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We undertook Zr isotope measurements on zircon, titanite, biotite, amphibole, and whole rocks from the La Posta pluton (Peninsular Ranges, southern California) together with trace element analyses and U-Pb age measurements to understand the controls on Zr isotope fractionation in igneous rocks, including temperature, crystallization sequence, and kinetic effects. We find large (>0.6‰) Zr isotope fractionations (expressed as δ⁹⁴/⁹⁰Zr) between titanite and zircon forming at approximately the same temperature. Using equilibrium fractionation factors calculated from ionic and ab initio models, we infer the controls on Zr isotope evolution to include the relative order in which phases appear on the liquidus, with titanite fractionation resulting in isotopically lighter melt and zircon fractionation resulting in isotopically heavier melt. While these models of Zr fractionation can explain δ⁹⁴/⁹⁰Zr variations in zircon of up to ∼1.5‰, crystallization order, temperature and presence of co-crystallizing phases do not explain all aspects of the intracrystalline Zr isotopic distribution in zircons in the La Posta pluton or the large range of Zr isotopic values among zircons (>2‰). Without additional constraints, such as knowledge of co-crystallizing phases and a better understand of the true causes of Zr isotope fractionation, Zr isotopes in zircon remains an ambiguous proxy of magmatic evolution.

Tectonic deformation can influence spatiotemporal patterns of erosion by changing both base level and the mechanical state of bedrock. Although base-level change and the resulting erosion are well understood, the impact of tectonic damage on bedrock erodibility has rarely been quantified. Eastern Tibet, a tectonically active region with diverse lithologies and multiple active fault zones, provides a suitable field site to understand how tectonic deformation controls erosion and topography. In this study, we quantified erosion coefficients using the relationship between millennial erosion rates and the corresponding channel steepness. Our work shows a twofold increase in erosion coefficients between basins within 15 km of major faults compared to those beyond 15 km, suggesting that tectonic deformation through seismic shaking and rock damage significantly affects eastern Tibet erosion and topography. This work demonstrates a field-based, quantitative relationship between rock erodibility and fault damage, which has important implications for improving landscape evolution models.

Accurately quantifying the composition of continental crust on Hadean and Archean Earth is critical to our understanding of the physiography, tectonics, and climate of our planet at the dawn of life. One longstanding paradigm involves the growth of a relatively mafic planetary crust over the first 1 to 2 billion years of Earth history, implying a lack of modern plate tectonics and a paucity of subaerial crust, and consequently lacking an efficient mechanism to regulate climate. Others have proposed a more uniformitarian view in which Archean and Hadean continents were only slightly more mafic than at present. Apart from complications in assessing early crustal composition introduced by crustal preservation and sampling biases, effects such as the secular cooling of Earth’s mantle and the biologically driven oxidation of Earth’s atmosphere have not been fully investigated. We find that the former complicates efforts to infer crustal silica from compatible or incompatible element abundances, while the latter undermines estimates of crustal silica content inferred from terrigenous sediments. Accounting for these complications, we find that the data are most parsimoniously explained by a model with nearly constant crustal silica since at least the early Archean.

The Hadean Jack Hills zircons represent the oldest known terrestrial material, providing a unique and truly direct record of Hadean Earth history. This zircon population has been extensively studied via high-spatial-resolution high-throughput in situ isotopic and elemental analysis techniques, such as secondary ionization mass spectrometry (SIMS), but not by comparatively destructive, high-temporal-precision ( % two-sigma) thermal ionization mass spectrometry (TIMS). In order to better understand the lead loss and alteration history of terrestrial Hadean zircons, we conduct stepwise chemical abrasion–isotope dilution–thermal ionization mass spectrometry with trace element analysis (CA-ID-TIMS-TEA) on manually microfractured Hadean Jack Hills zircon fragments previously dated by SIMS. We conducted three successive HF leaching steps on each individual zircon fragment, followed by column chromatography to isolate U–Pb and trace element fractions. Following isotopic and elemental analysis, the result is an independent age and trace element composition for each leachate of each zircon fragment. We observe ∼50 Myr of age heterogeneity in concordant residues from a single zircon grain, along with a protracted history of post-Hadean Pb loss with at least two modes circa ∼0 and 2–4 Ga. Meanwhile, stepwise leachate trace element chemistry reveals enrichments of light rare earth elements, uranium, thorium, and radiogenic lead in early leached domains relative to the zircon residue. In addition to confirming the efficacy of the LREE-I alteration index and providing new insight into the mechanism of chemical abrasion, the interpretation and reconciliation of these results suggest that Pb loss is largely driven by low-temperature aqueous recrystallization and that regional thermal events may act to halt – not initiate – Pb loss from metamict domains in the Hadean Jack Hills zircons.

