Part 2 - Analyze the data
- Open the file you have just downloaded in Excel. Notice the huge amount of data! This is only a subset of the MORB samples available on PetDB. Those marine geologists have definitely been busy!
- Now you'll make a series of plots of the consolidated global data. Start by making an x-y plot of longitude (on the x-axis) vs. latitude (on the y-axis). (Because of the large amount of data, you may want to decrease the size of the datapoint symbol used on this graph). Does the resulting graph look familiar? If not, look at a map of the world's mid-ocean ridges provided here:
- Now make a plot of SiO2(on the x-axis) vs. MgO (on the y-axis). The easiest way to do this is to simply change the variables on your lat-long plot. Clicking on any datapoint on a plot will highlight the two columns used by surrounding those columns with colored boxes. Dragging those colored boxes to other columns will change the variables shown on the plot. Note which color surrounds the data for the x-coordinate, and which surrounds the data for the y-coordinate.
Question 1. According to your plot and ignoring outlying samples, what is the range in SiO2 of the main cluster of MORB glasses? Is this variation real?--i.e., is it greater than the ~1 wt.% error for SiO2 in volcanic glasses?
Question 2. Ignoring outlying samples, what seems to be the range of MgO for the global MORB dataset? Is this variation real?--i.e., is it above or below the ~0.5 wt.% error for MgO analysis in volcanic glasses?
Question 3. What is the likely cause for most of this geochemical variation in MgO (and SiO2)?
Question 4. In terms of MgO content, which glass compositions will be more closely related to their original mantle sources?--low MgO, medium MgO, or high MgO?
- It will be of interest now to focus on the geochemical variation among the more primitive MORB liquids, but to do this, we need to find a way to correct for the effects of fractionation. Well, we're in luck! A method of fractionation correction has been developed by [Klein and Langmuir, 1987] which is based on the observation that low-pressure fractionation of a primitive basaltic liquid within an individual volcanic suite (or at a specific location) results in a more-or-less linear array ("liquid lines of descent") on major-element variation diagrams, at least over certain ranges of MgO.
On your spreadsheet, create three additional columns with these headings: "Fe8.0", "Na8.0", and "CaO/Al2O3", where
- Fe8.0 = FeOt + 1.664*(MgO) - 13.313
- Na8.0 = Na2O + 0.373*(MgO) - 2.98
- Fe8.0, Na8.0, and CaO/Al2O3 are not meaningful for all values of MgO. To be able to select a range of the samples for plotting in the next step, "Select All" the cells in the spreadsheet and sort by increasing MgO.
- Now make three graphs showing the variation of the above three variables vs. Elevation (depth below sealevel). Plot only Fe8.0 and Na8.0 values for samples with 5.0-8.5 wt.% MgO (this is the limited range for which this calculation is valid). Similarly, plot only CaO/Al2O3 values for samples with > 5.0 wt.% MgO.
You have successfully completed the data analysis portion of this assignment. Now its time to make sense of those plots you made and learn something about MORB geochemistry! Hang in there--you're almost done!