Oxygen isotope heterogeneity of the mantle beneath the Canary Islands: insights from olivine phenocrysts

A relatively narrow range of oxygen isotopic ratios (?? ¹⁸O?=?5.0?C5.4??) is preserved in olivine of mantle xenoliths, mid-ocean ridge (MORB), and most ocean island basalts (OIB). The values in excess of this range are generally attributed either to the presence of a recycled component in the Earth??s mantle or to shallow level contamination processes. A viable way forward to trace source heterogeneity is to find a link between chemical (elemental and isotopic) composition of the earlier crystallized mineral phases (olivine) and the composition of their parental magmas, then using them to reconstruct the composition of source region. The Canary hotspot is one of a few that contains ~1- to 2-Ga-old recycled ocean crust that can be traced to the core-mantle boundary using seismic tomography and whose origin is attributed to the mixing of at least three main isotopically distinct mantle components i.e. HIMU, DMM, and EM. This work reports ion microprobe and single crystal laser fluorination oxygen isotope data of 148 olivine grains also analyzed for major and minor elements in the same spot. The olivines are from 20 samples resembling the most primitive shield stage picrite through alkali basalt to basanite series erupted on Gran Canaria, Tenerife, La Gomera, La Palma and El Hierro, Canary Islands, for which shallow level contamination processes were not recognized. A broad range of ?? ¹⁸O_olivine values from 4.6 to 6.1?? was obtained and explained by stable, long-term oxygen isotope heterogeneity of crystal cumulates present under different volcanoes. These cumulates are thought to have crystallized from mantle-derived magmas uncontaminated at crustal depth, representing oxygen isotope heterogeneity of source region. A relationship between Ni?×?FeO/MgO and ?? ¹⁸O_olivine values found in one basanitic lava erupted on El Hierro, the westernmost island of the Canary Archipelago, was used to estimate oxygen isotope compositions of partial melts presumably originated from peridotite (HIMU-type component inherited its radiogenic isotope composition from ancient, ~1 to 2?Ga, recycled ocean crust) and pyroxenite (young, <1?Ga, recycled oceanic crust preserved as eclogite with depleted MORB-type isotopic signature) components of the Canary plume. The model calculations yield 5.2 and 5.9?±?0.3?? for peridotite- and pyroxenite-derived melts, respectively, which appeared to correspond closely to the worldwide HIMU-type OIB and upper limit N-MORB ?? ¹⁸O values. This difference together with the broad range of ?? ¹⁸O variations found in the Canarian olivines cannot be explained by thermodynamic effects of oxygen isotopic fractionation and are believed to represent true variations in the mantle, due to oceanic crust and continental lithosphere recycling.     

Oxygen isotope heterogeneity of the mantle deduced from global 18O systematics of basalts from different geotectonic settings

Based upon a compilation and analysis of O-isotope data for Neogene volcanic rocks worldwide, the ¹⁸O variation for 743 basalts (historic lavas, submarine glasses, and lavas with <0.75 wt% H₂O) is +2.9 to +11.4. Mid-ocean-ridge basalt (MORB) has a uniform O-isotope composition with ¹⁸0=+5.7±0.2. Basalts erupted in different tectonic settings have mean ¹⁸O/¹⁶O ratios that are both lower and higher than MORB, with continental basalts enriched in ¹⁸O by ca. 1 over oceanic basalts. The ¹⁸O range for the subset of 88 basalts with Mg# [100·Mg(Mg+Fe²⁺)] 75–68, considered to be unmodified primary mantle partial melts, is +3.6 to +8.7. These features are a clear indication that: (1) the Earth's upper mantle is heterogeneous with respect to its O-isotope composition; (2) that both low-¹⁸O and high-¹⁸O reservoirs have contributed to basalt petrogenesis. Large-ion lithophile element-enriched basalts associated with subduction at convergent plate margins are slightly enriched in ¹⁸O, a characteristic that is considered to be an intrinsic feature of the subduction process. Intraplate oceanic and continental basalts have highly variable ¹⁸O/¹⁶O ratios, with individual localities displaying ¹⁸O ranges in excess of 1.5 to 2. Systematic co-variations between O-, Sr-, Nd-, and Pb-isotope ratios reflect the same principal intramantle end-member isotopic components (DMM, HIMU, EM-I, EM-II) deduced from radiogenic isotope considerations and, therefore, imply that a common process is responsible for the origin of upper mantle stable and radiogenic isotope heterogeneity, namely the recycling of lithospheric material into the mantle.     

