Spinel Websterites Research Articles

Distribution of Trace Elements in Minerals from Anhydrous Spinel Peridotites and Websterites From the Ronda Peridotite: Implications for the Nature of LILE, REE and HFSE Reservoirs in the Subcontinental Lithospheric Mantle

Intermineral distribution coefficients (DXt/r We report dissolution ICP-MS analyses ofRb, Ba, Th, U, Nb, Ta, REE, Sr, Zr, Hf and Sc in leached separates of clinopyroxene, orthopyroxene, olivine and spinel in anhydrous spinel peridotites (12 samples) and websterites (6 samples) from the Ronda peridotite (S. Spain). None of the selected samples display evidence of cryptic metasomatism. This data-set provides a good example of the distribution of a large group of trace elements in anhydrous minerals from the subcontinental lithospheric mantle, as exemplified by orogenic massifs. The average partition coefficients (relative to clinopyroxene: D xt/cpx) of the studied trace elements are shown in Fig. 1. For most elements, there are no statistical differences in the D ~ between spinel peridotites and websterites. For the REE, Zr and Hf, the small standard deviation is suggestive of the lack of an important compositional effect in the partitioning of the studied compositions ('fertiles' to 'depleted'). As previously suggested, clinopyroxene is the main host of REE and other incompatible elements in anhydrous spinel mantle rocks. The D ~ for Zr and Hf is a factor 10 higher than for MREE, indicating the potential of orthopyroxene to fractionate these High Field Strength Elements (HFSEZr, Hf, Nb, Ta) relative to REE 1. When plotted in increasing order of compatibility in a melt/peridotite system (Fig. 1), the D xt/cpx of the Highly Incompatible trace Elements (HIE = Rb, Ba, Th, U, Nb, Ta) systematically increases from the MREE (Fig. 1). This feature is more marked in the olivine/cpx and spinel/cpx partitioning (Fig.l). Although the distribution of MREE, HREE, Zr and Hf in clinopyroxenes, orthopyroxenes and, partly, in olivines might be accounted for by crystallographic control in these minerals. However, the distribution of the HIE is inconsistent with a crystallographic control, especially in the case of spinels and olivines. The fact that the relative increase of the D xtjcpx of the HIE is correlated with their degree of incompatibility in a melt/peridotite system, and that olivines and spinels from different samples display similar normalized trace element patterns and abundances of HIE, indicates that an important part of their budget may be hosted in melt/fluid inclusions within these minerals. In Ronda minerals, two-phase fluid/(melt?) inclusions (~ lgm) typically occur as inclusions trails similar to secondary fluid/melt inclusions documented in peridotite xenoliths worldwide 2.

Tectonic Blocks of the Precambrian Lower Crust and Upper Mantle, Southern Sayan Mountains, East Siberia

Two large blocks of Precambrian lower-crustal and upper-mantle rocks in East Siberia, the Arbansky and Saramtinsky massifs, were tectonically emplaced into the Late Archean Onotsky Greenstone Belt and the Early Proterozoic Sharyzhalgay Granulite Belt approximately 2 Ga. The Arbansky massif is an exposure of deep lithospheric rocks formed at the base of a Precambrian granulite belt. It consists of garnet granulite and eclogite formed at 20 to 34 kb and layers of enderbite (alkali charnockite) gneiss and garnet-kyanite schist. These rocks were intruded by spinel pyroxenite and spinel-hornblende peridotite while at pressures of 6 to 10 kb. The Saramtinsky massif is an exposure of Precambrian mantle material that consists of spinel harzburgite equilibrated at 10 to 12 kb but that hosts lensoidal bodies of spinel websterite and garnet websterite equilibrated at 8 to 11 kb and 24 to 31 kb, respectively. These observations are best reconciled with an evolutionary model in which deep lithospheric rocks that equil...

