Two billion years of magmatism recorded from a single Mars meteorite ejection site

Abstract

The timing and nature of igneous activity recorded at a single Mars ejection site can be determined from the isotope analyses of Martian meteorites. Northwest Africa (NWA) 7635 has an Sm-Nd crystallization age of 2.403 ± 0.140 billion years, and isotope data indicate that it is derived from an incompatible trace element-depleted mantle source similar to that which produced a geochemically distinct group of 327- to 574-million-year-old "depleted" shergottites. Cosmogenic nuclide data demonstrate that NWA 7635 was ejected from Mars 1.1 million years ago (Ma), as were at least 10 other depleted shergottites. The shared ejection age is consistent with a common ejection site for these meteorites. The spatial association of 327- to 2403-Ma depleted shergottites indicates >2 billion years of magmatism from a long-lived and geochemically distinct volcanic center near the ejection site.

Keywords: Early Amazonian Magmatism; Mars Geochemistry; Martian Mantle; Martian Meteorite.

Fig. 1. False-color x-ray compositional map showing the mineralogy and mineral textures of NWA 7635.

Mineral labels: O, olivine; P, plagioclase (maskelynite); C, clinopyroxene (augite). Chemical compositions: Fe (purple), Mg (green), Ca (blue), Ti (magenta), and S (yellow). Purple colors in the mesostasis represent Fe-rich augite.

Fig. 2. Seven-point Sm-Nd isochron for NWA 7635 using an Isoplot model 1 solution (40).

A weighted average of four separate dilute leaches is calculated as one phosphate-dominated leachate measurement, and a re-integration of three hydrogen fluoride (HF)–based sequential dissolution fractions is calculated as one measurement of whole-rock residue (see the Supplementary Materials for details). MSWD, mean square weighted deviation. The inset shows the analytical uncertainty and scatter in epsilon units of individual points that define the isochron.

Fig. 3

(A) Source mixing array for shergottite Lu-Hf and Sm-Nd source compositions calculated using equations of Nyquist et al. (8) and mantle end-member compositions of Debaille et al. (13). Inset shows a best-fit mixing hyperbola for source Rb/Sr and Sm/Nd compositions of shergottites. Mixing depleted deep mantle (high Lu/Hf, high Sm/Nd, and low Rb/Sr) with 0 to 7% depleted shallower mantle material (low Lu/Hf, high Sm/Nd, and low Rb/Sr) and adding 0.5 to 12% enriched mantle material (low Lu/Hf, low Sm/Nd, and high Rb/Sr) can account for the compositions of depleted, intermediate, and enriched shergottites. NWA 7635 extends the observed range in depleted source compositions for all three isotopic systems. The source composition of ALH 84001 is not used in the regression or modeling but falls within error of the enriched end-member composition in Lu/Hf-Sm/Nd source space and on the mixing hyperbola for Rb/Sr-Sm/Nd source mixtures. Data sources are listed in table S3; ALH 84001 data are obtained from Lapen et al. (12) and Beard et al. (36). (B) Best-fit shergottite mixing line for long- and short-lived Sm-Nd isotope systems with 2σ error envelope. All data, except ALH 84001 and nakhlites (orange), are used in the regression. Average terrestrial standards define zero value for μ¹⁴²Nd (μ = 100 × ε). DS, depleted shergottites; IS, intermediate shergottites; ES, enriched shergottites. Data sources: black (3), green (11), orange (16), and blue (12).

Fig. 4. Summary of Mars ejection ages for depleted, intermediate, and enriched shergottites.

Data (with modified classification terminology) from Fig. 14 of Herzog and Caffee (41), with the addition of data from Wieler et al. (17) and this work. The group of 11 depleted shergottites with 1-My ejection ages (including NWA 7635) defines an average age of 1.1 ± 0.2 My (95% confidence) shown as the vertical gray box. These data show that only depleted shergottites were ejected at 1.1 Ma.