Ar–Ar age determinations of eclogitic clinopyroxene and garnet inclusions in diamonds from the Venetia and Orapa kimberlites

Ar–Ar age measurements are reported for selected eclogitic clinopyroxene and garnet inclusions in Orapa diamonds and clinopyroxene inclusions in Venetia diamonds. Laser drilling of encapsulated clinopyroxene inclusions within Venetia diamonds released a maximum of 3% of the total ⁴⁰Ar, indicating little diffusive transfer and storage of radiogenic ⁴⁰Ar at the diamond–inclusion boundary. Apparent ages obtained during stepped heating of three diamonds are consistent with diamond crystallisation occurring just prior to the kimberlite eruption 520 Ma ago. Stepped heating of three clinopyroxene-bearing Orapa diamonds gave ages of 906–1032 Ma, significantly above the eruption age, but consistent with previously determined isotopic ages. A few higher apparent ages hint at the presence an older generation of Orapa diamonds that formed >2500 Ma ago. Orapa garnets also contain measurable K contents, and record a range of ages between 1000 and 2500 Ma. The old apparent ages and lack of significant interface ⁴⁰Ar released by the laser probe, suggests that pre-eruption radiogenic ⁴⁰Ar and mantle-derived ⁴⁰Ar components are trapped in microinclusions within the pyroxene and garnet inclusions.     

Very fast exhumation of high-pressure metamorphic rocks with excess Ar and inherited Sr, Betic Cordilleras, southern Spain

In order to attempt to further constrain the age of the early Alpine tectonic evolution of the Mulhacén Complex and to explore the influence of inherited isotopes, micas from a small number of well-characterised rocks from the Sierra de los Filábres, with a penetrative tectonic fabric related to the exhumation of eclogite-facies metamorphic rocks, were selected for ⁴⁰Ar/³⁹Ar and Rb–Sr dating.

A single phengite grain from an amphibolite yielded an ⁴⁰Ar/³⁹Ar laser step heating plateau age of 86.9±1.2 Ma (2σ; 70% ³⁹Ar released) and an inverse isochron age of 86.2±2.4 Ma with an ³⁶Ar/⁴⁰Ar intercept within error of the atmospheric value. Induction furnace step heating of a biotite separate from a gabbro relic in an eclogite yielded a weighted mean age of 173.2±6.3 Ma (2σ; 95% ³⁹Ar released). These ages are diagnostic of excess argon (⁴⁰Ar_XS) incorporation, as they are older than independent age estimates for the timing of eclogite-facies metamorphism and intrusion of the gabbros. ⁴⁰Ar_XS incorporation probably resulted from restricted fluid mobility in the magmatic rocks during their metamorphic recrystallisation.

Rb–Sr whole-rock–phengite ages of graphite-bearing mica schists from Paleozoic rocks (Secano unit) show a dramatic variation (66.1±3.2, 40.6±2.6 and 14.1±2.2 Ma). An albite chlorite mica schist from the Mesozoic series of the Nevado–Lubrín unit has a whole-rock–mica–albite age of 17.2±1.9 Ma, which is within error of an ⁴⁰Ar/³⁹Ar plateau age published previously and of the youngest Rb–Sr age of the Paleozoic series obtained in this study. The significant spread in Rb–Sr ages implies that progressive partial resetting of an older isotopic system has occurred. The microstructure of the samples with pre-Miocene Rb–Sr ages reveals incomplete recrystallisation of white mica and inhibited grain growth due to the presence of graphite particles. This interpretation agrees with previously published, disturbed and slightly dome-shaped ⁴⁰Ar/³⁹Ar age spectra that may point similarly to the presence of an older isotope component. The progressively reset Rb–Sr system is a relic of Variscan metamorphism of the Paleozoic series of the Mulhacén Complex. In contrast, the origin of the ca. 17.2 Ma old sample from the Mesozoic series precludes any isotopic inheritance, in agreement with its pervasive tectono-metamorphic recrystallisation during the Miocene.

Exhumation of the eclogite-facies Mulhacén Complex occurred in two stages with contrasting rates of about 22.5 mm/year during the early phase and 9–10 mm/year during the late phase; the latter with a cooling rate in the order of 330 °C/Ma.     

Diamonds and their mineral inclusions from the A154 South pipe, Diavik Diamond Mine, Northwest territories, Canada

Mineral inclusions recovered from 100 diamonds from the A154 South kimberlite (Diavik Diamond Mines, Central Slave Craton, Canada) indicate largely peridotitic diamond sources (83%), with a minor (12%) eclogitic component. Inclusions of ferropericlase (4%) and diamond in diamond (1%) represent “undetermined” parageneses.

