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HADEAN ZIRCON ARE NOT FROM HELL: EVIDENCE FROM ATOM PROBE TOMOGRAPHY & SIMS

WEDNESDAY SEPTEMBER 11 – 4 PM

JOHN W. VALLEY
DEPT. OF GEOSCIENCES, UNIVERSITY OF WISCONSIN-MADISON

ABSTRACT: SIMS analysis of terrestrial zircons (ZrSiO4) yields U-Pb ages nearly as old as the Earth, but these Hadean Eon ages (4000 – 4400 Ma, million years ago) have been challenged as possibly biased by mobility of Pb atoms. Questions about “Pb-migration” have plagued U-Pb geochronology for over 100 years and were first resolved at atomic scale in 2014 by APT of a 4374 Ma zircon that shows clustering of radiogenic Pb. These new results show that unaltered domains in zircon with low degrees of radiation damage can be recognized, strengthening the interpretation, based on oxygen isotopes, that most of the Hadean Eon was not “hell-like” as commonly believed and implied by the name. The earliest Earth was indeed hot, violent and inhospitable at 4500 Ma, but by 4300 Ma it’s surface had cooled and the steam atmosphere condensed to form habitable oceans. Thus, it is possible that life emerged almost 1 billion years earlier than the oldest known microfossils.

BIO: John W. Valley is the Charles R. Van Hise Distinguished Professor of Geology at the Department of Geoscience, University of Wisconsin Madison, where he is also the chair of the WiscSIMS (Wisconsin Secondary Ion Mass Spectrometer) Laboratory. He was recently elected member of the National Academy of Sciences (2019) and received the Day Medal of the Geological Society of America (2019). The new mineral Valleyite (Ca4Fe6O13) was named after him to honour his contributions to geology and mineralogy. His research interests span many fields in Earth Science, including the igneous and metamorphic evolution of the crust during orogenesis, Precambrian geology and Early Earth, Astrobiology and new technologies for microanalysis of stable isotope ratios.