Time-scales for magmatic differentiation at the Snaefellsjökull central volcano, western Iceland: Constraints from U-Th-Pa-Ra disequilibria in post-glacial lavas

We present major and trace element and Sr-Nd-Pb and U-Th-Pa-Ra isotope data for a small sample suite of primarily post-glacial, mildly alkalic volcanic rocks from the Snaefellsjökull central volcano situated off the main rift systems in western Iceland. The volcanic rocks are crystal-poor and range from olivine alkali basalt to trachyte and show tight correlations of major and trace elements that are explained by fractional crystallization involving removal of olivine, plagioclase, clinopyroxene, Fe-Ti oxide and apatite. Sr-Nd-Pb isotopes are practically invariant, consistent with derivation from the same source region. During fractionation from primitive basalt to evolved trachyte, (²³⁰Th/²³²Th), (²³⁰Th/²³⁸U) and (²³¹Pa/²³⁵U) decrease progressively at broadly constant (²³⁸U/²³²Th). A continuous closed-system fractionation model that assumes constant initial (²³⁰Th/²³²Th) in the basaltic precursor melt indicates that hawaiite was derived from olivine basalt by ∼50% fractional crystallization within and trachyte by ∼80% fractionation within . An overrepresentation of evolved basalts and hawaiites with young inferred magma ages in the dataset is consistent with the parental precursor to these magmas intruded into the sub-volcanic magma plumbing system as a consequence of lithospheric rebound caused by deglaciation. Lavas affected by apatite removal have higher (²³¹Pa/²³⁵U) than predicted for simple radioactive decay, suggesting apatite significantly fractionates U from Pa. The proposed fractionation model consistently explains our U-series data assuming and and . If applicable, these D values would indicate that the effect of apatite fractionation must be adequately considered when assessing differentiation time scales using (²³¹Pa/²³⁵U) disequilibria data.     

Oxygen isotope ratios of phenocrysts from alkali basalts of the Pannonian basin: Evidence for an O-isotopically homogeneous upper mantle beneath a subduction-influenced area

Oxygen isotope compositions of olivine and pyroxene phenocrysts and pyroxene and amphibole megacrysts from Neogene alkali basalts of the Pannonian basin (0.5–11 Ma) have been determined by laser fluorination. Measured δ¹⁸O values in olivine and clinopyroxene phenocrysts show rather restricted variations from 5.00 to 5.20‰ and from 5.07 to 5.34%., respectively, with cpx-ol fractionations Δ¹⁸O(cpx-ol) ranging from + 0.04 to + 0.29‰. These δ¹⁸O values are significantly lower than those of the corresponding whole rocks, suggesting that low temperature alteration has increased the ¹⁸O/¹⁶O ratios of the groundmass of host rocks, even in fresh looking samples, whereas their phenocrysts have retained original oxygen isotope compositions. The uniform oxygen isotope ratio in the phenocrysts suggests that the mantle source of the alkali basalts was also homogeneous with respect to its oxygen isotope composition, which is in contrast to the relatively wide variation of Sr, Nd and Pb isotope ratios in the source. Variations in radiogenic isotope compositions in the basalts have been explained by the interaction of subduction-related fluids with the mantle source of the basalts. If this is the case, then the fluids which caused significant changes in the Sr and Pb isotope ratios of the mantle source clearly did not noticeably modify its oxygen isotope composition. These data support the opinion that the upper mantle is more homogeneous with respect to its oxygen isotope composition than it was previously considered.     

辽东中—新生代玄武岩的橄榄石斑晶和捕虏晶氧同位素组成及其岩石圈地幔演化启示

华北克拉通东部岩石圈地幔性质在中—新生代时期发生了重大转变,但细节还不清楚。本文对辽东半岛早白垩世(克拉通破坏峰期)小岭组玄武岩和第四纪(克拉通破坏后)宽甸玄武岩中橄榄石斑晶/捕虏晶进行了主量元素和氧同位素组成研究。早白垩世小岭组玄武岩斑晶橄榄石Fo为79～88,CaO>0.1%,具有高Ni/Mg(0.4～1.2)、低Mn/Fe(1.3～1.6)和低Ca/Fe比值(0.2～1.5)的特征,指示岩浆源区是辉石岩和橄榄岩混合的岩石圈地幔;高于正常地幔橄榄石的δ¹⁸O(4.77‰～5.96‰,平均值5.4‰)显示低温热液蚀变洋壳熔体/流体组分对地幔源区的影响。第四纪宽甸玄武岩捕虏晶橄榄石Fo值为88～92(平均值90),具有高NiO(0.3%～0.4%)和低CaO(<0.1%)、MnO(0.1%～0.2%)含量,显示主体饱满、与少量过渡型和难熔型并存的地幔组成特征;其δ¹⁸O(4.58‰～5.38‰,平均值5.3‰)与正常地幔值接近。结合华北其他地区地幔橄榄石氧同位素数据,发现早白垩世破坏峰期有较多俯冲洋壳来源的熔/流体交代岩石圈地幔,...     

