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Zircons Are Forever (figures)

John Valley, William H. Peck, Elizabeth M. King

New techniques for oxygen isotope analysis, developed at UW-Madison, have lead to the first studies of oxygen isotope geochemistry of zircons (1, 2). Zircon is shown to be highly retentive of oxygen isotope ratios, preserving the best record of igneous composition even in samples that have enjoyed (not suffered!) high grade metamorphism or hydrothermal alteration.

Zircon is a ubiquitous trace mineral (ZrSiO4) in many igneous, metamorphic, and clastic sedimentary rocks. Its ability to concentrate uranium and exclude lead forms the basis of U-Pb geochronology and its refractory nature and concentric growth patterns create robust records of crystallization age.

The ability to analyze oxygen isotope ratios in a sample that has been dated, directly links the magmatic composition to known geologic events and has opened many new and exciting avenues for study. Several recent graduate theses at UW have incorporated some aspect of zirconology: Carrie Gilliam (MS 1996) and Salma Monani (MS 1999) showed that shallowly intruded, sub-volcanic Tertiary granites from the Isles of Skye and Arran, Scotland formed by melting of anomalous rocks in the deep crust (3, 4). Liz King (MS 1997, PhD cand.) completed a comprehensive survey of Archean igneous rocks in Canada (5) that lead to a reevaluation of the genesis of one of the world’s richest base metal ore deposits (6). William Peck (MS 1996, PhD cand.) is studying anorthosite and related Proterozoic granitoids in the Grenville Province and has discovered oxygen isotope provinciality that appears to identify a cryptic continental suture and an accretionary plate margin that is now buried in the deep crust (7).

Another advantage of oxygen isotope studies of zircons stems from the fact that oxygen is the most abundant element in the crust. Thus oxygen is affected by different processes than trace elements or radiogenic isotopes that are commonly employed to study crustal growth and evolution. This difference is dramatically revealed by a comparison all existing data for zircons (8) from the Canadian Shield (Fig. 1). Archean age samples (2.7 to 3.0 Ga) have a very restricted range of d18O value (5.7±0.6‰) that is consistent with high temperature equilibrium with a primitive mantle reservoir (Fig. 2). In contrast, the Proterozoic samples from the Grenville show a much larger range and a higher average value (8.2±1.7‰). These anomalous values in the younger terrane result from processes that can only occur at the Earth’s surface: weathering and low temperature alteration. The high d18O signature of surfical processes must be buried in the form of sediments and altered rocks to near the base of the crust in order to become incorporated in granitic magmas by melting and assimilation. Thus, the deep crust beneath the Grenville Province is dramatically different from the more primitive, early crust of the Superior Province. The quantity of high d18O supracrustals is much smaller in the Archean. These results lead to speculation that the differences seen in N. America may be worldwide trends and that sedimentation may have been less voluminous and of a different character during early Earth history. The Wisconsin group is now actively studying zircons from many other terranes, including: less than 1Ma rhyolites at Yellowstone (Bindeman and Valley, unpbd), the world’s oldest known zircons from Australia (>4.3 Ga, Peck, Wilde and Valley, unpbd), and mantle-derived megacrysts from Kimberlite pipes (9). The results suggest that this trend is worldwide and that a major, non-uniformitarian change occurred either at the end of the Archean (2.7 Ga) or with the evolution of oxygen-rich atmospheres and weathering (~2 Ga). Since the zircons are forever, we have plenty of time to work this out.

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