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.

Keywords: Mars; geodynamics; meteorites; zircon.

Fig. 1.

(A) A U–Pb concordia diagram for 41 zircon and baddeleyite grains from the NWA 7533 meteorite. All depicted grains have U–Pb ages that are less than 5% discordant. (B) A U–Pb concordia diagram for four zircons extracted from clast C27. Labels on concordia curves represent time B.P. in millions of years. (C) Plot of the ²⁰⁷Pb/²⁰⁶Pb ages of the zircon and baddeleyite depicted in A in a histogram with bin sizes of 5 Myr together with NWA7034 zircons (4). (D) A U–Pb concordia diagram for the eight zircons belonging to the young detrital grain population (Table 1). Data-point error ellipses are 2σ in all concordia diagrams. U–Pb isotope data are reported in full in Dataset S1 and SI Appendix, Table S3. Note that the ²⁰⁶Pb/²⁰⁴Pb ratios of the zircons described here (Dataset S1) are within a similar range of values to that reported in earlier studies (2, 16).

Fig. 2.

Hf isotope evolution diagrams. (A) The initial εHf values for ancient individual zircons extracted from the bulk rock NWA 7533 as well as one zircon extracted from the C27 basaltic clast together with previously published data from NWA 7034 (4). The blue zone represents the time evolution of a crustal reservoir extracted at 4547 Ma and characterized by ¹⁷⁶Lu/¹⁷⁷Hf values ranging from 0.004 to 0.014. The upper boundary of the forbidden region represents a reservoir with a ¹⁷⁶Lu/¹⁷⁷Hf = 0 and a formation age defined by the age of the Solar System at 4567.3 ± 0.16 Ma (38). Uncertainty on the εHf values reflects the internal precision (2σ) of the individual measurements. Uncertainty on the ²⁰⁷Pb/²⁰⁶Pb ages (2σ) is smaller than symbols. (B) The initial εHf values for the young zircon population (Table 1) plotted together with Martian meteorites of comparable ages (–24) as well as two bulk aliquots of NWA 7034 (39) and their back-projected time evolution as stippled lines.

Fig. 3.

Schematic representation of Mars’s main geochemical reservoirs at the time of formation of the <600-Ma shergottite lavas. The young zircons with chondritic-like initial Hf isotope compositions (εHf_T ∼0) are inferred to have been ultimately derived from plume-related magmatism sampling the convecting mantle. Note that the location of the source reservoir of enriched and intermediate shergottites is uncertain and could be located in the lithospheric mantle, crust, or both (2, 39).