choose Lndscp UR ). e) To discriminate the different rock types, select Symbols from the Edit menu. In the window, select the Kcode in the upper left. You may also need to refresh the symbols to see the types. If you want to reduce their size, click on New Size and reduce the default size (0.7) to a lower value. Close the window and different symbols should appear corresponding to your Kcode selections e) Save this plot as a Word Metafile (.wmf) by selecting Save diagram to wmf file diagram from the File menu (it will place it in the same folder as the data). Give it a name like Si Mg. f) Make more Harker diagrams for SiO2 vs. FeOt*, Al2O3, CaO, TiO2, K2O, and P2O5 and save them as WMF files. (*You want to plot total Fe, which may be as Fe2O3t or FeO+Fe2O3 which you can make as a new parameter when the menu in d) appears ). Compile your data into a series of stacked Harker plots in either Word or Illustrator. Draw regression lines showing the main trends of the data Comment on whether your data define a reasonable magma series as discussed in the lecture on Monday. If any inflections appear in your data trends, can you speculate as to a possible explanation (e.g. a change in the fractionating mineral assemblage)? If your data show significant scatter, can you proffer any explanations for that? 2

3 4) Making AFM Plots in IGPET. IGPET has a number of canned discrimination plots of well accepted petrologic significance. First you will determine whether you data are alkaline or subalkaline using an SiO2 Alkali plot. Then for the subalkaline data, you will use an Alkali Iron Magnesium (AFM) ternary plot to discriminate tholeiitic from calc alkaline magma series based on the work of Irvine and Baragar (1970). a) With your GEOROC data table loaded, select Diagrams from the Plot menu. b) This will open a window asking you to find a.dgm file. You will need to browse to the IGPET 2001 folder. Opening this, you will see several.dgm files; select the Irvbar.dgm file. c) A select diagram menu will pop up; select SiO2 alk alk vs subalk ; a ternary plot of your data should appear. Again, you may need to adjust the diagram position to see it entirely. d) Save this diagram as a wmf file. e) ID the samples that plot in the alkaline field; reopen your text file in Excel and assign those data plotting in the alkaline field an Lcode of 0; assign the subalkaline rocks an Lcode of 1. f) Select the Irvbar.dgm file again and this time select AFM thol vs. calc alk ; a ternary plot of your data should appear. From the edit menu, select symbols and have data sorted by the L code (you may also want to reduce the size of the symbols). This will then show the data that are subalkaline as large symbols and the alkaline data as small symbols. d) Save this diagram as a wmf file. Place all your figures in a word document and answer the questions below. Is your subalkaline data mostly tholeiitic or calc alkaline? Does this make sense knowing the tectonic environment of your data set? If you replot your data on the AFM diagram and discriminate by rock type (Kcode), do certain rock types tend to plot in one field or the other? Add a plot of this diagram to your Word document. 5) Tectonic Discrimination Diagrams in IGPET. IGPET also has various tectonic discrimination diagrams based on various trace element abundances and ratios. a) With your GEOROC data table loaded, again select Diagrams from the Plot menu; browse for the IGPET 2001 folder and this time select discrim.dgm b) This will open a select diagram window of displaying various discrimination diagrams. c) Plot your data on at least three different diagrams and save each as a wmf file. Place all your figures in a word document and comment on whether the indicated tectonic environments are consistent with the known tectonic environment of your data. Part 2 Comparing Data from Different Tectonomagmatic Environments with Spidergrams 1) Download Database From the class website, download the text file BSLT ANDST. This lists average compositions of primitive basalt compositions from various island arc (IAB) and continental arc (CAB) settings complied by Keleman et al. (2004). It also lists their average compositions for MORB, oceanic basalt (OIB), continental basalt (CFB), oceanic andesite (OA), continental andesites (CA) and oceanic bonninite (OBn). The text table is already formatted for being read into IGPET. 3

4 2) Make REE plots IGPET has several normalization data sets to create REE spider diagrams for you data. We will use the plots normalized to chondrites as determined by Sun and McDonough (1989). a) Open the BSLT ANDST text file in IGPET, then select Spider from the Plot menu; choose the first plot REEs Sun and McD chondrites b) The next window will ask you select specific samples to be plotted (double click to select). Assignment You should plot (and save as wmf files) the following data pairs: A) MORB, Avg Oceanic Basalt (OIB) B) Avg Oceanic Basalt (OIB), Avg Continental Basalt (CFB) C) Avg Oceanic Basalt (OIB), Avg Oceanic Andesite D) Avg Cont Basalt, Avg Aleutian Andesite E) Suite of Island Arc Basalts (IAB), Suite of Continental Arc Basalts (CAB) F) Avg Aleutian Andesite, Avg Boninite Andesite Paste the six plots into Word and comment on; 1) how the REE patterns differ from each other in terms of shape and enrichment relative to bulk earth (chondrite) composition, and 2) what minerals might be involved in the generation of the magmas either through partial melting or fractional crystallization. Realize that all of the basalt compositions are relatively primitive, such that their signatures may be most representative of partial melting processes, whereas andesitic compositions were like generated by fractionation of basalts. The figure on the next page may be helpful in this evaluation. Also, you should look into the origin of boninitic magma to evaluate Data Pair F. 3) Make Spidergram plots To compare a broader range of incompatible trace elements, we will plot spidergrams using Thompson (1982) element suite normalized to chondrite. a) Using the BSLT ANDST text file, select the Thompson, R.N., 1982 double chondrite Spider diagram. Assignment Again, make, save, and paste into Word, plots of the same six data pairs as for the REE Comment on the contrasting patterns and degrees of enrichment Point out any positive or negative anomalies 4

5 From Rollinson (1996) 5