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1.
Ultramafic xenoliths entrained in the late Miocene alkali basalts and basanites from NW Turkey include refractory spinel-harzburgites and dunites accompanied by subordinate spinel-lherzolites. Whole-rock major and trace element characteristics indicate that the xenoliths are mostly the solid residues of varying degrees of partial melting (～4–～15%), but some have geochemical signatures reflecting the processes of melt/rock interaction. Mantle-normalized trace element patterns for the peridotites vary from LREE-depleted to strongly LREE-enriched, reflecting multistage mantle processes from simple melt extraction to metasomatic enrichment. Rhenium and platinum group element (PGE) abundances and 187Os/188Os systematics of peridotites were examined in order to identify the nature of the mantle source and the processes effective during variable stages of melt extraction within the sub-continental lithospheric mantle (SCLM). The peridotites are characterized by chondritic Os/Ir and Pt/Ir ratios and slightly supra-chondritic Pd/Ir and Rh/Ir ratios, representing a mantle region similar in composition to the primitive mantle (PM). Moderate enrichment in PPGE (Pd–Pt–Rh)/IPGE (Ir–Os–Ru) ratios with respect to the PM composition in the metasomatized samples, however, reflects compositional modification by sulphide addition during possible post-melting processes. The 187Os/188Os ratios of the peridotites range from 0.11801 to 0.12657. Highly unradiogenic Os isotope compositions (γOs at 10 Ma from –7.0 to –3.2) in the chemically undisturbed mantle residues are accompanied by depletion in Re/Os ratios, suggesting long-term differentiation of SCLM by continuous melt extraction. For the metasomatized peridotites, however, systematic enrichments in PPGE and Re abundances, and the observed positive covariance between 187Re/188Os and γOs can most likely be explained by interaction of solid residues with basaltic melts produced by melting of relatively more radiogenic components in the mantle. Significantly, the wide range of 187Os/188Os ratios characterizing the entire xenolith suite seems to be consistent with multistage evolution of SCLM and suggests that parts of the lithospheric mantle contain materials that have experienced ancient melt removal (～1.3 Ga) which created time-integrated depletion in Re/Os ratios; in contrast, some other parts display evidence indicative of recent perturbation in the Re–Os system by sulphide addition during interaction with metasomatizing melts.  相似文献   

2.
《Chemical Geology》2007,236(3-4):323-338
Serpentinized garnet peridotites from the Xugou peridotite body of the Sulu ultrahigh-pressure (UHP) metamorphic terrane, central eastern China, are refractory (olivines have Fo91.7–93.1), indicating their origin as residual mantle. Negative correlations between whole-rock MgO and TiO2, Al2O3, total Fe2O3 and CaO (r = − 0.90 to − 0.95) and positive correlations between whole-rock Al2O3 and CaO and incompatible elements [Li, V, Cu, Ga, Sr, Y, Zr, heavy rare earth elements (HREEs), Hf, Pb and U] (r = 0.69 to 0.98) likely reflect melt depletion trends. Four highly refractory samples were selected for Re–Os isotopic analysis. Although they show evidence of variable enrichment of incompatible elements during serpentinization/metasomatism, no correlations exist between 187Re/188Os or 187Os/188Os with either La or Re (r = 0.00 to 0.17). These results indicate that any Re addition was fairly recent and did not affect the Os isotopic composition significantly. The correlation between 187Os/188Os and 187Re/188Os ratios thus, most likely reflects an ancient melt extraction event.The TRD, TMA and errorchron ages of the Xugou peridotites are all similar, suggesting that these peridotites formed around 2.0 Ga ago. This age is similar to Os model ages of mantle peridotites from the Dabie terrane, but contrasts markedly with the Archean ages of the continental lithospheric mantle (CLM) beneath the eastern block of the North China craton (NCC). If we assume that the Dabie–Sulu belt formed by the Triassic collision of the Yangtze craton with the eastern block of NCC and that the Archean aged CLM of the latter persisted until the Triassic, the Paleoproterozoic ages suggest derivation of these Dabie–Sulu mantle peridotites from the Yangtze craton. A Yangtze craton origin is consistent with the existing tectonic model of the Dabie–Sulu UHP belt. Our results support the hypothesis that the crust and underlying lithospheric mantle of the Yangtze craton were subducted to depths of > 180–200 km to form the world's largest UHP belt.  相似文献   

3.
凉城、四子王旗、三义堂和大同的地幔包体的岩石学和矿物的主、微量元素成分显示华北克拉通中、西部北缘总体为过渡型岩石圈地幔,为原始地幔经过不同程度的熔体抽取和后期交代富集作用的残留.凉城岩石圈地幔经历的熔体抽取程度最低,后期交代富集作用比较强烈,这与其橄榄岩中尖晶石的Cr#较低,并且共存的单斜辉石的Mg#较低一致.三义堂岩石圈地幔经历的熔体抽取程度最高,后期交代富集作用最强烈,不同样品甚至同一样品中不同单斜辉石颗粒的La/Yb和LREE配分模式变化范围非常大,且矿物边部比核部更富集微量元素.大同、四子王旗岩石圈地幔经历的熔体抽取程度介于凉城和三义堂之间,但后期的交代富集作用明显不如凉城和三义堂强烈.研究区的岩石圈地幔主要受硅酸盐熔体交代作用的影响,只有三义堂岩石圈地幔还可能也受到了碳酸盐交代作用的影响.  相似文献   

