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Bravo-Y mudstone samples were analysed for their major and trace elemental composition. Complete major and trace element concentrations of the individual mudstone samples along with the ratios of selected pairs of major and trace elements and calculated weathering indices can be found in Appendix IV. Overall, in accord with the consistent bulk mineralogy major and trace elements show relatively little variability among the samples.

The amount of SiO2 varies between 48.88 and 62.83% (average of 55.65%), Al2O3 varies

between 12.38 and 18.48% (average of 15.78%), TiO2 varies between 0.59 and 0.79% (average of 0.71%), Fe2O3 varies between 5.41 and 7.18% (average of 6.28%), MnO varies

between 0.08 and 0.14% (average of 0.11%), MgO varies between 1.22 and 2.09% (average of 1.57%), CaO varies between 0.52 and 2.05% (average of 0.88%), Na2O varies

between 0.5 and 0.94% (average of 0.68%), K2O varies between 1.93 and 2.93% (average

of 2.49%) and P2O5 varies between 0.08 and 0.12% (average of 0.10). Total iron is

expressed as Fe2O3. The anomalously high loss on ignition (LOI) values of the samples

XM1 and XM3 can be attributed to the presence of organic rich, coaly material in the cutting samples. Further evidence is presented in Table 4.4 which shows strong correlation between the measured LOI and TOC values.

115

Figure 4.16. SiO2-Al2O3-TOC ternary diagram showing mixing relationships of the main sediment

components in Bravo-Y samples (red dots). Green and brown stars indicate the average compositions of smectite and illite respectively; the field outlined by dashed lines represents the compositional variation of illites and smectites (based on Weaver, 1989).

It should be noted that due to the heterogenous nature of the provided Bravo-Y mudstone cuttings these samples are not truly representatives of “pure”, homogeneous mudstones. Hence these results only represent an average composition of the fine- grained lithologies present in the sampled depth intervals. Figure 4.16 was constructed to show the mixing relationships of the main sediment components in Bravo-Y samples. The relatively high SiO2/Al2O3 ratios (average of 3.6) of the samples (considering that the

dominant aluminosilicate phase is illite) can be attributed to the relatively high detrital quartz content.

Correlation coefficient values for major elements are given in Table 4.4. Al2O3 shows good

positive correlations with K2O and TiO2. This indicates that both potassium and titanium

reside primarily in clay minerals and are dominantly terrigenous components. K2O and

TiO2 also show weak positive correlation with SiO2 that can be attributed to small

amounts of detrital feldspar and anatase. Positive correlation between Fe2O3 and Al2O3

indicates that iron was transported to the basin with clay minerals either as primary constituents in the crystal structure or on crystal surfaces as oxides and oxyhydroxides

116 (Carroll, 1958; Hofstetter et al., 2003). MnO shows positive correlation with Fe2O3

indicating that Mn primarily resides in siderite. Good correlation between CaO and MgO can be attributed to the presence of dolomite. Correlation between K2O and MgO can be

attributed to the presence of chlorite.

Values of K2O/Al2O3 ratio of the Bravo-Y samples show very little variation between 0.14

and 0.18 (average of 0.16). These values are consistent with the preponderance of clay minerals (illite) over K-feldspar in these samples (Cox et al., 1995).

Similarly values of Si/Al, K/Al and Na/Al show relatively little variation and correlate well with QXRD established mineral abundances (Figure 4.18). In accordance with an illite dominated clay mineralogy Na/Al shows strong correlation with the plagioclase content. This indicates that sodium resides almost exclusively in plagioclase feldspar (Figure 4.18e, f).

Calculated Index of Compositional Variability (ICV) values of the samples range from 0.72 to 0.94. Calculated Chemical Index of Alteration (CIA) and Chemical Index of Weathering (CIW) values range from 72 to 79 and 90 to 96 respectively. Calculated CIA and CIW values show strong positive correlation indicating that the two indices are matchable (Figure 4.17).

