Strontium, Nd, Pb, Hf, Os, and O isotope compositions for 30 Quaternary lava flows from the Mount Adams stratovolcano and its basaltic periphery in the Cascade arc, southern Washington, USA indicate a major component from intraplate mantle sources, a relatively small subduction component, and interaction with young mafic crust at depth. Major- and trace-element patterns for Mount Adams lavas are distinct from the rear-arc Simcoe volcanic field and other nearby volcanic centers in the Cascade arc such as Mount St. Helens. Radiogenic isotope (Sr, Nd, Pb, and Hf) compositions do not correlate with geochemical indicators of slab-fluids such as (Sr/P) (n) and Ba/Nb. Mass-balance modeling calculations, coupled with trace-element and isotopic data, indicate that although the mantle source for the calc-alkaline Adams basalts has been modified with a fluid derived from subducted sediment, the extent of modification is significantly less than what is documented in the southern Cascades. The isotopic and trace-element compositions of most Mount Adams lavas require the presence of enriched and depleted mantle sources, and based on volume-weighted chemical and isotopic compositions for Mount Adams lavas through time, an intraplate mantle source contributed the major magmatic mass of the system. Generation of basaltic andesites to dacites at Mount Adams occurred by assimilation and fractional crystallization in the lower crust, but wholesale crustal melting did not occur. Most lavas have Tb/Yb ratios that are significantly higher than those of MORB, which is consistent with partial melting of the mantle in the presence of residual garnet. delta (18)O values for olivine phenocrysts in Mount Adams lavas are within the range of typical upper mantle peridotites, precluding involvement of upper crustal sedimentary material or accreted terrane during magma ascent. The restricted Nd and Hf isotope compositions of Mount Adams lavas indicate that these isotope systems are insensitive to crustal interaction in this juvenile arc, in stark contrast to Os isotopes, which are highly sensitive to interaction with young, mafic material in the lower crust.

Sulfide inclusions in diamonds from the 90-Ma Jagersfontein kimberlite, intruded into the southern margin of the Kaapvaal Craton, were analyzed for their Re-Os isotope systematics to constrain the ages and petrogenesis of their host diamonds. The latter have delta C-13 ranging between -3.5 and -9.8aEuro degrees and nitrogen aggregation states (from pure Type IaA up to 51% total N as B centers) corresponding to time/temperature history deep within the subcontinental lithospheric mantle. Most sulfides are Ni-poor ([Ni + Co]/Fe = 0.05-0.25 for 15 of 17 inclusions), have elevated Cu/[Fe + Ni + Co] ratios (0.02-0.36) and elemental Re-Os ratios between 0.5 and 46 (12 of 14 inclusions) typical of eclogitic to more pyroxenitic mantle sources. Re-Os isotope systematics indicate two generations of diamonds: (1) those on a 1.7 Ga age array with initial Os-187/Os-188 (Os-187/Os-188(i)) of 0.46 +/- A 0.07 and (2) those on a 1.1 Ga array with Os-187/Os-188(i) of 0.30 +/- A 0.11. The radiogenic initial Os isotopic composition for both generations of diamond suggests that components with high time-integrated Re-Os are involved, potentially by remobilization of ancient subducted oceanic crust and hybridization of peridotite. A single sulfide with higher Os and Ni content but significantly lower Os-187/Os-188 hosted in a diamond with less aggregated N may represent part of a late generation of peridotitic diamonds. The paucity of peridotitic sulfide inclusions in diamonds from Jagersfontein and other kimberlites from the Kaapvaal craton contrasts with an overall high relative abundance of diamonds with peridotitic silicate inclusions. This may relate to extreme depletion and sulfur exhaustion during formation of the Kaapvaal cratonic root, with the consequence that in peridotites, sulfide-included diamonds could only form during later re-introduction of sulfur.

Osmium isotopes for volcanic rocks from Grande Comore, the youngest island of the Comores Archipelago, located between East Africa and Madagascar, show systematic differences between the two volcanoes of the island. Os-187/Os-188 ratios (0.15-0.16) in shield-stage alkali basalts from Karthala are among the highest yet measured in ocean island basalts with >40 ppt Os. They are uniform over the whole range of Os abundances of 20-100 ppt and form nearly linear correlations with Sr-87/Sr-86, Nd-143/Nd-144 and He-3/He-4 ratios. In contrast. (OS)-O-187/Os-188 ratios in alkaline lavas from La Grille volcano are more variable (0.129-0.159) and negatively correlate with Os abundances. but not with Sr-Nd-He isotope ratios.

