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.

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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

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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

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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

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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

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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.

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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: 30% Seprentine + Opaque Fe oxides after olivine

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Thin Section 1228 Sample Number P757 Rock Name: Augite Spessartite - a subtype of lamprophyre Location: ?

Thin Section Description: Texture: Porphyritic, with poikilitic groundmass

7% Phenocrysts: Hornblende, up to 2 cm long, euhedral, pleochroic from yellow-green to green, Often with inclusions of Cpx, and partly replaced by low relief chlorite.

93% Groundmass, relatively coarse-grained (grains up to 0.3 mm): 60% Augite, euhedral, with good cleavage, grey-greenish 5% Hornblende, euhedral, pleochroic from yellow-green to green, with perfect cleavage at 120o, and partly replaced by a low relief fibrous mineral 28% Plagioclase, in interstitial anhedral grains, poikilitic, encloses Aug. Polysynthetically twinned. Partly replaced by sericite and a sericite-bearing aggregate. Sphene - yellow- grey grain with high relief

Secondary Minerals: 5% - An aggregate of secondary minerals with high relief and first order interference colours. The aggregate may contain fine-grained sphene (?) or epidote (?) that give it a high relief, and sericite that accounts for the birefringence. The aggregate replaces plagioclase and amphibole.

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Thin Section 1229 Sample Number P758 Rock Name: Olivine Spessartite - a subtype of lamprophyre Location: near Boulder Dam, Arizona.

Thin Section Description: Texture: Porphyritic, amygdaloidal

5% Phenocrysts: Hornblende, reaching 3 cm in length, euhedral, pleochroic from yellow to light brown. Some rims show recrystallization into microlites of Bi. One grain of plagioclase with a faint polysynthetic twinning

10% Vesicles of irregular shapes and veins. Infilled with secondary coarse-grained carbonate and zeolite.

85% Groundmass: 10% Olivine, in euhedral microphenocrysts, almost totally replaced by reddish-brown iddingsite. Only in larger crystals grain cores remain unaltered. 10% Opaque mineral in tiny euhedral crystals 5% Carbonate, forms occasional segregations (?) devoid of all other groundmass minerals 60% Plagioclase, in needle-like long crystals or skeletal crystals with simple Albite twinning. Calcic andesine. Occasional grains of euhedral apatite, colourless garnet and anhedral quartz. The latter two may be xenocrysts.

Secondary Minerals: 7% - Carbonate in amygdales and veins 3% Zeolite, with very low birefringence, dark-grey, almost isotropic, with polysynthetic twinning and a low negative relief. 10% Iddingsite on olivine rims

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Thin Section 98KL076, 98KL13-2, 98Kl-12B, 98KL-21C, KL05-02A515A, 98KL- 19B, KL05-5B2 Sample Number KL Rock Name: Ultramafic lamprophyre Location: West Greenland

Thin Section Description: Texture: Porphyritic

0-10% Phenocrysts: Olivine, subhedral, rounded

90-100% Groundmass: ~40% Olivine microphenocrysts, subhedral, rounded ~20% Phlogopite is present in large interstitial grains and in smaller euhedral zoned crystals with orange rims. Pleochroic from colourless to light-yellow. ~15% Carbonate in large poikilitic grains, with pearl interference colours ~10% Serpentine, forms fine-grained aggregates of light-green colour. 3% Perovskite, in small euhedral rhombic crystals, dark brown, isotropic, with high relief. 3% Opaque mineral, in euhedral isometric microlites, concentrate around Ol microphenocrysts 1% Opaque mineral in long rods, most likely Ilmenite.