Geochemical data on widely distributed igneous rocks of southern Tibet are used to reconstruct paleo-crustal thickness during the 50+ million years that have elapsed since the onset of the India-Asia collision. We use two approaches, one based on Nd isotopes and an assimilation-recharge model for granitic magma genesis and another empirical method based on trace element geochemistry (La/Yb). The focus is on granitic rocks of two age ranges in a segment of the southern Lhasa Block between approximately 89.5° and 92.5°E longitude. One age range, 45 to 62 Ma, spans the time of the onset of collision and for which we infer the geochemistry of granitic rocks reflects mainly pre-collision structure. The other age range is 21 to 9 Ma for the Nd isotopic approach, and 32 to 9 Ma for La/Yb, where the geochemistry must reflect post-collision structure. Our results suggest that the pre- and syn-collision southern margin of the Lhasa block, that portion now located within 50–60 km of the Indus-Yarlung suture (IYS) and south of 29.8°N latitude was relatively thin, about 25–35 km thick until 45 Ma. At approximately 29.8°-29.9°N latitude there was a pronounced crustal discontinuity, and north of that latitude (for a distance that we cannot constrain), the inferred crustal thickness was greater, at least 50–55 km, as indicated by latest Cretaceous and Early Tertiary granitoids and ignimbrites that have large fractions of assimilated continental crust and high La/Yb ratios. Post-collision Nd isotopic and La/Yb data from granitoids younger than 32 Ma suggest that the southern margin south of 29.8°N was thickened substantially to at least 55–60 km (based on Nd isotopes) and possibly as much as 70–75 km (based on La/Yb) by Early to mid-Miocene time. These observations require that thickening of the southern Lhasa Block margin in the period 45–32 Ma was non-uniform; the crust now within 60 km of the suture was thickened by approximately 40 km whereas the crust north of 29.9°N latitude was thickened much less, or not at all. The region currently between 29.8°N and the YTS may have been the highest elevation mountain terrane in the period from roughly 30 to 20 Ma. The amount of Miocene denudation reflects this difference, as there is evidence of substantially more denudation near the IYS than in the region north of 29.9°N. Some of the difference in thickening could be due to magmatic additions from the mantle in the region south of 29.8°N, but there is need for at least 30 km of tectonic thickening between 45 and 32 Ma. The non-uniform thickening suggests that the high elevations at the southern margin of the Himalaya-Tibet orogen propagated southward by about 200 km, from north of Lhasa to their present position, during the period from 50 to 20 Ma. Present crustal thickness requires an additional 10–15 km of more uniform post-Miocene thickening.

Trace elements in magmatic zircon provide a wealth of petrogenetic information about host magmas. However, they are susceptible to alteration through post-magmatic interaction with hydrothermal fluids. Trace element analyses can also be biased by the inadvertent inclusion of exotic materials, such as mineral or glass inclusions, in analyzed volumes of zircon. In order to screen out samples with altered chemical signatures, zircons with high, flat light rare earth element (LREE) patterns are typically considered to be altered. However, visual selection of such patterns is qualitative and does not address ambiguous cases. The light rare earth element index (LREE-I = Dy/Nd + Dy/Sm) provides an approach for quantitative screening for aqueous alteration and contamination of zircon by exotic materials and was used to assess secondary processes in the greenschist facies Jack Hills detrital zircon suite. However, in addition to aqueous alteration, the LREE-I is also sensitive to melt compositional evolution, and its applicability to alteration in settings other than the Jack Hills quartzite is thus far undetermined. We investigate igneous zircon populations from a variety of geologic settings that show evidence for alteration by contact metamorphism during magma intrusion and deuteric fluid interactions during pluton crystallization. In suites with a high proportion of texturally altered zircons, low LREE-I values are common and this parameter correlates well with other contamination indicators (based on the observed secondary phases deposited in the zircons during fluid flow). Filtering zircon trace element compositions based on the LREE-I appears to remove the majority of chemically altered zircon, in many cases revealing previously obscured magmatic signals.

We show that the existence and evolution of the geodynamo before 3.5 Ga ago remains unknown., The time of origin of the geodynamo has important implications for the thermal evolution of the planetary interior and the habitability of early Earth. It has been proposed that detrital zircon grains from Jack Hills, Western Australia, provide evidence for an active geodynamo as early as 4.2 billion years (Ga) ago. However, our combined paleomagnetic, geochemical, and mineralogical studies on Jack Hills zircons indicate that most have poor magnetic recording properties and secondary magnetization carriers that postdate the formation of the zircons. Therefore, the existence of the geodynamo before 3.5 Ga ago remains unknown.

The study of Earth as an object whose history can be understood by application of physical laws dates back 200 years. This tradition is, however, rife with missteps related to as yet undiscovered physics or fundamentally incorrect assumptions. While the former is unavoidable, the latter amounts to self-inflicted wounds that may have forestalled scientific progress. Even in the absence of knowledge of initial conditions, linear mathematical relationships such as first order loss (e.g., radioactive decay) have proved useful in predicting Hadean conditions. However, more complex physical systems cannot be uniquely extrapolated back in time. For example, mantle convection, a highly non-linear, dispersive, chaotic system is, by its very nature, uninvertible. This fact has not inhibited generations of modeler’s from making ab initio predictions regarding early Earth evolution. Their results were initially limited by technological impediments and adoption of assumptions regarding the relationship between interior temperature and planetary heat loss that narrowed possible solutions. Radically new proposals regarding both the latter issue and discontinuous transitions between modes of heat loss have tempered earlier conclusions that plate-tectonic-like behavior could not arise on early Earth. Physical calculations have an important role to play in assessing the plausibility of Hadean geodynamic models, but should best be seen as “convenient fictions”.

