Clinopyroxene microtextures reveal incompletely extracted melts in abyssal peridotites.

ABSTRACT Textural evidence is interpreted to suggest that in regions where upwelling rates of the mantle are slow to very slow, a small amount (ϳ2%) of melt was present when plagioclasefree abyssal peridotites entered the conductive regime at the base of the oceanic lithosphere. Upon crystallization, this melt appears to have been undersaturated in orthopyroxene, but precipitated clinopyroxene, Al-rich and Ti-poor spinel, and sulfides. Furthermore, the primary clinopyroxene grains have rare earth element patterns typical of residues of fractional melting, suggesting that the interstitial liquids were incremental partial melts rather than having mid-oceanic-ridge basalt compositions

The effects of melt depletion and metasomatism on highly siderophile and strongly chalcophile elements: S–Se–Te–Re–PGE systematics of peridotite xenoliths from Kilbourne Hole, New Mexico

The composition of the Earth’s upper mantle is a function of melt depletion and subsequent metasomatism; the latter obscuring many of the key characteristics of the former, and potentially making predictions of Primitive Upper Mantle (PUM) composition problematic. To date, estimates of PUM abundances of highly siderophile element (HSE = platinum group elements (PGE) and Re) and the strongly chalcophile elements Se and Te, have been the subject of less scrutiny than the lithophile elements. Critically, estimates of HSE and strongly chalcophile element abundances in PUM may have been derived by including a large number of metasomatized and refertilized samples whose HSE and chalcophile element abundances may not be representative of melt depletion alone. Unravelling the effects of metasomatism on the S–Se–Te–HSE abundances in peridotite xenoliths from Kilbourne Hole, New Mexico, USA, potentially provides valuable insights into the abundances of HSE and strongly chalcophile element abundances in PUM. Superimposed upon the effects of melt depletion is the addition of metasomatic sulfide in approximately half of the xenoliths from this study, while the remaining half have lost sulfide to a late S-undersaturated melt. Despite these observations, the Kilbourne Hole peridotite xenoliths have HSE systematics that are, in general, indistinguishable from orogenic peridotites and peridotite xenoliths used for determination of PUM HSE abundances. This study represents the first instance where Se-Te-HSE systematics in peridotite xenoliths are scrutinized in detail in order to test their usefulness for PUM estimates. Despite earlier studies attesting to the relative immobility of Se during supergene weathering, low S, Se, Os and Se/Te in peridotite xenoliths suggests that Se may be more mobile than originally thought, and for this reason, peridotite xenoliths may not be suitable for making predictions of the abundance of these elements in PUM. Removal of Se, in turn, lowers the Se/Te in basalt-borne xenolithic peridotites to subchondritic values. This is in contrast to what has been recently reported in kimberlite-borne peridotite xenoliths. Moreover, Te added to melt depleted peridotite in metasomatic sulfide is more difficult to remove in a S-undersaturated melt than the HSE and Se due to the generation of micron-scale tellurides. The effects of these processes in orogenic peridotites and xenoliths, from which PUM abundances have been calculated, require further scrutiny before unequivocal Se-Te-Re-PGE values for PUM can be derived

Highly Siderophile Element and 187 Os Signatures in Non-cratonic Basalt-hosted Peridotite Xenoliths: Unravelling the Origin and Evolution of the Post-Archean Lithospheric Mantle

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Pétrologie des sulfures de Fe-Ni-Cu et géochimie des éléments fortement sidérophiles (étude couplée dans les péridotites abyssales de la zone de fracture Kane (zone MARK, 20-24N ride médio-atlantique) et de la campagne EDUL (49-70E, ride sud-ouest indienne))

