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Sublithospheric diamonds and the inclusions they may carry crystallize in the asthenosphere, transition zone, or uppermost lower mantle (from 300 to ∼800 km), and are the deepest minerals so far recognized to form by plate tectonics. These diamonds are distinctive in their deformation features, low nitrogen content, and inclusions of these major mantle minerals: majoritic garnet, clinopyroxene, ringwoodite, CaSi perovskite, ferropericlase, and bridgmanite or their retrograde equivalents. The stable isotopic compositions of elements within these diamonds (δ11B, δ13C, δ15N) and their inclusions (δ18O, δ56Fe) are typically well outside normal mantle ranges, showing that these elements were either organic (C) or modified by seawater alteration (B, O, Fe) at relatively low temperatures. Metamorphic minerals in cold slabs are effective hosts that transport C as CO3 and H as H2O, OH, or CH4 below the island arc and mantle wedge. Warming of the slab generates carbonatitic melts, supercritical aqueous fluids, or metallic liquids, forming three types of sublithospheric diamonds. Diamond crystallization occurs by movement and reduction of mobile fluids as they pass through host mantle via fractures—a process that creates chemical heterogeneity and may promote deep focus earthquakes. Geobarometry of majoritic garnet inclusions and diamond ages suggest upward transport, perhaps to the base of mantle lithosphere. From there, diamonds are carried to Earth's surface by eruptions of kimberlite magma. Mineral assemblages in sublithospheric diamonds directly trace Earth's deep volatile cycle, demonstrating how the hydrosphere of a rocky planet can connect to its solid interior. Sublithospheric diamonds from the deep upper mantle, transition zone, and lower mantle host Earth's deepest obtainable mineral samples. Low-temperature seawater alteration of the ocean floor captures organic and inorganic carbon at the surface eventually to become some of the most precious gem diamonds. Subduction transports fluids in metamorphic minerals to great depth. Fluids released by slab heating migrate, react with host mantle to induce diamond crystallization, and may trigger earthquakes. Sublithospheric diamonds are powerful tracers of subduction—a plate tectonic process that deeply recycles part of Earth's planetary volatile budget. [ABSTRACT FROM AUTHOR]

Geological pathways for the recycling of Earth's surface materials into the mantle are both driven and obscured by plate tectonics.sup.1-3. Gauging the extent of this recycling is difficult because subducted crustal components are often released at relatively shallow depths, below arc volcanoes.sup.4-7. The conspicuous existence of blue boron-bearing diamonds (type IIb).sup.8,9 reveals that boron, an element abundant in the continental and oceanic crust, is present in certain diamond-forming fluids at mantle depths. However, both the provenance of the boron and the geological setting of diamond crystallization were unknown. Here we show that boron-bearing diamonds carry previously unrecognized mineral assemblages whose high-pressure precursors were stable in metamorphosed oceanic lithospheric slabs at depths reaching the lower mantle. We propose that some of the boron in seawater-serpentinized oceanic lithosphere is subducted into the deep mantle, where it is released with hydrous fluids that enable diamond growth.sup.10. Type IIb diamonds are thus among the deepest diamonds ever found and indicate a viable pathway for the deep-mantle recycling of crustal elements.Mineral inclusions in blue boron-bearing diamonds reveal that such diamonds are among the deepest diamonds ever found and indicate a viable pathway for the deep-mantle recycling of crustal elements., Author(s): Evan M. Smith [sup.1] , Steven B. Shirey [sup.2] , Stephen H. Richardson [sup.3] , Fabrizio Nestola [sup.4] , Emma S. Bullock [sup.5] , Jianhua Wang [sup.2] , Wuyi [...]

The emplacement of the 2.05-billion-year-old Bushveld complex, the world's largest layered intrusion and platinum-group element (PGE) repository (1), is a singular event in the history of the Kaapvaal craton of [...]

New [sup.187]Os/[sup.188]Os ratios of Quaternary Mount Adams volcanic rocks from the Cascade arc in southern Washington vary by >300% ([sup.187]Os/[sup.188]Os = 0.165-0.564) and fall into high (>0.319) and low (0.166 to 0.281) groups of [sup.187]Os/[sup.188]Os ratios that are substantially more radiogenic than mantle values. These Os isotope compositions and groupings are interpreted to reflect recycling of discrete intracrustal domains with high [sub.187]Os/[sup.188]Os ratios but differing ages, thus recording the process of crustal hybridization and homogenization. Os isotope compositions provide new constraints on amounts of intracrustal recycling in young subduction-zone environments that reflect the magmatic history of the arc. Sr, Nd, Hf, and Pb isotope variations in this young, mafic arc complex are too small to allow such constraints. Keywords: Mount Adams, Cascade Range, crust, recycling, osmium, hybridization.

Ultramafic rocks from the center of the Vredefort structure previously have been interpreted either as Early Proterozoic igneous rocks genetically related to the Bushveld complex, or as Archean upper mantle rocks. Re-Os isotopic date from these rocks display very low (super.187)Os/(Super.188)Os and (super.187)Re/(super.188)Os, suggesting that they are not magma chamber cumulates, but rather the residues of melt extraction, similar to Re-Os systematics to subcontinental lithospheric mantle. Whole-rock Re depletion model ages for two samples are 3.3 and 3.5 Ga, indicating that the postulated melt extraction event must have occurred in the Archean. On the basis of these data we conclude that the ultramafic rocks are not related to the Bushveld event, but rather represent upper mantle that rebounded to crustal levels following an impact event. This conclusion strengthens evidence for a model whereby the rocks exposed in the core of the Vredefort structure represent the lower parts of a crust on edge profile that possibly contains a paleo-Moho very near the Earth's surface.

Mafic deep crustal and shallow upper mantle xenoliths hosted in the Chino Valley and Camp Creek latites of central Arizona have 1870s/1880s (0.216 to 1.51) ratios dramatically higher than values typical of mantle peridotite (

A relatively high-temperature oxidizing digestion using aqua regia has been developed for

Geochemical and isotopic studies of the Cretaceous (106 Ma) Prairie Creek olivine lamproites in southwestern Arkansas indicate that these diamond-bearing ultrapotassic magmas were derived from metasomatized Proterozoic subcontinental lithospheric mantle. Re-Os isotopic data yield a nonradiogenic initial [[Gamma].sub.Os] = -3.2 to -3.6 (percent deviation in 187Os/188Os from chondritic mantle) and mantle model ages ([T.sub.MA]) of 0.9 to 1.2 Ga. Sm-Nd isotopic data yield a nonradiogenic initial [[Epsilon].sub.Nd] = -10 and a depleted mantle model age ([T.sub.DM) of 1.2 Ga. These data are consistent with initial stabilization of low-density lithospheric mantle at [approximately] 1.2 Ga, followed closely in time by reenrichment in incompatible trace elements. This Middle Proterozoic lithospheric mantle source is younger than overlying Early Proterozoic crust of the southern U.S. midcontinent. Pre-Grenville subduction of oceanic lithosphere may have juxtaposed 1.2 Ga mantle beneath 1.6 to 2.0 Ga crust and imparted the necessary slab-derived chemical components for the growth of diamonds.