Yoichi Usui

Rock magnetism of tiny exsolved magnetite in plagioclase from a Paleoarchean granitoid in the Pilbara craton

Granitoids are widespread in Precambrian terranes as well as the Phanerozoic orogenic belts, but they have garnered little attention in paleomagnetic studies, because granitoids often contain abundant coarse-grained, magnetically unstable oxides. In this study, the first example of tiny, needle-shaped, exsolved oxides in plagioclase in a Paleoarchean granitoid is reported. The magnetic properties of single plagioclase crystals with the exsolved oxide inclusions have been studied to determine their paleomagnetic recording fidelity. Demagnetization experiments and hysteresis parameters indicate that the oxide inclusions are near stoichiometric magnetite and magnetically very stable. First-order reversal curve (FORC) diagrams reveal negligible magnetostatic interactions. Minimal interactions are also reflected by very efficient acquisition of anhysteretic remanent magnetization. Single plagioclase crystals exhibit strong magnetic remanence anisotropies, which require corrections to their paleodirectional and paleointensity data. Nonetheless, quantitative consideration of anisotropy tensors of the single plagioclase crystals indicates that the bias can be mitigated by properly averaging data from a few tens of single crystals. From the nonlinear thermoremanence acquisition of the plagioclase crystals, we estimate that the plagioclase crystals can reconstruct paleointensity up to 50 μT. Local metamorphic condition suggests that those magnetite may carry remanence of ∼3.2 to 3.3 Ga. We suggest that exsolved magnetite in granitoids is potentially a suitable target for the study of the early history of the geomagnetic field, and prompt detailed microscopic investigations as well as paleomagnetic tests to constrain the age of remanence.

The tremendous potential of deep-sea mud as a source of rare-earth elements

Potential risks of supply shortages for critical metals including rare-earth elements and yttrium (REY) have spurred great interest in commercial mining of deep-sea mineral resources. Deep-sea mud containing over 5,000 ppm total REY content was discovered in the western North Pacific Ocean near Minamitorishima Island, Japan, in 2013. This REY-rich mud has great potential as a rare-earth metal resource because of the enormous amount available and its advantageous mineralogical features. Here, we estimated the resource amount in REY-rich mud with Geographical Information System software and established a mineral processing procedure to greatly enhance its economic value. The resource amount was estimated to be 1.2 Mt of rare-earth oxide for the most promising area (105 km2 × 0–10 mbsf), which accounts for 62, 47, 32, and 56 years of annual global demand for Y, Eu, Tb, and Dy, respectively. Moreover, using a hydrocyclone separator enabled us to recover selectively biogenic calcium phosphate grains, which have high REY content (up to 22,000 ppm) and constitute the coarser domain in the grain-size distribution. The enormous resource amount and the effectiveness of the mineral processing are strong indicators that this new REY resource could be exploited in the near future.

Paleointensity estimates from oceanic gabbros: Effects of hydrothermal alteration and cooling rate

Gabbros recovered by previous ocean drillings were investigated in paleointensity and rock magnetic experiments. The young ages of the samples (ca. 0.78–1.3 Ma) enable a direct comparison between the plutonic paleointensity and volcanic data. Microscopic observations revealed two kinds of magnetite: needle-shaped exsolution in plagioclase and aggregate associated with the hydrothermal alteration of olivine. In Shaw paleointensity experiments, some samples revealed reasonable estimates, while some others showed an anomalously low ratio of natural remanent magnetization (NRM) versus thermoremanent magnetization (TRM). First-order reversal curve (FORC) diagrams indicated that the reasonable NRM/TRM were from non-interacting single domain magnetite exsolved in plagioclase, while the anomalously low NRM/TRM were from secondary magnetite associated with olivine. From the paleointensity results, the mean virtual axial dipole moment (VADM) was calculated to be 8.2 ±2.1 [1022Am2]. Volcanic records in the PINT database for 0.78–1.3 Ma revealed a mean virtual dipole moment (VDM) of 6.3 ± 0.73 [1022Am2]. The difference between them is consistent with the theoretical prediction of the cooling rate effect on paleointensity. These results indicate that oceanic gabbros are reliable paleointensity recorders.