4.
Elevated 187Os/188Os ratios compared to ambient oceanic mantle, i.e.,187Os/188Os>0.13, have been reported for both arc lavas and mantle wedge xenoliths, which have been ascribed to the addition of crustal Os through slab dehydration or melting. By contrast, much lower 187Os/188Os ratios of spinels from Izu‐Bonin‐Mariana boninites indicate slight or no crustal Os was transferred from the slab to the forearc mantle. Here we report Os isotopic compositions of peridotites from New Caledonia ophiolites, which represent relics of a forearc mantle. Some New Caledonia peridotites are characterized by Os concentrations of <1 ppb, yet have187Os/188Os ratios comparable to the ambient oceanic mantle (i.e., 187Os/188Os<0.13). This confirms that little crustal Os was transported to the forearc mantle via slab dehydration. Contrasting Os isotopes between forearc peridotites and mantle wedge xenoliths may reflect the changing behavior of Os in diverse agents released from the descending slab as a function of depth, which is mainly controlled by the stability of sulfides in the slabs. During dehydration at shallow depths, sulfides keep stable and thus little Os is transported to the overlying mantle. In comparison, sulfides become unstable and tend to break down at deeper depths where slab melting or supercritical fluid generation occurs, and thus Os behaves like a mobile element.  相似文献   

5.
Dunitic xenoliths from late Palaeogene, alkaline basalt flows on Ubekendt Ejland, West Greenland contain olivine with 100 × Mg/(Mg + Fe), or Mg#, between 92.0 and 93.7. Orthopyroxene has very low Al2O3 and CaO contents (0.024–1.639 and 0.062–0.275 wt%, respectively). Spinel has 100 × Cr/(Cr + Al), or Cr#, between 46.98 and 95.67. Clinopyroxene is absent. The osmium isotopic composition of olivine and spinel mineral separates shows a considerable span of 187Os/188Os values. The most unradiogenic 187Os/188Os value of 0.1046 corresponds to a Re-depletion age of ca. 3.3 Gy, while the most radiogenic value of 0.1336 is higher than present-day chondrite. The Os isotopic composition of the xenoliths is consistent with their origin as restites from a melt extraction event in the Archaean, followed by one or more subsequent metasomatic event(s). The high Cr# in spinel and low modal pyroxene of the Ubekendt Ejland xenoliths are similar to values of some highly depleted mantle peridotites from arc settings. However, highly depleted, arc-related peridotites have higher Cr# in spinel for a given proportion of modal olivine, compared to cratonic xenolith suites from Greenland, which instead form coherent trends with abyssal peridotites, dredged from modern mid-ocean ridges. This suggests that depleted cratonic harzburgites and dunites from shallow lithospheric mantle represent the residue from dry melting in the Archaean.  相似文献   

6.
Major elements, highly siderophile elements (HSE) and Re-Os isotope ratios were analysed in situ on individual sulfide grains in spinel peridotite xenoliths hosted by Miocene intraplate basalts from the Penghu Islands, Taiwan. The xenoliths represent texturally and compositionally different mantle domains, and the geochemical characteristics of the sulfides show changes in HSE distribution and Re-Os isotope systematics, produced as their host rocks were metasomatised by percolating fluids/melts. In prophyroclastic and partly metasomatised peridotites from the Kueipi (KP) locality, the sulfides have subchondritic to superchondritic 187Re/188Os and 187Os/188Os ratios. Many of these sulfides reflect fluid/melt interaction with residual MSS and/or crystallization of fractionated sulfide melts, which produced high contents of Cu and PPGEs and high Re/Os; inferred melt/rock ratios are low. In contrast, sulfides in equigranular and extensively metasomatised peridotites from the Tungchiyu (TCY) locality are mainly more sulfur-rich Ni-(Co)-rich MSS, with subchondritic to chondritic 187Os/188Os and subchondritic 187Re/188Os. These sulfides are interpreted as products of interaction between pre-existing MSS and percolating silicate melts. Melt/rock ratios were high and the percolating melt was less differentiated than the melt that percolated the KP peridotites. Sulfides in a TCY pyroxenite are mainly MSS; they have the lowest HSE contents, subchondritic to superchondritic 187Os/188Os and subchondritic 187Re/188Os, and may have precipitated from sulfide melts that segregated from basaltic melts under S-saturated conditions. In most sulfides melt percolation appears to have induced fractionation among the HSEs and disturbed Re-Os isotope compositions. Despite the metasomatic effects, rare residual MSS, sulfides that from crystallised sulfide melts and sulfides modified by addition of Re (with no evidence for Os addition) can still provide useful chronological information. Such sulfides yield TRD age peaks of 1.9, 1.7-1.6, 1.4-1.3 and 0.9-0.8 Ga, which may record the timing of melt extraction and/or metasomatic events in the mantle. These periods are contemporaneous with the major crustal events recorded by U-Pb dates and Nd and Hf model ages in the overlying crust. This close correspondence indicates that the sulfide TRD ages reflect the timing of lithosphere-scale tectonothermal events (such as melting and metasomatism) that affected both the lithospheric mantle and the overlying crust. The sulfide TRD ages, taken together with the crustal data, suggest that most of the Cathaysia block had formed at least by Paleo-Proterozoic time, and that some domains are Archean in age.  相似文献   

