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1.
The Mesozoic lithospheric mantle beneath the North China craton remains poorly constrained relative to its Palaeozoic and Cenozoic counterparts due to a lack of mantle xenoliths in volcanic rocks. Available data show that the Mesozoic lithospheric mantle was distinctive in terms of its major, trace element, and isotopic compositions. The recent discovery of mantle peridotitic xenoliths in Late Cretaceous mafic rocks in the Jiaodong region provides an opportunity to further quantify the nature and secular evolution of the Mesozoic lithospheric mantle beneath the region. These peridotitic xenoliths are all spinel-facies nodules and two groups, high-Mg# and low-Mg# types, can be distinguished based on textural and mineralogical features. High-Mg# peridotites have inequigranular textures, high Mg# (up to 92.2) in olivines, and high Cr# (up to 55) in spinels. Clinopyroxenes in the high-Mg# peridotites are generally LREE-enriched ((La/Yb)N>1) with variable REE concentrations, and have enriched Sr–Nd isotopic compositions (87Sr/86Sr = 0.7046–0.7087; 143Nd/144Nd = 0.5121–0.5126). We suggest that the high-Mg# peridotites are fragments of the Archaean and/or Proterozoic lithospheric mantle that underwent extensive interaction with both carbonatitic and silicate melts prior to or during Mesozoic time. The low-Mg# peridotites are equigranular, are typified by low Mg# ( < 90) in olivines, and by low Cr# ( < 12) in spinels. Clinopyroxenes from low-Mg# peridotites have low REE abundances (ΣREE = 12 ppm), LREE-depleted REE patterns ((La/Yb)N < 1), and depleted Sr–Nd isotopic features, in contrast to the high-Mg# peridotites. These geochemical characteristics suggest that the low-Mg# peridotites represent samples from the newly accreted lithospheric mantle. Combined with the data of mantle xenoliths from the Junan and Daxizhuang areas, a highly heterogeneous, secular evolution of the lithosphere is inferred for the region in Late Cretaceous time.  相似文献   

2.
Several spinel peridotite xenoliths from Spitsbergen have Sr–Ndisotopic compositions that plot to the right of the ‘mantlearray’ defined by oceanic basalts and the DM end-member(depleted mantle, with low 87Sr/86Sr and high 143Nd/144Nd).These xenoliths also show strong fractionation of elements withsimilar compatibility (e.g. high La/Ce), which cannot be producedby simple mixing of light rare earth element-depleted peridotiteswith ocean island basalt-type or other enriched mantle melts.Numerical simulations of porous melt flow in spinel peridotitesapplied to Sr–Nd isotope compositions indicate that thesefeatures of the Spitsbergen peridotites can be explained bychemical fractionation during metasomatism in the mantle. ‘Chromatographic’effects of melt percolation create a transient zone where thehost depleted peridotites have experienced enrichment in Sr(with a radiogenic isotope composition) but not in Nd, thusproducing Sr–Nd decoupling mainly controlled by partitioncoefficients and abundances of Sr and Nd in the melt and theperidotite. Therefore, Sr–Nd isotope decoupling, earlierreported for some other mantle peridotites worldwide, may bea signature of metasomatic processes rather than a source-relatedcharacteristic, contrary to models that invoke mixing with hypotheticalSr-rich fluids derived from subducted oceanic lithosphere. Pbisotope compositions of the Spitsbergen xenoliths do not appearto be consistently affected by the metasomatism. KEY WORDS: Spitsbergen; lithospheric mantle; metasomatism; radiogenic isotopes; theoretical modelling  相似文献   

3.
汉诺坝玄武岩中地幔岩捕掳体REE和Sr,Nd同位素地球化学   总被引：20，自引：3，他引：20  
刘丛强  解广轰 《岩石学报》1996,12(3):382-389
本文报道汉诺坝玄武岩中地幔岩捕掳体的ＲＥＥ丰度和Ｓｒ、Ｎｄ同位素组成。不同岩石类型的ＲＥＥ配分模式和同位素组成反映地幔部分熔融程度和交代作用过程。二辉橄榄岩亏损轻稀土，是原始地幔经不同程度部分熔融的残留体。方辉橄榄岩具Ｕ型ＲＥＥ配分模式，是强烈亏损的地幔岩被熔体非化学平衡交代的结果。二辉岩脉状体富轻、中稀土，它同与脉状体接触的二辉橄榄岩可达化学平衡或近于化学平衡，而二辉岩脉状体的形成与玄武岩岩浆无成因关系。据对二辉岩脉状体和不含脉状体橄榄岩的Ｓｍ－Ｎｄ同位素定年，这种脉状体形成于３００Ｍａ左右。  相似文献   

