Postmagmatic Geochemical Processes in Kimberlites

L.G. Kuznetsova

Institute of Geology and Mineralogy, Siberian Branch of the Russian Academy of Sciences, Novosibirsk, 630090 Russia

Kimberlites are of special interest as derivatives of the deepest magmatic melts. They contain rock and mineral fragments from the upper mantle and high-pressure accessories, of which diamond is the most important.

In addition to common studies of the xenogenous matter in kimberlites, much attention is given to the petrochemistry of kimberlites themselves; particularly, to variations in contents of rock-forming oxides, rare earths, and diamonds. These studies, though, are hampered by the fact that kimberlites, like other alkali and ultrabasic rocks, undergo considerable postmagmatic alteration. Little is known about mass transfer accompanying these alterations, and satisfactory criteria of alteration degree have not been developed.

We studied quantitative indices of mass transfer during postmagmatic kimberlite alteration and calculated the equivalent insoluble constituent, quartz group minerals. The amount of this constituent, normative secondary quartz, can be calculated by simple algorithms suitable for any silicate analysis of a kimberlite. It can be used as an index of postmagmatic alteration intensity.

The main rock-forming kimberlite minerals are olivine, calcite, and phlogopite. Other minerals typically constitute less than 1% of the rock volume. Olivine and phlogopite distributions determine the distributions of silica and magnesium oxide, which show clear positive correlation. The correlation between mean contents of silica and magnesium oxide in 90 kimberlite bodies of the Yakutian Diamondiferous Province is quite close: r= +82.

Both eucrystalline and cryptocrystalline secondary quartz varieties can be found in altered kimberlites. Their direct quantitation is difficult; therefore, excess silica in altered kimberlites can be expressed as normative secondary quartz, Q. This index can be calculated on the base of mean contents of main rock-forming minerals of kimberlite and mineral impurities: diopside and dolomite.

The following oxide ratios (wt%) in minerals were taken for calculating Q: in calcite ๓ม๏ : ๓๏₂ = 1.27; in diopside MgO : CaO = 0.86; in diopside SiO₂ : CaO = 2.69; in phlogopite MgO : K₂O = 3.72; in phlogopite SiO₂ : K₂O = 5.81; in olivine SiO₂ : MgO = 0.81; and in dolomite MgO : CO₂ = 0.45.

The algorithms listed below were used depending on initial data:

A.ššššššššššššš ๓ม๏ > CO₂ , not accounting dolomite. In this case, ๓ม๏_clc = ๓๏_{2 kmb} × 1.27;
๓ม๏_diops = ๓ม๏_kmb – ๓ม๏_clc; MgO_diops = ๓ม๏_diops × 0.86 ; SiO_2diops = ๓ม๏_diops × 2.69;
MgO_phl = K₂O_kmb × 3.72; SiO_{2 phl} = K₂O_kmb × 5.81;
MgO_ol = MgO_kmb – MgO_diops – MgO_phl;
SiO_{2 ol} = MgO_ol × 0.81; Q = SiO_{2 kmb} – SiO_{2 ol} – SiO_{2 diops} – SiO_{2 phl}.

According to algorithm (แ): Q = SiO₂ – 0.81 MgO – 2.0 CaO + 2.54 CO₂ – 2.8 K₂O.

B.ššššššššššššš At ๓๏₂>CaO and ๓๏₂ = Ca๏, not accounting diopside. In this case, ๓๏_{2 clc}= ๓ม๏_kmb /1.27;
MgO_dol=(๓๏_{2 kmb} – ๓๏_{2 clc}) × 0.45; MgO_phl = K₂O_kmb × 3.72;
MgO_ol = MgO_kmb – MgO_dol – MgO_phl;
SiO_{2 phl} = K₂O_kmb × 5.81; SiO_{2 ol} = MgO_ol × 0.81; Q = SiO_{2 kmb} – SiO_{2 ol} – SiO_{2 phl}.

According to algorithm (B): Q = SiO₂ – 0.81 MgO – 0.28 CaO + 0.36 CO₂ – 2.8 K₂O.