Figure 4.17. Strong positive correlation between calculated Chemical Index of Alteration (CIA) and Chemical Index of Weathering (CIW) values of Bravo-Y mudstones.

117

Figure 4.18. Variation of (a) Si/Al, (c) K/Al and (e)Na/Al with depth in comparison with the variation of (b) quartz, (d) illite-smectite and (f) plagioclase content.

118

SiO2 Al2O3 Fe2O3(T) MnO MgO CaO Na2O K2O TiO2 P2O5 LOI TOC

SiO2 1.00 Al2O3 0.06 1.00 Fe2O3(T) -0.31 0.64 1.00 MnO -0.43 0.09 0.66 1.00 MgO -0.11 0.53 0.11 -0.46 1.00 CaO 0.06 0.11 -0.20 -0.49 0.74 1.00 Na2O -0.28 -0.05 -0.12 -0.37 0.65 0.39 1.00 K2O 0.32 0.85 0.29 -0.34 0.74 0.45 0.09 1.00 TiO2 0.52 0.78 0.41 0.07 0.12 0.03 -0.54 0.72 1.00 P2O5 -0.44 0.70 0.93 0.61 0.35 0.06 0.01 0.40 0.38 1.00 LOI -0.77 -0.62 -0.09 0.38 -0.42 -0.36 0.09 -0.82 -0.80 -0.10 1.00 TOC -0.62 -0.72 -0.20 0.31 -0.54 -0.48 0.04 -0.87 -0.81 -0.26 0.97 1.00

119

SiO2 Al2O3 CaO TiO2 Sc V Ba Sr Y Zr Cr Co Ni Zn Rb Nb Pb Th U

SiO2 1.00 Al2O3 0.06 1.00 CaO 0.06 0.11 1.00 TiO2 0.52 0.78 0.03 1.00 Sc 0.05 0.91 0.38 0.68 1.00 V 0.03 0.91 0.43 0.62 0.97 1.00 Ba -0.26 0.49 0.70 0.05 0.64 0.69 1.00 Sr -0.08 0.38 0.54 0.14 0.64 0.59 0.77 1.00 Y 0.46 0.39 -0.53 0.70 0.18 0.06 -0.35 -0.16 1.00 Zr 0.92 -0.10 -0.14 0.43 -0.18 -0.22 -0.46 -0.36 0.57 1.00 Cr -0.75 -0.09 0.06 -0.46 0.03 0.09 0.25 0.31 -0.64 -0.86 1.00 Co -0.21 0.56 0.26 0.17 0.75 0.68 0.69 0.82 0.00 -0.40 0.22 1.00 Ni -0.31 0.29 0.05 0.10 0.52 0.42 0.23 0.65 -0.10 -0.49 0.57 0.71 1.00 Zn -0.31 0.26 0.08 0.14 0.27 0.20 0.33 0.42 0.03 -0.31 0.21 0.32 0.38 1.00 Rb 0.18 0.90 0.43 0.72 0.96 0.98 0.60 0.56 0.12 -0.08 0.03 0.59 0.41 0.22 1.00 Nb 0.62 0.58 0.17 0.82 0.50 0.50 0.11 0.23 0.48 0.40 -0.35 0.00 -0.06 -0.03 0.61 1.00 Pb -0.15 0.51 0.06 0.43 0.40 0.45 0.34 0.22 0.11 -0.18 0.37 0.10 0.18 0.57 0.53 0.33 1.00 Th 0.31 0.89 0.01 0.96 0.80 0.74 0.17 0.21 0.64 0.22 -0.31 0.35 0.25 0.17 0.80 0.69 0.48 1.00 U 0.70 0.41 -0.03 0.77 0.26 0.20 -0.20 -0.25 0.76 0.76 -0.89 -0.15 -0.41 -0.20 0.28 0.60 -0.05 0.66 1.00

