Igneous petrology EOSC 321 Laboratory 7 Mafic volcanic rocks
Learning Goals. After this Lab, you should be able: Identify fine-grained igneous rock-forming minerals observed in the groundmass Estimate modal mineral abundances for minerals of different grain sizes (phenocrysts and groundmass) Name mafic rocks based on their textures and groundmass mineralogy
Material Needed: a) Microscope, b) a Manual on Optical Mineralogy (i.e. Minerals in Thin Section by Perkins and Henke); c) lab assignment printed out
Lab Organization. NO TEST TODAY!
Introduction to Mafic volcanic rocks collection: This collection demonstrates mafic volcanic rocks, i.e. basalts. Basalt is the most common terrestrial volcanic rock. Volcanic rocks classified in thin sections as basalts can have substantial variations in chemical compositions. Chemical analysis is absolutely necessary to distinguish, for example, between trachybasalt and basanite. Both of these chemical sub-types of basalt are comprised of plagioclase, augite, olivine and Fe-Ti oxide. Note that in the absence of chemical data we call all rocks in our reference thin sections "basalts". If basalt has olivine in the groundmass (not as phenocryst!), it should be called Olivine basalt. Olivine basalt is Si-undersaturated, alkaline in character.
From textures and experiments on natural samples, the common crystallization sequence for basalts is olivine ( Mg-Spl) => Ol +Plag ( Mg-Spl) =>Ol+Plag+Cpx. Thus, augite clinopyroxene is rare as a phenocrystal phase, but comprises up to 50% of the groundmass in basalts.
An important distinction between silica-saturated basalts and basalts of alkaline series can be made through petrographic observations. Alkali olivine basalt has olivine in the groundmass, but rarely has glass in the groundmass. Olivine phenocrysts tell us nothing about alkalinity.
Picrite is an olivine-rich basalt, and rocks with more than 20% of olivine could possibly be termed picrites or picro-basalts. However, a correct classification of a rock as picrite cannot be done petrographically and should be confirmed chemically.
Note that clinopyroxene-plagioclase fine-grained rocks are called basalts if the plagioclase is labradorite-anorthite, or andesites if the plagioclase is andesine.
Make sure that you see the following important rock characteristics in the Reference Collection: 2
1. Mineralogical differences between an alkali basalt and a silica-saturated basalt 2. Mineralogical differences between a basalt and more ultrabasic picrite/ankaramite 3. Mineralogical differences between a basalt and more felsic andesite 4. Crystal habits of crystobalite common in volcanic rocks 5. Trachytic, intergranular and intersertal textures 6. Characteristic alteration of olivine phenocrysts to iddingsite 7. Characteristic alteration of clinopyroxene to uralite
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Activity I There are several hand specimens of mafic volcanic rocks available in the lab. Examine all of them and write down a list of the rock-forming minerals you can observe in hand specimen. Are there any other features of these rocks that you can observe? Can you see phenocrysts, if so what are they? Group the samples into similar rock types. What general rock names would you give your different groups?
P469, P64, P2613, P2612, P1737, P619, P1852, P999, P515, MU 72 9A, 935B 4A, MU 74
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Activity II Now we will examine these same mafic volcanic rocks under the microscope. Examine one thin section from each of the following groups and provide a sketch, rock description and rock name.
1) 953, 732, 728, 597 olivine basalt 2) 974, 871, 659, 1257 basalt 3) 1942A1, 1935B-2B, 1935A1(2), 1935A2-2A (note some have blue epoxy) 4) A, Mu 74-1(A)
How do these grouping and rock names compare to the ones you came up with for the hand specimens? What groundmass, textures, and accessory minerals are you able to observe under the microscope that you can’t see in hand specimen? What are the similarities and differences between these rocks?
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Thin section ______Magnification______Field of view width ______
Description:
Rock name:
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Thin section ______Magnification______Field of view width ______
Description:
Rock name:
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Thin section ______Magnification______Field of view width ______
Description:
Rock name:
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Thin section ______Magnification______Field of view width ______
Description:
Rock name:
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Activity III Now your TA will hand out a list containing a brief petrographic description of all the thin sections and their associated hand samples for you to compare against your own. Please make sure you understand the reference petrographic descriptions and can find all of the minerals and textures mentioned. You should also be able to understand why a rock is given a particular name. In addition make sure you take a look at the following thin sections and their associated hand specimens:
1) 1114 basalt with crystoballite 2) 924 andesite
Make sure you can assess groundmass mineralogy and mineral modes under a microscope, and can plot these on rock classification triangles. Look through thin sections of different rock types so that by the end of the lab period you will have examined a variety of basalts. Your knowledge will be tested at the start of the next Lab, which begins with a 30 minute independent assignment on a thin section of an unknown fine-grained mafic rock.
Lab 7-2014 Igneous Petrology EOSC 321