There is, I think, something ethereal about i —the square root of minus one. I remember first hearing about it at school. It seemed an odd beast at that time—an intruder hovering on the edge of reality.

Usually familiarity dulls this sense of the bizarre, but in the case of i it was the reverse: over the years the sense of its surreal nature intensified. It seemed that it was impossible to write mathematics that described the real world in …

I and i

Summary Trace-element data for mid-ocean ridge basalts (MORBs) and ocean island basalts (OIB) are used to formulate chemical systematics for oceanic basalts. The data suggest that the order of trace-element incompatibility in oceanic basalts is Cs ≈ Rb ≈ (≈ Tl) ≈ Ba(≈ W) > Th > U ≈ Nb = Ta ≈ K > La > Ce ≈ Pb > Pr (≈ Mo) ≈ Sr > P ≈ Nd (> F) > Zr = Hf ≈ Sm > Eu ≈ Sn (≈ Sb) ≈ Ti > Dy ≈ (Li) > Ho = Y > Yb. This rule works in general and suggests that the overall fractionation processes operating during magma generation and evolution are relatively simple, involving no significant change in the environment of formation for MORBs and OIBs. In detail, minor differences in element ratios correlate with the isotopic characteristics of different types of OIB components (HIMU, EM, MORB). These systematics are interpreted in terms of partial-melting conditions, variations in residual mineralogy, involvement of subducted sediment, recycling of oceanic lithosphere and processes within the low velocity zone. Niobium data indicate that the mantle sources of MORB and OIB are not exact complementary reservoirs to the continental crust. Subduction of oceanic crust or separation of refractory eclogite material from the former oceanic crust into the lower mantle appears to be required. The negative europium anomalies observed in some EM-type OIBs and the systematics of their key element ratios suggest the addition of a small amount (⩽1% or less) of subducted sediment to their mantle sources. However, a general lack of a crustal signature in OIBs indicates that sediment recycling has not been an important process in the convecting mantle, at least not in more recent times (⩽2 Ga). Upward migration of silica-undersaturated melts from the low velocity zone can generate an enriched reservoir in the continental and oceanic lithospheric mantle. We propose that the HIMU type (eg St Helena) OIB component can be generated in this way. This enriched mantle can be re-introduced into the convective mantle by thermal erosion of the continental lithosphere and by the recycling of the enriched oceanic lithosphere back into the mantle.

Chemical and isotopic systematics of oceanic basalt : implications for mantle composition and processes

Chemical and isotopic systematics of oceanic basalts. Implications for Mantle Composition and Processes

We present the first finding of continental crust-derived Precambrian zircons in garnet/spinel pyroxenite veins within mantle xenoliths carried by the Neogene Hannuoba basalt in the central zone of the North China Craton (NCC). Petrological and geochemical features indicate that these mantle-derived composite xenoliths were formed by silicic melt^lherzolite interaction. The Precambrian zircon ages can be classified into three age groups of 2·4^2·5 Ga, 1·6^2·2 Ga and 0·6^1·2 Ga, coinciding with major geological events in the NCC. These Precambrian zircons fall in the field of continental granitoid rocks in plots of U/Yb vs Hf and Y. Their igneous-type REE patterns and metamorphic zircon type CL images indicate that they were not crystallized during melt^peridotite interaction and subsequent high-pressure metamorphism.The 2·5 Ga zircons have positive eHf(t) values (2·9^10·6), whereas the younger Precambrian zircons are dominated by negative eHf(t) values, indicating an ancient continental crustal origin.These observations suggest that the Precambrian zircons were xenocrysts that survived melting of recycled continental crustal rocks and were then injected with silicate melt into the host peridotite. In addition to the Precambrian zircons, igneous zircons of 315 3 Ma (2 ), 80^170 Ma and 48^64 Ma were separated from the garnet/spinel pyroxenite veins; these provide evidence for lower continental crust and oceanic crust recycling-induced multi-episodic melt^peridotite interactions in the central zone of the NCC. The combination of the positive eHf(t) values (2·91^24·6) of the 315 Ma zircons with the rare occurrence of 302^324 Ma subduction-related diorite^granite plutons in the northern margin of the NCC implies that the 315 Ma igneous zircons might record melt^peridotite interactions in the lithospheric mantle induced by Palaeo-Asian oceanic crust subduction. Igneous zircons of age 80^170 Ma generally coexist with the Precambrian metamorphic zircons and have lower Ce/Yb and Th/U ratios, higher U/Yb ratios and greater negative Eu anomalies.The eHf(t) values of these zircons vary greatly from ^47·6 to 24·6.The 170^110 Ma zircons are generally characterized by negative eHf(t) values, whereas the 110^100 Ma zircons have positive eHf(t) values.These observations suggest that melt^peridotite interactions at 80^170 Ma were induced by partial melting of recycled continental crust. The 48^64 Ma igneous zircons are characterized by negligible Ce anomalies, unusually high REE, U and Th contents, and positive eHf(t) values.These features imply that the melt^peridotite interactions at 48^64 Ma could be associated with a depleted mantle-derived carbonate melt or fluid.

