Evolution of Mafic Lavas from the Main Ethiopian Rift Research Scholar, Erica Campbell Research Mentor, Dr. Tanya Furman Abstract Understanding continental rifting processes means understanding plate tectonics, and hence the physics and geochemistry within the earth. Mineral chemical data in this study is acquired directly from crystals found within the basalt lavas of the Main Ethiopian Rift (MER). Microanalysis gives us a geochemical signature of the parental magma and mantle source. Through application of instrumental tools and techniques including the polarized microscope, electron microprobe, geochemical modeling, geothermometry, geobarometry, and MS excel worksheets it is possible to obtain the data needed to develop an accurate interpretation. Olivines, pyroxenes and plagioclase feldspars were the three main minerals analyzed in this study. It was determined rather are not those minerals grew under thermodynamic equilibrium conditions in the lavas. By calculating the pressure and the temperature at the time of crystal growth, it helps to determine the mechanics of the volcanic plumbing system beneath the MER volcanoes Introduction One important way to understand the processes occurring in the interior of the earth is through study of volcanoes and their eruptive products. The East African Rift System (EARS) is an area of special interest because it is the best example of modern continental rifting. Divergent boundaries driven by one or more mantle plumes are causing three plates of the Afar Triangle to pull apart. When continental plates are pulled apart, hot molten rock seeps onto the earth s surface and becomes the foundation of a new sea floor. Eventually the rifting process will lead to Africa splitting apart and the creation of new ocean along the EARS itself. The recent volcanic eruptions of the (MER) are key sites for obtaining samples of lavas that document the rifting process. The purpose of this study is to obtain geochemical data from the MER lava samples and the crystals within them. This work enables us to document differences between MER lavas and those generated in different portions of the EARS. Our goal is to determine the complexity of the volcanic plumbing system underlying individual MER volcanoes. This knowledge enables us to understand how the earth operates at sub-continental depths, and to describe the thermal, mineralogical and chemical structure of deep earth beneath the MER. Methodology Petrographic Analysis Each sample used in this study was examined petrographically prior to chemical analysis. Petrographic analysis involves the microscopic observation of mineral types and textures. The rock of interest is glued to a microscope slide, sliced to a thickness of 3 microns and then polished. The resulting thin section is then viewed through a polarized microscope. (Figure 3) is an example of what a pyroxene looks like when viewed under a polarized 1
microscope. A petrographic (or polarizing) microscope employs a plane polarizing filter located in the light pathway prior to encountering the sample, and a second filter located between the sample and the objective lens. The polarized microscope is unique because when the two polarized filters are turned at right angles to one other, all light is blocked out. A key characteristic that assists with identifying crystals is extent to which individual minerals interact with the orientation and speed of individual light rays, enabling them to be seen as brightly colored in the cross-polarized light. The minerals of interest in the MER lavas include olivine, clinopyroxene, plagioclase feldspar and various Fe-Ti-Cr oxide phases. Electron Microprobe Micro-analysis of mineral chemistry is performed using an electron microprobe housed in the Materials Characterization Laboratory at Penn State University. The Cameca SX50 Electron Microprobe is an instrumental tool that yields the elemental analysis of rockforming minerals. The analyses are done at a micro-scale level; a good example would be taking a grain of salt and then viewing it as the size of a small car. Figure 4 shows photomicrographs of plagioclase feldspar and olivine