http://www.researchonline.mq.edu.au/vital/access/services/Feed ${session.getAttribute("locale")} 5 http://www.researchonline.mq.edu.au/vital/access/manager/Repository/mq:5501 Mantle xenoliths and xenocrysts were retrieved from three of the 88–86 Ma Buffalo Hills kimberlites (K6, K11, K14) for a reconnaissance study of the subcontinental lithospheric mantle (SCLM) beneath the Buffalo Head Terrane (Alberta, Canada). The xenoliths include spinel lherzolites, one garnet spinel lherzolite, garnet harzburgites, one sheared garnet lherzolite and pyroxenites. Pyroxenitic and wehrlitic garnet xenocrysts are derived primarily from the shallow mantle and lherzolitic garnet xenocrysts from the deep mantle. Harzburgite with Ca-saturated garnets is concentrated in a layer between ~135–165 km depth. Garnet xenocrysts define a model conductive paleogeotherm corresponding to a heat flow of 38–39 mW/m². The sheared garnet lherzolite lies on an inflection of this geotherm and may constrain the depth of the lithosphere–asthenosphere boundary (LAB) beneath this region to ca 180 km depth. A loss of >20% partial melt is recorded by spinel lherzolites and up to 60% by the garnet harzburgites, which may be related to lithosphere formation. The mantle was subsequently modified during at least two metasomatic events. An older metasomatic event is evident in incompatible-element enrichments in homogeneous equilibrated garnet and clinopyroxene. Silicate melt metasomatism predominated in the deep lithosphere and led to enrichments in the HFSE with minor enrichments in LREE. Metasomatism by small-volume volatile-rich melts, such as carbonatite, appears to have been more important in the shallow lithosphere and led to enrichments in LREE with minor enrichments in HFSE. An intermediate metasomatic style, possibly a signature of volatile-rich silicate melts, is also recognised. These metasomatic styles may be related through modification of a single melt during progressive interaction with the mantle. This metasomatism is suggested to have occurred during Paleoproterozoic rifting of the Buffalo Head Terrane from the neighbouring Rae Province and may be responsible for the evolution of some samples toward unradiogenic Nd and Hf isotopic compositions. Disturbed Re–Os isotope systematics, evident in implausible model ages, were obtained in situ for sulfides in several spinel lherzolites and suggest that many sulfides are secondary (metasomatic) or mixtures of primary and secondary sulfides. Sulfide in one peridotite has unradiogenic ¹⁸⁷Os/¹⁸⁸Os and gives a model age of 1.89±0.38 Ga. This age coincides with the inferred emplacement of mafic sheets in the crust and suggests that the melts parental to the intrusions interacted with the lithospheric mantle. A younger metasomatic event is indicated by the occurrence of sulfide-rich melt patches, unequilibrated mineral compositions and overgrowths on spinel that are Ti-, Cr- and Fe-rich but Zn-poor. Subsequent cooling is recorded by fine exsolution lamellae in the pyroxenes and by arrested mineral reactions. If the lithosphere beneath the Buffalo Head Terrane was formed in the Archaean, any unambiguous signatures of this ancient origin may have been obliterated during these multiple events. 2010-01-27T22:30:01.808Z ]]> http://www.researchonline.mq.edu.au/vital/access/manager/Repository/mq:5651 Analyses of mineral inclusions, carbon isotopes, nitrogen contents and nitrogen aggregation states in 29 diamonds from two Buffalo Hills kimberlites in northern Alberta, Canada were conducted. From 25 inclusion bearing diamonds, the following paragenetic abundances were found: peridotitic (48%), eclogitic (32%), eclogitic/websteritic (8%), websteritic (4%), ultradeep? (4%) and unknown (4%). Diamonds containing mineral inclusions of ferropericlase, and mixed eclogitic-asthenospheric-websteritic and eclogitic-websteritic mineral associations suggests the possibility of diamond growth over a range of depths and in a variety of mantle environments (lithosphere, asthenosphere and possibly lower mantle). Eclogitic diamonds have