Patrick Boehnke

Potentially biogenic carbon preserved in a 4.1 billion-year-old zircon

Significance Evidence for carbon cycling or biologic activity can be derived from carbon isotopes, because a high 12C/13C ratio is characteristic of biogenic carbon due to the large isotopic fractionation associated with enzymatic carbon fixation. The earliest materials measured for carbon isotopes at 3.8 Ga are isotopically light, and thus potentially biogenic. Because Earth’s known rock record extends only to ∼4 Ga, earlier periods of history are accessible only through mineral grains deposited in later sediments. We report 12C/13C of graphite preserved in 4.1-Ga zircon. Its complete encasement in crack-free, undisturbed zircon demonstrates that it is not contamination from more recent geologic processes. Its 12C-rich isotopic signature may be evidence for the origin of life on Earth by 4.1 Ga. Evidence of life on Earth is manifestly preserved in the rock record. However, the microfossil record only extends to ∼3.5 billion years (Ga), the chemofossil record arguably to ∼3.8 Ga, and the rock record to 4.0 Ga. Detrital zircons from Jack Hills, Western Australia range in age up to nearly 4.4 Ga. From a population of over 10,000 Jack Hills zircons, we identified one >3.8-Ga zircon that contains primary graphite inclusions. Here, we report carbon isotopic measurements on these inclusions in a concordant, 4.10 ± 0.01-Ga zircon. We interpret these inclusions as primary due to their enclosure in a crack-free host as shown by transmission X-ray microscopy and their crystal habit. Their δ13CPDB of −24 ± 5‰ is consistent with a biogenic origin and may be evidence that a terrestrial biosphere had emerged by 4.1 Ga, or ∼300 My earlier than has been previously proposed.

Warm storage for arc magmas

Significance The increasingly popular notion that steady-state magma chambers are highly crystallized, and thus only capable of erupting during brief (<1 ka) reheatings, implies that melt detection beneath volcanoes warns of imminent eruption. By integrating the microgeochronology and geochemistry of zircons from lavas with those from components crystallized within the magma chamber and incorporated during eruption, we show that the Soufrière (Saint Lucia) volcanic reservoir was instead eruptible over long (>100 ka) timescales. Together with data from other volcanic complexes, we show that arc magmas may generally be stored warm (are able to erupt for >100 ka). Thus geophysical detection of melt beneath volcanoes represents the normal state of magma storage and holds little potential as an indicator of volcanic hazard. Felsic magmatic systems represent the vast majority of volcanic activity that poses a threat to human life. The tempo and magnitude of these eruptions depends on the physical conditions under which magmas are retained within the crust. Recently the case has been made that volcanic reservoirs are rarely molten and only capable of eruption for durations as brief as 1,000 years following magma recharge. If the “cold storage” model is generally applicable, then geophysical detection of melt beneath volcanoes is likely a sign of imminent eruption. However, some arc volcanic centers have been active for tens of thousands of years and show evidence for the continual presence of melt. To address this seeming paradox, zircon geochronology and geochemistry from both the frozen lava and the cogenetic enclaves they host from the Soufrière Volcanic Center (SVC), a long-lived volcanic complex in the Lesser Antilles arc, were integrated to track the preeruptive thermal and chemical history of the magma reservoir. Our results show that the SVC reservoir was likely eruptible for periods of several tens of thousands of years or more with punctuated eruptions during these periods. These conclusions are consistent with results from other arc volcanic reservoirs and suggest that arc magmas are generally stored warm. Thus, the presence of intracrustal melt alone is insufficient as an indicator of imminent eruption, but instead represents the normal state of magma storage underneath dormant volcanoes.

Early formation of the Moon 4.51 billion years ago

Data on lunar zircons require the formation of the Moon by 4.51 Gy, therefore within the first 60 My of the solar system. Establishing the age of the Moon is critical to understanding solar system evolution and the formation of rocky planets, including Earth. However, despite its importance, the age of the Moon has never been accurately determined. We present uranium-lead dating of Apollo 14 zircon fragments that yield highly precise, concordant ages, demonstrating that they are robust against postcrystallization isotopic disturbances. Hafnium isotopic analyses of the same fragments show extremely low initial 176Hf/177Hf ratios corrected for cosmic ray exposure that are near the solar system initial value. Our data indicate differentiation of the lunar crust by 4.51 billion years, indicating the formation of the Moon within the first ~60 million years after the birth of the solar system.

