The internal structure and geodynamics of Mars inferred from a 4.2-Gyr zircon record

Abstract

Combining U–Pb ages with Lu–Hf data in zircon provides insights into the magmatic history of rocky planets. The Northwest Africa (NWA) 7034/7533 meteorites are samples of the southern highlands of Mars containing zircon with ages as old as 4476.3 ± 0.9 Ma, interpreted to reflect reworking of the primordial Martian crust by impacts. We extracted a statistically significant zircon population (n = 57) from NWA 7533 that defines a temporal record spanning 4.2 Gyr. Ancient zircons record ages from 4485.5 ± 2.2 Ma to 4331.0 ± 1.4 Ma, defining a bimodal distribution with groupings at 4474 ± 10 Ma and 4442 ± 17 Ma. We interpret these to represent intense bombardment episodes at the planet’s surface, possibly triggered by the early migration of gas giant planets. The unradiogenic initial Hf-isotope composition of these zircons establishes that Mars’s igneous activity prior to ∼4.3 Ga was limited to impact-related reworking of a chemically enriched, primordial crust. A group of younger detrital zircons record ages from 1548.0 ± 8.8 Ma to 299.5 ± 0.6 Ma. The only plausible sources for these grains are the temporally associated Elysium and Tharsis volcanic provinces that are the expressions of deep-seated mantle plumes. The chondritic-like Hf-isotope compositions of these zircons require the existence of a primitive and convecting mantle reservoir, indicating that Mars has been in a stagnant-lid tectonic regime for most of its history. Our results imply that zircon is ubiquitous on the Martian surface, providing a faithful record of the planet’s magmatic history.

40Ar/39Ar ages of Northwest Africa 7034 and Northwest Africa 7533

“…And while we have attributed younger ages to what amounts to selective resetting within the intact protometeorite, Cassata et al (2018) suggested instead that the constituents of NWA 7034 and its pairs came together only within the last~200 Ma, which would allow grains to have independent histories and a variety of ages. This scenario also readily explains the young, concordant zircon ages of Costa et al (2020). If lithification occurred late, then temperatures during the process could not have exceeded 250°C for long without resetting the 1.40 Ga step ages of the K-rich samples.…”

“…They concluded that "Collectively, the data suggest that metamorphism at 1500-1200 Ma is associated with protracted magmatic activity, and brecciation occurred much later (after~225 Ma)." While this work was in the later stages of review, Costa et al (2020) reported six concordant Pb/Pb ages between 200 and 1200 Ma for zircons extracted from NWA 7533. Costa and coworkers interpreted these ages as supporting lithification within the last 200 Ma.…”

“…If due to heating alone, these young ages would imply 40 Ar losses of~50%. One possible explanation is that 22239 and 22242 were heated as separate objects and incorporated late in the history of the meteorite, as suggested by Cassata et al (2018) and Costa et al (2020). Alternatively, rather than loss of 40 Ar at higher temperatures, these two samples may have experienced hydrothermal alteration at lower temperatures with the addition of potassium.…”

“…11) pointed out indications of a disturbance within the last 1.4 Ga of the U-Pb ages of "young" zircons. Costa et al (2020) reported six concordant zircon ages between 0.2 and 1.2 Ga. To this partial list, we add the younger apparent 40 Ar/ 39 Ar step ages of 22239,~700 Ma; 22242,~700 Ma; and 22348, 700-1000 Ma. We now examine the maximum extent of heating compatible with the amounts of argon that the samples retained.…”

“…Breccias record complex temporal sequences that may include crystallization ages of their clasts, a lithification age, and subsequent thermal disturbances. The spectrum of ages reported for NWA 7034 and pair NWA 7533 (see Table 1) includes Pb/Pb crystallization ages of >4.4 Ga and later disturbance ages of up to 1.4 Ga for zircons and apatites (Humayun et al 2013;Bellucci et al 2015;McCubbin et al 2016;Bouvier et al 2018;Costa et al 2020), an Sm-Nd mineral isochron of 4.39 Ga (Nyquist et al 2016), disturbed Rb-Sr isochrons of 1.7-3.8 Ga, Ar-Ar ages that range from 3.4 to 0.2 Ga, and U-Th-Sm- 4 He ages indicating more recent losses of He (Cassata et al 2018). Connecting each of these ages to specific events in the history of the breccia helps in understanding the processes that shaped the breccia.…”