Trace element analysis

Fifty-two samples from the Eyreville-B borehole core were analysed for trace elements by XRF (Table 3.5). Trace elements for the samples of the W series and CB6 series, also obtained by XRF analysis, have been included for comparison purposes (Bartosova et al., 2009; Schmitt et al., 2009). Trace element data for all samples obtained by XRF analysis are presented in Appendix 2a.

Figures 3.10 and 3.11 show selected trace elements plotted against depth for the eight target rock units of the Eyreville-B borehole core. These same trace elements have been plotted against SiO2 in Figure 3.12.

Table 3.5: Mean and standard deviation (data in ppm) of trace element abundances obtained by XRF analysis for the eight target rock units of the Eyreville-B borehole core. Abbr.: n = number of samples; SD = standard deviation; bdl = below detection limit.

Trace

Figure 3.10: Variations in trace element concentrations (ppm) with depth (m) for the four target rock units in the lower basement-derived section of the Eyreville-B borehole core. Not all samples could be plotted for Zr, Ba, V and Zn graphs as their concentrations were below the lower limit of detection. Samples from the CB6 and W series (after Schmitt et al., 2009) have been included to increase the data set size. The geologic column, as described in Chapters 1 and 2 (after Horton et al., 2009a), has been placed on the right side for reference purposes. Dashed lines represent observed groupings.

Figure 3.11: Variations in trace element concentrations (ppm) with depth (m) for the four target rock units in the upper granite and amphibolite megablocks of the Eyreville-B borehole core. The geologic column, after Horton et al. (2009), has been placed on the right side for reference purposes. Not all samples could be plotted for Rb, Cr, Ba, V and Zn graphs as their concentrations were below the lower limit of detection. Samples from the CB6 series (after Schmitt et al., 2009) have been included to increase the data set size. The geologic column, as described in Chapters 1 and 2 (after Horton et al., 2009a), has been placed on the right side for reference purposes. Dashed lines represent observed groupings.

3.4.1 Mica schists

Concentrations of Rb, Sr, Cr, V and Ni tend to increase with depth towards approximately 1610 m in the lower basement-derived mica schists, and then decrease with depth below 1660 m (Figure 3.10a, b, e, f and g). Zr concentrations in the basement mica schists differ in that there is a negative trend from around 1550 m to 1610 m, which changes to a positive trend from around 1610 m to 1640 m (the mylonite zone; Figure 3.10c). Zr concentrations return to a negative trend below 1670 m (Figure 3.10c). This signature is similar to that observed in the major element oxides SiO2 and K2O (Figure 3.1a and i) whereas Fe2O3 shows opposite trends at similar depths (Figure 3.1d).

The basement mica schists generally decrease in trace element abundance with increasing SiO2, although no trend is observed in Zr and Zn abundances (Figure 3.12c and h). This negative correlation is mirrored in major element oxides TiO2, Al2O3, Fe2O3, MgO, Na2O and K2O (Figure 3.3a, b, c, e, g and h). Rb, Ba, Cr and V abundances increase with increasing Al2O3

concentrations in the biotite schist xenoliths (Figure 3.10a, b, c and d). Cr and V also increase in abundance with increasing Fe2O3 (Figure 3.13e and f) and Rb and Ba increase with increasing K2O (Figure 3.14c and d). Rb and Zr have negative trends with increasing Na2O (Figure 3.14a and b).

Other than for V, average trace element concentrations for the lower basement-derived mica schists do not tend to agree with the NASC and PAAS trace element compositions (Table 3.5).

Average Rb (215 ppm, NASC = 125 ppm; PAAS = 160 ppm) and Cr (136 ppm; NASC = 125 ppm; PAAS = 110 ppm) concentrations are higher while Sr (120 ppm; NASC = 142; PAAS = 200), Zr (163 ppm; NASC = 200; PAAS = 210), Ba (364 ppm; NASC = 636; PAAS = 650) and Ni concentrations (38 ppm; NASC = 58; PAAS = 55) are lower (Gromet et al., 1984; Taylor and McLennan, 1985).

The biotite schist xenoliths of the upper granite megablock show the most scatter with depth in terms of trace element abundance (Figure 3.11). Rb, Zr, V and Ni abundances in the biotite

schist xenoliths tend to decrease with depth (Figure 3.11a, c, f and g), similar to that seen in the major element oxides TiO2 and Fe2O3 (Figure 3.2b and d). Two outliers that occur in Sr, Ba and Cr abundances are samples CB6-078 and W39b (Figure 3.11b, d and e), and a single outlier in Zn is sample CB6-076 (Figure 3.11h).

Figure 3.12: Harker diagrams showing trace element (Rb, Sr, Zr, Ba, Cr, V, Ni and Zn) concentrations (in ppm) plotted against SiO2 for the eight target rocks units of the Eyreville-B borehole core. Abbr.: NASC = North American Shale Composite (after Gromet et al., 1984); PAAS = Post Archean Australian Shale (after Taylor and McLennan, 1985). Dashed lines represent observed groupings.

Figure 3.12 (continued): Harker diagrams showing trace element (Rb, Sr, Zr, Ba, Cr, V, Ni and Zn) concentrations (in ppm) plotted against SiO2 for the eight target rocks units of the Eyreville-B borehole core. Abbr.: NASC = North American Shale Composite (after Gromet et al., 1984); PAAS = Post Archean Australian Shale (after Taylor and McLennan, 1985). Dashed lines represent observed groupings.

