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THE NAKHLITES SAMPLE MULTIPLE IGNEOUS UNITS : EVIDENCE FROM 40 Ar / 39 Ar CHRONOLOGY AND
| Content Provider | Semantic Scholar |
|---|---|
| Author | Cohen, Benjamin E. Mark, Darren F. Lee, Martin R. Cassata, William S. Griffin, Sammy Cohen Smith Tomkinson |
| Copyright Year | 2018 |
| Abstract | Introduction: Mars hosts the solar system’s largest volcanoes. Here we present Ar/Ar ages of six nakhlites, meteorites that were ejected from Mars by a single impact event. These chronologic results were recently published in Nature Communications [1]. These age data can also be used to inform the search for the nakhlite source crater, and through the geochemistry of the nakhlite meteorites, to show how the magmatic system evolved through time. If the location of the nakhlite source crater can be confirmed, the chronological data for the nakhlites (cosmogenic exposure age for the crater, and age of the source terrain lavas) will also provide an ‘absolute’ temporal anchor for the surface of Mars that could underpin a Martian crater counting calibration model, while the chemical data could provide a useful ground-truth point for remote-sensing studies. Methods: High-resolution (n43–45 steps) laser step-heating Ar/Ar dating was undertaken on multiple aliquots (n2–5) of six nakhlites [1]. Cosmogenic (Ar) exposure ages were also obtained [1]. The cosmogenic exposure ages allow for accurate correction of cosmogenic and chlorine-derived contributions to the bulk isotope measurements and for determination of accurate Ar/Ar age data [2]. The REE data were compiled from published sources. Results: Analyses of unirradiated fragments of each of the six nakhlites yielded concordant cosmogenic exposure ages, and a weighted mean of 10.7 ± 0.8 Ma (2σ). These indistinguishable cosmogenic exposure ages confirm that the nakhlites are launch-paired, i.e., ejected from Mars in a single impact event. Ar/Ar ages reveal that the nakhlites sample a stratigraphically layered volcanic sequence, and temporally constrain at least four discrete eruptive events spanning 93 ± 11 Ma (1,416 ± 7 Ma to 1,322 ± 9 Ma [2σ], Fig. 1a). Our robust Ar/Ar results are a product of the high-resolution approach to the step-heating experiments, and employment of procedures to make appropriate corrections for cosmogenic and chlorinederived Ar and Ar [2] – which is important given the abundance of chlorine in the nakhlites (often >1,000 ppm Cl [3]). The highly reproducible age data (Fig. 1a) are consistent with the near-pristine character of the meteorites and associated low degrees of shock metamorphism [4]. Discussion: Our stratigraphic model is consistent with the geology of Martian volcanoes. Highresolution satellite imagery has revealed sequences of lava flows, with individual layers typically 4–26 m thick [5]. Our model of a layered volcanic sequence (Fig. 1) differs from a previous interpretation of the nakhlites that invoked sampling from a single thick flow/intrusive unit [e.g., 6]. Such a model would require all of the nakhlites to have the same cooling age, which is inconsistent with our Ar/Ar data (Fig. 1). |
| File Format | PDF HTM / HTML |
| Alternate Webpage(s) | https://www.hou.usra.edu/meetings/lpsc2018/pdf/1892.pdf |
| Language | English |
| Access Restriction | Open |
| Content Type | Text |
| Resource Type | Article |
