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40Ar/39Ar and cosmic ray exposure ages of plagioclase-rich lithic fragments from Apollo 17 regolith, 78461
Earth, Planets and Spacevolume 68, Article number: 11 (2016)
Argon isotopic data is used to assess the potential of low-mass samples collected by sample return missions on planetary objects (e.g., Moon, Mars, asteroids), to reveal planetary surface processes. We report the first 40Ar/39Ar ages and 38Ar cosmic ray exposure (CRE) ages, determined for eleven submillimeter-sized (ranging from 0.06 to 1.2 mg) plagioclase-rich lithic fragments from Apollo 17 regolith sample 78461 collected at the base of the Sculptured Hills. Total fusion analysis was used to outgas argon from the lithic fragments. Three different approaches were used to determine 40Ar/39Ar ages and illustrate the sensitivity of age determination to the choice of trapped (40Ar/36Ar)t. 40Ar/39Ar ages range from ~4.0 to 4.4 Ga with one exception (Plag#10). Surface CRE ages, based on 38Ar, range from ~1 to 24 Ma. The relatively young CRE ages suggest recent re-working of the upper few centimeters of the regolith. The CRE ages may result from the effect of downslope movement of materials to the base of the Sculptured Hills from higher elevations. The apparent 40Ar/39Ar age for Plag#10 is >5 Ga and yielded the oldest CRE age (i.e., ~24 Ma). We interpret this data to indicate the presence of parentless 40Ar in Plag#10, originating in the lunar atmosphere and implanted in lunar regolith by solar wind. Based on a chemical mixing model, plagioclase compositions, and 40Ar/39Ar ages, we conclude that lithic fragments originated from Mg-suite of highland rocks, and none were derived from the mare region.
The regolith of planetary objects (e.g., Moon, Mars, and asteroids) can be defined as the boundary layer between the solid crust and outer space. The regolith evolution on planetary objects is a function of the continuous flux of impactors of various sizes, volcanism, as well as the constant bombardment by solar and galactic energetic particles. Regolith generally consists of fragmental and unconsolidated rock material. The hyper-velocity of the impacts and the continuous bombardment of energetic particles and micrometeorites reduce the initially rocky planetary surface to increasingly finer grain-sized regolith. For example, on the lunar surface, the mean grain size of lunar regolith ranges from 40 to 800 μm and averages between 60 and 80 μm (Lucey et al. 2006). A record of impact events and interactions with cosmic rays preserved in lunar regolith can be understood by geochemical and chronological studies of lunar regolith samples. The outcome of these studies provide constraints to understand the evolution of the Earth-Moon system and inner solar system (e.g., Lucey et al. 2006; Stöffler et al. 2006). The geochemical and chronological constraints developed based on lunar regolith studies can be applied to the surfaces of other solar system objects and used as a guide for future and ongoing missions like Hayabusa (Tsuda et al. 2013; Okada et al. 2015) to a C-type asteroid 1999 JU3, OSIRIS-REx sample return mission (Lauretta and Team 2012) to the asteroid 101955 Bennu, and the Mars Science Laboratory (Grotzinger et al. 2012), currently conducting experiments on the Martian surface. Kring (2015) describes how the success of NASA’s Orion crew vehicle, as well as the outcome of current lunar orbiters, has revitalized future interest in robotic and human exploration programs to the moon and beyond.
Regolith sample collected at station 8, Apollo 17
The Apollo 17 landing site is located in the center of the Taurus-Littrow valley. One of the prominent geological features of the valley is the highland unit found near the north massif, the Sculptured Hills. Apollo 17 astronauts sampled the regolith at the base of the Sculptured Hills during their last surface traverse at station 8 (latitude 20.278° N, longitude 30.848° S; Robinson and Jolliff 2002). They dug a 25-cm-deep trench (Wolfe et al. 1981) that revealed the best preserved lunar regolith stratigraphy collected during the Apollo missions (Mitchell et al. 1973; Fig. 1). Based on the chemical composition of the deepest layer of the trench (i.e., sample 78421), Korotev and Kremser (1992) concluded that the regolith represents six chemical components. Among them, three chemical components are of mare origin: (1) high-Ti (HT) mare basalt, (2) very-low-Ti (VLT) basalt, and (3) pyroclastic orange glass. The other three chemical components are of highland origin: (4) impact-melt noritic breccia (NB), (5) anorthositic norite (AN) breccia, and (6) Mg-suite (high Mg/Fe troctolites and norites). A similar study by Simon et al. (1981) found that mare volcanics and highland-derived fragments were present in equal proportions in Apollo 17 regolith sample 78221. A mixing model based on the major and trace-element composition of the sample from the top layer of trench regolith (i.e., 78481) suggest mixing of 40–50 % mare and 52–59 % highland components (Rhodes et al. 1974). Goswami and Lal (1974) performed nuclear track studies on the samples from the trench and observed that the density of tracks observed in the bottom sample (i.e., 78121) is only slightly less than the track density observed in the surface sample (i.e., 78481) and concluded that the track density distribution pattern, along with the knowledge of the geological setting at station 8, can be used to characterize the macroscopic lunar surface transport processes including regolith mixing and down slope movement. Prior to our study, the crystallization and metamorphic ages on this trench regolith sample were unknown (Meyer 2010). We present the first 40Ar/39Ar ages and 38Ar cosmic ray exposure (CRE) ages on plagioclase-rich lithic fragments separated from the 78461 trench regolith sample collected at station 8. These chronology data are used together with plagioclase compositions and a mixing model to determine the origin and duration that the regolith resided on the lunar surface.
