M.S.A. PRESIDENTIAL ADDRESS: NEW TOOLS FOR OLD MINERALS

The oldest minerals known from Earth are zircons that crystallized from magma at 4.4 to 4.0 Ga. While zircon is a common trace mineral with advantageous crystal chemistry that is frequently exploited in petrologic and geochemical studies, the rarity, small grain size, and detrital history of the >4 Ga zircon suite presents special challenges. Microanalysis of these typically 100-300 micron crystals has pushed the limits of many techniques in order to obtain: images; isotope ratios; major and trace element compositions; Raman and IR spectra; and identification of inclusions. Although these data have spawned a wide range of interpretations about the Early Earth, a moderate conclusion is that stable crust started to form by 4.4 Ga (necessary to preserve zircons) and water oceans existed by 4.2 Ga (indicated by mildly elevated δ¹⁸O(Zrc) = 6.3-7.5 permil). Thus the fiery Hadean ("hell-like") conditions subsided earlier than previously thought, setting the stage for emergence of life nearly 400 Myr. before the earliest carbon isotope evidence and 700 Myr. before the first microfossils.

In situ microanalysis is required to measure oxygen isotope ratios in the >4 Ga detrital zircons that provide evidence for the end of Hadean conditions. The WiscSIMS CAMECA 1280 is a large radius, multicollector ion microprobe that analyzes δ¹⁸O (and δ¹⁷O) from ~10 micron spots in situ from a polished surface. One analysis of δ¹⁸O consumes a sample over one million times smaller than competitive laser fluorination techniques (ng vs. mg) and recently has achieved comparable precision and accuracy. CL and BSE images guide placement of analysis pits, allowing δ¹⁸O to be correlated to U-Pb age, inclusions, and trace element chemistry. All data can be from a single ~20 micron domain. This correlation is important for proper interpretation of small, zoned crystals like zircon.

The development of procedures for precise and accurate stable isotope analysis of minerals has many other applications for study of precious, small or zoned materials at 3 to 15 micron-scale in spot mode and at <1 micron in depth profile. Smaller spot sizes of 250 nm are possible for isotopically labeled experiments where high precision is unnecessary. New studies are possible in Earth, Materials, and Biological Sciences.