Cu‐Isotope Evidence for Subduction Modification of Lithospheric Mantle
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Abstract Ultramafic xenoliths from southeastern Arizona, USA, provide evidence for Cu‐isotope heterogeneity in the lithospheric mantle. We report new data on Type I (Cr‐, Mg‐rich) peridotites, but also the first Cu‐isotope data for Fe‐Ti‐Al‐rich Type II pyroxenite (±amphibole) xenoliths. Whole rock δ 65 Cu values of the pyroxenites and cryptically metasomatized Type I lherzolites range to isotopically heavier compositions than asthenospheric mantle (i.e., up to +1.44‰ and +1.12‰, respectively, vs. ∼ 0‰ ± 0.2‰). Copper leached from the xenoliths using aqua regia, assumed to be hosted in interstitial sulfides, is even more variable (δ 65 Cu −0.78 to +3.88‰), indicating considerable isotopic heterogeneity within individual samples. Host basalts have low δ 65 Cu (−0.23‰ to −1.30‰), so basalt—xenolith interactions are not responsible for the compositional variations observed. While mass‐dependent fractionation may be partly responsible, metasomatism by fluids derived from recycled crustal materials is the predominant control on isotopic variations observed. Amphibole megacrysts and amphiboles separated from Type II amphibole‐bearing clinopyroxenite have normal, mantle‐like 18 O/ 16 O ratios but H‐isotope compositions (δ 2 H SMOW −82‰ to −45‰) that range between that of nominally anhydrous mantle (−80 ± 10‰) and seawater (0‰). Host basalts are also enriched in 34 S relative to depleted asthenospheric mantle, having δ 34 S CDT values up to +8‰, i.e., compositions commonly attributed to a component of recycled seawater or hydrated oceanic crust. These new data suggest that formation of Type II metasomes in the lithospheric mantle beneath the Basin and Range Province was associated with subduction of the Farallon plate and not alkali basalt magmatism associated with Basin and Range extension.Keywords:
Amphibole
Xenolith
Metasomatism
Peridotite
Ultramafic rock
Metasomatism
Xenolith
Petrogenesis
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Spinel-peridotite xenoliths entrained by Plio-Pleistocene alkaline basic lavas from Sardinia (Italy) indicate a complex petrological history of the uppermost lithospheric mantle. They mostly show protogranular textures and are characterised by a four-phase equilibrated assemblage, ranging in composition from lherzolites (up to 18% of Cpx) to harzburgites, suggesting that the Sardinian subcontinental mantle underwent partial melting episodes with extraction of basic magmas. Trace element analyses (LAM-ICP-MS) and Sr- Nd isotope data, carried out on clinopyroxene (Cpx) separates, indicate a multistage history of depletion and enrichment processes. Clinopyroxene from Cpx-rich lherzolites are characterised by a LREE depletion, with low 87Sr/86Sr (0.70262-0.70391) and high 143Nd/144Nd (0.51323 - 0.51286) values, while clinopyroxene from less fertile lherzolites and harzburgites show LREE enrichments, higher 87Sr/86Sr (0.70410-0.70461) and lower 143Nd/144Nd (0.51288-0.51251).
Nd model ages (relative to CHUR) of the most LREE-depleted samples, with 87Sr/86Sr<0.703, suggest that partial melting events occurred during Pre-Palaeozoic times. Modelling of the HREE distribution in clinopyroxene indicates that the Cpx-rich lherzolites could be interpreted as a residue after low (< 5%) melting degrees of an inferred fertile source, while higher melting degrees (up to 20-25%) are necessary to fit the Cpx composition of the most refractory harzburgites. Subsequent metasomatic processes are testified by the isotopic/ LREE enrichments, mainly recorded in the Cpx-poor peridotites. This fact implies that the most refractory domains of the mantle are more easily percolated by fluids, while Cpx-rich domains are less permeable to metasomatic agents, as indicated by experiments on melt connectivity in peridotite materials. Geochemical modelling suggests that the above mentioned enriched compositions can be obtained by metasomatising previously depleted mantle peridotite with a small amount (< 3%) of a strongly alkaline silicate melt.
