GLY306 Petrology: Lab 7-Petrology Lab: Whole-rock geochemistry

| March 14, 2016

Question
GLY306 Petrology: Lab 7

NAME: _____________________ DAY: ______

Petrology Lab: Whole-rock geochemistry
Objectives: Geochemistry is the study of the chemical properties of Earth
systems, and how we can glean valuable data about Earth processes from
them. In petrology, the chemical analysis of igneous rocks illuminates the
evolution of the host magma, and reveals much about its history. This lab
will explore how major and trace element variations can tell us about magma
genesis and evolution in a way difficult to parse from just the samples
themselves.
Part A: Major element variation
In this exercise, you will use real geochemical data from the Thingmuli
volcano (Iceland) to construct major element variation (Harker) diagrams.
The data in this exercise come from analyses of volcanic rocks (primarily
lavas). Although much of the field location is made up almost exclusively of
basaltic samples, a wide range of rocks, including basalt, andesite, and
rhyolite, can be found. Many of the rocks are aphanitic, but those that are
porphyritic contain phenocrysts of plagioclase, olivine, augite, magnetite,
ilmenite, and apatite. The major question you are seeking to answer in this
exercise is “how are these different rocks related to one another?” In
particular, you will use variation diagrams to determine whether this rock
suite can be related by fractional crystallization or magma mixing.
1. Open the Excel File available on the UB Learns website for our course.
The tab labeled “Thingmuli Major” contains the data for Part A.
2. Generate a series of variation diagrams for each major element oxide
presented. This should be a simple X-Y scatter plot, with the
appropriate axes titles and legend. Don’t forget to scale the axes
appropriately so that you can see trends in the data (such as the Liquid
Line of Decent).
3. Plot points showing the composition of olivine, clinopyroxene, and
plagioclase on each graph as well, for reference.
4. Print off the diagrams for MgO, FeO, CaO, Na2O, K2O, and P2O5 to hand
in with this worksheet (you can put multiple diagrams on one sheet, as
long as they are legible).
5. Based on your knowledge of mineralogy and igneous petrology (from
both lecture and chapter 8 in Winter), answer the following questions:
1. Look at your Harker diagrams for MgO and CaO. What mineral(s) are
responsible for the chemical trends shown by these elements as silica
increases?

1

GLY306 Petrology: Lab 7

NAME: _____________________ DAY: ______

2. FeO shows a slightly different behavior early on in the magma evolution.
Why does it not perfectly mirror the MgO plot, even though FeO and MgO
both fit into the chemistry of early crystallizing phases?

3. What type of trend would an incompatible element have on a Harker
diagram? Do any of the elements you plotted display incompatible behavior?
If so, which one(s)?

4. What is the explanation for the “hump” you see in the plot of P 2O5 versus
SiO2? (Hint: What mineral will control the behavior of P in a magma?)

5. Sodium and potassium are both alkali elements, but they have somewhat
different trends on these plots. Describe how the trends of these elements
differ in appearance. Why does this difference occur?

6. In what order (from first to last) did the MINERALS (not the oxides) in this
magma begin to crystallize? Explain how you can tell from a Harker diagram
when a mineral begins to crystallize.

2

GLY306 Petrology: Lab 7

NAME: _____________________ DAY: ______

7. Manganese does not form its own mineral in lavas, but instead it
substitutes into other minerals. In the case of these lavas, what mineral(s) is
Mn substituting into? How did you determine this? (Hint: what other
element(s) show trends similar to that of Mn?)

Part B: Trace element normalization and variation
In this exercise, we will normalized trace element data from the geologic
reference standard QLO. Reference standards are used in geochemistry to
ensure quality control of an analysis. QLO is well characterized, and any
deviations from the known values during an analytical session can be
corrected for in our unknowns. The rock is a quartz latite, collected in Lake
County, Oregon. The sample is derived from a lava flow on the flanks of an
extrusive dome and is probably of late Miocene or early Pliocene age. The
rock is greasy black and aphanitic, containing <1% microphenocrysts of
plagioclase feldspar, pyroxene, and magnetite.
1. Open the Excel file and select the tab labeled “QLO trace”
2. You are provided with two normalization factors from Sun and
McDonough (1989). The authors conducted many analyses of
important materials, from C1 carbonaceous chondrite meteorites to
Mid Ocean Ridge Basalts. To normalize our QLO trace element values,
we need to divide the measured sample value by one of the
normalization factors. You can write a simple formula to do this, and
drag it into the appropriate cells for each element. Normalize the QLO
data for C1 and MORB in a separate row.
3. Plot both normalized trace element data sets on a line graph, and set
the X axis labels to the elements. The Y axis values are unitless – we
divided [sample ppm] by [reference ppm] after all, and we display the
Y axis on a logarithmic scale. You may also want to set the X axis
intercept to 0.1 instead of 1 so that your plots do no overlap with the
axis labels. Be sure to include a legend and appropriate titles. Add
this plot to the print offs to turn in with the assignment, and answer
the following questions:
1. Generally speaking, which side (left or right) of the plot shows the
greatest enrichment relative to the normalization references?

3

GLY306 Petrology: Lab 7

NAME: _____________________ DAY: ______

2. Can you think of a process that would cause the dip in Ti in the trace
element plots?

3. On the MORB normalized plot, are any of the trace elements compatible
with regards to the QLO source? What about strongly incompatible?
How can you tell?

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