Mechanisms of Mantle Metasomatism: Geochemical Evidence from the Lherz Orogenic Peridotite

Detailed study of a 65 cm harzburgite section perpendicularto an amphibole pyroxenite vein from the Lherz massif revealsa strong mineralogical and chemical zonation with distance fromthe vein-host boundary. At less than 20 cm, the host peridotiteis modally metasomatized and displays patterns of increasingFe, Ti, Mn, Al, Ca, Na, and HREE, and decreasing Mg and Ni towardthe vein contact This zone is also relatively impoverished inCr but is enriched in K and Sr. It is characterized by relativelyunfractionated, mainly convex-upward, chondrite-normalized REEpatterns. At a distance over 20 cm, the host peridotite displaysthe typical feature of cryptic metasomatism, i.e., selectiveLREE enrichment in otherwise anhydrous mineralogy. The chondrite-normalizedREE patterns vary from U-shaped in the range 15–25 cmto strongly fractionated in the range 25–65 cm. Thesevariations encompass the whole range reported from metasomatizedperidotite nodules in alkali basalts. They may be accountedfor by a single, silicate-melt, metasomatic event associatedwith infiltration of the Pyrenean alkali basalts into the mostrefractory peridotites, during their ascent through the subcontinentallithosphere, 100 Ma ago. The proposed model involves a chemicalevolution of the infiltrated melt with increasing distance inthe host. At < 15–20 cm, the melt composition wouldbe strongly influenced by the proximity of the vein conduit,because of the existence of advective chemical fluxes throughgrain boundaries (short-range porous flow; distance of percolation< 1 m) and into small branching cracks, and a possibly dominantdiffusive flux within the infiltrated melt. This may explainthe reactivity of the melt towards the anhydrous peridotitemineralogy, the existence of chemical gradients for most elements,and the lack of REE chromato-graphic fractionation. At a distanceof > 20–25 cm, chemical exchange with the conduit wouldbe negligible and the melt composition would be mainly controlledby re-equilibration of the peridotite matrix during long-range(> 1 m) porous flow percolation. Thus the melt would be bufferedby the amphibole peridotite mineralogy, except for LREE. Thismay explain the lack of mineralogical reaction and chemicalenrichments (except for REE) in this zone, and the chromatographicfractionation of REE. We propose a quantitative model of diffusionand percolation-controlled metasomatism associated with infiltrationof alkali basalts into peridotites hosting vein-conduits. Wealso suggest that silicate-melt percolation may explain mineraldisequilibrium features observed in mantle xenoliths.     

Copper and Noble Metal Enrichments Across the Lithosphere--Asthenosphere Boundary of Mantle Diapirs: Evidence from the Lanzo Lherzolite Massif

The Lanzo lherzolite massif shows that the top of asthenosphericdiapirs is a zone of preferential S, Cu, Au, and platinum-groupelement (PGE) enrichment. Residual plagioclase lherzolites whichunderwent a limited extraction of mid-ocean ridge basalt (MORB)melt during diapiric uprise are enriched in Ru, PPGE (Pt andPd), and Au (3–10 times the pristine asthenospheric mantle),whereas they are moderately enriched in Cu (up to 38 ppm), depletedin Ir (Ir–Ni_mn=1–0.1), and have S contents rangingbetween 95 and 215 ppm. The behaviour of chalcophile elementsin the lherzolites cannot be modelled by equilibrium batch melting.The precious metals vary independently of lithophile elementcontent and modal rock composition. It is suggested that theexcess of PPGE, Au, and Cu was introduced either by a plume-typemagma which cross-cut the Lanzo massif before rifting or bydownward percolation of sulphides segregated from the MORB magmasextracted from the asthenospheric diapir. Calculated chalcophile element compositions of the extractedmelts show high Cu/Pd ratios (27–145) typical of meltsthat have experienced early S saturation. This adds straightforwardevidence to theoretical modelling and indirect assumptions basedon extruded MORB, i.e., it is possible to produce high D-elementdepleted magma compositions simply by low degrees of mantlemelting under S-saturated conditions without fractionation inmagma chambers. The MORB magma has circulated via ariegite andgabbro dykes as well as dunitic bands. All of these rocks arestrongly depleted in Ir. Their Ir/Ni_mnratios range between 0192in the gabbros and 0068–0168 in the dunites. The depletionin Ir (and probably Os) is attributed to segregation of Ir-bearingalloys from the MORB melt before it cross-cut the peridotites. The dunitic bands are distinguished from the harzburgites anddunites analysed so far by a large range of total precious metalcontent (17–77 ppb), positive Pd/Ir_mn ratios, and an excessof S (up to 210 ppm) and/or Cu (up to 87 ppm). Chalcophile elementdata support a model whereby the dunitic bands have formed fromreaction with percolating S-saturated melts. The progressiveenrichments in Cu and PPGEs observed in the latest percolationstages (Pd/Ir_mn=16–28; Cu/S=0–4) may be modelledby increasing the degree of melting that produced the percolatingmelt(s). A similar model may account for strong Cu and PPGEenrichments in the dunitic transition zone of some ophioliticcomplexes. ^* Present address; Laurentian University, Ramsey Lake Road, Sudbury, Ontario, P3E 2C6, Canada     

