Igneous petrology EOSC 321 Laboratory 2:

Ultramafic plutonic and volcanic rocks

Learning Goals. After this Lab, you should be able:

Identify igneous rock-forming minerals based on their optical properties Demonstrate the operation of the petrographic microscope in plane and crossed polarized light Determine how many minerals are present in a thin section and describe their abundance and

optical properties Identify the key rock-forming minerals in ultramafic rocks Name ultramafic rocks based on their textures and mineral modes using triangle diagrams

Material Needed: a) Microscope, b) Glossary of rock names and textures (see Pages 24- 25 and 43 of Winter); c) Lab Manual printed off the course website; d) tables to aid determination of mineral modes in thin sections; e) classification triangles; f) a Manual on Optical Mineralogy (i.e. Minerals in Thin Section by Perkins and Henke)

Microscopes: You were assigned a microscope last week. The microscope will be shared with students from the other laboratories. Please keep the microscopes in working condition and, should a problem arise, let your TA know immediately so that repairs can be made.

Introduction: This lab introduces ultramafic rocks formed as intrusives or in volcanic bodies. Ultramafic rocks consist mainly of olivine, clinopyroxene and orthopyroxene with accessory spinel (chromite or magnetite) and plagioclase. Typical alteration products of these primary phases are serpentine, chlorite, talc, amphibole and mica. Your TA’s will demonstrate these minerals at the beginning of the Lab and remind you of optical properties that can be used for their identification.

Activity I From last week’s lab here are a list of the minerals in the different thin sections. If you check against your descriptions and make sure you were able to identify all the minerals properly. Any you didn’t look at, take a quick look now. This a box contains reference thin sections for igneous rock-forming minerals that may be useful to refer to throughout the course.

Reference Mineral Collection

37 Olivine (Ol), Orthopyroxene (Opx), Spinel (Spl) 101 Olivine (Ol), Orthopyroxene (Opx) 87 Talc (Ta)

108 Serpentine after Olivine (Serp)

105 Serpentine after Olivine (Serp) 135 Hornblende (Hb), apatite (Ap) 56 Hornblende (Hb)

208 Clinopyroxene (Cpx) Clinopyroxene (Cpx). Plagioclase (Pl) and

Light yellow serpentine replacing Ol phenocrysts Black serpentine ( Serp mixture with fine magnetite) replacing olivine along fractures

150

148

quartz (Qz) are also present Clinopyroxene (Cpx), biotite (Bi). Feldspar (Fsp) also present

1535 Garnet (Gar), Chlorite (Chl), Muscovite (Mus) Chlorite is green pleochroic 166 Plagioclase (Plag), biotite (Bi), garnet (Gar)

75 Biotite (Bi)

P2713 Plagioclase (Pl). Ol, Cpx are also present

Note black haloes of metamict alteration around zircons in Bi

45 Plagioclase (Pl), volcanic glass Zoned Pl pjenocrysts in brown volcanic glass 79 Spherulites Texture of quick crystallization of lava.

59 Sericite ( Ser), Quartz (Qz).

158 Microcline, Perthites, Green alkaline Cpx

K feldspar, Sericite, Quartz. Perhtites and 49 microclines are present

42 Plagioclase (Pl), Quartz (Qz)

55 K-feldspar (K-Fsp), Quartz (Qz). Perthites. 210 Quartz (Qz), Mus (Mus), Biotite (Bi)

Note that sericite (fine grained muscovite that looks like brown dust) replaces feldspars (Pl and Ksp), but quarts is completely fresh Microcline is K-feldspar ( KFsp) polymorph with tartan twinning Note that sericite (fine grained muscovite that looks like brown dust) replaces Ksp, but quarts is completely fresh. Perhtites and microclines are present Note the difference between these minerals: Pl is twinned, while the quartz is not. Note the different reliefs of the minerals. Note the difference between these minerals: Pl is twinned, while the quartz is not. Note the different reliefs of the minerals.

23 Quartz (Qz) Round grains in sandstone 72 Carbonate (Carb), Apatite ( Ap) Note euhedral shapes of apatite 14 Carbonate (Carb)

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Activity II

There are several hand specimens of ultramafic rocks available in the lab. Examine all of them and write down a list of the ultramafic rock-forming minerals you can observe in hand specimen. Group the samples into similar rock types. What general rock names would you give your different groups?

P1607, P1606, P612, P3404, P984, P1657, P3384, P1340, P1738, P622, P332, P1663, P624,

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Activity III Now we will examine some ultramafic rocks under the microscope. Examine one thin section from each of the following groups and provide a sketch, rock description and rock name. 1) 1614, 1615, 920, 1859, 1298B, 1408 2) 1655, 1864, 1457, 805, 804, 840, 927, 1027 3) 1652, 1653, 930, 790 4) KM-8, KM-2

How do these grouping and rock names compare to the ones you came up with for the hand specimens?

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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 IV 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 mentioned. You should also be able to understand why a rock is given a particular name. For these, you should recap how to assess mineral modes under a microscope, and how to 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 peridotite, pyroxenite, anorthosite and komatiite. Your knowledge will be tested in the next Lab, which starts with an independent assignment on a thin section of an unknown ultramafic rock.

CLASSIFICATION SCHEMES FOR PERIDOTITIC ROCKS