UNE ETUDE COUPLEE DE LA PETROLOGIE DES SULFURES DE FE-NI-CU ET DE LA GEOCHIMIQUE DES ELEMENTS FORTEMENT SIDEROPHILES (OS, IR, RU, RH, PT, PD ET AU) A ETE REALISEE DANS LES PERIDOTITES ABYSSALES DRAGUEES DE LA ZONE DE FRACTURE KANE (ZONE MARK 20-24\N, RIDE MEDIO-ATLANTIQUE) ET DANS LE SITE EDUL (49-70\E RIDE SUD-OUEST INDIENNE). CES ECHANTILLONS SONT DES LHERZOLITES, DES LHERZOLITES PAUVRES EN CPX ET DES HARZBURGITES ; REFERTILISEES PAR DES FRACTIONS DE LIQUIDES NON EXTRAITES DES RESIDUS ET RECRISTALLISEES A LA BASE DE LA LITHOSPHERE OCEANIQUE. TOUTES LES PERIDOTITES CONTIENNENT DES SULFURES MAGMATIQUES SOUS FORME D'INCLUSIONS COMPOSEES DE PENTLANDITE, CHALCOPYRITE ET BORNITE ET DE PLAGES INTERGRANULAIRES RICHES EN CHALCOPYRITE. LES SULFURES SE DISTRIBUENT DE FACON HETEROGENE A L'ECHELLE DE LA LAME MINCE MAIS SONT FREQUEMMENT ASSOCIES AU CPX ET AU SPINELLE CRISTALLISES A PARTIR DES FRACTIONS DE LIQUIDES NON EXTRAITES. LEUR ABONDANCE, VARIANT INDEPENDAMMENT DES INDICES DE FERTILITE (AL 2O 3 EN ROCHE TOTALE ET CR SPINELLE), NE PEUT ETRE EXPLIQUEE PAR UN MODELE DE FUSION PARTIELLE. LES ARGUMENTS TEXTURAUX, MINERALOGIQUES ET GEOCHIMIQUES DEMONTRENT L'ORIGINE METASOMATIQUE DES SULFURES MAGMATIQUES. LES PROFILS D'ABONDANCE DES ELEMENTS FORTEMENT SIDEROPHILES SONT CHONDRITIQUES SAUF POUR RU (RU N/IR N = 1,210,1, N : NORMALISE AUX CHONDRITES CI) ET PD (0,08<PD N/IR N<17,4). LES LHERZOLITES ONT DES RAPPORTS PD N/IR N INFRACHONDRITIQUES (0,26-0,81) EN PARTIE LIES AU DEPART DU PALLADIUM LORS DU LESSIVAGE PAR L'EAU DE MER FROIDE. LES HARZBURGITES ONT DES RAPPORTS PD N/IR N INFRACHONDRITIQUES (0,08-0,48) TYPIQUES DES RESIDUS AYANT SUBI UN FORT TAUX DE FUSION. LES LHERZOLITES PAUVRES EN CPX ONT DES RAPPORTS PD N/IR N CHONDRITIQUES A SUPRACHONDRITIQUES (0,88-2,04) SE CORRELANT AVEC L'ABONDANCE DES SULFURES. LES RAPPORTS RH N/IR N DANS UNE MOINDRE MESURE SE CORRELENT AUSSI AVEC L'ABONDANCE DES SULFURES. LES SULFURES EXERCENT DONC UN FORT CONTROLE SUR PD ET RH. LES RAPPORTS SUPRACHONDRITIQUES PD/IR ET RH/IR OBSERVES DANS LES PERIDOTITES ABYSSALES ONT DONC UNE ORIGINE PETROGENETIQUE (PRECIPITATION DE SULFURES RICHES EN CU ET NI DANS LA COLONNE DE MANTEAU OCEANIQUE) ET NE RESULTENT ALORS PAS DE PROCESSUS TERRESTRES GLOBAUX (ECHANGES NOYAU-MANTEAU).PARIS-Museum Hist.Naturelle (751052304) / SudocSudocFranceF

Chalcophile and Siderophile Elements in Mantle Rocks: Trace Elements Controlled By Trace Minerals

International audienc

Platinum-group element micronuggets and refertilization process in Lherz orogenic peridotite (northeastern Pyrenees, France)

International audienceHighly siderophile elements (Platinum-group elements, Au and Re) are currently assumed to reside inside base metal sulfides (BMS) in the convecting upper mantle. However, fertile lherzolites sampled by Pyrenean orogenic peridotite massifs are unexpectedly rich in 0.5-3 mu m large micronuggets of platinum-group minerals (PGM). Among those, sulfides from the laurite-erlichmanite series (Ru, Os(Ir)S(As)2), Pt-Ir-Os alloys and Pt-Pd-Te-Bi phases (moncheite-merenskyite) are predominant. Not only the BMS phases but also the PGM micronuggets must be taken into account in calculation of the PGE budget of orogenic fertile lherzolites. Laurite is a good candidate for equilibrating the whole-rock budget of Os, it and Ru while accounting for supra-chondritic Ru/Ir-N. Textural relationships between PGMs and BMS highlight heterogeneous mixing between refractory PGMs (laurite/Pt-Ir-Os alloys) inherited from ancient refractory lithospheric mantle and late-magmatic metasomatic sulfides precipitated from tholeiitic melts. "Low-temperature" PGMs, especially Pt-Pd bismuthotellurides should be added to the list of mineral indicators of lithosphere refertilization process. Now disseminated within fertile lherzolites, "lithospheric" PGMs likely account for local preservation of ancient Os model ages (up to 2 Ga) detected in BMS by in-situ isotopic analyses. These PGMs also question the reliability of orogenic lherzolites for estimating the PGE signature of the Primitive Silicate Earth

Platinum-group element systematics and petrogenetic processing of the continental upper mantle: A review

