GLY 3311C Second Term Project: Introduction

The rest of the semester will be devoted to the examination of igneous and metamorphic rocks. So, what we’re going to do by way of assessing your performance and learning is conduct a study of the rocks that have been recovered during a recent IODP Expedition.

The focus of the project will be on samples recovered from the following drilling sites:

Sites U1439C and U1442A (boninites) from IODP Expedition 352

We have at our disposal “field” summaries of these sites, in the Expedition Report volume for 352 (Reagan et al 2015; 2017), along with all the papers published on these samples thus far, which are all in our Canvas site. What you will do is study the samples from these two sites petrographically and in terms of mineral chemistry, toward defining their igneous Petrogenesis and Geologic History.

How will this work? We have thin sections of a selection of the Expedition 352 samples. Each of you will be individually responsible for describing one of the thin sections. A subset of these samples have been analyzed via electron microprobe, and you’ll get to analyze several more (up to six additional samples), so there will be new mineral chemistry data to work with as well.

The samples chosen for microprobe study will be up to you and your classmates who are working on rocks from each site – you’ll want to select the couple that are most representative of what you’re seeing in the set of rocks you’re working on. For this, you will have to talk to each other, share your observations, and cooperatively make a judgment as to which samples will go.

Electron microprobe analyses of these samples will give us the chemical compositions of their major and minor minerals, which you’ll use to talk about the origins of these samples

It is absolutely OK for you to share your observations, to work together on your data collection, and brainstorm with each other about what you think is going on. We’ll set up lab time for this to happen. However, each of you will be responsible for your own project writeup and your own interpretations!

What will you produce?

You’re going to prepare a report. This report will include the following:

2) Background: Information based on your reading of the relevant literature on these places (this includes, but should not be limited to, the literature I have provided about Expedition 352 and boninites generally. The literature will also propose models for the origin of these rocks, which you will be testing in your own studies.

What is known about the beginnings of the Izu-Bonin subduction zone and arc?

Are there boninites in other places in the arc? If so where, relative to the Expedition 352 sites?

Why have others suggested that boninites are found in this place?

3) Field/Core relations: You will include a short description of the field site(s) where are samples were recovered. To do this you are going to have to draw from the Expedition Report, Preliminary Report, Cruise prospectus, and the other materials available on the site, as well as any and all published literature on this part of the world. You’ll also be able to draw upon geophysical and observational data for the region which can be obtained via the Interdisciplinary Earth Data Alliance (IEDA) portal.

What do we in fact know about the subsurface geology at these sites?

4) Petrography: As a piece of this assignment you’ll be asked to collect some specific information, including pictures, of the petrography of your assigned sample. This information will be shared, and you’ll be expected to compare your results to those who have samples from the same depth segment of the same core.

What is the overall mineralogy of the samples from your site, based on your hand specimen and thin section identifications?

Which mineral are Primary (that is, formed during igneous melting and crystallization) and

Which minerals are Secondary (formed long after eruption due to seawater chemical exchanges)?

Are there any unusual features evident in these minerals, such as Zoning or Overgrowth textures?

5) Mineral Chemistry: You’ll report on new mineral chemistry results for samples from your core segment – and you’ll be expected to look at and compare these to those that have been analyzed in past years, which will be available on Canvas for you to use.

What are the minerals you identified via the Electron Microprobe, how do these compare with your optical/visual observations?

What constraints may they provide about the pressures and temperatures of melting and crystallization, and/or their metamorphism? (more on that in your activities…).

If any of your identified minerals show zoning, how do their compositions change from core to rim, or zone to zone?

6) Interpretation: You will be expected to make an interpretation of the igneous origins of the rocks from your site, based on the chemistry, texture, mineralogy and fabric of the samples from your core and core section. In doing this, you’ll be expected to look at your findings in light of published work, and compare what appears to be happening, based on your samples, to what others have proposed. We have and will be learning a variety of tools for doing just this in class, so throughout the term, you’ll want to “try out” what you’ve learned on your sample. What you want to do make an argument, based on your work, at to whether or not hypotheses you found proposed in the literature for the origins of these three igneous rock units are reasonable.

Doing this kind of report takes some time, so we’re going to be working on it in pieces the rest of the semester. Specific pieces will have specific due dates, so you can get some feedback from me on them before you proceed.

10/19: Expedition 352 samples assigned.

11/2: Preliminary Petrographic Description of the sample is due (see the Petrographic Description assignment for details on the specifics)

11/9: Group assignment on sample selections for EPMA study are due (see the EPMA prep assignment for specifics)

11/16: Draft of your Introduction/Background and Field/Core Relations section is due (see the Introduction/Background assignment for specifics)

*11/16, 11/18, 11/23, (*Tentative – TBD): EPMA analysis days

11/30: Complete Results section (Petrography and Mineral Chemistry descriptions) for review.

12/9: Full report (with Discussion/Interpretation and a brief Introduction) due by 11:59 PM

Your grade will depend to a great degree on level of effort put into the project – the more thorough you are, the better! It will also be important to understand the various interpretive tools we are going to be learning, and to use them correctly. I will maintain a record of the chemical data on the rocks via Canvas. The IODP samples will be collected at the end, both for grading and to ultimately return to IODP.

The Second Term Project will be due on December 9, 2021, if all goes well. [If all goes less than well, i.e., the microprobe misbehaves, or COVID messes up the end of the term even more than they expect – we’ll talk!]

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