H and Cl isotope characteristics of indigenous and late hydrothermal fluids on the differentiated asteroidal parent body of Grave Nunataks 06128

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Abstract

The paired achondrites Graves Nunataks (GRA) 06128 and 06129
are samples of an asteroid that underwent partial melting within a few
million years after the start of Solar System formation. In order to
better constrain the origin and processing of volatiles in the early
Solar System, we have investigated the abundance of H, F and Cl and the
isotopic composition of H and Cl in phosphates in GRA 06128 using
secondary ion mass spectrometry. Indigenous H in GRA 06128, as recorded
in magmatic merrillite, is characterised by an average dD of ca. -152 ±
330‰, which is broadly similar to estimates of the H isotope composition
of indigenous H in other differentiated asteroidal and planetary bodies
such as Mars, the Moon and the angrite and eucrite meteorite parent
bodies. The merrillite data thus suggest that early accretion of locallyderived
volatiles was widespread for the bodies currently populating the
asteroid belt. Apatite formed at the expense of merrillite around 100
million years after the differentiation of the GRA 06128/9 parent body,
during hydrothermal alteration, which was probably triggered by an impact
event. Apatite in GRA 06128 contains 5.4-5.7 wt.% Cl, 0.6-0.8 wt.% F, and
~20 to 60 ppm H2O, which is similar to the H2O abundance in merrillite
from which apatite formed. The apatite dD values range between around
+100‰ and +2000‰ and are inversely correlated with apatite H2O contents.
The Cl isotope composition of apatite appears to be homogeneous across
various grains, with an average d37Cl value of 3.2 ± 0.7‰. A possible
scenario to account for the apatite chemical and isotopic characteristics
involves interaction of GRA 06128/9 with fumarole-like fluids derived
from D- and HCl-rich ices delivered to the GRA 06128/9 parent-body by an
ice-rich impactor.

Bibliographical metadata

Original languageEnglish
JournalGeochimica et Cosmochimica Acta
Early online date25 Jan 2019
DOIs
Publication statusPublished - 2019

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