4.
Ultramafic (lherzolites, metasomatized peridotites, harzburgites,websterites and clinopyroxenites) and mafic igneous (basalts,dolerites, diorites and gabbros) rocks exposed at the sea-flooralong the West Iberia continental margin represent a rare opportunityto study the transition zone between continental and oceaniclithosphere. The igneous rocks are enriched in LREE, unlikeNorth Atlantic MORB. A correlation between their 143Nd/144Ndisotopic composition and Ce/Yb ratio suggests that they originatefrom mixing between partial melts of a depleted mantle sourcesimilar to DMM and of an enriched mantle source which may residewithin the continental lithosphere. Clinopyroxenes and amphibolesin the ultramafic rocks are LREE depleted and have flat HREEpatterns with concentrations higher than those of abyssal peridotites.Clinopyroxenes in the harzburgites are less LREE depleted buthave lower HREE concentrations. The clinopyroxenes in the GaliciaBank (GB) lherzolites have radiogenic Nd (143Nd/144Nd rangingfrom 0·512937 to 0·513402) and unradiogenic Sr(87Sr/86Sr ranging from 0·702100 to 0·702311)isotopic ratios similar to, or higher than, DMM (Depleted MORBMantle) whereas the clinopyroxenes in the Iberia Abyssal Plainwebsterites have low-Nd isotopic compositions (143Nd/144Nd rangingfrom 0·512283 to 0·512553) with high-Sr isotopicratios (87Sr/86Sr ranging from 0·704170 to 0·705919).Amphiboles in Galicia Bank lherzolites and diorites have Nd–Srisotopic compositions (143Nd/144Nd from 0·512804 to 0·512938and 87Sr/86Sr from 0·703243 to 0·703887) intermediatebetween those of the clinopyroxenes from the Galicia Bank andthe Iberia Abyssal Plain, but similar to the clinopyroxenesin the 5100 Hill harzburgite (143Nd/144Nd = 0·512865and 87Sr/86Sr = 0·703591) and to the igneous rocks (143Nd/144Ndranging from 0·512729 to 0·513121 and 87Sr/86Srranging from 0·702255 to 0·705109). The majorand trace element compositions of cpx in the Galicia Bank spinellherzolites provide evidence for large-scale refertilizationof the lithospheric upper mantle by MORB-like tholeiitic melts.The associated harzburgites did not undergo partial meltingduring the rifting stage, but, in earlier times, probably during,or even before, the Hercynian orogeny. Iberia Abyssal Plainwebsterites are interpreted as high-pressure cumulates formedin the mantle. Their high Sm/Nd ratios (from 0·43 to0·67) coupled with very low-Nd isotopic compositionsare best explained by a two-stage history: formation of thecumulates from the percolation of enriched melts long beforethe rifting, followed by low-degree partial melting of the pyroxenites,accounting for their LREE depletion. This last event probablyoccurs during the rifting episode, 122 Myr ago. The isotopicheterogeneities observed in the ultramafic rocks of the Iberiamargin were already present at the time of the rifting event.They reflect a long and complex history of depletion and enrichmentevents in an old part of the mantle, and provide strong argumentsfor a sub-continental origin of this part of the upper mantle. KEY WORDS: Iberia margin; mantle peridotites; igneous rocks; petrology; geochemistry  相似文献   

5.
A total of 17 alkali basalts (alkali olivine basalt, limburgite, olivine nephelinite) and quartz tholeiites, and of 10 peridotite xenoliths (or their clinopyroxenes) were analyzed for Nd and Sr isotopes. 143Nd/144Nd ratios and 87Sr/86Sr ratios of all basalts and of the majority of ultramafic xenoliths plot below the mantle array with a large variation in Nd isotopes and a smaller variation in Sr isotopes. The tholeiites were less radiogenic in Nd than the alkali basalts. Volcanics from the Eifel and Massif Central regions contain Nd and Sr, which is more radiogenic than that of the basalts from the Hessian Depression. Nd and Sr isotopic compositions of all rocks from the latter area, with the exception of one tholeiite and one peridotite plot in the same field of isotope ratios as the Ronda ultramafic tectonite (SW Spain), which ranges in composition from garnet to plagioclase peridotite. The alkali basaltic rocks are products of smaller degrees of partial melting of depleted peridotite, which has undergone a larger metasomatic alteration compared with the source rock of tholeiitic magmas. For the peridotite xenoliths such metasomatic alteration is indicated by the correlation of their K contents and isotopic compositions. We assume that the upper mantle locally can acquire isotopic signatures low in radiogenic Nd and Sr from the introduction of delaminated crust. Such granulites low in radiogenic Nd and Sr are products of early REE fractionation and granite (Rb) separation.  相似文献   

