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We investigate the implications of prolonging the equilibrium crystallization (EQX) stage of lunar magma ocean (LMO) solidification beyond the oft-modeled 50% volume solids, to 60%. Most models of two-stage LMO crystallization halt the EQX phase once 50% of a molten Moon (post-core formation) solidifies, after which the remaining 50% of the LMO solidifies via fractional crystallization (FRX). We quantitatively show through a simple scaling analysis that compares crystal settling velocity to vertical convective velocity that the early EQX regime can operate up to (and possible even slightly beyond) 60% volume solids. Phases that stabilize during the EQX and FRX regimes are then computed using Perple_X (thermodynamic calculator) along with the hp633ver database and associated activity-composition relations for solid solutions, and consider an adiabat that remains between the liquidus and solidus. Early results show two key findings: 1) only low volumes (~2%) of ilmenite form over ~50-km thick upper mantle layers for both 50% and 60% EQX regimes, suggesting that a mantle overturn may have been sluggish and/or limited in depth (dense ilmenite is thought to have been a critical driver of late-stage mantle mixing); and 2) contrary to most published two-stage LMO models, a refractory-enriched (i.e. high Al2O3) bulk silicate Moon is not required to produce garnet in the lunar mantle, assuming an Earth-like bulk silicate Moon composition with an alumina content of ~4 wt.%. To complement and test these numerical phase equilibria model results, a series of piston-cylinder experiments is underway that simulate the pressures and temperatures experienced by an FeO+TiO2-rich residual LMO in order to assess the volume and distribution of ilmenite produced during LMO solidification. These results are compared to those of the numerical phase equilibria models. Despite the model-dependent nature of these results, they provide a unique insight into potential LMO crystallization that has not been previously considered in the literature. 

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 cocrystallized from 1150 ◦C to 850 ◦C at 1 GPa in a Y-REE-P2O5-NaCl-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 (Π XLREE Xnt ×XY Π Mzt XLREE Mzt ×XY Mzt ) (at. %) and 104/T(K) and our preferred calibration is: log (ΠXLREE Xnt × XY Π Mzt XLREE Mzt × XY Mzt ) = ( 􀀀 6714 ± 2264) T (K) 􀀀 (0.79±1.76) where LREE = La, Ce and Pr, α = activity of a cation in a phase, and ΠαY/REE Mzt/Xnt 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. 

Quantifying the oxygen fugacity (fo2) of high temperature lithospheric fluids, including hydrothermal systems, presents a challenge because these fluids are difficult to capture and measure in the same manner as quenched glasses of silicate melts. The chemical properties of fluids can however be inferred through mineral proxies that interacted with the fluids through precipitation or recrystallization. Here, we present hydrothermal experiments to quantify the partition coefficients of rare earth elements (REEs) – including redox-sensitive Ce and Eu – between zircon and fluid. Experiments were conducted in a piston cylinder device at temperatures that range from 1200 to 800 ◦C under fo2-buffered conditions in a SiO2-ZrO2-NaCl-REE-oxide system, and similar experiments were performed in the absence of NaCl (31 total experiments). The fo2 was buffered to values that range from approximately 3 log units below to 7 log units above the fayalite magnetite quartz equilibrium. Zircon REE concentrations were quantified using laser ablation inductively coupled plasma mass spectrometry whereas the quenched fluids were extracted and measured by solution-based inductively coupled plasma mass spectrometry. Zircon Ce anomalies, quantified relative to La and Pr, exhibit sensitivity to oxygen fugacity and temperature and our preferred calibration is: log [􀀀 Ce Ce* ) D 􀀀 1 ] = (0.237 ± 0.040)× log(fo2) + 9437±640 T(K) 􀀀 5.02 ± 0.38 where the Ce anomalies are calculated from the partition coefficients for La, Ce, and Pr. Zircon Eu anomalies are also a function of oxygen fugacity though they exhibit no systematic dependence on T. Our preferred calibration is described by: 􀀀 Eu Eu* ) D = 1 1+100.30±0.04􀀀 [0.27±0.03]×ΔFMQ We performed additional calculations, in which lattice strain parabolas were fit to all non-redox sensitive rare earth elements that were added to the starting composition (i.e., La, Pr, Sm, Gd, Dy, Ho, Tm, Lu) as an alternate means to calculate anomalies. This method yields broadly similar results, though we prefer the La-Pr calibrations due to the non-systematic REE patterns frequently encountered with hydrothermal zircons; e.g., LREE zircon enrichment relative to other REEs. These experiments are applied to quantify the fo2 of fluids during mineralization of critical element-bearing systems, and separately to calculate the oxygen fugacity values of fluids formed during plate boundary processes. 

The early Earth may have had oxidizing fluids feeding hydrothermal pools. 

Abstract Partition coefficients for rare earth elements (REEs) between apatite and basaltic melt were determined as a function of oxygen fugacity (fO2; iron-wüstite to hematite-magnetite buffers) at 1 bar and between 1110 and 1175 °C. Apatite-melt partitioning data for REE3+ (La, Sm, Gd, Lu) show near constant values at all experimental conditions, while bulk Eu becomes more incompatible (with an increasing negative anomaly) with decreasing fO2. Experiments define three apatite calibrations that can theoretically be used as redox sensors. The first, a XANES calibration that directly measures Eu valence in apatite, requires saturation at similar temperature-composition conditions to experiments and is defined by: ( E u 3 + ∑ E u ) Apatite  = 1 1 + 10 - 0.10 ± 0.01 × l o g ⁡ ( f o 2 ) - 1.63 ± 0.16 . The second technique involves analysis of Sm, Eu, and Gd in both apatite and coexisting basaltic melt (glass), and is defined by: ( Eu E u * ) D Sm × Gd = 1 1 + 10 - 0.15 ± 0.03 × l o g ⁡ ( f o 2 ) - 2.46 ± 0.41 . The third technique is based on the lattice strain model and also requires analysis of REE in both apatite and basalt. This calibration is defined by ( Eu E u * ) D lattice strain = 1 1 + 10 - 0.20 ± 0.03 × l o g ⁡ ( f o 2 ) - 3.03 ± 0.42 . The Eu valence-state partitioning techniques based on (Sm×Gd) and lattice strain are virtually indistinguishable, such that either methodology is valid. Application of any of these calibrations is best carried out in systems where both apatite and coexisting glass are present and in direct contact with one another. In holocrystalline rocks, whole rock analyses can be used as a guide to melt composition, but considerations and corrections must be made to either the lattice strain or Sm×Gd techniques to ensure that the effect of plagioclase crystallization either prior to or during apatite growth can be removed. Similarly, if the melt source has an inherited either a positive or negative Eu anomaly, appropriate corrections must also be made to lattice strain or Sm×Gd techniques that are based on whole rock analyses. This being the case, if apatite is primary and saturates from the parent melt early during the crystallization sequence, these corrections may be minimal. The partition coefficients for the REE between apatite and melt range from a maximum DEu3+ = 1.67 ± 0.25 (as determined by lattice strain) to DLu3+ = 0.69 ± 0.10. The REE partition coefficient pattern, as observed in the Onuma diagram, is in a fortuitous situation where the most compatible REE (Eu3+) is also the polyvalent element used to monitor fO2. These experiments provide a quantitative means of assessing Eu anomalies in apatite and how they be used to constrain the oxygen fugacity of silicate melts. 

Abstract 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 ≤ δ³⁰Si_NBS28 ≤ 0.53 ‰; 5.23 ≤ δ¹⁸O_VSMOW ≤ 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.