Sample and experimental details
Regolith sample 78460 was collected at a depth interval of 1–6 cm at station 8 at the base of the Sculptured Hills (Meyer 2010), and ~20 m above the Taurus-Littrow valley floor from the Apollo 17 landing site (Fig. 1). Plagioclase-rich lithic fragments comprise ~7–9 modal percentage of the 1000–90 μm (Heiken and McKay 1974) size fractions of station 8 regolith samples. The median grain size of regolith sample 78460 is 47 μm (Graf 1993). The eleven plagioclase-rich lithic fragments analyzed in this study were handpicked from the 1000–90-μm fraction of a 5-g aliquot of 78461, which is the <1 mm size fraction of 78460 (Butler 1973). The sieved fraction was ultrasonically cleaned in acetone, dried, handpicked, and weighed. Out of eleven handpicked lithic fragments, eight fragments were less than 0.52 mg. The other three fragments, Plag#21, Plag#23, and Plag#24 weighed 1.20, 1.08, and 0.72 mg, respectively (Table 1).
Chemical characterization of plagioclase-rich lithic fragments
Plagioclase-rich fragments were individually mounted in CrystalBond© adhesive, ground, polished, and carbon coated. Samples were analyzed for major element composition using a CAMECA SX-100 electron microprobe at Rensselaer Polytechnic Institute with run condition of 15 kV, 20 nA, and 30-s counting times with a 10-μm-diameter beam. Samples were subsequently polished with 0.3-μm alumina powder to remove the carbon coating and warmed to ~100 °C for ~1 min to soften the CrystalBond© adhesive for removal of each lithic fragment. These were then ultrasonically cleaned in acetone to remove adhering CrystalBond©. Major elemental compositions of the eleven plagioclase-rich lithic fragments are given in Table 1. The lithic fragments contain Ca-rich feldspar (i.e., anorthite), with compositions ranging from An93 to An96 (Table 1). The K2O contents range from 0.03 to 0.28 wt.% suggesting 200 to 2300 ppm of K in the fragments. The Fe/(Fe + Mg) ratio varies from 0.19 (Plag#1) to 0.84 (Plag#24). Given the size of the plagioclase lithic fragments (<48 μm), we were unable to obtain a pure plagioclase separate using standard mineral separation techniques.
Irradiation of plagioclase-rich lithic fragments
Following electron probe analysis, each plagioclase-rich lithic fragment was individually packed in Al foil and irradiated for 100 h in the cadmium-lined in-core irradiation tube (CLICIT) facility of the TRIGA reactor at Oregon State University, USA. Cadmium shielding was used to minimize the production of 38Ar from 37Cl and undesirable isotopic interference reactions. The neutron flux was monitored using Hb3gr (hornblende, PP-20) hornblende (Turner 1971; Jourdan and Renne 2007; Schwarz and Trieloff 2007). These flux monitor standards were placed between the plagioclase-rich lithic fragments to monitor the fast neutron dose. In addition, CaF2 and KSiO3 glasses were also placed between the samples to derive correction factors for reactor-produced 37Ar and 39Ar.
Gas extraction and mass spectrometric measurements
Following irradiation, the plagioclase-rich lithic fragments, international standard Hb3gr (hornblende, PP-20) flux monitors, CaF2, and KSiO3 were unwrapped from Al foil. The samples were outgassed using a CO2 laser (Synrad model J48) with beam diameter of 3.5 mm and ~8 W of power. Extracted Ar from each fused sample was measured using a VG5400 noble gas mass spectrometer in Syracuse University’s Noble Gas Isotope Research Laboratory. Based on the measurement of 8 PP-20 grains (1073.6 ± 4.6 (1σ); Schwarz and Trieloff (2007)), the mean J value for the samples is calculated to be 0.02643 ± 0.00021 (1σ). These values are similar to those reported for 100 h irradiation at the CLICIT facility (Fernandes et al. 2013; Gombosi et al. 2015). The correction factors for reactor-produced isotopes are as follows: (39Ar/37Ar)Ca = (7.57 ± 0.34) × 10−4, (36Ar/37Ar)Ca = (3.22 ± 1.26) × 10−4, and (40Ar/39Ar)K = 0.01576 ± 0.0004. After extraction, reactive gases were removed using SAES ST-707 getters. The purified gas was then expanded into the VG5400 mass spectrometer, and 11 cycles at m/e = 40, 39, 38, 37, 36, and 35 were measured in peak jumping mode. Blank measurements were performed before and after every sample analysis to determine the background for Ar isotopes in the extraction system. Typical blank values in volts, during the course of these experiments, including 1σ errors are as follows: 35Ar = (1.064 ± 0.330) × 10−8, 36Ar = (1.521 ± 0.004) × 10−7, 37Ar = (1.933 ± 0.414) × 10−8, 38Ar = (2.849 ± 1.156) × 10−8, 39Ar = (8.923 ± 7.049) × 10−10, and 40Ar = (1.320 ± 0.001) × 10−5. For 40Ar and 39Ar, the blank seldom exceeded greater than 5 % of sample signal during the experiments. Air standards were analyzed before and after the sample analyses to determine the Ar sensitivity, as well as mass discrimination. The Ar sensitivity determined during the course of these experiments was 1.340 × 1011 V/mol. The mass discrimination correction factors for 36Ar, 37Ar, 38Ar, and 39Ar relative to 40Ar were 0.973 ± 0.15 %, 0.981 ± 0.33 %, 0.987 ± 0.16 %, and 0.993 ± 0.57 %, respectively. Based on the present Ar sensitivity and low K content in the plagioclase-rich lithic fragments, individual fragments were outgassed in a single-step total fusion. Multiple step extraction would have resulted in the Ar signals below the present detection limit of the VG5400 mass spectrometer.