6.
Ultramafic xenoliths from alkali basalts in the Perjani Mountainsin the Eastern Transylvanian Basin (ETB) of Romania are mainlyspinel Iherzolites, although spinel harzburgites, websterites,clinopyroxenites and amphibole pyroxenites are also present.Amphibole veins cut some spinel peridotite samples. All arederived from the shallow lithospheric upper mantle. In general,textural variations are restricted to protogranular and porphyroclastictypes, compared with the more varied textures found in mantlexenoliths from the alkali basalts of the neighbouring PannonianBasin. Also, ETB peridotites are richer in amphibole. Thus,the mantle beneath the edge of the ETB is less deformed butmore strongly metasomatized than the mantle closer to the centreof the Pannonian Basin.Mineralogical and bulk-rock geochemicalvariations resemble those of spinel Iherzolites from other sub-continentalshallow mantle xenolith suites. There is no apparent correlationbetween deformation and geochemistry, and much of the majorand trace element variation is due to variable extraction ofpicritic melts. The REE patterns of separated clinopyroxenesfrom the peridotite xenoliths are mostly LREE depleted, althoughclinopyroxenes from regions adjacent to amphibole veins haveexperienced an enrichment in La and Ce and a change in theirSr and Nd isotopic values towards those of the vein, while stillretaining an overall LREE depletion. Clinopyroxenes from thewebsterites and clinopyroxenites are more variable. Amphibolein the hydrous pyroxenites and amphibole veins is strongly LREEenriched and is considered to be metasomatic in origin. 87Sr/86Srand 143Nd/l44Nd isotopic ratios of the xenoliths vary between07018 and 07044, and 051355 and 0 51275, respectively. Thesevalue are more depleted than those obtained for xenoliths fromthe Pannonian Basin. The lower l43Nd/l44Nd and higher 87Sr/Sr86values are found in anhydrous pyroxenites, metasomatic amphibolesin veins and amphibole pyroxenites, and in the only exampleof an equigranular spinel Iherzolite in the suite.The ETB xenolithswere brought to the surface in alkaline vokanism which post-dateda period of Miocene to Pliocene subduction-related cak-alkalinevolcanism. However, the effects of the passage of either slab-derivedfluids or cak-alkaline magmas through the ETB lithospheric mantlecannot be discerned in the chemistry of the xenoliths. The metasomaticamphibole has 87Sr/Sr86 and 143Sr/Sr144 ratios similar to thehost alkali basalts, but the least evoked cak-alkaline magmasalso have similar Sr and Nd isotope compositions. The REE patternsof the amphibole resembk those of amphiboles considered to havecrystallized from alkaline melts. No preferential enrichmentin elements typically associated with slab-derivedfluids (K,Rb and Sr) relative to elements typically depleted in cak-alkalinemagmas (Ti, 2jr and Nb) has been observed in the vein amphiboles,although some interstitial amphibole is depleted in all incompatibletrace elements, including LREE. Thus, despite its position closeto the calc-alkaline volcanic arc of the Eastern Carpathians,we cannot readily detect any interaction between the lithosphericupper mantle beneath the ETB and subduction-related magmas orfluids. Metasomatism in the lithospheric mantle is instead largelyrelated to the passage of a primitive alkaline magma similarto the host alkali basal *corresponding author  相似文献   

7.
韩江伟  熊小林  朱照宇 《岩石学报》2009,25(12):3208-3220
对雷琼地区21个晚新生代玄武岩样品的主量、微量元素和Sr、Nd、Pb同位素分别用湿化学法、ICP-MS和MC-ICPMS进行了测定.这些玄武岩主要为石英拉斑玄武岩,其次为橄榄拉斑玄武岩和碱性玄武岩.大多数样品的微量元素和同位素成分与洋岛玄武岩(OIBs)相似,而且随着SiO_2不饱和度增加,不相容元素含量也增加.除R4-1可能受到地壳混染外,其他样品相对均一的Nd同位素(ε_(Nd)=2.5-6.0)以及变化明显但范围有限的Sr同位素(0.703106～0.704481),可能继承了地幔源区的特征.~(87)Sr/~(86)Sr与~(206)Pb/~(204)Pb的正相关和~(143)Nd/~(144)Nd与~(206)Pb/~(204)Pb的负相关特征暗示DM(软流圈地幔)与EM2(岩石圈地幔)的混合.地幔捕虏体的同位素特征暗示EM2成分不可能存在于尖晶石橄榄岩地幔,而La/Yb和Sm/Yb系统表明岩浆由石榴石橄榄岩部分熔融产生,这意味着EM2成分可能存在于石榴石橄榄岩地幔.雷琼地区玄武岩的地球化学变化可以用软流圈地幔为主的熔体加入不同比例石榴石橄榄岩地幔不同程度熔融产生的熔体来解释:碱性玄武岩和橄榄拉斑玄武岩是软流圈熔体与石榴石橄榄岩地幔较低程度(7%～9%)熔融体混合,而石英拉斑玄武岩是软流圈熔体与石榴石橄榄岩地幔较高程度(10%～20%)熔融体的混合.  相似文献   

