Читать книгу Earth Materials онлайн

100 страница из 155

1 Source rocks from which magmas are derived.

2 Origin of water on Earth's surface.

3 Timing of mountain building events involving igneous intrusions and metamorphism.

4 Timing of unroofing of such rocks and the dispersal of their erosional products by sedimentary agents.

5 Source rocks for petroleum and natural gas.

6 Changes in ocean water temperatures, biological productivity and circulation.

7 History of ice age glacial expansions and contractions.

8 Climate change.

Stable isotopes

Three isotopes of oxygen occur in Earth materials (ssss1): oxygen‐18 (¹⁸O), oxygen‐17 (¹⁷O), and oxygen‐16 (¹⁶O). Each oxygen isotope contains eight protons in its nucleus; the remaining mass results from the number of neutrons (10, 9, or 8 respectively) in the nucleus.¹⁶O constitutes >99.7% of the oxygen on Earth,¹⁸O constitutes ~0.2%, and¹⁷O is relatively rare. The ratio¹⁸O/¹⁶O is widely used to infer important information concerning Earth history.

3

2

¹⁸O/¹⁶O ratios are generally expressed with respect to a standard in terms of δ¹⁸O. One standard is the¹⁸O/¹⁶O ratio in a belemnite from the Cretaceous Pee Dee Formation of South Carolina, called PDB. δ¹⁸O is usually expressed in parts per thousand (mils) and calculated from: