The Li concentrations in natural zircons vary by over six orders of magnitude, and Li intra-zoning within zircon has been used to model temperature–time histories in the crust, where the inputs to ...these T–t models presume some initial Li content of intra-zircon domains. This potential wealth of information may be further improved with experimental studies that characterize Li partitioning between zircon and melt. Here we report experiments that systematically quantify Li partitioning between zircon and different silicate melts as a function of temperature. We also explored how Li in zircon is structurally accommodated and the possible element(s) that influence Li incorporation. Zircon–melt partitioning is reported in four different melt compositions, with ASI values (molar ratio of Al
2
O
3
/(Na
2
O + K
2
O + CaO)) ranging from 0.56 to 1.44. The experiments were performed in a piston–cylinder device at 1000, 1100, 1200, and 1300 °C, and at a constant pressure of 1 GPa. Results of experiments show that the zircon–melt Li partition coefficient is not dependent on melt composition, but shows a negative relationship to temperature. This means the large range of Li concentrations in natural zircon is probably due to the combined factors of crystallization temperature and the original melt Li contents. We used our data to derive an empirical equation to describe the relationship of Li partitioning between zircon and melt (D
Li
) and temperature (
K
): log
10
(D
Li
) = (5918 ± 1479)/
T
− (6.91 ± 1.05). Through experiments with or without P added to the starting rock mix, we found that P-doped experiments yield lower D
Li
than those without P, which implies that Li competes with
P
for rare earth element (REE
3+
) charge balance. To test the influence of H
2
O content of the melt on Li partitioning in zircon, we also explored melts with varying H
2
O contents (nominally 0, 5 wt.%, and 10 wt.%); results indicate that Li partitioning in zircon is not dependent on the H
2
O content of the melt. This method lays the groundwork to estimate the concentration of Li in melt that crystallized zircon, given the concentration of Li in zircon and an independent temperature estimate, provided that no post-crystallization modification of Li in zircon has occurred. We applied this equation to Hadean, Archean, and modern zircons and found that the predicted Li
melt
for Jack Hills zircons, Archean TTG and sanukitoid are one order magnitude higher than the modern crust.
Chondrite normalized rare earth element (REE) patterns of zircons generally have enriched Ce values relative to La and Pr, and depleted Eu values relative to Sm and Gd. High Ce contents in zircon may ...imply oxidizing conditions (Ce4+ is more compatible than Ce3+), whereas depleted Eu contents may imply reducing conditions (Eu2+ does not substitute into the zircon lattice). We report 41 experiments in which temperature, melt composition, and oxygen fugacity (fO2) were varied in order to explore the details of Ce and Eu incorporation into zircon. Crystals were synthesized in hydrous silicate melts at 10kbar and 800–1300°C. Synthetic rock mixes were doped with La+Ce+Pr (±P) or Sm+Eu+Gd and buffered at oxygen fugacities ranging from ∼IW (iron–wüstite) to >MH (magnetite–hematite); the run products were analyzed by electron microprobe to obtain crystal/melt partition coefficients. Cerium anomalies increase with higher oxygen fugacities and lower crystallization temperatures. In agreement with other experimental studies, peralkaline melts yield the largest zircon grains but show only modest Ce anomalies even at fO2s>MH. The same reason that zircons grown in peralkaline melts are easy to synthesize in the laboratory (these melts are capable of dissolving wt.% levels of Zr before zircon saturation due to high alkali content) makes the melt structure/composition atypical and not representative of most natural magmas. With this in mind, we synthesized zircons in a granitic melt with more modest alkali contents that require geologically plausible Zr contents for saturation. We obtained the following empirical relationship: lnCeCe∗D=(0.1156±0.0050)×ln(fO2)+13,860±708T(K)-6.125±0.484where (Ce/Ce∗)D is the Ce anomaly in zircon calculated from partition coefficients, and T is the zircon crystallization temperature in K. Europium anomalies from the same melt composition are more negative at lower oxygen fugacities, but with no resolvable temperature dependence, and can be described by the following empirical relationship: EuEu∗D=11+10-0.14±0.01×ΔNNO+0.47±0.04where (Eu/Eu∗)D is the Eu partitioning anomaly and ΔNNO is the difference in log units from the NNO buffer. If both Eu and Ce anomalies in zircons can be used as proxies for the oxidation state of Ce and Eu in the host melts, then it is clear that Eu2+ and Ce4+ can coexist in most zircon-saturated magmas. This implies that depletion of Eu melt contents by feldspar crystallization fractionation prior to (or during) zircon crystallization is not required to produce Eu anomalies. Thus, zircon Eu anomalies are a function of the oxygen fugacity and the Eu anomaly of the melt. Cerium anomalies of natural melts are not predicted to be as common because no major rock-forming phase depletes or enriches magmas in Ce compared to neighboring elements La and Pr. Thus, (Ce/Ce∗)D may be most readily applied to constrain the oxidation state of natural melts.
