The tholeiitic magma series is one of two main
magma series
In geology, igneous differentiation, or magmatic differentiation, is an umbrella term for the various processes by which magmas undergo bulk chemical change during the partial melting process, cooling, emplacement, or eruption. The sequence of (u ...
in
subalkaline igneous rocks
Volcanic rock (often shortened to volcanics in scientific contexts) is a Rock (geology), rock formed from lava erupted from a volcano. In other words, it differs from other igneous rock by being of Volcano, volcanic origin. Like all rock types, ...
, the other being the
calc-alkaline
The calc-alkaline magma series is one of two main subdivisions of the subalkaline magma series, the other subalkaline magma series being the tholeiitic series. A magma series is a series of compositions that describes the evolution of a mafic mag ...
series. A magma series is a chemically distinct range of magma compositions that describes the evolution of a
mafic
A mafic mineral or rock is a silicate mineral or igneous rock rich in magnesium and iron. Most mafic minerals are dark in color, and common rock-forming mafic minerals include olivine, pyroxene, amphibole, and biotite. Common mafic rocks include ...
magma into a more evolved, silica rich end member. Rock types of the tholeiitic magma series include tholeiitic basalt, ferro-basalt,
tholeiitic basaltic andesite,
tholeiitic andesite,
dacite
Dacite () is a volcanic rock formed by rapid solidification of lava that is high in silica and low in alkali metal oxides. It has a fine-grained (aphanitic) to porphyritic texture and is intermediate in composition between andesite and rhyolite. ...
and
rhyolite
Rhyolite ( ) is the most silica-rich of volcanic rocks. It is generally glassy or fine-grained (aphanitic) in texture, but may be porphyritic, containing larger mineral crystals (phenocrysts) in an otherwise fine-grained groundmass. The mineral ...
. The variety of basalt in the series was originally called ''tholeiite'' but the
International Union of Geological Sciences
The International Union of Geological Sciences (IUGS) is an international non-governmental organization devoted to international cooperation in the field of geology.
About
The IUGS was founded in 1961 and is a Scientific Union member of the Inte ...
recommends that ''tholeiitic basalt'' be used in preference to that term.
[Le Maitre ''et al.'' 2002]
Tholeiitic rock types tend to be more enriched in iron and less enriched in aluminium than calc-alkaline rock types. They are thought to form in a less oxidized environment than calc-alkaline rocks. Tholeiitic basalt is formed at
mid-ocean ridges
A mid-ocean ridge (MOR) is a seafloor mountain system formed by plate tectonics. It typically has a depth of about and rises about above the deepest portion of an ocean basin. This feature is where seafloor spreading takes place along a diverge ...
and makes up much of the oceanic crust. Almost all the basalt found on the Moon is tholeiitic basalt.
Geochemical characterization
Rocks in the tholeiitic magma series are classified as
subalkaline (they contain less sodium than some other basalts) and are distinguished from rocks in the
calc-alkaline
The calc-alkaline magma series is one of two main subdivisions of the subalkaline magma series, the other subalkaline magma series being the tholeiitic series. A magma series is a series of compositions that describes the evolution of a mafic mag ...
magma series by the
redox
Redox (reduction–oxidation, , ) is a type of chemical reaction in which the oxidation states of substrate (chemistry), substrate change. Oxidation is the loss of Electron, electrons or an increase in the oxidation state, while reduction ...
state of the magma they crystallized from (tholeiitic magmas are reduced; calc-alkaline magmas are oxidized
). When the parent magmas of
basalt
Basalt (; ) is an aphanite, aphanitic (fine-grained) extrusive igneous rock formed from the rapid cooling of low-viscosity lava rich in magnesium and iron (mafic lava) exposed at or very near the planetary surface, surface of a terrestrial ...
s crystallize, they preferentially crystallize the more magnesium-rich and iron-poor forms of the silicate minerals
olivine
The mineral olivine () is a magnesium iron silicate with the chemical formula . It is a type of nesosilicate or orthosilicate. The primary component of the Earth's upper mantle, it is a common mineral in Earth's subsurface, but weathers quickl ...
and
pyroxene
The pyroxenes (commonly abbreviated to ''Px'') are a group of important rock-forming inosilicate minerals found in many igneous and metamorphic rocks. Pyroxenes have the general formula , where X represents calcium (Ca), sodium (Na), iron (Fe II) ...
, causing the iron content of tholeiitic magmas to increase as the melt is depleted of iron-poor crystals. However, a calc-alkaline magma is oxidized enough to precipitate significant amounts of the iron oxide
magnetite
Magnetite is a mineral and one of the main iron ores, with the chemical formula Fe2+Fe3+2O4. It is one of the oxides of iron, and is ferrimagnetic; it is attracted to a magnet and can be magnetized to become a permanent magnet itself. With the ...
