Evolution of the Lithospheric Mantle beneath the Marsabit Volcanic Field (Northern Kenya): Constraints from Textural, P-T and Geochemical Studies on Xenoliths

Xenoliths hosted by Quaternary basanites and alkali basaltsfrom Marsabit (northern Kenya) represent fragments of Proterozoiclithospheric mantle thinned and chemically modified during riftingin the Mesozoic (Anza Graben) and in the Tertiary–Quaternary(Kenya rift). Four types of peridotite xenoliths were investigatedto constrain the thermal and chemical evolution of the lithosphericmantle. Group I, III and IV peridotites provide evidence ofa cold, highly deformed and heterogeneous upper mantle. Textures,thermobarometry and trace element characteristics of mineralsindicate that low temperatures in the spinel stability field(750–800°C at <1·5 GPa) were attained bydecompression and cooling from initially high pressures andtemperatures in the garnet stability field (970–1080°Cat 2·3–2·9 GPa). Cooling, decompressionand penetrative deformation are consistent with lithosphericthinning, probably related to the development of the Mesozoicto Paleogene Anza Graben. Re-equilibrated and recrystallizedperidotite xenoliths (Group II) record heating (from 800°Cto 1100°C). Mineral trace element signatures indicate enrichmentby mafic silicate melts, parental to the Quaternary host basanitesand alkali basalts. Relationships between mineral textures,P–T conditions of equilibration, and geochemistry canbe explained by metasomatism and heating of the lithosphererelated to the formation of the Kenya rift, above a zone ofhot upwelling mantle. KEY WORDS: East African Rift System; Anza Graben; in situ LA-ICPMS; peridotite xenoliths; thermobarometry     

Mg-Al Sapphirine- and Ca-Al Hibonite-bearing Granulite Xenoliths from the Chyulu Hills Volcanic Field, Kenya

Basanites of the Chyulu Hills (Kenya Rift) contain mafic Mg–Aland Ca–Al granulite xenoliths. Their protoliths are interpretedas troctolitic cumulates; however, the original mineral assemblageswere almost completely transformed by subsolidus reactions.Mg–Al granulites contain the minerals spinel, sapphirine,sillimanite, plagioclase, corundum, clinopyroxene, orthopyroxeneand garnet, whereas Ca–Al granulites are characterizedby hibonite, spinel, sapphirine, mullite, sillimanite, plagioclase,quartz, clinopyroxene, corundum, and garnet. In the Mg–Algranulites, the first generation of orthopyroxene and some spinelmay be of igneous origin. In the Ca–Al granulites, hibonite(and possibly some spinel) are the earliest, possibly igneous,minerals in the crystallization sequence. Most pyroxene, spineland corundum in Mg–Al and Ca–Al granulites formedby subsolidus reactions. The qualitative P–T path derivedfrom metamorphic reactions corresponds to subsolidus cooling,probably accompanied, or followed by, compression. Final equilibrationwas achieved at T 600–740°C and P <8 kbar, inthe stability field of sillimanite. The early coexistence ofcorundum and pyroxenes (± spinel), as well as the associationof sillimanite and sapphirine with clinopyroxene and the presenceof hibonite, makes both types of granulite rare. The Ca–Alhibonite-bearing granulites are unique. Both types enlarge thespectrum of known Ca–Al–Mg-rich granulites worldwide. KEY WORDS: granulite xenoliths; corundum; sapphirine; hibonite; Kenya Rift     

Insights into Petrological Characteristics of the Lithosphere of Mantle Wedge beneath Arcs through Peridotite Xenoliths: a Review

