Igneous petrology EOSC 321

Laboratory 8:

Exotic volatile-rich rocks -

Kimberlites, lamprophyres, carbonatites

Duration: This lab is to be completed within the lab period. The results will be evaluated and returned to you in the following lab period.

Material Needed: a) Microscope, b) a Manual on Optical Mineralogy (i.e. Minerals in Thin Section by Perkins and Henke)

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, Kimberlites are divided into two groups, or types, based on their chemistry and mineralogy. Group I kimberlite is the most abundant, it lacks phlogopite in fine-grained groundmass. Group II kimberlite is called micaceous since it has phlogopite in groundmass. Because kimberlites are hybrid rocks, 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 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 as 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, a kimberlite contains numerous smaller (<0.5 mm) euhedral olivines that are usually termed microphenocrysts. All smaller olivine crystallizes from the host magma.

Lamprophyres are defined by the IUGS 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 using a Nomenclature Table (enclosed). Lamprophyres encompass a wide range of compositions, from ultramafic to silisic, but they all feature the high H₂O content and the resulting 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 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, the IUGS recommends the use of the 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. The zeolites (T/s 1229) are a group of minerals, hydrated aluminolsilicates, 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.

Make sure that you see the following important rock characteristics in the Reference Collection:

1.      Mineralogical differences between Type I and Type II kimberlites

2.      Mineralogical differences between kimberlites (no Plag) and mafic lamprophyres (Plag in the groundmass)

3.      Primary carbonate in all rock types

4.      Differences between primary  and secondary serpentine in lamprophyres and kimberlite

In the third hour you will receive a thin section of an unknown kimberlite/lamprophyre/carbonatite for your independent examination. Write its petrographic description and give a rock name. Your petrographic report should be completed and handed to the TA by the end of the Lab.       

Reference collection

Exotic volatile-rich rocks -

Kimberlites, lamprophyres, carbonatites

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, anisoropic. 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. Montichellite is not recognizable under an optical microscope and was determined ander a Scanning Electron Microscope (SEM).

15%     Opaque minerals (most common spinel± perovskite±magnetite±ilmenite) in euhedral microlitic grains

2%       Carbonate, interstitial

5%       Chlorite, in radiating fibres

Megacrysts:      Olivine

±brown minerals of Fe oxides and hydroxides, mainly on rims and in fractures;

± perovskite±magnetite±ilmenite) in euhedral microlitic grains

± perovskite±magnetite±ilmenite) in euhedral microlitic grains

Thin Section Description:

Texture:          Hipidiomorphic

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.

Thin Section   KL 056 - 3B

Sample Number         KL 056

Rock Name:               Ultramafic lamprophyre

Location:  West Greenland