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Thin Section BAK39 270m Sample Number BAK39 270m (4 thin sections) Rock Name: Phlogopite kimberlite Location: Kennady Lake kimberlite, NWT

Thin Section Description: Texture: Macrocrystal

15% Macrocrysts: Olivine, subhedral, rounded, partly (5%) replaced by serpentine

85% Groundmass: 10% Microphenocrysts of olivine in euhedral grains 35% Phlogopite, in interstitial anhedral grains poikilitically enclosing opaques. Pleochroic from light yellow to orange. Zoned. 20% Fine-grained aggregate of groundmass serpentine, bluish-green, anisotropic. The aggregate is present in interstities between an opaque mineral, phlogopite and microphenocrysts. 15% Monticellite, in euhedral high-relief grains unevenly distributed throughout the groundmass. Light green to colourless under 1 polar, interference colours up to 2nd order yellow. Readily visible on thin section margins. 15% Opaque minerals (most common spinel± perovskite±magnetite±ilmenite) in euhedral microlitic grains 2% Carbonate, interstitial 5% Chlorite, in radiating fibres

Megacrysts: Olivine

Secondary Minerals: Chlorite

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Thin Section JD35 132’ JD30 150’5” (8 thin sections) Sample Number Rock Name: Serpentine carbonate kimberlite Location: Jericho pipe, NWT

Thin Section Description: Texture: Macrocrystal, with segregational groundmass

20% Macrocrysts: Olivine, subhedral, rounded, 10-100% replaced by seprentine±brown minerals of Fe oxides and hydroxides, mainly on rims and in fractures; Garnet with kelyphitic rims

80% Groundmass: 37% Microphenocrysts of olivine and opaque mineral (most likely spinel) in euhedral grains 25% Fine-grained aggregate of groundmass serpentine and carbonate. Serpentine is light-green, fine-grained and almost isotropic. Carbonate is not recognizable under an optical microscope and was determined under a Scanning Electron Microscope (SEM). The aggregate is present in interstities between an opaque mineral and microphenocrysts. 15% Opaque minerals (most common spinel± perovskite±magnetite±ilmenite) in euhedral microlitic grains 3% Segregations of coarse-grained serpentine and carbonate

Megacrysts: Olivine, Clinopyroxene with exsolution lamellae of Opx. Cpx is rimmed by a brown reaction rim

Xenoliths: Country Rocks and mantle peridotites

Secondary Minerals: Serpentine after olivine, light green, isotropic or anisotropic in fine-grained aggregates

Note: There are 2 variety of serpentine in this rock: primary and secondary. Primary is present in the groundmass, the secondary replaces primary olivine. Their optical properties are similar.

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Thin Section SI-1 Sample Number SI-1 Rock Name: Carbonate kimberlite Location: Somerset Island, NWT

Thin Section Description: Texture: Macrocrystal

7% Macrocrysts: 5% Olivine, subhedral, rounded. 20-50% replaced by a black opaque mineral 2% Phlogopite, subhedral, rounded. Pleochroic from colourless to light brown. Often zoned, with darker brown rims, rarely recrystallized into fine-grained opaque-rich aggregate on rims.

93% Groundmass: 10% Microphenocrysts of olivine, phlogopite and opaque mineral (most likely spinel) in euhedral grains 30% Carbonate, in elongate euhedral rectangular laths, with high pearl interference colour. 30% Fine-grained aggregate of groundmass carbonate and serpentine (?). Carbonate has high pearl interference colour. The aggregate is present in interstities between opaque mineral, carbonate laths and microphenocrysts. 23% Opaque minerals (most common spinel± perovskite±magnetite±ilmenite) in euhedral microlitic grains

Xenoliths: 3% Country Rocks

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Thin Section 989 (Caution! Thicker than normal thin section! ) Sample Number P121 Rock Name: Augite Kersantite - a subtype of lamprophyre Location: Franklin Furnace

Thin Section Description: Texture: Porphyritic

25% Phenocrysts: 20% Augite, euhedral, tan colour. Shows strong dispersion, i.e. does not extinct entirely, but instead becomes dark-grey-blue. 5% Phlogopite, in euhedral long laths, pleochroic from colourless to dark brown.

75% Groundmass: 20% Plagioclase, low (+) relief, in interstitial anhedral grains. Forms wavy radiate aggregates. Probably largely altered - some grains of secondary calcite and quartz seen. 20% Augite, euhedral, tan colour. Shows strong dispersion, i.e. does not extinct entirely, but instead becomes dark -grey-blue. 10% Phlogopite, in euhedral long laths, pleochroic from colourless to dark brown. 10% Grey fine-grained mineral, most likely secondary epidote replacing laths of mica and Aug grains 5% Opaque mineral, euhedral to anhedral, often rimmed by secondary epidote. Apatite

Secondary Minerals: Epidote abundant in the groundmass, around opaque mineral and replacing mica and/or Aug. Epidote is slightly pleochroic from colourless to yellow, with the characteristic very bright interference colours of the 2nd order.

