Basalt

A basalt is a fine-grained basic igneous rock containing essential calcic plagioclase feldspar and pyroxene (usually Augite), with or without olivine. Basalts can also contain quartz, hornblende, biotite, hypersthene (an orthopyroxene) and feldspathoids. Basalts are often porphyritic and can contain mantle xenoliths. As basic rocks they contain between 45-50% silica, abundant Fe, Mg and Ca, and little Na and K. The coarse and medium-grained equivalents of basalt are gabbro and dolerite respectively. Basalt is distinguished from pyroxene andesite by its more calcic plagioclase.
There are two main chemical subtypes of basalt: tholeiites which are silica saturated to oversaturated and alkali basalts that are silica undersaturated. Tholeiites dominate the upper layers of oceanic crust and oceanic islands, alkali basalts are common on oceanic islands and in continental magmatism. Basalts can occur as both shallow hypabyssal intrusions or as lava flows. Picrites are basalts containing abundant olivine. Basalts with alkali feldspar in addition to plagioclase are known as the trachybasalts.

Basalts are erupted in a wide variety of tectonic environments on Earth (e.g. mid–ocean ridges, island arcs, back-arc basins, intra -plate oceanic islands, large igneous provinces and intra - continental rifts), and collectively they are found on the Earth’s surface in greater volume than any other volcanic rock type. Basalts also occur on other terrestrial planets and the Moon and constitute an important class of meteorites (basaltic achondrites).

Terrestrial basalt magmas are the products of melting in the Earth’s mantle, and therefore their geochemistry, and the inclusions they sometimes contain, can tell us a great deal about the composition and mineralogy of the upper mantle and – many petrologists would argue–may provide information about the composition of the lower mantle too. Basalt magmas are parental to most of the more evolved magma types involved in continental and oceanic igneous activity, that develop through fractional crystallization in the crust. In view of their parental role, and also because of their abundance and relatively simple mineralogy, basalts provide the most logical starting point for a systematic study of igneous rocks. Many basalts are too fine - grained to permit confident microscopic identification of every mineral present, and may indeed have a glassy matrix from which one or more latent minerals have failed to crystallize.

Basal classification in the Yoder & Tilley tetrahedron

On a chemical basis, basalts can be classified into three broad groups based on the degree of silica saturation. On this basis, most basalts consist predominantly of the normative minerals - Olivine, Clinopyroxene, Plagioclase, and Quartz or Nepheline. These minerals are in the 4 component normative system Ol-Ne-Cpx-Qtz, (Yoder & Tilley tetrahedron). In the tetrahedron, plagioclase plots between Ne and Qtz, and Opx plots between Ol and Qtz. The basalt tetrahedron can be divided into three compositional volumes, separated by planes:

- The plane Cpx-Plag-Opx is the critical plane of silica saturation. Compositions that contain Qtz in their norms plot in the volume Cpx-Plag- Opx-Qtz, and would be considered silica oversaturated. Basalts that plot in this volume are called Quartz Tholeiites.

- The plane Ol - Plag - Cpx is the critical plane of silica undersaturation. Normative compositions in the volume between the critical planes of silica undersaturation and silica saturation are silica saturated compositions (the volume Ol - Plag - Cpx - Opx). Silica saturated basalts are called Olivine Tholeiites.

- Normative compositions that contain no Qtz or Opx, but contain Ne are silica undersaturated (the volume Ne-Plag-Cpx-Ol). Alkali Basalts, Basanites, Nephelinites, and other silica undersaturated compositions lie in the silica undersaturated volume.

The critical plane of silica undersaturation appears to be a thermal divide at low pressure. This means that compositions on either side of the plane cannot produce liquids on the other side of the plane by crystal fractionation. To see this, look at the front two faces of the basalt tetrahedron. These are in the three component systems Ol-Cpx-Qtz and Ol-Cpx-Ne. The critical plane of silica undersaturation is a thermal divide at low pressures (less thanabout 10 kb) and is not a thermal divide at higher pressures.

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The basalt tetrahedron (after Yoder and Tilley, 1962)



Mid Ocean Ridge Basalts (MORBs)
N-MORBs derive from an (upper), depleted mantle.
E-MORBs (Enriched) derive from a (deeper) enriched source.
T-MORBs (Transitional) derive by mixing of N- and E- magmas during ascent and/or in shallow chambers or mixing of sources before melting.

