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1. Heat Transfer and Production in Cratonic Continental Crust: U‐Pb Thermochronology of Xenoliths From the Siberian Craton

   https://doi.org/10.1029/2022GC010497  

   Apen, Francisco E.; Rudnick, Roberta L.; Ionov, Dmitri A.; Cottle, John M.; Moyen, Jean‐Franҫois; Golovin, Alexander V.; Korsakov, Andrey V. (October 2022, Geochemistry, Geophysics, Geosystems)

   Abstract Coupled U‐Pb and trace‐element analyses of accessory phases in crustal xenoliths from the Late Devonian Udachnaya kimberlite (Siberian craton, Russia) are used to constrain Moho temperature and crustal heat production at the time of kimberlite eruption. Rutile and apatite in lower‐crustal garnet granulites record U‐Pb dates that extend from 1.8 Ga to 360 Ma (timing of kimberlite eruption). This contrasts with upper‐crustal tonalites and amphibolites that contain solely Paleoproterozoic apatite. Depth profiling of rutile from the lower‐crustal xenoliths show that U‐Pb dates increase gradually from rim to core over μm‐scale distances, with slower‐diffusing elements (e.g., Al) increasing in concentration across similar length‐scales. The U‐Pb and trace element gradients in rutile are incompatible with partial Pb loss during slow cooling, but are consistent with neocrystallization and re‐heating of the lower crust for <1 Myr prior to eruption. Because Paleoproterozoic rutile and apatite dates are preserved, we infer that long‐term ambient lower‐crustal temperatures before this thermal perturbation were cooler than the Pb closure temperature of rutile and probably apatite (<400°C). The lower‐crustal temperature bounds from these data are consistent with pressure‐temperature arrays of Udachnaya peridotite xenoliths that suggest relatively cool geothermal gradients, signifying that the mantle xenoliths accurately capture the thermal state of the lithosphere prior to eruption. Combined, the xenolith data imply low crustal heat production for the Siberian craton (∼0.3 μW/m³). Nevertheless, such values produce surface heat flow values of 20–40 mW/m², higher than measured around Udachnaya (average 19 mW/m²), suggesting that the surface heat flow measurements are inaccurate. 

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2. Pacific Lithosphere Evolution Inferred from Aitutaki Mantle Xenoliths

   https://doi.org/10.1093/petrology/egz047  

   Snortum, Eric; Day, James_M D; Jackson, Matthew G (September 2019, Journal of Petrology)

   Abstract Highly siderophile element (HSE: Os, Ir, Ru, Pt, Pd, Re), major and trace element abundances, and 187Re–187Os systematics are reported for xenoliths and lavas from Aitutaki (Cook Islands), to investigate the composition of Pacific lithosphere. The xenolith suite comprises spinel-bearing lherzolites, dunite, and harzburgite, along with olivine websterite and pyroxenite. The xenoliths are hosted within nephelinite and alkali basalt volcanic rocks (187Os/188Os ∼0·1363 ± 13; 2SD; ΣHSE = 3–4 ppb). The volcanic host rocks are low-degree (2–5%) partial melts from the garnet stability field and an enriched mantle (EM) source. Pyroxenites have similar HSE abundances and Os isotope compositions (Al2O3 = 5·7–8·3 wt %; ΣHSE = 2–4 ppb; 187Os/187Os = 0·1263–0·1469) to the lavas. The pyroxenite and olivine websterite xenoliths directly formed from—or experienced extensive melt–rock interaction with—melts similar in composition to the volcanic rocks that host the xenoliths. Conversely, the Aitutaki lherzolites, harzburgites and dunites are similar in composition to abyssal peridotites with respect to their 187Os/188Os ratios (0·1264 ± 82), total HSE abundances (ΣHSE = 8–28 ppb) and major element abundances, forsterite contents (Fo89·9±1·2), and estimated extents of melt depletion (<10 to >15%). These peridotites are interpreted to sample relatively shallow Pacific mantle lithosphere that experienced limited melt–rock reaction and melting during ridge processes at ∼90 Ma. A survey of maximum time of rhenium depletion ages of Pacific mantle lithosphere from the Cook (Aitutaki ∼1·5 Ga), Austral (Tubuai’i ∼1·8 Ga), Samoan (Savai’i ∼1·5 Ga) and Hawaiian (Oa’hu ∼2 Ga) island groups shows that Mesoproterozoic to Neoproterozoic depletion ages are preserved in the xenolith suites. The variable timing and extent of mantle depletion preserved by the peridotites is, in some instances, superimposed by extensive and recent melt depletion as well as melt refertilization. Collectively, Pacific Ocean island mantle xenolith suites have similar distributions and variations of 187Os/188Os and HSE abundances to global abyssal peridotites. These observations indicate that Pacific mantle lithosphere is typical of oceanic lithosphere in general, and that this lithosphere is composed of peridotites that have experienced both recent melt depletion at ridges and prior and sometimes extensive melt depletion across several Wilson cycles spanning periods in excess of two billion years. 

