Temporal stress variations along a seismogenic megasplay fault in the subduction zone: An example from the Nobeoka Thrust,southwestern Japan

The Nobeoka Thrust, an ancient megasplay fault in the Shimanto Belt, southwestern Japan, contains fault rocks from the seismogenic zone, providing an accessible analog of active megasplay faults in deep subduction settings. In this study, the paleostress along the Nobeoka Thrust was analyzed using multiple inversion techniques, including k‐means clustering of fault datasets acquired from drillcores that intersected the thrust. The six resultant stress orientation clusters can be divided into two general groups: stress solutions with north–south‐trending σ₁ axes, and those with east–west‐trending σ₁ axes. These groups are characterized by the temporal changes for the orientations of the σ₁ and σ₃ principal stress axes that involve alternation between horizontal and vertical. The findings are probably due to a change in stress state before and after earthquakes that occurred on the fault; similar changes have been observed in active tectonic settings, such as the 2011 Tohoku‐Oki earthquake (Japan).     

Variations in stress and driving pore fluid pressure ratio using vein orientations along megasplay faults : Example from the Nobeoka Thrust,Southwest Japan

The Nobeoka Thrust of Southwest Japan is an on‐land example of an ancient megasplay fault that provides an excellent record of deformation and fluid flow at seismogenic depths. The present study reports: (i) temporal stress changes for the seismogenic period of the Nobeoka Thrust; and (ii) spatial heterogeneities in driving pressure ratios P* obtained from mineral veins around the Nobeoka Thrust fault zone. Many quartz veins that filled mode I cracks can be observed in the hanging wall and footwall of the thrust. Inversion for stress orientation suggests that normal faulting dominated in both the hanging wall and footwall, with similar stress axis orientations in both. The orientation of σ₃ for the estimated stress regime is parallel to the slip direction of the Nobeoka Thrust. The detected normal‐faulting‐type stress regimes likely resulted from post‐seismic stress buildup after megathrust earthquakes. The hanging wall of the Nobeoka Thrust has smaller P* values than the footwall. Two possible explanations are proposed for the observed spatial variations in the driving pore fluid pressure ratio, P*: spatial variations in pore fluid pressure P_f are directly responsible for P* variations, or P* variations are controlled by differences in mechanical properties between the hanging wall and footwall.     

A Method to Suppress Isobaric and Polyatomic Interferences for Measurements of Highly Siderophile Elements in Desilicified Geological Samples

Sample decomposition using inverse aqua regia at elevated temperatures and pressures (e.g., Carius tube or high‐pressure asher) is the most common method used to extract highly siderophile elements (HSEs: Ru, Rh, Pd, Re, Os, Ir, Pt and Au) from geological samples. Recently, it has been recognised that additional HF desilicification is necessary to better recover HSEs, potentially contained within silicate or oxide minerals in mafic samples, which cannot be dissolved solely by inverse aqua regia. However, the abundance of interfering elements tends to increase in the eluent when conventional ion‐exchange purification procedures are applied to desilicified samples. In this study, we developed an improved purification method to determine HSEs in desilicified samples. This method enables the reduction of the ratios of isobaric and polyatomic interferences, relative to the measured intensities of HSE isotope masses, to less than a few hundred parts per million. Furthermore, the total procedural blanks are either comparable to or lower than conventional methods. Thus, this method allows accurate and precise HSE measurements in mafic and ultramafic geological samples, without the need for interference corrections. Moreover, the problem of increased interfering elements, such as Zr for Pd and Cr for Ru, is circumvented for the desilicified samples.     

Boron isotopic constraints on the source of Hawaiian shield lavas

Boron isotopic compositions of lavas from three representative Hawaiian shield volcanoes (Kilauea, Mauna Loa, and Koolau) were analyzed by thermal ionization mass spectrometry. The boron isotopic composition of each sample was analyzed twice, once with and once without acid leaching to evaluate the effect of posteruptive boron contamination. Our acid-leaching procedure dissolved glass, olivine, secondary zeolite, and adsorbed boron; this dissolved boron was completely removed from the residue, which was comprised of plagioclase, pyroxenes, and newly formed amorphous silica. We confirmed that an appropriate acid-leaching process can eliminate adsorbed and incorporated boron contamination from all submarine samples without modifying the original ¹¹B/¹⁰B ratio. On the other hand, when the sample was weathered, i.e., the olivine had an iddingsite rim, ¹¹B/¹⁰B of the acid-resistant minerals are also modified, thus it is impossible to get the preeruptive ¹¹B/¹⁰B value from the weathered samples. Through this elimination and evaluation procedure of posteruptive contamination, preeruptive δ¹¹B values for the shield lavas are −4.5 to −5.4‰ for Koolau (N = 8), −3.6 to −4.6‰ for Kilauea (N = 11), and −3.0 to −3.8‰ for Mauna Loa (N = 6).Historical Kilauea lavas show a systematic temporal trend for B content and Nb/B coupled with other radiogenic isotopic ratios and trace element ratios, at constant δ¹¹B, indicating little or no assimilation of crustal materials in these lavas. Uncorrelated B content and δ¹¹B in Koolau and Mauna Loa lavas may also indicate little or no effect of crustal assimilation in these lavas. The source of KEA-component (identical to the so-called Kea end member in Hawaiian lavas) of the Hawaiian source mantle, represented by Kilauea, should be derived from lower part of subducted oceanic crust or refractory peridotite in the recycled subducted slab. The systematic trend from Kilauea to Koolau—decreasing δ¹¹B coupled with decreasing εNd as well as increasing ⁸⁷Sr/⁸⁶Sr and ²⁰⁶Pb/²⁰⁴Pb—is consistent with involvement of subducted sediment components in the EMK(enriched Makapuu)-component, represented by Makapuu-stage of Koolau lavas.     

Two new eucrite breccias from Northwest Africa

Abstract– This work describes two newly discovered eucrite breccias: three presumably paired meteorites, all named Northwest Africa (NWA) 6105, and NWA 6106. For each meteorite, major‐ and minor‐element compositions of minerals were determined using the electron microprobe. Pyroxene Fe‐Mn co‐variations and bulk‐rock oxygen isotope compositions confirm their classification as eucrites. Variations in mineral compositions and textures are attributed to differences in clast types present (i.e., basaltic or cumulate eucrite). The pyroxene compositions support the hypothesis that samples NWA 6105,1; 6105,2; and 6105,3 are paired polymict eucritic breccias, whereas sample NWA 6106 is a monomict basaltic eucritic breccia. Two‐pyroxene geothermometry yields temperatures too low for igneous crystallization. The variation in temperatures among samples suggests that metamorphism occurred prior to brecciation.