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Crustal growth and mantle differentiation through Earth's history are often traced using two radiogenic isotope systems - 176Lu-176Hf and 147Sm-143Nd. Unlike most post-Archean igneous rocks that show correlated initial Hf and Nd isotopic compositions, many ancient rocks have broadly chondritic zircon initial εHf values but highly variable whole-rock initial εNd values. These features have classically been interpreted as differences in the behavior of the Lu-Hf and Sm-Nd isotope systems during either deep magma ocean crystallization, subduction zone processes, or post-crystallization metamorphism. To clarify the cause of early Archean Hf-Nd isotope relationships, which are essential for understanding early Earth's evolution, we investigated the in situ U-Th-Pb and Sm-Nd isotope systematics of co-existing titanite, apatite, and allanite—the major Sm-Nd carriers in early Archean felsic rocks—in a representative early Archean (3.5–3.4 Ga) tonalite-trondhjemite-granodiorite (TTG) suite from the Minnesota River Valley (MRV) terrane, northern USA. These rocks exhibit multiple generations of closed-system zircon growth with chondritic initial zircon Hf isotope signatures, and apparent decoupled zircon initial Hf and whole-rock Nd isotopic compositions, and thus serve as an useful test of the role of accessory minerals in controlling the whole rock isotopic signatures. Our new U-Pb results show that the REE-rich accessory phases were reset during the 2.60–2.58 Ga Sacred Heart Orogeny, representing the accretion of the MRV terrane to the southern margin of the Superior Craton. Additionally, two samples contain apatite that yields significantly younger U-Pb dates of 1.9–1.8 Ga, suggesting that a portion of the MRV basement was thermally overprinted by the ∼1.85 Ga Penokean Orogeny. The ∼2.6 Ga Sm-Nd array recorded by apatite and titanite in these rocks suggest terrane-wide Sm-Nd isotopic re-equilibration in the MRV during the Sacred Heart Orogeny, with no Penokean disturbance of the Sm-Nd isotope systematics. Nevertheless, allanite in one sample has survived the post-crystallization 147Sm-143Nd resetting event, and yielded a Sm-Nd isochron date broadly consistent with the crystallization age of the host rock (∼3.4 Ga), with chondritic initial εNd. Therefore, allanite appears to be an important target in order to obtain the primary Nd isotopic signature of early Archean rocks in ancient terranes. In contrast to the ambiguous whole-rock data, the Nd isotopic compositions of the coexisting apatite, titanite and allanite collectively reconstruct a clear crustal evolution history in which the crust was repeatedly re-melted in a closed-system since formation at 3.5 Ga, initially from a source evolving with broadly chondritic Lu-Hf and Sm-Nd systematics. This suggests a minimally differentiated, or already rehomogenized, mantle at 3.5 Ga. Our study highlights the importance of using REE-rich accessory phases to obtain a clear Nd isotopic record to constrain the history of continent formation on the early Earth. [ABSTRACT FROM AUTHOR]

The emplacement of the 2.05-billion-year-old Bushveld complex, the world’s largest layered intrusion and platinum-group element (PGE) repository, is a singular event in the history of the Kaapvaal craton of southern Africa, one of Earth’s earliest surviving continental nuclei. In the prevailing model for the complex’s mineralization, the radiogenic strontium and osmium isotope signatures of Bushveld PGE ores are attributed to continental crustal contamination of the host magmas. The scale of the intrusion and lateral homogeneity of the PGE-enriched layers, however, have long been problematical for the crustal contamination model, given the typically heterogeneous nature of continental crust. Furthermore, the distribution of Bushveld magmatism matches that of seismically anomalous underlying mantle, implying significant interaction before emplacement in the crust. Mineral samples of the ancient 200-km-deep craton keel, encapsulated in macrodiamonds and entrained by proximal kimberlites, reveal the nature of continental mantle potentially incorporated by Bushveld magmas. Here we show that sulphide inclusions in ∼2-billion-year-old diamonds from the 0.5-billion-year-old Venetia and 1.2-billion-year-old Premier kimberlites (on opposite sides of the complex) have initial osmium isotope ratios even more radiogenic than those of Bushveld sulphide ore minerals. Sulphide Re–Os and silicate Sm–Nd and Rb–Sr isotope compositions indicate that continental mantle harzburgite and eclogite components, in addition to the original convecting mantle magma, most probably contributed to the genesis of both the diamonds and the Bushveld complex. Coeval diamonds provide key evidence that the main source of Bushveld PGEs is the mantle rather than the crust. [ABSTRACT FROM AUTHOR]

