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1.
Tectonically emplaced peridotites from North Hebei Province, North China Craton, have retained an original harzburgite mineral assemblage of olivine(54%–58%) + orthopyroxene(40%–46%)+minor clinopyroxene(1%)+spinel. Samples with boninite-like chemical compositions also coexist with these peridotites. The spinels within the peridotites have high-Al end-members with Al_2O_3 content of 30 wt % –50 wt %, typical of mantle spinels. When compared with experimentally determined melt extraction trajectories, the harzburgites display a high degree of melting and enrichment of SiO_2, which is typical of cratonic mantle peridotites. The peridotites display variably enriched light rare earth elements(REEs), relatively depleted middle REEs and weakly fractionated heavy REEs, which suggest a melt extraction of over 25% in the spinel stability field. The occurrence of arc-and SSZ-type chromian spinels in the peridotites suggests that melt extraction and metasomatism occurred mostly in a subduction-related setting. This is also supported by the geochemical data of the coexisting boninite-like samples. The peridotites have ~(187)Os/~(188)Os ratios ranging from 0.113–0.122, which is typical of cratonic lithospheric mantle. These ~(187)Os/~(188)Os ratios yield model melt extraction ages(TRD) ranging from 981 Ma to 2054 Ma, which may represent the minimum estimation of the melt extraction age. The Al_2O_3-~(187)Os/~(188)Os-proxy isochron ages of 2.4 Ga–2.7 Ga suggest a mantle melt depletion age between the Late Achaean and Early Paleoproterozoic. Both the peridotites and boninite-like rocks are therefore interpreted as tectonically exhumed continental lithospheric mantle of the North China Craton, which has experienced mantle melt depletion and subduction-related mantle metasomatism during the Neoarchean-Paleoproterozoic.  相似文献   

2.
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   

3.
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   

4.
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   

5.
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   

6.
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   

7.
《Gondwana Research》2015,28(4):1560-1573
We used Os isotopic systematics to assess the geochemical relationship between the lithospheric mantle beneath the Balkans (Mediterranean), ophiolitic peridotites and lavas derived from the lithospheric mantle. In our holistic approach we studied samples of Tertiary post-collisional ultrapotassic lavas sourced within the lithospheric mantle, placer Pt alloys from Vardar ophiolites, peridotites from nearby Othris ophiolites, as well as four mantle xenoliths representative for the composition of the local mantle lithosphere. Our ultimate aim was to monitor lithospheric mantle evolution under the Balkan part of the Alpine-Himalayan belt. The observations made on Os isotope and highly siderophile element (HSE) distributions were complemented with major and trace element data from whole rocks as well as minerals of representative samples. Our starting hypothesis was that the parts of the lithospheric mantle under the Balkans originated by accretion and transformation of oceanic lithosphere similar to ophiolites that crop out at the surface.Both ophiolitic peridotites and lithospheric mantle of the Balkan sector of Alpine-Himalayan belt indicate a presence of a highly depleted mantle component. In the ophiolites and the mantle xenoliths, this component is fingerprinted by the low clinopyroxene (Cpx) contents, low Al2O3 in major mantle minerals, together with a high Cr content in cogenetic Cr-spinel. Lithospheric mantle-derived ultrapotassic melts have high-Fo olivine and Cr-rich spinel that also indicate an ultra-depleted component in their mantle source. Further resemblance is seen in the Os isotopic variation observed in ophiolites and in the Serbian lithospheric mantle. In both mantle types we observed an unusual increase of Os abundances with increase in radiogenic Os that we interpreted as fluid-induced enrichment of a depleted Proterozoic/Archaean precursor. The enriched component had suprachondritic Os isotopic composition and its ultimate source is attributed to the subducting oceanic slab. On the other hand, a source–melt kinship is established between heterogeneously metasomatised lithospheric mantle and lamproitic lavas through a complex vein + wall rock melting relationship, in which the phlogopite-bearing pyroxenitic metasomes with high 187Re/188Os and extremely radiogenic 187Os/188Os > 0.3 are produced by recycling of a component ultimately derived from the continental crust.We tentatively propose a two-stage process connecting lithospheric mantle with ophiolites and lamproites in a geologically reasonable scenario: i) ancient depleted mantle “rafts” representing fragments of lithospheric mantle “recycled” within the convecting mantle during the early stages of the opening of the Tethys ocean and further refertilized, were enriched by a component with suprachondritic Os isotopic compositions in a supra-subduction oceanic environment, probably during subduction initiation that induced ophiolite emplacement in Jurassic times. Fluid-induced partial melts or fluids derived from oceanic crust enriched these peridotites in radiogenic Os; ii) the second stage represents recycling of the melange material that hosts above mantle blocks, but also a continental crust-derived terrigenous component accreted to the mantle wedge, that will later react with each other, producing heterogeneously distributed metasomes; final activation of these metasomes in Tertiary connects the veined lithospheric mantle and lamproites by vein + wall rock partial melting to generate lamproitic melts. Our data are permissive of the view that the part of the lithospheric mantle under the Balkans was formed in an oceanic environment.  相似文献   

