In situ origin for glass in mantle xenoliths from southeastern Australia: insights from trace element compositions of glasses and metasomatic phases
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Keywords:
Phlogopite
Amphibole
Metasomatism
Peridotite
Xenolith
Trace element
Incompatible element
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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Primitive mantle
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Peridotite
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Peridotite xenoliths from the Nógrád–Gömör Volcanic Field (NGVF) record the geochemical evolution of the subcontinental lithospheric mantle beneath the northern margin of the Pannonian Basin. This study is focused on spinel lherzolites and presents petrography, and major and trace element geochemistry for 51 xenoliths selected from all xenolith-bearing localities of the NGVF. The xenoliths consist of olivine, orthopyroxene, clinopyroxene and spinel ± amphibole. No correlations between modal composition and textures were recognized; however, major and trace element geochemistry reveals several processes, which allow the distinction of xenolith groups with different geochemical evolution. The xenoliths have undergone varying degrees (∼7–25%) of partial melting with overprinting by different metasomatic processes. Based on their Mg#, the xenoliths can be subdivided into two major groups. Group I has olivine Mg# between 89 and 91, whereas Group II has Mg# <89, significant enrichment of Fe and Mn in olivine and pyroxenes, and high Ti in spinel. Trace element contents of the xenoliths vary widely, allowing a further division based on light rare earth element (LREE) enrichment or depletion in pyroxenes. REE patterns of amphiboles match those of clinopyroxenes in each xenolith where they appear, and are inferred to have different origins based on their Nb (and other high field strength element) contents. It is proposed that Nb-poor amphiboles record the oldest metasomatic event, caused by subduction-related volatile-bearing silicate melts or fluids, followed by at least two further metasomatic processes: one that resulted in U–Th–(Nb–Ta)–LREE enrichment and crystallization of Nb-rich amphibole, affecting selective domains under the entire NGVF, and another evidenced by Fe–Mn–Ti–LREE enrichment, which overprinted early geochemical signatures. We suggest that the metasomatic agents in both cases were basaltic silicate melts, compositionally similar to the host basalts. These melts were generated during the Miocene extension of the Pannonian Basin. The effects of heating and subsequent cooling are evident in significantly different equilibration temperatures.
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Fractional crystallization (geology)
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Carbonatite
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The metasomatised continental mantle may play a key role in the generation of some ore deposits, in particular mineral systems enriched in platinum-group elements (PGE) and Au. The cratonic lithosphere is the longest-lived potential source for these elements, but the processes that facilitate their pre-concentration in the mantle and their later remobilisation to the crust are not yet well-established. Here, we report new results on the petrography, major-element, and siderophile- and chalcophile-element composition of native Ni, base metal sulphides (BMS), and spinels in a suite of well-characterised, highly metasomatised and weakly serpentinised peridotite xenoliths from the Bultfontein kimberlite in the Kaapvaal Craton, and integrate these data with published analyses. Pentlandite in polymict breccias (failed kimberlite intrusions at mantle depth) has lower trace-element contents (e.g., median total PGE 0.72 ppm) than pentlandite in phlogopite peridotites and Mica-Amphibole-Rutile-Ilmenite-Diopside (MARID) rocks (median 1.6 ppm). Spinel is an insignificant host for all elements except Zn, and BMS and native Ni account for typically <25% of the bulk-rock PGE and Au. High bulk-rock Te/S suggest a role for PGE-bearing tellurides, which, along with other compounds of metasomatic origin, may host the missing As, Ag, Cd, Sb, Te and, in part, Bi that are unaccounted for by the main assemblage. The close spatial relationship between BMS and metasomatic minerals (e.g., phlogopite, ilmenite) indicates that the lithospheric mantle beneath Bultfontein was resulphidised by metasomatism after initial melt depletion during stabilisation of the cratonic lithosphere. Newly-formed BMS are markedly PGE-poor, as total PGE contents are <4.2 ppm in pentlandite from seven samples, compared to >26 ppm in BMS in other peridotite xenoliths from the Kaapvaal craton. This represents a strong dilution of the original PGE abundances at the mineral scale, perhaps starting from precursor PGE alloy and small volumes of residual BMS. The latter may have been the precursor to native Ni, which occurs in an unusual Ni-enriched zone in a harzburgite and displays strongly variable, but overall high PGE abundances (up to 81 ppm). In strongly metasomatised peridotites, Au is enriched relative to Pd, and was probably added along with S. A combination of net introduction of S, Au +/− PGE from the asthenosphere and intra-lithospheric redistribution, in part sourced from subducted materials, during metasomatic events may have led to sulphide precipitation at ~80–120 km beneath Bultfontein. This process locally enhanced the metallogenic fertility of this lithospheric reservoir. Further mobilisation of the metal budget stored in these S-rich domains and upwards transport into the crust may require interaction with sulphide-undersaturated melts that can dissolve sulphides along with the metals they store.
