Pervasive remagnetization of detrital zircon host rocks in the Jack Hills, Western Australia and implications for records of the early geodynamo

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

Field Guide for the Geology of Central Park and New York City

Teachers guide for geology of Central Park. Supplement to: Jaret, S. J., et. al. (2021). Geology of Central Park, Manhattan, New York City, USA: New geochemical insights. Geological Society of America bulletin. https://doi.org/10.1130/2020.0061(02

New experimental techniques for studying : (i) trace element substitution in minerals, and (ii) determining S-L-V relationships in silicate-H2O systems at high pressure

Ti site occupancy in zircon (ZrSiO{u2074}) is fundamental to thermobarometry because substitution mechanisms control Ti content-temperature relations. Described here are the results of three independent methods used to demonstrate that Ti predominately substitutes for Si and not Zr in zircon. Zircons were synthesized from oxide powders held in a Na{u2082}WO{u2084} flux at 1 bar and 1,100-1,400 {u00B0}C. Fully buffered experiments run at 1,300 {u00B0}C show zircons equilibrated with rutile + cristobalite have Ti contents (1201 ppm) nearly half that of zircons equilibrated with srilankite + tetragonal zirconia (2640 ppm). The lower Ti content of zircons produced under silica-saturated conditions indicates that Ti substitution predominately occurs on the Si site. Moreover, the higher Ti contents of silica-saturated experiments at 1 bar (1201 ppm), relative to those at 10 kbar (457 ppm, Ferry and Watson, 2007), indicate a substantial pressure effect on Ti solubility in zircon (correcting for different silica species present). Measured Ti K-a edge X-ray Absorption Near Edge Structure (XANES) spectra of synthetic zircons show energies and normalized intensities akin to those seen among tetrahedrally coordinated Ti-bearing standard minerals, strongly suggesting that Ti occupies the Si site. Density functional theory (DFT) calculations confirm that Ti substitution is most likely to occur on the Si site and predict a Ti-O bond length of 1.797 {u00C5} (compared to an average of 2.160 A for substitution on the Zr site), in excellent agreement with X-ray Absorption Fine Structure (EXAFS) spectra of experimentally grown zircons which indicate a value of 1.76(1) {u00C5}. The software FEFF8.4 was used to simulate XANES spectra from the defect structures determined by OFT for Ti substituting on both the Si and Zr sites. The predicted spectrum for Ti on the Si site reproduces all the key features of the experimental zircon spectra, whereas Ti on the Zr site is markedly different. All applied methods confirm that Ti predominately substitutes for Si in zircon. Consequently, the Ti content of zircon is not only a function of temperature, but increases with decreasing silica activity. Because elements that activate or quench cathodoluminescence (CL) in zircon are incorporated into the Zr site, a decoupling of CL signal and Ti distribution (which occurs on the Si site in zircon) is expected. This hypothesis has been verified by a systematic CL-trace element study of natural and experimental zircon