8.
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   

9.
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   

10.
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   

11.
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   

12.
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   

13.
R.N. Thompson 《Lithos》1975,8(1):9-14
An intensely-welded ignimbrite has been identified in the upper part of the thick Palaeocene lava pile of Ubekendt Ejland, west Greenland. It consists of 2.3 volume present of phenocrysts and 2 percent of lithic fragments (basalt and trachyte) in a compact nitreous matrix of flattened and welded rhyolitic pumice fragments and glass shards. The phenocrysts are mostly of plagioclase (An17), together with minor amounts of sanidine (Or87), hydrothermally-altered olivine and augite, ilmenite and zircon. The major elements chemistry of the magmatic fraction of the rock is deduced from microprobe analyses of its constituent phases. It is suggested that this erupted pyroclastic rock may be a surface expression of high-level granite emplacement in southern Ubekendt Ejland. The source could have been an early acid member of the currently-exposed Sarqâta qáqâ plutonic complex. Recent Rb/Sr isotopic studies have shown that the acid and basic rocks of this intrusive centre, together with the surrounding basaltic lavas, define a single isochron (65 m.y.) with an initial 87Sr/86Sr ratio of 0.7045, appropriate to the upper mantle. The ultimate source of the Ubekendt Ejland ignimbr.es may therefore be sub-crustal.  相似文献   

14.
Pyroxenitic layers are a minor constituent of ultramafic mantle massifs, but are considered important for basalt generation and mantle refertilization. Mafic spinel websterite and garnet-spinel clinopyroxenite layers within Jurassic ocean floor peridotites from the Totalp ultramafic massif (eastern Swiss Alps) were analyzed for their highly siderophile element (HSE) and Os isotope composition.Aluminum-poor pyroxenites (websterites) display chondritic to suprachondritic initial γOs (160 Ma) of −2 to +27. Osmium, Ir and Ru abundances are depleted in websterites relative to the associated peridotites and to mantle lherzolites worldwide, but relative abundances (Os/Ir, Ru/Ir) are similar. Conversely, Pt/Ir, Pd/Ir and Re/Ir are elevated.Aluminum-rich pyroxenites (clinopyroxenites) are characterized by highly radiogenic 187Os/188Os with initial γOs (160 Ma) between +20 and +1700. Their HSE composition is similar to that of basalts, as they are more depleted in Os, Ir and Ru compared to Totalp websterites, along with even higher Pt/Ir, Pd/Ir and Re/Ir. The data are most consistent with multiple episodes of reaction of mafic pyroxenite precursor melts with surrounding peridotites, with the highest degree of interaction recorded in the websterites, which typically occur in direct contact to peridotites. Clinopyroxenites, in contrast, represent melt-dominated systems, which retained the precursor melt characteristics to a large extent. The melts may have been derived from a sublithospheric mantle source with high Pd/Ir, Pt/Ir and Re/Os, coupled with highly radiogenic 187Os/188Os compositions. Modeling indicates that partial melting of subducted, old oceanic crust in the asthenosphere could be a possible source for such melts.Pentlandite and godlevskite are identified in both types of pyroxenites as the predominant sulfide minerals and HSE carriers. Heterogeneous HSE abundances within these sulfide grains likely reflect subsolidus processes. In contrast, large grain-to-grain variations, and correlated variations of HSE ratios, indicate chemical disequilibrium under high-temperature conditions. This likely reflects multiple events of melt-rock interaction and sulfide precipitation. Notably, sulfides from the same thick section for the pyroxenites may display both residual-peridotite and melt-like HSE signatures. Because Totalp pyroxenites are enriched in Pt and Re, and depleted in Os, they will develop excess radiogenic 187Os and 186Os, compared to ambient mantle. These enrichments, however, do not possess the requisite Pt-Re-Os composition to account for the coupled suprachondritic 186Os-187Os signatures observed in some Hawaiian picrites, Gorgona komatiites, or the Siberian plume.  相似文献   

