LAB 9: ULTRAMAFIC ROCKS, CUMULATES AND MELT SOURCES

Transcription

1 Geology 316 (Petrology) (03/26/2012) Name LAB 9: ULTRAMAFIC ROCKS, CUMULATES AND MELT SOURCES INTRODUCTION Ultramafic rocks are igneous rocks containing less than 10% felsic minerals (quartz + feldspars + feldspathoids) and more than 90% mafic minerals such as olivine, pyroxenes and hornblende. Chemically, ultramafic rocks are very poor in silica (< 45 wt. % SiO 2 ) corresponding to their ultramafic mineralogy. Ultramafic rocks are much denser than most crustal rocks with a density of about 3.3 g/cm 3 (granite is about 2.7 g/cm 3 ). You can simply feel it when you compare an ultramafic hand sample to a granite. Ultramafic rocks sometimes represent the petrologic composition of mantle and can be found in exposed mantle sections, or ophiolites, in a number of places on the Earth. Ultramafic intrusive rocks also can be found in the bottom part of layered intrusions, in which mafic minerals recrystallize first and sink down to the bottom of the magma chamber forming layers of cumulated ultramafic minerals. In the later case, we call such ultramafic rocks cumulates. Cumulates sometimes are associated with ore deposits (such as chromium and platinum group elements) and thus are economically important. Ultramafic extrusive rocks are much less common than their intrusive counterparts. Kimberlites are K- rich hybrid ultramafic extrusive rocks that occur in ancient cratons. They usually contain diamond xenocrysts when the ultramafic magma rises from the deep mantle. Another type of ultramafic extrusive rocks is komatiite. In this lab, you will practice how to use the IUGS classification to name ultramafic rocks. Several hand samples and thin sections are also available for you to explore. Finally, we will use a computer program to study the genesis of two types of basaltic magmas. PART I: Naming an ultramafic rock The following Ol-Opx-Cpx triangle (Fig 1.) is recommended by the IUGS for the classification and nomenclature of the ultramafic rocks with phanaritic textures. This method works for intrusive rocks in which modal percentages of minerals can be estimated. For extrusive rocks of fine-grained/glassy textures, the classification is based on their chemistry compositions and normative mineralogy. We won t deal with them in this lab. Question 1 In the Table 1, I list modal percentages of minerals of a few igneous rocks. Some of them are ultramafic rocks and some of them are not. Please (1) identify the ultramafic ones by circling the sample number; (2) only plot the ultramafic ones in Fig 1 and give them appropriate names. 1

2 Number Volcanic/ Plutonic Modal percentages Quartz Plag K-spar Nepheline Olivine Opx Cpx Hbl Others 1 Plutonic Plutonic Plutonic Plutonic Plutonic Plutonic Tab. 1 Fig. 1 2

3 PART II: Hand samples and thin sections In this part of the lab, you will look at several hand samples and thin sections. You need to identify the mineralogy and textures of the rocks, give them their proper names and answer related questions. Question 2: You will look at 2 hand samples in this part. Please (1) estimate modal percentages of each mineral and name it according to the IUGS systematics; (2) describe the textures of the sample. (3) Both of them have thin sections. If you are not sure about the mineralogy, check the thin sections. Sample 316-5D: Name Minerals and modal percentages: Texture: Sample 316-5A: Name Minerals and modal percentages: Texture: 3

4 Question 3: You will look at 3 thin sections in this part. Answer questions related to each thin section. The thin sections below are generally in the sequence from most primitive to more evolved. Primitive means the rock has not undergone differentiation and evolved means that the rock has been produced by differentiation from a primitive composition. When you look at thin sections, however, you don t have to follow such sequence. Thin section: Meteorite. Here is our guest thin section of meteorite! This one is called a chondrite, which means a meteorite containing chondrules, nearly spherical silicate inclusions between 0.1 and 3 mm in diameter. The chondrules are considered to be droplets of liquid/glass that have subsequentially crystallized into silicate minerals. These droplets still keep their spherical shape because they are not modified by later melting and differentiation. In this sense, they are the most primitive things having the compositions closest to the original solar nebular! People think the composition of such meteorites represent the bulk composition of the Earth. Even mantle has more evolved composition compare to the chondrite. How cool! What mineral makes up the chondrules? Please draw a sketch of one of them. Thin section: 316-5D. This is probably the most mafic rock in the whole collection! (1) What is the dominant mineral? Is there any other minerals besides the dominant mineral? Give modal percentages of the minerals and name it according to the IUGS systematics. Compare your mode estimation in thin section to the one you made based on hand sample. Do the two results match? (2) What is the overall texture of the rock? (3) Based on the mineralogy of the thin section. Which part of the Earth do you think it comes from? 4