7.
The Yarlung Zangbo suture zone (YZSZ) in southern Tibet includes the remnants of Neo‐Tethyan oceanic lithosphere and marks a major suture between the Indian plate to the south and the Lhasa terrane of Tibet to the north. The upper mantle section of the Cuobuzha ophiolite in the northern subbelt of the western YZSZ comprises mainly clinopyroxene (cpx)‐rich and depleted harzburgites. Spinels in the cpx‐harzburgites show lower Cr# values (12.6–15.1) than the spinels in the harzburgites (26.1–34.5), and the cpx‐harzburgites display higher heavy rare earth element concentrations than the depleted harzburgites. The harzburgites have subchondritic Os isotopic compositions (0.11624–0.11699), yielding Re‐depletion model ages (TRD) ages from 1.8 to 1.7 Ga, indicating that the Cubuzha mantle underwent at least one ancient melt extraction event ca. 1.8‐1.7Ga; whereas the cpx‐harzburgites have suprachondritic 187Os/188Os ratios (0.12831–0.13125) with higher Re concentrations (0.380–0.575 ppb), indicating subsequent addition of Re following the last partial melting event that occurred during mid‐ocean ridge melt evolution processes. Although these geochemical and isotopic signatures suggest that both peridotite types in the ophiolite represent mid‐oceanic ridge–type upper mantle units, their melt evolution trends reflect different mantle processes. The cpx‐harzburgites formed from low‐degree partial melting of a primitive mantle source, and they were subsequently modified by melt‐rock interactions in a mid‐oceanic ridge environment. The depleted harzburgites, however, were produced by remelting of the cpx‐harzburgites, which later interacted with mid‐oceanic ridge basalt– or island‐arc tholeiite–like melts, possibly in a trench–distal backarc spreading center. Our new isotopic and geochemical data from the Cuobuzha peridotites confirm that the Neo‐Tethyan upper mantle had highly heterogeneous Os isotopic compositions as a result of multiple melt production and melt extraction events during its seafloor spreading evolution.  相似文献   

8.
Peridotitic sulphide inclusions in diamonds from the central Slave craton constrain the age and origin of their subcontinental lithospheric mantle (SCLM) sources. These sulphides align with either a ca. 3.5 Ga (shallow SCLM) or a ca. 3.3 Ga isochron (deep SCLM) on a Re–Os ischron diagram, with variably enriched initial 187Os/188Os. Since some Archaean to recent plume-derived melts carry a subducted crust (eclogite) signature and some cratonic SCLM may have been generated in plumes by extraction of komatiitic liquids, we explain these data by subduction of evolved lithospheric material (shallow SCLM) and melting in a hybrid mantle plume that contains domains of recycled eclogite (deep SCLM), respectively. In upwelling hybrid mantle, eclogite-derived melts react with olivine in surrounding peridotites to form aluminous orthopyroxene, convert peridotite to pyroxenite and confer their crustal isotope signatures. We suggest that it is subsequent to orthopyroxene enrichment of peridotite in an upwelling plume that partial melting of this Al- and Si- enriched source generated komatiites and complementary ultradepleted cratonic mantle residues. Although subduction is needed to explain some cratonic features, melting of a hybrid plume source satisfies several key observations: (1) suprachondritic initial 187Os/188Os in subsets of lithospheric mantle samples and in some coeval Archaean komatiites; (2) variable enrichment of cratonic mantle by high-temperature aluminous orthopyroxene; (3) high Mg# combined with high orthopyroxene content in cratonic mantle due to higher melt productivity of an Al- and Si-richer source; (4) variable orthopyroxene enrichment possibly linked to varying mantle potential temperatures (Tp), plume buoyancy and resultant eclogite load and/or variable availability of subducted material in the source; and (5) absence of younger analogues due to a secular decrease in Tp. Most importantly, this model also alleviates a mass balance problem, because it predicts a hybrid mantle source with variably higher SiO2 and Al2O3 than primitive mantle, and, contrary to a primitive mantle source, is able to reconcile compositions of komatiites and complementary cratonic mantle residues.  相似文献   

9.
High-Mg# peridotite xenoliths in the Cenozoic Hebi basalts from the North China Craton have refractory mineral compositions (Fo > 91.5) and highly heterogeneous Sr–Nd isotopic compositions (87Sr/86Sr = 0.7031–0.7048, 143Nd/144Nd = 0.5130–0.5118) ranging from MORB-like to EM1-type mantle, which are similar to those of peridotites from Archean cratons. Thus, the high-Mg# peridotites may represent relics of the ancient lithospheric mantle. Published Re–Os isotopic data for Cenozoic basalt-borne xenoliths show TRD ages of 3.0–1.5 Ga for the peridotites from Hebi (the center of the craton), 2.2–0 Ga for those from Hannuoba and Jining (north margin of the craton), and 2.6–0 Ga for those from Fanshi and Yangyuan (midway between the center and north margin of the craton). In situ Re–Os data of sulfides in Hannuoba peridotites suggest that whole-rock Re–Os model ages represent mixtures of multiple generations of sulfides with varying Os isotopic compositions. These observations indicate that initial lithospheric mantle beneath the Central Zone of the North China Craton formed during the Archean and was refertilized by multiple melt additions after its formation. The refertilization became more intensive from the interior to the margin of the craton, leading to the high heterogeneity of the lithospheric mantle: more ancient and refractory peridotites with highly variable Sr–Nd isotopic compositions in the interior, and more young and fertile peridotites with depleted Sr–Nd isotopic composition in the margin. Our data, coupled with published petrological and geochemical data of peridotites from the Central Zone of the North China Craton, suggest that the lithospheric mantle beneath this region is highly heterogeneous, likely produced by refertilization of Archean mantle via multiple additions of melts/fluids, which were closely related to the Paleoproterozoic collision between the Eastern and the Western Blocks and subsequent circum-craton subduction events.  相似文献   