15.
Iron isotopes, together with mineral elemental compositions of spinel peridotite xenoliths and clinopyroxenites from Hannuoba and Hebi Cenozoic alkaline basalts, were analyzed to investigate iron isotopic features of the lithospheric mantle beneath the North China Craton. The results show that the Hannuoba spinel peridotite xenoliths have small but distinguishable Fe isotopic variations. Overall variations in δ57Fe are in a range of ?0.25 to 0.14‰ for olivine, ?0.17 to 0.17‰ for orthopyroxene, ?0.21 to 0.27‰ for clinopyroxene, and ?0.16 to 0.26‰ for spinel, respectively. Clinopyroxene has the heaviest iron isotopic ratio and olivine the lightest within individual sample. No clear linear relationships between the mineral pairs on “δ-δ” plot suggest that iron isotopes of mineral separates analyzed have been affected largely by some open system processes. The broadly negative correlations between mineral iron isotopes and metasomatic indexes such as spinel Cr#, (La/Yb)N ratios of clinopyroxenes suggest that iron isotopic variations in different minerals and peridotites were probably produced by mantle metasomatism. The Hebi phlogopite-bearing lherzolite, which is significantly modified by metasomatic events, appears to be much heavier isotopically than clinopyroxene-poor lherzolite. This study further confirms previous conclusions that the lithospheric mantle has distinguishable and heterogeneous iron isotopic variations at the xenoliths scale. Mantle metasomatism is the most likely cause for the iron isotope variations in mantle peridotites.  相似文献   

16.
Previous studies on iron isotope compositions of subduction zone magmas have revealed significant and complex variations that have great bearings on petrogenetic processes in the mantle wedge, e.g., partial melting, fluid metasomatism and redox state. However, interpretations for the fractionations are highly debatable and lack direct constraints from mantle wedge peridotites. This study presents iron isotope compositions for whole rocks and mineral separates in fresh forearc peridotites from the Yushigou ophiolite, North Qilian orogen in northern Tibet. Major and trace element compositions of whole rock and mineral indicate that the peridotites are highly depleted forearc peridotites overprinted by melt metasomatism, in contrast to the long‐holding opinion that the peridotites are derived from mid‐oceanic ridges. The minerals fall on a line with a slope of ～1 on the plot of δ56Fe vs. δ56Fe, indicating isotope equilibrium between minerals. δ56Fe fractionation between olivine and orthopyroxene is within the range of 0～0.05, while fractionation between olivine and spinel is about 0.05～0.10. The fractionation trend between olivine and spinel is opposite to previous theoretical and experimental constraints, which may be due to substantial Cr substitution into the spinel. This indicates that negative correlations between spinel Cr#, fO2 and spinel δ56Fe in previous studies are probably a reflection of gradual Cr enrichment in spinel during melt extraction, and spinel δ56Fe values are not a proxy for oxygen fugacity. Whole rock δ56Fe values are well correlated with mineral δ56Fe values, varying from overlapping with depleted mantle to slightly lower than depleted mantle. Therefore, variations in iron isotope compositions of subduction zone magmas are probably due to combined effect of source heterogeneity and partial melting fractionation.  相似文献   