a broad range of C-isotopic composition (δ¹³C=−21‰ to −5‰). Peridotitic, websteritic and ultradeep diamonds have typical mantle C-isotope values (δ¹³C=−4.9‰ av.), except for two ¹³C-depleted peridotitic (δ¹³C=−11.8‰, −14.6‰) and one ¹³C-depleted websteritic diamond (δ¹³C=−11.9‰). Infrared spectra from 29 diamonds identified two diamond groups: 75% are nitrogen-free (Type II) or have fully aggregated nitrogen defects (Type IaB) with platelet degradation and low to moderate nitrogen contents (av. 330 ppm-N); 25% have lower nitrogen aggregation states and higher nitrogen contents (~30% IaB; <1600 ppm-N). The combined evidence suggests two generations of diamond growth. Type II and Type IaB diamonds with ultradeep, peridotitic, eclogitic and websteritic inclusions crystallised from eclogitic and peridotitic rocks while moving in a dynamic environment from the asthenosphere and possibly the lower mantle to the base of the lithosphere. Mechanisms for diamond movement through the mantle could be by mantle convection, or an ascending plume. The interaction of partial melts with eclogitic and peridotitic lithologies may have produced the intermediate websteritic inclusion compositions, and can explain diamonds of mixed parageneses, and the overlap in C-isotope values between parageneses. Strong deformation and extremely high nitrogen aggregation states in some diamonds may indicate high mantle storage temperatures and strain in the diamond growth environment. A second diamond group, with Type IaA–IaB nitrogen aggregation and peridotitic inclusions, crystallised at the base of the cratonic lithosphere. All diamonds were subsequently sampled by kimberlites and transported to the Earth's surface. 2010-01-27T22:28:13.631Z ]]> http://www.researchonline.mq.edu.au/vital/access/manager/Repository/mq:5652 Melt inclusions in clinopyroxenes from lherzolitic xenoliths from the deep lithospheric mantle beneath the Slave Craton (Lac de Gras area, Canada) reveal multiple origins for carbonatitic melts. One type of inclusions consists of a series of silicate–carbonate–silicate concentric layers, interpreted to have unmixed under disequilibrium conditions during rapid ascent to the surface. Bulk major- and trace-element compositions are typical of Group 1 kimberlites and quantitative nuclear microprobe imaging of the globules reveals fractionation of related elements (e.g. F–Br, Nb–Ta) between the silicate and carbonate components. The globules probably formed by partial melting of carbonated peridotite, consistent with results of melting experiments and some models for the generation of kimberlite magmas. They provide evidence for a genetic relationship between some carbonate-rich magmas and ultramafic silicate magmas, and for the possibility of unmixing processes of these melts during their evolution. The second inclusion type comprises carbonate-rich globules interpreted as samples of Mg-carbonatite melt that quenched on ascent to the surface. Bulk major- and trace-element compositions indicate that the melts were derived from a carbonate-rich source and oxygen, carbon, and strontium isotope data are consistent with the involvement of recycled crustal material and suggest that some mantle-derived carbonatites are unrelated to kimberlites. 2010-01-27T22:28:11.923Z ]]> http://www.researchonline.mq.edu.au/vital/access/manager/Repository/mq:5653 10 page(s) 2010-01-27T22:28:11.804Z ]]> http://www.researchonline.mq.edu.au/vital/access/manager/Repository/mq:5654 Fe–Ni–Cu sulfide mineralogy has been investigated along with bulk-rock and in-situ PGE analyses by ICP MS and LA-ICP-MS in eight lherzolites from the Internal (IL) and External Liguride (EL) ophiolites (Italy). The two EL lherzolites are fertile (2–4% partial melting) and slightly serpentinized while the six IL cpx-poor lherzolites have experienced 5–10% of partial melting, impregnation by instantaneous melt fractions [Geochim. Cosmochim. Acta 61 (1997) 4557] and have been highly serpentinized. The EL lherzolites show broadly chondritic PGE relative abundances with a slight to pronounced