Missing western half of the Pacific Plate: Geochemical nature of the Izanagi-Pacific Ridge interaction with a stationary boundary between the Indian and Pacific mantles

The source mantle of the basaltic ocean crust on the western half of the Pacific Plate was examined using Pb–Nd–Hf isotopes. The results showed that the subducted Izanagi–Pacific Ridge (IPR) formed from both Pacific (180–∼80 Ma) and Indian (∼80–70 Ma) mantles. The western Pacific Plate becomes younger westward and is thought to have formed from the IPR. The ridge was subducted along the Kurile–Japan–Nankai–Ryukyu (KJNR) Trench at 60–55 Ma and leading edge of the Pacific Plate is currently stagnated in the mantle transition zone. Conversely, the entire eastern half of the Pacific Plate, formed from isotopically distinct Pacific mantle along the East Pacific Rise and the Juan de Fuca Ridge, largely remains on the seafloor. The subducted IPR is inaccessible; therefore, questions regarding which mantle might be responsible for the formation of the western half of the Pacific Plate remain controversial. Knowing the source of the IPR basalts provides insight into the Indian–Pacific mantle boundary before the Cenozoic. Isotopic compositions of the basalts from borehole cores (165–130 Ma) in the western Pacific show that the surface oceanic crust is of Pacific mantle origin. However, the accreted ocean floor basalts (∼80–70 Ma) in the accretionary prism along the KJNR Trench have Indian mantle signatures. This indicates the younger western Pacific Plate of IPR origin formed partly from Indian mantle and that the Indian–Pacific mantle boundary has been stationary in the western Pacific at least since the Cretaceous

Trends of Sr and Nd isotopes through time near the Japan Sea in northeastern Japan

Nd and Sr isotopic analyses were made on selected volcanic rocks of known stratigraphic age from a limited area in the “Green Tuff” region, northeastern Japan, and in adjacent areas. Two ultramafic inclusions from Ichinomegata were also analyzed. The volcanic rocks range in age from 27 to 6 Ma, and vary in ε_(Nd) from −0.9 to 5.7 and in ε_(Sr) from 14.1 to −16.3. The isotopic data plot generally along the region of the “mantle array”. When plotted against time, the isotopic ratios are found to be a regular function of geologic age. The results indicate that: (1) the volcanic rocks in the area are part of a “related” sequence or evolutionary pattern over a time period of 27 Ma; (2) the “evolutionary” pattern goes from an initially enriched source to that of a very depleted source. The shift in isotopic composition appears to reflect the ongoing lining of magma conduits or the decreasing extraction of lower crustal contributions with increasing infiltration of oceanic magma types in an environment where the crust is thinning. Based on the geotectonic development of this area due to the back-arc spreading of the Japan Sea, we identify two different stages: the pre-opening stage (> 15 Ma) and the post-opening stage (< 15 Ma). The volcanic rocks of the pre-opening stage are characterized by an enriched isotopic signature, and those of the post-opening era by a depleted signature. The host rocks of the Kuroko ore deposit lie within this evolutionary trend and contain a substantial amount of continentally derived materials processed through an oceanic environment. In general, we infer that the degree of continental crust involvement is high during the pre-opening stages of a back-arc basin when it is associated with a rifted continental mass

Systematic variation in Sr-, Nd- and Pb-isotopes with time in lavas of Mauritius, Reunion Hotspot

We report Sr-, Nd- and Pb-isotopic compositions for the lavas of Mauritius, the second youngest volcanic island in the Réunion hotspot. The lavas of the Older Series (7·8–5·5 Ma) have identical isotopic compositions (87Sr/86Sr = 0·70411 to 0·70422,143Nd/144Nd = 0·512865 to 0·512854, and 206Pb/204Pb = 19·016 to 19·041) to those of Réunion, where the center of volcanic activity is currently located. The lavas of the Intermediate Series (3·5–1·9 Ma) and Younger Series (0·70–0·17 Ma) are shifted to lower Sr-isotopic compositions (0·70364–0·70394, with 143Nd/144Nd = 0·512813 to 0·512948 and 206Pb/204Pb = 18·794 to 18·984). The Intermediate Series lavas have similar trace-element characteristics (e.g. Zr–Nb, Ba–Y) to those of Rodrigues, in both cases requiring the involvement of an enriched mantle-like component in the mantle source. During the volcanic history of Mauritius, the magmas lost the principal isotopic characteristics of the Réunion hotspot with time, and became gradually imprinted with the isotopic signature of a shallower mantle source that produced the Central Indian Ridge basalts

Evidence from the Rb-Sr system for 4.4 Ga alteration of chondrules in the Allende (CV3) parent body

The timing and processes of alteration in the CV parent body are investigated by the analysis of Sr isotopes, major and trace elements, and petrographic type and distribution of the secondary minerals (nepheline and sodalite) in 22 chondrules from the Allende (CV3) chondrite. The Sr isotopic compositions of the chondrules are scattered around the 4.0 Ga reference line on the 87Sr/ 86Sr evolution diagram, indicating that the chondrules have been affected by late thermal alteration event(s) in the parent body. The degree of alteration, determined for individual chondrules based on the distribution of nepheline and sodalite, is unrelated to the disturbance of the Rb-Sr system, suggesting that the alteration process that produced nepheline and sodalite is different from the thermal process that disturbed the Rb-Sr system of the chondrules. Considering the geochemical behavior of Rb and Sr, the main host phase of Sr in chondrules is likely to be mesostasis, which could be most susceptible to late thermal alteration. As there is a poor connection between the alteration degree determined from abundances of nepheline and sodalite and the disturbance of Rb-Sr isotopic system, we consider the mesostasis to provide a constraint on the late parent body alteration process. From this point of view, 23 mesostasis-rich chondrules, including those from literature data, were selected. The selected chondrules are closely correlated on the 87Sr/86Sr evolution diagram, with an inferred age of 4.36 +/- 0.08 Ga. This correlation would represent an age of the final major Sr isotopic redistribution of the chondrules in the parent body.The Meteoritics & Planetary Science archives are made available by the Meteoritical Society and the University of Arizona Libraries. Contact [email protected] for further information.Migrated from OJS platform February 202