, causing the iron content of the magma to remain more steady as it cools than with a tholeiitic magma.
The difference between these two magma series can be seen on an AFM diagram, a
ternary diagram
A ternary plot, ternary graph, triangle plot, simplex plot, Gibbs triangle or de Finetti diagram is a barycentric plot on three variables which sum to a constant. It graphically depicts the ratios of the three variables as positions in an equ ...
showing the relative proportions of the oxides Na
2O + K
2O (A), FeO + Fe
2O
3 (F), and MgO (M). As magmas cool, they precipitate out significantly more iron and magnesium than alkali, causing the magmas to move towards the alkali corner as they cool. In the tholeiitic magma, magnesium-rich crystals are produced preferentially, the magnesium content of the magma plummets, causing the magma to move away from the magnesium corner until it runs low on magnesium and simply moves towards the alkali corner as it loses iron and any remaining magnesium. With the calc-alkaline series, however, the precipitation of magnetite causes the iron-magnesium ratio to remain relatively constant, so the magma moves in a straight line towards the alkali corner on the AFM diagram.
[Philpotts and Ague 2009, pp. 143-146]
The AFM plot distinguishes the intermediate members of the tholeiitic and calc-alkali magma series quite well. However, the felsic end members of the two series are nearly indistinguishable, so granitic rocks are generally assigned to the calc-alkali magma series. The mafic end members may be distinguished by the aluminium content, with tholeiitic basalts containing 12% to 16% Al
2O
3 versus 16% to 20% Al
2O
3 for calc-alkali basalts.
[Philpotts and Ague 2009, p.144]
Petrography
Like all basalt, the rock type is dominated by
olivine
The mineral olivine () is a magnesium iron silicate with the chemical formula . It is a type of nesosilicate or orthosilicate. The primary component of the Earth's upper mantle, it is a common mineral in Earth's subsurface, but weathers quickl ...
,
clinopyroxene
The pyroxenes (commonly abbreviated to ''Px'') are a group of important rock-forming inosilicate minerals found in many igneous and metamorphic rocks. Pyroxenes have the general formula , where X represents calcium (Ca), sodium (Na), iron (Fe II) ...
and
plagioclase
Plagioclase is a series of tectosilicate (framework silicate) minerals within the feldspar group. Rather than referring to a particular mineral with a specific chemical composition, plagioclase is a continuous solid solution series, more prope ...
, with minor iron-
titanium
Titanium is a chemical element with the symbol Ti and atomic number 22. Found in nature only as an oxide, it can be reduced to produce a lustrous transition metal with a silver color, low density, and high strength, resistant to corrosion in ...
oxides.
["Polarized Light Microscopy Digital Image Gallery: Tholeiitic Basalt" (Accessed 4/1/06)](_blank)
/ref> Orthopyroxene
The pyroxenes (commonly abbreviated to ''Px'') are a group of important rock-forming inosilicate minerals found in many igneous and metamorphic rocks. Pyroxenes have the general formula , where X represents calcium (Ca), sodium (Na), iron (Fe II) ...
or pigeonite
Pigeonite is a mineral in the clinopyroxene subgroup of the pyroxene group. It has a general formula of . The calcium cation fraction can vary from 5% to 25%, with iron and magnesium making up the rest of the cations.
Pigeonite crystallizes in th ...
may also be present in tholeiitic basalt, and olivine, if present, may be rimmed by either of these calcium-poor pyroxenes. Tridymite
Tridymite is a high-temperature polymorph of silica and usually occurs as minute tabular white or colorless pseudo-hexagonal crystals, or scales, in cavities in felsic volcanic rocks. Its chemical formula is Si O2. Tridymite was first described ...
or quartz
Quartz is a hard, crystalline mineral composed of silica (silicon dioxide). The atoms are linked in a continuous framework of SiO4 silicon-oxygen tetrahedra, with each oxygen being shared between two tetrahedra, giving an overall chemical form ...
may be present in the fine-grained groundmass of tholeiitic basalt, and feldspathoid
The feldspathoids are a group of tectosilicate minerals which resemble feldspars but have a different structure and much lower silica content. They occur in rare and unusual types of igneous rocks, and are usually not found in rocks containing prim ...
s are absent. Tholeiitic rocks may have a fine, glassy groundmass
The matrix or groundmass of a rock is the finer-grained mass of material in which larger grains, crystals, or clasts are embedded.
The matrix of an igneous rock consists of finer-grained, often microscopic, crystals in which larger crystals, ca ...
, as may other types of basalt.
Geologic context
Tholeiitic rocks are the most common igneous
Igneous rock (derived from the Latin word ''ignis'' meaning fire), or magmatic rock, is one of the three main rock types, the others being sedimentary and metamorphic. Igneous rock is formed through the cooling and solidification of magma or ...
rocks in Earth's crust
Earth's crust is Earth's thin outer shell of rock, referring to less than 1% of Earth's radius and volume. It is the top component of the lithosphere, a division of Earth's layers that includes the crust and the upper part of the mantle. The ...