8.
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  相似文献   

9.
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  相似文献   

10.
湖南锡矿山锑矿矿区煌斑岩的地球化学特征   总被引：9，自引：3，他引：9  
对湖南锡矿山矿区煌斑岩脉进行了系统的元素和同位素地球化学研究，来探讨煌斑岩的成因。本区煌斑岩形成时代大约为120Ma，早于锑成矿时间，并与锑成矿有密切相关。其微量元素MORB蛛网图，以富集高场强元素，Nb-Ta亏损和Ti不亏损，Th强富集和Ce弱富集为特征，具有岛弧型玄武岩微量元素分配模式，构造环境可能为大陆内部破坏板块边缘。稀土元素具有总量高，无明显的负铕异常，配分模式呈轻稀土富集的右倾型特征。同位素以高^87Sr/^86Sr、低^143Nd/^144Nd为特征。结合元素地球化学、同位素特征和地质发展史分析，煌斑岩可能为富含REE和高场强元素的俯冲带流体交代过的富集地幔部分熔融所产生的岩浆，在上升过程中同化和混合很少量的矿区隐伏花岗岩浆和灰岩的产物。  相似文献   

11.
Spinel peridotite xenoliths found in the Monte Vulture carbonatite-melilitite volcano have been derived from the subcontinental lithospheric mantle beneath central southern Italy. Clinopyroxene-poor lherzolites and harzburgites are the most common rock types, with subordinate wehrlites and dunites. Small quantities of phlogopite and carbonate are present in a few samples. The peridotites record a large degree of partial melting and have experienced subsequent enrichment which has increased their LILE and LREE contents, but in most cases their HFSE contents are low. Despite being carried to the surface by a carbonatite-melilitite host, the whole-rock and clinopyroxene compositions of the xenoliths have a trace-element signature more closely resembling that of silicate-melt metasomatised mantle rather than carbonatite-metasomatised peridotites. 87Sr/86Sr and 143Nd/144Nd isotopic ratios for clinopyroxene from the Vulture peridotites are 0.7042-0.7058 and 0.51260-0.5131 respectively. They form a trend away from the depleted mantle to the composition of the host magmas, and show a significant enrichment in 87Sr/86Sr compared with most European mantle samples. The mantle beneath Monte Vulture has had a complex evolution - we propose that the lithosphere had already undergone extensive partial melting before being affected by metasomatism from a silicate melt which may have been subduction-related.  相似文献   

12.
中国大陆科学钻探工程预先导孔(CCSD-PP1) 打在苏鲁超高压变质带芝麻坊超镁铁岩体上,钻孔穿透超镁铁岩体115m.超镁铁岩体由二辉橄榄岩、方辉橄榄岩和少量单辉橄榄岩和纯橄岩组成,与上下围岩接触的橄榄岩被强烈蛇纹石化.多数橄榄岩含有石榴石或其假象,普遍含有金云母和菱镁矿,少量样品中有钛斜硅镁石.在化学成分上,橄榄岩的Mg#指数变化于90.392.6之间,MgO含量(36.61%49.15%,平均45.17%) 与Na2O (0.01%0.25%)、Al2O3 (0.07%3.71%,多数 < 2.0%,平均1.46%)和CaO (0.12%2.53%,一个高达3.30%,平均1.00%) 呈负相关关系.与主量易熔元素相对亏损的特点相反,橄榄岩中显示了稀土元素富集、分馏和配分曲线显示近于平行和相似的特点,(La/Lu) N比值为3.1833.05;此外,多数样品具有高Ba (最高比原始地幔高100多倍) 含量,在蛛网图上显示Rb、Nb、Ta、Zr、Hf和Sr的负异常,Ti/Eu比值均低于1300.岩相学特征和难熔主量元素与不相容元素之间的无相关性表明橄榄岩至少受到了形成金云母和菱镁矿的2次交代作用.富含金云母的橄榄岩(例如C25-143-61,C32-149-71) 具有富钾趋势,并且显示K2O与Rb、Ba和Th等大离子亲石元素的正相关关系,未见K2O和稀土元素、Sr和Ca之间的相关关系.这些特征表明橄榄岩被含水、硅铝质碱性熔体交代,之后又被高Ba低Rb和高场强元素的镁质(菱镁矿) 碳酸岩熔体交代,并且强烈地改变了Ba的丰度和显示了特定地幔碳酸盐的稀土元素配分型式.全岩具有不均一的高放射性Sr (87Sr/86Sr=0.70840.7201)和低放射性Nd (εNd (t) =-1.14-8.55) 组成,结合已有的氧同位素研究资料,表明预先导孔PP1中的橄榄岩所代表的地幔可能在早期就遭受了来自深部的介质的交代作用.   相似文献   