Estimates of when plate tectonics began range from the last 20% of Earth history to within the first 5%. While there is no observation that precludes plate tectonics from operating at 4.3 Ga, evidence that it was is indirect. Although subduction initiation is a robust feature of the modern plate tectonic system and we can calculate with some accuracy when oceanic lithosphere attains negative buoyancy, we don’t yet understand how strong the lithosphere weakens sufficiently for subduction to initiate. Most approaches used to estimate when Earth first entered the mobile lid regime—preservation of modern plate tectonic features, detrital zircon age spectra, trace element and radiogenic isotope geochemistry, atmosphere-crust-mantle exchange, and model-based estimates—can be interpreted in multiple ways and are all underlain by assumptions that cannot be independently tested. All share the flaw that absence of evidence is not evidence of absence. Of special concern is that the Precambrian geologic record is likely biased to rock compositions most likely to resist deformation and thus exposure to erosion at newly rifted continental margins where loss to subduction erosion could occur. Thus any look-back comparison is flawed to some degree by a preservation bias. A more recently recognized limitation is the failure to consider how a hotter, early Earth would differ petrologically from, say, Phanerozoic behavior (e.g., lower incompatible trace element concentrations in mantle magmas, higher geothermal gradients). Historically, computational limitations in early geophysical modelling methods led to skepticism regarding the possibility of plate tectonics on early Earth. Influenced by this view, the geologic community was reluctant to take a dynamic view of the preserved crustal record, instead inferring the apparent absence of a Hadean rock record as evidence that there never was one. The unknown extent to which ancient continental crust was recycled into the mantle and thoroughly mixed, the abovementioned selection biases in the rock record, and the assumption of uniformitarian conditions throughout Earth history limit virtually all continental growth estimates to providing only lower age bounds and thus minimum estimate on the initiation age of subduction.

Hadean zircons have been documented from fifteen terrestrial localities in Australia, Asia, Africa, and North and South America, in stony and martian meteorites, and in lunar rocks. Extraterrestrial zircons are characterized by the absence of the positive Ce anomaly, seen in virtually all terrestrial zircons, much higher formation temperatures, and a unique suite of mineral inclusions. Remarkably little effort has been directed toward characterizing the geochemical nature of Hadean zircons from terrestrial localities beyond the Jack Hills region and thus it remains unclear how representative it is of the Hadean world. A massive analysis campaign is indicated to better understand Earth’s last true ‘dark age’.

Evidence from short- and long-lived radioisotope systems indicates that Earth was largely built from volatile-depleted planetesimals and planetary embryos during runaway accretion that occurred within 1–10 Ma of formation of the first solar system solids at 4.567 ± 0.001 Ga. Both long- and short-lived radionuclides leave isotopic signatures in mantle rocks that bear on when and how the silicate Earth formed and differentiated. The longstanding view that the Hadean mantle was compositionally undepleted appeared to be contradicted by differences between mantle and chondrite Nd isotopes which suggested a very early enriched terrestrial reservoir. Although recent work indicates this difference reflects differing irradiation histories of Earth-forming-materials and meteorites, and thus has little bearing on the timing of silicate differentiation, both terrestrial and lunar Lu–Hf zircon data appear to require global silicate differentiation by 4.50 ± 0.02 Ga billion years. Tungsten isotopic data from mantle rocks provides evidence of core formation by about 4.53 Ga and either very early isotopic isolation of silicate reservoirs or disturbance by a late chondritic veneer. Despite evidence that Moon formed substantially from proto-Earth material between about 4.53 and 4.50 Ga, the exact mechanism by which this occurred remains controversial. The once widely accepted model of collision of a Mars-sized body has lost support in light of contradictory evidence in the form of indistinguishable isotopic compositions of volatile and refractory elements between Earth and Moon. Models that appear to transcend this problem (hit-and-run collision, synestia, successive smaller collisions, magma ocean heating, etc.) are currently being evaluated. Although geochemical evidence requiring an early terrestrial magma ocean is almost entirely lacking, the sources of thermal energy available during accretion make such an appearance appear inevitable. If solidification proceeded from the bottom up, vigorous convection would have caused the lower mantle to rapidly crystallize with the upper mantle becoming largely solidified within several million years. The high abundance of highly siderophile elements in the upper mantle is strong evidence that Earth added at least half a percent of its present mass following core formation but prior to the Mesoarchean. Preservation of mantle isotopic anomalies throughout the Hadean-Archean seem unlikely to reflect sluggish mantle convention in a stagnant lid tectonic regime during that period as the plate tectonic era is associated with a large range of isolated mantle isotopic domains.

We don’t know with confidence the mechanisms by which primitive arc basalts are modified to produce the broadly granodioritic continental crust but there is widespread agreement that plate tectonics has been doing just that for at least the past billion years. We also don’t fully understand the structure of the continental crust; popular layered models lack mechanisms to produce such structures or to recover them following tectonic homogenization. The geochemical community long favored the view that early crust was mafic, in part owing to misconceptions regarding feldspar buoyancy on a hydrous magmatic substrate and the deep stabilization of garnet (which retards crystallization of more buoyant aluminous phases from the magma). But early felsic crusts with the potential for long term stability could have emerged via crystallization of tonalitic liquids fractionated from ultramafic magmas in equilibrium with olivine or differentiating magma sheets following large impacts into early basaltic crusts. The remarkable range of estimates of the growth history of continental crust reflects a number of influences but, generally speaking, earlier growth has been increasingly favored as new age survey methodologies became available and as the effects that crustal reworking and recycling have on apparent surface age provinces became better appreciated. Isotopic data once thought to support rapid growth at ~2.7 Ga are now recognized as equally consistent with constant volume continental crust. The longstanding misapprehension that the present-day distribution of crust formation ages is equivalent to the growth history of continents strongly influenced some estimates. Instead, today’s crust represents a running balance between new growth, internal overprinting, and crustal recycling. The difficulty in deconvolving these processes is one of the two principal challenges in establishing the growth history of continental crust. The other is that crust recycled back into the mantle and thoroughly mixed leaves no trace of its past incarnations. Although the rock record has yet to yield clear, direct evidence from which to constrain the magnitude of Hadean continental crust, optimal solutions to modelling mantle isotopic data are at least as consistent with constant volume continental crust since ca. 4.4 Ga as with slow monotonic growth. Radiogenic isotopic data used to argue for an early mafic crust are contradicted by stable isotopic results that appear to support a continuously felsic continental crust of unknown volume.