enhancement of the light PGE (Ru, Rh and Pd) relative to the heavy PGE (Os, Ir and Pt) (RuN/IrN=1.13; RhN/IrN=1.08–1.10; PdN/IrN=1.24–1.62; N=CI-chondrite normalized). Their magmatic sulfide modal abundances and S contents, similar to the orogenic peridotites values, are consistent with their very low degree of partial melting. The occurrence of Cu–Rh–Pd-rich pentlandite, however, demonstrates that, even for low degree of partial melting, a Cu–Ni-rich sulfide liquid can segregate, leaving the residual monosulfide solid solution (Mss) (now transformed into Cu-poor pentlandite) depleted in Rh and Pd (RhN/IrN and PdN/IrN<1). The IL cpx-poor lherzolites display a broadly flat PGE patterns from Os to Pt with a slight enhancement of Ru and Rh (RuN/IrN=1.05–1.38; RhN/IrN=1.01–1.31). PdN/IrN ratios range from chondritic to superchondritic (1.02–2.99) and cannot be interpreted in terms of partial melting models. Rh–Pd–Cu–Ni-rich pentlandite grains are associated with large corroded cpx crystals ascribed to exotic melt percolation by Rampone et al. [Geochim. Cosmochim. Acta 61 (1997) 4557]. It is concluded that precipitation of Cu–Ni–Rh–Pd-rich sulfides has significantly enhanced the Pd concentrations as well as the magmatic sulfide modal abundances. Such processes, previously documented in abyssal peridotites from slow-spreading mid-oceanic ridges, characterize residues from low to moderate melting degrees (5–10%) of the oceanic mantle as a whole, either from mature ocean or from short-lived oceanic basins. 2010-01-27T22:28:09.962Z ]]> http://www.researchonline.mq.edu.au/vital/access/manager/Repository/mq:5690 Mantle xenoliths from Tenerife show evidence of metasomatism and recrystallization overprinting the effects of extensive partial melting. The evidence includes: recrystallization of exsolved orthopyroxene porphyroclasts highly depleted in incompatible trace elements into incompatible-trace-element-enriched, poikilitic orthopyroxene with no visible exsolution lamellae; formation of olivine and REE–Cr-rich, strongly Zr–Hf–Ti-depleted clinopyroxene at the expense of orthopyroxene; the presence of phlogopite; whole-rock CaO/Al₂O₃ >> 1 (Ca metasomatism) in recrystallized rocks; and enrichment in incompatible elements in recrystallized rocks, relative to rocks showing little evidence of recrystallization. The ‘higher-than-normal’ degree of partial melting that preceded the metasomatism probably results from plume activity during the opening of the Central Atlantic Ocean. Sr–Nd isotopic compositions are closely similar to those of Tenerife basalts, indicating resetting from the expected original mid-ocean ridge basalt composition by the metasomatizing fluids. Metasomatism was caused by silicic carbonatite melts, and involved open-system processes, such as trapping of elements compatible with newly formed acceptor minerals, leaving residual fluids moving to shallower levels. The compositions of the metasomatizing fluids changed with time, probably as a result of changing compositions of the melts produced in the Canary Islands plume. Spinel dunites and wehrlites represent rocks where all, or most, orthopyroxene has been consumed through the metasomatic reactions. 2010-01-27T22:27:41.795Z ]]> http://www.researchonline.mq.edu.au/vital/access/manager/Repository/mq:5725 Over 700 apatite grains from a range of rock types have been analysed by laser-ablation microprobe ICPMS for 28 trace elements, to investigate the potential usefulness of apatite as an indicator mineral in mineral exploration. Apatites derived from different rock types have distinctive absolute and relative abundances of many trace elements (including rare-earth elements (REE), Sr, Y, Mn, Th), and chondrite-normalised trace-element patterns. The slope of chondrite-normalised REE patterns varies systematically from ultramafic through mafic/intermediate to highly fractionated granitoid rock types. (Ce/Yb)cn is very high in apatites from carbonatites and mantle-derived lherzolites (over 100 and over 200, respectively), while (Ce/Yb)cn values in apatites from granitic pegmatites