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Central America Arc Presentation materials for instructor to show in association with class exercises Bob Stern (U TX Dallas), Ben Edwards (Dickinson College), Mike Carr (Rutgers U), Jim Walker ( N. Illinois U) Dec. 2013 Slide 2 Part one: background Slide 3 Handout Fig. 1 Modified from Le Maitre et al 2002 Slide 4 (Courtesy of Eric Calais, UMR CNRS http://web.ics.purdue.edu/~ecalais/haiti/context/)http://web.ics.purdue.edu/~ecalais/haiti/context/ Central American arc (~1100 km long) lies along the western margin of the Caribbean plate. Handout Fig. 2 Slide 5 Simplified cross section across the Central American subduction zone Handout Fig. 3 Slide 6 Nations of Central America Handout Fig. 4 (Source Wikipedia) Slide 7 Mayan civilization Wikipedia http://www.latinamericanstudies.org/mayas.htm Slide 8 Simplified tectonic map of the Central American convergent plate boundary Handout #5 Slide 9 http://www.geomapapp.org Slide 10 Cocos Plate Sediment (Hole 495) Modified from Plank & Langmuir 1998 Miocene & younger pelagic sediment Plank et al. 2002 Slide 11 Central American Subduction Zone is a hot subduction zone (subducts young ocean crust) Slide 12 Tectonic setting CRDB oblique Note: DSDP drillsite 495 sampled sediments on downgoing Cocos Plate. Well-defined volcanic front defined by linear segments with offsets. Scattered behind volcanic front volcanoes. Volcano-tectonic depressions, marked by two lakes and a gulf Gulf of Fonseca; Lake Managua; Lake Nicaragua; DSDP 495 (Courtesy of Mike Carr). Slide 13 Gravity anomalies show that the NW arc is built on continental crust (gravity low) but the SE arc is built on oceanic crust (gravity high) Modified from: http://www.ifg.uni-kiel.de/1267.html Slide 14 Prof. Mike Carr (Rutgers) keeping dry. Arenal (Costa Rica) in background Rutgers students at the La Paz waterfall (on the N side of Pos, on the trail to Arenal (Costa Rica) Central American volcanoes are often rained on (tropical rain forest). A wide range of people study Central American volcanoes: Professors like Mike Carr of Rutgers and students (Images Courtesy of Mike Carr). Slide 15 Smooth slab-red contours at 50 Km interval Slightly oblique convergence-green lines Discontinuity in contours in Costa Rica, Quesada sharp contortion (QSC) is controversial Slab dip increases from Mexico through Nicaragua, then decreases Beneath Costa Rica. Convergence vectors, DeMets 2001 From Syracuse and Abers, 2006 QSC Slide 16 (Source LaGeo, 2009) Slide 17 Part two: Modes, Major Elements, and Norms: Cerro Negro and Ilopango Slide 18 Paleo Granola recipe 1 cup raw almonds 1 cup raw cashews 1/4 cup raw pepitas/pumpkin seeds 1/4 cup raw sunflower seeds (no shell) 1/2 cup unsweetened coconut flakes 1/4 cup coconut oil 1/2 cup raw honey 1 tsp. vanilla extract 1 tsp. sea salt 1 cup raisins 1.Preheat oven to 275 F. Place the almonds, cashews and coconut flakes in a food processor or blender and pulse a few times to break into smaller chunks. 2.Microwave the coconut oil, raw honey and vanilla extract together in a medium sized mixing bowl for about 30-45 seconds. Add the seeds, nuts and coconut flakes and stir to coat. 3.Spread the granola mixture evenly onto a baking sheet lined with parchment paper and cook for about 20-25 minutes or until lightly browned, stirring once or twice. Remove from heat and add the raisins and sprinkle with sea salt, pressing the mixture together to form a flat surface (see photo below). 