advanced igneous petrology eosc 530 · advanced igneous petrology eosc 530 laboratory 1: mantle...

EOSC 530 Labs 1 Instructor: Kopylova

Arbatishzhe:Teaching:Advanced Igneous 2012:LabXen.doc

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Advanced Igneous petrology EOSC 530 Laboratory 1: Mantle Xenoliths

Introduction: Upper mantle rocks can be found in ultramafic massifs or as xenoliths in

basalts and kimberlite. You will examine the following hand specimens and thin sections

of mantle xenoliths in kimberlite:

coarse garnet and spinel peridotite;

deformed garnet peridotite (porphyroclastic and granuloblastic);

metasomatized nodules that contain phlogopite and amphibole;

eclogite;

coarse-and megacrystalline websterite.

The above assemblage is representative of the upper mantle, which contains lenses and

patches of deep metamorphic rocks (eclogites), young magmatic intrusions (websterites

and pyroxenites) in the peridotitic mantle wall-rock.

In the peridotites you would normally see the following textures: panidiomorphic

(bulk of the crystals are euhedral and of uniform size), hypidiomorphic (bulk of the

crystals are subhedral and of uniform size); and allotriomorphic (bulk of the crystals are

anhedral and of uniform size). Less common are inequigranular textures resulting from

deformation and breakdown of olivine crystals - the most plastic mineral in a peridotitic

assemblage. An example of such inequigranular deformation texture is a porphyroclastic

texture, where big strained porphyroclasts coexist with neoblasts (smaller grains free of

dislocations and deformation signatures). Porphyroclasts commonly show undulatory

extinction, may contain kink bands and subboundaries. Neoblasts exhibit both euhedral,

tabular, and equant polygonal habits. Porphyroclastic and granuloblastic peridotite often

has a moderate to strong preferred orientation of olivine and pyroxenes.

Websterite may exhibit all of the above textures. However, pyroxenite in

kimberlite mantle sections, including websterite, commonly have magmatic

allotriomorphic texture.

Eclogite is metabasite composed of garnet and omphacitic clinopyroxene (no Pl!),

with accessory quartz, kyanite, amphibolite, rutile, and sulfides. Eclogite has

metamorphic granoblastic texture with 120o triple junctions of grains.



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Reference collection - Mantle Xenoliths Thin Section Rock description M2Bult (6 T's's) Amphibole-Phlogopite Peridotite M1Bult (6 T's's) Glimmerite (predominantly phlogopite rock) Metasomatized Opx-Rut-Phl rock GBult Glimmerite (predominantly phlogopite rock) 100% Phl Mx3 LGS8 Coarse Gar lherzolite, 50% serpentenized Mx7B LGS8 Spl lherzolite with tabular granuloblastic texture. Strongly deformed, secondary texture.

Note an orientation of Spl grains and a strong preferred orientation of Ol.

Mx12 LGS9 Spl lherzolite with tabular granuloblastic texture. Strongly deformed, secondary texture.

Note a strong preferred orientation of Ol.

Mx1 LGS8 Coarse Spl-Gar lherzolite

Mx9 LGS8 Deformed Gar peridotite. Mosaic-Porphyroclastic fluidal disrupted texture. Mx2 LGS11 Deformed Gar lherzolite. Porphyroclastic fluidal non-disrupted texture. Mx5 LGS20 Deformed Carbonatized peridotite. Porphyroclastic non-fluidal non-disrupted texture.

Most Ol neoblasts are replaced by carbonate LGS26 Mx2 Deformed olivine megacryst with spectacular subgrains Per2 LGS8 Deformed Porphyroclastic peridotite. LGS9 Mx10 (7 T's's) Megacrystalline Gar websterite

Mx14 LGS8 Opx megacryst with inclusions of Cpx and Ol. LGS9 Mx1 (2 T's's) Eclogite (Gar+Cpx) with accessory Rutile

Mx12 LGS8 Eclogite (Gar+Cpx) with accessory Rutile Mx17 LGS11 Eclogite (Gar+Cpx) with accessory Rutile



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Peridotites in basalts 10 t/s

Home Problem:

You have received two xenolith thin sections for your independent examination. Prepare their full petrographic descriptions and give rock names. Use Harte's (1977) rock nomenclature to describe deformation textures of "sheared" peridotites ( attached at the back of this document).

Examples of thin section descriptions: Thin Section: 920 Sample: Rock Type: Harzburgite Location: Thin Section Description: Texture: Hypidiomorphic granular, as grains of all minerals are subhedral

~90% Olivine replaced by serpentine (10-50 vol %) with opaque microcrystalline hematite.

5% Orthopyroxene. Distinguished by relief lower than that in Ol, cleavage, low birefringence, and presence of lamellae (twinning? Cpx exsolution?) . Colourless.

3% Chromite, opaque, in euhedral rhombic grains in interstities, 1% Clinopyroxene. Distinguished by relief lower than that in Ol, cleavage,

and presence of lamellae (twinning? Opx exsolution?). Light green colour. Secondary minerals: Serpentine, colourless to yellow, with a grey, first order interference colour,

fibrous under cross polars, replaces Ol and present in veins Hematite microcrystalline, occurs in serpentine, formed by oxidation of olivines



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Sample: P2009 Rock Type: Harzburgite Location: Great Dyke, H.C.G. Thin Section Description:

Texture: Poikilitic, as the rock contains Opx oikocrysts and Ol chadacrysts. In poikilitic texture, relatively large crystals of one mineral enclose numerous smaller crystals of other minerals. Host crystals is known as an oikocryst, and the enclosed crystals are chadacrysts.

50-90% Olivine, euhedral, of cumulate origin (?). 80% of Ol is replaced by serpentine with fine-grained opaque hematite.

0-50% Orthopyroxene. One big grain in a fresh part of the rock. 1% Phlogopite. Pleochroic from colourless to brown, interstitial grains. <1% Chromite or Magnetite, black opaque, in euhedral interstitial grains Secondary minerals:

Serpentine, hematite, talc Note: Secondary alteration and mineral abundances of the rock vary from one part of the thin section to another. The part with fresh Opx contains 50%Ol and 50% Opx. In the altered part, Opx (10%) is entirely replaced by an intergrowth of serpentine (outer part of grains) and talc (inner part of former Opx grains).

Thin Sections: 979, 1456 Sample: P1339 Rock Type: Clinopyroxenite Location: N of Keremeos (Ollala) Thin Section Description:

Texture: Hypidiomorphic, all minerals are subhedral ~90% Clinopyroxene, 2V=56 to 60o, pleochroic from light yellow to grey,

subhedral, aften twinned. 5% Orthopyroxenite 5% Black opaque mineral (magnetite?) with habits from euhedral to anhedral 1% Biotite, pleochroic from khaki to dark brown, associated with magnetite A few alteration patches, possibly magnetite, talc, carbonates.



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Thin Section: 1859 Sample: Rock Type: Deformed dunite Location: Thin Section Description:

Texture: Porphyroclastic where larger deformed grains (porphyroclasts) coexist with new grains free of dislocations (neoblasts). In this sample, deformation is uneven and concentrates in lenses and veins comprised by 60-70% of fine-grained neoblasts. The rock outside of these veins and lenses is practically neoblasts-free. ~98% Olivine, present as porphyroclasts and neoblasts. Porphyroclasts show

undulose extinction and subgrains (domains with slightly different optical orientation separated by subboundaries).

1% Orthopyroxene. Distinguished by a low birefringence and the presence of cleavage

3% Chromite, brown, in euhedral rhombic and anhedral grains

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