The petrological characteristics of peridotite xenoliths exhumedfrom the lithospheric mantle below the Western Pacific arcs(Kamchatka, NE Japan, SW Japan, Luzon–Taiwan, New Irelandand Vanuatu) are reviewed to obtain an overview of the supra-subductionzone mantle in mature subduction systems. These data are thencompared with those for peridotite xenoliths from recent orolder arcs described in the literature (e.g. New Britain, WesternCanada to USA, Central Mexico, Patagonia, Lesser Antilles andPannonian Basin) to establish a petrological model of the lithosphericmantle beneath the arc. In currently active volcanic arcs, thedegree of partial melting recorded in the peridotites appearsto decrease away from the fore-arc towards the back-arc region.Highly depleted harzburgites, more depleted than abyssal harzburgites,occur only in the frontal arc to fore-arc region. The degreeof depletion increases again to a degree similar to that ofthe most depleted abyssal harzburgites within the back-arc extensionalregion, whether or not a back-arc basin is developed. Metasomatismis most prominent beneath the volcanic front, where the magmaproduction rate is highest; silica enrichment, involving themetasomatic formation of secondary orthopyroxene at the expenseof olivine, is important in this region because of the additionof slab-derived siliceous fluids. Some apparently primary orthopyroxenes,such as those in harzburgites from the Lesser Antilles arc,could possibly be of this secondary paragenesis but have beenrecrystallized such that the replacement texture is lost. TheTi content of hydrous minerals is relatively low in the sub-arclithospheric mantle peridotites. The K/Na ratio of the metasomatichydrous minerals decreases rearward from the fore-arc mantleas well as downward within the lithospheric mantle. The lithosphericmantle wedge peridotites, especially metasomatized ones frombelow the volcanic front, are highly oxidized. Shearing of themantle wedge is expected beneath the volcanic front, and isrepresented by fine-grained peridotite xenoliths. KEY WORDS: mantle wedge; lithospheric mantle; peridotite xenoliths; melting; metasomatism     

Regional Variations in the Mineralogy of Metasomatic Assemblages in Mantle Xenoliths from the West Eifel Volcanic Field, Germany

Xenoliths record two distinct events in the mantle below theQuarternary West Eifel Volcanic Field, Germany. The first, duringthe Hercynian Orogeny, led to widespread formation of secondary,Ti-poor amphibole, clinopyroxene and phlogopite. The signatureof the second event, related to Quaternary volcanism, variesacross the field. At Dreiser Weiher and Meerfelder Maar, thisevent is characterized by amphibole–phlogopite–clinopyroxeneveins, hosted in lherzolite and harzburgite xenoliths broughtto the surface by sodic olivine nephelinite–basanite suitelavas. These veins formed from crystallization of sodic magmathat flowed along fractures in the mantle. At Rockeskyller Kopf,Gees and Baarley, the Quaternary event is characterized by wehrlitexenoliths, many of which have phlogopite–clinopyroxeneveins, that were transported by potassic foid suite lavas. Wehrliteformed by reaction of lherzolite–harzburgite, with a largevolume of potassic magma that flowed along grain boundariesrather than in fractures. During reaction, orthopyroxene wasconsumed and secondary clinopyroxene, olivine and phlogopiteprecipitated. Veins formed in wehrlites only during periodicover-pressure events. The composition of the magmas parentalto the veins is similar to the lavas that carried the xenolithsto surface, indicating that the source of foid and olivine nephelinite–basanitesuite magma is domainal, as was the flow regime and magma flux. KEY WORDS: Eifel; mantle xenoliths; metasomatism; trace elements     

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 (     

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     

Geochemistry and Evolution of Subcontinental Lithospheric Mantle in Central Europe: Evidence from Peridotite Xenoliths of the Kozakov Volcano, Czech Republic