Magmatic outgassing of volatiles from Earth's interior probably played a critical part in determining the composition of the earliest atmosphere, more than 4,000 million years (Myr) ago. Given an ...elemental inventory of hydrogen, carbon, nitrogen, oxygen and sulphur, the identity of molecular species in gaseous volcanic emanations depends critically on the pressure (fugacity) of oxygen. Reduced melts having oxygen fugacities close to that defined by the iron-wüstite buffer would yield volatile species such as CH(4), H(2), H(2)S, NH(3) and CO, whereas melts close to the fayalite-magnetite-quartz buffer would be similar to present-day conditions and would be dominated by H(2)O, CO(2), SO(2) and N(2) (refs 1-4). Direct constraints on the oxidation state of terrestrial magmas before 3,850 Myr before present (that is, the Hadean eon) are tenuous because the rock record is sparse or absent. Samples from this earliest period of Earth's history are limited to igneous detrital zircons that pre-date the known rock record, with ages approaching ∼4,400 Myr (refs 5-8). Here we report a redox-sensitive calibration to determine the oxidation state of Hadean magmatic melts that is based on the incorporation of cerium into zircon crystals. We find that the melts have average oxygen fugacities that are consistent with an oxidation state defined by the fayalite-magnetite-quartz buffer, similar to present-day conditions. Moreover, selected Hadean zircons (having chemical characteristics consistent with crystallization specifically from mantle-derived melts) suggest oxygen fugacities similar to those of Archaean and present-day mantle-derived lavas as early as ∼4,350 Myr before present. These results suggest that outgassing of Earth's interior later than ∼200 Myr into the history of Solar System formation would not have resulted in a reducing atmosphere.
Constraining the lithological diversity and tectonics of the earliest Earth is critical to understanding our planet's evolution. Here we use detrital Jack Hills zircon (3.7 - 4.2 Ga) analyses coupled ...with new experimental partitioning data to model the silica content, Si+O isotopic composition, and trace element contents of their parent melts. Comparing our derived Jack Hills zircons' parent melt Si+O isotopic compositions (-1.92 ≤ δ
Si
≤ 0.53 ‰; 5.23 ≤ δ
O
≤ 9.00 ‰) to younger crustal lithologies, we conclude that the chemistry of the parent melts was influenced by the assimilation of terrigenous sediments, serpentinites, cherts, and silicified basalts, followed by igneous differentiation, leading to the formation of intermediate to felsic melts in the early Earth. Trace element measurements also show that the formational regime had an arc-like chemistry, implying the presence of mobile-lid tectonics in the Hadean. Finally, we propose that these continental-crust forming processes operated uniformly from 4.2 to at least 3.7 Ga.