Trace element abundances relative to SiO2 also show the most scatter in the biotite schist xenoliths (Figure 3.12), mirroring what is observed in Figure 3.11. The xenoliths show positive correlations in Sr and Ba (Figure 3.12b and d) and correlate negatively in Rb and V abundance with increasing SiO2 (Figure 3.12a and f). The biotite schist xenoliths tend to increase in V with

increasing Fe2O3 (Figure 3.13e and f), and Zr abundance with increasing Na2O (Figure 3.14b).

Negative correlations are between V and Al2O3 (Figure 3.10d), Cr and Fe2O3 (Figure 3.13e) and Rb and Na2O (Figure 3.14a).

Unlike the basement-derived mica schists, the biotite schist xenoliths do generally agree with NASC and PAAS trace element concentrations for Sr, Ba, V and Ni (Table 3.5). However, the Rb and Zr concentrations (342 and 427 ppm, respectively) are more than double the NASC (Rb

= 125 ppm; Zr = 200 ppm) and PAAS (Rb = 160; Zr = 210 ppm). Average Cr concentration (66 ppm) in the biotite schist xenoliths is half of that observed for the NASC (125 ppm) and PAAS (110 ppm) (Gromet et al., 1984; Taylor and McLennan, 1985). A single biotite schist xenolith sample (W36b) is the most enriched in Rb (507 ppm), V (318 ppm) and Ni (198 ppm) of all the mica schist samples studied (Figure 3.11a, f and g; see Appendix 2a).

3.4.2 Amphibolites and calc-silicate rock

Trace element abundances in the lower basement-derived amphibolite and calc-silicate show similar major element concentrations with depth, with Sr, Zr, V, Ni and Zn showing the most trends in the basement amphibolite, while Rb and Ba show positive trends with increasing K2O (Figure 3.14c and d).

The calc-silicate tends to have the opposite trend (or no trend at all) to that seen in the basement amphibolite, with the exception of Sr (Figure 3.12b). The calc-silicate tends to be more chemically associated, in terms of trace element abundance with increasing SiO2, to the upper amphibolite megablock than that of the lower basement-derived section (Figure 3.12).

This association can be seen in Figure 3.14c and d where Rb and Ba abundances are plotted

Figure 3.13: Harker diagrams showing trace element concentrations (in ppm) plotted against Al2O3

(Rb, Ba, Cr and V) and against Fe2O3 (Cr and V) for the eight target rocks units of the Eyreville-B borehole core. Abbr.: NASC = North American Shale Composite (after Gromet et al., 1984); PAAS = Post Archean Australian Shale (after Taylor and McLennan, 1985). Dashed lines represent observed groupings.

against K2O. Interestingly, the calc-silicate tends to be spatially associated with the basement amphibolite where trace elements have been plotted against other major element oxides (Figure 3.13 and 3.14).

Although samples from the upper amphibolite megablock show similar trace element concentrations to each other, there are no specific trends with depth (Figure 3.11) unlike those seen in the major element oxides (Figure 3.2). The upper megablock amphibolite shows more scatter in terms of trace element abundance (Figure 3.12) than that observed in the major (RG06) occurs as an outlier in Zr versus Na2O (Figure 3.14b).

3.4.3 Granites

The lower basement-derived granite typically does not show any trends in trace element concentrations with depth (Figure 3.10); however, a notable negative trend in Rb is observed below 1730 m depth (Figure 3.10a). The basement granite shows much scatter in trace element abundance with increasing SiO2 in comparison to the granite varieties of the upper granite megablock (Figure 3.12). Sr, Zr, Ni and Zn show the least scatter in the basement granite, with comparably low abundances (Figure 3.12b, c, g and h). The only significant trend of trace element abundance plotted against other major element oxides in the basement granite is that Rb tends to increase in abundance with increasing K2O (Figure 3.14c).

The massive and gneissic megablock granites show the least scatter in trace element concentrations with depth, with the exceptions of Zr and Ba (Figure 3.11c and d). A strong positive correlation in Ba with depth is observed in the gneissic granite (Figure 3.11d). Both the

Figure 3.14: Harker diagrams showing trace element concentrations (in ppm) plotted against Na2O (Rb, Zr and Cr) and against K2O (Rb and Ba) for the eight target rocks units of the Eyreville-B borehole core. Abbr.: NASC = North American Shale Composite (after Gromet et al., 1984); PAAS = Post Archean Australian Shale (after Taylor and McLennan, 1985). Dashed lines represent observed groupings.

massive and gneissic granites show similar trace element abundances, with increasing SiO2

and are enriched in Sr Zr and Ba compared to the basement granite (Figure 3.11b, c and d).

The gneissic megablock granite shows depletion in Zr and Ba abundance with SiO2 compared to the massive megablock granite (Figure 3.12c and d) as well as where Rb and Zr are plotted

against Na2O (Figure 3.14a and b) and Rb against K2O (Figure 3.14c). Both megablock granite rock units show a negative trend in Zr and Ba abundance with increasing SiO2 (Figure 3.12c and d) and a positive trend in Ba abundance with increasing Al2O3 (Figure 3.13b).