Sources of Ar in lunar regolith: radiogenic, cosmogenic, and trapped argon components
The top layer of lunar regolith is constantly bombarded by energetic particles originating from solar wind, solar cosmic rays, and galactic cosmic rays. The three stable isotopes of Ar (36Ar, 38Ar, and 40Ar) are affected in different ways due to the interaction of lunar regolith with outer space. For example, 36Ar resides close to the surface of individual grains and is mainly implanted by solar wind; this component is the trapped component (e.g., Wieler and Heber 2003). Cosmogenic 38Ar is produced by interaction of energetic protons from solar and galactic cosmic rays with target elements such as Fe, Ca, K, and Ti. 40Ar is produced by radioactive decay of 40K; however, an “excess or parentless 40Ar” component has also been observed in lunar regolith samples (Manka and Michel 1971). On the lunar surface, 40Ar/36Ar is 2–14 (e.g., Norman et al. 2003), and solar wind 40Ar/36Ar is 10−4 (Lodders 2010). The 40Ar atoms present in the lunar atmosphere is photoionized and then accelerated by solar wind. About 50 % of the 40Ar ions escape to space, and the remaining 50 % of 40Ar atoms are driven back to the lunar surface (Signer et al. 1977). Only a few percent of ions are trapped in the lunar regolith while most 40Ar ions are recycled to the lunar atmosphere. The small fraction of trapped lunar atmospheric 40Ar has been proposed to be the source of excess 40Ar in lunar regolith (e.g., Manka and Michel 1971; Wieler and Heber 2003). Below, we discuss procedures for calculating the abundance of radiogenic 40Ar and cosmogenic 38Ar to determine 40Ar/39Ar ages and 38Ar cosmic ray exposure ages, respectively, for the plagioclase-rich lithic fragments.
Calculating the abundance of radiogenic 40Ar
In the case of lunar regolith samples, the non-radiogenic 36Ar originates from solar wind. Measured 40Ar (40Arm) is corrected for non-radiogenic 40Ar using the non-radiogenic trapped (40Ar/36Ar)t and assuming the measured 36Ar is of trapped Ar. Generally, a standard isochron plot (40Ar/36Ar vs. 39Ar/36Ar) is used where the y-intercept of a linear regression is assumed to be the trapped argon composition (e.g., Merrihue and Turner 1966; McDougall and Harrison 1999). We use two approaches to determine the trapped 40Ar/36Ar composition from total fusion analyses of plagioclase-rich lithic fragments. The first approach assumes that all the fragments contain the same trapped (40Ar/36Ar) composition. The second approach assumes that a trapped 40Ar/36Ar = 0.
Calculating the abundance of cosmogenic 38Ar
The measured 38Ar abundances in the plagioclase-rich lithic fragments result from (1) nucleogenic (i.e., reactor produced), (2) trapped, and (3) cosmogenic components. During neutron irradiation, 38Ar can be produced from Ca (i.e., 42Ca(n, nα)38Ar) and K (i.e., 39K(n,d)38Ar). Based on CaF2 crystals that were also irradiated with plagioclase-rich lithic fragments, the (38Ar/37Ar)Ca is (4.50 ± 0.02) × 10−5, a negligible contribution to total 38Ar. To estimate the 38Ar produced from K, we determined (38Ar/39Ar)K = 0.0120 ± 0.0002, on KSiO3 glasses irradiated with the plagioclase-rich lithic fragments. 38Ar can also be produced during irradiation from reactions on Cl (i.e., 37Cl(n, γ)38Cl(β−)38Ar). However, in lunar regolith, the amount of Cl is very low, averaging 20 ppm (e.g., Taylor and Hodges 1981). Because cadmium shielding was used during irradiation, 38Ar produced from 37Cl can be neglected.
The trapped 38Ar in lunar surface samples results from solar wind implantation ((38Ar/36Ar)t = 0.188; Eberhardt et al. (1972)). Following previous studies (e.g., Fernandes et al. 2013), the cosmogenic 38Ar (38Arc) in individual plagioclase-rich lithic fragments was determined using the following equation:
where 39ArK = 39Ar* = 39Arm − 37Arm × (39Ar/37Ar)Ca; (39Ar/37Ar)Ca = (7.57 ± 0.34) × 10−4; (38Ar/39Ar)K = 0.012 ± 0.0002; and c, m, and t subscripts denote cosmogenic, measured, trapped (i.e., solar wind), respectively; and K and Ca subscripts indicate reactor-produced components.
Argon data for the plagioclase-rich lithic fragments, corrected for blank contribution, and reactor-produced interferences are given in Table 2.
Solar wind-implanted 36Ar
On the lunar surface, solar wind is the most dominant source of 36Ar (i.e., ions of 36Ar directly implanted in lunar regolith). Since the implantation of solar wind is depth dependent and can penetrate only a few microns on the lunar surface, the variation in abundance of 36Ar is a function of solar wind implantation duration and/or the depth at which fragments were exposed to solar wind. The measured 36Ar abundance (36Arm) in eleven plagioclase-rich lithic fragments, corrected for blank contribution and mass discrimination, varies from 0.41 × 10−6 cm3 STP/g (i.e., Plag#23) to 4.1 × 10−6 cm3 STP/g (i.e., Plag#10) (Table 2).