phenocrysts. An electron beam is directed onto the mineral of interest, where it excites the electrons of each atom located within a 10 micron footprint. The resulting energy is measured by five spectrometers, each calibrated for specific elements found in the mineral. The most common elements analyzed are Si, Ti, Al, Fe, Mn, Mg, Ca, Na, K, and Cr. A software package unique to the Cameca microprobe translates the amount and energy of electrons emitted to a quantitative elemental analysis of the chosen mineral. One advantage of the Cameca SX50 electron microprobe is that it provides a small degree of error. Having five spectrometers increases productivity by increasing speed and the quantity of major and minor elements detected. Geochemical Modeling Mineral chemistry data obtained from the electron microprobe will be compared to major element data collected on bulk rock samples. The goal of this phase of the research is to determine whether minerals present in each rock crystallized locally under equilibrium conditions, or whether they represent fragments of other rocks that were entrained during magma ascent. Thermobarometric modeling of olivine and clinopyrozene mineral chemistry is also used to constrain the ambient temperature and pressure at which these minerals grew. Results Petrographic Description of MER Samples Table 1 shows results of the analysis. Textures ranged from holocrystalline, ophitic seriate to phorphyritic, typical of tholeiitic basalt and alkali olivine basalts. The most common textures were tholeiitic basalt and alkali olivine basalt. While Kone samples have abundant in olivine crystals, Fantale samples have abundant plagioclase feldspar phenocrysts. Samples from Gedemsa contained small proportions of crystals with the average crystal being 3/8 mm in length. Boseti samples are also rich in olivine phenocrysts and have a moderate amount of plagioclase feldspar. Tulu Moye samples are abundant in plagioclase feldspars. 2
MER Electron Microprobe Data The data obtained from the electron microprobe were converted to MS Excel Files. The Weight percents (Wt %) of the major elements were found and that data was used to determine Kd the equilibrium Fe-Mg exchange coefficent, Fo Fosterite content in olivines, and An Anorthite content in plagioclase feldspars. The Boseti samples had abundant olivine with composition Fo 76-82. The average An level was approximately 80 in the plagioclase feldspars of the Boseti samples. The Gedemsa samples consisted of olivines with Fo 70-71 and some relatively high An levels up to 93. Kone samples have several populations of olivines with Fo values ranging from 62-82. Also in the Kone samples, the plagioclase feldspar An ranges from 82-92 and the clinopyroxenes were rich in calcium. Fantale samples consisted of olivines in equilibrium with a Fo 80 and some olivines in disequilibrium with Fo 63-64. Clinopyroxenes from Fantale are rich in calcium. Tulu Moye samples were dominantly plagioclase feldspars with An 69-82, there are very few olivines in this sample with Fo 68-72. Sample Equilibrium Calculation Looking at the data it is obvious that the analyses reflect crystals that formed under equilibrium conditions and crystals that formed under disequilibrium conditions. Below are some data taken on an olivine crystal the (Kd) represents the Fe-Mg exchange coefficient that determines if the minerals were in equilibrium with the host lava in which they are found. Table 2 shows average (Kd) values of the olivines and pyroxenes. Type MgO FeO Kd Wt%-mineral 36.29 22.55 0.38 BRA-rock 6.68 10.98 The following calculation is the method used to determine if the mafic minerals formed under equilibrium conditions (FeO mineral * Mg rock) / FeO rock * MgO mineral) = x Note BRA=Bulk Rock Analysis A sample problem: using point # 1 from sample N-14 (22.547 * 6.68) / (10.98 * 36. 292) = 0.378 The sample calculation yielded to a result of 0.378. This particular point is from an olivine crystal; if this sample to crystallized under equilibrium conditions it should yield a value ~ 0.30. The calculated value of 0.38 indicates that the MgO content of lava is too low to have enabled that particular crystal to grow. Thus, the crystal is not at equilibrium with the host lava, but rather was picked up by that lava as it erupted. Discussion Plate Tectonics Plate tectonic theory explains continental drifting and formation of new ocean floor. Figure 1 helps to visualize tectonic plate boundaries throughout the world. The earth is 3