10.
位于安徽省境内的女山新生代碱性玄武岩中含有大量而且类型丰富的地幔橄榄岩包体,主要类型有尖晶石相、石榴石相、尖晶石-石榴子石过渡相二辉橄榄岩以及少量的方辉橄榄岩,其中部分尖晶石二辉橄榄岩样品中出现富含挥发分的角闪石、金云母和磷灰石。本文选择该区的尖晶石二辉橄榄岩和方辉橄榄岩包体进行了较为详细的岩石学、矿物学、地球化学研究工作。结果显示,除2个方辉橄榄岩表现难熔特征外,其它25件尖晶石相二辉橄榄岩均具有饱满的主量元素组成。二辉橄榄岩样品的Sr-Nd-Hf同位素均表现为亏损地幔的性质,不同于古老克拉通型难熔、富集的岩石圈地幔。富含挥发份交代矿物的出现以及轻稀土元素不同程度的富集,表明女山岩石圈地幔经历了较为强烈的交代作用,然而Re-Os同位素及PGE分析结果表明交代作用并没有显著改变Os同位素组成。二辉橄榄岩样品均具有较高的Os同位素组成,结合其饱满的主量元素组成,亏损的同位素特征,表明女山地区岩石圈地幔整体为新生岩石圈地幔。但1个方辉橄榄岩样品给出了较低的Os同位素比值0.1184,其Re亏损年龄为1.5Ga,它可能来自于软流圈中残留的古老难熔地幔。  相似文献   

11.
本文对马关地区新生代碱性玄武岩中的地幔包体进行了系统的岩石学和地球化学研究,并首次进行了包体的Re-Os同位素测试。马关地区的橄榄岩包体主量成分上表现为饱满肥沃的特征;具有不同程度的轻稀土亏损特征,亏损Nb、Ti和Zr等高场强元素(HFSE)以及Ba等大离子亲石元素(LILE);橄榄岩包体的Nd同位素特征表明橄榄岩包体代表的是不均一的亏损地幔。5个橄榄岩全岩样品的Re-Os同位素分析结果表明,样品的Os含量总体较高(3.29×10-9~3.78×10-9),接近于造山带橄榄岩体的Os含量,Re含量变化范围较大(0.24×10-9~0.54×10-9),与Re的迁移能力较强有关。样品的187Os/188Os值在0.12295~0.12530之间变化,与187Re/188Os值和Al2O3含量之间都不存在较好的相关性,说明Re-Os体系不单纯由熔体抽取过程所控制。橄榄岩包体的Re亏损年龄tRD为254~604Ma,说明马关地区岩石圈地幔形成的时代应该在新元古代之前。马关地区岩石圈地幔并非是由软流圈上涌新增生的地幔,而是经历了如下演化历史:在新元古代之前,由原始地幔的部分熔融和熔体抽取作用形成了岩石圈地幔,之后经历了熔/流体交代和改造而发生了再富集作用,导致部分地幔橄榄岩逐渐从亏损难熔的特征向饱满肥沃转变,而未遭受熔/流体的改造的橄榄岩仍然保持了难熔亏损的特征。这种熔/流体交代和改造作用很可能与晚二叠纪峨眉山地幔柱的活动有关,而新生代以来印度-亚洲大陆碰撞导致地幔物质向东南方向的侧向流动,诱发软流圈上涌和马关地区的钾质岩浆的活动,也对马关地区岩石圈地幔的改造具有重要的影响,但由于喷发时间较新对Os同位素组成的影响还未显现出来。  相似文献   