13.
We present here Sr, Nd, and Pb-isotopic data from harzburgite (group I) and dunite-pyroxenite (group II) suite mantle xenoliths from the island of Hierro, one of the youngest and westernmost of the Canary Islands. A progressive leaching technique has been developed and applied to the whole-rock powder samples in order to identify and remove as far as possible any recent additions (host basalt and/or sea-water). Isotopic analyses of the leached residues show significant systematic differences between these two suites. Dunite-pyroxenite suite xenoliths (olivine pyroxenites, dunites and wehrlites) exhibit a relatively small range of isotopic compositions (87Sr/86Sr from 0.70292 to 0.70315; 143Nd/144Nd from 0.51295 to 0.51302; 206Pb/204Pb from 19.18 to 19.40) compared to the harzburgite suite (87Sr/86Sr from 0.70295 to 0.70320; 143Nd/144Nd from 0.51285 to 0.51296; 206Pb/204Pb from 18.85 to 19.41). In all isotope correlation diagrams the leached dunite-pyroxenite suite xenoliths plot between the Hierro basalt field and a hypothetical depleted mantle suggesting that these xenoliths may have been strongly infiltrated by Hierro-type basalt. Progressive leaching of this suite of samples showed removal of a component with more enriched Sr (higher 87Sr/86Sr relative to depleted mantle) and Nd (lower 143Nd/144Nd) isotopic compositions that is probably host basalt glass. The leached harzburgite suite xenoliths extend to more enriched Sr and Nd isotopic compositions than Hierro-type basalt but always have more depleted Pb. This relationship can best be explained if this suite has been subject to infiltration by earlier magmas of the Canary Island suite (in particular, those from Gran Canaria show appropriate compositional ranges), although additional infiltration by Hierro basalt cannot be ruled out. The leaching experiments for this suite mostly show removal of a radiogenic Sr component only (? seawater) which supports the interpretation of early infiltration and subsequent recrystallisation and equilibration prior to the Hierro event. Isotopic data presented in this study show that complex interaction with percolating basaltic melts of varying composition was occurring in the upper mantle beneath Hierro prior to and during the volcanic event and was probably related to the generation of earlier Canary Island magmas.  相似文献   

14.
Spinel–peridotite facies mantle xenoliths in Cenozoic alkali basalts of the Pico Cabuji volcano (Rio Grande do Norte State, Northeast Brazil) and the adjacent South Atlantic oceanic island of Fernando de Noronha are studied for: (1) the information they provide on the composition of the lithospheric component in the erupted basalt geochemistry, and (2) to check the effects of the Fernando de Noronha plume track on the mantle lithosphere. Xenoliths from Pico Cabuji are protogranular lherzolites and porphyroclastic harzburgites recording average equilibrium temperatures of 825 ± 116 and 1248 ± 19 °C, respectively. Pressure in the porphyroclastic xenoliths ranges from 1.9 to 2.7 GPa (Ca-in-olivine geobarometer). Both groups show major element chemical variation trends in whole-rock and Ti and HREE (Er, Yb) variations in clinopyroxene consistent with fractional melting and basalt extraction. REE (rare earth element) profiles of clinopyroxenes vary from LREE (La, Ce) enriched (spoon shaped) to LREE depleted in the protogranular group, whereas they are slightly convex upward in most porphyroclastic clinopyroxenes. HFSE (Ti and Zr) negative anomalies are in general modest in the clinopyroxenes of both groups. Xenoliths from Fernando de Noronha have textural variations similar to those of Pico Cabuji. Protogranular and porphyroclastic samples have similar temperature (1035 ± 80 °C) and the pressure is 1–1.9 and 2.3 GPa, respectively. Whole-rock chemical variation trends overlap and extend further than those of Pico Cabuji. The trace element profiles of the clinopyroxenes of the porphyroclastic xenoliths are enriched in La up to 30 × PM and are smoothly fractionated from LREE to HREE, with deep, negative, Zr and Ti anomalies. The geochemical heterogeneities of the xenoliths from both localities are interpreted in terms of reactive porous percolation. The porphyroclastic xenoliths from Pico Cabuji represent the lower part of a mantle column (the head of a mantle diapir, at the transition conductive–adiabatic mantle), where OIB infiltration triggers melting, and the protogranular xenoliths the top of the mantle column, chromatographically enriched by percolation at a low melt/rock ratio. This interpretation may also apply for Fernando de Noronha, but the different geochemical signature recorded by the clinopyroxenes requires a different composition of the infiltrated melt. Nd and Sr isotopes of the Pico Cabuji porphyroclastic clinopyroxenes (143Nd/144Nd= 0.51339–0.51255, 87Sr/86Sr=0.70275–0.70319) and of Fernando de Noronha (143Nd/144Nd=0.51323–0.51285, 87Sr/86Sr=0.70323–0.70465) plot on distinct arrays originating from a similar, isotopically depleted composition and trending to low Nd–low Sr (EMI) and low Nd–high Sr (EMII), respectively. Correlation of the isotope variation with geochemical parameters indicates that the isotopic variation was induced by the metasomatic component, of EMI type at Pico Cabuji and of EMII type at Fernando de Noronha. These different components enriched a lithosphere isotopically similar to DMM (depleted MORB mantle) at both localities. At Fernando de Noronha, the isotopic signature of the metasomatic component is similar to that of the ∼ 8 Ma old lavas of the Remedios Formation, suggesting that this is the age of metasomatism. At Pico Cabuji, the mantle xenoliths do not record the high 87Sr/86Sr component present in the basalts. We speculate that the EMII component derives from a lithospheric reservoir, which was not thermally affected during mantle metasomatism at Pico Cabuji, but was mobilized by the hotspot thermal influence at Fernando de Noronha. This interpretation provides a plausible explanation for the presence of distinct metasomatic components at the two localities, which would be difficult to reconcile with their genetic relationship with the same plume. Received: 12 June 1999 / Accepted: 13 December 1999  相似文献   