Neogene basanite lavas of Kozákov volcano, located alongthe Lusatian fault in the northeastern Czech Republic, containabundant anhydrous spinel lherzolite xenoliths that providean exceptionally continuous sampling of the upper two-thirdsof central European lithospheric mantle. The xenoliths yielda range of two-pyroxene equilibration temperatures from 680°Cto 1070°C, and are estimated to originate from depths of32–70 km, based on a tectonothermal model for basalticunderplating associated with Neogene rifting. The sub-Kozákovmantle is layered, consisting of an equigranular upper layer(32–43 km), a protogranular intermediate layer that containsspinel–pyroxene symplectites after garnet (43–67km), and an equigranular lower layer (67–70 km). Negativecorrelations of wt % TiO₂, Al₂O₃, and CaO with MgO and clinopyroxenemode with Cr-number in the lherzolites record the effects ofpartial fusion and melt extraction; Y and Yb contents of clinopyroxeneand the Cr-number in spinel indicate 5 to 15% partial melting.Subsequent metasomatism of a depleted lherzolite protolith,probably by a silicate melt, produced enrichments in the largeion lithophile elements, light rare earth elements and highfield strength elements, and positive anomalies in primitivemantle normalized trace element patterns for P, Zr, and Hf.Although there are slight geochemical discontinuities at theboundaries between the three textural layers of mantle, theretends to be an overall decrease in the degree of depletion withdepth, accompanied by a decrease in the magnitude of metasomatism.Clinopyroxene separates from the intermediate protogranularlayer and the lower equigranular layer yield ¹⁴³Nd/¹⁴⁴Nd valuesof 0·51287–0·51307 (_Nd = +4·6 to+8·4) and ⁸⁷Sr/⁸⁶Sr values of 0·70328–0·70339.Such values are intermediate with respect to the Nd–Srisotopic array defined by anhydrous spinel peridotite xenolithsfrom central Europe and are similar to those associated withthe present-day low-velocity anomaly in the upper mantle beneathEurope. The geochemical characteristics of the central Europeanlithospheric mantle reflect a complex evolution related to Devonianto Early Carboniferous plate convergence, accretion, and crustalthickening, Late Carboniferous to Permian extension and gravitationalcollapse, and Neogene rifting, lithospheric thinning, and magmatism. KEY WORDS: xenoliths; lithospheric mantle; REE–LILE–HFSE; Sr–Nd isotopes; Bohemian Massif     

The Evolution of the Upper Mantle beneath the Canary Islands: Information from Trace Elements and Sr isotope Ratios in Minerals in Mantle Xenoliths

Laser ablation microprobe data are presented for olivine, orthopyroxeneand clinopyroxene in spinel harzburgite and lherzolite xenolithsfrom La Palma, Hierro, and Lanzarote, and new whole-rock trace-elementdata for xenoliths from Hierro and Lanzarote. The xenolithsshow evidence of strong major, trace element and Sr isotopedepletion (⁸⁷Sr/⁸⁶Sr 0·7027 in clinopyroxene in themost refractory harzburgites) overprinted by metasomatism. Thelow Sr isotope ratios are not compatible with the former suggestionof a mantle plume in the area during opening of the AtlanticOcean. Estimates suggest that the composition of the originaloceanic lithospheric mantle beneath the Canary Islands correspondsto the residues after 25–30% fractional melting of primordialmantle material; it is thus significantly more refractory than‘normal’ mid-ocean ridge basalt (MORB) mantle. Thetrace element compositions and Sr isotopic ratios of the mineralsleast affected by metasomatization indicate that the upper mantlebeneath the Canary Islands originally formed as highly refractoryoceanic lithosphere during the opening of the Atlantic Oceanin the area. During the Canarian intraplate event the uppermantle was metasomatized; the metasomatic processes includecryptic metasomatism, resetting of the Sr–Nd isotopicratios to values within the range of Canary Islands basalts,formation of minor amounts of phlogopite, and melt–wall-rockreactions. The upper mantle beneath Tenerife and La Palma isstrongly metasomatized by carbonatitic or carbonaceous meltshighly enriched in light rare earth elements (REE) relativeto heavy REE, and depleted in Zr–Hf and Ti relative toREE. In the lithospheric mantle beneath Hierro and Lanzarote,metasomatism has been relatively weak, and appears to be causedby high-Si melts producing concave-upwards trace element patternsin clinopyroxene with weak negative Zr and Ti anomalies. Ti–Al–Fe-richharzburgites/lherzolites, dunites, wehrlites and clinopyroxenitesformed from mildly alkaline basaltic melts (similar to thosethat dominate the exposed parts of the islands), and appearto be mainly restricted to magma conduits; the alkali basaltmelts have caused only local metasomatism in the mantle wall-rocksof such conduits. The various metasomatic fluids formed as theresults of immiscible separations, melt–wall-rock reactionsand chromatographic fractionation either from a CO₂-rich basalticprimary melt, or, alternatively, from a basaltic and a siliceouscarbonatite or carbonaceous silicate melt. KEY WORDS: mantle xenoliths; mantle minerals; trace elements; depletion; carbonatite metasomatism     

