Quartz Oxygen Isotopic Stability in Relation to Isolation from Sediments and Diversity of Source
- K. Sridhar,
- M. L. Jackson and
- R. N. Clayton
The NaOH boiling treatment to remove amorphous silica relics in the quartz isolation procedure is unnecessary because the subsequent H2SiF6 treatment dissolves both the amorphous silica relics and feldspars. Washing with 0.1N HF, water, and saturated boric acid in the new procedure removed fluorates that precipitate during the H2SiF6 treatment. Failure to remove the fluorates depressed the oxygen yield (to as much as 85%) of theoretical SiO2 and slightly raised the quartz weight percent, but the oxygen isotopic ratio of quartz derived from various sources was not affected by these washings, by Na2S2O7 fusion, or by H2SiF6 treatment.
When 35 to 45% of the outer shells of the 1–3.5, 3.5–7, and 7–10 µm quartz size fractions from Cretaceous (Pierre) marine shales was dissolved with 3N HF, the quartz δ18O decreased by 0.8, 2.1, and 2.6 o/oo for a central-basin shale and 0.7, 1.2, and 0.8 o/oo for a near-shore shale, respectively. The trend, the greater the particle size the greater the δ18O decrease, is the opposite of that expected (finer particles, higher specific surface) if the δ18O decrease by HF treatment is a result of dissolving an isotopically exchanged portion and/or overgrowths in outer shells. The decrease appeared by scanning electron microscopy (SEM) to result from preferential dissolution of the cements holding fine quartz crystals (formed at low temperature) together as silt-size clusters. A systematic increase in δ18O with decreasing grain size occurred in the quartz from the near-shore shale (> 10 µm quartz, 16.1 o/oo; < 1 µm quartz, 21.8 o/oo). The increase was 6.7 o/oo from the central-basin shale (> 10 µm quartz, 18.8 o/oo; < 1 µm quartz, 25.5 o/oo). The wide variation appears, from quartz particle morphology (SEM) and the response to partial dissolution, to result mainly from a mixing within each size fraction of different proportions of individually homogeneous quartz grains formed at various temperatures. Because of quartz oxygen isotopic stability, the isotopic ratio can be reliably used to trace source or provenance of fluvial and eolian sediments and soil parent materials.Please view the pdf by using the Full Text (PDF) link under 'View' to the left.
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