Neither the inferred metasomatic agents nor the Plio- Pleistocene Sardinian lavas show the HIMU geochemical imprint which, in addition to enriched mantle EM components, is recognised in Cenozoic anorogenic magmas throughout Central Europe (Wilson and Downes, 1991). The available data therefore indicate that the lithospheric mantle beneath Sardinia is heterogeneously enriched mainly by EM components, which reflect the complex multistage evolution occurring over the last 500 Ma.
Similar geochemical features are observed in other samples of the European lithospheric mantle, such as the peridotite xenoliths entrained in alkaline lavas from the Massif Central (Zangana et al., 1997), and Tallante (Southern Spain; unpublished data). Analogous metasomatic enrichments can also be recognised in the most residual peridotites from the Pyrenean and Lanzo massifs (Bodinier et al., 1991; Downes et al., 1991). This suggests that the observed geochemical features were probably acquired during pre-Middle Mesozoic times, due to the repeated percolation of uprising EM metasomatic fluids in the European lithosphere.
It should be emphasised that this metasomatic signature has not generally been observed for the lithospheric mantle of the African plate, where Cenozoic anorogenic magmas and associated mantle xenoliths are characterised by a prevalent HIMU metasomatic component (Beccaluva et al., 1998 and reference therein).
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Radiogenic nuclide
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Alkali basalt
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Abstract Ultramafic xenoliths in alkali basalts from Jeju Island, Korea, are mostly spinel lherzolites with subordinate amounts of spinel harzburgites and pyroxenites. The compositions of major oxides and compatible to moderately incompatible elements of the Jeju peridotite xenoliths suggest that they are residues after various extents of melting. The estimated degrees of partial melting from compositionally homogeneous and unfractionated mantle to form the residual xenoliths reach 30%. However, their complex patterns of chondrite‐normalized rare earth element, from light rare earth element (LREE)‐depleted through spoon‐shaped to LREE‐enriched, reflect an additional process. Metasomatism by a small amount of melt/fluid enriched in LREE followed the former melt removal, which resulted in the enrichment of the incompatible trace elements. Sr and Nd isotopic ratios of the Jeju xenoliths display a wide scatter from depleted mid‐oceanic ridge basalt (MORB)‐like to near bulk‐earth estimates along the MORB–oceanic island basalt (OIB) mantle array. The varieties in modal proportions of minerals, (La/Yb) N ratio and Sr‐Nd isotopes for the xenoliths demonstrate that the lithospheric mantle beneath Jeju Island is heterogeneous. The heterogeneity is a probable result of its long‐term growth and enrichment history.
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Alkali basalt
Ultramafic rock
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Primitive mantle
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Xenolith
Metasomatism
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Ultramafic xenoliths entrained in the late Miocene alkali basalts and basanites from NW Turkey include refractory spinel-harzburgites and dunites accompanied by subordinate spinel-lherzolites. Whole-rock major and trace element characteristics indicate that the xenoliths are mostly the solid residues of varying degrees of partial melting (∼4–∼15%), but some have geochemical signatures reflecting the processes of melt/rock interaction. Mantle-normalized trace element patterns for the peridotites vary from LREE-depleted to strongly LREE-enriched, reflecting multistage mantle processes from simple melt extraction to metasomatic enrichment. Rhenium and platinum group element (PGE) abundances and 187Os/188Os systematics of peridotites were examined in order to identify the nature of the mantle source and the processes effective during variable stages of melt extraction within the sub-continental lithospheric mantle (SCLM). The peridotites are characterized by chondritic Os/Ir and Pt/Ir ratios and slightly supra-chondritic Pd/Ir and Rh/Ir ratios, representing a mantle region similar in composition to the primitive mantle (PM). Moderate enrichment in PPGE (Pd–Pt–Rh)/IPGE (Ir–Os–Ru) ratios with respect to the PM composition in the metasomatized samples, however, reflects compositional modification by sulphide addition during possible post-melting processes. The 187Os/188Os ratios of the peridotites range from 0.11801 to 0.12657. Highly unradiogenic Os isotope compositions (γOs at 10 Ma from –7.0 to –3.2) in the chemically undisturbed mantle residues are accompanied by depletion in Re/Os ratios, suggesting long-term differentiation of SCLM by continuous melt extraction. For the metasomatized peridotites, however, systematic enrichments in PPGE and Re abundances, and the observed positive covariance between 187Re/188Os and γOs can most likely be explained by interaction of solid residues with basaltic melts produced by melting of relatively more radiogenic components in the mantle. Significantly, the wide range of 187Os/188Os ratios characterizing the entire xenolith suite seems to be consistent with multistage evolution of SCLM and suggests that parts of the lithospheric mantle contain materials that have experienced ancient melt removal (∼1.3 Ga) which created time-integrated depletion in Re/Os ratios; in contrast, some other parts display evidence indicative of recent perturbation in the Re–Os system by sulphide addition during interaction with metasomatizing melts.