汉诺坝玄武岩Re-Os同位素地球化学——Re的挥发性丢失和壳-幔相互作用的证据

本文报道了采自汉诺坝玄武岩区周坝和白龙硐剖面以及白布洛张20井等地29个玄武岩样品的Re、Os含量和~(187)Os/~(188)Os比值。Os含量为11×10~(-12)～314×10~(-12),Re含量为40×10~(-12)～238×10~(-12),Re和Os含量有正相关趋势。碱性玄武岩(AK)的Re、Os含量高于拉斑玄武岩(TH)和过渡玄武岩(TR),玄武岩Os含量变化与分离结晶作用有关,玄武岩的低Re含量与地面喷发的火山岩浆脱气过程中Re的挥发性丢失作用有关。玄武岩的~(187)Os/~(188)Os比值为0.14735～0.61136,AK的~(187)Os/~(188)Os比值比TH和TR低且变化小。玄武岩的~(187)Os/~(188)Os比值与Os含量有负相关性。随着Os含量降低到小于75×10~(-12),~(187)Os/~(188)Os比值迅速升高,反映了地壳混染在TH和TR成因中的贡献。在以往的研究中没有观察到类似的地壳混染作用,说明了Re-Os同位素体系在示踪壳源物质上的优势。一些Os含量较高的TH的~(187)Os/~(188)Os比值表明其地幔源区既非亏损的又非经交代富集的SCLM,可能是混入了地壳俯冲物质的"Marble cake"型地幔。总之,汉诺坝玄武岩的Re-Os同位素地球化学研究支持了以往研究的主要成果,两类玄武岩地球化学差异性和异源成因论;分离结晶和部分熔融过程在玄武岩成因中的重要作用;碱性玄武岩的成因与地幔柱的关系等。同时揭示了一些新的现象:汉诺坝玄武岩形成中存在少量的地壳混染作用;地面喷发的火山熔岩在脱气过程中Re的挥发性丢失;拉斑玄武岩的源区更有可能为"Marble cake"型地幔。     

The origin and evolution of the parental magmas of frontal volcanoes in Kamchatka: Evidence from magmatic inclusions in olivine from Zhupanovsky volcano

The paper presents data on naturally quenched melt inclusions in olivine (Fo 69–84) from Late Pleistocene pyroclastic rocks of Zhupanovsky volcano in the frontal zone of the Eastern Volcanic Belt of Kamchatka. The composition of the melt inclusions provides insight into the latest crystallization stages (∼70% crystallization) of the parental melt (∼46.4 wt % SiO₂, ∼2.5 wt % H₂O, ∼0.3 wt % S), which proceeded at decompression and started at a depth of approximately 10 km from the surface. The crystallization temperature was estimated at 1100 ± 20°C at an oxygen fugacity of ΔFMQ = 0.9–1.7. The melts evolved due to the simultaneous crystallization of olivine, plagioclase, pyroxene, chromite, and magnetite (Ol: Pl: Cpx: (Crt-Mt) ∼ 13: 54: 24: 4) along the tholeiite evolutionary trend and became progressively enriched in FeO, SiO₂, Na₂O, and K₂O and depleted in MgO, CaO, and Al₂O₃. Melt crystallization was associated with the segregation of fluid rich in S-bearing compounds and, to a lesser extent, in H₂O and Cl. The primary melt of Zhupanovsky volcano (whose composition was estimated from data on the most primitive melt inclusions) had a composition of low-Si (∼45 wt % SiO₂) picrobasalt (∼14 wt % MgO), as is typical of parental melts in Kamchatka and other island arcs, and was different from MORB. This primary melt could be derived by ∼8% melting of mantle peridotite of composition close to the MORB source, under pressures of 1.5 ± 0.2 GPa and temperatures 20–30°C lower than the solidus temperature of “dry” peridotite (1230–1240°C). Melting was induced by the interaction of the hot peridotite with a hydrous component that was brought to the mantle from the subducted slab and was also responsible for the enrichment of the Zhupanovsky magmas in LREE, LILE, B, Cl, Th, U, and Pb. The hydrous component in the magma source of Zhupanovsky volcano was produced by the partial slab melting under water-saturated conditions at temperatures of 760–810°C and pressures of ∼3.5 GPa. As the depth of the subducted slab beneath Kamchatkan volcanoes varies from 100 to 125 km, the composition of the hydrous component drastically changes from relatively low-temperature H₂O-rich fluid to higher temperature H₂O-bearing melt. The geothermal gradient at the surface of the slab within the depth range of 100–125 km beneath Kamchatka was estimated at 4°C/km.     