Hydrosphere interactions and alteration of the terrestrial crust likely played a critical role in shaping Earth’s surface, and in promoting prebiotic reactions leading to life, before 4.03 Ga (the ...Hadean Eon). The identity of aqueously altered material strongly depends on lithospheric cycling of abundant and water-soluble elements such as Si and O. However, direct constraints that define the character of Hadean sedimentary material are absent because samples from this earliest eon are limited to detrital zircons (ZrSiO₄). Here we show that concurrent measurements of Si and O isotope ratios in Phanerozoic and detrital pre-3.0 Ga zircon constrain the composition of aqueously altered precursors incorporated into their source melts. Phanerozoic zircon from (S)edimentary-type rocks contain heterogeneous δ18O and δ30Si values consistent with assimilation of metapelitic material, distinct from the isotopic character of zircon from (I)gneous- and (A)norogenic-type rocks. The δ18O values of detrital Archean zircons are heterogeneous, although yield Si isotope compositions like mantle-derived zircon. Hadean crystals yield elevated δ18O values (vs. mantle zircon) and δ30Si values span almost the entire range observed for Phanerozoic samples. Coupled Si and O isotope data represent a constraint on Hadean weathering and sedimentary input into felsic melts including remelting of amphibolites possibly of basaltic origin, and fractional addition of chemical sediments, such as cherts and/or banded iron formations (BIFs) into source melts. That such sedimentary deposits were extensive enough to change the chemical signature of intracrustal melts suggests they may have been a suitable niche for (pre)biotic chemistry as early as 4.1 Ga.
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.
Key Points
Approximately 800 Al concentrations in zircons measured in 19 different peraluminous and metaluminous rocks
Al concentrations in zircons from peraluminous rocks are distinctly higher than zircons from metaluminous rocks
Al in zircon proposed as a tool to discriminate peraluminous and metaluminous rocks within the detrital zircon record
The widespread presence of ribonucleic acid (RNA) catalysts and cofactors in the Earth′s biosphere today suggests that RNA was the first biopolymer to support Darwinian evolution. However, most ...“path‐hypotheses” to generate building blocks for RNA require reduced nitrogen‐containing compounds not made in useful amounts in the CO2−N2−H2O atmospheres of the Hadean. We review models for Earth′s impact history that invoke a single ∼1023 kg impactor (Moneta) to account for measured amounts of platinum, gold, and other siderophilic (“iron‐loving”) elements on the Earth and Moon. If it were the last sterilizing impactor, by reducing the atmosphere but not the mantle Moneta, would have opened a “window of opportunity” for RNA synthesis, a period when RNA precursors rained from the atmosphere onto land holding oxidized minerals that stabilize advanced RNA precursors and RNA. Surprisingly, this combination of physics, geology, and chemistry suggests a time when RNA formation was most probable, ∼120±100 million years after Moneta′s impact, or ∼4.36±0.1 billion years ago. Uncertainties in this time are driven by uncertainties in rates of productive atmosphere loss and amounts of sub‐aerial land.
Timing prebiotic RNA formation: Models for Earth′s impact history explain the late delivery of platinum, gold, and other siderophiles via a ∼1023 kg impactor (Moneta) circa 4.48 billion years ago. The iron core from this impactor would have reduced the atmosphere above a relatively oxidized mantle, opening a window of opportunity for RNA precursor synthesis. Surprisingly, this suggests that RNA formation was most probable ∼4.36±0.1 billion years ago.
We present experimental measurements of monazite/melt partition coefficients (Kmonz/melt) for thorium (Th) and uranium (U) in haplogranitic Na2O–Al2O3–SiO2 melts (with 6–10wt.% H2O) at 900°C–1100°C ...and 1GPa pressure. Our results show the following trends: (1) KTh and KU in felsic melts are strongly dependent on melt Al/Na. Both partition coefficients reach maxima in mildly peraluminous region and decrease toward strongly peraluminous (Al/Na>1) and peralkaline (Al/Na<1) melt compositions. KTh and KU vary by 2 orders of magnitude for Al/Na in the range of 0.4 to ~2. (2) For identical melt compositions, the effect of temperature on KU is pronounced: A 200°C increase in temperature results in a 6-fold decrease in KU. In contrast, the same temperature change results in only 50% decrease of the less temperature-sensitive KTh. (3) One experiment with 10wt.% H2O added does not change K, which may suggest little dependence of K upon water content from 6 to 10wt.%. (4) KTh is much larger than 1 at all natural magmatic temperature, so the removal of monazite during fractionation crystallization or its retention as a residual phase during partial melting has a significant effect on the content of Th in the coexisting melt phase. (5) U is much less compatible than Th, and could even behave incompatibly at high temperatures (e.g., at 1000°C, KU is 0.01 for melts containing 75wt.% SiO2 and Al/Na~0.4).