Mantle Petrology and Geochemistry Beneath the Nógrád-Gömör Volcanic Field, Carpathian-Pannonian Region

Neogene to Quaternary alkali basalts within the Nograd-Gomor Volcanic Field (NGYF) contain abundant mafic and ultramafic xenoliths and megacrysts, yielding insight into the nature and evolution of the mantle beneath this region. The Cr-diopside suite of ultramafic nodules has been classified by texture, mineralogy, and chemistry into two groups: Group 1—spinel lherzolite and spinel websterite, with protogranular to porphyroclastic or equigranular textures and small amounts of pargasitic amphibole; and Group 2—dunite and spinel lherzolite, with “secondary” recrystallized textures and minute amounts of phlogopite. Cr-diopside-rich veinlets occur in Groups 1 and 2, forming composite xenoliths. Calculated bulk-rock compositions and most mineral compositions vary distinctly with texture. “Secondary” recrystallized xenoliths (Group 2) are depleted in “basaltic” major elements, which decrease gradually through equigranular-textured xenoliths towards protogranular to porphyroclastic xenoliths. Nevertheless, withi...

Gabbroic and Pyroxenite Layers in the Tinaquillo, Venezuela, Peridotite: Succession of Melt Intrusions in a Rising Mantle Diapir

The Tinaquillo alpine-type peridotite (Venezuela) contains an unusual abundance of pyroxenite, gabbroic granulite, and homblendite layers (about 15% of the massif). Of the anhydrous mafic layers, the metaclinopyroxenites and spinel websterites have cumulate-type compositions, with high MgO contents (14 to 18%). They originated in the upper mantle, in the spinel lherzolite facies and underwent the same two deformation events as the peridotite. Their petrography, mineralogy, and geochemistry are typical of the Al-augite suite pyroxenite layers in subcontinental lherzolite massifs and alkaline basalt nodules. Orthopyroxenite layers, characterized by low-alumina content, could represent earlier Cr-diopside suite pyroxenites. In contrast, the gabbroic hornblende granulites were only affected by the second deformation-recrystallization episode, at about 7 kb, have lower MgO contents (7-11%), and are enriched in Fe, Ti, and incompatible elements. Their normative compositions are those of tholeiitic and mildly undersaturated basalts. Based on their chemistry, three groups of hornblende granulites have been defined: (1) Group A form a moderate-P (5-7 kb) sequence enriched in iron and silica; (2) Group B is richer in silica, magnesium, and chromium but poorer in iron than group A. Their normative compositions are compatible with an origin as primary melt at about 8-10 kb; (3) Garnet-hornblende granulites are iron-rich, mildly alkaline rocks, that could result from a high P (>7-8 kb) evolution of a tholeiitic magma. Trace element concentrations in the gabbroic granulites suggest that they intruded the massif during the early stage of the formation of a back-arc basin.

Distribution of trace elements during breakdown of mantle garnet: an example from Zabargad

Ion probe investigations on mineral phases forming the Al-Di pyroxenites from the Zabargad peridotite body indicate that porphyroclastic pyroxenes in composite mafic layers record an unusual HREE, Zr, Sc enrichment not registered by pyroxenes in spinel websterites. Orthopyroxene in the opx+sp clusters forming the inner, cpx-free zone of layered pyroxenites shows strongly fractionated REE patterns (HREEN/LREEN>1000; Yb>100xch) and very high Zr, Sc and Y abundances (up to 30,672 and 60ppm, respectively). In the outer, cpx-rich zone porphyroclastic clinopyroxene is strongly HREE enriched (HREEN/LREEN∼29; Yb∼ 269xch) and displays very high Sc and Zr abundances (up to 819 and 164 ppm, respectively). It is suggested that the unusual trace element abundances are inherited from a precursor garnet. Composite pyroxenite layers are interpreted as former garnet clinopyroxenites characterized by gnt/cpx modal zoning. The sp+opx(cpx-free) assemblage in the inner part is a product of the break-down reaction of garnet upon decompression, with Ca of the original garnet completely entering the enstatite solid solution. The temperature at which the breakdown reaction occurred is estimated to be higher than 1000°C (P in the range 20–30 kbar). In the outer part, decompression caused the garnet to form a sp+opx assemblage; however, the grossularite component participated in the formation of new clinopyroxene which reacted with the clinopyroxene present in the original mode before the decompression reaction, thus forming a cpx2+sp+opx assemblage. As a result of garnet breakdown, pyroxenes have peculiar HFSE anomalies. Progressive upwelling during the Red Sea rifting produced incomplete reaction under pl-facies conditions. The geochemical signatures of precursor garnet in pyroxenes were partially crased during the recrystallization from granular spincl-bearing to granoblastic plagioclase-bearing assemblages, being preserved only in a few porphyroclast relies. The finding of pyroxenes with trace element characteristics of precursor garnet has important geodynamic and geochemical implications. Al-Di pyroxenite layers had a long history within the mantle, before the continental lithosphere rifting and thinning took place in the region. It is suggested that Al-Di pyroxenites were formed by deep-seated tholeiitic magmatism unrelated to the Red Sea evolution, thus representing the earliest event in the Zabargad upper mantle. Garnet breakdown significantly preceded the metasomatism induced by hydrous fluids (crystallization of Ti-rich pargasite) and the later intrusion of hydrous (Cr-Di) pyroxenite dykes. During the stages of mantle evolution, the HFSE anomalies in pyroxenes varied significantly. We note that the study of HFSE anomalies in mineral phases reveals complex geochemical histories which are not recorded by the whole-rock system.