Orogenic Lherzolites and Mantle Processes: Editorial

The papers in this thematic issue of Journal of Petrology weredelivered at the Fourth International Workshop on Orogenic Lherzolitesand Mantle Processes, which was held in Samani, Hokkaido, Japan,between August 26 and September 3, 2002. Fifty oral presentationswere given and 45 posters were displayed during the meeting,and the research papers in this issue provide an     

Mechanisms and Sources of Mantle Metasomatism: Major and Trace Element Compositions of Peridotite Xenoliths from Spitsbergen in the Context of Numerical Modelling

Mineral and whole-rock chemical data for peridotite xenolithsin basaltic lavas on Spitsbergen are examined to reassess mechanismsof melt–fluid interaction with peridotites and their relativerole versus melt composition in mantle metasomatism. The enrichmentpatterns in the xenoliths on primitive mantle-normalized diagramsrange from Th–La–Ce ‘inflections’ inweakly metasomatized samples (normally without amphibole) toa continuous increase in abundances from Ho to Ce typical foramphibole-bearing xenoliths. Numerical modelling of interactionbetween depleted peridotites and enriched melts indicates thatthese patterns do not result from simple mixing of the two end-membersbut can be explained by chromatographic fractionation duringreactive porous melt flow, which produces a variety of enrichmentpatterns in a single event. Many metasomatized xenoliths havenegative high field strength element and Pb anomalies and Srspikes relative to rare earth elements of similar compatibility,and highly fractionated Nb/Ta and Zr/Hf. Although amphiboleprecipitation can produce Nb–Ta anomalies, some of thesefeatures cannot be attributed to percolation-related fractionationalone and have to be a signature of the initial melt (possiblycarbonate rich). In general, chemical and mineralogical fingerprintsof a metasomatic medium are strongest near its source (e.g.a vein) whereas element patterns farther in the metasomatic‘column’ are increasingly controlled by fractionationmechanisms. KEY WORDS: Spitsbergen; lithospheric mantle; metasomatism; trace elements; theoretical modelling     

Ultramafic Xenoliths from the Bearpaw Mountains, Montana, USA: Evidence for Multiple Metasomatic Events in the Lithospheric Mantle beneath the Wyoming Craton

