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Chronology of Martian Breccia NWA 7034 and the Formation of the Martian Crustal Dichotomy

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Chronology of Martian Breccia NWA 7034 and the Formation of the Martian Crustal Dichotomy

William S Cassata et al. Sci Adv.

Abstract

Martian meteorite Northwest Africa (NWA) 7034 and its paired stones are the only brecciated regolith samples from Mars with compositions that are representative of the average martian crust. These samples therefore provide a unique opportunity to constrain the processes of metamorphism and alteration in the martian crust, which we have investigated via U-Pu/Xe, 40Ar/39Ar, and U-Th-Sm/He chronometry. U-Pu/Xe ages are comparable to previously reported Sm-Nd and U-Pb ages obtained from NWA 7034 and confirm an ancient (>4.3 billion years) age for the source lithology. After almost 3000 million years (Ma) of quiescence, the source terrain experienced several hundred million years of thermal metamorphism recorded by the K-Ar system that appears to have varied both spatially and temporally. Such protracted metamorphism is consistent with plume-related magmatism and suggests that the source terrain covered an areal extent comparable to plume-fed edifices (hundreds of square kilometers). The retention of such expansive, ancient volcanic terrains in the southern highlands over billions of years suggests that formation of the martian crustal dichotomy, a topographic and geophysical divide between the heavily cratered southern highlands and smoother plains of the northern lowlands, likely predates emplacement of the NWA 7034 source terrain-that is, it formed within the first ~100 Ma of planetary formation.

Figures

Fig. 1
Fig. 1. Chronometric ages and the history of NWA 7034.
(A) Summary of NWA 7034 radiometric ages. The horizontal widths of the boxes are defined by the maximum and minimum ages obtained from a given method plus or minus the associated 2σ uncertainties, respectively. U-Th-Sm/He, 40Ar/39Ar, and U-Pu/Xe ages are from this study. U-Pb in phosphate ages are from the studies of McCubbin et al. (6), Yin et al. (8), and Bellucci et al. (9). U-Pb in young zircon ages are from the studies of McCubbin et al. (6), Tartèse et al. (7), and Yin et al. (8). U-Pb in old zircon ages are from the studies of Humayun et al. (2), McCubbin et al. (6), Tartèse et al. (7), and Yin et al. (8). U-Pb in baddelyite ages are from Tartèse et al. (7). Sm-Nd data are from Nyquist et al. (5). The whole-rock 40Ar/39Ar age from the study of Lindsay et al. (11) is included with the data from this study. 40Ar/39Ar ages from feldspars are not shown. (B) Chronology of major events in the history of NWA 7034 inferred from isotopic measurements (see main text for additional details). (i) The source lithology formed at 4420 ± 70 Ma. (ii) The source lithology was subject to impact events that created ancient impact melt rocks at ~4400 Ma. (iii) After almost 3000 Ma of quiescence, the terrain experienced several hundred million years of thermal metamorphism from 1500 to 1200 Ma. (iv) Following metamorphism, another ~1000 Ma of quiescence prevailed until brecciation at 225 Ma or later. (v) A final impact event after brecciation ejected NWA 7034 from the surface of Mars.
Fig. 2
Fig. 2. 40Ar/39Ar isochron diagrams.
Data shown in blue are derived from relatively K-rich (felsic) phases. Filled symbols were included in isochron regressions (see the Supplementary Materials for details regarding isochron regressions). Data shown in gray are derived from relatively K-poor phases that appear to be affected by 39Ar recoil and were excluded from isochron regressions. Error ellipses reflect the uncertainty correlation and ±2σ analytical uncertainties. The confidence intervals on the isochron regressions (red lines) are shown at 2 SE.
Fig. 3
Fig. 3. 40Ar/39Ar age spectra.
Age and Ca/K spectra obtained from whole-rock fragments and feldspar separates. 40Ar/39Ar ages are shown without and with martian atmospheric corrections as gray and red boxes, respectively, and are plotted against the primary y axis. Ca/K spectra are shown in blue and are plotted against the secondary y axis. Each spectrum is plotted against the cumulative release fraction of 39Ar released. Vertical dimensions of the boxes reflect the ±2σ analytical uncertainties. The horizontal dashed black lines and associated gray bands reflect the plateau ages and their associated 2 SE uncertainties, respectively. Horizontal arrows denote steps that were included in plateau ages (see the Supplementary Materials for details regarding plateau calculations).

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