Rates and Timescales of Fractional Crystallization from 238U-230Th-226Ra Disequilibria in Trachyte Lavas from Longonot Volcano, Kenya

A suite of peralkaline trachytes from Longonot volcano, Kenya,which erupted during the last 6000 years, has been analysedfor major and trace elements, Pb and Nd isotopes, and U–Th–Radisequilibria. The lavas are divided into three stratigraphicgroups of trachytes (Lt²a, Lt²b and Lt³), and hybrid lavas,designated LMx¹ and LMx², which, respectively, pre-date andpost-date the Lt² lavas. Major and trace elements are consistent,with up to 37% within-group fractional crystallization of predominantlyalkali feldspar. The parental magma for the different trachytegroups had a more mafic composition—probably hawaiitic.Nd and Pb isotopes show minimal variation, both within and betweenmagma groups, and indicate that up to 10% comendite magma fromthe neighbouring Olkaria volcanic field may have intermixedwith the Longonot magma. (²³⁰Th/²³⁸U) disequilibria indicatethat limited U/Th fractionation occurred during the past 10kyr, whereas (²²⁶Ra/²³⁰Th) disequilibria reflect the effectof alkali feldspar fractionation >8 kyr ago in the Lt²a lavas,between 3 and 7 kyr ago in the Lt²b lavas and in the past 3kyr for the Lt³ lavas. (²²⁶Ra/²³⁰Th) disequilibria in the Lt²blavas are interpreted using a model that combines the equationsof radioactive decay and in-growth with Rayleigh crystallizationto give fractionation rates of about 0·2 x 10^–4/yearfor the evolution of hawaiite to trachyte, but more rapid ratesof up to 3 x 10^–4/year for fractionation within the trachytesequence. (²²⁶Ra/²³⁰Th) from two whole-rock–alkali feldsparpairs are interpreted to show the crystals formed at 5800 yearsBP (Lt²b) and 2800 years BP (Lt³), implying that phenocrystformation continued almost up to the time of eruption. The resultsstrongly indicate that fractionated magmas can be stored forperiods on the order of 1000–2500 years prior to eruption,whereas other magmas were erupted as fractionation was proceeding. KEY WORDS: trachyte; magma chambers; u-series; Kenya     

Petrography and Geochemistry of Peridotite Xenoliths from Hannuoba and Significance for Lithospheric Mantle Evolution

Mantle-derived xenoliths and xenocrysts in Pale-ozoic diamondiferous ki mberlites in Mengyin (Shan-dong Province) and Fuxian (Liaoning Province) showthe presence of a cold,thick lithospheric mantle be-neath the North China craton ( NCC) in the MiddleOrdovician ( Griffin et al ., 1998 ; Menzies et al .,1993 ;Fan and Menzies ,1992) . However ,studies onmantle peridotites captured in the Tertiary to Neo-gene basalts of the NCC have revealed the existenceof a thin, hot and fertile lithosph…     

Tracing Lithosphere Evolution through the Analysis of Heterogeneous G9-G10 Garnets in Peridotite Xenoliths, II: REE Chemistry