Ultramafic xenoliths in Eocene minettes of the Bearpaw Mountainsvolcanic field (Montana, USA), derived from the lower lithosphereof the Wyoming craton, can be divided based on textural criteriainto tectonite and cumulate groups. The tectonites consist ofstrongly depleted spinel lherzolites, harzburgites and dunites.Although their mineralogical compositions are generally similarto those of spinel peridotites in off-craton settings, somecontain pyroxenes and spinels that have unusually low Al₂O₃contents more akin to those found in cratonic spinel peridotites.Furthermore, the tectonite peridotites have whole-rock majorelement compositions that tend to be significantly more depletedthan non-cratonic mantle spinel peridotites (high MgO, low CaO,Al₂O₃ and TiO₂) and resemble those of cratonic mantle. Thesecompositions could have been generated by up to 30% partialmelting of an undepleted mantle source. Petrographic evidencesuggests that the mantle beneath the Wyoming craton was re-enrichedin three ways: (1) by silicate melts that formed mica websteriteand clinopyroxenite veins; (2) by growth of phlogopite fromK-rich hydrous fluids; (3) by interaction with aqueous fluidsto form orthopyroxene porphyroblasts and orthopyroxenite veins.In contrast to their depleted major element compositions, thetectonite peridotites are mostly light rare earth element (LREE)-enrichedand show enrichment in fluid-mobile elements such as Cs, Rb,U and Pb on mantle-normalized diagrams. Lack of enrichment inhigh field strength elements (HFSE; e.g. Nb, Ta, Zr and Hf)suggests that the tectonite peridotites have been metasomatizedby a subduction-related fluid. Clinopyroxenes from the tectoniteperidotites have distinct U-shaped REE patterns with strongLREE enrichment. They have ¹⁴³Nd/¹⁴⁴Nd values that range from0·5121 (close to the host minette values) to 0·5107,similar to those of xenoliths from the nearby Highwood Mountains.Foliated mica websterites also have low ¹⁴³Nd/¹⁴⁴Nd values (0·5113)and extremely high ⁸⁷Sr/⁸⁶Sr ratios in their constituent phlogopite,indicating an ancient (probably mid-Proterozoic) enrichment.This enriched mantle lithosphere later contributed to the formationof the high-K Eocene host magmas. The cumulate group rangesfrom clinopyroxene-rich mica peridotites (including abundantmica wehrlites) to mica clinopyroxenites. Most contain >30%phlogopite. Their mineral compositions are similar to thoseof phenocrysts in the host minettes. Their whole-rock compositionsare generally poorer in MgO but richer in incompatible traceelements than those of the tectonite peridotites. Whole-rocktrace element patterns are enriched in large ion lithophileelements (LILE; Rb, Cs, U and Pb) and depleted in HFSE (Nb,Ta Zr and Hf) as in the host minettes, and their Sr–Ndisotopic compositions are also identical to those of the minettes.Their clinopyroxenes are LREE-enriched and formed in equilibriumwith a LREE-enriched melt closely resembling the minettes. Thecumulates therefore represent a much younger magmatic event,related to crystallization at mantle depths of minette magmasin Eocene times, that caused further metasomatic enrichmentof the lithosphere. KEY WORDS: ultramafic xenoliths; Montana; Wyoming craton; metasomatism; cumulates; minette     

The Recrystallization Front of the Ronda Peridotite: Evidence for Melting and Thermal Erosion of Subcontinental Lithospheric Mantle beneath the Alboran Basin

Evidence for a major heating event accompanied by decompressionwas recently reported from crustal rocks drilled in the Alboranbasin. The metamorphic evolution recorded by these rocks impliescomplete removal of lithospheric mantle during the Cenozoic,a process that is confirmed by geophysical modelling indicatingthin lithosphere beneath the Alboran domain. In this region,the Ronda lherzolite massif (Betic Cordillera, southern Spain)provides a unique opportunity for the observation of mantleprocesses associated with lithospheric thinning. A strikingfeature of the Ronda peridotite is a narrow recrystallizationfront, which has been ascribed to kilometre-scale porous meltflow through the massif. The front separates the spinel tectonitedomain, interpreted as old, veined lithospheric mantle, fromthe granular domain where the lithospheric microstructures,mineralogical assemblages and geochemical signatures were obliteratedby grain growth coeval with pervasive infiltration of basalticmelts. On the basis of trace-element abundances in peridotitescollected over a distance of 12 km along the recrystallizationfront, our study confirms that the front is a relatively sharp(     

Evidence for Modal Metasomatism in the Orogenic Spinel Lherzolite Body from Caussou (Northeastern Pyrenees, France)