This chapter summarizes what is known about the timing of the emergence of life on Earth from the morpho- and chemo-fossil (chemical and isotopic signals remaining from the decomposition of living organisms) records. The geologic record back to ca. 3.5 billion years includes low grade sedimentary rocks in which organic residues of microbiota present during deposition have remained substantially intact. As different metabolic mechanisms variably fractionate carbon isotopes toward isotopically light values, a longstanding strategy has been to measure δ13C in these organic residues, or kerogens, for biologic signatures. When compared to carbon isotopes in inorganic carbonate rocks, a consistent offset is seen throughout the past 3.5 billion years with inorganic carbon averaging δ13C close to 0‰ and kerogens yielding δ13C of approximately −25‰. As the latter value is broadly characteristic of oxygenating photosynthesis, this relationship has been seen as evidence of past biologic activity. However, as metamorphic grade increases, kerogens are reacted to simpler hydrocarbons, ultimately yielding graphitic residues. The discovery of isotopically light carbon isotopes in microscopic graphite inclusions in rocks as old as ca. 3.83 billion years and in a 4.1 Ga zircon extends the possible emergence of life on this planet back into the Hadean eon. Although inorganic mechanisms exist that could potentially produce light δ13C signatures, these isotopic data are consistent with molecular clock calibrations of genomic mutations which suggest a lower bound for the time of life’s origin between 4.1 and 4.4 billion years.

Given the absence of a macroscopic Hadean rock record, evaluating terrestrial habitability is largely a thought experiment, but data from Hadean zircons can provide some constraints. We are certain that life as we know it would not be possible without four requirements; soluble bioactive elements (carbon, hydrogen, oxygen, nitrogen, sulfur and phosphorous), free energy, liquid water, and time. Beyond these essential ingredients, there is broad agreement that there are ten secondary factors that separate us from the other, uninhabited terrestrial planets and maintain our planet’s homeostasis. They are: (1) a galactic and planetary sanctuary for life; (2) liquid water at the planetary surface to mediate biochemistry and efficiently cool the planet; (3) dissolved water in the deep planetary interior to enhance mantle circulation and catalyze the eclogite transition; (4) a broadly solar chemical composition to provide sufficient metallicity for a stable surface platform; (5) sufficient planetary mass to retain an atmosphere and heat; (6) planetary satellite(s) to stabilize climate zones; (7) extra-planetary impactors to introduce organic building blocks and water and to create satellites; (8) long-term interior heat generation to maintain mantle circulation and the geodynamo; (9) a self-sustaining dynamo to protect the atmosphere is erosion; and (10) a mechanism to recycle surface carbon into the interior and back. Evaluating how these various factors interact is complicated but our speculations can be guided by inferences from Hadean zircon geochemistry which potentially bear on six of the ten ingredients for life—the presence of surface and interior water, the role of impacts on early Earth, internal heat generation, surface recycling, and the existence of a Hadean geodynamo. Knowledge of the geochemistry and inclusion population of Hadean zircons also permits constraints to be placed on whether mineral phases and trace elements key to biopoiesis were present during the Hadean eon.

Geochemical analysis of zircons older than 4 billion years, found in Early Archean metasediments at Jack Hills, Western Australia, provide insights into the nature of Hadean Earth. Oxygen isotopes have been interpreted as indicating that protoliths of magmas from which Hadean zircons crystallized were formed in the presence of water at or near Earth’s surface. Apparent crystallization temperatures of Hadean zircons cluster at 680 °C. Given the low porosity expected in rocks under anatectic conditions, dehydration melting of micas as the principal source of the melts from which these zircons crystallized can be ruled out. Instead, a regulated mechanism producing near minimum-melting conditions during the Hadean is inferred. Combined, these results have been interpreted to reflect chemical weathering and sediment cycling in the presence of liquid water shortly after Earth accretion. 176Hf/177Hf ratios of Hadean Jack Hills zircons show large heterogeneities indicating a major differentiation of the silicate Earth by 4.50 Ga. A possible consequence of this differentiation is the formation of continental crust of similar order to the present. Studies of mineral inclusions within Hadean zircons indicate their crystallization from hydrous, granitoid magmas at pressures greater than 6 kbars, implying low near-surface geothermal gradients which in turn suggests their origin in underthrust environments. Given general agreement that life could not have emerged until liquid water appeared at or near Earth’s surface, a significant implication is that our planet may have been habitable as much as 500 Ma earlier than previously thought. Indeed, carbon isotopic evidence obtained from inclusions in a Hadean zircon is consistent with life having emerged by 4.1 Ga, or several 100 million years earlier that the hypothesized lunar cataclysm. Trace element analyses of aluminum, halogens, sulfur, phosphorus, rare earth elements in Hadean zircons are consistent with their origin in a range of granitoid magma types and redox conditions. Although some of the above interpretations remain subject to debate, there is now a widespread consensus that molecular water was present at or near Earth’s surface since at least 4.3 Ga. Perhaps the most remarkable feature of inferences drawn from investigations of these ancient zircons is that none were predicted from theory, underscoring the importance of observations in testing models of early Earth.