Abstract: Os–Hf–Sr–Nd isotopes and PGE were determined in peridotite xenoliths carried to the surface by Quaternary alkali basaltic magmas in the Tokinsky Stanovik Range on the Aldan shield. These data constrain the timing and nature of partial melting and metasomatism in the lithospheric mantle beneath SE Siberian craton. The xenoliths range from the rare fertile spinel lherzolites to the more abundant, strongly metasomatised olivine-rich (70–84%) rocks. Hf–Sr–Nd isotope compositions of the xenoliths are mainly within the fields of oceanic basalts. Most metasomatised xenoliths have lower 143Nd/ 144Nd and 176Hf/ 177Hf and higher 87Sr/ 86Sr than the host basalts indicating that the metasomatism is older and has distinct sources. A few xenoliths have elevated 176Hf/ 177Hf (up to 0.2838) and plot above the Hf–Nd mantle array defined by oceanic basalts. 187Os/ 188Os in the poorly metasomatised, fertile to moderately refractory (Al2O3 ≥1.6%) Tok peridotites range from 0.1156 to 0.1282, with oldest rhenium depletion ages being about 2 Ga. The 187Os/ 188Os in these rocks show good correlations with partial melting indices (e.g. Al2O3, modal cpx); the intercept of the Al–187Os/ 188Os correlation with lowest Al2O3 estimates for melting residues (∼0.3–0.5%) has a 187Os/ 188Os of ∼0.109 suggesting that these peridotites may have experienced melt extraction as early as 2.8 Gy ago. 187Os/ 188Os in the strongly metasomatised, olivine-rich xenoliths (0.6–1.3% Al2O3) ranges from 0.1164 to 0.1275 and shows no apparent links to modal or chemical compositions. Convex-upward REE patterns and high abundances of heavy to middle REE in these refractory rocks indicate equilibration with evolved silicate melts at high melt/rock ratios, which may have also variably elevated their 187Os/ 188Os. This inference is supported by enrichments in Pd and Pt on chondrite-normalised PGE abundance patterns in some of the rocks. The melt extraction ages for the Tok suite of 2.0 to 2.8 Ga are younger than oldest Os ages reported for central Siberian craton, but they must be considered minimum estimates because of the extensive metasomatism of the most refractory Tok peridotites. This metasomatism could have occurred in the late Mesozoic to early Cenozoic when the Tok region was close to the subduction-related Pacific margin of Siberia and experienced large-scale tectonic and magmatic activity. This study indicates that metasomatic effects on the Re–Os system in the shallow lithospheric mantle can be dramatic. [Copyright &y& Elsevier]

Two decades of diamond research in southern Africa allow the age and average composition (C isotope and N abundance) of diamonds and the dominant paragenesis (peridotitic versus eclogitic) of their syngenetic silicate and sulfide inclusions to be reviewed on a cratonwide scale. Individual eclogitic sulfide inclusions in diamonds from the Kimberley area kimberlites. Koffiefontein, Orapa and Jwaneng have Re-Os isotopic ages that ranges from ∼ 2.9 Ga to Proterozoic and display little correspondence with the prominent variations in the P-wave velocity (± 1%) that the mantle lithosphere shows at depths within the diamond stability field (150 to 225km). Silicate inclusions in diamonds and their host diamond compositions for the above kimberlites, Finsch, Jagersfontein, Roberts Victor, Premier, Venetia and Letlhakane show a regional relationship to the seismic velocity of the lithosphere. Mantle lithosphere with slower P-wave velocity relative to the craton average correlates with a greater proportion of eclogitic versus peridotitic silicate inclusions in diamond, a greater incidence of younger Sm-Nd ages of silicate inclusions, a greater proportion of diamonds with lighter C isotopic composition, and a lower percentage of low-N diamonds. The converse is true for diamonds from higher velocity mantle. The oldest formation ages of diamonds support a model whereby mantle that became part of the keel of the oldest continental nuclei was created by middle Archean (∼3.3 to ∼3.2 Ga or older) mantle depletion events with high degrees of melting and early harzburgite formulation. The predominance of eclogitic sulfide inclusions in the ∼2.9 Ga age population links late Archean subduction-accretion events involving an oceanic lithosphere component to craton stabilization. These events resulted in a widely-distributed, late Archean generation of eclogitic diamonds in an amalgamated craton. Subsequent Proterozoic tectonic and magmatic events altered the... [ABSTRACT FROM AUTHOR]

The article explores the interpretation and the process in obtaining the ages of diamonds. It states that the diamond ages can be secured through the radiogenic isotopic analysis of the mineral inclusions like pyroxene, garnet and sulfide. Also emphasized are the significance of the mineral inclusions in ensuring the accurate diamond age and the influence of any protogenetic mineral inclusion processes on mineralogical and geological uncertainties.