Metasomatic mineral-bearing and/or trace element-enriched ultramaficassemblages have been reported from very few Alpine-type massifs.The small ultramafic body from Caussou (Ari?ge, northeasternPyrenees) compared with other north Pyrenean ultramafic complexesshows distinctive features which are similar to those of modallymetasomatized mantle xenoliths found in alkali basalts. It ismainly composed of clinopyroxene-rich spinel lherzolites (cpx/opxratios 1), with subordinate titanian pargasite-rich peridotites,both greatly depleted in orthopyroxene. Moreover the Caussouperidotites differ from other Ari?ge peridotites in the presenceof ilmenite, the abundance of sulfide inclusions in pyroxenesand amphiboles, higher Al, Ca, Na, K, Ti, and lower Mg contents,and enrichment in incompatible trace elements (ITE). Such mineralogicaland geochemical features are interpreted as resulting from modalmetasomatism produced by influxes of silicate melt into theperidotites. At Caussou, the metasomatic assemblage comprisesTi-pargasite+Ti-bearing clinopyr oxene+ilmenite+Ti-phlogopite+sulphide+fluid,suggesting that K, Ti, Na, ITE (including S, H₂O CO₂ and possiblyFe and Ca, were introduced by the metasomatizing agent. Thismetasomatism was probably imposed on an ultramafic associationdominated by LREE-depleted peridotites similar to the northPyrenean spinel lherzolites. These features indicate that, underupper lithospheric mantle conditions, a mafic melt locally infiltratedlherzolites by a grain-boundary percolation process and reactedwith the original mineral assemblage. The infiltration of alkali-basalticliquids into spinel peridotite led to: (1) partial dissolutionof orthopyroxene and, locally, spinel; (2) crystallization ofclinopyroxene directly from introduced melts; and (3) re-crystallization/equilibrationof pre-existing clinopyroxene with these magmatic liquids. Inthe last stage of the metasomatism, segregation of more fractionatedsilicate liquids, coexisting with a (CO₂+H₂O) fluid phase, mayhave been responsible for the crystallization of titanian pargasite,possibly by means of hydro-fracturing mechanism. The pervasive modal metasomatism at Caussou was contemporaneouswith the segregation of amphibole-bearing dykes in the Lherz-Freychin?debodies (northeast Pyrenees) (101–103 Ma). They representtwo manifestations of the same magmatic event in the lithosphericmantle, probably related the Middle Cretaceous alkaline magmatisrnof the Pyrenees.     

Silicate, Hydrous and Carbonate Metasomatism at Lherz, France: Contemporaneous Derivatives of Silicate Melt-Harzburgite Reaction

Complex multi-stage models involving silicate, hydrous and carbonatemelts of distinct provenance have been invoked to explain themetasomatism observed in mantle rocks. In contrast, relativelysimple models requiring polybaric crystallization of alkalinesilicate melts have been proposed to explain the occurrenceof veined mantle rocks. To address the spatial and temporalrelationships between veins and wall-rocks, a sequence of drillcores was obtained from Lherz, France. In outcrop the vein (amphibole–garnetpyroxenite dyke) is spatially associated with hornblendite veinlets(lherzite), and proximal amphibole-bearing and distal apatite-bearingwall-rock peridotite. Considerable elemental and isotopic heterogeneityexists in these wall-rock peridotites, in many instances equivalentto, or greater than, that observed in mantle xenoliths fromworldwide localities. A single stage of reactive porous flowbest explains the elemental and isotopic heterogeneity in thewall-rock. In essence it is proposed that emplacement of thesilicate melt (dyke) was inextricably linked to chromatographicfractionation/reaction of derivatives which led to the coexistence,in space and time, of silicate, hydrous and carbonate melts.This model elegantly and simply describes the formation of complexmetasomatic aureoles around mantle veins and negates the need,in the case of basalt-hosted (and kimberlite-hosted) xenoliths,for complex multi-stage models involving several episodes ofmelt influx with each melt being of different provenance. KEY WORDS: mantle metasomatism; trace-element enrichment; isotopic contamination; wall-rock peridotite; Lherz peridotite     

Contrasting Enrichments in High- and Low-Temperature Mantle Xenoliths from Nushan, Eastern China: Results of a Single Metasomatic Event during Lithospheric Accretion?

Distinct equilibration temperatures, deformation and trace elementcharacteristics are observed in amphibole-bearing and amphibole-freeperidotite xenoliths from Nushan, Sino-Korean Craton, easternChina. Amphibole-free peridotites are predominantly deformed,fine-grained (