As much as half of lunar surface rocks may have originated between 4.4 and 3.9 billion years and thus observations of, and samples from, Moon could attest to conditions then extant in the inner solar system. The concept of a lunar cataclysm at ~3.9 Ga grew from seemingly contradictory observations of elemental fractionation in lunar highland rocks. U–Pb—and some Rb–Sr—data suggested recrystallization occurred between about 4.0 and 3.8 Ga. The Late Heavy Bombardment (LHB) concept that emerged appeared supported by ~3.9 Ga 40Ar/39Ar “plateau ages” of lunar impact melt rocks, although no similar spike in ages was seen in the likely more globally distributed lunar meteorites. While the 40Ar/39Ar step-heating method can reveal intragrain isotope variations, this capability has several method-specific requirements that, if not met, preclude thermochronologic interpretations. Three such issues effectively rule out the use of virtually all lunar 40Ar/39Ar data as support for the LHB hypothesis: (1) the “plateau age” approach used is an aphysical concept for the thermally disturbed samples typical of most lunar impact melt rocks, (2) laboratory artifacts destroy preserved diffusion information, or create false apparent age gradients; and (3) obtaining meaningful thermal history information from extraterrestrial samples that have differing activation energies for Ar diffusion in their K-bearing phases requires a different laboratory protocol than was used on lunar rocks. Possibly due to these issues, no case in which multiple chronometric techniques have yielded intrasample concordancy of a lunar melt rock has yet been documented. Advancements in mass spectrometry now permit 40Ar/39Ar and U–Pb dating to be undertaken on small (10 s-of-μm diameter) in situ spots on glasses and accessory minerals in lunar rocks. This approach has the potential to transcend the analytical challenge posed by the continuous impact reworking of the lunar regolith that produces fine-scale polygenetic breccias of multiple age and origins. The longstanding assumption that lunar melt rocks originated from discrete, basin-forming events is obviated by lunar imaging that show impact melts formed in small highland craters and clusters of ‘light plains’ deposits radiating outward >2000 km from large impact basins. The latter underscores how poorly the spatial relationships between large basins and their surrounding deposits were understood when impact chronologies were developed in the 1970s. The assumption that a specific lunar melt rock from a given landing site is representative of one of the basin-forming impacts is deeply flawed. Establishing a reliable, quantitative planetary impact chronology requires that all analyzed rocks be equally suitable for the application of specific chronometers. This may not be possible given the large contrasts in incompatible trace element distributions across the lunar surface (e.g., Procellarum KREEP terrane, South Pole Aiken basin). A conservative view of the lunar chronological record is that the large nearside basins are older than 3.82 Ga but these data are consistent with most of them being older than 3.92 Ga and possibly older than 4.35 Ga.

Any successful geodynamic or environmental model for early Earth must be consistent with ten robust lines of evidence derived from geochemical and petrologic observations of Hadean Jack Hills zircons. These are: (1) a zircon sub-population enriched in 18O and depleted in 30Si relative to mantle values; (2) low crystallization temperatures; (3) the presence of primary hydrous mineral inclusions; (4) the predominance of magmatic muscovite, quartz, and biotite inclusions; (5) zircon formation in relatively low heat flow environments; (6) sub-chondritic initial 176Hf/177Hf ratios consistent with source isolation as early as 4.50 Ga; (7) fission Xe isotope compositions indicating variable fractionation of Pu from U; (8) the absence of ultra-high pressure mineral inclusions; (9) zircon formation under a wide range of redox conditions; and (10) geochemical signatures diagnostic of felsic continental crust. Numerous models have been proposed to explain these characteristics, including an origin similar to Icelandic rhyolites or lunar KREEP terranes, crystallization from mafic igneous rocks, formation in impact melts or sagduction, plate boundary and heat pipe tectonic environments, and multi-stage scenarios involving several of these mechanisms. While an origin of Jack Hills Hadean zircons in felsic and intermediate granitoids in a plate-boundary-type setting is consistent with all ten geochemically-derived constraints, competitor models are either only partially consistent or inconsistent with the evidence.

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.

Mineral inclusions are common in magmatic zircon and a potentially rich source of petrologic information. Controls on the relative proportions of inclusion phases, specifically early-crystallizing minerals such as apatite and late-crystallizing phases such as quartz, K-feldspar, and muscovite, have not been systematically studied. For instance, apatite dominates many magmatic zircon inclusion suites, and selective replacement of apatite over other phases has been proposed as a mechanism for generating apatite-poor inclusion assemblages in detrital zircons. However, the extent to which apatite inclusion abundance is influenced by source rock composition has not been established. The preservation of characteristic minerals in granite series, such as differences in magnetite and ilmenite abundances due to varying redox, have also not been systematically explored as inclusion phases in zircon. We surveyed zircon inclusion assemblages in Phanerozoic granitoids of a range of compositions and found a broadly inverse relationship between the presence of apatite in the inclusion suite and whole-rock SiO2 content. Selective loss of apatite is evident from deficits in apatite content among inclusions in contact with cracks in both detrital zircons and some granitoid zircons with independent evidence for fluid ingress (i.e., secondary phases filling open cracks). In cases where microstructural observations can identify primary inclusion assemblages (versus those impacted by fluid ingress), the relative occurrence of apatite can be used to broadly predict source rock SiO2 content. There is little relationship between whole-rock chemistry and the abundance of late-crystallizing phases or the relative proportions of quartz, plagioclase, and alkali feldspar. The presence of ilmenite not in contact with cracks in the host zircon likely points to origins in ilmenite-series (i.e., reduced) magmas, but the presence of magnetite is less diagnostic. Using the apatite abundance metric, detrital Hadean zircons from Jack Hills (Western Australia), appear to derive from felsic rather than intermediate or mafic granitoids and detrital Eoarchean zircons from Nuvvuagittuq Supracrustal Belt (Quebec) may derive from intermediate to felsic granitoids. No Fe-Ti oxides have been identified in Nuvvuagittuq zircons, but rare ilmenite in Jack Hills zircons points to origins of at least some part of the population in ilmenite-series magmas.

