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Zealandia is a largely submerged, continental fragment in the southwest Pacific, generally considered to be derived from East Gondwana, but whose origins, age, structure, and relationships with other continental masses are poorly known. To explore the development of this microcontinent, a suite of mantle xenoliths was assembled from 12 localities throughout New Zealand, an emergent part of Zealandia. The 187 Re- 188 Os isotopic systematics of the xenoliths yield model ages (T RD2 ) between 0 and 2.3 Ga. Six samples from the newly defined Waitaha domain, South Island, have a narrow range of T RD2 ages from 1.6 to 1.9 Ga, in agreement with an aluminochron model age for this mantle domain of ca. 1.95 Ga, and with a three-point Re-Os isochron age of 2.26 ± 0.10 Ga. These ages are >500 m.y. older than T RD2 ages preserved in other regions of mantle lithosphere from the eastern margin of Gondwana (e.g., southeastern Australia and Marie Byrd Land, Antarctica) and >1 b.y. older than the oldest crustal rocks exposed in New Zealand. Thus, the lithospheric mantle of Zealandia has a complex age structure, including a region of Paleoproterozoic cratonic mantle with a minimum extent of ∼45,000 km 2 . This ancient mantle resided at the margins of several supercontinents during the past ∼2 b.y., attesting to the durability of subcontinental lithospheric mantle domains, even when decoupled from overlying contemporaneous crust and in an oceanic setting distanced from stable cratonic nuclei.

Northeastern Queensland, a part of the Phanerozoic composite Tasman Fold Belt of eastern Australia, has a Paleozoic to Mesozoic history dominated by subduction zone processes. A suite of 13 peridotite xenoliths from the

Initial 187Os/188Os isotopic compositions for geochronologically and geologically well -constrained 3.8-Ga spinel peridotites from the Itsaq Gneiss Complex of southern West Greenland and chromite separates from 3.46-Ga komatiites from the Pilbara region of Western Australia have been determined to investigate the osmium isotopic evolution of the early terrestrial mantle. The measured compositions of 187Os/188Os(0) = 0.10262 ± 2, from an olivine separate, and 0.10329 ± 3, for a spinel separate from ∼3.8-Ga peridotite G93/42, are the lowest yet reported from any terrestrial sample. The corrections for in situ decay over 3.8 Ga for these low Re/Os phases are minimal and change the isotopic compositions by only 0.5 and 2.2% for the spinel and the olivine, respectively, resulting in 187Os/188Os(3.8 Ga) = 0.1021 ± 0.0002 and 0.1009 ± 0.0002, respectively. These data extend direct measurement of Os isotopic compositions to much earlier periods of Earth history than previously documented and provide the best constraints on the Os isotopic composition of the early Archean terrestrial mantle. Analyses of Pilbara chromites yield 3.46-Ga mantle compositions of 0.1042 ± 0.0002 and 0.1051 ± 0.0002. These new data, combined with published initial Os isotopic compositions from late Archean and early Proterozoic samples, are compatible with the mantle, or at least portions of it, evolving from a solar system initially defined by meteorites to a modern composition of 187Os/188Os(0) = 0.1296 ± 0.0008 as previously suggested from peridotite xenolith data ( Meisel et al., 2001); the associated 187Re/188Os(0) = 0.435 ± 0.005. Thus, chondritic 187Os/188Os compositions were a feature of the upper mantle for at least 3.8 billion years, requiring chondritic Re/Os ratios to have been a characteristic of the very early terrestrial mantle. In contrast, nonchondritic initial compositions of some Archean komatiites demonstrate that Os isotopic heterogeneity is an ancient feature of plume materials, reflecting the development of variable Re/Os mantle sources early in Earth history. The lower average 187Os/188Os = 0.1247 for abyssal peridotites (Snow and Reisberg, 1995) indicate that not all regions of the modern mantle have evolved with the same Re/Os ratio. The relative sizes of the various reservoirs are unknown, although mass balance considerations can provide some general constraints. For example, if the unradiogenic 187Os/188Os modern abyssal peridotite compositions reflect the prevalent upper mantle composition, then the complementary high Re/Os basaltic reservoir must represent 20 to 40% by mass of the upper mantle (taken here as 50% of the entire mantle), depending on the mean storage age. The difficulties associated with efficient long-term storage of such large volumes of subducted basalt suggest that the majority of the upper mantle is not significantly Re-depleted. Rather, abyssal peridotites sample anomalous mantle regions. The existence of 3.8-Ga mantle peridotites with chondritic 187Os/188Os compositions and with Os concentrations similar to the mean abundances measured in modern peridotites places an upper limit on the timing of a late accretionary veneer. These observations require that any highly siderophile element -rich component must have been added to the Earth and transported into and grossly homogenized within the mantle by 3.8 Ga. Either large-scale mixing of impact materials occurred on very short (0–100 myr) timescales or (the interpretation preferred here) the late veneer of highly siderophile elements is unrelated to the lunar terminal cataclysm estimated to have occurred at ∼3.8 to 3.9 Ga.