Lithium isotope analysis of olivine by SIMS: Calibration of a matrix effect and application to magmatic phenocrysts

High-spatial resolution analysis of light element isotope variations by secondary-ion mass spectrometry (SIMS) has numerous applications in geochemistry and cosmochemistry. Recent attention has focused on ⁷Li/⁶Li variations in magmatic phenocrysts to infer the volatile degassing history of their parent magmas, and on minerals from mantle samples to determine source-region processes and the recycling history of mantle reservoirs. In these studies the effect of mineral composition on the ⁷Li/⁶Li ratio measured by SIMS has been considered secondary, and generally disregarded. We show, using a suite of nine olivines analyzed by MC-ICP-MS or TIMS, that there is a substantial effect of composition on the ⁷Li/⁶Li ratio of olivine measured by SIMS. For magnesian olivine (74 < Fo < 94) the effect is a linear function of composition, with δ⁷Li increasing by 1.3‰ for each mole percent decrease in forsterite component. At higher Fe contents, the relationship ceases to be linear. The composition range over which linear behavior is exhibited appears to depend on instrumental conditions. A calibration of this matrix effect over the linear range is presented, assuming the measurement of ⁷Li/⁶Li relative to an olivine standard of known composition. Application of this calibration to a suite of olivines separated from basaltic lavas from Ko'olau, Hawai'i demonstrates that the matrix effect is responsible for a geologically spurious correlation between δ⁷Li and Mg#. However, after correction, the olivines retain evidence of Li isotope heterogeneity, the degree and nature of which differs in each of the four separates studied. These results emphasize the importance of compositional correction for SIMS measurement of δ⁷Li in olivine, particularly in zoned crystals, and support previous conclusions that Li isotope variability in igneous materials is subject to late-stage disturbance. The significant matrix effect demonstrated for olivine suggests that matrix effects in other minerals require further evaluation.     

Nitrogen isotopes in the recent solar wind from the analysis of Genesis targets: Evidence for large scale isotope heterogeneity in the early solar system

We have analyzed nitrogen, neon and argon abundances and isotopic ratios in target material exposed in space for 27 months to solar wind (SW) irradiation during the Genesis mission. SW ions were extracted by sequential UV (193 nm) laser ablation of gold-plated material, purified separately in a dedicated line, and analyzed by gas source static mass spectrometry. We analyzed gold-covered stainless steel pieces from the Concentrator, a device that concentrated SW ions by a factor of up to 50. Despite extensive terrestrial N contamination, we could identify a non-terrestrial, ¹⁵N-depleted nitrogen end-member that points to a 40% depletion of ¹⁵N in solar-wind N relative to inner planets and meteorites, and define a composition for the present-day Sun (¹⁵N/¹⁴N = [2.26 ± 0.67] × 10⁻³, 2σ), which is indistinguishable from that of Jupiter’s atmosphere. These results indicate that the isotopic composition of nitrogen in the outer convective zone of the Sun has not changed through time, and is representative of the protosolar nebula. Large ¹⁵N enrichments due to e.g., irradiation, low temperature isotopic exchange, or contributions from ¹⁵N-rich presolar components, are therefore required to account for inner planet values.     