15.
In order to provide mantle and crustal constraints during the evolution of the Colombian Andes, Sr and Nd isotopic studies were performed in xenoliths from the Mercaderes region, Northern Volcanic Zone, Colombia. Xenoliths are found in the Granatifera Tuff, a deposit of Cenozoic age, in which mantle- and crustal-derived xenoliths are present in bombs and fragments of andesites and lamprophyres compositions. Garnet-bearing xenoliths are the most abundant mantle-derived rocks, but websterites (garnet-free xenoliths) and spinel-bearing peridotites are also present in minor amounts. Amphibolites, pyroxenites, granulites, and gneisses represent the lower crustal xenolith assemblage. Isotopic signatures for the mantle xenoliths, together with field, petrographic, mineral, and whole-rock chemistry and pressure–temperature estimates, suggest three main sources for these mantle xenoliths: garnet-free websterite xenoliths derived from a source region with low P and T (16 kbar, 1065 °C) and MORB isotopic signature, 87Sr/86Sr ratio of 0.7030, and 143Nd/144Nd ratio of 0.5129. Garnet-bearing peridotite and websterite xenoliths derived from two different sources in the mantle: i) a source with intermediate P and T (29–35 kbar, 1250–1295 °C) conditions, similar to that of sub-oceanic geotherm, with an OIB isotopic signature (87Sr/86Sr ratio of 0.7043 and 143Nd/144Nd ratio of 0.5129); and ii) another source with P and T conditions similar to those of a sub-continental geotherm (>38 kbar, 1140–1175 °C) and OIB isotopic characteristics (87Sr/86Sr ratio=0.7041 and 143Nd/144Nd ratio=0.5135).  相似文献   

16.
Major and trace-element whole rock data, Nd and Sr isotopic data, and microprobe data have been collected from a suite of basanites, olivine nephelinites, and olivine melilite nephelinites from the Raton-Clayton volcanic field, New Mexico. Most of the lavas have geochemical characteristics that suggest they are primary upper mantle derived melts. The previously unreported occurrence of Type I and Type II ultramafic xenoliths in some of the lava flows supports this conclusion. All the lavas are strongly enriched in light REE, Sr, Ba, U, Th, and P2O5. 87Sr/86Sr ratios are 0.70394 to 0.70412 and 143Nd/144Nd ratios are equal to an epsilon value of +1.4; the data fall within the Nd-Sr correlation field. Trace-element modeling indicates that the lavas were last in equilibrium with a light-REE enriched mantle with a (La/Yb)N of two to nine. However, the Nd isotopic data indicate a source with a time integrated, chondritic normalized, Sm/Nd ratio of 1.01. To account for this discrepancy a metasomatic enrichment of the source is proposed. The timing of the enrichment event can only be constrained to less than 1 AE ago, and the isotopic composition of the premetasomatized source and the metasomatizing agent cannot be specified. However, geochemical constraints suggest a CO2-rich fluid enriched in incompatible elements as the likely metasomatizing agent  相似文献   