C.ššššššššššššš Simplified algorithm not accounting diopside or dolomite: MgO_phl =๋₂๏_kmb × 3.72;
SiO_{2 phl}=๋₂๏_kmb × 5.81; MgO_ol = MgO_kmb – MgO_phl; SiO_{2 ol} = MgO_ol × 0.81;
Q = SiO_{2 kmb} – SiO_{2 phl} – SiO_{2 ol}

According to algorithm (C): Q = SiO₂ – 0.81 MgO – 2.8 K₂O.

Algorithms A and B are best used if CO₂ data are available. Algorithm C is recommended for the most common X-ray fluorescence analyses without determining volatile components.

By these algorithms, Q can be calculated for each analysis. The set of Q values for a particular kimberlite body allows construction of a postmagmatic transfer model, where the distribution of Q value frequencies corresponds to frequencies of contents of other elements. The class interval width for Q distributions is chosen arbitrarily to remain constant for all kimberlite bodies. Six stages with rock alteration degrees increasing with Q are recognized. At stage 0, Q varies from –1.0% to large negative values. Rocks of this stage are considered kimberlites dolomitized during secondary alteration. This is confirmed by their substantially dolomitic composition and presence of minerals of the hypergenesis zone: gypsum, dioctahedral micas, and, probably, terrigenous quartz. Other stages include Q values from –0.99% to 85.00%. Stage 1 includes Q values from –0.99 to 3.99%; stage 2, from 4.00 to 5.99%; stage 3, from 6.00 to 11.99%, stage 4, from 12.00 to 19.99%; and stage 5, ≥ 20.00%. The Aikhal pipe (Table 1) illustrates the model of mass transfer during postmagmatic kimberlite alteration.

Table 1. Postmagmatic mass transfer in kimberlites of the Aikhal pipe (n*=331)

* n, number of analyses; ** n^F proportion of analyses falling to the corresponding cluster.

The results of studies of qualitative mass transfer indices in Yakutian kimberlites experiencing postmagmatic alteration based on Q can be summarized as follows:

Rock-forming oxides. The increase in Q is accompanied by decrease in MgO. Profound alteration is accompanied by all other components except SiO₂ and Al₂O₃.

Iron group elements (FeO, Fe₂O₃, Cr₂O₃, NiO, and CoO) can be accumulated in reaction zones or subtracted from them. Generally, secondary rock alteration ends in increasing basicity (predominance of FeO over Fe₂O₃).

Rare earths. The contents of rare earth elements (REEs) rapidly change at the postmagmatic stage. Thisš can be clearly traced with increasing Q. The main petrochemical factors of REE accumulation are contents of phosphorus and titanium in kimberlites. 'The greatest REE subtraction is recorded in rocks rich in phosphorus and poor in titanium.

Isotope ratios. The ¹⁴⁷Sm/¹⁴⁴Nd, ¹⁴³Nd/¹⁴⁴Nd, and ⁸⁷Rb/⁸⁶Sr ratios decrease with accumulation of normative secondary quartz (Table 2).

Table. 2. Mean isotope ratios in kimberlites with various secondary alteration degrees (the pipes Internatsional'naya, Aikhal, Udachnaya-West, Udachnaya-East, Komsomol'skaya-magnitnaya, and Poiskovaya).

Diamond presence. Change of the contents of diamonds in altered kimberlites was described most comprehensively for the Botuobinskaya pipe. It has been shown on the base of a large body of facts that the increase in Q is accompanied by a decrease in diamond grain size; as a result, the diamond potential of rocks decreases dramatically. We observed a similar effect in the Nyurbinskaya pipe. Thus, postmagmatic metasomatism causes depauperation of diamondiferous ores.

To conclude, we note that secondary kimberlite alteration is more notable in subsurface parts of pipes but also involves deeper kimberlites.

As shown above, secondaryš alteration of kimberlites profoundly changes their geochemical indices. This fact can be taken into account in petrological reconstructions. We consider kimberlites with –0.99<Q<3.99% to be unaltered. Just these kimberlites should be used in geochemical correlations.