Indirect crustal contamination: evidence from isotopic and chemical disequilibria in minerals from alkali basalts and nephelinites from northern Tanzania

 Alkali basalts and nephelinites from the volcanic province of northern Tanzania contain pyroxene and nepheline that show evidence for chemical and/or isotopic disequilibria with their host magmas. Olivine, pyroxene, nepheline and plagioclase all appear to be partially xenocrystic in origin. Five whole rock/mineral separate pairs have been analyzed for Sr, Nd, and Pb isotopic compositions. The ²⁰⁶Pb/²⁰⁴Pb ratios are distinct by as much as 20.94 (whole rock) vs. 19.10 (clinopyroxene separate). The Sr and Nd isotopic disequilibria vary from insignificant in the case of nepheline, to Δ ⁸⁷Sr/⁸⁶Sr of 0.0002 and Δɛ_Nd of 0.7 in the case of clinopyroxene. The mineral chemistry of 25 samples indicates the ubiquitous presence of minerals that did not crystallize from a liquid represented by the host rock. The northern Tanzanian magmas are peralkaline and exhibit none of the xenocrystic phases expected from crustal assimilation. The disequilibria cannot be the result of mantle source variations. Rather the xenocrystic phases present appear to have been derived from earlier alkali basaltic rocks or magmas that were contaminated by the crust. Material from this earlier magma was then mixed with batches of magma that subsequently erupted on the surface. Disequilibrium in volcanic rocks has potentially serious consequences for the use of whole rock data to identify source reservoirs. However, mass balance calculations reveal that the ²⁰⁶Pb/²⁰⁴Pb isotopic compositions of the erupted lavas were changed by less than 0.25% as a result of this indirect crustal contamination. Received: 15 February 1995 / Accepted: 4 May 1996     

Stable isotope studies and processes of carbonate formation in Hungarian alkali basalts and lamprophyres: evolution of magmatic fluids and magma-sediment interactions

Processes of carbonate formation have been related to C and O isotopic compositions in the Mesozoic alkali basalt (Mecsek Mts.) and lamprophyre (Transdanubian Range) suites of Hungary. In the studied magrnatic rocks, carbonates are present as ocelli, amygdales, xenoliths, veins and groundmass carbonate. C and O isotope studies of these types of carbonate have yielded information on the origin of the carbonates and indicated the following processes of formation that determined the δ¹³C and δ¹⁸O values of the carbonates:(1)Crystallization of magmatic carbonate. Textural characteristics and δ¹³C values suggest formation of magmatic carbonate in alkali basalt and lamprophyre dikes, whereas the δ¹⁸O compositions of these carbonates indicate low temperature oxygen isotope exchange with magmatic fluids.(2) Assimilation of sedimentary carbonate by silicate magmas. Even completely recrystallized amygdales and ocelli of basalts and lamprophyres have preserved their sedimentary δ¹³C values. In contrast, variations in the extent of mobilization and isotope exchange with magmatic fluids are reflected in differences in the ranges of the δ¹⁸O values of amygdales, ocelli and veins, and can be attributed to different amounts of fluids involved in the magmatic events.(3) Low temperature alteration of magmatic rocks caused only ¹⁸O-enrichment in the carbonate amygdales of basalts and the groundmass carbonates of lamprophyres, indicating that no externally-derived CO₂ was present in the alteration fluids.(4) Degassing of magma and magmatic fluid. Correlations between δ¹³C and δ¹⁸O data, magma crystallization depths and amygdale sizes in the alkali basalts suggest that CO₂ degassing has been responsible for the negative δ¹³C and positive δ¹⁸O shifts observed. A similar trend was found in the lamprophyres, but the extent of the δ¹⁸O shift indicates that in these rocks H₂O degassing also played an important role.     

神农架大九湖晚更新世与全新世之交的气候变化——来自孢粉的信息

对神农架大九湖泥炭地层进行了孢粉分析,建立了晚更新世与全新世之交(14 031～9 625 aBP)高分辨率的孢粉组合带.根据孢粉植被反映的气候信息推断大九湖地区在晚更新世与全新世之交的气候大致以冷湿为特点,同时气候变化剧烈,表现为冷暖干湿波动频繁,可划分为3个气候旋回:第一旋回(14 031～12 581 aBP)气候变化经历了由温凉偏干-温凉湿润-温暖偏干的过程;第二旋回(12 581～11 046 aBP)气候变化经历了由寒冷偏干-温凉湿润-温凉偏干的过程;第三旋回(11 406～9 625 aBP)气候变化经历了由温凉偏干-寒冷湿润-温凉偏干的过程.     