Following previous publication of major–minor elementdata, this paper presents rare earth element (REE) data forheterogeneous (chemically zoned) garnets belonging to the peridotitesuite of mantle xenoliths from the Jagersfontein kimberlitepipe, South Africa. The rim compositions of the garnets in thehighest temperature–pressure (deepest) deformed peridotitesshow a typical megacryst-like pattern, of very low light REE(LREE) increasing through the middle REE (MREE) to a plateauof heavy REE (HREE) at c. 20 times chondrite; these compositionswould be in equilibrium with small-volume melts of the mid-oceanridge basalt (MORB) source (asthenosphere). With decreasingdepth the garnet rims show increasing LREE and decreasing HREE,eventually resulting in humped relative abundance patterns.A set of compositions is calculated for melts that would bein equilibrium with the garnet rims at different depths. Theseshow decreasing relative abundance of each REE from La to Lu,and the La/Lu ratio of the melts increases with decreasing depthof formation. Modelling of the effects of crystal fractionationshows that this process could largely generate the sequenceof garnet rim and melt compositions found with decreasing depth,including the humped REE patterns in high-level garnets. Consideringthe behaviour of major–minor elements as well as REE,a process of percolative fractional crystallization is advocatedin which megacryst source melts percolate upwards through peridotitesand undergo fractionation in conjunction with exchange withthe peridotite minerals. The initial megacryst melt probablyincludes melt of lithospheric origin as well as melt from theMORB source, and it is suggested that the process of percolativefractional crystallization may form a variety of metasomaticand kimberlitic melts from initial megacryst melts. Repeatedmetasomatism of the lower lithosphere by such differentiatingmelts is suggested by consideration of garnet core compositions.Such metasomatism would progressively convert harzburgites tolherzolites by increasing their CaO content, and this may accountfor the fact that the Cr-rich diamond–garnet harzburgiteparagenesis is commonly preserved only where it has been encapsulatedin diamonds. KEY WORDS: cratonic lithosphere; garnet zoning; mantle xenoliths; megacryst magma; metasomatic melt     

Chemical Variations in Peridotite Xenoliths from Vitim, Siberia: Inferences for REE and Hf Behaviour in the Garnet-Facies Upper Mantle

Peridotite xenoliths in a Miocene picrite tuff from the Vitimvolcanic province east of Lake Baikal, Siberia, are samplesof the off-craton lithospheric mantle that span a depth rangefrom the spinel to garnet facies in a mainly fertile domain.Their major and trace element compositions show some scatter(unrelated to sampling or analytical problems), which is notconsistent with different degrees of partial melting or metasomatism.Some spinel peridotites and, to a lesser degree, garnet-bearingperidotites are depleted in heavy rare earth elements (HREE)relative to middle REE (MREE), whereas some garnet peridotitesare enriched in HREE relative to MREE, with Lu abundances muchhigher than in primitive mantle estimates. Clinopyroxenes fromseveral spinel peridotites have HREE-depleted patterns, whichare normally seen only in clinopyroxenes coexisting with garnet.Garnets in peridotites with similar modal and major elementcompositions have a broad range of Lu and Yb abundances. Overall,HREE are decoupled from MREE and Hf and are poorly correlatedwith partial melting indices. It appears that elements withhigh affinity to garnet were partially redistributed in theVitim peridotite series following partial melting, with feweffects for other elements. The Lu–Hf decoupling may disturbHf-isotope depletion ages and their correlations with meltingindices. KEY WORDS: garnet peridotite; lithospheric mantle; Lu–Hf isotope system; Siberia; trace elements     