Argon isotopic composition
The comparison of (40Ar/36Ar)m and (38Ar/36Ar)m ratios for eleven plagioclase-rich lithic fragments, corrected for blank and mass discrimination, is shown in Fig. 2. Generally, 40Ar/36Ar values are explained as a result of the mixture between non-radiogenic trapped and radiogenic argon compositions. Non-radiogenic trapped argon on the lunar surface is mostly solar wind-implanted argon as discussed above. The (40Ar/36Ar)m varies from 0.62 (i.e., Plag#14) to 15.5 (i.e., Plag#23) with higher values suggesting the presence of radiogenic and/or excess parentless 40Ar in the samples. The (38Ar/36Ar)m value for individual plagioclase-rich lithic fragments varies from ~0.191 to 0.6 (Fig. 2) and can be explained as a mixture of non-radiogenic trapped (i.e., 38Ar/36Ar = 0.188; Eberhardt et al. (1972)) and cosmogenic Ar (i.e., 38Ar/36Ar = 1.54; Hohenberg et al. (1978)). For the Plag#23 sample, the concentration of 36Ar is lowest when compared to all other plagioclase-rich lithic fragments. We argue that the high 38Ar/36Ar ratio is a result of 36Ar loss from Plag#23. The retentivity of noble gas components generally have an order: solar wind implanted (e.g., 36Ar) <radiogenic (e.g., 40Ar) <cosmogenic (e.g., 38Ar). In general, solar wind-implanted gases are released at low temperature when stepwise heating is performed on lunar regolith samples (e.g., Hohenberg et al. 1970). This observation supports the argument that the solar wind-implanted Ar is more susceptible to loss compared to radiogenic and cosmogenic components of Ar.
(40Ar/36Ar)m vs. (39Ar/36Ar)m correlation
The (40Ar/36Ar)m and (39Ar/36Ar)m ratios were determined by correcting the measured abundances of 36Ar, 40Ar, and 39Ar for blank contributions. In addition, the abundance of 36Ar was corrected for mass discrimination, and the abundance of 39Ar was corrected for radioactive decay since the time of sample irradiation. The (40Ar/36Ar)m and (39Ar/36Ar)m for eleven plagioclase-rich lithic fragments yield a strong linear correlation (r 2 = 0.99) (Fig. 3). Based on this linear regression, the 40Ar/36Ar intercept is 0.146 ± 0.002 and the slope is 335.44 ± 16.77. For the x-y error-weighted linear regression (York 1968), the intercept is 0.050 ± 0.104 and the slope is 336.49 ± 17.78. A linear correlation is generally observed in the case of isochron analysis of step heat experiments on undisturbed samples (e.g., Merrihue and Turner 1966; McDougall and Harrison 1999). A similar correlation is also observed for grain size fractions separated from regolith (e.g., Bogard and Nyquist 1975; Eberhardt et al. 1976), where the y-intercept represents the non-radiogenic composition of trapped (40Ar/36Ar)t and slope is used to determine the isochron age. Here, we discuss whether the apparent linear correlation observed in the case of the plagioclase-rich lithic fragments analyzed can be used to argue for the lithic fragments having (a) the same 40Ar/39Ar age and (b) the same (40Ar/36Ar)t (i.e., implanted solar wind).
40Ar/39Ar ages are calculated for the plagioclase-rich lithic fragments using three different approaches as described below. For all age determinations, a half-life of 1.250 ± 0.002 × 109 for 40K is used (Steiger and Jäger 1977; Renne et al. 2010).
Assuming the same trapped (40Ar/36Ar)t composition
In this first approach, we assume that all the plagioclase-rich lithic fragments contain the same trapped (40Ar/36Ar)t = 0.146 (i.e., the y-intercept value on the (40Ar/36Ar) m versus (39Ar/36Ar)m regression (Fig. 3)). We prefer to use a slope obtained through normal regression as the correlation is observed for individual samples and the correlation between errors of 39Ar/36Ar (x-axis) and 40Ar/36Ar (y-axis) is not expected unlike the case of an age isochron where x-y error-weighted linear regression is required. The 40Ar/39Ar ages determined by this approach range from 3.96 to 5.03 Ga. (Table 2, Fig. 4).
Assuming the absence of trapped 40Ar
For this second approach, we assume that the plagioclase-rich lithic fragments are devoid of any trapped 40Ar (i.e., (40Ar/36Ar)t = 0). In this case, it is assumed that all the measured 40Ar is radiogenic 40Ar (e.g., Norman et al. 2006; Fernandes et al. 2013). The calculated 40Ar/39Ar ages determined in this way range from 4.13 to 5.26 Ga (Table 2, Fig. 4).
40Ar/39Ar age based on (40Ar/36Ar) m vs. (39Ar/36Ar)m correlation
A third approach does not assume a trapped (40Ar/36Ar)t composition. As discussed in the “(40Ar/36Ar) m vs. (39Ar/36Ar)m correlation” section, we obtained a 40Ar/39Ar age of 4.13 ± 0.09 Ga for all the lithic fragments based on the slope of the linear regression (i.e., 40Ar*/39ArK = 335.44 ± 16.77) observed on the correlation plot (Fig. 3). If we use the slope of linear regression (based on the York method), the 40Ar/39Ar age =4.135 ± 0.086 Ga, similar to the age derived using a slope obtained through linear regression. In Fig. 4, this age is compared with individual 40Ar/39Ar ages determined using the approaches outlined in the “Assuming the same trapped (40Ar/36Ar)t composition” and “Assuming the absence of trapped 40Ar” sections above. The comparison of 40Ar/39Ar ages derived from three different approaches clearly indicate the sensitivity of age determination to the choice of trapped (40Ar/36Ar)t.