12.
The Raobazhai ultramafic massif of the ultrahigh pressure Sulu–Dabie orogenic belt, central China, is thought to be a segment of subcontinental lithospheric mantle that was subducted and exhumed during the Triassic collision of the North China and Yangtze cratons. We performed a Re–Os isotopic study of peridotites from the massif, associated with major and trace element analysis and textural examination. Os (1.02 to 6.28 ppb) and Re (0.004 to 0.376 ppb) concentrations are typical of orogenic lherzolite values, and 187Os/188Os ratios (0.1157 to 0.1283) are all similar to or lower than the proposed primitive upper mantle value. 187Os/188Os is roughly correlated with 187Re/188Os, and strongly correlated with Al2O3. These correlations can be explained by radiogenic ingrowth of 187Os since an ancient partial melting event. TMA model ages (1.7 to 2.0 Ga) of refractory peridotites from the lower massif are consistent with the model age (1.8 Ga) obtained from the 187Os/188Os vs. Al2O3 correlation at ~1% Al2O3. This age cannot distinguish the cratonic provenance of the Raobazhai massif, since similar Re–Os model ages have been obtained from both the North China and the Yangtze cratons. The poor quality of the 187Os/188Os vs. 187Re/188Os correlation indicates that the Re/Os ratios were disturbed, perhaps during Triassic subduction. The mainly lherzolitic samples of the upper massif, which were most strongly affected by this process, have porphyroclastic textures with fine-grained olivine, pyroxene and amphibole neoblasts, suggesting Re mobility during recrystallization in the presence of fluids.Previous studies of ultramafic xenoliths from arc volcanics demonstrate that slab-derived melts or fluids can both scavenge mantle Os and add substantial amounts of radiogenic Os to the suprasubduction mantle. In Raobazhai, both trace element patterns and the abundance of hydrous phases provide evidence for extensive interaction with fluids during subduction and/or exhumation. Nevertheless, the strong correlation between 187Os/188Os and Al2O3, and the high Os concentrations of these rocks indicate that Os isotopic ratios, and probably even Os concentrations, were essentially unaffected by this process. Assuming that the arguments favoring a suprasubduction setting for the Raobazhai massif are valid, these data provide evidence that Os systematics are sometimes surprisingly robust, even above subduction zones.  相似文献   

13.
We present major element and PGE (platinum-group-element) abundances in addition to Re–Os isotope data for 11 spinel-facies whole rock peridotites from a single maar from the Middle Atlas Mountains, Morocco.Major element systematics of these xenoliths are generally correlated with indices of depletion. FeO–MgO systematics appear to suggest spinel-facies melting in the range of 5 to 25%. However, Al2O3 abundances in these xenoliths appear elevated relative to primitive mantle (Prima). The Al2O3 abundances in conjunction with other major elements require distinct re-enrichment of the Middle Atlas continental mantle root due to melt/rock reaction and precipitation of amphibole and/or clinopyroxene from passing silicate melts akin to MORB or OIB that evolved in reverse direction along the melting curves in e.g. FeO–MgO space. Sc and V confirm the range of apparent depletion and also indicate that the currently preserved fO2 in these peridotites is distinctly different from fO2 conditions observed in subduction zones.The majority of these xenoliths have low Os and Ir (I-PGEs) concentrations relative to Prima and modelled sulphide- and clinopyroxene-depleted residues of mantle melting under low fO2, mid-ocean ridge-like conditions. Moreover, Pt and Pd (P-PGE) abundances are elevated when compared to their expected abundances after substantial melt extraction. Importantly, the systematically low Ir abundances in the majority of samples show well-correlated trends with Al2O3, MgO and Cu that are inconsistent with established melting trends. Os isotopes in the Middle Atlas xenoliths range from 187Os/188Os = 0.11604 to 0.12664 although most samples are close to chondritic. The Os isotope ratios are decoupled from 187Re/188Os but, together with Re abundances, also exhibit a good correlation with Al2O3, MgO and Cu.The major element, I-PGE and Os isotope correlations suggest that the initial melt depletion led to the exhaustion of sulphide and clinopyroxene (20 to 30%) without significant stabilization of I-PGE-rich alloys. During later modal metasomatism of the refractory Middle Atlas continental mantle root with silicate melts akin to MORB or OIB the introduction of clinopyroxene/amphibole reduced the volume of the melt inducing sulphur saturation in these melts causing precipitation of secondary sulphides. This coupled crystallization of pyroxenes and sulphides (chalcopyrite) resulted in the two-component mixing systematics exhibited by I-PGEs, Os isotopes with major elements and Cu preserved in the Middle Atlas continental mantle root.  相似文献   

14.
The Zedong ophiolites in the eastern Yarlung–Zangbo suture zone of Tibet represent a mantle slice of more than 45 km~2. This massif consists mainly of mantle peridotites, with lesser gabbros, diabases and volcanic rocks. The mantle peridotites are mostly harzburgite, lherzolite; a few dike-like bodies of dunite are also present. Mineral structures show that the peridotites experienced plastic deformation and partial melting. Olivine(Fo89.7–91.2), orthopyroxene(En_(88–92)), clinopyroxene(En_(45–49) Wo_(47–51) Fs_(2–4)) and spinel [Mg~#=100×Mg/(Mg+Fe)]=49.1–70.7; Cr~#=(100×Cr/(Cr+Al)=18.8–76.5] are the major minerals. The degree of partial melting of mantle peridotites is 10%–40%, indicating that the Zedong mantle peridotites may experience a multi–stage process. The peridotites are characterized by depleted major element compositions and low REE content(0.08–0.62 ppm). Their "spoon–shaped" primitive–mantle normalized REE patterns with(La/Sm)_N being 0.50–6.00 indicate that the Zedong ultramafic rocks belong to depleted residual mantle rocks. The PGE content of Zedong peridotites(18.19–50.74 ppb) is similar with primary mantle with Pd/Ir being 0.54–0.60 and Pt/Pd being 1.09–1.66. The Zedong peridotites have variable, unradiogenic Os isotopic compositions with ~(187)Os/~(188)Os=0.1228 to 0.1282. A corollary to this interpretation is that the convecting upper mantle is heterogeneous in Os isotopes. All data of the Zedong peridotites suggest that they formed originally at a mid-ocean ridge(MOR) and were later modified in supra–subduction zone(SSZ) environment.  相似文献   

