[IGPP Everyone] [EPSS Everyone] Geocheminar this Friday

[Kevin McKeegan]

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*GEOCHEMINAR*

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*Dr. Beth Ann Bell*

*UCLA*


*Zircon alteration: the good, the bad, and the ugly*

Owing to both its ability to record U-Pb ages and its mechanical and
chemical resilience, zircon is the premier crustal geochronometer in most
geologic settings.  Its ability to survive multiple sedimentary cycles
makes it a key tracer for continental evolution and indeed, zircon is to
date the only material known to survive from Earth's first 500 million
years.  However, zircon's ability to serve as a "time capsule" preserving
information about this period is only commensurate with its ability to
resist later alteration, and radiation damage can render it susceptible to
recrystallization or hydrothermal alteration.  By examining zircon from
known geologic settings with well-constrained tectonothermal histories, we
are able to establish patterns of alteration affecting many of the lines of
evidence useful for constraining continental evolution: zircon trace
element chemistry and the cargo of mineral inclusions which most zircons
contain.  Using the Jack Hills detrital zircons, we have developed textural
criteria for recognizing altered inclusion assemblages, and using zircon
from a suite of recent granites, we can further constrain the frequency of
inclusion alteration and telltale signs.   We have also used the Jack Hills
zircons to develop an index for chemical alteration of zircon.  Application
to Phanerozoic granite zircon reveals its general applicability.
Alteration to zircon trace chemistry and mineral inclusions appears to
usually be identifiable either texturally or chemically, respectively.
Secondary features in zircon can in some circumstances yield their own
useful information -- for example, secondary inclusions which are amenable
to U-Pb dating, when examined on a 5-10 micron scale, provide a chronicle
of the metamorphic events undergone by their host zircons throughout their
residence in the crust.

*3853 Slichter (conference room)*


*Friday, October **27th **at 12:00*
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Kevin D. McKeegan
Professor of Cosmochemistry & Geochemistry
Dept. of Earth, Planetary, and Space Sciences
UCLA
Los Angeles, CA 90095-1567
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mckeegan at epss.ucla.edu
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