DEFINICIÓN DE CARGOS, PERFILES Y FUNCIONES

Alone, a unit cell parameter measurement cannot unambiguously discriminate between kimberlitic and non-kimberlitic chromite. The primary reason for this is that multiple chemical compositions, and to a lesser extent ordering states, will result in identical unit cell parameters. If the chemical compositions can be constrained, for example, due to a known kimberlitic origin, unit cell can be used as a first approximation for chemical

Figure 4-15: Cr2O3 vs Al2O3 discrimination diagram. This plot shows

the diamond inclusion compositional field <8 wt% Al2O3 and >61 wt%

Cr2O3. (see boxed area) (Sobolev, 1977)

Al

2

O

(wt%)

Cr

O

(wt%)

composition. The diamond inclusion field in the three bi-variant plots defines the composition range of chromites associated with diamonds. All bivariant discrimination plots can distinguish the diamond inclusion field composition on the basis of unit cell size, with values between 8.31 Å and 8.33 Å, when constrained to lower Fe

compositions. These results are consistent with those of Lenaz et al. (2009) on kimberlites from Siberia

4.4.5.1

Bi-variant discrimination plots

Three commonly-used chemical discrimination plots have been overlaid with unit cell data, to demonstrate the applicability of unit cell as a proxy for composition, for discrimination purposes. The areas that are typically associated with the diamond inclusion field are overlaid in the diagrams.

4.4.5.1.1 Cr

O

vs Al

O

Discrimination plot

The Cr2O3 vs Al2O3 discrimination diagram, Figure 4-15, shows the diamond-associated

Figure 4-16: Kimberlite discrimination can be done on the basis of those grains that are above the Ti-Cr discrimination field (defined by the black line). This is likely due to Fe2+ rather than Ti. Note the diamond inclusion field by a box. (Sobolev, 1977)

Cr

O

(wt%)

TiO

2

(wt%)

There are, however; some kimberlitic samples mixed in this area. The diamond inclusion field samples have unit cells between 8.31 and 8.33 Å.This discrimination plot shows one general trend from high Al2O3 to high Cr2O3. This trend is associated with Cr-Al

substitution, as there is increasing Cr as described above. At low Al2O3 levels, there is an

increase in unit cell size as Cr2O3 decreases. This is a result of spread between

chromite/magnetite and magnesiochromite due to the Fe-Mg substitution leading to the increased unit cell size of the chromite/magnetite end members.

4.4.5.1.2 Cr

O

vs TiO

Discrimination Plot

The Cr2O3 vs TiO2 discrimination plot, Figure 4-16, is commonly used to discriminate

kimberlites from non-kimberlites by separating the field that is created by the transect from 8-9 wt% TiO2 to 60 wt% Cr2O3 (Nowicki 2014; Grütter and Apter 1998). Currently

those grains in the “kimberlite” trend have unit cell parameter dimensions above 8.34 Å (Roeder 2001). This is likely due to the grains also having high Fe2+in conjunction with the high Cr, as seen in Figure 4-15.

It is unlikely that Ti is driving the unit cell size significantly enough to conclude that it will be able to the unit cell parameter will be able to discriminate TiO2 once more data is

added to the plot. Ti does partition into spinels with Fe3+ meaning it may trace the Fe3+. This could partially explain the unit cell size discrimination combined with the larger unit cell size of high Ti grain. Figure 14 shows the diamond inclusion field below 0.5 wt% TiO2 and greater than ~62 wt% Cr2O3. Among the chromite grains below 8.31 Å there is

a mix of non-kimberlitic and kimberlitic samples.

4.4.5.1.3 Cr

O

vs MgO Discrimination Plot

The Cr2O3 vs MgO diagram, Figure 4-17, demonstrates some of the limitations to the use

of unit cell as a compositional proxy. The ‘phenocryst trend’ is found to have increasing unit cell size from 8.33 Å with decreasing MgO content along the positive linear

chemical trend. The trend is linked to an increase in unit cell size toward more Fe2+ rich spinels, mainly from chromite to magnesiochromite, because of the larger radius for Fe2+ than Mg, as discussed previously. The mixed xenocryst and non-kimberlite field

represents unit cells of less than 8.31 Å and the negative linear trend toward higher MgO contents, as seen from chromite to spinel in the chemical composition, correlates with the decreasing unit cell. The diamond inclusion field ranges from 8.31 to 8.33 Å and is shown in the region above ~61 wt% Cr2O3 and 10-16 wt% MgO(Fipke et al. 1989; Fipke

et al. 1995). The technique is, however, unable at this time to discriminate between chemical substitutions driving identical unit cell changes between different populations. This makes it impossible in the general case (i.e., without any chemical constraint or context) to discern kimberlite-specific trends within unit cell data, which is essential for KIM discrimination (Nowicki et al. 2004).

MgO (wt%)

Cr

2

O

(wt%)

Figure 4-17 :Cr

O

vs MgO discrimination diagram showing the typical

kimberlite trends for phenocryst (large unit cells - red) and xenocryst

(small unit cells - blue), as exhibited by Koala/Sheiba/Misery, as

compared to Matsitami exhibiting the classic flat non-kimberlite trend.

Note that the non-kimberlitic Matsitami samples have similar unit cell

size to those in the diamond inclusion field (yellow). Diamond inclusion

field is indicated by the shaded box. Thus, the use of unit cell for these

purposes is limited without context or other constraints.

4.4.5.2

3D-variant discrimination plots

The Cr2O3 vs MgO and Cr2O3 vs TiO2 diagrams were combined to see the net effect of

these two discrimination plots on unit cell, Figure 4-19. The result was that the diamond inclusion field became highly defined and unit cells of 8.31-8.33 Å described the area. The kimberlitic trend of the Cr2O3 vs TiO2 diagram and the Cr2O3 vs MgO are clearly

discriminated using the >8.33 Å unit cells. This diagram has the same caveats as all unit cell compositional proxies, but displays data in a clear manner. The xenocryst and non- kimberlite field of the Cr2O3 vs MgO diagram is spread upward by the TiO2 content, as

unit cell increases with TiO2, although unlikely driven by it except in the Sheiba

population.

Figure 4-18: Cr2O3 vs TiO2 vs MgO, a 3D diagram that provides

a convenient method to display chemical compositions for discrimination purposes. It also demonstrates the utility of unit cell as a discriminator in kimberlitic terrains.