A radio-frequency plasma ion source, the Hyperion-II, has been commissioned on a CAMECA ims1290, a high resolution/high transmission secondary ion mass spectrometer at UCLA. Performance characteristics (e.g., beam density, spot size, etc.) of the primary oxygen beam are documented and application to isotopic analyses requiring high lateral resolution with high secondary ion transmission are described. The Hyperion source delivers on average 10 times and 6 times the current density delivered by the CAMECA duoplasmatron for 16O− and 16O2− beams, respectively. This significantly enhanced current density allows for reduction of the analytical spot size by at least a factor of three while maintaining the beam intensity, making it possible to perform isotopic measurements at smaller scales without sacrificing analytical precision. In addition, the smaller Hyperion beam spot size also reduces the crossover size and spherical aberrations in the secondary ion optics, thereby improving secondary ion transmission at mass resolving power of 3000–12,000 by 25–80% compared to that achieved with a duoplasmatron.

Natural thermoluminescence (TL) signals from feldspar crystals extracted from thermally stable drill cores ( T = − 4.1 − 60.2 ∘ C) exhibit a strong dependence on geologic and laboratory thermal conditions. As burial temperature increases, the position of the TL glow curve at half-maximum intensity (i.e., the T 1 / 2 parameter) shifts to higher measurement temperatures. This shift is also observed following isothermal treatments in the laboratory. This relationship can be explained using a kinetic model originally developed for the optical luminescence dating of feldspar grains. The thermal history of a sample is preserved in the degree of electron trap saturation as a function of thermal detrapping probability, which varies with recombination distance. A natural feldspar sample contains a range of thermal stabilities: the least stable traps will remain empty, the most stable will be full, and those traps which are partially filled will, in the case of thermal equilibrium, be diagnostic of the storage temperature. The T 1 / 2 parameter of a TL glow curve reflects which sites remain occupied. This interpretation is further borne out by additive dose measurements which illustrate that samples buried at lower temperatures are fully saturated at lower TL measurement temperatures ( ∼ 200 − 300 ∘ C) relative to warmer samples. This signal is estimated to be useful in rapidly-cooling bedrock and should grow measurably for ∼ 10 2 − 10 6 years.

Detrital zircons are the only confirmed surviving remnants of >4.03 Ga crust while younger detrital zircons provide a parallel record of more recent crustal evolution to that preserved in crystalline rocks. Zircons often preserve inclusions that may provide clues as to the origins of out-of-context grains in the sedimentary record. Previous studies have established that inclusions of biotite in magmatic zircon are compositionally well-matched to biotite in the source rock matrix, although a direct application to ancient detrital zircons has not been made. A number of studies have documented variations in the Fe, Mg, and Al contents of magmatic biotite from different source rocks and tectonic settings, suggesting that biotite inclusions may indeed serve as provenance indicators for detrital zircons. Consistent with earlier studies, we find that the FeO⁎/MgO ratio of magmatic biotite from continental arcs, collisional, and within-plate settings varies with relative oxidation state as well as whole-rock FeO⁎/MgO, while its Al2O3/(FeO⁎ + MgO) varies with whole-rock A/CNK (molar Al/(2 ⋅ Ca + Na + K)). Biotite from oxidized metaluminous and reduced S-type granitoids can be readily distinguished from each other using FeO⁎/MgO and Al2O3/(FeO⁎ + MgO), while biotite from reduced I-type and oxidized peraluminous granites may in some cases be more ambiguous. Biotite from peralkaline and reduced A-type granites are also distinguishable from all other categories by Al2O3/(FeO⁎ + MgO) and FeO⁎/MgO, respectively. Biotite inclusions in Hadean zircons from Jack Hills, Western Australia indicate a mixture of metaluminous and reduced S-type host rocks, while inclusions in 3.6–3.8 Ga detrital zircons from the Nuvvuagittuq Supracrustal Belt indicate more oxidized peraluminous magmas. These results highlight the diversity of felsic materials on the early Earth and suggest that biotite inclusions are applicable to zircon provenance throughout the sedimentary record.

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.