Rocks exposed at the Earth's surface provide an incomplete record of the varied and complex processes that have acted to form and modify the continents. The continental lithospheric mantle (CLM), sampled as xenoliths carried in explosive volcanics, is an alternative reservoir of age information that has only recently begun to be tapped. Data from southeastern Australia, a technically complex region, illustrate that the CLM can record important aspects of continental evolution that are not indicated by the exposed crustal rocks. Re-Os isotopic data and the resultant model ages for 15 spinel peridotite xenoliths from five localities in southeastern Australia show that parts of the CLM are ∼ 1960 Myr old, some 1400 Myrs older than the oldest crust exposed immediately in the sampled region. As the five sampled localities are east of the presumed eastern edge of the Precambrian interior of Australia, the preservation of Proterozoic ages in the lithosphere indicates that the supposed Precambrian edge of the Australian continent (the Tasman Line) is a surficial feature rather than a lithospheric scale boundary. This conclusion is consistent with recent Skippy seismic tomography models for Australia. The mantle xenolith age distribution also suggests a significant age change from Proterozoic to dominantly Phanerozoic, which appears coincident with the surface expression of the boundary between two major fold belts (Lachlan and Delamerian) and with a deep lithospheric boundary previously proposed on the basis of differences in Sr and Pb isotopic systematics of southeast Australian basalts.

Increasing recognition of complexities in the Platinum-group element (PGE) and Re concentration patterns in mantle samples are challenging the view of chondritic relative abundances in the upper mantle. To investigate the possible causes of PGE abundance variations, a suite of east Australian, mantle-derived, spinel peridotite xenoliths, ranging from fertile lherzolites to depleted harzburgites, and including apatite ± phlogopite ± amphibole bearing samples, have been analysed for their whole rock PGE and Re abundances. Whole rock abundances for 21 samples, combined with mineral separate analyses of 2 xenoliths, are presented to constrain the distribution of the PGEs and Re, their inherent heterogeneity at difference scales, and their behaviour during both melt extraction and metasomatism. Fertile (>2.9 wt% Al2O3) xenoliths have broadly chondritic relative PGE abundances, with the significant exception of positive Rh anomalies and variable negative Os anomalies. The high Rh abundances cannot be attributed to melt extraction or metasomatism. Bulk mineral separate PGE-Re analyses of 2 fertile xenoliths indicate less than 6% of the whole rock PGE budget resides in either silicate or oxide (spinel) phases. The remainder of the PGEs, and at least 80% of the whole rock Re budget, are sited in acid-leachable sulfides and less soluble trace phases such as PGE-sulfides or alloys. Individual PGEs partition into different trace phases resulting in small scale heterogeneity of both PGE ratios and concentrations on the order of 8%–20%. Although these trace phases may be present within the mantle, it is more likely at least some exsolved from monosulfide solid solutions at low temperatures. Ir and Rh abundances are consistent with compatible behaviour during melt extraction, whereas Ru, Pt and Pd abundances are consistent with slightly incompatible behaviour and can be modeled by assuming all reside in sulfides within the mantle, with DsulfRu ∼ DsulfPt > DsulfPd. Comparison of PGE abundances between ‘dry’ xenoliths and modally metasomatised xenoliths, suggests the PGEs are not significantly mobilised during interaction with carbonate melts or during metasomatism leading to hydrous mineral growth. Given the problems of various types of secondary alteration processes, including melt extraction and surficial alteration that commonly affect xenoliths, and as within-locality heterogeneity is on a comparable order to any proposed regional heterogeneity, it may be premature to define significant regional differences, or ‘primary’ non-chondritic PGE patterns in lithospheric peridotites.

The ReOs systematics of a suite of spinel peridotite xenoliths from southeast Australia provide insight into the effects of melt extraction and metasomatism on Re and Os and place strong constraints on the evolution and long-term stability of post-Archean lithospheric mantle in a tectonically complex region. Data from variably melt-depleted and non-modally metasomatised xenoliths demonstrate that Re abundances are largely controlled by melt extraction, with Re similarly distributed to Os. Ratios of ReOs correlate strongly with indices of melt extraction (e.g. Al2O3, Ni and Yb), and with the calculated bulk partition coefficient of Re, comparable to that of Yb over a large range of melt extraction (∼ 4–20%). Hence, if Re is controlled by sulfide phases, sulfur:clinopyroxene ratios should remain essentially constant over large degrees of melt extraction. Eight of the 24 samples analysed were wehrlites or apatite-bearing peridoties, representing residual peridotite which has interacted with a carbonatitic melt. In comparison with the non-modally metasomatised xenoliths, these samples show no evidence for disturbance of Os isotopic composition, or addition of Re or Os during metasomatism. The entire suite provides a 220 km long, WNW-ESE lithospheric mantle transect, east of, and perpendicular to, the presumed Australian Precambrian shield margin. The Os model ages indicate at least three episodes of mantle depletion: ca. 1960 Ma, 800–1000 Ma and < 500 Ma. The older age is found only in the two westernmost localities where a subset of four samples define a ReOs age of 1959 ± 100 Ma, with an initial γOs = +0.2. Although the oldest exposed rocks in the region are of Cambrian age, and the presence of early Proterozoic basement is highly contentious, the Os isotopic data require that early Proterozoic basement extends some 400 km further east than the easternmost exposed early Proterozoic crust. Model ages of 800–1000 Ma are common to all but one locality, indicating at least two melt extraction events in the western localities. Paleozoic ages are only identified in the eastern localities, suggesting the lithospheric mantle becomes younger to the east. Importantly, this and other ReOs isotopic studies provide increasing evidence for the long-term stability and persistence of lithospheric mantle of Proterozoic as well as of Archean age.

Like many granites, the Late Cretaceous intrusives of the eastern Mojave Desert, California, have heretofore provided useful but poorly focused images of their source regions. New studies of lower crustal xenoliths and inherited accessory minerals are sharpening these images. Xenoliths in Tertiary dykes in this region are the residues of an extensive partial melting event. Great diversity in their composition reflects initial heterogeneity (both igneous and sedimentary protoliths) and varying amounts of melt extraction (from 70%)