The Oceanic Ridges are probably the largest producers of magma on Earth. Magma is both erupted and intruded near the central depressions that form the oceanic ridges. Thus, both basalts and gabbros are produced. The main melting mechanism is likely decompression melting as rising convection cells move upward through the mantle beneath the ridges. At most oceanic ridges the basalts that are erupted are tholeiitic basalts sometimes referred to as NMORBs (normal MORBs). Mid-ocean ridge basalts (MORB) have long been classified into at least two different geochemical types, so-called "N-MORB" (normal) and "E-MORB" (enriched MORB). in the early literature the enriched MORB were usually called "plume-ridge" or "P-MORB".

In modern literature "normal" actually refers to the very common, incompatible-element depleted variety, whereas "enriched" refers to the much less frequent, but nevertheless not uncommon, incompatible element enriched basalts. Many E-MORB (or "P-MORB") are clearly associated with obvious hot spots or plumes such as Iceland, Others have been interpreted to be formed by channels or offshoots of plume material into the asthenosphere.
But some E-MORB, and this includes many small seamounts, cannot be attributed to input from any obvious plumes. The cause of enrichment can be classified roughly into two categories, recycled oceanic crust (with or without minor amounts of continental input) and recycled subcrustal, usually also oceanic, lithosphere that has been enriched "metasomatically", that is, by infiltration of low-degree melts at some point during its journey from the ridge through the subduction zone.

OIB
Ocean island basalts or OIBs are basaltic rocks found on many volcanic islands away from tectonic plate boundaries typically associated with hot spots. Islands hosting ocean island basalts always lie above the oceanic crust and are not limited to volcanic islands but occur also on volcanoes of any size under the sea. The chemical compostition of these basalts can vary from tholeiite to alkali basalt within the same island group, but is never calc-alkaline. During the shield volcano stage of many hotspot islands, tholeiitic OIBs build up most of the volcano's structure. Following the post-erosional stage this is usually accompanied by violent eruptions of alkali basalt and other more evolved volcanic rocks with high alkali content.

Orogenic Basalts
These basalts typically occur near convergent plate boundaries on island arcs and seismically active continental margins. At present, the volume of magmatic rocks produced in this tectonic environment is huge, it is second only to the material produced on the Mid-ocean ridges. There is also a great diversity in the type of magmatic rocks that evolve in this tectonic environment. The orogenic basalts belong to:

a) Boninite association
b) Island-arc basalt association (IAB)
c) Calc-alkaline association
d) High-K association
e) Shoshonitic association

Subduction is unifying tectonic feature that is characteristic of magmas generated along convergent plate boundaries. during early stages in the evolution of an Island-arc, most of the extrusive rocks are likely to belong to the Boninite association and/or the Tholeiite association, but during the main phase in the evolution of active continental margins, andesites and other cal-alkaline rocks are usually extruded.

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A proposed model for subduction zone magmatism with particular reference to island arcs. Dehydration of slab crust causes hydration of the mantle (violet), which undergoes partial melting as amphibole (A) and phlogopite (B) dehydrate. From Tatsumi (1989).



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QAPF Diagram Basalt field in Blue



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Aldeyjarfoss basalt columns, Iceland





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Columnar Basalt, Sheepeater Cliffs, Yellowstone National Park. From Natalie Teager, Arizona State University.

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Olivine crystals (green) in a vesiculated Basalt





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Olivine crystals in a Basalt from Peridot Mesa, Arizona. From Natalie Teager, Arizona State University.

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Fresh basalt from Hawaii. From Washington University



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Basalt specimen from the Pu,o'o' eruption on the Big Island of Hawaii. From Natalie Teager, Arizona State University.


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Dunitic Xenoliths in a basalt, San Carlos Indian Reservation, Arizona. From Natalie Teager, Arizona State University.


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Sandstone Xenolith in Basalt, Coconino County, Arizona. From Natalie Teager, Arizona State University.