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The trace element and radiogenic isotope systematics of clinopyroxene have frequently been used to characterise mantle metasomatic processes, because it is the main host of most lithophile elements in the lithospheric mantle. To further our understanding of mantle metasomatism, both solution-mode Sr-Nd-Hf-Pb and in situ trace element and Sr isotopic data have been acquired for clinopyroxene grains from a suite of peridotite (lherzolites and wehrlites), MARID (Mica-Amphibole-Rutile-Ilmenite-Diopside), and PIC (Phlogopite-Ilmenite-Clinopyroxene) rocks from the Kimberley kimberlites (South Africa). The studied mantle samples can be divided into two groups on the basis of their clinopyroxene trace element compositions, and this subdivision is reinforced by their isotopic ratios. Type 1 clinopyroxene, which comprises PIC, wehrlite, and some sheared lherzolite samples, is characterised by low Sr (~100–200 ppm) and LREE concentrations, moderate HFSE contents (e.g., ~40–75 ppm Zr; La/Zr < 0.04), and restricted isotopic compositions (e.g., 87Sr/86Sri = 0.70369–0.70383; εNdi = +3.1 to +3.6) resembling those of their host kimberlite magmas. Available trace element partition coefficients can be used to show that Type 1 clinopyroxenes are close to being in equilibrium with kimberlite melt compositions, supporting a genetic link between kimberlites and these metasomatised lithologies. Thermobarometric estimates for Type 1 samples in this study indicate equilibration depths of 135–160 km within the lithosphere, thus showing that kimberlite melt metasomatism is prevalent in the deeper part of the lithosphere beneath Kimberley. In contrast, Type 2 clinopyroxenes occur in MARID rocks and coarse granular lherzolites in this study, which derive from shallower depths (<135 km), and have higher Sr (~350–1000 ppm) and LREE contents, corresponding to higher La/Zr of > ~ 0.05. The isotopic compositions of Type 2 clinopyroxenes are more variable and extend from compositions resembling the "enriched mantle" towards those of Type 1 rocks (e.g., εNdi = −12.7 to −4.4). To constrain the source of these variations, in situ Sr isotope analyses of clinopyroxene were undertaken, including zoned grains in Type 2 samples. MARID and lherzolite clinopyroxene cores display generally radiogenic but variable 87Sr/86Sri values (0.70526–0.71177), which are correlated with Sr contents and La/Zr ratios, and which might be explained by the interaction between peridotite and melts from different enriched sources within the lithospheric mantle. Most notably, the rims of these Type 2 clinopyroxenes trend towards compositions similar to those of the host kimberlite and Type 1 clinopyroxene from PIC and wehrlites. These results are interpreted to represent clinopyroxene overgrowth during late-stage (shortly before/during entrainment) metasomatism by kimberlite magmas. Our study shows that a pervasive, alkaline metasomatic event caused MARID to be generated and harzburgites to be converted to lherzolite in the lithospheric mantle beneath the Kimberley area, which was followed by kimberlite metasomatism during Cretaceous magmatism. This latter event is the time at which discrete PIC, wehrlite, and sheared lherzolite lithologies were formed, and MARID and granular lherzolites were partly modified.
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