120

SiO2 Al2O3 CaO TiO2 La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb

SiO2 1.00 Al2O3 0.06 1.00 CaO 0.06 0.11 1.00 TiO2 0.52 0.78 0.03 1.00 La 0.60 0.73 0.07 0.98 1.00 Ce 0.61 0.71 0.03 0.97 1.00 1.00 Pr 0.59 0.73 0.04 0.99 0.99 0.99 1.00 Nd 0.54 0.72 0.02 0.96 0.99 0.99 0.98 1.00 Sm 0.55 0.73 -0.11 0.97 0.95 0.95 0.97 0.93 1.00 Eu 0.43 0.07 -0.41 0.46 0.52 0.58 0.54 0.56 0.56 1.00 Gd 0.33 0.75 -0.05 0.96 0.91 0.90 0.92 0.90 0.93 0.47 1.00 Tb 0.54 0.52 -0.09 0.86 0.89 0.91 0.91 0.92 0.90 0.76 0.83 1.00 Dy 0.67 0.41 -0.20 0.80 0.84 0.87 0.85 0.86 0.82 0.76 0.76 0.89 1.00 Ho 0.62 0.51 -0.13 0.85 0.87 0.89 0.88 0.90 0.84 0.57 0.81 0.87 0.95 1.00 Er 0.52 0.58 -0.17 0.88 0.91 0.93 0.91 0.94 0.86 0.72 0.86 0.91 0.96 0.94 1.00 Tm 0.68 0.64 -0.08 0.95 0.96 0.97 0.97 0.95 0.96 0.60 0.88 0.92 0.93 0.95 0.93 1.00 Yb 0.74 0.51 -0.10 0.91 0.92 0.94 0.93 0.90 0.91 0.61 0.86 0.86 0.91 0.92 0.90 0.96 1.00

121 Correlation coefficient values for selected trace elements and rare earth elements (REE) are shown in Table 4.5 and 4.6 respectively. Among the analysed trace elements Sc, V, Rb, Nb and Th show strong positive correlation with Al2O3 indicating that their closely related

to clay minerals either as primary constituents or as adsorbed components on clay mineral surfaces. Zirconium and uranium show strong positive correlation with SiO2

indicating the presence of zircon. Yttrium and niobium shows positive correlation with TiO2 and are likely associated with anatase. Strontium positively correlates with CaO and

is most likely related to carbonate (calcite and dolomite). Chromium, cobalt and nickel are most likely associated with mafic mineral components. Rare earth elements show strong positive correlations with TiO2 and Al2O3 indicating that their mostly associated with clay

minerals.

Th/Sc ratios of the Bravo-Y samples range from 0.98 to 1.20 (average of 1.08). Zr/Sc ratios range from 8.93 to 17.53 (average of 10.89). La/Sc ratios of the samples range from 2.3 to 3.03 (average of 2.55). La/Th ratios range from 2.24 to 2.50 (average of 2.35). Zr/Cr ratios range from 0.91 to 3.8 (average of 1.64) and Y/Ni ratios range from 0.57 to 1 (average of 0.72).

4.6.5 Clay mineral fabric alignment

Quantitative assessment of the phyllosilicate alignment of the Bravo-Y mudstone samples was carried out using high-resolution X-ray textural goniometry (HRXTG) as described in 3.3.10.

Ideally the degree of preferred orientation is measured in a bedding-parallel direction. However, due to the inherent uncertainty of mudstone cuttings (often no indication where the bedding is), most of the results were considered erroneous. Features such as colour variations or preferred breakage were used to estimate the bedding direction. Figure 4.19 shows the results considered least erroneous.

Complete HRXTG data of individual samples (including the chosen 2theta angles) can be found in Appendix V.

122

Figure 4.19. Measured clay mineral fabric anisotropy values of Bravo-Y mudstone samples. Representative illite/mica and chlorite/kaolinite pole figures are displayed to the right. The letters of these samples are shown on the left figure.

The results shown in Figure 4.19 indicate moderate/strong alignment, even in the shallowest sample (XM2; 1515 m below seafloor). Even if we ignore possible error (real MRD values may be higher), overall these results indicate moderate to strong alignment in Bravo-Y mudstones.