made of 6 large plates and several smaller ones. Oceanic plates are composed of solid rock from cooled mantle material, and are approximately 10-16 kilometers which is an equivalent of 6-10 miles thick. The EARS is an area where continental rifting is taking place at a rate of nearly 1-2 centimeters annually. As the plates move apart (divergent boundaries) molten rock pushes up from underneath the surface and forms a ridge when cooled. A new spreading center is in development along the East African Rift system. Spreading centers are areas where new sea floor has formed or has the potential to form. Typically, divergent plate boundaries in one area will cause a subduction zone in another area. Geological Setting The MER is located in North East Africa in the EARS figure. EARS extends southward from the Afar triangle, which is also known as the triple junction for an estimated 4000 km. The Afar triangle consists of the junction between three plates; the Arabian plate, Nubian plate and the Somalian Plate. The Nubian plate and the Somalian plate are the two African plates that are pulling away from each other due to continental rifting. The plates pulling away from each other will eventual lead to Africa splitting apart. This process has been occurring for approximately 50 million years and it is predicted that the split will occur in about 15 million years. The MER has many active volcanoes as does the entire EARS. Mafic Lavas In the word mafic, the Ma stands for magnesium and the Fic stands for felsic, therefore defining mafic lava as being rich in magnesium and iron. Bowen s reaction series (Figure 2) classifies mafic lavas by the formation of specific minerals that form at specific temperatures in natural samples. Those minerals that usually form in mafic lavas are olivines, pyroxenes and plagioclase feldspars, and the temperature that they are formed at is around 1000 o C or higher. Olivine ((Mg,Fe) 2 SiO 4 ) gets the name from the olive-greenish color of the crystal. Pyroxene ((Si,Al) 2 O 6) ) is a crystal that forms before the lava erupts and is usually dark or black in color. Plagioclase feldspar ((Na,Ca)Al 1-2Si 3-2 O 8 ) varies in the amount of sodium and calcium and it is characterized by compositional zoning which happens when crystals grow over a range of temperatures. Other characteristics of mafic lavas are darkness in color and low viscosity due to their low silica contents. Bowen s Reaction Series classifies olivines and pyroxenes as discontinuous and plagioclases as continuous. What does this mean? A discontinuous reaction series is when different minerals grow at different temperatures and the continuous reaction series is when one mineral changes composition but undergoes no structural change. Significance of MER Mafic Lavas The Main Ethiopian Rift is an essential area to study mafic lavas because of its location in an area where continental rifting is evident and also because of its distinctive basalt rocks. Obtaining geochemical data from these mafic rocks allows us to explore what is occurring in the mantle sources underneath the MER. The samples from this experiment were obtained from these five young volcanoes referred to as the N-Series: Fantale, Kone, Boseti, Gedemsa, and Tulu Moye. From the bulk rock analysis Figure 5, Kone and 4