5 Thin section: 316-5A. You have looked at this hand sample. Now it s time to check it under microscope. (1) Give modal percentages of the minerals you can find and name it according to the IUGS systematics. Be sure you know how to identify two types of pyroxenes. Compare your mode estimation in thin section to the one you made based on hand sample. Do the two results match? (2) What is the overall texture of the rock? (3) Some of the pyroxene grains have the exsolution lamellae. Recall your mineralogy knowledge, low-ca orthopyroxene (Opx) separates from high-ca clinopyroxene (Cpx) as the unmixing occurs when temperature decreases (Figure 2). What are the compositions (Cpx or Opx) of exsolution lamellae and host crystals in this thin section? What does another common mineral usually have the exsolusion texture? Fig. 2. When pigeonite (Mg-Fe-pyroxene, Opx) or augite (Ca-Mg-Fe-pyroxene,Cpx) exsolve they may form exsolution lamellae that form parallel to the (001) plane. At lower temperature the exsolution of Opx or augite result in exsolution lamellae that are parallel to the (100) plane. (Modify from 5

6 PART III: Generate basaltic magmas from a mantle peridotite In this part, we will use a computer program named PhasePlot (Figure 3) to explore the genesis of basaltic magma generated from a mantle rock. Recall we have two types of basalts: alkaline and tholeiitic basalts. Both can be derived from a chemically uniform mantle of the same composition. Melting depths (thus the pressures) and the degrees of partial melting are two important factors controlling the types of basalt generated. We will test several melting scenarios using the PhasePlot. PhasePlot PhasePlot ( is a computational thermodynamics software package for visualizing equilibrium phase relations in application to the Earth and other silicate planetary bodies. It is a free Mac application only run on Mac OS. It can be downloaded from the Mac App Store: However, it only runs on the newest Mac OS (version 10.7 Lion); so make sure your computer can install this. In the lab, let s do this part together as a group. You may want to take some notes about the data generated by the program. Fig 3. Screenshot of PhasePlot. A: Start/Stop. B: Composition popup menu. C: Composition popover. D: Clear the display. E: Popup the phase selection dialog. F: Button for full screen mode. G: Configure phase color palette. H: Invoke grid overlay diaplays. I: Configure temperature and pressure grid. J: Current database display. More info: 6

7 Modal Setup Since we are going to melt a mantle rock, we need to tell the program the chemical composition of the rock and the P-T ranges we are interested in. We will use the mantle peridotite as our initial mantle composition. We will do the following steps to setup the modal: (1) Launch the PhasePlot. Click the PhasePlot at the meun. Choose Preferences (2) In the General setting window, choose pmelt for the calculation database. The pmelt database allows us to do an equilibrium calculation under a high pressure. (3) Click composition drop-down list. Choose Mantle peridotite (MM3). (4) Click the Mantle Peridotite to the right of composition drop-down list. A list of major oxides will pop up. You can enter your own values. But here, we will use the default setting as it shows. (5) At lower right corner of the window, there are places for you to enter the pressures and temperatures. Temperature is in the unit of degree Celsius; and pressure is in the unit of Mega Pascal (MPa). Let s enter 1200 (min)-1600(max) ºC for temperature and 1000 (min)-3000(max) MPa for pressure. (6) You should realize that by setting the maximum pressure at 3000 MPa, we are dealing with quite deep depth in mantle (~100 km). (7) Click start button on upper left. Individual pie chart corresponding to a P-T condition will be calculated one by one. The color palette on the right ride tells you that which color in the pie chart represent which mineral. The result is shown in Figure 4. You can click the pie chart to show more detailed information about the equilibrium phase. Fig. 4 7

8 Question 4 In the T=1200 ºC column, temperature is constant while pressure changes from 3000 MPa to 1000 MPa. Does the mineralogy of the mantle change with pressure? What is the typical high-p mineralogy of the solid-phase mantle and what is the typical low-p mineralogy of the solid-phase mantle? What causes the increasing percentage of liquid phase in the T=1400 ºC column? Question 5 What is the chemical compositions (major oxides) of liquid phase in the pie charts at 1400 ºC-1000 MPa (A liquid) and 1600 ºC-3000 MPa (B liquid)? Both liquids are ultra-basaltic to basaltic in terms of silica content. Which one is alkaline liquid and which one is tholeiitic? Please use Figure 5 and 6 to discriminate between them (ignore the existing dots on the diagrams) and plot A and B on diagrams. re F Mg ks o l Calc-alkaline Fig. 5. Total alkalis vs. silica diagram for the alkaline and sub- alkaline rocks. A M Fig. 6. AFM diagram. A=K2O+Na2O, F=FeO+Fe2O3 or FeO*. M= MgO. 8

9 Question 6 In the P=1571 MPa row, pressure is constant while temperature changes from 1200 to 1600 ºC. With the increasing temperature, the percentage of liquid phase is also increasing. Compare the chemical compositions of the liquid phases at 1571 MPa-1400 ºC (C liquid ) and 1571 MPa-1600 ºC (D liquid). Which liquid is more alkaline and which is more tholeiitic? Plot C and D on Figure 5 and 6. How does the composition relate to the degree of melting? Question 7 If the liquid phase is extracted from the source region, we call what is left as residuum (pl. residua). What is the typical mineralogy of the residua in this modal if more than 25% of the liquid phase is extracted? 9