On the lunar surface, 38Ar is produced by solar as well as galactic cosmic rays. With the knowledge of 38Ar production rate, it is possible to estimate the duration of cosmic ray exposure for individual plagioclase-rich lithic fragments. However, the production rate depends on the shielding depths. Hohenberg et al. (1978) calculated the production rates of 38Ar on the lunar surface, as well as various shielding depths. These production rates are for 2π exposure geometry and give a production rate of 38Ar, produced predominantly from the target element Ca, with smaller amounts contributed from Fe, Ti, and K. Assuming that the plagioclase-rich lithic fragments received the cosmic ray exposure on the lunar surface, we calculated production rates for 38Ar using the measured major elemental compositions following surface production rates given by Hohenberg et al. (1978). These production rates are the maximum production rate estimates and range from 0.73 to 0.79 × 10−8 cm3 STP g−1 Ma−1. However, the depths from which the plagioclase-rich lithic fragments were sampled is not known precisely. As noted previously, the samples analyzed were picked from ~410 g of regolith sampled from 1 to 6 cm depth. We calculated production rates using average production rates of 2, 5, and 10 g/cm2 corresponding to 1.3, 3.3, and 6.6 cm depths, respectively (Hohenberg et al. 1978). The production rate for surface (P38 sf) and average production rate for 1 to 6 cm depth (P38 avg) along with respective 38Ar cosmic ray exposure ages (T38 sf and T38 avg) are listed in Table 2 and plotted in Fig. 5. Because the abundance of the major target element (i.e., Ca) is similar for all the plagioclase-rich lithic fragments (Table 1), the range of the production rates is narrow as listed in Table 2. The surface exposure ages (T38 sf) for plagioclase-rich lithic fragments vary from ~1 to 24 Ma representing the minimum exposure duration for the fragments. On the other hand, the exposure ages based on the average production rate for 1 to 6 cm depth (T38 avg) range from ~5 to 110 Ma.
Interpretation of 40Ar/39Ar ages
Although the small sample weights (0.06–1.20 mg) precluded step heat experiments from being performed on the plagioclase-rich lithic fragments, the laser total fusion ages can be viewed as equivalent to total integrated ages based on step heat results (e.g., Lister and Baldwin 1996) assuming no loss or implantation of 40Ar. The relatively tight clustering of inferred total fusion 40Ar/39Ar ages (i.e., 3.96 to 4.28 Ga) for eleven plagioclase-rich lithic fragments as well as older age range (~4 Ga) for the fragments indicate a modest disturbance among the entire set of samples with negligible loss of radiogenic 40Ar. However, low temperature diffusive loss of Ar cannot be ruled out completely. Low temperature loss in the lunar samples have apparently occurred at a time closer to the formation ages. For example, Shuster et al. (2010) reported concordance in the initial step ages (ranging from 3.3–3.4 Ga) for 63503 samples which have different 40Ar/39Ar plateau ages and argued that the last significant loss of 40Ar* occurred 3.3–3.4 Ga ago. It should be noted that the initial step age provides an estimate for the timing associated with the last significant 40Ar* loss from a sample. Boehnke et al. (2015) also observed a broad peak at ~3.7 Ga for the Apollo 16 impact-melt samples based on initial minimum ages. This observation is inconsistent with the proposed late heavy bombardment (LHB) type event at ~3.85 Ga (e.g., Tera et al. 1974). Based on these observations, we argue that the lunar regolith most likely experienced impact events that peaked at ~3.7 Ga. Also, the loss of radiogenic 40Ar at a younger time would result in greater difference between the total fusion age and the plateau age derived by step heat experiments. For example, the 40Ar/39Ar data reveal partial resetting resulting in ~29 % of 40Ar loss at ~2.3 Ga for 77,017 samples (Hudgins et al. 2008). The total fusion age for 77017 is 3.741 ± 0.083 Ga while the plateau age is 4.018 ± 0.04 Ga, respectively. Similarly, the loss of around ~49 % 40Ar at 3.2 Ga was estimated for sample 60035 (Hudgins et al. 2008) while the total fusion age is 4.088 ± 0.100 Ga. We infer that the difference between total fusion age and the plateau age is proportional to the (1) fraction of 40Ar* loss and (2) time difference between crystallization of sample and time of partial resetting. In the present study, we find total fusion 40Ar/39Ar ages from 3.96 to 4.28 Ga for eleven plagioclase-rich lithic fragments. Despite a tight clustering and older 40Ar/39Ar ages, we cannot rule out the possibility of partial resetting of 40Ar/39Ar ages in the plagioclase lithic fragments from regolith.
The two approaches used to determine the 40Ar/39Ar ages (sections “Assuming the same trapped (40Ar/36Ar)t composition” and “Assuming the absence of trapped 40Ar”) highlight the fact that the trapped non-radiogenic 40Ar/36Ar chosen for age calculation impacts the calculated 40Ar/39Ar ages (Fig. 4) of plagioclase-rich lithic fragments (c.f., Norman et al. (2003)). For example, Plag#14 has the lowest 40Ar/39Ar age (~3.96 Ga, Fig. 4), if measured 40Ar is corrected using the trapped contribution ((40Ar/36Ar)t = 0.146), obtained from the y-intercept on the (40Ar/36Ar)m vs. (39Ar/36Ar)m diagram. In comparison, the age of Plag#14 is similar to that of other fragments (Fig. 4) when we assume (40Ar/36Ar)t = 0. On the contrary, the 40Ar/39Ar age for Plag#10 is ~5.0 Ga (assuming (40Ar/36Ar)t = 0.146) and ~5.3 Ga (assuming (40Ar/36Ar)t =0), and both approaches yielded geologically meaningless ages.