Bibliography



• David Shelley (1983): Igneous and metamorphic rocks under the microscope. Campman & Hall editori.
• Vernon, R. H. & Clarke, G. L. (2008): Principles of Metamorphic Petrology. Cambridge University Press
• Shelley D (1992): Igneous and Metamorphic Rocks under the Microscope: Classification, textures, microstructures and mineral preferred orientation
• Cox et al. (1979): The Interpretation of Igneous Rocks, George Allen and Unwin, London.
• Eric A.K. (1985): Middlemost Magmas and Magmatic Rocks. Longman, London
• Carmichael I.S.E., Turner F.J. & Verghoogen J. (1974): Igneous Petrology. McGraw-Hill.

Photo
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Plagioclase (colorless) and olivine (high relief) crystals within a fine-grained groundmass. PPL image, 2x (Field of view = 7mm)
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Plagioclase (I order gray) and olivine (high interference colors) crystals within a fine-grained groundmass. XPL image, 2x (Field of view = 7mm)
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Plagioclase (colorless) crystals within a fine-grained groundmass. PPL image, 2x (Field of view = 7mm)
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Plagioclase (I order gray) crystals within a fine-grained groundmass. XPL image, 2x (Field of view = 7mm)
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Plagioclase (I order gray) crystals within a fine-grained groundmass. XPL image, 2x (Field of view = 7mm)
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Plagioclase (colorless) crystals within a fine-grained groundmass. PPL image, 2x (Field of view = 7mm)
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Plagioclase (I order gray) crystals within a fine-grained groundmass. XPL image, 2x (Field of view = 7mm)
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Plagioclase (I order gray) crystals within a fine-grained groundmass. XPL image, 2x (Field of view = 7mm)
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Plagioclase (colorless) and olivine (high relief) crystals within a fine-grained groundmass. PPL image, 10x (Field of view = 2mm)
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Plagioclase (I order gray) and olivine (high interference colors) crystals within a fine-grained groundmass. XPL image, 10x (Field of view = 2mm)
basalto20917(11).jpg

Plagioclase (colorless) and olivine (high relief) crystals within a fine-grained groundmass. PPL image, 10x (Field of view = 2mm)
basalto20917(12).jpg

