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Widespread Miocene (24–8 Ma) ultrapotassic rocks and their entrained xenoliths provide information on the composition, structure, and thermal state of the sub-continental lithospheric mantle in southern Tibet during the India-Asia continental collision. The ultrapotassic rocks along the Lhasa block delineate two distinct lithospheric domains with different histories of depletion and enrichment. The eastern ultrapotassic rocks (89°E–92°E) reveal a depleted, young, and fertile lithospheric mantle (87Sr/86Srt = 0.704–0.707 [t is eruption time]; Hf depleted-mantle model age [TDM] = 377–653 Ma). The western ultrapotassic rocks (79°E–89°E) and their peridotite xenoliths (81°E) reflect a refractory harzburgitic mantle refertilized by ancient metasomatism (lavas: 87Sr/86Srt = 0.714–0.734; peridotites: 87Sr/86Srt = 0.709–0.716). These data integrated with seismic tomography suggest that upwelling asthenosphere was diverted away from the deep continental root beneath the western Lhasa block, but rose to shallower depths beneath a thinner lithosphere in the eastern part. Heating of the lithospheric mantle by the rising asthenosphere ultimately generated the ultrapotassic rocks with regionally distinct geochemical signatures reflecting the different nature of the lithospheric mantle.

The minimum oxygen fugacity ( f O2) of Earth’s upper mantle probably is controlled by metal saturation, as defined by the iron-wustite (IW) buffer reaction (FeO → Fe + O). However, the widespread occurrence of moissanite (SiC) in kimberlites, and a suite of super-reduced minerals (SiC, alloys, native elements) in peridotites in Tibet and the Polar Urals (Russia), suggest that more reducing conditions ( f O2 = 6–8 log units below IW) must occur locally in the mantle. We describe pockets of melt trapped in aggregates of corundum crystals ejected from Cretaceous volcanoes in northern Israel which contain high-temperature mineral assemblages requiring extremely low f O2 (IW < −10). One abundant phase is tistarite (Ti2O3), previously known as a single grain in the Allende carbonaceous chondrite (Mexico) and believed to have formed during the early evolution of the solar nebula. It is associated with other reduced phases usually found in meteorites. The development of super-reducing conditions in Earth’s upper mantle may reflect the introduction of CH4 + H2 fluids from the deep mantle, specifically related to deep-seated volcanic plumbing systems at plate boundaries.

We suggest a new explanation for the presence of crustally derived zircons in the upper-mantle rocks of ophiolitic complexes, as an alternative to subduction-related models. Integrated isotopic (U-Pb, Hf, and O isotopes) and trace-element data for zircons from the Tumut ophiolitic complex (southeast Australia) indicate that these grains are related to granitic magmatism and were introduced into the mantle rocks after their emplacement into the crust. These observations emphasize that a clear understanding of the origin of individual zircon populations and their relationship to the host rock is essential to interpretations of the tectonic history of upper-mantle rocks and the dynamics of crust-mantle interactions.

The Os-isotope compositions of platinum-group minerals (PGMs) in ophiolite chromitites are commonly regarded as resistant to fluid-related processes, and have been used to track the evolution of Earth’s convecting mantle. However, we have found significant differences in 187Os/188Os between primary and secondary PGMs from metamorphosed ophiolite chromitites of the Dobromirtsi Ultramafic Massif, in the Central Rhodope Metamorphic Core Complex of southeastern Bulgaria. Primary (magmatic) PGMs hosted in unaltered chromite cores have 187Os/188Os from 0.1231 to 0.1270, and 187Re/188Os ≤ 0.002. T MA and T RD model ages, calculated relative to the Enstatite Chondrite Reservoir, cluster around three main peaks: ca. 0.3, 0.4, and 0.6 Ga. Secondary PGMs, produced by alteration of magmatic PGMs, have a wider range of variation (187Os/188Os = 0.1124–0.1398, 187Re/188Os ≤ 0.024); these grains yield T MA and T RD model ages from –1.7 Ga up to 2.2 Ga. The larger range in 187Os/188Os in the secondary PGMs is interpreted as due to reactions between the primary PGMs and infiltrating metamorphic-hydrothermal fluids with a range of Os-isotope compositions. This redistribution of Os in PGMs during metamorphism has significant implications for the interpretation of both whole-rock and in situ Os-isotope data in mantle-derived rocks.