REE and Sr-Nd Isotopic Geochemistry of Eastern Pyrenean Peridotite Massifs: Sub-Continental Lithospheric Mantle Modified by Continental Magmatism

Separated and leached minerals from >40 samples from Eastern Pyrenean ultramafic massifs have been analysed for rare earth elements (REEs), and Sr and Nd isotope ratios. The sub-continental lithospheric mantle in this region has interacted with several different episodes of niagmatic activity since at least the Palaeozoic. The range of isotopic composition in the massifs is extreme for mantle material (87Sr/86Sr =0·7020−0·7050; 143Nd/144Nd = 0·5139−0·5121), although the dominant lithologies, spinel lherzolite and spinel websterite, are isotopically much more homogeneous. Light rare earth element (LREE)-depleted patterns occur in clinopyroxenes from protogranular and porphyroclastic spinel lherzolites. These rocks have high ɛNd and lowɛSr values (mid-ocean ridge basalt—MORB-type) and represent lithospheric mantle unaffected or only slightly affected by subsequent magmatic events. With increasing refractory nature of the peridotites, heavy rare earth element (HREE) content and ɛNd of clinopyroxene decrease, and ɛSr and (Ce/Yb)N increase. Harzburgite bands may be formed by the passage of tholeitic melts unrelated to the host peridotites. Layered pyroxenites originated as crystal segregates from tholeiitic magmas. Clinopyroxenes from garnet clinopyroxenites have bell-shaped REE patterns, but websterites have reacted with host peridotites, acquiring similar REE patterns. Layered pyroxenites generally have higher ɛSr values than spinel peridotites, and show a wide range of ɛNd, resulting in part from time-integrated Sm/Nd fractionation. Some layered pyroxenites have extremely low ɛNdvalues (−5·6 to −10·2) resembling enriched mantle 1 (EMI); others have high ɛNd values (+ 25 to +28), although many have ɛNd values similar to those of their host peridotites. Cross-cutting amphibole pyroxenite and hornblendite veins were formed by 100-Ma-old alkali basaltic magmas which have reacted with pre-existing peridotite. The metasomatism of the Caussou massif is also related to this magmatism. REE patterns in clinopyroxene and amphibole from hydrous veins and metasomatized peridotites resemble those of megacrysts from alkali basalts. Magmatic minerals in hydrous veins have homogeneous #x025B;Nd values (+ 5·7 to +7·2), whereas metasomatic minerals have slightly higher values (+ 6·5 to +9·4). The highɛSr(+ 6·4 to +7·8) of some amphiboles from Caussou may be related to crustal fluids.

