Isotopic and elemental abundances of copper and zinc in lunar samples, Zagami, Pelé’s hairs, and a terrestrial basaltстатья
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Аннотация:We used ICP–MS to measure the elemental concentrations and isotopic abundances of Cu and Zn in: nine Ti-rich lunar
basalts (10017, 10022, 10024, 10057, 70215, 71055, 74255, 75055, and 75075); size-separated samples prepared by sieving of
pyroclastic black glass 74001, orange glass 74022, and the lunar soils 15021, 15231, 70181, and 79221; a basalt from the Piton
des Neiges volcano, Reunion Island; two samples of Pele’s hairs from the Nyiragongo volcano, Democratic Republic of Congo,
and the martian meteorite Zagami.
The isotopic fractionation of zinc in lunar basalts and Zagami is mass dependent relative to a terrestrial standard (JMC
400882B). These and published results imply that lunar, terrestrial, meteoritic, and perhaps martian zinc all come from one or
more reservoirs linked by mass-dependent fractionation processes. Relative to terrestrial basalts, Ti-rich lunar basalts are
enriched in the heavier isotopes of Cu and Zn: we find for Ti-rich lunar basalts the following ranges and averages ±1 r
(&): d65Cu/63Cu d65Cu, 0.1–1.4, 0.5 ± 0.1& (N = 7); d66Zn/64Zn d66Zn = 0.2–1.9, 1.2 ± 0.2& (N = 8; 10017 excluded).
For two terrestrial samples, we find d66Zn +0.3& and d65Cu 0&, which are consistent with published values. The differences
between the lunar basalts and terrestrial basalts could reflect minor, planetary-scale vaporization or igneous processes
on the Moon.
Data for size separates of the pyroclastic glasses 74001 and 74220 confirm the well-known surface correlation of Cu and
Zn, but modeling calculations reveal no sharp differences between either the elemental ratios or the isotopic composition of
grain interiors and exteriors. The absence of such differences indicates that the isotopic compositions for bulk samples are
dominated by a light-isotope-rich surface component.
Data for size separates of lunar soils also confirm the surface correlation of Cu and Zn, but an enrichment of heavy rather
than light isotopes. Averages for bulk lunar soils from this work and the literature are (&): d65Cu, from 1.4 to 4.1, average
3.0 ± 0.3 (N = 9); d66Zn, from 2.2 to 6.4, average 4.0 ± 0.3 (N = 14). As with the glasses, in all but soil 15231 our data show
no strong differences between the isotopic composition of soil sub-samples with small and large grains.
The size of the isotopic fractionation inferred for the surface component in the soils is 3 smaller than predicted by a published
model of sputtering primarily by solar particles. At the same time, the observed fractionation is larger than predicted by
calculations based on a model of micrometeorite impact heating and hydrodynamic quenching. Because impact heating
appears unable to explain the observations, we conclude that sputtering must be important even though samples with very
large isotopic fractionation of Cu and Zn have not yet been found.