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Phlogopite
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Ultramafic xenoliths from alkali basalts in the Perjani Mountains in the Eastern Transylvanian Basin (ETB) of Romania are mainly spinel Iherzolites, although spinel harzburgites, websterites, clinopyroxenites and amphibole pyroxenites are also present. Amphibole veins cut some spinel peridotite samples. All are derived from the shallow lithospheric upper mantle. In general, textural variations are restricted to protogranular and porphyroclastic types, compared with the more varied textures found in mantle xenoliths from the alkali basalts of the neighbouring Pannonian Basin. Also, ETB peridotites are richer in amphibole. Thus, the mantle beneath the edge of the ETB is less deformed but more strongly metasomatized than the mantle closer to the centre of the Pannonian Basin. Mineralogical and bulk-rock geochemical variations resemble those of spinel Iherzolites from other sub-continental shallow mantle xenolith suites. There is no apparent correlation between deformation and geochemistry, and much of the major and trace element variation is due to variable extraction of picritic melts. The REE patterns of separated clinopyroxenes from the peridotite xenoliths are mostly LREE depleted, although clinopyroxenes from regions adjacent to amphibole veins have experienced an enrichment in La and Ce and a change in their Sr and Nd isotopic values towards those of the vein, while still retaining an overall LREE depletion. Clinopyroxenes from the websterites and clinopyroxenites are more variable. Amphibole in the hydrous pyroxenites and amphibole veins is strongly LREE enriched and is considered to be metasomatic in origin. 87Sr/86Sr and 143Nd/l44Nd isotopic ratios of the xenoliths vary between 0·7018 and 0·7044, and 0·51355 and 0 51275, respectively. These value are more depleted than those obtained for xenoliths from the Pannonian Basin. The lower l43Nd/l44Nd and higher 87Sr/Sr86 values are found in anhydrous pyroxenites, metasomatic amphiboles in veins and amphibole pyroxenites, and in the only example of an equigranular spinel Iherzolite in the suite. The ETB xenoliths were brought to the surface in alkaline vokanism which post-dated a period of Miocene to Pliocene subduction-related cak-alkaline volcanism. However, the effects of the passage of either slab-derived fluids or cak-alkaline magmas through the ETB lithospheric mantle cannot be discerned in the chemistry of the xenoliths. The metasomatic amphibole has 87Sr/Sr86 and 143Sr/Sr144 ratios similar to the host alkali basalts, but the least evoked cak-alkaline magmas also have similar Sr and Nd isotope compositions. The REE patterns of the amphibole resembk those of amphiboles considered to have crystallized from alkaline melts. No preferential enrichment in elements typically associated with slab-derivedfluids (K, Rb and Sr) relative to elements typically depleted in cak-alkaline magmas (Ti, 2jr and Nb) has been observed in the vein amphiboles, although some interstitial amphibole is depleted in all incompatible trace elements, including LREE. Thus, despite its position close to the calc-alkaline volcanic arc of the Eastern Carpathians, we cannot readily detect any interaction between the lithospheric upper mantle beneath the ETB and subduction-related magmas or fluids. Metasomatism in the lithospheric mantle is instead largely related to the passage of a primitive alkaline magma similar to the host alkali basal
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