final exam_Petrology 2023

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Oct 30, 2023

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Petrology GEOL 3003C/7011C spring 2023 Final Exam Your name here: _Olivia Girten___ 1. THIN SECTION PETROGRAPHY Listed below are a series of rocks their thin sections. Answer the question or questions for each sample. F3-22, eclogite – As can be seen in thin section, garnet is commonly in contact with high-relief, blocky, very pale green omphacitic pyroxene. Garnet is also in contact with lower relief, colorless chlorite (in XPL, the birefringent color of this chlorite is charcoal gray; the one excellent “mica cleavage” in chlorite can be readily seen). Meta-basalt in the eclogite facies is characterized by garnet + omphacite. Chlorite isn’t a mineral which characterizes meta-basalts metamorphosed in the eclogite facies. Give one explanation of how chlorite got into this rock. Since chlorite isn’t found typically in the meta-basalts that are metamorphosed in the eclogite facies a possibility could be that it was formed during retrograde metamorphism where the existing meta-basalt was uplifted, lowering the P and T conditions and possibly allowing fluids to react with the minerals leading to the chlorites formation. hand sample OC.89.5.10 and thin section OC.95.1 (one and the same despite the different labels ) – Aside from sphene (this is the out-of-date name, the current name for the same mineral is “titanite”, CaTi(SiO 4 )O) – this rock is nearly entirely composed of two minerals. Titanite occurs as a small, high- relief, colorless mineral with a “flattened diamond” shape. Very minor epidote can be seen in some sections. What are the two dominant minerals? What name would you give this rock? What might be its protolith? The two dominant minerals are plagioclase and hornblende, I would name this rock a titanite hornblende, the protolith would be a mafic igneous rock. VT 2 – This is a rather spectacular schist from Vermont composed of the assemblage bt + ms + chl + st + grt + ky + qtz + opq . The staurolite in this rock is obvious and stunning (remember, staurolite has yellow pleochroism). The identification card for this rock describes it as a “garnet-sillimanite schist” but I think muscovite intergrown with biotite and small muscovite laths were misidentified as sillimanite. Consider the diagram below. Some explanation of the figure is given.

This P-T diagram shows some of the limits of various minerals and mineral assemblages found in meta- shales. The horizontal lines show the pressure limits (zones given numerical labels) of various minerals assemblages; for example, the upper pressure limit of andalusite + biotite (+ vapor) is about 4 kilobars. Chlorite + muscovite + staurolite + quartz + sillimanite OR kyanite has an upper pressure limit a bit over 6 kilobars; whether sillimanite or kyanite is stable depends on temperature. (The kyanite-sillimanite boundary has its label in the upper right-hand corner of the diagram.) Notice that the reaction labelled (16.6) limits the upper temperature limit of chlorite + muscovite + staurolite + quartz. Reaction (16.8) shows the limit of the assemble muscovite + staurolite + quartz (without chlorite). Again, the assemblage in Vt-2 is bt + ms + chl + st + grt + ky + qtz + opq . What limits can you put on the P-T conditions of metamorphism of this rock? a. in zone 4, and above reaction 16.6 b. in zone 5, but below reaction 16.6 c. in zone 5, and above reaction 16.8 d. any zone above reaction 16.8

RE (Reunion Island)-3 – What is the phenocryst in this rock? The phenocryst in RE (reunion Island)-3 is olivine RE-8 – What is the phenocryst in this rock? The phenocryst in RE-8 is plagioclase, there is some chemical zoning in the plagioclase as well. The yellowish color that some of the plagioclase has could be because the section is too thick. Could these two rocks have been derived from a single magma chamber? Briefly explain why or why not. Yes, they could have, due to crystal fractionation crystal settling and floating can occur. The higher density crystals like olivine sink to the floor of the magma chamber, in turn the lower density crystals like plagioclase rise to the top of the magma chamber and accumulate. Match the following plutonic rocks and their names. Each rock has one or more thin sections. ___ D __ S-196 ___ A __ BAR (note: half of the thin section has been stained for plagioclase feldspar) __ _C __ S-321 ___ B __ PS-23 A. biotite granite B. diorite C. olivine gabbro D. anorthosite 2. TWO GEOCHEMICAL DIAGRAMS