Evolution of the Upper Mantle beneath the Pyrenees: Evidence from Orogenic Spinel Lherzolite Massifs

About 40 spinel lherzolite massifs occur as small isolated bodies throughout the North Pyrenean Metamorphic Zone. Peridotites are foliated and range in composition from harzburgite to lherzolite. Pyroxenites (spinel websterites to garnet pyroxenites) define a layering concordant to the foliation plane. Less commonly, the layering is cross-cut by dykes of amphibole pyroxenites and hornblendites (e.g., Lherz and Freychinède). The oldest event of the complex pre-emplacement evolution is a lowdegree partial melting (< 10% melt extracted) which was initiated in the garnet lherzolite stability field. This event may have been associated with asthenospheric upwelling of Palaeozoic age. Two magmatic episodes are superimposed on partial melting features. The first gave rise to the layered pyroxenites, interpreted as crystal segregates which separated within feeder dykes of tholeiites. These melts intruded the peridotites when the massifs were located at the top of mantle diapirs (i.e., 45–50 km). Subsequently, both peridotites and pyroxenites underwent deformation with tectonic rotation of the dykes and recrystallized within the spinel lherzolite stability field during an isobaric cooling stage from 1200 to 900 °C. Peridotites and pyroxenites contain trace amounts of Ti-pargasite which occur as interstitial grains, suggesting the arrival of volatile-rich, strongly evolved melt before the end of the isobaric cooling stage. The amphibole-rich veins correspond to crystal segregates which crystallized within feeder dykes of Mid-Cretaceous alkali basaltic magmas. The injection of alkali basalts was contemporaneous with high-stress, low-temperature plastic deformation which produced mylonitic textures in the peridotites and tectonic rotation of the amphibole-rich veins. These intra-lithospheric shear zones are related to the Mid-Cretaceous transcurrent movements of the Iberian Peninsula relative to the European Plate, which were also responsible for the tectonic introduction of the massifs into the lower crust. Several arguments suggest a slightly older age of crustal emplacement for the Western Pyrenean massifs compared with the eastern Pyrenean massifs.

Metasomatised lower crustal and upper mantle xenoliths from north Queensland: Chemical and isotopic evidence bearing on the composition and source of the fluid phase

A diverse range of ultramafic and mafic granulite xenoliths from cone #32, north Queensland display mineralogical and chemical evidence for enhanced fluid activities in the lowermost crust and uppermost mantle. Metasomatism of spinel lherzolite, garnet and spinel websterite and garnet granulite produced variable amounts of amphibole ± mica, locally resulting in development of hornblendite as the end-product of reaction between spinel websterite and C-O-H fluids. In contrast, associated felsic and 2-pyroxene granulite xenoliths have remained relatively unaffected. The garnet granulites and spinel websterites are interpreted as products of underplating of the lower crust by Cenozoic magmatic activity, whereas Nd-Sr isotope systematics for the 2-pyroxene and felsic granulite xenoliths suggest that they may be of Proterozoic age. The development of amphibole and mica in the mafic and ultramafic xenoliths was accompanied by increases in TiO 2, Na 2O, K 2O, A1 2O 2, Sr, Ba, Zr, Nb, LREE and Fe 2O 3/FeO, and decreased MgO, SiO 2, Cr and Ni. Virtually anhydrous spinel websterite xenoliths have 87Sr 86Sr and 143Nd 144Nd almost identical to amphibole-rich variants and hornblendite, and very similar values to the host nepheline mugearite and an amphibole megacryst. The chemical and isotopic data suggest that the metasomatism is a relatively young feature caused by oxidized fluids exsolved from alkaline magmas similar to the host as they ascended through the lithosphere. This style of metasomatism is considered a relatively localized phenomenon which is unlikely to provide an important contribution to the source regions of alkali basaltic magmas.