15.
Podiform chromite deposits occur in the mantle sequences of many ophiolites that were formed in supra-subduction zone (SSZ) settings. We have measured the Re-Os isotopic compositions of the major chromite deposits and associated mantle peridotites of the Dongqiao Ophiolite in the Bangong-Nujiang suture, Tibet, to investigate the petrogenesis of these rocks and their genetic relationships.The 187Os/188Os ratios of the chromite separates define a narrow range from 0.12318 to 0.12354, less variable than those of the associated peridotites. Previously-reported 187Os/188Os ratios of the Os-rich alloys enclosed in the chromitites define two clusters: 0.12645 ± 0.00004 (2 s; n = 145) and 0.12003 to 0.12194. The ultra-depleted dunites have much lower 187Os/188Os (0.11754, 0.11815), and the harzburgites show a wider range from 0.12107 to 0.12612. The average isotopic composition of the chromitites (187Os/188Os: 0.12337 ± 0.00001) is low compared with the carbonaceous chondrite value (187Os/188Os: 0.1260 ± 0.0013) and lower than the average value measured for podiform chromitites worldwide (0.12809 ± 0.00085). In contrast, the basalts have higher 187Os/188Os, ranging from 0.20414 to 0.38067, while the plagioclase-bearing harzburgite and cumulates show intermediate values of 187Os/188Os (0.12979 ~ 0.14206). Correspondingly, the basalts have the highest 187Re/188Os ratios, up to 45.4 ± 3.2, and the chromites have the lowest 187Re/188Os ratios, down to 0.00113 ± 0.00008. We suggest that melts/fluids, derived from the subducting slab, triggered partial melting in the overlying mantle wedge and added significant amounts of radiogenic Os to the peridotites. Mass-balance calculations indicate that a melt/mantle ratio of approximately 15:1 (melt: 187Re/188Os: 45.4, 187Os/188Os: 0.34484; mantle peridotite: 187Re/188Os: 0.0029, 187Os/188Os: 0.11754) is necessary to increase the Os isotopic composition of the chromitite deposits to its observed average value. This value implies a surprisingly low average melt/mantle ratio during the formation of the chromitite deposits. The percolating melts probably were of variable isotopic composition. However, in the chromitite pods the Os from many melts was pooled and homogenized, which is why the chromitite deposits show such a small variation in their Os isotopic composition. The results of this study suggest that the 187Os/188Os ratios of chromitites may not be representative of the DMM, but only reflect an upper limit. Importantly, the Os-isotope compositions of chromitites strongly suggest that such deposits can be formed by melt/mantle mixing processes.  相似文献   

16.
This study focuses on the origin of the Os isotope heterogeneities and the behaviour of Os and Re during melt percolation and partial melting processes in the mantle sequence of the Troodos Ophiolite Complex. The sequence has been divided into an eastern (Unit 1) and a western part (Unit 2) (Batanova and Sobolev, 2000). Unit 1 consists mainly of spinel-lherzolites and a minor amount of dunites, which are surrounded by cpx-bearing harzburgites. Unit 2 consists of harzburgites, dunites, and contains chromitite deposits.Unit 1 (187Os/188Os: 0.1169 to 0.1366) and Unit 2 (187Os/188Os 0.1235 to 0.1546) peridotites both show large ranges in their Os isotopic composition. Most of the 187Os/188Os ratios of Unit 1 lherzolites and harzburgites are chondritic to subchondritic, and this can be explained by Re depletion during ancient partial melting and melt percolation events. The old Os isotope model ages (>800 Ma) of some peridotites in a young ophiolitic mantle show that ancient Os isotopic heterogeneities can survive in the Earth upper mantle. Most harzburgites and dunites of Unit 2 have suprachondritic 187Os/188Os ratios. This is the result of the addition of radiogenic Os during a younger major melt percolation event, which probably occurred during the formation of the Troodos crust 90 Ma ago.Osmium concentrations tend to decrease from spinel-lherzolites (4.35 ± 0.2 ng/g) to harzburgites (Unit 1: 4.06 ± 1.12 ng/g; Unit 2: 3.46 ± 1.38 ng/g) and dunites (Unit 1: 2.71 ± 0.84 ng/g; Unit 2: 1.85 ± 1.20 ng/g). Therefore, this element does not behave compatibly during melt percolation as it is observed during partial melting, but becomes dissolved and mobilized by the percolating melt. The Os contents and Re/Os ratios in the mantle peridotites can be explained if they represent mixing products of old depleted mantle with cpx- and opx-veins, which are crystallization products of the percolating melt. This mixing occurred during the melting of a continuously fluxed mantle in a supra-subduction zone environment.This study shows that Unit 1 and Unit 2 of the Troodos mantle section have a complex and different evolution. However, the Os isotopic characteristics are consistent with a model where the harzburgites and dunites of both units belong to the same melting regime producing the Troodos oceanic crust.  相似文献   

