The platinum-group elements contain three radioisotope systems that have been used in many and varied ways in geo- and cosmochemistry. Unique chronological applications include dating the formation of such diverse materials as sulfides, gold, organic-rich sediments, iron meteorites, and sulfide inclusions in diamonds. These systems also serve as isotope tracers for processes such as continental erosion, the deposition of extraterrestrial materials on Earth's surface, crust-mantle differentiation, recycling of subducted crust into the mantle, core-mantle exchange, and volatile-element depletion of planets and planetesimals. Although these systems have been in use for only a short time, the discoveries they have provided bode well for their incorporation as staples in the geochemical toolbox.

A suite of 23 basaltic to dacitic lavas erupted over the last 350 kyr from the Mount Adams volcanic field has been analyzed for U-Th isotope compositions to evaluate the roles of mantle versus crustal components during magma genesis. All of the lavas have (Th-230/U-238) > 1 and span a large range in (Th-230/Th-232) ratios, and most basalts have higher (Th-230/Th-232) ratios than andesites and dacites. Several of the lavas contain antecrysts (crystals of pre-existing material), yet internal U-Th mineral isochrons from six of seven lavas are indistinguishable from their eruption ages. This indicates a relatively brief period of time between crystal growth and eruption for most of the phenocrysts (olivine, clinopyroxene, plagioclase, magnetite) prior to eruption. One isochron gave a crystallization age that is similar to 20-25 ka older than its corresponding eruptive age, and is interpreted to reflect mixing of older and juvenile crystals or a protracted period of magma storage in the crust. Much of the eruptive volume since 350 ka consists of lavas that have small to moderate Th-230 excesses (2-16%). which are likely inherited from melting of a garnet-bearing intraplate ("OIB-like") mantle source. Following melt generation and subsequent migration through the upper mantle, most Mt. Adams magmas interacted with young, mafic lower crust, as indicated by Os-187/Os-188 ratios that are substantially more radiogenic than the mantle or those expected via mixing of subducted material and the mantle wedge. Moreover, Os-Th isotope variations suggest that unusually large Th-230 excesses (25-48%) and high Os-187/(OS)-O-188 ratios in some peripheral lavas reflect assimilation of small degree partial melts of pre-Quaternary basement that had residual garnet or Al-rich clinopyroxene. Despite the isotopic evidence for lower crustal assimilation, these processes are not generally recorded in the erupted phenocrysts, indicating that the crystal record of the deep-level 'cryptic' processes has been decoupled from shallow-level crystallization. (c) 2008 Elsevier B.V. All rights reserved.

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.