西藏拿若隐爆角砾岩中岩浆岩成因:来自锆石Hf同位素证据

西藏拿若铜(金)矿床是多龙矿集区重要矿床之一,矿体边部存在一岩浆作用的隐爆角砾岩筒。隐爆角砾岩中存在两种岩浆岩:一种是早期花岗闪长斑岩;一种是引起隐爆作用的岩浆热液形成的胶结物。此次工作以两种岩浆岩为对象,研究其锆石的Hf同位素特征。花岗闪长斑岩的锆石~(176)Hf/~(177)Hf值介于0.282789~0.282905之间,176Lu/177Hf值介于0.000406~0.001042之间,ε_(Hf)~(t)值介于3.17~7.24之间,二阶模式年龄(tDM2)为713~978Ma。胶结物锆石的~(176)Hf/~(177)Hf介于0.282777~0.282858之间,~(176)Lu/~(177)Hf值介于0.000441~0.001572之间,ε_(Hf)~(t)值为2.69~5.54,二阶模式年龄(tDM2)为823~1005M。两期岩体的ε_(Hf)~(t)值都是较小的正值,都具有年轻二阶模式年龄。拿若隐爆角砾岩中锆石Hf同位素特征显示,两期岩浆均具有壳幔混源的特征。拿若矿床的形成受控于特提斯洋壳俯冲作用。     

Tectonic Subdivision of Dabie Orogenic Belt,Central China：Evidence from Pb Isotope Geochemistry of Late Mesozoic Basalts

It has long been debated that the Dabie orogenic belt belongs to the North China or Yangtze craton. In recent years, eastern China has been suggested, based on the Pb isotopic compositions of Phanerozoic ore and Mesozoic granitoid K-feldspar (revealing the crust Pb) in combination with Meso-Cenozoic basalts (revealing the mantle Pb), being divided into the North China and Yangtze Pb isotopic provinces, where the crust and mantle of the Yangtze craton are characterized by more radiogenic Pb. In this sense, previous researchers suggested that the pro-EW-trending Dabie crogenic belt with less radiogenic Pb in the crust was part of the North China craton. In this paper, however, the Late Cretaceous basalts in the central and southern parts of the Dabie orogenic belt are characterized by some more radiogenic Pb (²⁰⁶Pb/²⁰⁴Pb=17.936−18.349,²⁰⁷Pb/²⁰⁴Pb=15.500−15.688,²⁰⁸Pb/²⁰⁴Pb=38.399−38.775) and a unique U-Th-Pb trace element system similar to those of the Yangtze craton, showing that the Mesozoic mantle is of the Yangtze type. In addition, the decoupled Pb isotopic compositions between crust and mantle were considerably derived from their rheological inhomogeneity, implying a complicated evolution of the Dabie orogenic belt. The study was funded by the National Natural Science Foundation of China (No. 49794043) and the Open Laboratory of Constitution, Interaction and Dynamics of the Crust-Mantle System, China.     

OIB-type basalts of the Irbistu River basin (Southeast Mountain Altai): Evidence for the HIMU component in the magmatic source

It has been found that basalts of the Irbistu River basin (southeast Altai Mountains) belong to the intraplate OIB type. The peculiarities of their material composition do not indicate interaction effects between the basic melt and continental crust, and signifies that there was a magmatic source in the moderately depleted mantle comparable to the HIMU domain. It is suggested that basalts of the Irbistu River basin, along with other manifestations of similar volcanites in the Altai Mountains, reflect the Neoproterozoic-Early Paleozoic stage of the North Asian plume’s activity during closure of the Paleoasian Ocean.     

西藏纳如松多铅锌矿床成矿岩体形成机制:岩浆锆石证据

纳如松多铅锌矿床位于拉萨地块中部隆格尔-工布江达断隆带中段,以发育隐爆角砾岩型和矽卡岩型铅锌矿化为特征.西矿段与矽卡岩型铅锌矿化相关的岩体为粗斑和细斑两种石英正长斑岩,对其锆石进行的U-Pb 定年、稀土元素、Lu-Hf同位素和锆石群型特征分析表明,粗斑石英正长斑岩侵位于(62.54±0.77) Ma,细斑石英正长斑岩侵位于(62.47±0.91)Ma;锆石稀土元素具有相似的左倾配分模式和Ce正异常、Eu负异常,在U/Yb-Y图解上均落于陆壳锆石范围;粗斑石英正长斑岩的176Hf/177Hf介于0.282577～0.282803,εHf(t)变化于-5.58～+2.21,反映岩浆来源于地壳物质的部分熔融,并有地幔物质的加入;锆石群型特征显示粗斑和细斑石英正长斑岩为地壳地幔混合岩浆成因的花岗岩.上述结果说明纳如松多铅锌矿床的岩浆侵入与成矿作用发生于印度-亚洲大陆碰撞造山的主碰撞期.由于印度陆壳随回转的新特提斯洋壳板片一起向拉萨地块之下陡俯冲,并产生异常热源,诱发了地幔物质上涌和上覆地壳部分熔融,形成的地幔地壳混合成因岩浆经结晶分异演化后上升侵位,形成矿区内粗斑和细斑两种石英正长斑岩及相关的铅锌矿化.     

