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Kimberlites, Lamprophyres, Carbonatites EOSC 321 Instructor: Kopylova Laboratory 8: Exotic volatile-rich rocks - Kimberlites, lamprophyres, carbonatites Learning Goals. After this Lab, you should be able: •! Identify the key rock-forming minerals in exotic volatile-rich rocks •! Classify kimberlites, lamprophyres, carbonatites •! Identify primary serpentine and carbonate in igneous rocks Material Needed: a) Microscope, b) a Manual on Optical Mineralogy (i.e. Minerals in Thin Section by Perkins and Henke). Lab Organization: You will start with a 45 min test on alkaline rocks. In the next two hours of the lab period you will examine hand specimens and reference thin sections of exotic, rare magmatic rocks rich in volatiles. Each thin section has a brief petrographic description to assist you in the identification of minerals, textures and rock classification. 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. Your knowledge will be tested in the next Lab, which starts with an independent assignment on a thin section of a volatile-rich rock. After you have reviewed the reference collection, do a short assignment, which will be checked for completion, but not marked. Introduction: This lab introduces exotic, rare magmatic rocks rich in volatiles. Kimberlites are potassic ultramafic, CO2-rich rocks with a characteristic inequigranular texture, where larger crystals set in a fine-grained groundmass. Because kimberlites are hybrid rocks and contain a lot of large xenocrysts, we describe them using terms with no genetic connotations. Thus, we cannot call kimberlitic textures porphyritic, as it implies a cognate origin of phenocrysts. Instead, we refer to kimberlitic texture as macrocrystal. Macrocrystal texture is inequigranular, where larger crystals are set in a fine-grained groundmass. It is analogous to porphyritic texture in common igneous rocks. Large grains of unknown origin present in kimberlites are called macrocrysts or megacrysts. Macrocryst is a crystal of unknown origin 0.5-10 mm in size set in a fine-grained matrix of volcanic rocks. In kimberlites, olivine, phlogopite, Cr-diopside, chromite, ilmenite, and garnet are common macrocrysts. Macrocrysts in kimberlite have dual origin: they could have crystallized from the host magma, or they could be xenocrysts. Megacryst is a big (1-10 cm) crystal of unknown origin in volcanic rocks. In kimberlites they are usually olivine, phlogopite, pyroxenes, ilmenite, or garnet. Two generations of olivine are present in kimberlites; in addition to macrocrystal olivine, kimberlite contains numerous smaller (<0.5 mm) euhedral olivines that are usually termed microphenocrysts. All smaller olivine crystallizes from the host magma. In contrast to most igneous rocks, serpentine and carbonate in kimberlites are primary, having crystallized in fine intergrown crystals on late stages of groundmass crystallization. - 1 - EOSC 321 Instructor: Kopylova Lamprophyres are defined as a group of rocks which are strongly porphyritic in mafic minerals, typically biotite, amphiboles and pyroxenes, with any feldspar being confined to the groundmass. Lamprophyres could be further classified into different rock types (such as spessartine, kersantite, melilitite etc.), but because lamprophyres are relatively rare, we do not require you to memorize all names of lamprophyric rocks. Lamprophyres encompass a wide range of compositions, from ultramafic to silisic, but they all have high H2O content and therefore crystallize abundant-mica-amphibole phenocrysts. Ultramafic lamprophyres mineralogically resemble kimberlites and their classification is highly ambiguous. Carbonatites are the most unusual type of igneous rocks. More than 50 modal % of the rock is primary magmatic carbonate (calcite, dolomite or ankerite). It is impractical to distinguish between these carbonate species in thin sections without special staining. Carbonatites commonly has varied amounts of clinopyroxene, alkali amphibole, biotite, magnetite and apatite. For carbonate-bearing rocks with 10 to 50% carbonates, we should use modifying terms "calcitic' or "dolomitic" before the igneous rock name based on the remaining silicate assemblage. Over 280 minerals are known to occur in various carbonatites, reflecting the diverse and exotic carbonatite chemistry. You will not be asked to identify these minerals in Assignments. Exotic rocks we see today in the lab host many minerals that are new to us. Phlogopite is a mafic Mg-rich mica. Petrographers call any mica that exhibits brown or yellow to colourless pleochroism a phlogopite. Monticellite is Ca olivine (CaMgSiO4) and it resembles olivine in thin sections. It has a high relief, parallel extinction, high birefringence, and can be