, produced by submarine volcanism at mid-ocean ridge
A mid-ocean ridge (MOR) is a seafloor mountain system formed by plate tectonics. It typically has a depth of about and rises about above the deepest portion of an ocean basin. This feature is where seafloor spreading takes place along a diverge ...
s and make up much of the ocean crust. Tholeiitic basaltic magmas are initially generated as partial melts of peridotite
Peridotite ( ) is a dense, coarse-grained igneous rock consisting mostly of the silicate minerals olivine and pyroxene. Peridotite is ultramafic, as the rock contains less than 45% silica. It is high in magnesium (Mg2+), reflecting the high prop ...
(olivine
The mineral olivine () is a magnesium iron silicate with the chemical formula . It is a type of nesosilicate or orthosilicate. The primary component of the Earth's upper mantle, it is a common mineral in Earth's subsurface, but weathers quickl ...
and pyroxene
The pyroxenes (commonly abbreviated to ''Px'') are a group of important rock-forming inosilicate minerals found in many igneous and metamorphic rocks. Pyroxenes have the general formula , where X represents calcium (Ca), sodium (Na), iron (Fe II) ...
) produced by decompression melting of the Earth's mantle
Earth's mantle is a layer of silicate rock between the crust and the outer core. It has a mass of 4.01 × 1024 kg and thus makes up 67% of the mass of Earth. It has a thickness of making up about 84% of Earth's volume. It is predominantly sol ...
. Tholeiitic basalt constituting the oceanic crust is termed ''MORB:'' ''m''id-''o''cean-''r''idge ''b''asalt. Throughout the process of igneous differentiation
In geology, igneous differentiation, or magmatic differentiation, is an umbrella term for the various processes by which magmas undergo bulk chemical change during the partial melting process, cooling, emplacement, or eruption. The sequence of (u ...
, the oceanic crust acts to reduce the magma, producing the tholeiitic trend. In contrast, alkali basalts
Alkali basalt or alkali olivine basalt is a dark-colored, porphyritic volcanic rock usually found in oceanic and continental areas associated with volcanic activity, such as oceanic islands, continental rifts and volcanic fields. Alkali basalt ...
are not typical of ocean ridges, but are erupted on some oceanic islands and on continents, as also is tholeiitic basalt. Because the Moon
The Moon is Earth's only natural satellite. It is the fifth largest satellite in the Solar System and the largest and most massive relative to its parent planet, with a diameter about one-quarter that of Earth (comparable to the width of ...
is extremely reduced, all of its basalts are tholeiitic.
Type locality
''Tholeiite'' is named for its type locality near the municipality of Tholey
Tholey () is a municipality in the district of Sankt Wendel, in Saarland, Germany. It is situated approximately west of Sankt Wendel, and north of Saarbrücken.
History
Local history
The first traces of settlement in the area of today's Thole ...
, Saarland
The Saarland (, ; french: Sarre ) is a state of Germany in the south west of the country. With an area of and population of 990,509 in 2018, it is the smallest German state in area apart from the city-states of Berlin, Bremen, and Hamburg, and ...
, Germany
Germany,, officially the Federal Republic of Germany, is a country in Central Europe. It is the second most populous country in Europe after Russia, and the most populous member state of the European Union. Germany is situated betwe ...
.[Mindat Location]
/ref>
See also
* Alkaline magma series
The alkaline magma series is a chemically distinct range of magma compositions that describes the evolution of an alkaline mafic magma into a more evolved, silica-rich end member.
Geochemical characterization
Rocks in the alkaline magma series a ...
* Petrology
Petrology () is the branch of geology that studies rocks and the conditions under which they form. Petrology has three subdivisions: igneous, metamorphic, and sedimentary petrology. Igneous and metamorphic petrology are commonly taught together ...
* Igneous differentiation
In geology, igneous differentiation, or magmatic differentiation, is an umbrella term for the various processes by which magmas undergo bulk chemical change during the partial melting process, cooling, emplacement, or eruption. The sequence of (u ...
References
Citations
Sources
* R. W. Le Maitre (editor), A. Streckeisen, B. Zanettin, M. J. Le Bas, B. Bonin, P. Bateman, G. Bellieni, A. Dudek, S. Efremova, J. Keller, J. Lamere, P. A. Sabine, R. Schmid, H. Sorensen, and A. R. Woolley, Igneous Rocks: A Classification and Glossary of Terms, Recommendations of the International Union of Geological Sciences, Subcommission of the Systematics of Igneous Rocks. Cambridge University Press, 2002. .
*
* American Geological Institute. ''Dictionary of Geological Terms.'' New York: Dolphin Books, 1962.
{{basalt
Basalt
Igneous petrology
Industrial minerals