Both these plagioclase-rich lithic fragments (i.e., Plag#10 and Plag#14) have the lowest (40Ar/36Ar)m and (39Ar/36Ar)m (Fig. 3). Assuming non-radiogenic trapped (40Ar/36Ar)t = 0.143, the percentage of radiogenic 40Ar (40Ar*) released from Plag#10 and Plag#14 is approximately 87 and 76 %, respectively, while the rest of the samples yielded >94 % 40Ar* (Table 2). Plag#10 and Plag#14 have higher 36Arm compared to that of other fragments; however, the 40Ar/39Ar ages do not appear to be a function of the abundance of measured 36Ar (Fig. 6). As we have assumed the same value of non-radiogenic trapped 40Ar/36Ar (i.e., 0.143), the correction for trapped contribution in measured 40Ar is higher for Plag#10 and Plag#14, owing to their higher abundance of 36Ar. Overcorrection for (40Ar)m could explain the lowest 40Ar/39Ar age for Plag#14 (i.e., 3.96 Ga). It is possible that Plag#14 fragment may have a lower or no trapped (40Ar/36Ar)t component. On the other hand, the highest apparent 40Ar/39Ar age for Plag#10 can be due to the higher amount of non-radiogenic trapped 40Ar/36Ar. The 40Ar*/39ArK ratio for the Plag#10 fragment is the highest (i.e., ~578) when compared to that of other plagioclase-rich lithic fragments including Plag#14 (ranging from 303 to 391) (Table 2). We conclude that Plag#10 sample contains parentless 40Ar. This can explain the geologically meaningless apparent age (i.e., an age of a sample cannot be older than the age of solar system). The Plag#10 fragment also has the highest CRE age (i.e., 24 to 110 Ma, depending on the choice of production rate; Table 2), indicating longer residence on the lunar surface. We propose that during residence on the lunar surface, 40Ar that was present in the lunar atmosphere was ionized and implanted in the Plag#10 fragment by solar wind, resulting in entrapment of excess or parentless 40Ar.
Results yielded total fusion 40Ar/39Ar ages that are interpreted as minimum crystallization ages of the plagioclase-rich lithic fragments. If samples have been partially outgassed due to heating following crystallization (e.g., due to impacts on lunar surface), this would result in loss of radiogenic 40Ar from the lithic fragments, therefore lowering the 40Ar/39Ar ages. On the other hand, implantation of unknown amount of parentless 40Ar from lunar atmosphere through SW would result in higher 40Ar/39Ar ages. Therefore, it is difficult to characterize the amount of loss of 40Ar and/or implantation of 40Ar in single-millimeter fragments. Although we were unable to resolve possible details of the thermal history following crystallization, results are comparable to an integrated 40Ar/39Ar age for step heat experiments, assuming no loss of 40Ar or implantation of 40Ar, and thus provide minimum ages of crystallization (e.g., Lister and Baldwin 1996). However, it is highly possible that lunar regolith could have experienced complicated history of 40Ar degassing and implantation as discussed below.
During the early lunar evolution (>4 Ga), a greater amount of 40K was available to decay to 40Ar in the lunar crust and this radiogenic 40Ar escaped to lunar atmosphere (e.g., Eugster et al. 2001). A fraction of the escaped 40Ar was ionized and accelerated by solar wind. Eventually, a small fraction of 40Ar (5–8.5 %; Manka and Michel 1971) was implanted by the solar wind into the lunar surface. The 40Ar/36Ar ratio of trapped Ar in lunar regolith has decreased throughout lunar history, roughly in phase with the decreasing 40K; however, the initial 40Ar/36Ar composition of argon trapped in the regolith is uncertain and dependent on the ambient environment at the time of crystallization. This is especially relevant if impact ejecta are the source of regolith. On the other hand, the lunar regolith experiences gardening processes and impact events by micrometeorites that may have a variable effect dependent on regolith size. For example, grains that spent more time on the lunar surface are susceptible to impacts by micrometeorites unlike grains at a few centimeters depth. The gardening process can randomly excavate small grains multiple times. These processes affect the lunar regolith and therefore the trapped 40Ar/36Ar composition of regolith material.
What is the trapped non-radiogenic 40Ar/36Ar composition in lunar regolith?
The selection of trapped non-radiogenic 40Ar/36Ar is critical for the determination of 40Ar/39Ar ages. However, in the case of total fusion experiments and step heat experiments where isochron plot does not yield a meaningful y-intercept (i.e., 67215 clast; Norman et al. 2003), it is difficult to determine the trapped non-radiogenic 40Ar/36Ar composition. For lunar regolith, the trapped 40Ar/36Ar composition has a range of values. For example, orange glass from 74001/2 core drilled near the Shorty crater at the Apollo 17 landing site has 40Ar/36Ar composition ranging from 8 to 10 (Eugster et al. 1980). Two Apollo 14 breccia samples (14301 and 14318) have trapped 40Ar/36Ar composition ranging between 10 and 14 (Megrue 1973; Reynolds et al. 1974; Bernatowicz et al. 1980). 40Ar/36Ar values for breccia samples from Apollo 16 regolith lie in the range of <1 to 12 (McKay et al. 1986). In the present study, the measured 40Ar/36Ar ratio in plagioclase-rich lithic fragments ranges from 0.62 to 15.5, which is similar to the ratios reported.
