Petrography and Petrogenesis of a Mid-Ocean Ridge Lava Suite
Department of Geological Sciences; University of Florida
This activity is designed to accompany a set of thin sections available from the authors. Students investigate mid-ocean ride basalt petrography and relate observed mineralogic changes to relevant phase diagrams and major element chemical evolution. Using this information and data provided on the tectonic location of the samples they develop hypotheses regarding the petrogenesis of the suite.
Skills and concepts that students must have mastered
1. Students should have basic petrographic skills using a polarizing light microscope, including the ability to identify the following minerals, at a minimum: olivine, plagioclase, and clinopyroxene. Other minerals identification of use includes pigeonite, orthopyroxene, and zircon. One part of this investigation involves students optically estimating plagioclase feldspar compositions.
2. Students should have a basic, qualitative understanding of ternary phase diagrams.
3. Students should be able to plot bivariate data on an x-y plot (preferably using Excel or another spreadsheet/graphic analysis program)
How the activity is situated in the course
This investigation is used as a beginning lab assignment and is the first in a series of assignments using this same rock suite to illustrate magmatic evolution and the investigation of genetic relationships among igneous rocks. Ideally, we revisit this same rock suite (or at least some data from it) several times during the course. The simple mineralogy and well-constrained crystallization path and liquid line of decent of MORB provide a simple context to investigate petrogenesis. Our experience with the exercise suggests that approximately two weeks (two lab sessions) are needed to cover all of the material. Some instructors may want to delete some sections to allow it to be completed in one lab session. As with most petrology lab classes we are familiar with, it will take additional time for students outside of lab class time to complete the required work.
Content/concepts goals for this activity
In this investigation students examine the petrography and major-element geochemistry of 6 samples of mid-ocean ridge basalt (MORB) and related differentiated lavas (tholeiitic andesites and dacites) recovered from the Cleft segment of southern Juan de Fuca Ridge (JdFR), a medium spreading-rate MOR in the northeast Pacific Ocean. The goals of this investigation are to:
1. Investigate magmatism in a mid-ocean ridge environment
2. Examine petrography and mineralogy of tholeiitic igneous systems
3. Relate observed textures and mineral phases to relevant basalt phase diagrams and magma evolution
4. Correlate observed mineralogic changes with magma chemical evolution
5. Investigate how chemical variations may correlate with MOR/tectonic environment.
Higher order thinking skills goals for this activity
Several higher-order thinking skill are involved in completing this investigation. These include: making inferences from observations, data analysis and pattern recognition, formulation of hypotheses and the synthesis of ideas.
Other skills goals for this activity
Other goals include: assisting students develop petrographic skills and the ability to use a polarizing light microscope, become familiar with the use and interpretation of ternary phase diagrams, and to use computer spreadsheet/graphics programs to generate and interpret bivariate data plots.
Description of the activity/assignment
This is a laboratory-style investigation wherein students examine the petrography and major-element geochemistry of 6 samples of mid-ocean ridge basalt and related differentiated lavas recovered from the Cleft segment of southern Juan de Fuca Ridge, a medium spreading-rate MOR in the northeast Pacific Ocean. Lava types range from basalt to dacite.
After some initial background information on basalts, the MOR environment, and the study area students investigate four thin sections, beginning with typical basalts and ending with a dacite. They are led through a series of directed questions that help them gain familiarity with commonly occurring minerals and textures in mid-ocean ridge lavas. Questions direct students toward the interpretation of quench-related textural features and crystallization sequence, as well as a few other textural observations and petrographic techniques. After proceeding through the initial four thin sections and associated questions student are then asked to undertake "full thin-section descriptions" of the remaining two samples.
After investigating the thin-sections and determining a possible crystallization sequence from the petrographic data gathered (plagioclase followed by olivine followed by augitic clinopyroxene followed by pigeonite), students examine a P-T phase diagram to constrain possible pressures of formation. Discovering that crystallization pressures were low (less than ~ 0.75 GPa) students then examine a phase diagram of the olivine-plagioclase-augite-quartz system (olivine-quartz-augite ternary, projected from the plane of plagioclase saturation) [Walker, 1979]. Students draw 2 possible liquid lines of descent (LLD) onto the diagram, and then use their petrographic observations to qualitatively plot the samples along that LLD, determining a relative sequence of chemical evolution for the suite of samples. Lastly, given those determinations, student graph the major element data for the lava suite to infer paths of chemical evolution and the effects of fractional crystallization (possibly coupled with magma mixing).