International audienceThe platinum-group element (PGE) systematics of continental mantle peridotites show large variability, reflecting petrogenetic processing of the upper mantle during partial melting and melt/fluid percolation inside the lithosphere. By removing Pd-Cu-Ni rich sulfides, partial melting events that have stabilized the sub-continental mantle lithosphere fractionated PPGEs (Palladium-group PGE; Pt, Pd) relative to IPGEs (Iridium-group PGE; Os, Ir, Ru, Rh). Residual base-metal sulfides (BMS) survive as enclosed IPGE-enriched Monosulfide Solid Solutions (Mss), which otherwise decompose into Ru-Os-Ir-rich refractory platinum-group minerals (PGMs) once the partial melts become S-undersaturated. The small-scale heterogeneous distribution of these microphases may cause extreme nugget effects, as seen in the huge variations in absolute PGE concentrations documented in cratonic peridotites. Magmas fluxing through the lithospheric mantle may change the initial PGE budgets inherited from the melting events, resulting in the great diversity of PGE systematics seen in peridotites from the sub-continental lithosphere. For instance, melt-rock reactions at increasing melt/rock ratios operate as open-system melting processes removing residual BMS/PGMs. Highly percolated peridotites are characterized by extreme PGE depletion, coupled with PGE patterns and Os-isotope compositions that gradually evolve toward that of the percolating melt. Reactions at decreasing melt-rock ratios (usually referred to as "mantle metasomatism") precipitate PPGE-enriched BMS that yield suprachondritic Pd/Ir and occasionally affect Pt/Ir and Rh/Ir ratios as well. Moreover, volatile-rich, small volume melts fractionate Os relative to Ir and S relative to Se, thereby producing rocks with supra-chondritic Os/Ir and S/Se coupled with supra-chondritic Pd/Ir and Pt/Ir. Major magmatic inputs at the lithosphere-asthenosphere boundary may rejuvenate the PGE systematics of the depleted mantle. Integrated studies of "refertilized" peridotites with worldwide provenance provide evidence for mixing between old PGM-rich harzburgitic protoliths and newly-precipitated BMS. Long-lived PGMs carry the Os-isotope compositions of ancient melt‐depletion events into seemingly undepleted fertile lherzolites. Another diagnostic feature of major refertilization processes is the increasing modal abundance of Pt-Pd-Te-Bi or Pt-As-S microphases. Due to regional-scale refertilization processes, sizeable (> 100 km) domains of the upper lithospheric mantle are now significantly enriched in Pd, Au, Cu, Se, and other incompatible chalcophile elements that are of considerable importance in PGE-ore forming events

Sulfur and selenium systematics of the subcontinental lithospheric mantle : inferences from the Massif Central xenolith suite (France)

Selenium has been analyzed in addition to S in 58 spinel peridotite xenoliths collected in Cenozoic alkali basalts from the Massif Central (France). The S concentration range now available for this suite, calculated from 123 samples, is the largest ever reported for alkali basalt-hosted xenoliths (<3–592 ppm). Likewise, the Se concentrations range between 0.2 and 67 ppb. No partial melting signature can be identified from the S and Se systematic. Half of the analyzed xenoliths have lost S during supergene weathering. By contrast, neither surficial alteration, nor loss of chalcophile elements during eruption can explain the regional-scale variations of S and Se concentrations. A first group of lherzolite xenoliths sampled in Southern Massif Central, from volcanic centers older and spatially unrelated to the Massif Central plume that triggered the Cenozoic volcanism, contains between 20 and 250 ppm S (with occasional S concentrations up to 592 ppm) and 12–67 ppb Se. It is clear that the highest S values, originally interpreted as representing S abundances in the primitive mantle, were in fact enriched by metasomatism. Highly variable S and Se contents (<5–360 ppm; 9–52 ppb) have also been observed in peridotite xenoliths collected in the Northern Massif Central, from volcanic centers mostly older than the plume. Like Group I xenoliths, these Group II xenoliths were strongly metasomatized by volatile-rich carbonated/silicated melts which precipitated Cu-rich sulfides. A third group of xenoliths from Plio-Quaternary basalts spatially related to the Massif Central Plume are uniformly poor in S (10–60 ppm) and Se (9–29 ppb). In this Group III, poikiloblastic textured xenoliths have lost most of their S and Se budget by peridotite-melt interactions at high melt/rock ratios. Taken as a whole, the Massif Central xenolith suite provides further evidence for strong heterogeneities in the S and Se budget of the sub-continental lithospheric mantle. However, the few LREE-depleted fertile lherzolites that escaped strong metasomatic alterations suggest a S- and Se-depleted primitive mantle reservoir compared to currently accepted primitive mantle estimates.15 page(s