Boseti volcanoes have the highest MgO content. The bulk rock analysis also shows Gedemsa and Tulu Moye having a higher content. Fantale has moderate abundances of MgO. Determining Equilibrium during crystal growth Factors that affect equilibrium of crystal growth include temperature, pressure and cooling rate. Thus, different types of crystals develop under different conditions. An example of temperature effect on crystal growth is zoning in plagioclase feldspars which is caused by continuous growth during temperature changes which results in a crystal having more calcium in areas that grew at high temperature and more sodium in areas of lower temperature growth. So what does this say about crystal growth that is not in equilibrium? Crystals found to be not in equilibrium with the host lava must have grown in another environment and been incorporated into the lava after they grew. Implications for Magma Chamber Structure Examination of bulk rock compositions, clinopyroxenes, plagioclase feldspars and olivine phenocrysts from the MER lavas demonstrates that the lavas originated from temperatures approximately 1188 o C ( =32-289 o C). The lower temperature values are obtained from plagioclase composed higher wt% of Ca, SiO 2, and Al 2 O 3. The higher temperature values in clinopyroxenes come from crystals composed of higher wt% in CaO and SiO 2. The temperatures of the olivine phenocryst lie between the clinopyroxenes and the plagioclase feldspars. The calculated pressures were used to model the depth underneath the earth s surface at which the minerals crystallized. The plagioclase feldspars crystallized at pressures ~ 6.0 kb and at depths ~ 20 km. The clinopyroxenes crystallized at pressures ~ 11kb and depths ~36 km. The pressure of olivine could not be determined but the overlap in calculated temperatures between olivines and pyroxenes suggests these two minerals grew at comparable conditions. The model magma chamber structure is shown in figure 7. This structure appears to be found at all volcanoes, even though the composition of lavas erupted differs, particularly between Kone and Gedemsa. I propose the existence of two magma chambers, one at a depth of about 20 km (near the base of the crust) and the other at the base of the lithosphere at a depth of about 35-40 km. Conclusions The higher the Fo level in olivine the more stability during crystal growth and the lower the Fo the more it shifts towards disequilibrium. Observation of the geochemical and petrological data of the Gedemsa samples showed distinct differences from the other MER volcanoes in temperature, high levels of An and by the general appearance when viewed as a backscattered image. The data suggest that the magmas from Gedemsa reside for a long time period in the shallow crust prior to eruption where they cool and grow low- 5
temperature minerals. During eruptions, basalt magma from the deeper chamber gets mixed with the more silica-rich lava, and as a result the minerals we see are typically at disequilibrium. Mafic minerals grow in deep chambers, crystallization of plagioclase feldspars takes place at more shallow depths. MER volcanoes are typically underlain by at least two magma chambers that are connected and that communicate during eruptions. In areas where the supply of basalt is low (such as Gedemsa), silica-rich lavas evolve in the shallow magma chambers and mixed lavas are erupted. In areas where the supply of basalt is high (such as Kone), basalt lavas are present in both chambers and no mixing of magma is observed. 6
Figure 1. Display of tectonic plate boundaries (adapted from NASA, 1999). Black box indicates location of study area shown below. Inset: This map shows the location of the study area immediately south of the Afar triangle. Samples were analyzed from five volcanoes: Fantale, Kone, Boseti, Gedemsa and Tulu Moye (located off the map towards the southeast). 7
Figure 2. Bowen s Reaction Series (Figure adapted from University of Oregon s Department of Physics webpage, 1998) Figure 3. Compositionally zoned plagioclase feldspar in an alkali basalt sample. The darker sections are richer in Ca and Si, the lighter sections are enriched in Na and Al. Groundmass includes glass and plagioclase Figure 4. Photomicrographs showing back-scattered images obtained from the Cameca SX50 Electron Microscope. Image A: scattered feldspars of different sizes. Image B: centered are olivine phenocrysts, the bright white spots are oxide, feldspars in the groundmass. Image A Image B 8
