EOSC 321 Labs Page 2 Instructor: Kopylova
Igneous petrology EOSC 321
Laboratory 9:
Exotic volatile-rich rocks -
Kimberlites, lamprophyres, carbonatites
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 1h test on alkaline rocks. Descibe a thin section and write a short explanation what petrographic observations made you think the rock has an alkaline affinity. In the next two hours of the lab period you will examine 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.
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 kimbelites are primary, having crystallized in fine intergrown crystals on late stages of groundmass crystallization.
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 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 cabonatites, reflecting the diverse and exotic carbonatite chemistry. We have some of those exotic minerals in the Reference Thin Sections: pyrochlore (T/s 58) and monticellite (T/s 262). 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 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) and the absence of serpentinization. Monticellite is a common mineral in the kimberlite groundmass, where it forms tiny fresh crystals atypical of olivine (T/s BAK39). 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 very 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:
1. Mineralogical differences between kimberlites (no Plag) and mafic lamprophyres (Plag in the groundmass)
2. Primary carbonate in all rock types
3. Differences between primary and secondary serpentine in lamprophyres and kimberlite
Reference collection (in two boxes)
Exotic volatile-rich rocks -
Kimberlites, lamprophyres, carbonatites
Thin Section 98KL 20B, 98Kl- no name, KL07, 98KL-19A, 98KL18, 98KL16, 98KL23P-3, 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.
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
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.
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.
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
Thin Section 98KL076, 98KL13-2, 98KL13-1, 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.
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.
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
Thin Section JD30 150’5”, JD35 (11+ 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.
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
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.