17.
Anhydrous spinel peridotite xenoliths from the Ray Pic Quaternary alkali basalt volcano (French Massif Central) show a wide range of mineralogical and geochemical compositions, reflecting significant heterogeneities in the shallow sub-continental lithospheric mantle. Variations in modal mineralogy, mineral chem istry, REE patterns and radiogenic isotope data suggest that depletion by partial melting and enrichment by cryptic metasomatism were the major mantle processes which caused the heterogeneity. The lithospheric mantle beneath Ray Pic contains two contrasting types of peridotite: (i) lherzolites with LREE-depleted compositions, high 143Nd/144Nd, low 87Sr/86Sr and unradiogenic Pb isotope ratios; (ii) lherzolites, harzburgites and a wehrlite with LREE-enriched patterns, low 143Nd/144Nd, high 87Sr/86Sr and radiogenic Pb isotope ratios. The former closely resemble depleted MORB-source mantle. The latter are related to enrichment by recent infiltration of small degree partial melts or fluids from the asthenospheric mantle, possibly related to the “low velocity component” observed by Hoernle et al. (1995) in European Neogene alkaline magmas. Thus, the Ray Pic peridotite xenoliths represent interaction between asthenospheric mantle-derived melts/fluids and depleted lithospheric mantle. This is probably linked to the upwelling mantle plume imaged beneath the Massif Central (Granet et al. 1995). A relationship between textural deformation, equilibration temperature and geochemistry of the xenoliths suggests that the hotter (> 900 °C) undeformed regions are LREE-enriched and tend to have more enriched isotope ratios, whereas the cooler (< 900 °C) regions have undergone more deformation and are more depleted both in LREE and in isotope compositions. Received: 27 July 1996 / Accepted: 25 November 1996  相似文献   

18.
Northeastern Queensland, a part of the Phanerozoic composite Tasman Fold Belt of eastern Australia, has a Paleozoic to Mesozoic history dominated by subduction zone processes. A suite of 13 peridotite xenoliths from the <3 Ma Atherton Tablelands Volcanic Province, predominantly from Mount Quincan, comprise fertile (1.8-3.4 wt.% Al2O3 and 38.7-41.9 wt.% MgO) spinel lherzolites free from secondary volatile-bearing phases and with only weak metasomatic enrichment of incompatible trace elements (SmN/YbN = 0.23-1.1; LaN/YbN = 0.11-4.9). The suite is isotopically heterogeneous, with measured Sr (87Sr/86Sr = 0.7027-07047), Nd (143Nd/144Nd = 0.51249-0.51362), and to a lesser extent, Os (187Os/188Os = 0.1228-0.1292) compositions broadly overlapping MORB source mantle (DMM) and extending to more depleted compositions, reflecting evolution in a time-integrated depleted reservoir. Major and rare earth element systematics are consistent with mantle that is residual after low to moderate degrees of melt extraction predominantly in the spinel facies, but with a few samples requiring partial melting at greater pressures in the garnet field or near the garnet-spinel transition. In contrast to most previously studied suites of continental lithospheric mantle samples, the incompatible trace element contents and Sr and Nd isotopic systematics of these samples suggest only minimal modification of the sampled lithosphere by metasomatic processes.Five of six Mount Quincan xenoliths preserving depleted middle to heavy REE patterns form a whole rock Sm-Nd isochron with an age of ∼275 Ma (εNdi = +9), coincident with widespread granitoid emplacement in the overlying region. This isochron is interpreted to indicate the timing of partial melting of a DMM-like source. Xenoliths from other Atherton localities scatter about the isochron, suggesting that the sampled mantle represents addition of DMM mantle to the lithosphere in the Permian, when the region may have broadly been within a subduction zone setting. A sixth middle to heavy REE-depleted Mount Quincan xenolith has a distinct Nd and Os isotopic composition consistent either with an earlier, possibly Precambrian melt extraction event, or with Permian derivation from a mantle source with a less depleted (time-averaged lower Sm/Nd) Nd isotopic composition, but a more depleted (low Re/Os) Os isotopic composition.The range in measured whole rock Os isotopic compositions cannot solely be the result of time-integrated effects of variable melt extraction, especially considering the coherent Sm-Nd systematics of the suite. The Os heterogeneity more likely reflects either a heterogeneous ∼275 Ma DMM source that would have a present-day Os composition (187Os/188Os ∼ 0.1265-0.1287) overlapping both abyssal peridotites and chondrites, or significant and variable enrichment within the lithospheric mantle by secondary sulfides carrying radiogenic Os in a cryptic chalcophile enrichment event. Regardless of the origin of the Os isotopic variability, these data highlight the mantle Re-Os isotopic heterogeneity that may be present over small length scales where the lithophile Sm-Nd system may be relatively homogeneous.  相似文献   