Continental and Oceanic Crust Recycling-induced Melt^Peridotite Interactions in the Trans-North China Orogen: U^Pb Dating, Hf Isotopes and Trace Elements in Zircons from Mantle Xenoliths

Geochemical fingerprinting of oceanic basalts with applications to ophiolite classification and the search for Archean oceanic crust

K–Ar analyses of the post-caldera lavas of Bratan volcano in Bali Island, Indonesia — Ar isotope mass fractionation to light isotope enrichment

An unusual hercynite-rich inclusion was found in the Vigarano CV3 chondrite. Pure and coarse-grained hercynite crystals occupy more than 50 vol.% of the inclusion, and it contains small amount of melilite (1 vol.%) and Mg-bearing spinel. Moreover, there is calcium monoaluminate CaAl2O4, which has been found only in a refractory inclusion of a CH chondrite [1, 2], in the core of this inclusion. However, it consists of grossite, corundum, and perovskite, which are common in normal refractory inclusions. Therefore, it obviously has a close relationship with normal refractory inclusions. The crystallization sequence of the hercynite-rich inclusion is consistent with equilibrium condensation sequence in the solar nebular gas, except for corundum. It is possible that corundum was produced by a reaction between grossite and FeO-bearing fluids during aqueous alteration on the parent body. The oxygen isotopic compositions of the minerals lie along the mass-dependent terrestrial fractionation line rather than the massindependent fractionation line obtained from normal CAIs in carbonaceous chondrites. Hercynite and calcium monoaluminate is the lightest phases in this inclusion (δ17O ~ -12‰, δ18O ~ -20‰), heavier than spinels in normal refractory inclusions. The hercynite-rich inclusion is indicative of unusual nebular conditions where it formed.

A Hercynite-rich Inclusion in the Vigarano CV3 Chondrite

Abstract All life on Earth contains amino acids and carbonaceous chondrite meteorites have been suggested as their source at the origin of life on Earth. While many meteoritic amino acids are considered indigenous, deciphering the extent of terrestrial contamination remains an issue. The Ryugu asteroid fragments (JAXA Hayabusa2 mission), represent the most uncontaminated primitive extraterrestrial material available. Here, the concentrations of amino acids from two particles from different touchdown sites (TD1 and TD2) are reported. The concentrations show that N,N-dimethylglycine (DMG) is the most abundant amino acid in the TD1 particle, but below detection limit in the other. The TD1 particle mineral components indicate it experienced more aqueous alteration. Furthermore, the relationships between the amino acids and the geochemistry suggest that DMG formed on the Ryugu progenitor body during aqueous alteration. The findings highlight the importance of aqueous chemistry for defining the ultimate concentrations of amino acids in primitive extraterrestrial samples. 

/pdf/insights-into-the-formation-and-evolution-of-2kbpyjhu.pdf

Insights into the formation and evolution of extraterrestrial amino acids from the asteroid Ryugu

Maximum depositional ages and evolving provenance of Franciscan metagraywackes, NW California: LA-ICPMS zircon U-Pb data

High-grade metamorphic rocks such as garnet amphibolite and garnet-biotite gneiss were found in Ohshima, Yawatahama city, western Shikoku. They occur in the Sanbagawa metamorphic belt and are in fault contact with greenstones of the pumpellyite-actinolite facies. Common occurrences of almandine garnet in the amphibolite and cummingtonite-quartz assemblage suggest that they belong to the amphibolite facies of the intermediate-pressure type metamorphism, distinct from the surrounding Sanbagawa metamorphic rocks. Two biotites and two amphiboles from the high-grade rocks have a narrow range of K-Ar age from 90 to 100 Ma. Although present occurrence of the high-grade rocks may be explained by thrusting over the Sanbagawa metamorphic rocks, there has been so far no report of a suitable candidate corresponding to the parental body of the high-grade metamorphic rocks.

https://www.jstage.jst.go.jp/article/ganko1988/85/1/85_1_19/_pdf

Eizo Nakamura

Papers

K–Ar analyses of the post-caldera lavas of Bratan volcano in Bali Island, Indonesia — Ar isotope mass fractionation to light isotope enrichment

A Hercynite-rich Inclusion in the Vigarano CV3 Chondrite

Insights into the formation and evolution of extraterrestrial amino acids from the asteroid Ryugu

Maximum depositional ages and evolving provenance of Franciscan metagraywackes, NW California: LA-ICPMS zircon U-Pb data

Mineral assemblages and K-Ar ages of metamorphic rocks in Ohshima Yawatahama city, Ehime Prefecture