Magnesium number

The magnesium number Mg # is a petrological parameter that shows the ratio of the divalent cations of iron and magnesium in rocks . It can generally be used to characterize igneous rocks , but it is most useful to classify mafites and ultramafites .

definition

The magnesium number Mg #, engl. magnesium number (Mg #) or magnesium value (Mg ') , is defined as follows:

{\ displaystyle Mg ^ {'} = {\ frac {Mg} {Mg + Fe}}}

or

{\ displaystyle Mg ^ {'} = {\ frac {molMg ^ {2 +}} {molMg ^ {2 +} + molFe ^ {2+}}}}

This ratio provides a number between 0 and 1, with iron-rich rocks being closer to 0, while magnesium-rich rocks are closer to 1.

Often the basic equation is also multiplied by 100, the resulting values are between 0 and 100:

${\ displaystyle Mg ^ {'} = 100 {\ frac {Mg} {Mg + Fe}}}$

or

${\ displaystyle Mg ^ {'} = 100 {\ frac {molMg ^ {2 +}} {molMg ^ {2 +} + molFe ^ {2+}}}}$

calculation

After the rock analysis has been completed, the magnesium number Mg # is usually calculated in the course of rock normalization using the molecular ratios. If rock analyzes are available that do not separate the two iron oxides from one another, the following formula can be used:

${\ displaystyle Mg ^ {'} = {{\ frac {1} {40.32}} MgO \ over {{\ frac {1} {40.32}} MgO + {\ frac {0.9} {79, 8}} Fe_ {2} O_ {3} (Total)}}}$

The divisor 40.32 is the molar mass of . The molecular weights of and are 71.85 and 159.69, respectively. The divisor 79.8 therefore takes into account a small proportion of Fe ₂ O ₃ in this calculation and at the same time reduces the iron component by 10%. ${\ displaystyle MgO}$ ${\ displaystyle FeO}$ ${\ displaystyle Fe_ {2} O_ {3}}$

Examples

To illustrate, the chemical composition of some rock types (mostly average values from more than 300 analyzes) with their characteristic magnesium number is given:

Oxide wt.%	Trachyte	Phonolite	granite	Granodiorite	Andesite	Diorite	basalt	Basanite	N-MORB	Island arc holeiit	Boninite	Nephelinite	Harzburgite	MORB pyrolite	Lherzolite	Dunite
SiO ₂	62.31	57.43	71.84	66.91	58.7	58.34	49.97	45.16	50.35	49.00	53.00	41.81	43.73	44.74	44.16	41.03
Fe ₂ O ₃	3.04	2.85	1.22	1.40	3.31	2.54	3.85	4.02				5.64	6.0
FeO	2.33	2.07	1.65	2.76	4.09	4.99	7.24	7.65	11.30	9.79	7.54	6.35	7.09	7.55	8.14	6.26
MgO	0.94	1.09	0.72	1.76	3.37	3.77	6.84	8.71	8.65	11.62	13.08	6.58	36.34	39.57	41.05	51.88
Mg #	0.28	0.33	0.36	0.48	0.50	0.52	0.58	0.62	0.63	0.72	0.79	0.82	0.86	0.92	0.92	0.95

Basalts usually have magnesium numbers of 0.68 to 0.75 (or 68 to 75), which corresponds to a molar mass ratio of MgO / FeO of 0.4 to 0.7. Peridotites are around 0.90 (or 90).

Remarks

During the melting process of igneous rocks, the magnesium number Mg # is not or only slightly changed. However, the subsequent fractional crystallization , especially of olivine , as well as any crust contamination , have a decisive influence on them.

application

Binary variation diagram of igneous rocks of average composition, plotted the magnesium number Mg # against the SiO ₂ content . The negative correlation to SiO _{2 is} clearly visible . Rocks that are undersaturated with SiO ₂ such as nephelinite or basanite and rocks that are relatively rich in SiO ₂ such as boninite are off the trend. The strong SiO ₂ increase towards the granitoids is explained by the increased plagioclase crystallization .

The magnesium number Mg # is widely used in petrogenetic studies. After the rock analysis has been completed, it is usually shown in binary variation diagrams of the main elements Si, Ti, Al, Ca, Na, K and P as well as the trace elements Sc, V, Cr, Co, Ni, Rb, Sr, Y, Zr, Nb and Ba Plotted abscissa . This allows element correlations to be recognized well. Basaltic rocks usually show negative correlations for the main elements Ti, Al, Na (indistinctly), K and P, whereas the Ca content remains more or less constant.

Discontinuities can indicate magmas of different origins (primary and differentiated magmas) or fractional crystallization differentiation and crust contamination.

The same diagrams can also be used for single crystal analysis , especially for peridotic rocks.

Individual evidence

↑ J. Harvey et al. a .: Ancient melt extraction from the oceanic upper mantle revealed by Re-Os isotopes in abyssal peridotites from the Mid-Atlantic ridge . In: Earth and Planetary Science Letters . tape 244 , 2006, pp. 606-621 .
^ MG Best and EH Christiansen: Igneous Petrology . Blackwell Science, 2001, ISBN 0-86542-541-8 .
↑ M. Wilson: Igneous Petrogenesis . Chapman & Hall, 1989, ISBN 0-412-53310-3 .
↑ Caroline Jung: Geochemical and isotope-geochemical investigations on tertiary volcanic rocks of the Hocheifel - A contribution to the identification of the mantle sources of rift-related volcanic rocks . 2003, p. 132 (dissertation from the Philipps University of Marburg).
↑ Kolja Stremmel: Geology and petrology of the mafic plutons in the Trinity Ophiolite, California (USA) . 2012, p. 399 (inaugural dissertation from the University of Cologne).

[1] J. Harvey et al. a .: Ancient melt extraction from the oceanic upper mantle revealed by Re-Os isotopes in abyssal peridotites from the Mid-Atlantic ridge . In: Earth and Planetary Science Letters . tape 244 , 2006, pp. 606-621 .

[2] MG Best and EH Christiansen: Igneous Petrology . Blackwell Science, 2001, ISBN 0-86542-541-8 .

[3] M. Wilson: Igneous Petrogenesis . Chapman & Hall, 1989, ISBN 0-412-53310-3 .

[4] Caroline Jung: Geochemical and isotope-geochemical investigations on tertiary volcanic rocks of the Hocheifel - A contribution to the identification of the mantle sources of rift-related volcanic rocks . 2003, p. 132 (dissertation from the Philipps University of Marburg).

[5] Kolja Stremmel: Geology and petrology of the mafic plutons in the Trinity Ophiolite, California (USA) . 2012, p. 399 (inaugural dissertation from the University of Cologne).