HR: 1330h
AN: V32C-1031 [PDF]
TI: (U-Th)/He Dating of Merrillites and Chlorapatites From Martian Meteorite Los Angeles
AU: * Min, K K
EM: kyle.min@yale.edu
AF: Dept of Geology & Geophysics, Yale University, P.O. Box 208109, New Haven, CT 06511 United States
AU: Reiners, P W
EM: peter.reiners@yale.edu
AF: Dept of Geology & Geophysics, Yale University, P.O. Box 208109, New Haven, CT 06511 United States
AU: Nicolescu, S
EM: stefan.nicolescu@yale.edu
AF: Dept of Geology & Geophysics, Yale University, P.O. Box 208109, New Haven, CT 06511 United States
AU: Greenwood, J P
EM: james.greenwood@yale.edu
AF: Dept of Geology & Geophysics, Yale University, P.O. Box 208109, New Haven, CT 06511 United States
AB:
Los Angeles (LA), one of the basaltic shergottitic Martian meteorites, has an igneous formation age of $\sim$170 Ma, and is
thought to have been ejected from Mars by collision at $\sim$3.1 Ma. The medium to high temperature thermal history of LA is
documented based on Rb/Sr and Sm/Nd chronometers, but essentially no thermochronologic data are available to constrain its
low temperature thermal history. To understand the thermal effects of collision-related shock metamorphism and subsequent
low-T thermal history of LA, we measured (U-Th)/He ages of single crystal phosphates (igneous merrillite and chlorapatite)
from the meteorite.
Merrillites and chlorapatites yielded distinctive U and Th concentrations ($\sim$2 ppm and $\sim$13 ppm for merrillites;
$\sim$4 ppm and $\sim$5 ppm for chlorapatites) with apparently different U/Th ratios. Thirteen merrillite grains have ages
between 0.8-3.5 Ma with a weighted mean age of 3.1 Ma. Five other merrillite ages are widely distributed between 5.1-32 Ma.
Six chlorapatite grains yielded ages clustering in the range of 1.9-2.9 Ma (alpha-recoil uncorrected) with a weighted mean
of 2.2 Ma, and three other grains produced older and scattered ages up to 5.8 Ma. The cumulative probability plots show
highest peaks at 2.3 Ma and 3.3 Ma for chlorapatites and merrillites, respectively. The peak age or weighted mean age of
merrillite is indistinguishable from the cosmogenic exposure ages (3.1 $\pm$ 0.2 Ma) previously determined from $^{3}$He,
$^{10}$Be, $^{21}$Ne, $^{38}$Ar and $^{81}$Kr systems, suggesting that the timing of the shock metamorphism is represented by
the merrillite (U-Th)/He system. The average age difference between the merrillite and chlorapatite ages is probably due to
the typically smaller grain size of apatite crystals and therefore greater proportion of alphas lost by recoil ejection.
Assuming a peak shock temperature of 450 $\deg$C (consistent with previous studies), the observed complete He degassing for
most of the phosphates by shock metamorphism suggests that the radius of the original body ejected from Mars was probably
larger than $\sim$100 cm. The few old and variable ages observed may be due to exceptionally large ($>$600 micrometers)
phosphate grains found in the LA, or incomplete He degassing during the shock metamorphism caused by heterogeneous heating as
revealed in the other Martian meteorites. This study shows that phosphate (U-Th)/He thermochronometry can be used to
understand the timing and dynamics of collision-related shock metamorphic processes in other meteorites, such as ALH84001.
DE: 1035 Geochronology
DE: 1040 Isotopic composition/chemistry
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2003 Fall Meeting