Compared to inclusions in diamonds from the Kaapvaal Craton, overall higher CaO contents (2.6 to 6.0 wt.%) of harzburgitic garnets and lower Mg-numbers (90.6 to 93.6) of olivines indicate diamond formation in a chemically less depleted environment. Peridotitic diamonds at A154 South formed in an exceptionally Zn-rich environment, with olivine inclusions containing more than twice the value (of  52 ppm) established for normal mantle olivine. Harzburgitic garnet inclusions generally have sinusoidal rare earth element (REE_N) patterns, enriched in LREE and depleted in HREE. A single analyzed lherzolitic garnet is re-enriched in middle to heavy REE resulting in a “normal” REE_N pattern. Two of the harzburgitic garnets have “transitional” REE_N patterns, broadly similar to that of the lherzolitic garnet. Eclogitic garnet inclusions have normal REE_N patterns similar to eclogitic garnets worldwide but at lower REE concentrations.

Carbon isotopic values (δ¹³C) range from − 10.5‰ to + 0.7‰, with 94% of diamonds falling between − 6.3‰ and − 4.0‰. Nitrogen concentrations range from below detection (< 10 ppm) to 3800 ppm and aggregation states cover the entire spectrum from poorly aggregated (Type IaA) to fully aggregated (Type IaB). Diamonds without evidence of previous plastic deformation (which may have accelerated nitrogen aggregation) typically have < 25% of their nitrogen in the fully aggregated B-centres. Assuming diamond formation beneath the Central Slave to have occurred in the Archean [Westerlund, K.J., Shirey, S.B., Richardson, S.H., Gurney, J.J., Harris, J.W., 2003b. Re–Os systematics of diamond inclusion sulfides from the Panda kimberlite, Slave craton. VIIIth International Kimberlite Conference, Victoria, Canada, Extended Abstracts, 5p.], such low aggregation states indicate mantle residence at fairly low temperatures (< 1100 °C). Geothermometry based on non-touching inclusion pairs, however, indicates diamond formation at temperatures around 1200 °C. To reconcile inclusion and nitrogen based temperature estimates, cooling by about 100–200 °C shortly after diamond formation is required.     

延边海沟金矿床石英流体包裹体激光探针40Ar/39Ar测年与成矿背景

海沟金矿为一大型贫硫化物石英脉型金矿床,长期以来其形成时代一直存在争议,致使矿床成因研究受到较大影响,成矿预测等方面研究进展缓慢。采用石英流体包裹体的⁴⁰Ar/³⁹Ar激光探针定年法对该金矿床进行了精细的年代学测定,获得主成矿阶段石英流体包裹体的⁴⁰Ar/³⁹Ar正、反等时线年龄分别为（170±20） Ma和（172±16） Ma,MSWD分别为0.31和0.35,⁴⁰Ar/³⁶Ar比值分别为298±4和299±4;进一步剔除接近大气的成分点和过剩氩高的成分点,计算获得等时线年龄为（170±38） Ma（⁴⁰Ar/³⁶Ar Int.=299±10）和（165±52） Ma（⁴⁰Ar/³⁶Ar Int.=303±23）。结合已有同位素年代学研究成果,进一步明确了海沟金矿床主成矿期发生在早侏罗世晚期,成矿作用与中生代库拉板块向亚洲板块俯冲、特别是由此引发的华北板块东缘与兴蒙造山带东段的强烈拼贴并诱发的岩石圈加厚、减薄作用密切相关。     

Excess Ar-free phengite in ultrahigh-pressure metamorphic rocks from the Lago di Cignana area, Western Alps

Ar/Ar analyses of phengites and paragonites from the ultrahigh-pressure metamorphic rocks (zoisite–clinozoisite schist, garnet–phengite schist and piemontite schist) in the Lago di Cignana area, Western Alps were carried out with a laser probe step-heating method using single crystals and a spot dating method on thin sections. Eight phengite and two paragonite crystals give the plateau ages of 37–42 Ma with 96–100% of ³⁹Ar released. Each rock type also contains mica crystals showing discordant age spectra with age fractions (20–35 Ma) significantly younger than the plateau ages. Phengite inclusions in garnet give ages of 43.2 ± 1.1 Ma and 44.4 ± 1.5 Ma, which are significantly older than the spot age (36.4 ± 1.4 Ma) from the matrix phengites, and the plateau ages from the step-heating analyses. Inclusion ages (43 and 44 Ma) are consistent with a zircon SHRIMP age (44 ± 1 Ma) in this area. These results suggest that the oceanic materials that underwent a simple subduction related UHPM, form excess ⁴⁰Ar-free phengite and that the peak metamorphism is ca. 44 Ma or little older. We suggest that matrix phengites experienced a retrogression reaction changing their chemistry contemporaneously with deformation related to the exhumation of rocks releasing significant radiogenic ⁴⁰Ar from the crystals. This has lead to the apparent ages of the matrix phengites that are significantly younger than the inclusion age.     

