Capítulo IV: Evaluación de la implementación
4.3. Relación de los estudiantes con el conocimiento
5.4.3 Detrital monazite geochronology
Detrital monazites were separated from a medium-grained quartz sandstone near Sulphur Creek (Fig. 5.3) using standard crushing, milling, and separation techniques. U-Pb isotopes were measured via LA-ICPMS using the same instrumental and analytical procedures used for detrital zircon analysis. Placement of 29 µm analysis spots was guided by high-resolution BSE images to avoid inclusions, cracks, and overlapping of distinct chemical or textural zones. Downhole fractionation, instrument drift, and mass bias corrections were made using the primary standard 14971 Monazite (in-house standard) and secondary standards RGL4B (Rubatto et al., 2001) and Banaeira (Gonçalves et al., 2016). Full analytical results are presented in Appendix 5.5
5.5 Results
5.5.1 U-Pb apatite geochronology
Apatite in the Cooee Dolerite occurs as pristine, euhedral crystals, up to 1 mm long that are intergrown with magmatic phases including pyroxene, plagioclase, and hornblende (Figs. 5.5B and 5.5C). The apatite grains typically display faint simple zoning in BSE images (inset, Fig. 5.5C) and are interpreted to be a magmatic phase.
Apatite can incorporate significant amounts of common Pb in its crystal structure, which can result in the calculation of discordant U-Pb ages (e.g., Chew et al., 2011). Time-resolved analysis of U and Pb isotopes of apatite analysed during this study indicates that common Pb is heterogeneously distributed both within the ablation volume of individual analyses and throughout the population of grains analysed. To better visualise the full spectrum of mixing between common Pb and radiogenic Pb within the population of grains analysed, each analysis is split into 8 equal-time segments and shown on the Tera-Waserburg Plot in Figure 5.7. This approach has the benefit of increasing the dispersion of data points along the common Pb-radiogenic Pb mixing line, which improves the precision of the calculated age (e.g., Davidson et al., 2007; Kamenetsky et al., 2016). Data from 241 splits derived from 38 individual apatite grains show variable mixing of radiogenic and common Pb and define a discordia array with a lower intercept age of 733 ± 9 Ma (Fig. 5.7). Using the average 207Pb/206Pb and 238U/206Pb composition of analysis spots yields a similar intercept age of 736 ± 14 Ma that is also consistent with our interpretations (inset, Fig. 5.7).
0.0 0.2 0.4 0.6 0.8 1.0 1.2 0 2 4 6 8 10 12 207
Pb/
206Pb
238U/
206Pb
Intercepts at Upper: 4860 ± 47 Ma Lower: 736 ± 14 Ma MSWD = 2.7 n=38 Intercepts at Upper: 4841 ± 16 Ma Lower: 733 ± 9 Ma MSWD = 1.4 n=241Apatite U-Pb splits
Apatite U-PbAverage ages
Figure 7
Figure 5.7: Terra-Wasserberg plots of U-Pb data from magmatic apatite from the Cooee Dolerite. Main diagram shows U-Pb composition of analyses split into 8 segments of equal time, inset shows average U-Pb composition for each analysis. Error bars are 1 sigma.
5.5.2 Detrital zircon geochronology
A total of 175 detrital zircons from the Oonah Formation with analyses between 80% and 105% concordant are presented as a probability distribution diagram in Figure 5.8, which was constructed using Isoplot version 3.1 (Ludwig, 2003). For grains older than 1000 Ma the 207Pb/206Pb ages are plotted in Figure 5.8 whereas 207Pb corrected 238U/206Pb ages are used for grains younger than 1000 Ma (c.f. Gehrels et al., 2008). The majority of detrital zircons analysed during this study have Paleoproterozoic ages (75% of total analyses) forming age peaks between 1800 and 1700 Ma. Archean detrital zircons include ages between 3100 and 2500 Ma and form a small age peak at 2990 Ma (3 grains). Mesoproterozoic ages comprise 16% of the new data set and form age peaks at 1590 Ma and 1435 Ma. The youngest detrital zircon has a moderately concordant (85%) age of 728 ± 10 Ma. Reanalysis of this grain in a separate analytical session produced a similar moderately concordant (89%) age of 723 ± 6 Ma. Combining the most concordant intervals of this grain from both analytical sessions produces a 207Pb corrected 206Pb/238U age of 730 ± 8 Ma. Two additional Neoproterozoic detrital zircons analysed during this study have ages of 738 ± 11 Ma (94% concordant) and 767 ± 8 Ma (100% concordant) respectively.
New detrital zircon data from the Forest Conglomerate and Quartzite includes 183 analyses with ages between 80% and 105% concordant (Fig. 5.9). The detrital zircons from the Forest Conglomerate and Quartzite include abundant Paleoproterozoic grains with ages between 1800 and 1600 Ma (80% of total population), which form a prominent age peak at 1790 Ma. Archean ages comprise 6% of the total dataset and form a scatter of small age peaks between 2800 and 2500 Ma. Mesoproterozoic detrital zircons include 3 analyses at ca. 1550 Ma and a cluster of ages between 1490 and 1415 Ma (13% of total analyses). Neoproterozoic detrital zircons from the Forest Conglomerate and Quartzite include two grains with ages of 725 ± 7 Ma (98% concordant) and 729 ± 8 Ma (97% concordant) respectively.
5.5.3 Detrital monazite geochronology
Detrital monazites from the Oonah Formation are mostly rounded or subhedral grains between 20 and 100 μm that typically have homogenous or faint patchy zoning in BSE images. All 51 detrital monazite grains analysed fall within the concordance limits used in this study (80—105% concordant), with 70% of the analysed grains having ages between 95% and 105% concordant. The probability distribution plot of detrital monazite 207Pb corrected 206Pb/238U ages is dominated by a large age peak at 750 Ma, which includes 49% of the total analyses (Fig. 5.9). The remainder of the detrital monazite dataset have mostly <1400 Ma ages (47% of total analyses) that form age peaks at 1270 Ma, 1220 Ma, 1170 Ma, and 1065 Ma (Fig. 5.9). Older grains include individual analyses at 1630 Ma and 1410 Ma.
5.6 Discussion