Illusory Late Heavy Bombardments

Significance The vast majority of evidence marshaled for the Late Heavy Bombardment comes from 40Ar/39Ar age spectra of Apollo samples, interpreted through “plateau” ages, which show an apparent cluster at ∼3.9 Ga. Whether such data can be uniquely inverted to constrain impact histories in the Earth−Moon system has never been tested. We show that diffusive loss of 40Ar from a monotonically declining impactor flux coupled with the early and episodic nature of lunar crust formation tends to create clustered distributions of apparent 40Ar/39Ar ages at ca. 3.9 Ga. Instead, these 40Ar/39Ar data can be reconciled with a continuously decreasing bollide flux. Thus, impacts may have played a minimal role in terrestrial habitability, early Earth dynamics, and the formation of Hadean zircons. The Late Heavy Bombardment (LHB), a hypothesized impact spike at ∼3.9 Ga, is one of the major scientific concepts to emerge from Apollo-era lunar exploration. A significant portion of the evidence for the existence of the LHB comes from histograms of 40Ar/39Ar “plateau” ages (i.e., regions selected on the basis of apparent isochroneity). However, due to lunar magmatism and overprinting from subsequent impact events, virtually all Apollo-era samples show evidence for 40Ar/39Ar age spectrum disturbances, leaving open the possibility that partial 40Ar* resetting could bias interpretation of bombardment histories due to plateaus yielding misleadingly young ages. We examine this possibility through a physical model of 40Ar* diffusion in Apollo samples and test the uniqueness of the impact histories obtained by inverting plateau age histograms. Our results show that plateau histograms tend to yield age peaks, even in those cases where the input impact curve did not contain such a spike, in part due to the episodic nature of lunar crust or parent body formation. Restated, monotonically declining impact histories yield apparent age peaks that could be misinterpreted as LHB-type events. We further conclude that the assignment of apparent 40Ar/39Ar plateau ages bears an undesirably high degree of subjectivity. When compounded by inappropriate interpretations of histograms constructed from plateau ages, interpretation of apparent, but illusory, impact spikes is likely.

A meta-analysis of geochronologically relevant half-lives: what’s the best decay constant?

Twenty-first century advances in both the analytical procedures and instrumentation used in geochronology promise age accuracy better than ±1‰, but realizing this potential requires knowledge of decay constants (λ) that exceed this level. Given the paucity of improved recent measurements of λ, the community has experimented with hybrid methodologies utilizing data largely generated during the 1970s. In this article, we perform a systematic review of laboratory decay constant determinations relevant to geochronology (i.e. 87Rb, 147Sm, 176Lu, 230Th, 232Th, 235U, and 238U), focusing on methodological consistency. For radioisotopes for which multiple studies are available, results are combined through a random effects model to yield the best available values and associated uncertainties. Unfortunately, despite its vital role in modern geochronology, only one experimental determination of 238U decay met our criteria for consideration, significantly limiting the ability to assess its reliability. Thus, utilizing λ238 as an anchor for establishing other decay constants (e.g. 40K, 176Lu, and spontaneous 238U fission) places an unverified result at the core of geochronology. For geochronology to attain its greatest potential, more and better laboratory determinations of decay constants are required, along with a community methodology that permits us to continuously take advantage of new data.

Differentiated impact melt sheets may be a potential source of Hadean detrital zircon: COMMENT

Kenny et al.’s (2016) ion microprobe zircon crystallization temperature data for the Sudbury impact crater adds to the existing database (Wielicki et al., 2012) for terrestrial impact melts. They note that zircons from the granophyre layer had not previously been analyzed by ion microprobe commensurate with its volumetric importance (20%–45%; Lightfoot et al., 1997) potentially biasing its comparator value when evaluating possible sources for the Hadean Jack Hills zircon population. However, the authors neglected to quantify the degree to which their data set further constrains this issue. Using data from their GSA Data Repository item 2016143, we tested the hypothesis that variants of the impact zircon record (as determined solely from ion microprobe data; Wielicki et al., 2012; Kenny et al., 2016) represent the same probability distribution (i.e., the null hypothesis) as the Hadean population (Harrison and Schmitt, 2007) through a Kolmogorov-Smirnov test (R Core Team, 2013). We reject the null hypothesis for both the case that the Kenny et al. data alone represent the Hadean population (p = 2 × 10), and that for all reported impact zircons (p < 2 × 10; Wielicki et al., 2012; Kenny et al., 2016). In fact, the hypothesis that the granophyre data alone are equivalent to the Hadean population can be rejected (p = 4 × 10). Taken at face value, these results appear to support the conclusion of Wielicki et al. (2012) that the Hadean Jack Hills zircon temperature distribution was not derived in any significant way from impact-derived zircons. However, Kenny et al. raise the prospect of a selection process preferentially destroying high-temperature Hadean zircons and thus biasing the detrital record to lower temperatures. In fact, nature does tend to bias the detrital zircon record, but that mechanism operates in exactly the opposite sense. Late crystallizing, thus low-temperature, granitoid zircons are known to contain elevated U and Th concentrations which lead to metamictization (Claiborne et al., 2010) and thus their likely removal from the detrital record, resulting in preferential preservation of higher-temperature zircons (Harrison and Schmitt, 2007). Thus, without the benefit of some as yet unknown selection mechanism, the probability of extracting the Hadean Ti-in-zircon temperature distribution from the data reported by Kenny et al., or any published data set of impactproduced zircons, remains vanishingly small.