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2a. The following diagram shows where Aleutian arc basalt (red triangles), MORB (gray circles), and ocean island basalt (small green squares) plot on a TAS diagram. Give one explanation why Aleutian arc basalt straddles the alkaline-subalkaline boundary, MORB is subalkaline, and OIB is alkaline. 35 40 45 50 55 60 65 0 2 4 6 8 10 12 Total alkalis vs. silica for MORB, OIB, and Aleutian arc basalts; data from GeoRoc Lin ear () wt% SiO2 wt% Na2O + K2O - MORBs are subalkaline due to their mantle source, they are generated by the partial melting at mid-ocean ridges and have low potassium and sodium and have a higher concentration of aluminum. The PT conditions also could have something to play in the reason as to why it is subalkaline, MORBs are generated at high P and are pretty anhydrous, limiting the production of alkali-rich melts. - The water content of OIBs might be a factor as to why they are alkaline, typically they are more hydrous than MORBs, since they are generated under lower PT conditions. They are higher in potassium and sodium due to the either the presence of mantle plumes or from subduction- related processes such as sediment and fluid being introduced to the melt. - The Aleutian arc is found near Alaska, where the North American plate subducts the Pacific plate, a reason as to why they are straddling the boundary could be the incorporation of crustal rocks interfering with the composition of the Aleutian arc basalts.

The following figure shows a measure of Fe-enrichment on the Y-axis vs. weight percent SiO 2 on the X- axis. Tuolumne is a pluton in the Sierra Nevada batholith; Mt. St. Helens is the famous volcano in Oregon; Tobago is a volcanic island in the Caribbean, and Seguan is one of the Aleutian islands. All are subduction-related rocks. 2b. Notice that the field of Tobago rocks has a maximum SiO 2 weight percent of about 65%, whereas the other three fields extend to 70% SiO 2 and higher. What could explain this difference? Magmatic differentiation, given that they are all subduction related rocks the weight percent of Tobago rocks being lower could be related to the temperature at which crystallization happened. Low silica content has a higher temp of crystallization. 2c. All four sets of rocks are magnesian. Are these magmatic environments more reducing or more oxidizing? Oxidizing conditions promote the igneous crystallization of iron oxide minerals; reducing conditions suppress the crystallization of iron oxides. These magmatic environments are more reducing, they are below the ferroan—magnesian line, indicating a lower iron oxide mineral composition, they as well show up lower on the Fe-index on the y- axis.

3. ONE QUESTION ABOUT PLAGIOCLASE Plagioclase is an essential mineral in all common igneous rocks. Mafic rocks commonly contain Ca-rich plagioclase whereas plagioclase in felsic rocks is sodium-rich. Show that anorthite (CaAl 2 Si 2 O 8 ) is silica- poor compared to albite (NaAlSi 3 O 8 ). The gram formula weights of the relevant oxides are: SiO 2 – 60.08 g/mol Al 2 O 3 -- 101.96 g/mol CaO – 40.08 g/mol Na 2 O – 61.98 Anorthite: 1x CaO – 40.08g/mol = 40.08 g/mol 1x Al 2 O 3 – 101.96 g/mol = 101.96 g/mol 2x SiO 2 – 60.08 = 120.16 g/mol + ______________ Total weight = 262.2 g/mol 120.16 g/mol  262.2 /mol = 45.8% SiO 2 Albite: .5x Na 2 O – 61.98 g/mol = 30.99 g/mol 1.5x Al 2 O 3 – 101.96 g/mol = 50.98 g/mol 3x SiO 2 – 60.08 g/mol = 180.24 g/mol +_______________ Total weight = 262.21 g/mol 180.24 g/mol  262.21 g/mol = 68.7% SiO 2 The total percentage of SiO 2 is less in anorthite than it is for albite.

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4. SHORT ESSAY QUESTION…REQUIRED FOR THE GRADUATE STUDENTS, EXTRA CREDIT FOR THE REST Crystalline rocks record crustal processes. Write 250-400 words describing what some of the crustal processes are that are recorded by igneous and metamorphic rocks. The crustal processes that are recorded by igneous and metamorphic rocks are magmatic, deformation, metamorphic and hydrothermal processes. For magmatic processes, analyzing igneous and metamorphic rocks can help form a picture of the history of the Earth’s crust and mantle; melting, uplift, assimilation, differentiation and fractional crystallization are processes that igneous petrologists use as tools to understand the Earth’s tectonic activity. For metamorphic processes, it can record deformation and the fracturing of rocks that underwent metamorphism, leading to the formation of faults and folds. Foliation is a process recorded in schists and gneiss that shows the directional pressure of minerals in a preferred orientation. For deformation processes in igneous rocks plutons can intrude the surrounding rock resulting in folds, fractures and shears. The cooling of the intruding magma also causes deformation such as jointing. Metamorphic rocks record deformation processes that occurred via pressure and temperature. Foliation and banding are some of the processes that metamorphic rocks record. Hydrothermal processes are the circulation of hot fluid through rock, this results in a change of composition due to the chemical reactions. By changing the composition of the rock, you inherently change the structure as well.