江苏东海芝麻房预先导孔（CCSD-PPl）橄榄岩及其反映的上地幔亏损和交代事件

芝麻房预先导孔橄榄岩岩石类型有二辉橄榄岩、方辉橄榄岩、单辉橄榄岩和纯橄榄岩，其中前3种多数含有石榴子石。这些岩石在垂向上显示成层性，没有某种方向的韵律变化，它们之间呈渐变过渡关系。对样品进行主量元素、微量元素、稀土元素和Sr、Nd、O同位素分析表明，随着样品MgO含量的增高，Al2O3、CaO逐渐降低，呈良好的相关关系，并且大致上从石榴橄榄岩类到不含石榴子石的橄榄岩类主量元素的亏损程度增高，两者一般均比原始地幔主量元素亏损。稀土元素配分型式表现为右倾的轻稀土富集型，在微量元素蛛网图上，也表现为强不相容元素和微量元素到适度不相容元素的总体右倾型，与原始地幔相比，均富集强不相容元素。富集程度的不同与主量元素的亏损无对应关系，说明主量元素和微量元素各自的特点不是同一种事件造成的，而是先前地幔部分熔融作用之后又发生了富含不相容元素流体的交代作用。Sr、Nd同位素具有富集特点，变化范围较大，而且主量元素较亏损的岩石更富集Sr同位素，显示了交代富集的特点。稀土元素(Ce／Yb)x比值和εNd之间的负相关关系和Rb丰度与Sr同位素之间的正相关关系表明，富集Sr、Nd同位素的流体和富集不相容微量元素的流体均来自地幔。     

汉诺坝橄榄岩捕虏体的单斜辉石LAM-ICPMS分析及其岩石圈地幔演化意义

橄榄岩成分及其中矿物(如单斜辉石)微量元素组成可以很好地揭示岩石圈地幔性质．在对汉诺坝新生代玄武岩中橄榄岩捕虏体做详细岩相学和岩石化学研究基础上,重点分析了单斜辉石的激光原位微量元素,并探讨了新生代华北克拉通北缘岩石圈地幔特征及地幔演化．汉诺坝地区岩石圈地幔是相当于原始地幔经过不同程度的部分熔融抽取形成的,除个别样品的部分熔融程度为15％-20％外,多数样品<5％．全岩主元素、单斜辉石成分体现出新生代汉诺坝地区的岩石圈地幔是很不均一的:在主体饱满型中有亏损型地幔的残留．这种共存现象可能是软流圈物质对古老地幔进行侵蚀、混合和改造置换的结果．单斜辉石微量元素组成所体现的碳酸岩岩浆交代作用和硅酸盐熔／流体的交代作用也支持这一认识．     

Layered Lithospheric Mantle Beneath the Ontong Java Plateau: Implications from Xenoliths in Alnoite, Malaita, Solomon Islands

A varied suite of mantle xenoliths from Malaita, Solomon Islands,was investigated to constrain the evolution of the mantle beneaththe Ontong Java Plateau. Comprehensive petrological and thermobarometricstudies make it possible to identify the dominant processesthat produced the compositional diversity and to reconstructthe lithospheric stratigraphy in the context of a paleogeotherm.P–T estimates show that both peridotites and pyroxenitescan be assigned to a shallower or deeper origin, separated bya garnet-poor zone of 10 km between 90 and 100 km. This zoneis dominated by refractory spinel harzburgites (Fo_91–92),indicating the occurrence of an intra-lithospheric depletedzone. Shallower mantle (     