17.
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   

18.
Rhenium (Re) is one of the least abundant elements in Earth, averaging 0.28 ppb in the primitive mantle. The unique occurrence of rheniite ReS2 (74.5 wt% of Re) in Kudryavy volcano precipitates raises questions about recycling of Re-rich reservoirs within the Kurile-Kamchatka volcanic Island arc setting. The sources of this unique Re enrichment have been inferred from studies of Re-Os isotope systematic and trace elements in volcanic gases, sulphide precipitates and host volcanic rocks. The fumarolic gas condensates are enriched in hydrophile trace elements relative to fluid-immobile elements and exhibit high Ba/Nb (133-204), Rb/Y (16-406) and Th/Zr (0.01-0.25) ratios. They are characterised by high Re (7-210 ppb) and Os abundances (0.4-0.9 ppb), with 187Os/188Os ratios in a range 0.122-0.152. This Os isotopic compositional range is similar to that of the peridotite xenoliths from the metasomatised mantle wedge above the subducted Pacific plate, the radiogenic isotopic signature of which is probably due to radiogenic addition from a slab-derived fluid.Re- and Os-rich sulphide and oxide minerals precipitate from volcanic gases within fumarolic fields. Molybdenite (MoS2), powellite (CaMoO4) and cannizzarite (Pb4Bi6S13) contain 1.5-1.7 wt%, 10 ppm, and 65-252 ppb of Re, respectively. Both molybdenite and rheniite contain normal Os concentrations, with total Os abundances in a range from 0.6 to 3.1 ppm for molybdenite, and 2.3-24.3 ppb for the rheniite samples. Repeated analyses of osmium isotope ratios for two rheniite samples form a best-fit line with an initial 187Os/188Os ratio of 0.32 ± 0.15 and an age of 79 ± 11 yr, which is the youngest age ever measured in natural samples. The high Re contents in molybdenite and rheniite led to high radiogenic 187Os values, even in the limited period of time, with 187Os/188Os ratios up to 3.3 for molybdenite and up to 4.4 for rheniite.The Os isotopic compositions of andesite-basaltic rocks from the Kudryavy volcano (187Os/188Os up to 0.326) are more radiogenic than those of residual peridotites and fumarolic gas condensates that are mainly constituted from magmatic vapor. Such radiogenic values can be attributed either to the addition of a radiogenic Os-rich subduction component to the depleted mantle, or to the assimilation of older dacitic caldera walls (187Os/188Os = 0.6) during arc magma ascent and emplacement. The latter hypothesis is supported by the correlation between 187Os/188Os ratio and indicators of fractionation such as MgO or Ni, and by low contents of potentially hydrophile trace elements such as Ba, Rb and Th relative to fluid-immobile elements such as Nb, Zr and Y. The high Re flux in the Kudryavy volcano (estimated at ∼46 kg/yr) can be explained by remobilisation of Re by Cl-rich water from an underplated mantle wedge and subducted organic-rich sediments of the Pacific plate.  相似文献   

19.
Iron isotopes, together with mineral elemental compositions of spinel peridotite xenoliths and clinopyroxenites from Hannuoba and Hebi Cenozoic alkaline basalts, were analyzed to investigate iron isotopic features of the lithospheric mantle beneath the North China Craton. The results show that the Hannuoba spinel peridotite xenoliths have small but distinguishable Fe isotopic variations. Overall variations in δ57Fe are in a range of −0.25 to 0.14‰ for olivine, −0.17 to 0.17‰ for orthopyroxene, −0.21 to 0.27‰ for clinopyroxene, and −0.16 to 0.26‰ for spinel, respectively. Clinopyroxene has the heaviest iron isotopic ratio and olivine the lightest within individual sample. No clear linear relationships between the mineral pairs on “δ-δ” plot suggest that iron isotopes of mineral separates analyzed have been affected largely by some open system processes. The broadly negative correlations between mineral iron isotopes and metasomatic indexes such as spinel Cr#, (La/Yb)N ratios of clinopyroxenes suggest that iron isotopic variations in different minerals and peridotites were probably produced by mantle metasomatism. The Hebi phlogopite-bearing lherzolite, which is significantly modified by metasomatic events, appears to be much heavier isotopically than clinopyroxene-poor lherzolite. This study further confirms previous conclusions that the lithospheric mantle has distinguishable and heterogeneous iron isotopic variations at the xenoliths scale. Mantle metasomatism is the most likely cause for the iron isotope variations in mantle peridotites.  相似文献   

20.
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   

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