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Magmatic differentiation produces positive correlations between  49 Ti and SiO2. The equilibrium Ti isotope fractionation factors of Ti-bearing minerals are essential for understanding the mechanisms driving this isotopic fractionation. We present ab initio derived mean force constants of Ti-bearing minerals (barium orthotitanate, potassium titanium oxide, fresnoite, diopside, geikielite, karooite, titanite, pseudobrookite, anatase, and titanium oxide) based on density functional theory (DFT) to calculate equilibrium isotopic fractionation factors. We find that the main driver for Ti isotopic fractionation is its coordination, with 4-, 5-, and 6-fold coordinated Ti characterized by mean force constants of 547, 462, and 310 N/m respectively. The coordination number of Ti in silicate melts is thought to be lower than in minerals, driving magmas towards higher  49 Ti values by fractional crystallization. The mineral-melt fractionation factors allow modeling of the observed Ti isotope trends in tholeiitic and calc-alkaline rocks. Our model results indicate that to first order, the steeper  49 Ti trend observed in tholeiitic vs. calc-alkaline magmas is most likely due to enhanced removal of Ti into sequestered minerals at low SiO2 concentration in tholeiitic series compared to calc-alkaline series. The  49 Ti-SiO2 differentiation trends however depend on Ti coordination in the melt and the strengths of Ti bonds in diverse Fe,Ti-oxides, which are still uncertain. Our resultsshow that Ti isotopes can be used to reconstruct the crystallization history and identify the magmatic series parentage of magmas that otherwise lack context, but further work is needed to identify the drivers behind Ti isotopic fractionation in igneous rocks.

The rise of molecular oxygen (O2) in the atmosphere and oceans was one of the most consequential changes in Earth's history. While most research focuses on the Great Oxidation Event (GOE) near the start of the Proterozoic Eon—after which O2 became irreversibly greater than 0.1% of the atmosphere—many lines of evidence indicate a smaller oxygenation event before this, at the end of the Archean Eon (2.5 billion years ago). Additional evidence of mild environmental oxidation—probably by O2—is found throughout the Archean. This emerging evidence suggests that the GOE might be best regarded as the climax of a broader First Redox Revolution (FRR) of the Earth system characterized by two or more earlier Archean Oxidation Events (AOEs. Understanding the timing and tempo of this revolution is key to unraveling the drivers of Earth's evolution as an inhabited world—and has implications for the search for life on worlds beyond our own. ▪ Many inorganic geochemical proxies suggest that biological O2 production preceded Earth's GOE by perhaps more than 1 billion years. ▪ Early O2 accumulation may have been dynamic, with at least two AOEs predating the GOE. If so, the GOE was the climax of an extended period of environmental redox instability. ▪ We should broaden our focus to examine and understand the entirety of Earth's FRR. Expected final online publication date for the Annual Review of Earth and Planetary Sciences, Volume 49 is May 2021. Please see http://www.annualreviews.org/page/journal/pubdates for revised estimates.

Evidence continues to emerge for the production and low-level accumulation of molecular oxygen (O 2 ) at Earth's surface before the Great Oxidation Event. Quantifying this early O 2 has proven difficult. Here, we use the distribution and isotopic composition of molybdenum in the ancient sedimentary record to quantify Archean Mo cycling, which allows us to calculate lower limits for atmospheric O 2 partial pressures (PO 2 ) and O 2 production fluxes during the Archean. We consider two end-member scenarios. First, if O 2 was evenly distributed throughout the atmosphere, then PO 2 > 10 -6.9 present atmospheric level was required for large periods of time during the Archean eon. Alternatively, if O 2 accumulation was instead spatially restricted (e.g., occurring only near the sites of O 2 production), then O 2 production fluxes >0.01 Tmol O 2 /year were required. Archean O 2 levels were vanishingly low according to our calculations but substantially above those predicted for an abiotic Earth system.

Constraining the rate at which sulfide minerals undergo oxidative weathering at low atmospheric O 2 is crucial for understanding the evolution of the Archean and Proterozoic biosphere when O 2 was a trace atmospheric gas. However, recent studies attempting to constrain sulfide oxidation rates, atmospheric O 2 sinks, and trace metal delivery to seawater under Archean conditions are limited by the need to extrapolate from experimental pyrite oxidation kinetics determined at much higher O 2 levels.Extrapolation of those data sets to Archean levels of O 2 (<10 -5 present atmospheric level or PAL prior to 2.4 Ga) leads to more than an order of magnitude uncertainty in sulfide mineral oxidation rates, hampering efforts to quantify oxidative weathering under early Earth conditions.To quantify sulfide oxidation kinetics at low pO 2 , we conducted aqueous pyrite and molybdenite oxidation experiments at ~2 -1200 nM dissolved O 2 and pH values 1.83, 5.08, and 8.58. Our experimental approach used LUMOS O 2 sensors to extend the pO 2 range explored by oxidation experiments down to 10 -5 PAL pO 2 , the limit of the sensors, which is up to three orders of magnitude lower than the pO 2 range explored in previous work.

Molybdenum isotopes in twenty-four composites of glacial diamictites spanning depositional ages of 2900 to 300 Ma show a systematic shift to lighter compositions and a decrease in Mo concentration over time. The diamictites fall into three age groups relative to the Great Oxidation Event (GOE): pre-GOE (2.43-2.90 Ga), syn-GOE (2.20-2.39 Ga), and post-GOE (0.33-0.75 Ga). Pre-GOE composites have an average δ 98 Mo NIST3134 of +0.03 (± 0.18), syn-GOE composites average −0.29 (± 0.60), and post-GOE composites average −0.45 (± 0.51). These groups are statistically different at p=0.05. We use the pre-GOE data to estimate the average Archean upper continental crust (UCC) δ 98 Mo signature as +0.03 ± 0.18 (2σ), which falls within the range of previous estimates of modern igneous rocks. As the diamictites represent a mixture of igneous and weathered crust, the shift to lighter Mo values over time likely reflects Mo isotope fractionation during oxidative weathering and increased retention of light Mo isotopes in weathered regolith and soils. We hypothesize that this fractionation is due to the mobilization of oxidized Mo following the GOE, and subsequent adsorption of light Mo onto Fe-Mn oxides and/or organic matter in weathered regolith. We conclude that Mo isotopes in continental weathering products record the rise of atmospheric oxygen and onset of oxidative weathering. As the regolith formed under oxidative conditions is isotopically lighter than average continental igneous rocks, mass balance dictates that Mo isotope fractionation during oxidative weathering should result in isotopically heavy groundwater and river water, which is observed in modern systems.

The formation of continental crust via plate tectonics strongly influences the physical and chemical characteristics of Earth’s surface and may be the key to Earth’s long-term habitability. However, continental crust formation is difficult to observe directly and is even more difficult to trace through time. Nontraditional stable isotopes have yielded significant insights into this process, leading to a new view both of Earth’s earliest continental crust and of what controls modern crustal generation. The stable isotope systems of titanium (Ti), zirconium (Zr), molybdenum (Mo), and thallium (Tl) have proven invaluable. Processes such as fractional crystallization, partial melting, geodynamic setting of magma generation, and magma cooling histories are examples of processes illuminated by these isotope systems.

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