are generally less than 1, reflecting both HREE enrichment and LREE depletion. Within a large suite of apatites from granitoid rocks, chemical composition is closely related to both the degree of fractionation and the oxidation state of the magma, two important parameters in determining the mineral potential of the magmatic system. Apatite can accept high levels of transition and chalcophile elements and As, making it feasible to recognise apatite associated with specific types of mineralisation. Multivariate statistical analysis has provided a user-friendly scheme to distinguish apatites from different rock types, based on contents of Sr, Y, Mn and total REE, the degree of LREE enrichment and the size of the Eu anomaly. The scheme can be used for the recognition of apatites from specific rock types or styles of mineralisation, so that the provenance of apatite grains in heavy mineral concentrates can be determined and used in geochemical exploration. 2010-01-27T22:27:13.228Z ]]> http://www.researchonline.mq.edu.au/vital/access/manager/Repository/mq:5735 Mineralogical, isotopic, geochemical and geochronological evidence demonstrates that the Friningen body, a garnet peridotite body containing garnet pyroxenite layers in the Seve Nappe Complex (SNC) of Northern Jämtland, Sweden, represents old, certainly Proterozoic and possibly Archean, lithosphere that became incorporated into the Caledonian tectonic edifice during crustal subduction into the mantle at c. 450 Ma. Both garnet peridotite and pyroxenite contain two (M₁ and M₂) generations of garnet-bearing assemblages separated by the formation of two-pyroxene, spinel symplectite around the M₁ garnet and the crystallization of low-Cr spinel1C in the matrix. These textures suggest initial high-pressure (HP) crystallization of garnet peridotite and pyroxenite succeeded by decompression into the spinel stability field, followed by recompression into the garnet peridotite facies. Some pyroxenite layers appear to be characterized solely by M₂ assemblages with stretched garnet as large as several centimeters. Laser ablation microprobe–inductively coupled plasma mass spectrometry Re–Os analyses of single sulfide grains generally define meaningless model ages suggesting more than one episode of Re and/or Os addition and/or loss to the body. Pentlandite grains from a single polished slab of one garnet peridotite, however, define a linear array on an Re–Os isochron diagram that, if interpreted as an errorchron, suggests an Archean melt extraction event that left behind the depleted dunite and harzburgite bodies that characterize the SNC. Refertilization of this mantle by melts associated with the development of the pyroxenite layers is indicated by enriched clinopyroxene Sr–Nd isotope ratios, and by parallel large ion lithophile-enriched trace element patterns in clinopyroxene from pyroxenite and the immediately adjacent peridotite. Clinopyroxene and whole-rock model Sm–Nd ages (TDM = 1·1–2·2 Ga) indicate that fertilization took place in Proterozoic times. Sm–Nd garnet₂–clinopyroxene₂–whole rock ± orthopyroxene₂ mineral isochrons from three pyroxenite layers define overlapping ages of 452·1 ± 7·5 and 448 ± 13 Ma and 451 ± 43 Ma (2σ). All ages are within error of Sm–Nd mineral ages from eclogite in the enclosing host gneiss, demonstrating that, whereas silicate intrusion and fertilization occurred in the mantle during the Proterozoic, the formation of at least the M₂ garnet-bearing assemblages occurred in the crust, as that crust was subducted into the mantle during the Caledonian orogenic cycle. By analogy we infer a similar origin for other mantle-derived lenses in HP nappes of the SNC in Northern Jämtland. 