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Laboratory 3:
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 mafic volcanic rocks, basalts. Basalt is the most common terrestrial volcanic rock. Volcanic rocks classified in thin sections as basalts can have substantial variations in chemical compositions. Chemical analysis is absolutely necessary to distinguish, for example, between trachybasalt and basanite. Both of these chemical sub-types of basalt comprise of plagioclase, augite, olivine and Fe-Ti oxide. Note that in the absence of chemical data we call all rocks in our reference thin sections "basalts". If basalt has olivine in the groundmass (not as phenocryst!), it should be called Olivine basalt. Olivine basalt is Si-undersaturated, alkaline in character.
From textures and experiments on natural samples, the common crystallization sequence for basalts is : olivine (± Mg-Spl) => Ol +Plag (± Mg-Spl) =>Ol+Plag+Cpx. Thus, augite clinopyroxene is rare as a phenocrystal phase, but comprises up to 50% of the groundmass in basalts.
An important distinction between silica-saturated basalts and basalts of alkaline series can be made through petrographic observations. Alkali olivine basalt has olivine in the groundmass, but rarely has glass in the groundmass. Olivine phenocrysts tell us nothing about alkalinity.
Picrite is an olivine-rich basalt, and rocks with more than 20% of olivine could possibly be termed picrites or picro-basalts. However, a correct classification of a rock as picrite cannot be done petrographically and should be confirmed chemically. Ankaramite is an alkaline analog to picrite, with lots of Ol and Cpx phenocrysts. An ultramafic character of the ankaramite thin section in the reference Hawaiian collection is suggested by 20% of Ol phenocrysts. However, the alkaline affinity of the rock is not evident in the thin section and can be proved only chemically.
Note that clinopyroxene-plagioclase fine-grained rocks are called basalts if the plagioclase is labradorite-anorthite, or andesites if the plagioclase is andesine.
Make sure that you see the following important rock characteristics in the Reference Collection:
1. Mineralogical differences between an alkali basalt and a silica-saturated basalt
2. Mineralogical differences between a basalt and more ultrabasic ankaramite
3. Mineralogical differences between a basalt and more felsic andesite
4. Crystal habits of crystobalite common in volcanic rocks
5. Trachytic, intergranular and intersertal textures
6. Characteristic alteration of olivine phenocrysts to iddingsite
7. Characteristic alteration of clinopyroxene to uralite
8. Characteristic alteration of plagioclase to saussurite
In the third hour you will receive a thin section of an unknown mafic rock for your independent examination. Write its petrographic description and give a rock name. Determination of the plagioclase composition is a necessary part of the petrographic description. Your petrographic report should be completed and handed to the TA by the end of the Lab.
Thin Section: 953 1
Sample:P 469
Rock Type: Basalt
Location: Stanford, California
Thin Section Description:
Texture: Porphyritic. The groundmass has domains of intersertal texture (glass occupies the wedge-shaped interstices between Plag laths), intergranular texture (the spaces between Plag laths are occupied by one or more grains of Px) and ophitic texture.
11% Phenocrysts of Alk Feldpar (7%) and Chloritized clinopyroxene (4%).
The Feldspar phenocrystas are corroded and embayed, have a speckled center with a thin rim of clear feldspar. The speckled core contains numerous inclusions of glass – the Fsp being a skeletal crystal.
89% Groundmass:
36% Plagioclase. Euhedral to subhedral shapes, An68
26% Clinopyroxene. Subhedral to anhedral shapes, forms small crystals and larger poikilocrystals enclosing Plag. Partly replaced by yellow Chl.
27% Glass, black, N<Nbalsam
Secondary Minerals: Chlorite after Cpx, yellow, has crude radiate to aggregate structure.
Thin Section: 974 2
Sample: P 64
Rock Type: Basalt
Location:
Thin Section Description:
Texture: Aphanitic with intergranular groundmass.
3% Phenocrysts of zoned Plag with glass inclusions. The amount of Plag phenocrysts is lower than the cut-off for the porphyritic texture (5%).