The inspiration for this volume arose in part from a shift in perception among U–Pb geochronologists that began to develop in the late 1980s. Prior to then, analytical geochronology emphasized progressively lower blank analysis of separated accessory mineral aggregates (e.g., Krogh 1982; Parrish 1987), with results generally interpreted to reflect a singular moment in time. For example, a widespread measure of confidence in intra-analytical reliability was conformity to an MSWD (a form of χ2 test; Wendt and Carl 1991) of unity. This approach implicitly assumed that geological processes act on timescales that are short with respect to analytical errors (e.g., Schoene et al. 2015). As in situ methodologies (e.g., Compston and Pidgeon 1986; Harrison et al. 1997; Griffin et al. 2000) and increasingly well-calibrated double spikes (e.g., Amelin and Davis 2006; McLean et al. 2015) emerged, geochronologists began to move away from interpreting geological processes as a series of instantaneous episodes (e.g., Rubatto 2002). At about the same time, petrologists developed techniques that permitted in situ chemical analyses to be interpreted in terms of continuously changing pressure–temperature–time histories (e.g., Spear 1988). The recognition followed that specific mineral reactions yielded products that could be directly dated or interpreted in terms of protracted petrogenetic processes. Part of this shift was due to an appreciation that trace elements in accessory phases could identify the changing nature of modal mineralogy during crystal growth (e.g., Pyle et al. 2001; Kohn and Malloy 2004) and thus potentially relate petrogenesis to absolute time. The transition to petrochronology was complete upon recognition that high MSWDs were in fact the expected case for most metamorphic minerals (Kohn 2009). One of the great frontiers for fundamental discovery in the geosciences is earliest Earth (DePaolo et al. 2008). However, investigations of the first five …

Author(s): Haproff, Peter J; Zuza, Andrew V; Yin, An; Harrison, T Mark; Manning, Craig E; Dubey, Chandra S; Ding, Lin; Wu, Chen; Chen, Jianlin

Results of whole-rock 40 Ar/ 39 Ar step-heating analyses of extra-terrestrial materials have been used to constrain the timing of impacts in the inner solar system, solidification of the lunar magma ocean, and development of planetary magnetic fields. Despite the importance of understanding these events, the samples we have in hand are non-ideal due to mixed provenance, isotopic disturbances from potentially multiple heating episodes, and laboratory artifacts such as nuclear recoil. Although models to quantitatively assess multi-domain, diffusive 40 Ar ⁎ loss have long been applied to terrestrial samples, their use on extra-terrestrial materials has been limited. Here we introduce a multi-activation energy, multi-diffusion domain model and apply it to 40 Ar/ 39 Ar temperature-cycling, step-heating data for meteoritic and lunar samples. We show that age spectra of extra-terrestrial materials, the Jilin chondrite (K-4) and Apollo 16 lunar breccia ( 67514 , 43 ) , yielding seemingly non-ideal behavior commonly interpreted as either laboratory artifacts or localized shock heating of pyroxene, are meaningful and can be understood in context of the presence of multi-diffusion domains containing multiple activation energies. Internally consistent results from both the meteoritic and lunar samples reveal high-temperature/short duration thermal episodes we interpret as due to moderate shock heating.

Despite the robust nature of zircon in most crustal and surface environments, chemical alteration, especially associated with radiation damaged regions, can affect its geochemistry. This consideration is especially important when drawing inferences from the detrital record where the original rock context is missing. Typically, alteration is qualitatively diagnosed through inspection of zircon REE patterns and the style of zoning shown by cathodoluminescence imaging, since fluid-mediated alteration often causes a flat, high LREE pattern. Due to the much lower abundance of LREE in zircon relative both to other crustal materials and to the other REE, disturbance to the LREE pattern is the most likely first sign of disruption to zircon trace element contents. Using a database of 378 (148 new) trace element and 801 (201 new) oxygen isotope measurements on zircons from Jack Hills, Western Australia, we propose a quantitative framework for assessing chemical contamination and exchange with fluids in this population. The Light Rare Earth Element Index is scaled on the relative abundance of light to middle REE, or LREE-I = (Dy/Nd) + (Dy/Sm). LREE-I values vary systematically with other known contaminants (e.g., Fe, P) more faithfully than other suggested proxies for zircon alteration (Sm/La, various absolute concentrations of LREEs) and can be used to distinguish primary compositions when textural evidence for alteration is ambiguous. We find that zircon oxygen isotopes do not vary systematically with placement on or off cracks or with degree of LREE-related chemical alteration, suggesting an essentially primary signature. By omitting zircons affected by LREE-related alteration or contamination by mineral inclusions, we present the best estimate for the primary igneous geochemistry of the Jack Hills zircons. This approach increases the available dataset by allowing for discrimination of on-crack analyses (and analyses with ambiguous or no information on spot placement or zircon internal structures) that do not show evidence for chemical alteration. It distinguishes between altered and unaltered samples in ambiguous cases (e.g., relatively high Ti), identifying small groups with potentially differing provenance from the main Jack Hills population. Finally, filtering of the population using the LREE-I helps to more certainly define primary correlations among trace element variables, potentially relatable to magmatic compositional evolution.

Determining the history of Earth's dynamo prior to the oldest known well-preserved rock record is one of the ultimate challenges in the field of paleomagnetism. Tarduno et al. (2015) argued that detrital zircons contain records of an active dynamo dating back to 4.2 billion years ago (Ga), 700 million years earlier than previously identified (Biggin et al., 2011 and Tarduno et al., 2010). However, this extraordinary claim requires evidence that the zircons have not been remagnetized during the intervening time since their formation. Weiss et al. (2015) argued that such evidence had yet to be provided, a conclusion that we find still firmly holds.