SiO2 vs. MgO 48.5 10 Figure 5. Kone and Boseti volcanoes have the highest MgO content, Gedemsa and Tulu Moye having a higher SiO 2 content. Fantale is moderate in both MgO and SiO 2. SiO2 48 47.5 47 46.5 9 8 7 6 5 4 MgO SiO2 MgO 46 3 45.5 2 1 45 Fantale Kone Boseti Gedemsa Tulu Moye 0 North South Figure 6. This is a P-T model showing a trend in all samples. As Temperature rises pressure increases and as Temperature decreases the pressure decreases Temperature vs. Pressure 1580 20 1560 18 1540 16 1520 14 Temperature (K) 1500 1480 1460 12 10 8 Pressure (kb) T P 1440 6 1420 4 1400 2 1380 1 2 3 4 5 6 7 8 9 Data points 0 9
Figure 7 Model magma chamber structure Earth s Surface 5 km 10 km 15 km 20 km Plagioclase forms 25 km Clinopyroxene forms 35 km 40 km 45 km Base of Lithosphere 10
Table 1. Petrographic Analysis Results of MER Samples Slide # Mineral Type Proportion Size (length in (mm)) Texture N-01 Feldspar 10% or less 1 ¾-2 ¼ mm Ophitic Olivine 35-40% 1/2-3mm basalt N-02 Small feldspars Less than 5% Ranging from 1/4-5/8 mm Ophitic basalt Small olivines App 10% Average size 1/2mm N-03 Feldspar (2) 3 % 1mm, 2 ¾ mm Porphoritic Very few olivines 2 % App 3/8 mm N-14 Large olivines 35% App 1mm Alkali Basalt N-15 Olivines 40% Ranging from 3/8-2 ½ mm Alkali Basalt N-18 Olivine 40% Ranging from 1/40-1 1/2mm Alkali Olivine Pyroxene 15% Ranging from 1/8-1 3/8 mm basalt Feldspar 10% Ranging from 1/8-1 7/8 mm N-19 Feldspar 30% Average Size 1/4-1 ¼ mm Olivines 5% Ranging from 1/40-1/2 mm Seriate texture basalt Gas cavities N-21 N-25 Olivine 30% Ranging 1/8-2 ½ mm Pyroxene 7% Ranging from 3/8-2 ½ mm Feldspar 1% Ranging from ½ -2 ¼ mm Olivine 2% Average size ½ mm Pyroxene 1% Average size 1/4-3/8 mm Feldspar 10% Ranging from 1/4-3 mm Phonolite Tholeiite Basalt Table 2 Average calculated Fe-Mg exchange coefficients for olivine and pyroxene from each bolcano Kd Averages Volcano Olivine Pyroxenes Boseti 0.53 n/a Gedemsa 0.27 0.64 Kone 0.45 0.71 Fantale 0.39 0.80 Tulu Moye 0.43 n/a 11
References Albarede, F., Introduction to Geochemical Modeling. New York, NY: Cambridge University Press. 1995. 543 pp Bursik M.I., Carey S. N., Glaze L.S., Glibert J. S, Sigurdsson H., and Woods A. W., Volcanic Plumes. New York, NY:John Wiley & Sons. 1997. 574 pp Caley D. and S. Russell, Bowen s Reaction Series. http://jersey.uoregon.edu/~mstrick/askgeoman/geoquerry32.html. accessed June 14, 2004 Condie K. C., Mantle Plumes and their record in Earth History. New York, NY: Cambridge University Press. 2001. 306 pp Flewitt, P.J., and Wild R. K. Physical Methods fro Materials Characterisation. 2 nd ed. Philadelphia, PA: Institute of Physics Publishing. 2003. 602 pp Hess, Paul C., Origins of Igneous Rock. Cambridge, MA:Havard University Press. 1989. 336 pp. Hughes, C.J., Igneous Petrology. New York, NY: Elsevier Science Publishing Company Inc. 1982. 551 pp. Loucks R.R. A precise olivine-augite Mg-Fe exchange geothermometer. Contrib Mineral Petrol 125 (1996): 140-150. NASA: Plate Tectonics NASA. 12 July 2004 http://observe.arc.nasa.gov/nasa/earth/tectonics/tectonics2.html. Nimis P. and P. Ulmer. Clinopyroxene geobarometry of magmatic rocks Part 1: An expanded structural geobarometer for anhydrous and hydrous, basic and ultra basic systems. Contrib Mineral Petrol 133 (1998): 122-135 Press, F. and Siever, R., Understanding Earth. 4 th ed. New York, NY: W.H. Freeman and Company. 2001. 573 pp. Robinson, H. Using Geochemical Data: Evaluation, presentation, interpretation. New York,NY: John Wiley & Sons. 1993. 352 pp Schall Joe. Style for Students: Effective Technical Writing in the Information Age. Eden Prairie MN: Outernet Publishing, LLC. 2002. 260pp. Acknowledgements I would like to thank the following individuals for all their support, encouragement and help with this research project: Dr. Tanya Furman, Department of Geosciences Dr. Evelyn Ellis and the Graduate School, Office of Educational Equity SROP Co-Directors o Nurian Mari Badillo-Vargas o Faye Hickman o Jonathan Stout Tyrone Rooney, Graduate Student, Department of Geosciences Cyndi Freeman-Fail, College of Earth and Mineral Sciences Myron McClure, College of Earth and Mineral Sciences Mark Angelone, Materials Characterization Lab Erin Manion, College of Earth Mineral Sciences College of Earth and Mineral Sciences SROP Scholars 12