97% Groundmass:
65% Plagioclase. Euhedral to subhedral shapes, An60
30% Clinopyroxene. Euhedral rhombic to anhedral shapes, partly altered to green Chl.
5% magnetite, euhedral, with red thin rims of secondary heamatite.
Secondary Minerals: Chlorite after Cpx, green
Haematite after magnetite
Sample: P1243
Thin Section: 1474
Rock Type: Basalt
Location: Cone Sheet, Near Mingary Castle
Sample: P384
Thin Section: 1042
Rock Type: Basalt. 35% Ol in the rock suggests that this rock could be picrite, but it should be confirmed chemically.
Location: Kilauea
Thin Section Description:
Texture: Vesicular (about 30% pore space) and porphyritic.
Phenocrysts:
35% Large euhedral olivines, fractured, up to 7mm. Note their extinction parallel to elongation that differentiates them from Cpx.
Groundmass: 65% (neglecting vesicles)
30% Clinopyroxene, anhedral to euhedral
29% Plagioclase, euhedral to anhedral. An53, zoned.
6% Anhedral elongate opaque mineral (ilmenite?).
Sample:P 2722
Thin Section: 1114
Rock Type: Basalt with Crystobalite
Location:
Sample: P 2613-2612
Thin Section: 732, 728
Rock Type: Vesicular Olivine Basalt (alkaline)
Location: Quilchena Creek
Thin Section Description:
Texture: Vesicular (30-50% vesicles), aphanitic with sub-ophitic groundmass
Groundmass:
5-10% Ol, euhedral. Larger crystals have reddish rims of iddingsite formed by oxidation in the process of extrusion and final quenching.
25-40% Plagioclase, euhedral, An66.
18-28% Cpx, anhedral
2% Glass, brown, interstitial
1% Opaque mineral in rod-like grains (ilmenite?)
Secondary Minerals: Iddingsite aggregate. i.e. fine-grained reddish- or yellowish-brown material that consists of goetite, clay, chlorite, quartz, talc, and other minerals. It is a characteristic alteration of olivine phenocrysts in response to higher oxygen fugacity and lower volatile content in quenched lavas.
Thin Section: 871 12
Sample: P 1737
Rock Type: Basalt
Location: Porcupine Mt., N.W. of Big Bar
Thin Section: 982 13
Sample: P99
Rock Type: Basalt
Location: Boulder, Colorado.
8% Clinopyroxene. Euhedral, often twinned, zoned, with numerous glass inclusions.
1% Olivine, in smaller grains, euhedral or rounded subhedral.
1% Plagioclase, euhedral, zoned
92% Groundmass:
8% Cpx, subhedral, round grains partly altered to green Chl
~80% Plagioclase, euhedral, An60. Most of Plagioclase is replaced by fibrous mineral with high releief – fibrous epidote (?) in saussurite.
3% Opaque mineral, euhedral, rhombic (magnetite?)
Secondary Minerals: Green Chlorite
Saussurite after Plag.
Saussurite is a fine-grained aggregate of epidote group minerals, albite, sericite and other minerals. Plagioclase is commonly altered either to sericite or saussurite.
Comment: Note that matrix, excluding the larger augites, is at least 80% feldspar – this is not in accord with the composition of a basaltic eutectic.
Thin Section: 924 15
Sample: P 619
Rock Type: Altered andesite
Location: Boulder, Montana
Sample: P 57, P 1852
Thin Section: 973, 597
Rock Type: Vesicular Olivine Basalt (alkaline)
Location: Snake River, Ropylava, Devil’s Ranch
Thin Section Description:
Texture: Vesicular (30% vesicles), hypohyaline (50-90% of rock is glass), microporphyritic
Phenocrysts:
1-4% euhedral olivines ½-1mm, rhombic or skeletal, often with iddingsite yellow rims (best to see in T/s 597)
Groundmass:
3% euhedral olivines, sometime skeletal
10% Plagioclase An50 in T/s 973, An 56-67 in T/s 597
0-3% Brown glass, n>balsam.