Determining whether students have met the goals
There is no formal evaluation component included with this investigation, however, given independent student work, there are many embedded assessment questions within the text of this investigation. Ultimately, student's determination of the sequence of crystallization in an evolving MORB magma and the subsequent interpretations that follow serve as the assessment. After completing this investigation students should be able to:
1. Identify the mineral phases occurring in this lava suite
2. Recognize petrographic evidence of magma quenching
3. Predict which minerals should be crystallizing at any point on a ternary phase diagram.
4. Predict the chemical evolution of major element oxides with changing MgO in tholeiitic systems
5. Have a better understanding of the environment of mid-ocean ridge volcanism
Teaching materials and tips
- Activity Description/Assignment (Microsoft Word 20MB May23 07)
- Major Element Data file (Table 1 of Assignment) (Excel 16kB May23 07)
- Brief step-by-step tutorial for plotting scatter diagrams in Microsoft Excel (Microsoft Word 30kB May23 07)
- PDF version of student assignment (Acrobat (PDF) 1.1MB May23 07)
A detailed reference list is included in the investigation. A few helpful references regarding the Juan de Fuca Ridge and mid-ocean ridge volcanism in general are included below.
Morse, S.A. (1980) Basalts and Phase Diagrams, Springer-Verlag, New York.
Natland, J.H. (1980), Crystal morphologies in basalts dredged and drilled from the East Pacific Rise near 9N and the Siqueiros Fracture Zone. In Init. Repts. DSDP, vol. 54 edited by Rosendahl, B.R., H'kinian, R., et al., Washington (U.S. Govt. Printing Office), 605-634.
Perfit, M.R., (1999). Earth's Oceanic Crust. in Encyclopedia of Geochemistry, C.P. Marshall and R.W. Fairbridge, eds., Kluwer Academic Publishers, Boston ;179-182.
Perfit, M.R., (2001). Sea-Floor Spreading- Mid-Ocean Ridge Geochemistry and Petrology. in Encyclopedia of Ocean Sciences, J. Steel, S. Thorpe and K. Turekian, eds., Academic Press, San Diego, CA, 1778-1788.
Perfit, M.R., and Chadwick, W.W. Jr., (1998). Magmatism at mid-ocean ridges: Constraints from volcanological and geochemical investigations. in Faulting and Magmatism at Mid-Ocean Ridges, ed. W.R. Buck, P. Delaney, and J.A. Karson. Geophys Monograph 92., Amer. Geophys Union, Washington D.C., 59-115.
Perfit, M.R. and Davidson, J.P. (2000). Plate Tectonics and Volcanism. in Encyclopedia of Volcanoes, H. Sigurdsson ed.. Academic Press, San Diego, CA. 89-113.
PetDB-The petrological database of the ocean floor (n.d.), Data retrieved in January, 2006 from https://www.earthchem.org/petdb.
Smith, M. C., M.R. Perfit, and I. R. Jonasson (1994), Petrology and geochemistry of basalts from the southern Juan de Fuca Ridge: controls on the spatial and temporal evolution of mid-ocean ridge basalt, J. Geophys. Res., 99, 4787-4812.
Stakes D. S., M. R. Perfit, M. A. Tivey, D. W. Caress, T. Ramirez, and N. Maher (2006), The Cleft revealed: Geologic, magnetic, and morphologic evidence for construction of upper oceanic crust along the southern Juan de Fuca Ridge, Geochem. Geophys. Geosyst., 7, Q04003, doi:10.1029/2005GC001038.
This project is a follow-on to the On the Cutting Edge workshop on Teaching Petrology in the 21st Century (2003). We would like to thank Debra Stakes, Bob Embley and Bill Chadwick for their expertise and support during our sea-going research programs that allowed us to collect hundreds of samples along the Juan de Fuca Ridge (including those used in this exercise). Laurie Cotsonika was instrumental in the initial stages of developing this project. We also acknowledge the support of the National Oceanic and Atmospheric Administration (NOAA) and the Monterey Bay Aquarium Research Institute. This project would not have been possible without support from the Marine Geology and Geophysics division of NSF who supported the field and lab work through NSF grant (OCE0221541) and development of this educational product through grant (OCE 0138088).