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3. Lithospheric mantle xenolith geochemistry from south-central Vietnam: Radiogenic isotopes

   https://doi.org/10.26022/IEDA/112848  

   Hobbs, Kirby; Elkins, Lynne Elkins; Lassiter, John Lassiter; Hoang, Nguyen; Burberry, Caroline Burberry (January 2023, Interdisciplinary Earth Data Alliance (IEDA))

   Radiogenic isotope data measured in lithospheric mantle xenolith whole rocks from south-central Vietnam. Samples are peridotite xenoliths from two alkali basalt locations in Vietnam, Pleiku and Xuan Loc. Data are radiogenic isotopes measured by TIMS and MC-ICP-MS in clinopyroxene mineral separates. 

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4. Formation of Spongy Clinopyroxene: Insights from Eclogitic Inclusions in Diamonds

   https://doi.org/10.1093/petrology/egaf036  

   Wang, Wenyu; Korolev, Nester M; Kiseeva, Ekaterina S; Davies, Rondi M; Weiss, Yaakov; Kopylova, Maya; Towbin, W Henry; Huang, Fang; Moore, Andy E; Vasilev, Evgeniy A (May 2025, Journal of Petrology)

   Abstract Spongy clinopyroxene is common in most mantle-derived xenoliths and megacrysts of eclogitic and peridotitic parageneses. Its formation is commonly attributed to the partial melting of a primary clinopyroxene in response to various factors, including changes in pressure and temperature or infiltration of external melts or fluids. In order to study the mechanism of spongy clinopyroxene formation in detail, we selected six eclogitic clinopyroxene inclusions in diamonds with varying amounts of spongy clinopyroxene (from ~10 to 100%). We employed computed tomography, electron microprobe analysis, and Raman spectroscopy to study the textural characteristics, major element concentrations, and the types of volatiles present in both phases. We also used pMELTS to model the compositions of spongy clinopyroxene and associated melts produced by the melting of primary clinopyroxene over a range of pressures and temperatures. We compare these results with estimates from major element thermobarometry of the spongy clinopyroxene. We conclude that the studied spongy clinopyroxene is the solid product of partial melting that occurs upon decompression of the primary clinopyroxene within the diamond in a near-closed system. Melting of the primary clinopyroxene occurred continuously or in pulses at different depths during the diamond’s ascent to Earth’s surface and produced variable spongy clinopyroxene and melt compositions even within the same inclusion. This is possible due to relatively rapid kimberlite ascent. The degrees of melting are various and unexpectedly high for mantle melting (between <10 and 60% with an average of ~20–30%). The produced melts are highly silicic, phonolitic, and alkali-rich. pMELTS modelling shows the spongy clinopyroxene compositions can be reproduced at pressures between 0.5–2.7 GPa and temperatures of 850–1300°C, with the majority of them satisfying the P–T conditions of 1–2 GPa and 1100–1300°C. This indicates decompression melting of primary clinopyroxene at shallow upper mantle or lower crustal conditions. 

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5. Sublithospheric Diamonds; Plate Tectonics from Earth's Deepest Mantle Samples

   https://doi.org/10.1146/annurev-earth-032320-105438  

   Shirey, Steven B; Pearson, D Graham; Stachel, Thomas; Walter, Michael J (July 2024, Annual Review of Earth and Planetary Sciences)

   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. 

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