The 520 Ma old Venetia kimberlite cluster is located in the Central Zone of the Limpopo Belt where Archean crust has experienced at least two major tectonothermal events at ca. 2.6 and 2.0 Ga, the second of which closely follows the 2.054 Ga emplacement of the Bushveld Complex. Peridotitic garnet inclusions in Venetia diamonds are harzburgitic to lherzolitic in composition with low Ca and high Cr contents spanning the entire G10 garnet field. The related garnet macrocrysts generally have less extreme Ca and Cr contents and represent the re-equilibrated mantle host rocks of at least some of the diamonds. The garnets encapsulated in diamonds have low Sm/Nd and Nd-143/Nd-144 directly correlated with Ca and moderate Sr-87/Sr-86 (0.704-0.706) inversely correlated with Ca. The garnet macrocrysts also show low though more scattered Sm/Nd and Nd-143/Nd-144 but much higher (SrSr)-Sr-87-Sr-/86 (up to 0.720). Three of four inclusion groups give a nominal Sm-Nd isochron age of 2.30 +/- 0.04 Ga with an unradiogenic initial (epsilon Nd = -8). However, Nd-143/Nd-144 and Sr-87/Sr-86 are also correlated with reciprocal Nd and Sr concentration, consistent with mixing between a low Ca, low Sm/Nd, harzburgitic end member with radiogenic Sr (<= 0.707) and a higher Ca, higher Sm/Nd, 'basaltic' end member with unradiogenic Sr (<= 0.702), which raised the initial Nd isotope ratios of the inclusions in proportion to the degree of mixing. Therefore, 2.3 Ga is a maximum age for the diamonds. The initial Nd composition and characteristics of the mixing array indicate a >3 Ga continental mantle harzburgite precursor to which a basaltic component was added at ca. 2 Ga, as suggested by the Re-Os isotope systematics of single sulfide inclusions in Venetia diamonds. In particular, four Venetia eclogitic sulfide inclusions describe a ca. 2.05 Ca Re-Cs array with elevated initial Os-187/Os-188 ratio even more radiogenic than that of Bushveld PGE mineralization. Combined silicate Sm-Nd and Rb-Sr and sulfide Re-Os isotope compositions indicate variable interaction of original convecting mantle magmas with harzburgitic and eclogitic SCLM components during genesis of both the diamonds and the Bushveld Complex. In this model, the Venetia peridotitic diamonds crystallized (or recrystallized) at ca. 2 Ga following modification of Archean harzburgitic SCLM by Bushveld type magmas beneath the Limpopo Belt. (C) 2009 Elsevier B.V. All rights reserved.

Sulphide inclusions from 35 eclogitic and 7 peridotitic diamonds from the Diavik kimberlites in the central Slave craton have been characterized to address questions of diamond age and craton formation. Eclogitic sulphide inclusions occur in diamonds with mantle-like delta C-13 (-4.94 +/- 0.72 1 sigma) and low N aggregation states (%N as B=8.2 +/- 10.0, average N contents of 720 ppm) indicative of relatively low mantle residence temperatures. A 1.86 +/- 0.19 Ga Re-Os age array for eclogitic sulphides with suprachondritic initial Os-187/Os-188 of 0.13 (+/- 0.10) indicates a close temporal link between eclogitic diamond formation, eclogite emplacement and collisional events affecting the Slave craton. Sulphides in peridotitic diamonds plot on older. previously established 3.3 and 3.5 Ga isochrons, consistent with higher average N aggregation states (similar to 20%) despite lower N contents (similar to 230 ppm) for their host diamonds compared to eclogitic diamonds.

The geochemistry of NW USA rhyolites correlates strongly with geography and the nature of the underlying basement terranes. Rhyolites from the Snake River Plain-Yellowstone (SRPY) province have higher Sr-87/Sr-86, Pb-207/Pb-206, and lower Nd-143/Nd-144 than those from the Oregon High Lava Plains (HLP) province, reflecting a dominant influence of Precambrian cratonic crust east of the western Idaho suture zone versus accreted oceanic terranes of Phanerozoic age to the west. Rhyolites from the cratonic domain show significant enrichments of Th, U, and LREE/HREE, whereas B concentration and especially B/Nb and B/Rb are systematically higher west of the tectonic boundary. This decoupling of B from the other incompatible elements is best explained in terms of distinctive magmatic sources east and west of the suture zone.

Diamonds form in the upper mantle during episodic events and have been transported to the Earth's surface from at least the Archean to the Phanerozoic. Small diamonds occur as inclusions in robust minerals in tectonically activated, ultrahigh-pressure metamorphosed crustal rock, establishing an association with subduction processes and recycled carbon but providing no economic deposits. Diamonds in economic deposits are estimated to be mainly (99%) derived from subcontinental lithospheric mantle and rarely (approx. 1%) from the asthenosphere. Harzburgite and eclogite are of roughly equal importance as source rocks, followed by lherzolite and websterite. Diamonds which provide evidence of extensive residence time in the mantle are, with minimal exceptions, smooth-surfaced crystalline diamonds (SCD) with potential commercial value. The oldest prolific SCD formation event documented on the world's major diamond producing cratons occurs in Archean lithospheric mantle harzburgite, metasomatized by likely subduction-related potassic carbonatitic fluids.

Sulphide-bearing diamonds recovered from the similar to 20 Ma Ellendale 4 and 9 lamproite pipes in north-western Australia were investigated to determine the nitrogen aggregation state of the diamonds and Re Os isotope geochemistry of the sulphide inclusions. The majority of diamond studies have been based on diamonds formed in the sub-continental lithospheric mantle (SCLM) below stable cratons, whereas the Ellendale lamproites intrude the King Leopold Orogen, south of the Kimberley craton. The sulphide inclusions consist of pyrrhotite pentlandite chalcopyrite assemblages, and can be divided into peridotitic and eclogitic parageneses on the basis of their Ni and Os contents. A lherzolitic paragenesis for the high-Ni sulphide inclusions is suggested from their Re and Os concentrations. Regression analysis of the Re Os isotope data for the lherzolitic sulphides yields an age of 1426 130 Ma, with an initial Os-187/Os-188 ratio of 0.1042 +/- 0.0034. The upper limit of the uncertainty on the Os-187/Os-188 initial ratio gives a Re depletion age of 2.96 Ga, indicating the presence of SCLM beneath Ellendale since at least the Mesoarchaean, with the lherzolitic diamond-forming event much younger and unrelated to the craton keel stabilisation. The nitrogen aggregation state of the diamonds and calculated mantle residence temperatures suggest an origin and storage of the Ellendale diamonds in a stable cratonic SCLM, consistent with the King Leopold Orogen being cratonised by about 1.8 Ga. The diamonds do not show evidence for pervasive deformation or platelet degradation, which suggests that the diamonds had a relatively undisturbed 1.4 billion year mantle storage history. (C) 2010 Elsevier Ltd. All rights reserved.