Plagioclase (I order gray) and olivine (high interference colors) crystals within a fine-grained groundmass. XPL image, 10x (Field of view = 2mm)
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Plagioclase (I order gray) and olivine (high interference colors) crystals within a fine-grained groundmass. XPL image, 10x (Field of view = 2mm)
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Plagioclase (I order gray) and olivine (high interference colors) crystals within a fine-grained groundmass. XPL image, 10x (Field of view = 2mm)
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Plagioclase (I order gray) and olivine (high interference colors) crystals within a fine-grained groundmass. XPL image, 10x (Field of view = 2mm)
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Vesiculated basalt, Antarctica. PPL image, 2x (Field of view = 7mm)
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Vesiculated basalt, Antarctica. PPL image, 2x (Field of view = 7mm)
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Vesiculated basalt, Antarctica. PPL image, 2x (Field of view = 7mm)
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Vesiculated basalt, Antarctica. PPL image, 2x (Field of view = 7mm)
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Vesiculated basalt, Antarctica. PPL image, 2x (Field of view = 7mm)
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Vesiculated basalt, Antarctica. PPL image, 2x (Field of view = 7mm)
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Olivine and Plagioclase crystals. PPL image, 2x (Field of view = 7mm)
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Olivine, clinopyroxene and Plagioclase crystals. XPL image, 2x (Field of view = 7mm)
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Olivine, clinopyroxene and Plagioclase crystals. XPL image, 2x (Field of view = 7mm)
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Olivine and Plagioclase crystals. PPL image, 2x (Field of view = 7mm)
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Olivine, clinopyroxene and Plagioclase crystals. XPL image, 2x (Field of view = 7mm)
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Twinned clinopyroxene. XPL image, 2x (Field of view = 7mm)
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Olivine, clinopyroxene and Plagioclase crystals. XPL image, 2x (Field of view = 7mm)
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Olivine, clinopyroxene and Plagioclase crystals. XPL image, 2x (Field of view = 7mm)
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Olivine, clinopyroxene and Plagioclase crystals. XPL image, 2x (Field of view = 7mm)
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Olivine alterd by Iddingsite, interstitial Ti-Augite and plagioclase crystals. XPL image, 10x (Field of view = 2mm)
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Olivine alterd by Iddingsite, interstitial Ti-Augite and plagioclase crystals. PPL image, 10x (Field of view = 2mm)
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Olivine alterd by Iddingsite, interstitial Ti-Augite and plagioclase crystals. XPL image, 10x (Field of view = 2mm)
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Olivine alterd by Iddingsite, interstitial Ti-Augite and plagioclase crystals. PPL image, 10x (Field of view = 2mm)
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Olivine alterd by Iddingsite, interstitial Ti-Augite and plagioclase crystals. XPL image, 10x (Field of view = 2mm)
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Olivine alterd by Iddingsite, interstitial Ti-Augite and plagioclase crystals. PPL image, 10x (Field of view = 2mm)
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Olivine alterd by Iddingsite, interstitial Ti-Augite and plagioclase crystals. XPL image, 10x (Field of view = 2mm)
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Olivine alterd by Iddingsite, interstitial Ti-Augite and plagioclase crystals. PPL image, 10x (Field of view = 2mm)
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Olivine alterd by Iddingsite, interstitial Ti-Augite and plagioclase crystals. XPL image, 10x (Field of view = 2mm)
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Plagioclase and olivine embedded in a glassy groundmass in a Basalt from Antarctica. XPL image, 2x (Field of view = 7mm)
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Plagioclase and olivine embedded in a glassy groundmass in a Basalt from Antarctica. XPL image, 2x (Field of view = 7mm)
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Plagioclase and olivine embedded in a glassy groundmass in a Basalt from Antarctica. XPL image, 2x (Field of view = 7mm)
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(100) section of Olivine. PPL image, 2x (Field of view = 7mm)
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(100) section of Olivine. XPL image, 2x (Field of view = 7mm)
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Olivine crystals partially altered by Iddingsite. PPL image, 2x (Field of view = 7mm)
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Olivine, altered by Iddingsite, clinopyroxene and Plagioclase crystals. PPL image, 2x (Field of view = 7mm)
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Olivine, altered by Iddingsite, clinopyroxene and Plagioclase crystals. PPL image, 2x (Field of view = 7mm)
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Olivine, altered by Iddingsite, clinopyroxene and Plagioclase crystals. XPL image, 2x (Field of view = 7mm)
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Olivine, altered by Iddingsite, clinopyroxene and Plagioclase crystals. PPL image, 2x (Field of view = 7mm)
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Olivine, altered by Iddingsite and Plagioclase crystals. PPL image, 2x (Field of view = 7mm)
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Olivine, altered by Iddingsite and Plagioclase crystals. XPL image, 2x (Field of view = 7mm)
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Olivine, altered by Iddingsite, clinopyroxene and Plagioclase crystals. PPL image, 2x (Field of view = 7mm)
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Olivine, altered by Iddingsite, clinopyroxene and Plagioclase crystals. PPL image, 2x (Field of view = 7mm)
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Olivine, altered by Iddingsite, clinopyroxene and Plagioclase crystals. PPL image, 2x (Field of view = 7mm)
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Olivine, altered by Iddingsite, clinopyroxene and Plagioclase crystals. PPL image, 2x (Field of view = 7mm)
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Olivine, altered by Iddingsite, clinopyroxene and Plagioclase crystals. PPL image, 2x (Field of view = 7mm)
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Olivine, altered by Iddingsite, clinopyroxene and Plagioclase crystals. PPL image, 2x (Field of view = 7mm)
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Olivine, altered by Iddingsite, clinopyroxene and Plagioclase crystals. PPL image, 2x (Field of view = 7mm)
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Olivine, altered by Iddingsite, clinopyroxene and Plagioclase crystals. PPL image, 2x (Field of view = 7mm)
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Olivine, altered by Iddingsite, clinopyroxene and Plagioclase crystals. PPL image, 2x (Field of view = 7mm)
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Olivine, altered by Iddingsite, clinopyroxene and Plagioclase crystals. PPL image, 2x (Field of view = 7mm)
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Olivine clinopyroxene and Plagioclase crystals. XPL image, 2x (Field of view = 7mm)
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Olivine clinopyroxene and Plagioclase crystals. XPL image, 2x (Field of view = 7mm)