17.
New major- and trace-element data of bulk-rocks and constituent minerals, and whole-rock Re-Os isotopic compositions of samples from the Lherz Massif, French Pyrenees, reveal complex petrological relationships between the dominant lithologies of lherzolite ± olivine-websterite and harzburgite. The Lherz peridotite body contains elongate, foliation parallel, lithological strips of harzburgite, lherzolite, and olivine-websterite cross-cut by later veins of hornblende-bearing pyroxenites. Peridotite lithologies are markedly bimodal, with a clear compositional gap between harzburgites and lherzolites ± olivine-websterite. Bulk-rock and mineral major-element oxide (Mg-Fe-Si-Cr) compositions show that harzburgites are highly-depleted and result from ∼20-25 wt.% melt extraction at pressures <2 GPa. Incompatible and moderately-compatible trace-element abundances of hornblendite-free harzburgites are analogous to some mantle-wedge peridotites. In contrast, lherzolites ± olivine-websterite overlap estimates of primitive mantle composition, yet these materials are composite samples that represent physical mixtures of residual lherzolites and clinopyroxene dominated cumulates equilibrated with a LREE-enriched tholeiitic melt. Trace-element compositions of harzburgite, and some lherzolite bulk-rocks and pyroxenes have been modified by; (1) wide-spread interaction with a low-volume LREE-enriched melt +/− fluid that has disturbed highly-incompatible elements (e.g., LREEs, Zr) without enrichment of alkali- and Ti-contents; and (2) intrusion of relatively recent, small-volume, hornblendite-forming, basanitic melts linked to modal and cryptic metasomatism resulting in whole-rock and pyroxene Ti, Na and MREE enrichment.Rhenium-Os isotope systematics of Lherz samples are also compositionally bimodal; lherzolites ± olivine-websterite have chondritc to suprachondritic 187Os/188Os and 187Re/188Os values that overlap the range reported for Earth’s primitive upper mantle, whereas harzburgites have sub-chondritic 187Os/188Os and 187Re/188Os values. Various Os-model age calculations indicate that harzburgites, lherzolites, and olivine-websterites have been isolated from convective homogenisation since the Meso-Proterozoic and this broadly coincides with the time of melt extraction controlled by harzburgite Os-isotope compositions. The association between harzburgites resulting from melting in mantle-wedge environments and Os-rich trace-phases (laurite-erlichmanite sulphides and Pt-Os-Ir-alloys) suggests that a significant portion of persistent refractory anomalies in the present-day convecting mantle of Earth may be linked to ancient large-scale melting events related to wide-spread subduction-zone processing.  相似文献   

18.
Initial 187Os/188Os isotopic compositions for geochronologically and geologically well -constrained 3.8-Ga spinel peridotites from the Itsaq Gneiss Complex of southern West Greenland and chromite separates from 3.46-Ga komatiites from the Pilbara region of Western Australia have been determined to investigate the osmium isotopic evolution of the early terrestrial mantle. The measured compositions of 187Os/188Os(0) = 0.10262 ± 2, from an olivine separate, and 0.10329 ± 3, for a spinel separate from ∼3.8-Ga peridotite G93/42, are the lowest yet reported from any terrestrial sample. The corrections for in situ decay over 3.8 Ga for these low Re/Os phases are minimal and change the isotopic compositions by only 0.5 and 2.2% for the spinel and the olivine, respectively, resulting in 187Os/188Os(3.8 Ga) = 0.1021 ± 0.0002 and 0.1009 ± 0.0002, respectively. These data extend direct measurement of Os isotopic compositions to much earlier periods of Earth history than previously documented and provide the best constraints on the Os isotopic composition of the early Archean terrestrial mantle. Analyses of Pilbara chromites yield 3.46-Ga mantle compositions of 0.1042 ± 0.0002 and 0.1051 ± 0.0002.These new data, combined with published initial Os isotopic compositions from late Archean and early Proterozoic samples, are compatible with the mantle, or at least portions of it, evolving from a solar system initially defined by meteorites to a modern composition of 187Os/188Os(0) = 0.1296 ± 0.0008 as previously suggested from peridotite xenolith data ( Meisel et al., 2001); the associated 187Re/188Os(0) = 0.435 ± 0.005. Thus, chondritic 187Os/188Os compositions were a feature of the upper mantle for at least 3.8 billion years, requiring chondritic Re/Os ratios to have been a characteristic of the very early terrestrial mantle. In contrast, nonchondritic initial compositions of some Archean komatiites demonstrate that Os isotopic heterogeneity is an ancient feature of plume materials, reflecting the development of variable Re/Os mantle sources early in Earth history.The lower average 187Os/188Os = 0.1247 for abyssal peridotites (Snow and Reisberg, 1995) indicate that not all regions of the modern mantle have evolved with the same Re/Os ratio. The relative sizes of the various reservoirs are unknown, although mass balance considerations can provide some general constraints. For example, if the unradiogenic 187Os/188Os modern abyssal peridotite compositions reflect the prevalent upper mantle composition, then the complementary high Re/Os basaltic reservoir must represent 20 to 40% by mass of the upper mantle (taken here as 50% of the entire mantle), depending on the mean storage age. The difficulties associated with efficient long-term storage of such large volumes of subducted basalt suggest that the majority of the upper mantle is not significantly Re-depleted. Rather, abyssal peridotites sample anomalous mantle regions.The existence of 3.8-Ga mantle peridotites with chondritic 187Os/188Os compositions and with Os concentrations similar to the mean abundances measured in modern peridotites places an upper limit on the timing of a late accretionary veneer. These observations require that any highly siderophile element -rich component must have been added to the Earth and transported into and grossly homogenized within the mantle by 3.8 Ga. Either large-scale mixing of impact materials occurred on very short (0-100 myr) timescales or (the interpretation preferred here) the late veneer of highly siderophile elements is unrelated to the lunar terminal cataclysm estimated to have occurred at ∼3.8 to 3.9 Ga.  相似文献   

