Rb-Sr, Sm-Nd ages of the Phalaborwa Carbonatite Complex, South Africa

We analyzed Rb-Sr and Sm-Nd isotopic compositions of whole-rock and minerals from syenite, biotite gneiss xenolith, dolerite dyke, phoscorite, and carbonatite from the Phalaborwa Carbonatite Complex located in northeastern Transvaal, South Africa. Syenite does not give significant Rb-Sr and Sm-Nd whole-rock and mineral isochron ages. Dolerite gives an Rb-Sr whole-rock and mineral isochron age of 2062±74Ma. This age overlaps with the timing of the magmatism of the Phalaborwa Carbonatite Complex. Biotite gneiss and phoscorite do not also give significant Rb-Sr and Sm-Nd isochrons. The Rb-Sr whole-rock and mineral isochron of phoscorite, however, gives an age of 2013±93Ma. The age is clearly the cooling age of this complex. Carbonatites are divided into two groups, having low and high initial Sr isotopic ratios. This coordinates with the result of S.C. Eriksson (Carbonatites, ed. by K. Bell, 1989). In addition, these groups indicate different initial Nd isotopic ratios. These suggest that carbonatite and related rocks were formed by mixing of two source magmas having different Sr and Nd isotopic compositions

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

Isotopic evidence for a link between the Lyra Basin and Ontong Java Plateau

The few geological and geophysical studies of the Lyra Basin at the western margin of 45 the Ontong Java Plateau (OJP; Pacific Ocean) revealed that it is underlain by thicker than normal 46 oceanic crust. The unusually thick oceanic crust is attributed to the emplacement of massive lava 47 flows from the OJP. Dredging was conducted to sample the inferred OJP crust on the Lyra Basin 48 but instead recovered younger extrusives that may have covered the older plateau lavas in the 49 area. The Lyra Basin extrusives are alkalic basalts with (87Sr/86Sr)t = 0.704513-0.705105, 50 (143Nd/144Nd)t = 0.512709-0.512749, εNd(t) = +3.0 to +3.8, (206Pb/204Pb)t = 18.488-18.722, 51 (207Pb/204Pb)t = 15.558-15.577, and (208Pb/204Pb)t = 38.467-38.680, values that are distinct from 52 those of the OJP tholeiites. They have age-corrected (187Os/188Os)t = 0.1263-0.1838 that overlap 53 with the range of values determined for the Kroenke-type and Kwaimbaita-type OJP basalts, but 54 their (176Hf/177Hf)t = 0.28295-0.28299 and εHf(t) = +7.9 to +9.3 values are lower. These isotopic 55 compositions do not match those of any Polynesian ocean island volcanics. Instead, the Lyra 56 Basin basalts have geochemical affinity and isotopic compositions that overlap with those of 57 some alkalic suite and alnoites in the island of Malaita, Solomon Islands. Although not directly 58 related to the main plateau volcanism at 120 Ma, the geochemical data and modeling suggest that 59 the origin of the Lyra Basin alkalic rocks may be genetically linked to the mantle preserved in 60 the OJP thick lithospheric root, with magmatic contribution from the Rarotongan hotspot