Comparison of results with previously reported isochron ages
The 40Ar/39Ar total fusion ages determined for the lithic fragments are the first ages ever determined for the trench soil collected at station 8 using any isotopic study. However, an approximately 0.5-m-diameter boulder also collected at station 8 (~50 m from the trench, Fig. 1), at the base of the Sculptured Hills, has been extensively studied. Jackson et al. (1975) concluded that the coarse-grained, cumulus-textured boulder formed at a depth of 8–30 km in the lunar crust and was excavated by a large basin-forming impact event. The boulder originated via impact generated ballistic transport to the present site and cannot be related to the geology of the Apollo 17 landing area or the Sculptured Hills. Samples 78235, 78236, 78238, 78255, and 78256 are derived from the same boulder. Figure 7 gives the compilation of ages derived by various isotopic dating methods. The ages range from 4.00 ± 0.09 Ga (metamorphic age for 78238 using Rb-Sr study (Edmunson et al. 2009) to ~4.43 ± 0.05 Ga (crystallization age based on Sm-Nd for most retentive phases (Nyquist et al. 1981)). The Rb-Sr metamorphic age is concordant with an 40Ar/39Ar age of 4.11 ± 0.02 for 78235/78236 (Aeschlimann et al. 1982). As can be seen in Fig. 7, the 40Ar/39Ar total fusion ages of plagioclase-rich lithic fragments (except Plag#10) fall within the age range observed for the station 8 boulder 78238. However, it should be noted that there is no apparent genetic relation between the lithic fragments and the boulder based on the origin of the boulder 78238 inferred by Jackson et al. (1975).
The crystallization ages of Mg-suite rock samples are also compared with the 40Ar/39Ar ages of the plagioclase-rich lithic fragments (Fig. 7). Sample 78155, also collected at station 8 (~20 m from the trench at the rim of Bowen-Apollo crater, Fig. 1), a thermally annealed polymict breccia of anorthositic norite composition (Bickel 1977), has a 40Ar/39Ar crystallization age of 4.19 ± 0.04 Ga (Fernandes et al. 2013), within the range observed for the plagioclase-rich lithic fragments reported here (Fig. 7). Delano and Bence (1977) reported two ~4-mm plagioclase-rich lithic fragments belonging to the Mg-suite that had been collected ~5 km from station 8 (station 3; regolith 73260) at the Apollo 17 site. These two lithic fragments, both belonging to the Mg-suite, had 40Ar/39Ar ages of ~4.2 Ga obtained by stepwise heating. A comparison of 40Ar/39Ar ages of plagioclase-rich lithic fragments with the crystallization age ranges observed for other lithologic components reveal that the lithic fragments overlap the age range observed for polymict breccia (anorthositic norite and noritic) and Mg-suite rocks, suggesting these lithologic components as possible origins for the lithic fragments as discussed in “Origin of the plagioclase-rich fragments” section below.
Implications for lunar surface processes based on CRE ages
The CRE ages of the lithic fragments represent integrated durations of exposure on the surface of the Moon and reveal an exposure and a burial history of the regolith samples. The variation in CRE ages (from ~1 to 24 Ma (based on 38Ar production rate for surface exposure) or ~5 to 110 Ma (based on average 38Ar production rate for 1 to 6 cm depth on lunar surface) indicates that individual plagioclase-rich lithic fragments have not remained stationary and instead have experienced movement within the lunar regolith for durations of 1 Ma. On the Moon, regolith from a depth of 9 mm is thought to overturn at least once in approximately 10 Ma (Hörz et al. 1991). The estimated CRE range, determined for the plagioclase-rich lithic fragments is consistent with previous suggested regolith overturn rates. The CRE ages for rock samples also collected at station 8 sample range from 17 to 292 Ma (Drozd et al. 1977; Hudgins et al. 2008; Fernandes et al. 2013). The CRE ages observed for some plagioclase-rich lithic fragments are similar to previously reported CRE age of ~17 to 21 Ma for the 78155 samples (Hudgins et al. 2008; Fernandes et al. 2013). Trench regolith from station 8 contained mature samples based on carbon content and maturity index (McKay et al. 1974). The deepest sample from trench (78421) is a mature regolith containing a high abundance of agglutinates (68 %). Based on the low albedo data (Apollo Lunar Geology Investigation Team 1973) and inferred regolith maturity on the basis of fossil track data (Goswami and Lal 1974), it was argued that a large fraction of the regolith at the Apollo 17 site was exposed and reworked at the lunar surface for significant periods of time. Since the plagioclase-rich lithic fragments were derived from the regolith at the base of Sculptured Hills, it is possible that the regolith at station 8 may have been affected by downslope movement of materials from higher elevations. The observed range for CRE ages, either ~1 to 24 Ma or ~5 to 110 Ma, for eleven plagioclase-rich lithic fragments suggests that there is a heterogeneous distribution of submillimeter-sized fragments with a range of cosmic ray exposure ages in the top layer of lunar regolith at station 8.