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Thin Section 262 Caution! One part of the thin section is thicker than normal! P2606 has the same texture and the same minerals but with different modes Sample Number P2605 Rock Name: Carbonate-Monticellite rock Location: ?

Thin Section Description: Texture: Hypidiomorphic

55% Monticellite, in large subhedral grains of rectangular shapes, birefringence up to second order blue, biaxial negative, high relief, extinction is parallel to elongation of the crystals. Optically resembles olivine. Shows brown alteration along fractures. 13% Opaque mineral (Magnetite?) in large round anhedral grains 13% Carbonate in large subhedral grains with pearl interference colours and 2 sets of perfect cleavage at 60o 8% Phlogopite, in euhedral crystals pleochroic from colourless to light-yellow. Often is associated with opaques. 7% Apatite, euhedral, in long crystals with a low birefringence and a parallel extinction, or in hexagonal and isometric cross-sections that look almost isotropic. Often found in radiate aggregates. 3% Perovskite in large dark-brown isotropic grains with high relief

Thin Section 837 Sample Number P1717 Rock Name: Carbonatite Location: Ring Dyke, Southern Zimbabwe

Thin Section Description: Texture: Hypidiomorphic. Rock shows veins of brecciation and mylonitization.

85% Carbonate in large subhedral grains with pearl interference colours and 2 sets of perfct cleavage at 60o 15% Apatite, euhedral, in long crystals with a low birefringence and a parallel extinction, or in hexagonal and isometric cross-sections that look almost isotropic. Forms veins, the orientation of larger crystals parallel to veins marks crude lineation of the rock. Distributed unevenly, in some zones apatite comprises up to 80%. 1% Phlogopite, in euhedral crystals pleochroic from colourless to light-yellow. Zoned. 5% Opaque mineral (Magnetite?) in large round anhedral grains 1% Quartz, in large poikilitic grains enclosing apatite in apatite-rich zones.

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Thin Section SWE/ALN/11 Sample Number Rock Name: Melilitite Locality: Alno, Sweden

Microscopic description: The rock has porphyritic texture, with ~10% coarse phenocrysts set in a medium-grained groundmass. The phenocrysts include: 3% Euhedral phlogopite, pleochroic from light yellow to brown, some grains are deformed; 3% Euhedral amphibole, 99% replaced by bright yellow serpentine with 2nd order interference colours; 1% clinopyroxene grains surrounded by a melilite-rich reaction rim, Occasional rare grains of colourless resorbed amphibole, low grey interference colours, perfect cleavage at 60o, high relief,

The phenocrysts are set in a a groundmass composed of laths of melilite completely replaced by brown cebollite (30%), euhedral and poikilitic, sometimes deformed phlogopite (20%), euhedral opaque mineral (most likely spinel, 20%), euhedral apatite (20%), olivine replaced by serpentine (10%). Melilite is distinguished by 1) its characteristic "pegs" separating simple twins along central lines parallel to the lath elongation; 2) its characteristic brown alteration product with high birefringence (cebollite).

Note: Two important things can be observed in this thin section: 1). A typical morphology and intense alteration of melilite in volcanic rocks; 2) Strange colour (or rather the absence of colour) of alkaline phenocrystal amphibole. This phenomenon is typical of alkaline rocks where familiar minerals acquire unfamiliar habits and colours.

Thin Section P2958 Sample Number Rock Name: Carbonatite Location: Fen Complex Thin Section Description: Texture: Hypidiomorphic

89% Carbonate in large subhedral grains with pearl interference colours and 2 sets of perfect cleavage at 60o 10% Apatite, euhedral, in long crystals with a low birefringence and a parallel extinction, or in hexagonal and isometric cross-sections that look almost isotropic. Forms veins; the orientation of larger crystals parallel to veins marks crude lineation of the rock. 1% Phlogopite, in euhedral crystals pleochroic from colourless to light-yellow.

One grain of Pyrochlore CaTa2O6(OH)2, dark brown anisotropic mineral, partly altered to black secondary mineral.

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