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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. 

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.