19.
Peridotite xenoliths entrained in Plio-Pleistocene alkali basalts from Sardinia represent fragments of the uppermost lithospheric mantle, and are characterised by an anhydrous four-phase mineral assemblage. They range in bulk rock composition from fertile spinel-lherzolites to residual spinel-harzburgites. The Sr-Nd isotope and trace element composition of clinopyroxene mineral separates varies between LREE-depleted samples with 87Sr/86Sr as low as 0.70262 and 143Nd/144Nd up to 0.51323 and LREE-enriched samples with 87Sr/86Sr up to 0.70461 and 143Nd/144Nd down to 0.51252. The available data suggest that all the studied peridotite samples suffered variable degrees of partial melting during Pre-Mesozoic times (based on Nd model ages relative to CHUR and DMM). The overprinted enrichment is related to a subsequent metasomatism, induced by fluids rising through the lithosphere that preferentially percolated the originally most depleted domains. Despite the occurrence of orogenic volcanism in the area, preferential enrichment in elements typically associated with slab derived fluids/melts (K, Rb, Sr, Th) relative to LREE has not been detected, and metasomatism seems to be more likely related to the infiltration of highly alkaline basic melts characterised by an EM-like Sr-Nd isotopic composition. Similar 87Sr/86Sr-143Nd/144Nd compositions, characterised by an EM signature, are observed in anorogenic mafic lavas and peridotite xenoliths from widespread localities within the "European" plate, whereas they have not previously been recorded in peridotite xenoliths and associated alkaline mafic lavas from the stable "African" lithospheric domain.  相似文献   

20.
Submarine volcanic rocks dredged during RV Meteor cruise M43-1 comprise alkali basalts, basanites, nephelinites and their differentiates representing both basement-shield and young post-shield volcanics of Gran Canaria, Tenerife, La Palma and El Hierro. The primitive lavas vary widely in trace element composition (e.g., Zr/Y=6.6-11.7, (La/Sm)N=2.3-5.4, and Ba/Yb=71-311), and they are characterized by steep, rare-earth element patterns with mean (La/Yb)N=16, and by pronounced, positive primitive mantle-normalized Nb and Ta and negative K anomalies similar to HIMU-type basalts. Rocks from the submarine flanks west and north of Gran Canaria are isotopically and geochemically identical to rocks of the subaerial Miocene shield stage, but they are distinct from rocks of the post-shield stages (Zr/Nb=6.3-8.9, 87Sr/86Sr=0.70327-0.70332, 143Nd/144Nd=0.51289-0.51293, 206Pb/204Pb=19.55-19.88). Most rocks dredged from the submarine flanks of Tenerife are isotopically and geochemically similar to rocks of the adjacent subaerial shield remnants, but a few resemble rocks of the subaerial post-shield stages (total range in Zr/Nb=4.6-6.1, 87Sr/86Sr=0.70300-0.70329, 143Nd/144Nd=0.51281-0.51292, 206Pb/204Pb=19.51-19.96). Rocks from the southern submarine ridge of La Palma cover the entire compositional range of the subaerial rocks of that ridge. Additionally, they comprise a high Zr/Nb group which resembles rocks of the ca. 1-Ma-old Taburiente shield of northern La Palma (total range in Zr/Nb=3.0-6.4, 87Sr/86Sr=0.70297-0.70314, 143Nd/144Nd=0.51288-0.51296, 206Pb/204Pb=19.21-19.79). Rocks from the southern submarine ridge of El Hierro compositionally resemble subaerial rocks of the island (Zr/Nb=4.1-6.2, 87Sr/86Sr=0.70296-0.70314, 143Nd/144Nd=0.51291-0.51297, 206Pb/204Pb=19.25-19.91). The degree of melting in the subcanarian mantle is interpreted to decrease from east to west across the archipelago whereas the proportion of depleted mantle component in the melting anomaly increases, as illustrated by Sr, Nd and Pb isotopes. The isotopic characteristics of the mantle source beneath the Canary Islands represents a mixture of HIMU, DMM and EM I. The overall isotopic signature is intermediate between that of Madeira to the north, which trends towards more depleted compositions, and that of the Cape Verde Islands to the south which shows a pronounced trend towards enriched mantle compositions. A clear trend towards the EM II component is only evident in more evolved rocks dredged from a seamount between Tenerife and Gran Canaria, some of which contain terrigenous sedimentary xenoliths. We propose a genetic model which relates similar mantle source signatures of volcanic archipelagos off West Africa to a common, large-scale lower mantle upwelling which, according to geophysical data, becomes more diffuse in the upper mantle. Narrow plumes or blobs feeding the volcanic centers along the passive margin may rise from this thermal anomaly due to upwelling in small, continent-parallel upper-mantle convection cells.  相似文献   

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