Aluminum saturation

In petrology, aluminum saturation is an important parameter for characterizing igneous rocks , which is mainly used for granitoids . A very similar parameter is the aluminosity , in which the elements iron , manganese and magnesium are taken into account in the calculation.

definition

The aluminum saturation is expressed in the aluminum saturation index ( English aluminum saturation index or ASI for short), the molecular quotient of [Al ₂ O ₃ ] / [Na ₂ O] + [K ₂ O] + [CaO], also Al / Na + in short notation K + approx. The ASI was first introduced by SJ Shand in 1943. The index was later supplemented, which takes into account the amount of calcium bound in the apatite (if present) . The redefinition is now as follows:

ASI = [Al ₂ O ₃ ] / [Na ₂ O] + [K ₂ O] + [CaO] -5/3 [P ₂ O ₅ ]

Compared to the original definition, however, it only has a minimal effect on the end result.

For magmas saturated with aluminum the ASI quotient is 1, for oversaturated magmas> 1 and for undersaturated magmas <1.

Supersaturated magmas are called peraluminos .

Undersaturated magmas are further subdivided on the basis of the molecular quotient [Al ₂ O ₃ ] / [Na ₂ O] + [K ₂ O] (or Al / Na + K) into:

metaluminos : [Al₂ O₃ ] / [Na₂ O] + [K₂ O]> 1 (or Al> Na + K)
per-alkaline : [Al₂ O₃ ] / [Na₂ O] + [K₂ O] <1 (or Al <Na + K)

Examples

The following rock analyzes should help illustrate the principle of aluminum saturation:

Oxide wt.%	Gouré granite	Delos granodiorite	Average granite
SiO ₂	74.05	68.00	71.84
TiO ₂	0.35	0.57	0.31
Al ₂ O ₃	11.20	15.20	14.43
Fe ₂ O ₃	2.10	0.61	1.22
FeO	1.70	2.20	1.65
MnO	0.10	0.05	0.05
MgO	0.45	1.24	0.72
CaO	0.45	3.62	1.85
Na ₂ O	4.60	3.09	3.71
K ₂ O	5.20	4.11	4.10
P ₂ O ₅	0.06	0.14	0.12
H ₂ O	0.42	0.65
ASI Al / Na + K + Ca - 5 / 3P	0.80	0.95	1.05
Al / Na + K	0.85	1.60	1.37

The Gouré granite is metaluminos and at the same time peralkaline, the Delos granodiorite is metaluminos and the average granite is peraluminos.

meaning

For feldspars and foids , the aluminum saturation index is exactly 1, ie feldspars and foids are saturated with aluminum. However, if a magma source is peraluminous and oversaturated with aluminum, then because of the excess of aluminum, besides feldspars and foids, other aluminum-rich minerals, especially mica, must also form . Examples are muscovite (ASI = 2 to 2.5), aluminum-rich biotite (ASI = 1 to 1.5), but also the aluminum silicates andalusite and sillimanite , as well as cordierite , aluminum oxide corundum , the garnets almandine and spessartine , tourmaline and topaz all of which can be found in peraluminous rocks. Pereraluminous rocks have corundum (c) in their standard . Clearly peraluminous melts usually arise from metasediments, but they can also arise from metaluminous, biotite-bearing, acidic magmatites. Even a peraluminous melt formation from mafic rocks is considered possible in the presence of water .

In order to compensate for their aluminum deficiency, magmas that are undersaturated with aluminum form hornblende (amphiboles, ASI = 0.3 to 0.5), low-aluminum biotite and titanite in addition to feldspars and foids . Metaluminous rocks are characterized in their norm by anorthite (an) and diopside (di) or wollastonite (wo). After the feldspars have crystallized out, metaluminous melts usually accumulate in calcium and then form calcium-rich mineral phases such as hornblende and augite , but not muscovite or sodium-rich Fe-Mg minerals.

In peralkaline rocks the very strong aluminum undersaturation (with simultaneous oversaturation with alkalis) is compensated by the formation of sodium-rich, mafic alkali minerals such as aegirine , arfvedsonite , riebeckite , Richterite and aenigmatite . Weakly peralkaline rocks can still lead to hornblende, but clearly peralkaline rocks are characterized by Na-amphiboles and Na-pyroxenes. Feldspars contain very little anorthite in peralkaline rocks. In their norm, peralkaline rocks can be recognized by the components Akmit (ac) or sodium metasilicate (ns), anorthite no longer occurs normatively.

application

The aluminum saturation index is mainly plotted in petrological diagrams with the SiO ₂ content .

Individual evidence

↑ Shand, SJ: Eruptive rocks, 2nd edition . John Wiley, New York 1943, p. 444 .
↑ Zen, E .: Phase relations of peraluminous granitic rocks and their petrogenetic implications . In: Annual Review of Earth and Planetary Sciences . tape 16 , 1988, pp. 21-52 .
↑ Chappell, BW and White, AJR: Two contrasting granite types . In: Pacific Geology . tape 8 , 1974, p. 173-174 .
↑ Miller, CF: Are strongly peraluminous magmas derived from pelitic sedimentary sources? In: Journal of Geology . tape 93 , 1985, pp. 673-689 .
↑ Ellis, DJ and Thompson, AB: Subsolidus and partial melting reactions in the quartz excess CaO + MgO + Al ₂ O ₃ + SiO ₂ + H ₂ O system under water excess and water deficient conditions to 10 kb: some implications for the origin of peraluminous melts from mafic rocks . In: Journal of Petrology . tape 27 , 1985, pp. 91-121 .

[1] Shand, SJ: Eruptive rocks, 2nd edition . John Wiley, New York 1943, p. 444 .

[2] Zen, E .: Phase relations of peraluminous granitic rocks and their petrogenetic implications . In: Annual Review of Earth and Planetary Sciences . tape 16 , 1988, pp. 21-52 .

[3] Chappell, BW and White, AJR: Two contrasting granite types . In: Pacific Geology . tape 8 , 1974, p. 173-174 .

[4] Miller, CF: Are strongly peraluminous magmas derived from pelitic sedimentary sources? In: Journal of Geology . tape 93 , 1985, pp. 673-689 .

[5] Ellis, DJ and Thompson, AB: Subsolidus and partial melting reactions in the quartz excess CaO + MgO + Al ₂ O ₃ + SiO ₂ + H ₂ O system under water excess and water deficient conditions to 10 kb: some implications for the origin of peraluminous melts from mafic rocks . In: Journal of Petrology . tape 27 , 1985, pp. 91-121 .