Origin of the plagioclase-rich fragments
The lunar regolith generally spans a large range of chemical compositions and the variations result from different proportions of major lithologic components that originate from mare basalt and highland rocks. Korotev and Kremster (1992) presented a mass-balance model to account for the compositional variation observed in the regolith samples collected during the Apollo 17 mission. Korotev and Kremster (1992) concluded that various proportions of three chemical components of mare origin (high-Ti and very-low-Ti basalts, pyrocloastic orange glass) and three components of highland origin (impact-melt noritic breccia, anorthositic norite breccia, Mg-suite) can account for most of the compositional variation observed for Apollo 17 regolith. A mixing model, based on major and trace-element compositions, reveals that the trench regolith sample collected at station 8 contains 25.8 % of HT basalt, 2.2 % of VLT basalt, 12 % of orange glass, 15.7 % of impact-melt NB, 35.7 % of AN breccia, 4.62 % of troctolite Mg-suite, and 3.1 % of norite Mg-suite (Table 3, Korotev and Kremser 1992). With the exception of orange glass, these components contain different proportions of plagioclase grains (Table 3). We use mole percent anorthite (i.e., CaAl2Si2O8) and Fe/(Fe + Mg) in plagioclase-rich lithic fragments to further constrain the origin of the lithic fragments. In Fig. 8, we compare An(%) and Fe/(Fe + Mg) values of plagioclase-rich lithic fragments (Table 1) with that of possible protoliths listed in Table 3. Based on this comparison, we conclude that none of the plagioclase-rich lithic fragments are inferred to be derived from HT mare basalt (Fig. 8a). This comparison also suggests multiple possible origins for a given plagioclase-rich lithic fragments due to overlapping values of An(%) and Fe/(Fe + Mg) for the lithologic components. We observe that the An(%) and Fe/(Fe + Mg) values of the Plag#1 lithic fragment do not overlap with that of any lithologic component. However, impact-melt NB could possibly contribute Plag#1 fragment. At least four lithic fragments may have originated from very-low-Ti mare basalt, while most of the plagioclase-rich lithic fragments may have originated from the highland region. We further constrain the lithologic origin of the plagioclase-rich lithic fragments based on the range of crystallization ages reported for different lithologic components (Table 3). The crystallization ages for highland polymict rocks (i.e., AN and NB) range from 3.75 ± 0.11 Ga to 4.16 ± 0.04 Ga (Stöffler et al. 2006). These overlaps with the younger age range are obtained for the plagioclase-rich lithic fragments (Fig. 7). Based on crystallization ages, it can also be argued that no lithic fragment originated from the VLT mare basalt, and most of the fragments may have been derived from the Mg-suite rocks. Simon et al. (1981) observed that mare volcanics and highland-derived fragments were present in similar proportions in Apollo 17 regolith sample 78221, the deepest part of the trench regolith collected at station 8. Similarly, mixing models based on major and trace-element composition indicate that the top layer of trench regolith at the station 8 (i.e., 78481) is composed of 40–50 % of mare and 52–59 % of highlands (Rhodes et al. 1974; Korotev and Kremser 1992). A recent study suggests that pure anorthosite is widely distributed over the entire Moon (Nagaoka et al. 2014). Contrary to thier suggestion, our results indicate that the fragments collected at the base of the Sculptured Hills are mostly derived from the Mg-suite rocks. However, it should be noted that the plagioclase-rich fragments analyzed in this work were selectively sampled and therefore should not be expected to yield similar results to previous studies based on bulk compositional data (Rhodes et al. 1974; Simon et al. 1981; Korotev and Kremser 1992). Due to the known lithologic complexity of the Sculptured Hills (Robinson and Jolliff 2002), and the plagioclase-rich fragments targeted for analysis, it is not surprising that the eleven plagioclase-rich lithic fragments were strongly biased against mare basaltic fragments.
We present the first 40Ar/39Ar ages on plagioclase-rich lithic fragments separated from trench regolith collected at station 8 during the Apollo 17 mission. The apparent 40Ar/39Ar ages for plagioclase-rich lithic fragments in general range from 4.0 to 4.4 Ga assuming a trapped 40Ar/36Ar of 0.146 (Fig. 3), the y-intercept of the regression between (40Ar/36Ar)m and (39Ar/36Ar)m. One sample (Plag#10) contains excess 40Ar interpreted to result from implantation of parentless 40Ar together with solar wind while exposed on the lunar surface. The CRE duration of Plag#10 is the longest among all the grains analyzed suggesting that this grain likely spent a longer duration on the lunar surface, most likely before 4 Ga and hence higher 40Ar was likely implanted with the solar wind. The variation in CRE ages (from ~1 to 24 Ma) in plagioclase-rich lithic fragments indicates that individual fragments have experienced recent movement within the lunar regolith during this time likely due to downslope movement at the base of the Sculptured Hills. Based on the chemical mixing model Korotev and Kremser (1992), chemical compositions, and 40Ar/39Ar ages, we argue that most of the plagioclase-rich lithic fragments analyzed in this study originated from Mg-suite of highland rocks. No plagioclase-rich fragments were derived from the mare region. The chemical and argon isotopic records preserved in Apollo 17 station 8 plagioclase-rich lithic fragments illustrate the wealth of information obtainable from submillimeter-sized samples returned from planetary objects including the Moon, Mars, and asteroids.
cadmium-lined in-core irradiation tube
cosmic ray exposure
high-titanium mare basalt
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This study was conducted in collaboration with the New York Center for Astrobiology. Financial support was provided by the NASA Astrobiology Institute (grant NNA09DA80A). We thank E. B. Watson for discussions during the course of this project. We thank Mark Norman for his constructive comments on an earlier draft of the manuscript. Jisun Park and an anonymous reviewer are thanked for their constructive comments that helped further improve the manuscript.
The authors declare that they have no competing interests.
SLB and JD conceived the project. JD separated the lithic fragments and characterized them. JPD designed and performed the experiments in consultation with SLB. JPD wrote the manuscript, together with SLB and JD. All authors read and approved the final manuscript.