Ancient cratons are typically characterized by thick lithosphericmantle roots that provide them with buoyancy and rigidity. Once formed, cratons, including their roots, move around Earth’s surface as parts of rigid plates that are normally unaffected by later magmatism or tectonic activity. Considerable geophysical and geochemical evidence, however, suggests that the ancient lithospheric mantle beneath the Chinese portion of the eastern Sino-Korean craton was replaced by thinner, hotter, juvenile mantle during the Jurassic to Early Cretaceous. The extent and cause of the changes to this lithospheric mantle have been debated. Keys to deciphering the history of the changes include knowledge of both the lateral extent of the lithospheric modifi cations and the relative timing of change across the craton. Discerning the age and structure of lithospheric mantle underlying North Korea, the easternmost part of the Sino-Korean craton, is thus particularly important. Here we report mineral compositions and Re-Os isotopic data for peridotite xenoliths from North Korean Triassic kimberlite and Tertiary basalts. The data suggest that, from the Triassic to the present, North Korea has been underlain by young, hot and fertile lithospheric mantle, unlike typical cratonic lithospheric roots, but similar to the juvenile lithospheric mantle underlying Chinese portions of the craton. Given the generally ancient nature of the crust composing North Korea, our fi ndings suggest that modifi cation of underlying cratonic roots extended to the eastern edge of the craton. The Triassic eruption age of the kimberlite suggests that the lithospheric changes may have occurred earlier than in China, indicating that lithospheric removal evolved from east to west. These results are most consistent with the conclusion that lithospheric loss was initially triggered by extension that followed the collision between the Sino-Korean and Yangtze cratons.

The age distribution of the crust is a fundamental parameter in modeling continental evolution and the rate of crustal accretion through Earth9s history, but this is usually estimated from surface exposures. The exposed Yangtze craton in eastern China consists mainly of Proterozoic rocks with rare Archean outcrops. However, the U-Pb ages and Hf isotope systematics of xenocrystic zircons brought to the surface in lamproite diatremes from three Proterozoic outcrop areas of the craton suggest the widespread presence of unexposed Archean basement, with zircon age populations of 2900–2800 Ma and 2600– 2500 Ma and Hf model ages of 2.6 to ca. 3.5 Ga or older. The zircons also record thermal events reworked on the craton ca. 2020 Ma (remelting of older crust) and 1000–850 Ma (addition of juvenile mantle material). The observation of deep crust significantly older than the upper crust will require revision of models for the rates of crustal generation through time.

The Hf isotope composition of rutile in mica-amphibole-rutile-ilmenite-diopside (MARID) xenoliths from the Kimberley area of South Africa has been analyzed in situ by laser ablation microprobe–multicollector–inductively coupled plasma mass spectrometry to investigate metasomatic processes in the lithospheric mantle. The 176Hf/177Hf ratio shows a wide range, from 0.2811 to 0.2858 (eHf = −55 to +110); much of this range is found within single samples and even within single grains. We suggest that the MARID rutiles initially resulted from the interaction of an asthenospheric melt with ancient depleted harzburgitic mantle with low 176Hf/177Hf ratios (

The Os isotope compositions of sulfides in mantle xenoliths from the Penghu Islands, Taiwan Strait, reveal the presence of Proterozoic subcontinental lithospheric mantle beneath the highly extended southeast margin of the South China block. Both T RD (Re depleted) model ages for individual sulfides and model ages estimated from the initial 187Os/188Os ratios of Re-Os mixing lines require that some volumes of the subcontinental lithospheric mantle formed prior to 2.3–1.9 Ga. Later events in the subcontinental lithospheric mantle may be recorded by T RD model ages of 1.5–1.2 Ga and ca. 0.9 Ga. The events recognized in the subcontinental lithospheric mantle are consistent with those known in the crust of the mainland South China block. The sulfide Os isotope data show that Proterozoic lithosphere beneath the South China block has survived the extensive Mesozoic Yanshanian magmatism on the continental margin and has not been delaminated even during the severe lithospheric extension that led to the subsidence of the Taiwan Strait.