On the iron isotope heterogeneity of lithospheric mantle xenoliths: implications for mantle metasomatism, the origin of basalts and the iron isotope composition of the Earth

With the aim to better understand the cause of the iron isotope heterogeneity of mantle-derived bulk peridotites, we compared the petrological, geochemical and iron isotope composition of four xenolith suites from different geodynamic settings; sub-arc mantle (Patagonia); subcontinental lithospheric mantle (Cameroon), oceanic mantle (Kerguelen) and cratonic mantle (South Africa). Although correlations were not easy to obtain and remain scattered because these rocks record successive geological events, those found between δ⁵⁷Fe, Mg#, some major and trace element contents of rocks and minerals highlight the processes responsible for the Fe isotope heterogeneity. While partial melting processes only account for moderate Fe isotope variations in the mantle (<0.2 ‰, with bulk rock values yielding a range of δ⁵⁷Fe ± 0.1 ‰ relative to IRMM-14), the main cause of Fe isotope heterogeneity is metasomatism (>0.9 ‰). The kinetic nature of rapid metasomatic exchanges between low viscosity melts/fluids and their wall-rocks peridotite in the mantle is the likely explanation for this large range. There are a variety of responses of Fe isotope signatures depending on the nature of the metasomatic processes, allowing for a more detailed study of metasomatism in the mantle with Fe isotopes. The current database on the iron isotope composition of peridotite xenoliths and mafic eruptive rocks highlights that most basalts have their main source deeper than the lithospheric mantle. Finally, it is concluded that due to a complex geological history, Fe isotope compositions of mantle xenoliths are too scattered to define a mean isotopic composition with enough accuracy to assess whether the bulk silicate Earth has a mean δ⁵⁷Fe that is chondritic, or if it is ~0.1 ‰ above chondrites as initially proposed.     

The oxygen isotope composition, petrology and geochemistry of mare basalts: Evidence for large-scale compositional variation in the lunar mantle

To investigate the formation and early evolution of the lunar mantle and crust we have analysed the oxygen isotopic composition, titanium content and modal mineralogy of a suite of lunar basalts. Our sample set included eight low-Ti basalts from the Apollo 12 and 15 collections, and 12 high-Ti basalts from Apollo 11 and 17 collections. In addition, we have determined the oxygen isotopic composition of an Apollo 15 KREEP (K - potassium, REE - Rare Earth Element, and P - phosphorus) basalt (sample 15386) and an Apollo 14 feldspathic mare basalt (sample 14053). Our data display a continuum in bulk-rock δ¹⁸O values, from relatively low values in the most Ti-rich samples to higher values in the Ti-poor samples, with the Apollo 11 sample suite partially bridging the gap. Calculation of bulk-rock δ¹⁸O values, using a combination of previously published oxygen isotope data on mineral separates from lunar basalts, and modal mineralogy (determined in this study), match with the measured bulk-rock δ¹⁸O values. This demonstrates that differences in mineral modal assemblage produce differences in mare basalt δ¹⁸O bulk-rock values. Differences between the low- and high-Ti mare basalts appear to be largely a reflection of mantle-source heterogeneities, and in particular, the highly variable distribution of ilmenite within the lunar mantle. Bulk δ¹⁸O variation in mare basalts is also controlled by fractional crystallisation of a few key mineral phases. Thus, ilmenite fractionation is important in the case of high-Ti Apollo 17 samples, whereas olivine plays a more dominant role for the low-Ti Apollo 12 samples.Consistent with the results of previous studies, our data reveal no detectable difference between the Δ¹⁷O of the Earth and Moon. The fact that oxygen three-isotope studies have been unable to detect a measurable difference at such high precisions reinforces doubts about the giant impact hypothesis as presently formulated.