0-1% Cpx, subhedral
80% Black opaque glass
Secondary Minerals: Iddingsite after Olivine in T/s 597
Sample: P 999
Thin Section: 659
Rock Type: Vesicular Basalt
Location: Near Risk Creek, Chilcotin
Sample:P515
Thin Section: 1257
Rock Type: Basalt
Location: Jamestown, California
Thin Section Description:
Texture: Porphyritic with intersertal groundmass
Phenocrysts: 25%
20% Plagioclase, with numerous glass inclusions, euhedral, An50
4% Olivine, 2V large and negative. Probably 40%+ Fayalite.
1% Clinopyroxene, (+)2V~60o. Brownish
Few grains of Orthopyroxene. Low birefringence, surrounded by clinopyroxene. 2V near 90o.
75% groundmass:
40% brown glass. Glass filled with scattered speckled globulites
30% Plagioclase, euhedral laths
5% Ol + clinopyroxene
1% Opaque mineral in triangular grains and in rods
Secondary Minerals: Talc (?) in fractures in Px and Ol
Hawaiian Basalts
Thin section: Mu 74-1(A)
Sample: MU 72 9A, 935B 4A, MU 74
Rock Type: Basalt
Location: Mauna Ulu vent, Kilauea Volcano, Hawaii (1974 flow)
Thin section description:
Texture: Vesicular, with 35-40% vesicles--subspherical with minor coalescence and deformation,
vitrophyric, intersertal
Phenocrysts:
10% olivine , mostly subhedral (few skeletal) no zoning or kink banding present. Some have spinel inclusions.
1-2% Plagioclase, sub-anhedral lath shaped plagioclase microphenocrysts
Groundmass: 48%:
50% glass, some with spherulite intergrowth (radiating fibrous crystals) sometimes growing around glomerocrysts of olivine and plagioclase microlites
Thin section: 1942A1, 1935B-2B, 1935A1(2), 1935A2-2A
(note some have blue epoxy)
Sample:
Rock Type: Basalt
Location: Mauna Loa, Hawaii
Thin section description:
Texture: microphyric with some intersertal glass,
Vesicular (5-30% vesicles depending on thin section)
Microphenocrysts:
20% plagioclase ; subhedral, lath-shaped (with trachytic texture)
5-10% clinopyroxene ; anhedral with inclined extinction (often elongate)
1-2% olivine phenocrysts;
groundmass:
plagioclase, clinopyroxene, and opaques with devitrified glass.
Thin section: A(4), HW-2(1)
Sample:
Rock Type: Basalt. Chemical composition classifies this rock as ankaramite
Location: Hawaii (Haleakala?)
Thin section description:
Texture: porphyritic
20% olivine; large anhedral phenocrysts some showing zoning
5-10% clinopyroxene phenocrysts; anhedral and more greenish in plane light than olivine, show inclined extinction
10% microphenocrysts of resorbed plagioclase
Groundmass:
devitrified glass, opaque mineral (ilmenite, magnetite), plagioclase, and clinopyroxene
Note: glomerocrysts of cpx, opaques, plag +/- olivine indicate sequence of cystallization (first minerals to crystallize are always internal to the later ones, if first minerals are not skeletal)
Thin section: P3651-1970 (4)
Sample: 3651
Rock Type: Basalt
Location: Mauna Loa, Hawaii
Thin section description:
Texture: porphyritic,
10% vesicles; irregular with some showing flow deformation
<5% olivine phenocrysts;
5% orthopyroxene glomerocrysts and phenocrysts;ahedral with low birefringence (1st order grays and yellows) parallel extinction and show inclined extinction
Groundmass: devitrified glass, opaque mineral (ilmenite, magnetite), and plagioclase
Thin section: HW3, HW11(3), HW12(2)
Sample: HW3
Rock Type: Alkali Olivine basalt
Location: Mauna Kea? Hawaii
Thin section description:
Texture: aphanitic, trachytic, vesicular
Vesicles elongated in flow direction
plagioclase, olivine, clinopyroxene, and opaques comprise a groundmass with flow orientation.
Note: The presence of olivine in the groundmass is significant in Hawaiian rocks and designates the difference between tholeiitic basalt and alkali olivine basalt. Alkali olivine basalts also rarely have glass in the groundmass.