4.Allow to cool for about 20 minutes or until hardened, and then break into chunks and enjoy. Slide 19 Paleo Granola 1) How many of the 10 ingredients can you see easily? Could you identify more with a hand lens? 2) Why cant you identify all of the ingredients visually? 3) What is the texture of this material? 4) What would you call the texture if it was an igneous rock? 5) What do you visually estimate as the % of this material that is raisins? Modal analysis of rocks involves visual estimation of visible minerals, by eye. This analysis is better looking with a hand lens, even better with a microscope and thin section. Slide 20 Lets do a modal analysis of Central American lava Olivine is honey colored, Clinopyroxenes are green and black. Plagioclase is white. Q1) What is texture? Q2) What is size and percentage of olivine, clinopyroxene, plagioclase, and groundmass? 5mm scale Courtesy of Bob Stern and Ben Edwards Slide 21 QAPF diagram for volcanic rocks (Q = quartz, A = alkali feldspar, P = plagioclase, F = feldspathoid (nepheline, leucite) IUGS rock classification system Depends on proportion of visible minerals of quartz, feldspars, or feldspathoids. Useless for aphyric lavas and problematic for porphyritic lavas Q: using modal analysis, what is name of Central American lava in last slide? QAPF classification diagram for volcanic rocks. (Source, http://bc.outcrop.org/ GEOL_B11/) Slide 22 Major elements Slide 23 Major Elements vs. Trace Elements Major elements comprise >0.1% of rock and are reported as oxides e.g., SiO 2, Al 2 O 3, TiO 2, etc. These are reported as weight percent (wt. %). Major element compositions are generally manifested by lava modes. We use major element compositions to classify lavas. Trace elements comprise Exercise: Calculate Mg# of Cerro Negro sample CN-4 5mm scale SiO 2 48.6 wt. % TiO 2 0.65 Al 2 O 3 14.4 FeO*11.0 MnO 0.2 MgO 9.97 CaO11.3 Na 2 O 1.78 K 2 O 0.35 P 2 O 5 0.06 LOI 0.08 Total 98.39 wt. % Primitive (unfractionated) basalts have Mg# >65 Q: Is this a primitive basalt? Courtesy of Bob Stern and Ben Edwards Slide 39 Answer: Mg# of Cerro Negro sample CN-4 5mm scale SiO 2 48.6 wt. % TiO 2 0.65 Al 2 O 3 14.4 FeO*11.0 MnO 0.2 MgO 9.97 CaO11.3 Na 2 O 1.78 K 2 O 0.35 P 2 O 5 0.06 LOI 0.08 Total 98.39 wt. % This sample has Mg# = 62. Almost primitive but not quite. Wait a minute! What about the oxidation state of iron? We should only be counting Fe 2+ atoms, not all Fe atoms, which includes Fe 3+ ! Q: What is the Mg# if we adjust according to Kelley & Cottrell (2009) who found Fe 3+ /Fe for arc basalts = 0.18 to 0.32). Redo calculation using Fe 3+ /Fe = 0.25 Courtesy of Bob Stern and Ben Edwards Slide 40 Cerro Negro, 1968 eruption Cerro Negro Volcano, Nicaragua. Ash and cinders erupt from the summit crater in 1968 as a vent near the base of the 150 meter-high cinder cone feeds a basaltic lava flow (R. Decker) Slide 41 Relative volumes of 39 Central American volcanic centers (Stoiber & Carr 1973). I: Ilopango; T: Telica; LP: Las Pila-El Hoyo (site of Cerro Negro). I I T LP Slide 42 Lets look at Cerro Negro and its basalts Slide 43 Cerro Negro, Nicaragua: Central Americas youngest volcano Established 1850, how many eruptions since? Slide 44 Cerro Negro is part of the Pilas-El Hoyo volcanic complex in western Nicaragua (also known as the Marabios Range) Walker & Carr 1986 Slide 45 Handout #6 Courtesy of Mike Carr Slide 46 Prof. Mike Carr at Cerro Negro, 1982 (thanks to J. Walker NIU) Slide 47 Volcanic Geology of Cerro Negro Walker & Carr 1986 Not locations where samples CN-4 and CN-6 were taken. Slide 48 Tourists enjoy sledding down scoria-covered slopes of Cerro Negro. Wikipedia Slide 49 Cerro Negro (CN-4) 1957-1960 eruption Olivine is honey colored, Clinopyroxenes are green and black. Plagioclase is white. This is a primitive basalt (MgO ~9%) although some phenocryst accumulation has occurred. 