Experimental Melting of Carbonated Peridotite at 6-10 GPa

Partial melting of magnesite-bearing peridotites was studiedat 6–10 GPa and 1300–1700°C. Experiments wereperformed in a multianvil apparatus using natural mineral mixesas starting material placed into olivine containers and sealedin Pt capsules. Partial melts originated within the peridotitelayer, migrated outside the olivine container and formed poolsof quenched melts along the wall of the Pt capsule. This allowedthe analysis of even small melt fractions. Iron loss was nota problem, because the platinum near the olivine container becamesaturated in Fe as a result of the reaction Fe₂SiO₄^Ol = Fe^Fe–Ptalloy + FeSiO₃^Opx + O₂. This reaction led to a gradual increasein oxygen fugacity within the capsules as expressed, for example,in high Fe³⁺ in garnet. Carbonatitic to kimberlite-like meltswere obtained that coexist with olivine + orthopyroxene + garnet± clinopyroxene ± magnesite depending on P–Tconditions. Kinetic experiments and a comparison of the chemistryof phases occasionally grown within the melt pools with thosein the residual peridotite allowed us to conclude that the meltshad approached equilibrium with peridotite. Melts in equilibriumwith a magnesite-bearing garnet lherzolite are rich in CaO (20–25wt %) at all pressures and show rather low MgO and SiO₂ contents(20 and 10 wt %, respectively). Melts in equilibrium with amagnesite-bearing garnet harzburgite are richer in SiO₂ andMgO. The contents of these oxides increase with temperature,whereas the CaO content becomes lower. Melts from magnesite-freeexperiments are richer in SiO₂, but remain silicocarbonatitic.Partitioning of trace elements between melt and garnet was studiedin several experiments at 6 and 10 GPa. The melts are very richin incompatible elements, including large ion lithophile elements(LILE), Nb, Ta and light rare earth elements. Relative to theresidual peridotite, the melts show no significant depletionin high field strength elements over LILE. We conclude fromthe major and trace element characteristics of our experimentalmelts that primitive kimberlites cannot be a direct productof single-stage melting of an asthenospheric mantle. They rathermust be derived from a previously depleted and re-enriched mantleperidotite. KEY WORDS: multianvil; carbonatite melt; peridotite; kimberlite; element partitioning     

Origin of Pyroxenite-Peridotite Veined Mantle by Refertilization Reactions: Evidence from the Ronda Peridotite (Southern Spain)

The Ronda orogenic peridotite (southern Spain) contains a varietyof pyroxene-rich rocks ranging from high-pressure garnet granulitesand pyroxenites to low-pressure plagioclase–spinel websterites.The ‘asthenospherized’ part of the Ronda peridotitecontains abundant layered websterites (‘group C’pyroxenites), without significant deformation, that occur asswarms of layers showing gradual modal transitions towards theirhost peridotites. Previous studies have suggested that theselayered pyroxenites formed by the replacement of refractoryspinel peridotites. Here, we present a major- and trace-element,and numerical modelling study of a layered outcrop of groupC pyroxenite near the locality of Tolox aimed at constrainingthe origin of these pyroxenites after host peridotites by pervasivepyroxene-producing, refertilization melt–rock reactions.Mg-number [= Mg/(Mg + Fe) cationic ratio] numerical modellingshows that decreasing Mg-number with increasing pyroxene proportion,characteristic of Ronda group C pyroxenites, can be accountedfor by a melt-consuming reaction resulting in the formationof mildly evolved, relatively low Mg-number melts (0·65)provided that the melt fraction during reaction and the time-integratedmelt/rock ratio are high enough (>0·1 and > 1,respectively) to balance Mg–Fe buffering by peridotiteminerals. This implies strong melt focusing caused by melt channellingin high-porosity domains resulting from compaction processesin a partial melted lithospheric domain below a solidus isothermrepresented by the Ronda peridotite recrystallization front.The chondrite-normalized rare earth element (REE) patterns ofgroup C whole-rocks and clinopyroxenes are convex-upward. Numericalmodeling of REE variations in clinopyroxene produced by a pyroxene-forming,melt-consuming reaction results in curved trajectories in the(Ce/Nd)_N vs (Sm/Yb)_N diagram (where N indicates chondrite normalized).Based on (Ce/Nd)_N values, two transient, enriched domains betweenthe light REE (LREE)-depleted composition of the initial peridotiteand that of the infiltrated melt may be distinguished in thereaction column: (1) a lower domain characterized by convex-upwardREE patterns similar to those observed in Ronda group C pyroxenite–peridotite;(2) an upper domain characterized by melts with strongly LREE-enrichedcompositions. The latter are probably volatile-rich, small-volumemelt fractions residual after the refertilization reactionsthat generated group C pyroxenites, which migrated throughoutthe massif—including the unmelted lithospheric spinel-tectonitedomain. The Ronda mantle domains affected by pyroxenite- anddunite- or harzburgite-forming reactions (the ‘layeredgranular’ subdomain and ‘plagioclase-tectonite’domain) are on average more fertile than the residual, ‘coarsegranular’ subdomain at the recrystallization front. Thisindicates that refertilization traces the moving boundariesof receding cooling of a thinned and partially melted subcontinentallithosphere. This refertilization process may be widespreadduring transient thinning and melting of depleted subcontinentallithospheric mantle above upwelling asthenospheric mantle. KEY WORDS: subcontinental mantle; refertilization; pyroxenite; peridotite; websterite; melt–rock reaction; plagioclase; trace elements     