Detrital zircons as old as nearly 4.4 Ga offer insights into the earliest moments of Earth history. Results of geochemical investigations of these grains have been interpreted to indicate their formation in near-H2O saturated meta- and peraluminous magmas under a relatively low (15–30 °C/km) geotherm. A key feature in pursuing a petrotectonic model that explains the full spectrum of these observations is their seeming contrast to most Phanerozoic magmatic zircons, specifically their low Ti-in-zircon crystallization temperatures and inclusion assemblages. The ~22 Ma Arunachal leucogranites of the eastern Himalaya appear, however, to be a rare exception to this generality. They show large-ion lithophile covariance trends indicative of wet basement melting together with a normal distribution of magmatic crystallization temperatures about an average of 660 °C. In the same fashion as Hadean zircons, Arunachal leucogranite and host gneiss zircons are dominated by muscovite + quartz inclusions that yield formation pressures of 5–15 kbars. We suggest that the Arunachal leucogranites originated in the hanging wall of a megathrust that carried H2O-rich foreland sediments to depths of >20 km whereupon de-watering reactions released fluids that fluxed hanging wall anatexis. Modeling suggests the thermal structure of this continental collision environment may have been broadly similar to a Hadean ocean-continent subduction zone. The similarity of these two environments, separated by over 4 Ga may explain seemingly common features of the Hadean and Arunachal leucogranite zircons. Their key difference is the absence of metaluminous magmas in the continental collision environment, which is shielded from juvenile additions.

It currently is unknown when Earth's dynamo magnetic field originated. Paleomagnetic studies indicate that a field with an intensity similar to that of the present day existed 3.5 billion years ago (Ga). Detrital zircon crystals found in the Jack Hills of Western Australia are some of the very few samples known to substantially predate this time. With crystallization ages ranging from 3.0–4.38 Ga, these zircons might preserve a record of the missing first billion years of Earth's magnetic field history. However, a key unknown is the age and origin of magnetization in the Jack Hills zircons. The identification of >3.9 Ga (i.e., Hadean) field records requires first establishing that the zircons have avoided remagnetization since being deposited in quartz-rich conglomerates at 2.65–3.05 Ga. To address this issue, we have conducted paleomagnetic conglomerate, baked contact, and fold tests in combination with U–Pb geochronology to establish the timing of the metamorphic and alteration events and the peak temperatures experienced by the zircon host rocks. These tests include the first conglomerate test directly on the Hadean-zircon bearing conglomerate at Erawandoo Hill. Although we observed little evidence for remagnetization by recent lightning strikes, we found that the Hadean zircon-bearing rocks and surrounding region have been pervasively remagnetized, with the final major overprinting likely due to thermal and/or aqueous effects from the emplacement of the Warakurna large igneous province at ∼1070 million years ago (Ma). Although localized regions of the Jack Hills might have escaped complete remagnetization, there currently is no robust evidence for pre-depositional (>3.0 Ga) magnetization in the Jack Hills detrital zircons.

Detrital igneous zircons from Jack Hills, Western Australia, range in age from ~ 3.0 to nearly 4.4 Ga and contain an inclusion assemblage dominated by quartz and muscovite, cited as evidence of their derivation from peraluminous granitoids. However, some phosphate inclusions in these zircons are known to be secondary from their post-depositional U–Pb ages and manifest mineralization along cracks. We undertook a survey of mineral inclusions in 4.3–3.0 Ga Jack Hills zircons with particular emphasis on their relationship to possible alteration features (e.g., cracks, disturbed internal zonation, and visual turbidity). Mineral inclusions revealed at polished surfaces show variations in modal mineralogy, mostly corresponding to their relationship with cracks. Muscovite is common both on and away from cracks, although the chemistry of muscovite inclusions shows little relationship with other potential alteration features. Inclusions filling cracks (secondary) and inclusions isolated from cracks differ in their modal mineralogy, although both suites are rich in muscovite and quartz. The higher incidence of crack-intersecting inclusions among younger zircons may reflect effects of the (generally larger) inclusion size among younger zircons. Mismatches between the isolated and crack-intersecting populations indicate selective loss of certain phases (e.g., feldspar, apatite) and over-representation of quartz and muscovite along cracks likely due to the effects of larger inclusion size and varying degrees of overpressure following zircon cooling and decompression. Inclusions not associated with cracks in magmatically zoned versus regions with disturbed zoning have similar phase proportions. This indicates only minor inclusion replacement away from cracks (i.e., the isolated assemblage is likely primary). This holds true also for inclusions within visually turbid versus clear volumes of zircon. Phase proportions within the inclusion assemblages differ with age indicating a provenance shift toward fewer mafic phases and apatite in

Exhumation of the southern Tibetan plateau margin reflects interplay between surface and lithospheric dynamics within the Himalaya-Tibet orogen. We report thermochronometric data from a 1.2-km elevation transect within granitoids of the eastern Lhasa terrane, southern Tibet, which indicate rapid exhumation exceeding 1 km/Ma from 17-16 to 12-11 Ma followed by very slow exhumation to the present. We hypothesize that these changes in exhumation occurred in response to changes in the loci and rate of rock uplift and the resulting southward shift of the main topographic and drainage divides from within the Lhasa terrane to their current positions within the Himalaya. At ∼17 Ma, steep erosive drainage networks would have flowed across the Himalaya and greater amounts of moisture would have advected into the Lhasa terrane to drive large-scale erosional exhumation. As convergence thickened and widened the Himalaya, the orographic barrier to precipitation in southern Tibet terrane would have strengthened. Previously documented midcrustal duplexing around 10 Ma generated a zone of high rock uplift within the Himalaya. We use numerical simulations as a conceptual tool to highlight how a zone of high rock uplift could have defeated transverse drainage networks, resulting in substantial drainage reorganization. When combined with a strengthening orographic barrier to precipitation, this drainage reorganization would have driven the sharp reduction in exhumation rate we observe in southern Tibet.