Fluid activity in the lower crust and upper mantle: mineralogical evidence bearing on the origin of amphibole and scapolite in ultramafic and mafic granulite xenoliths

AbstractXenoliths in an olivine nephelinite from the McBride Province, North Queensland, include Cr-diopside lherzolites, spinel and garnet websterites, felsic, 2-pyroxene and garnet granulites, and hornblendites. The spinel and garnet websterites are interpreted as crystal segregations from olivine basalt or alkali olivine basalt magma at ∼ 12 kbar followed by isobaric cooling (to approximately 900–1000°C) and subsolidus reequilibration. Garnet and 2-pyroxene granulites are mineralogically and texturally distinct and are considered to represent relatively large degrees of crystallization of basaltic magmas at comparable or slightly lower pressures (8–12 kbar). Mafic and ultramafic xenoliths have been modified to varying degrees following the relatively recent influx of a H2O- and CO2-bearing fluid. Variable amounts of amphibole and mica developed in response to the introduced fluid and it is argued that some hornblendites are the end-products of this process acting on spinel websterites. Felsic and 2-pyroxene granulite xenoliths display only minor evidence of increasedPH2O. Mineralogical and textural evidence indicates high-sulphur Ca-rich scapolite in several garnet granulites did not form in response to the increased fluid activities. It is proposed the scapolite was a primary cumulate phase precipitated from alkali basaltic magma under elevatedfo2andfso2conditions.

The eclogite-facies rocks of the Montagne Noire, France

Eclogite-facies rocks are very rare in the Montagne Noire, the local typical metamorphism being a low-pressure type with extensive development of migmatites. The Cabardès and Le Jounié eclogites (garnet + omphacite + quartz + rutile, 700–800°C, 9 ± 2 kbar) derive from tholeiitic basalts interbedded in paragneisses very close to the orthogneissic basement. Both are strongly retromorphosed and re-equilibrated in the conditions of the amphibolite-facies metamorphism of the surrounding gneisses. The eclogite-facies garnet ultramafite of the Airette pod derives from a ferromagnesian calc-alkalic cumulate; its reactional mineral associations show a complex polyphase evolution: a primary spinel websterite (Cpx + Opx + pleonaste + chromite) is reset in the eclogite facies, where garnet develops as coronas on the spinel. A further retromorphic evolution leads to a paragenesis (hornblende + tremolite + picotite + olivine (Fo 70) ± phlogopite) and finally to the paragenesis chlorite + amphibole. Those rocks may be interpreted either as tectonically emplaced slabs or as the only remnants of a medium- to high-pressure regional metamorphism. But it must be pointed out that all these rocks occur only in the southern part of the axial zone of the Montagne Noire, i.e. in the “root zone” of the large Penninic nappes which characterize this part of the Hercynian belt.

Garnet websterites and associated ultramafic inclusions from a nepheline mugearite in the Walcha area, New South Wales, Australia

AbstractA nepheline mugearite dyke in the Walcha area, New South Wales, contains xenoliths of Cr-diopside lherzolite, garnet- and spinel websterites, and two- pyroxene-garnet granulites. The Cr-diopside lherzolite xenoliths are regarded as refractory upper mantle which finally equilibrated at c. 950–980°C. The garnet websterites are considered to represent spinel websterite segregations from an Mg-rich olivine tholeiite magma at 14–16 kbar which on isobaric cooling to 1000–1050°C passed into the garnet pyroxenite stability field. Spinel-amphibole websterites which probably last equilibrated at c. 950°C and pressures of 9–12 kbar have dose mineralogical affinites with the Cr-diopside lherzolite xenoliths. The mafic granulites may represent metamorphosed crystal segregates resulting from relatively large degrees of crystallization of tholeiitic basaltic magma which finally equilibrated at c. 14 kbar and 1000°C Subcalcic clinopyroxene and spinel megacrysts crystallized from the host nepheline mugearite at c. 13–20 kbar.

Nikubuchi peridotite body in the sanbagawa metamorphic belt; thermal history of the ?Al-pyroxene-rich suite? peridotite body in high pressure metamorphic terrain

The Nikubuchi peridotite body occurs as a tectonic body in the Sanbagawa metamorphic belt. The peridotites consist of dunite, spinel wehrlite, spinel websterite and spinel-bearing metagabbro, and are characterized by the presence of aluminous pyroxenes and green spinel.