5mm scale Courtesy of Bob Stern and Ben Edwards Slide 50 CN-6 contains plagioclase with 75% anorthite endmember (AN75). Q: What is the name of plagioclase with this composition? CN-4 contains olivine composed of 77% forsterite endmember, 23% fayalite endmember. Shorthand for this is FO77. Handout Fig. 8 Courtesy of Bob Stern and Ben Edwards Slide 51 Cerro Negro CN-6 (1968 eruption) 5mm scale Olivine is honey colored, pyroxenes are green and black. Plagioclase is white. Q1: what is texture? Q2: what is mode? Courtesy of Bob Stern and Ben Edwards Slide 52 What happened between 1960 and 1968 beneath Cerro Negro? Schematic illustration of the magma storage system beneath Mount St. Helens showing earthquake locations (open circles) in country rock (high velocity) surrounding earthquake- free zones of magma storage (low velocity). From Cashman and Sparks 2013 1957-60: mantle melting and rapid rise of primitive magma to erupt (CN-4). 1960-1968: storage and fractionation of magma beneath volcano 1968: eruption of fractionated lava (CN-6) Cashman and Sparks, 2013 Slide 53 Photomicrographs of CN-6 Field of view = 4.6 mm What do you see? Top is plane-polarized light, bottom is with cross- polarized light (Images Courtesy of Mike Carr and Jim Walker NIU). Slide 54 Vesicles are clear in Plane-polarized light(PPL) but black (opaque) with X- polarized light (XPL). Several plagioclase phenocrysts. One is large (~3mm across). Note small (few hundred microns) olivine near top. It is more obvious under XPL because it has higher birefringence Note dusty rim around large plagioclase phenocryst. This indicates disequilibrium between magma and plagioclase at this time (late in the growth history of the plagioclase). Q: Something changed in the magma, what could it be? (Images Courtesy of Mike Carr and Jim Walker NIU). Slide 55 Jim Walker (NIU) and Sandino, Masaya Nicaragua, ~1982 (Image Courtesy of Jim Walker NIU). Slide 56 Pacaya, Guatemala 2006. Pacaya has been erupting since 1965. (Source http://volcano.si.edu/) Slide 57 Lets look at an example of a Central American felsic lava: Ilopango Slide 58 Ilopango (El Salvador) Courtesy of Mike Carr Slide 59 Lago Ilopango from above Lago Ilopango from the ground Ilopango is a broad, low volcano with a ~10 km diameter caldera Wikipedia http://blogs.laprensagrafica.com/scientia/?p=110 Slide 60 1880 eruption of Islas Quemadas, source of IL4 Wikipedia Slide 61 (Courtesy of Mike Carr). Slide 62 Slide 63 Slide 64 Slide 65 Additional slides (mostly about trace elements) Slide 66 Marine sediment is a key to some incompatible element variations that show up in the lavas lowest variance maximum in carbonate maximum in hemipelagic -------- Regional -------- --------- Local --------- moderate variance DSDP 495 Modified from Carr et al 2003 Slide 67 Geochemical changes with time at Cerro Negro 19201940196019802000 4 5 6 7 Na 2 O/K 2 O Age NIC-CN2000-1 NIC-CN1 NIC-CN2 NIC-CN3 NIC-CN4 NIC-CN5 NIC-CN6 NIC-CN7 NIC-CN8 NIC-CN9 NIC-CN10 NIC-CN11 NIC-CN12 NIC-CN13 NIC-CN-92-2 19201940196019802000 7 8 9 10 11 K/Ba Age NIC-CN2000-1 NIC-CN1 NIC-CN2 NIC-CN3 NIC-CN4 NIC-CN5 NIC-CN6 NIC-CN7 NIC-CN8 NIC-CN9 NIC-CN10 NIC-CN11 NIC-CN12 NIC-CN13 NIC-CN-92-2 Cerro Negro basalts show increase in K2O over time fractionation? (Courtesy of Mike Carr)