•26 experiments on Th and U partitioning behavior between monazite and melt•KTh and KU vary by 2 orders of magnitude for Al/Na in the range of 0.4 to ~2.•A 200°C increase in T results in a 6-fold decrease in KU and 50% decrease of KTh.
The oldest compiled U-Pb zircon ages for the Acasta Gneiss Complex in the Northwest Territories of Canada span about 4050-3850 Ma; yet older ca. 4200 Ma xenocrystic U-Pb zircon ages have also been ...reported for this terrane. The AGC expresses at least 25 km super(2) of outcrop exposure, but only a small subset of this has been documented in the detail required to investigate a complex history and resolve disputes over emplacement ages. To better understand this history, we combined new ion microprobe super(235,238)U- super(207,206)Pb zircon geochronology with whole-rock and zircon rare earth element compositions (REE sub(zirc)), Ti-in-zircon thermometry (Ti super(xln) ) and super(147)Sm- super(143)Nd geochronology for an individual subdivided similar to 60 cm super(2) slab of Acasta banded gneiss comprising five separate lithologic components. Results were compared to other variably deformed granitoid-gneisses and plagioclase-hornblende rocks from elsewhere in the AGC. We show that different gneissic components carry distinct Th/U sub(zirc) vs. Ti super(xln) and REE sub(zirc) signatures correlative with different zircon U-Pb age populations and WR compositions, but not with super(147)Sm- super(143)Nd isotope systematics. Modeled DWRzircon REE from lattice-strain theory reconciles only the ca. 3920 Ma zircons with the oldest component that also preserves strong positive Eu super(lowast) anomalies. Magmas which gave rise to the somewhat older (inherited) ca. 4020 Ma AGC zircon age population formed at similar to IW (iron-wuestite) to <FMQ (fayalite-magnetite-quartz) oxygen fugacities. A ca. 3920 Ma emplacement age for the AGC is contemporaneous with bombardment of the inner solar system. Analytical bombardment simulations show that crustal re-working from the impact epoch potentially affected the precursors to the Acasta gneisses.
Monazite and xenotime are common metamorphic phases that may be reliably U/Th-Pb dated to obtain absolute ages. The utility of these minerals is significantly enhanced if a crystallization ...thermometer can be developed and applied to better establish the temperature-time (T-t) paths of crustal terranes. Here we report experiments in which we have modeled the T-dependent Rare Earth element (REE) cationic exchange between coexisting monazite and xenotime to derive a new thermometer. We present a thermometer in which phosphates were co-crystallized from 1150 °C to 850 °C at 1 GPa in a Y-REE-P2O5NaCl-H2O system, with oxygen fugacity buffered at the Ni-NiO equilibrium. The composition of the phosphates was quantified using a laser ablation inductively coupled plasma mass spectrometer (LA-ICP-MS). Results reveal strong correlations between log10∏XXntLREE×XMztY∏XMztLREE×XMztY (at. %) and 104/T(K) and our preferred calibration is:log∏XXntLREE×XMztY∏XMztLREE×XMztY=−6714±2264TK−0.79±1.76
where LREE = La, Ce and Pr, α = activity of a cation in a phase, and ∏αMzt/XntY/REE refers to the product of activities of Y and/or REEs in a phosphate phase. The errors are 2 s.e. The greatest strength of this thermometer is its versatility. One can obtain derivative thermometers based on select elements rather than the entire suite of REEs. We showcase our thermometer's adaptability by applying it to two studies that have published REE data on monazite and xenotime from some quartzo-feldspathic psammites and garnet-bearing pelites that experienced amphibolite facies metamorphism from the Naver nappe in the Northern Highlands Terrane, Scotland. The main calibration shown above, as well as four derivative single-element thermometers (Y, La, Ce and Pr) were applied to the first study. Upon applying these thermometers, we find that the calculated metamorphic Ts agree well with the regional metamorphic facies. Thus, this versatile thermometer can be used in geologic environments where monazite and xenotime co-crystallized.
•Monazites and xenotimes can be U-Pb dated and used as a thermometer.•A new thermometer has been developed that can be used to derive T-t paths.•This thermometer is versatile and can be adjusted based on analytical data available.•We have applied our calibration to REE datasets with complete and sparse data.•The effects of P and fO2 on partitioning have also been explored (Supplementary).
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