distinguished from olivine only in large grains by slightly different grain habits (see T/s 262, P2606) and the absence of serpentinization. Monticellite is a common mineral in the kimberlite groundmass, but it is almost always replaced by serpentine. Perovskite (T/s KL05) is found in kimberlites and lamprophyres where it is identified easily due to a dark brown colour and its isotropy. Zeolites (T/s 1229) are a group of minerals (hydrated aluminosilicates) that encompasses more than 40 species. All zeolites have low indices of refraction and low birefringence. They commonly fill vesicles, veins and voids in volcanic rocks. It may be difficult to distinguish zeolite species based solely on their optical properties. The Reference collection for the Lab is large, but don’t feel intimidated. During the Lab, you need to thoroughly examine just one kimberlite, one lamprophyre and one carbonatite, and make sure you see the following important rock characteristics in the Reference Collection: •! Mineralogical differences between kimberlites (no Plag) and mafic lamprophyres (Plag in the groundmass) •! Primary carbonate in all rock types •! Differences between primary and secondary serpentine in lamprophyres and kimberlite - 2 - EOSC 321 Instructor: Kopylova TEST Alkaline rocks Thin section ______________ Your Name:__________ 45 minutes Magnification_____________ Student ID:___________ Field of view width ________ Texture: Description*: Comment on a what petrographic observations made you think the rock has an alkaline affinity: Rock name: * Should include determination of the plagioclase composition if Plag is present - 3 - EOSC 321 Instructor: Kopylova Activity II Now we will examine kimberlites, lamprophyres and carbonatites in hand specimens and under the microscope. After you finish, write two short notes to your friend who is taking an Introductory Petrology class and only a month ago has had an initial introduction to igneous rocks and the petrographic microscope. Note 1. – A Manual formatted as a table on how to distinguish primary from secondary serpentine in a thin section Secondary Serpentine Primary Serpentine Rock types where it can be found Minerals it Not applicable commonly replaces Textural position Not applicable Optical properties Factors controlling its formation in the rock - 4 - EOSC 321 Instructor: Kopylova Note 2. – A Manual formatted as a table on how to distinguish primary from secondary carbonate in a thin section Secondary Carbonate Primary carbonate Rock types where it can be found Minerals it Not applicable commonly replaces Textural position Optical properties Factors controlling its formation in the rock - 5 - EOSC 321 Instructor: Kopylova Reference collection (in two boxes) Exotic volatile-rich rocks - Kimberlites, lamprophyres, carbonatites Thin Section 98KL 20B, 98Kl- no name, KL07, 98KL-19A, 98KL18, 98KL16, 98KL-23P-2, 98KL-23P-1 Sample Number KL Rock Name: Ultramafic lamprophyre Location: West Greenland Thin Section Description: Texture: Porphyritic, with poikilitic groundmass 0-15% Phenocrysts: Olivine, subhedral, rounded 85-100% Groundmass: ~45% Olivine microphenocrysts, subhedral, rounded ~40% Carbonate in large poikilitic grains, with pearl interference colours 5% Phlogopite, in small laths, pleochroic from colourless to light-yellow. Some grains are bigger, anhedral, strongly zoned with orange rims 5% Opaque mineral, in euhedral isometric microlites and in large microphenocrysts 5% Serpentine, anhedral grains, light-yellow, with grey interference colours 1% Perovskite, in small euhedral rhombic crystals, dark brown, isotropic, with high relief. Comment: The rock is classified as lamprophyre based on major element chemistry and composition of minerals. In this case optical mineralogy alone cannot determine if the rock is kimberlite or ultramafic lamprophyre. - 6 - EOSC 321 Instructor: Kopylova Thin Section KL 056 - 3B Sample Number KL 056 Rock Name: Ultramafic lamprophyre Location: West Greenland Thin Section Description: Texture: Porphyritic, with poikilitic groundmass 5% Phenocrysts: Olivine, subhedral, rounded, 70% replaced by serpentine and a black powder of Fe oxides 95% Groundmass: 45% Olivine microphenocrysts, subhedral, rounded, 70% replaced by serpentine and a black powder of Fe oxides 24% Carbonate in large poikilitic grains, with pearl interference colours 10% Phlogopite, in long laths, pleochroic from colourless to light-yellow. Some grains are anhedral, interstitial, stongly zoned with orange rims 10% Opaque mineral, in euhedral microlites 5% Serpentine, anhedral grains, light-yellow, with grey interference colours 1% Perovskite, in small euhedral rhombic crystals, dark brown, isotropic, with high relief. Secondary Minerals:
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