Supra-subduction Zone Pyroxenites from San Jorge and Santa Isabel (Solomon Islands)

Peridotites associated with pyroxenites (with rare olivine andspinel) are exposed on the islands of San Jorge and Santa Isabelin the Solomon Islands. Orthopyroxenite occurs in large outcrops(100 m²) whereas websterite and clinopyroxenite occur as layersand veins/dykes in peridotites. The bulk compositions of thepyroxenites are characterized by high Mg²⁺/(Mg²⁺ + Fe²⁺) (0·78–0·91)and low Al₂O₃ (<2·7 wt %). Low rare earth elementabundances are coupled with large ion lithophile element enrichmentsand positive Sr and Pb anomalies (primitive mantle-normalized)relative to adjacent rare earths. Temperatures of equilibrationfor the pyroxenites are between 950 and 1050°C. These relativelylow temperatures, combined with the occurrence of primary fluidinclusions, suggest that the pyroxenites formed by interactionof peridotite protoliths with an aqueous fluid. Bulk-rock andmineral compositions of the orthopyroxenites are similar tothose of mantle-derived pyroxenites, whereas the websteriteshave closer chemical affinity with crustal arc cumulates. Nevertheless,field relationships plus petrological, textural and geochemicalevidence are consistent with formation of all pyroxenite typesin supra-subduction zone mantle, resulting from metasomatismof peridotite by subducted Pacific Plate-derived fluid. Sucha setting for pyroxenite has not previously been reported indetail. We propose that these processes produce mantle pyroxenitewith compositions similar to crustal pyroxenite. KEY WORDS: mantle metasomatism; pyroxenite; supra-subduction zone     

Diverse Origins of Xenoliths from Seamounts at the Continental Margin, Offshore Central California

A diverse assemblage of small mafic and ultramafic xenolithsoccurs in alkalic lava from Davidson and Pioneer seamounts locatedat the continental margin of central California. Based on mineralcompositions and textures, they form three groups: (1) mantlexenoliths of lherzolite, pyroxenite, and dunite with olivineof >Fo₉₀; (2) ocean crust xenoliths of dunite with olivine<Fo₉₀, troctolite, pyroxene-gabbro, and anorthosite withlow-K₂O plagioclase; (3) cumulates of seamount magmas of alkalicgabbro with primary amphibole and biotite and anorthosites withhigh-K₂O plagioclase. The alkalic cumulates are geneticallyrelated to, but more evolved than, their host lavas and probablycrystallized at the margins of magma reservoirs. Modeling andcomparison with experimentally derived phases suggest an originat moderate pressures (0·5–0·9 GPa). Thehigh volatile contents of the alkalic host lavas may have pressurizedthe magma chambers and helped to propel the xenolith-bearinglavas directly from deep storage at the base of the lithosphereto the eruption site on the ocean floor, entraining fragmentsof the upper mantle and ocean crust cumulates from the underlyingabandoned spreading center. KEY WORDS: basaltic magmatism; continental margin seamounts; geothermobarometry; mineral chemistry; xenoliths