2010-01-27T22:27:07.039Z ]]> http://www.researchonline.mq.edu.au/vital/access/manager/Repository/mq:5741 Pods of granulite facies dioritic gneiss in the Pembroke Valley, Milford Sound, New Zealand, preserve peritectic garnet surrounded by trondhjemitic leucosome and vein networks, that are evidence of high-P partial melting. Garnet-bearing trondhjemitic veins extend into host gabbroic gneiss, where they are spatially linked with the recrystallization of comparatively low-P two-pyroxene-hornblende granulite to fine-grained high-P garnet granulite assemblages in garnet reaction zones. New data acquired using a Laser Ablation Inductively Coupled Plasma Mass Spectrometer (LA-ICPMS) for minerals in various textural settings indicate differences in the partitioning of trace elements in the transition of the two rock types to garnet granulite, mostly due to the presence or absence of clinozoisite. Garnet in the garnet reaction zone (gabbroic gneiss) has a distinct trace element pattern, inherited from reactant gabbroic gneiss hornblende. Peritectic garnet in the dioritic gneiss and garnet in trondhjemitic veins from the Pembroke Granulite have trace element patterns inherited from the melt-producing reaction in the dioritic gneiss. The distinct trace element patterns of garnet link the trondhjemitic veins geochemically to sites of partial melting in the dioritic gneiss. 2010-01-27T22:27:02.871Z ]]> http://www.researchonline.mq.edu.au/vital/access/manager/Repository/mq:5744 Mantle-derived garnet and spinel peridotite xenoliths and xenocrysts from the Daoxian basalts (151–131 Ma) and the Anyuan lamprophyres (~44 Ma) provide insights into the nature and evolution of subcontinental lithospheric mantle beneath the Cathaysia block, SE China. The Daoxian area lies astride the translithospheric Ningyuan– Jianghua fault zone in western Cathaysia, whereas the Anyuan area lies east of the fault zone. The peridotite xenoliths from both localities have experienced melt extraction (generally 3–10% fractional or batch melting) and subsequent silicate melt metasomatism. Depleted coarse-textured spinel peridotites from Anyuan and localities in East Cathaysia have relatively magnesian olivine (Mg#≥91) and may be relics of the Proterozoic lithospheric upper mantle, preserved locally at shallow levels away from the translithospheric fault. At Daoxian coarse-textured xenoliths are absent, and some xenoliths have sheared microstructures. The minerals of the Daoxian xenoliths have lower Cr# and Mg# than those from Auyuan. The Daoxian xenoliths also have higher bulk CaO+Al₂O₃ contents and FeO/MgO, suggesting more pronounced metasomatism. The differences between the Daoxian and Anyuan xenolith suites reflect their spatial relationship to the translithospheric Ningyuan–Jianghua fault zone, which played an important role in the Mesozoic replacement of Proterozoic lithospheric mantle. The lithospheric mantle information from these localities is combined with published data from other localities to provide the first comprehensive survey of the evolution of the lithospheric mantle beneath the Cathaysia block. The xenolith suites of West and East Cathaysia are interpreted as sampling mixtures of older and newly accreted lithospheric mantle that replaced the Proterozoic lithosphere through extension, thermal erosion and melt metasomatism. 2010-01-27T22:27:01.350Z ]]> http://www.researchonline.mq.edu.au/vital/access/manager/Repository/mq:5747 The Storgangen orebody is a concordantly layered, sill-like body of ilmenite-rich norite, intruding anorthosites of the Rogaland Intrusive Complex (RIC), SW Norway. 17 zircon grains were separated from ca. 5 kg of sand-size flotation waste collected from the on-site repository from ilmenite mining. These zircons were analysed for major and trace elements by electron microprobe, and for U–Pb and Lu–Hf isotopes by laser ablation microprobe plasma source mass spectrometry. Eight of the zircons define a well-constrained (MSWD=0.37) concordant population with an age of 949±7 Ma, which is significantly older than the 920–930 Ma ages previously reported for zircon inclusions in orthopyroxene megacrysts from the RIC. The remaining zircons, interpreted as inherited grains, show a range of ²⁰⁷Pb/²⁰⁶Pb ages up to 1407±14 Ma, with an upper intercept age at ca. 1520 Ma. The concordant zircons have similar trace element patterns, and a mean initial Hf isotope composition of ¹⁷⁶Hf/¹⁷⁷Hf₉₄₉ Ma=0.28223±5 (εHf=+2±2). This is similar to the Hf-isotope composition of zircons in a range of post-tectonic Sveconorwegian granites from South Norway, and slightly more radiogenic than expected for mid-Proterozoic juvenile crust. The older, inherited zircons show Lu–Hf crustal residence ages in the range 1.85–2.04 Ga. One (undated) zircon plots well within the field of Hf isotope evolution of Paleoproterozoic rocks of the Baltic Shield. These findings indicate the presence of Paleoproterozoic components in the deep crust of the Rogaland area, but do not demonstrate that such rocks, or a Sveconorwegian mantle-derived component, contributed significantly to the petrogenesis of the RIC. If the parent magma was derived from a homogeneous, lower crustal mafic granulite source, the lower crustal protolith must be at least 1.5 Ga old, and it must have an elevated Rb/Sr ratio. This component would be indistinguishable in Sr, Nd and Hf isotopes from some intermediate mixtures between Sveconorwegian mantle and Paleoprotoerzoic felsic crust, but it cannot account for the initial ¹⁴³Nd/¹⁴⁴Nd of the most primitive, late Sveconorwegian granite in the region, without the addition of mantle-derived material. 2010-01-27T22:26:58.900Z ]]> http://www.researchonline.mq.edu.au/vital/access/manager/Repository/mq:5768 Major- and trace-element analyses of garnets from heavy-mineral concentrates have been used to derive the compositional and thermal structure of the subcontinental lithospheric mantle (SCLM) beneath 16 areas within the core of the ancient Laurentian continent and 11 areas in the craton margin and fringing mobile belts. Results are presented as stratigraphic sections showing variations in the relative proportions of different rock types and metasomatic styles, and the mean Fo content of olivine, with depth. Detailed comparisons with data from mantle xenoliths demonstrate the reliability of the sections. In the Slave Province, the SCLM in most areas shows a two-layer structure with a boundary at 140–160 km depth. The upper layer shows pronounced lateral variations, whereas the lower layer, after accounting for different degrees of melt-related metasomatism, shows marked uniformity. The lower layer is interpreted as a subcreted plume head, added at ca. 3.2 Ga; this boundary between the layers rises to <100 km depth toward the northern and southern edges of the craton. Strongly layered SCLM suggests that plume subcretion may also have played a role in the construction of the lithosphere beneath Michigan and Saskatchewan. Outside the Slave Province, most North American Archon SCLM sections are less depleted than similar sections in southern Africa and Siberia; this may reflect extensive metasomatic modification. In E. Canada, the degree of modification increases toward the craton margin, and the SCLM beneath the Kapuskasing Structural Zone is typical of that beneath Proterozoic to Phanerozoic mobile belts. SCLM sections from several Proterozoic areas around the margin of the Laurentian continental core (W. Greenland, Colorado–Wyoming district, Arkansas) show discontinuities and gaps that are interpreted as the effects of lithosphere stacking during collisional orogeny. Some areas affected by Proterozoic orogenesis (Wyoming Craton, Alberta, W. Greenland) appear to retain buoyant, modified Archean SCLM. Possible juvenile Proterozoic SCLM beneath the Colorado Plateau is significantly less refractory. The SCLM beneath the Kansas kimberlite field is highly melt-metasomatised, reflecting its proximity to the Mid-Continent Rift System. A traverse across the continent shows that the upper part of the cratonic SCLM is highly magnesian; the decrease in mg# with depth is interpreted as the cumulative effect of metasomatic modification through time. The relatively small variations in seismic velocity within the continental core largely reflect the thickness of this depleted layer. The larger drop in seismic velocity in the surrounding Proton and Tecton belts reflects the closely coupled changes in SCLM composition and geotherm. 2010-01-27T22:26:43.254Z ]]>