19.
The effects of mantle metasomatism on the sulfide phase in mantle xenoliths in general, and on the Os isotopic system in particular, have received increased attention in recent years. Here, we report on Os isotopic systematics of metasomatized mantle xenoliths from the late Quaternary Eifel (Dreiser Weiher and Meerfelder Maar) and neighboring Vogelsberg volcanic fields, which provide insight into the effects of melt extraction and metasomatism on Os isotopes and place constraints on the evolution of the lithospheric mantle component beneath central Europe. Sixteen harzburgite, lherzolite, and pyroxenite xenoliths from the Eifel and two lherzolite xenoliths from the Vogelsberg were analyzed for Os isotopes. Samples from the anhydrous peridotite suite (Ib) are highly variable in their Os isotopes, ranging from subchondritic values (187Os/188Os=0.1236) to suprachondritic values (187Os/188Os=0.1420), indicating that some of these samples have been overprinted by the addition of radiogenic Os and have lost the primary mantle Os that was presumably present. The suprachondritic values suggest a source for this Os in a reservoir with a time-integrated Re/Os ratio greater than that of the bulk Earth. Eifel samples with Os contents >1.5 ng/g from the hydrous suite (Ia) have relatively unradiogenic Os isotope compositions (187Os/188Os=0.1208-0.1237) and Al2O3-Os isotopic systematics consistent with ancient melt depletion and isolation from the convecting asthenospheric mantle for time periods similar to the age of the overlying crust (~1.5 Ga) as well with results from peridotite massifs in the European region. The LREE-metasomatism and the enrichment of Os (up to 6.47 ng/g) and As (sulfide metasomatism?) in the hydrous suite is strongly inversely correlated with the Os isotope ratios, demonstrating that mantle processes such as metasomatism can significantly modify the Os isotope chemistry of mantle xenoliths.  相似文献   

20.
The Qinling Orogenic Belt was formed by subduction and collision between the North and South China Blocks along the Shangdan suture. The Songshugou ultramafic massif located on the northern side of the Shangdan suture provides essential insights into the mantle origin and evolutionary processes during spreading and subduction of the Shangdan oceanic lithosphere. The ultramafic massif comprises harzburgite, coarse- and fine-grained dunites. The spinels from harzburgite exhibit low Cr# and high Mg# numbers, suggesting a mid-ocean ridge peridotite origin, whereas spinels from both coarse- and fine-grained dunites are indicated as resulted from melt-rock reaction due to their systematic higher Cr# and low Mg# numbers. This melt-rock reaction in the dunites is also indicated by the low TiO2 (mostly <0.4 wt%) in the spinel and high Fo (90–92) in olivines. Due to its relatively homogeneous nature in the mantle, oxygen isotopic composition is a sensitive indicator for the petrogenesis and tectonic setting of the Songshugou ultramafic rocks. Based on in-situ oxygen isotope analyses of olivines from twenty-six rock samples, most harzburgites from the Songshugou ultramafic massif show low δ18O values of 4.54–5.30‰, suggesting the olivines are equilibrium with N-MORB magmas and originally formed in a mid-ocean ridge setting. The coarse- and fine-grained dunites exhibit slightly higher olivine δ18O values of 4.69–6.00‰ and 5.00–6.11‰, respectively, suggesting they may have been modified by subduction-related boninitic melt-rock reaction. The δ18O values of olivines systematically increasing from the harzburgites, to coarse-grained dunites and fine-grained dunites may suggest enhancing of melt-rock reaction. The decreasing of Os concentration, 187Re/188Os and 187Os/188Os ratios from harzburgite to dunite suggest an 187Os-enriched, subduction zone melt was responsible for creating the melt channel for melt-rock reactions. Together with the high-temperature ductile deformation microstructures, these isotopic and mineral geochemical features suggest that the harzburgites represent mantle residues after partial melting at mid-ocean ridge or supra-subduction zone, while the dunites were probably resulted from reactions between boninitic melt and harzburgites in a supra-subduction zone. Re-Os geochronology yields a maximum Re depletion model age (TRD) of 805 Ma, constraining the minimum formation age of the harzburgites derived from oceanic mantle. Eight samples of whole rock and chromite yield a Re-Os isochron age of 500 ± 120 Ma, constraining the timing of melt-rock reactions. Combined with the regional geology and our previous investigations, the Songshugou ultramafic rocks favors a mantle origin at mid-ocean ridge before 805 Ma, and were modified by boninitic melt percolations in a SSZ setting at ca. 500 Ma. This long-term tectonic process from spreading to subduction might imply a huge Pan-Tethyan ocean between the Laurasia (e.g., North China Block) and Gondwana (e.g., South China Block) and/or a one-side subduction.  相似文献   

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