TITANIUM ISOTOPE SOURCE RELATIONS AND THE EXTENT OF MIXING IN THE PROTO-SOLAR NEBULA EXAMINED BY INDEPENDENT COMPONENT ANALYSIS

The Ti isotope variations observed in hibonites represent some of the largest isotope anomalies observed in the solar system. Titanium isotope compositions have previously been reported for a wide variety of different early solar system materials, including calcium, aluminum rich inclusions (CAIs) and CM hibonite grains, some of the earliest materials to form in the solar system, and bulk meteorites which formed later. These data have the potential to allow mixing of material to be traced between many different regions of the early solar system. We have used independent component analysis to examine the mixing end-members required to produce the compositions observed in the different data sets. The independent component analysis yields results identical to a linear regression for the bulk meteorites. The components identified for hibonite suggest that most of the grains are consistent with binary mixing from one of three highly anomalous nucleosynthetic sources. Comparison of these end-members show that the sources which dominate the variation of compositions in the meteorite parent body forming regions was not present in the region in which the hibonites formed. This suggests that the source which dominates variation in Ti isotope anomalies between the bulk meteorites was not present when the hibonite grains were forming. One explanation is that the bulk meteorite source may not be a primary nucleosynthetic source but was created by mixing two or more of the hibonite sources. Alternatively, the hibonite sources may have been diluted during subsequent nebula processing and are not a dominant solar system signatures.

Aluminum in zircon as evidence for peraluminous and metaluminous melts from the Hadean to present

Zircon structurally accommodates a range of trace impurities into its lattice, a feature which is used extensively to investigate the evolution of silicate magmas. One key compositional boundary of magmas is defined by whether the molar ratio of Al2O3/(CaO + Na2O + K2O) is larger or smaller than unity. Here we report ∼800 Al in zircon concentrations from 19 different rocks from the Lachlan Fold Belt (southeastern Australia), New England (USA), and Arunachal leucogranites (eastern Himalaya) with Al2O3/(CaO + Na2O + K2O) whole rock values that range from 0.88 to 1.6. Zircons from peraluminous rocks yield an average Al concentration of ∼10 ppm, which distinguishes them from crystals found in metaluminous rocks (∼1.3 ppm). This difference is related to the materials involved in the melting, assimilation, and/or magma differentiation processes; for example, magmas that assimilate Al-rich material such as metapelites are expected to produce melts with elevated alumina activities, and thus zircons with high Al concentrations. These observations are applied to the Archean and Hadean Jack Hills detrital zircon record. Detrital Archean zircons, with ages from about 3.30 to 3.75 Ga, yield Al in zircon concentrations consistent with origins in peraluminous rocks in ∼8% of the cases (n = 236). A single zircon from the pre-3.9 Ga age group (n = 39) contains elevated Al contents, which suggests that metaluminous crustal rocks were more common than peraluminous rocks in the Hadean. Weathered material assimilated into these Hadean source melts was not dominated by Al-rich source material.

Potassic, high-silica Hadean crust

Significance To understand early Earth’s habitability, we need to know when the continental crust first formed. However, due to the combined actions of plate tectonics and erosion, most of the evidence of the early crust has been destroyed. To shed light on this debate, we analyzed the strontium isotopic composition of apatite inclusions in zircons from Nuvvuagittuq, Canada, where independent evidence suggests a crust-forming event prior to 4.2 Ga, possibly as early as 4.4 Ga. Our results show that this early crust had a high Rb/Sr ratio and therefore a high silica content. This suggests that the early Earth was capable of forming continental crust within <350 million y of solar system formation. Understanding Hadean (>4 Ga) Earth requires knowledge of its crust. The composition of the crust and volatiles migrating through it directly influence the makeup of the atmosphere, the composition of seawater, and nutrient availability. Despite its importance, there is little known and less agreed upon regarding the nature of the Hadean crust. By analyzing the 87Sr/86Sr ratio of apatite inclusions in Archean zircons from Nuvvuagittuq, Canada, we show that its protolith had formed a high (>1) Rb/Sr ratio reservoir by at least 4.2 Ga. This result implies that the early crust had a broad range of igneous rocks, extending from mafic to highly silicic compositions.

Secondary magnetic inclusions in detrital zircons from the Jack Hills, Western Australia, and implications for the origin of the geodynamo

The time of origin of Earth’s dynamo is unknown. Detrital zircon crystals containing ferromagnetic inclusions from the Jack Hills of Western Australia have the potential to contain the oldest records of the geodynamo. It has recently been argued that magnetization in these zircons indicates that an active dynamo existed as far back as 4.2 Ga. However, the ages of ferromagnetic inclusions in the zircons are unknown. Here we present the first detailed characterization of the mineralogy and spatial distribution of ferromagnetic minerals in Jack Hills detrital zircons. We demonstrate that ferromagnetic minerals in most Jack Hills zircons are commonly located in cracks and on the zircons’ exteriors. Hematite is observed to dominate the magnetization of many zircons, while other zircons also contain significant quantities of magnetite and goethite. This indicates that the magnetization of most zircons is likely to be dominantly carried by secondary minerals that could be hundreds of millions to billions of years younger than the zircons’ crystallization ages. We conclude that the existence of the geodynamo prior to 3.5 Ga has yet to be established.