The trace element composition of silicate inclusions in diamonds: a review

On a global scale, peridotitic garnet inclusions in diamonds from the subcratonic lithosphere indicate an evolution from strongly sinusoidal REE_N, typical for harzburgitic garnets, to mildly sinusoidal or “normal” patterns (positive slope from LREE_N to MREE_N, fairly flat MREE_N–HREE_N), typical for lherzolitic garnets. Using the Cr-number of garnet as a proxy for the bulk rock major element composition it becomes apparent that strong LREE enrichment in garnet is restricted to highly depleted lithologies, whereas flat or positive LREE–MREE slopes are limited to less depleted rocks. For lherzolitic garnet inclusions, there is a positive relation between equilibration temperature, enrichment in MREE, HREE and other HFSE (Ti, Zr, Y), and decreasing depletion in major elements. For harzburgitic garnets, relations are not linear, but it appears that lherzolite style enrichment in MREE–HREE only occurs at temperatures above 1150–1200 °C, whereas strong enrichment in Sr is absent at these high temperatures. These observations suggest a transition from melt metasomatism (typical for the lherzolitic sources) characterized by fairly unfractionated trace and major element compositions to metasomatism by CHO fluids carrying primarily incompatible trace elements. Melt and fluid metasomatism are viewed as a compositional continuum, with residual CHO fluids resulting from primary silicate or carbonate melts in the course of fractional crystallization and equilibration with lithospheric host rocks.

Eclogitic garnet inclusions show “normal” REE_N patterns, with LREE at about 1× and HREE at about 30× chondritic abundance. Clinopyroxenes approximately mirror the garnet patterns, being enriched in LREE and having chondritic HREE abundances. Positive and negative Eu anomalies are observed for both garnet and clinopyroxene inclusions. Such anomalies are strong evidence for crustal precursors for the eclogitic diamond sources. The trace element composition of an “average eclogitic diamond source” based on garnet and clinopyroxene inclusions is consistent with derivation from former oceanic crust that lost about 10% of a partial melt in the garnet stability field and that subsequently experienced only minor reenrichment in the most incompatible trace elements. Based on individual diamonds, this simplistic picture becomes more complex, with evidence for both strong enrichment and depletion in LREE.

Trace element data for sublithospheric inclusions in diamonds are less abundant. REE in majoritic garnets indicate source compositions that range from being similar to lithospheric eclogitic sources to strongly LREE enriched. Lower mantle sources, assessed based on CaSi–perovskite as the principal host for REE, are not primitive in composition but show moderate to strong LREE enrichment. The bulk rock LREE_N–HREE_N slope cannot be determined from CaSi–perovskites alone, as garnet may be present in these shallow lower mantle sources and then would act as an important host for HREE. Positive and negative Eu anomalies are widespread in CaSi–perovskites and negative anomalies have also been observed for a majoritic garnet and a coexisting clinopyroxene inclusion. This suggests that sublithospheric diamond sources may be linked to old oceanic slabs, possibly because only former crustal rocks can provide the redox gradients necessary for diamond precipitation in an otherwise reduced sublithospheric mantle.     

K-Ar、40Ar/39定年中尼尔值和初始氩比值

K-Ar、⁴⁰Ar/³⁹Ar定年中, 用以计算囚禁⁴⁰Ar绝对量的⁴⁰Ar/³⁶Ar, 称为初始氩比值.现代大气中的初始氩比值叫尼尔值, 为295.5.K-Ar、⁴⁰Ar/³⁹Ar定年的表观年龄都是在假设初始氩比值与现代大气中的尼尔值一致的基础上得到的.事实证明, 地球样品的初始氩比值不总是尼尔值, 因此这种假定不可靠, 尤其在对年轻样品进行K-Ar、⁴⁰Ar/³⁹Ar定年时, 会带来错误的表观年龄结果.对于K-Ar、⁴⁰Ar/³⁹Ar精细年代学, 特别对于年轻样品和低钾含量样品, 只有初始氩比值才对定年研究有意义, 而初始氩比值只能通过⁴⁰Ar/³⁹Ar等时线法求得.所以严格地讲, 只有等时线年龄才是可靠的.结合实验流程, 本文分析了⁴⁰Ar/³⁹Ar定年中为什么大多数等时线获取的初始氩比值接近尼尔值, 一是因为常规⁴⁰Ar/³⁹Ar坪年龄的计算方法, 二是因为吸附氩的干扰.初始氩比值的一致性, 被用来证明样品同位素同源和封闭特性, 对年龄数据的精度及可靠性有非常大的影响, 尤其是年轻样品.因此, 在⁴⁰Ar/³⁹Ar定年中要选取氩同位素初始比值一致的同源样品, 并在实验流程中尽量克服样品吸附气体.本文对传统⁴⁰Ar/³⁹Ar方法中坪年龄、年龄坪的意义提出了质疑, 对提高⁴⁰Ar/³⁹Ar定年的精度具有重要意义, 并使之能够应用到极年轻的火山岩定年中.     

南岭中段主要锡矿床He、Ar同位素组成及其意义

对南岭中段主要锡矿床的黄铁矿等进行了流体包裹体的He、Ar同位素研究。结果表明,成矿流体的⁴⁰Ar/³⁶Ar≈288～371,³He/⁴He≈0.09～28.58Ra ,不同地区流体包裹体He、Ar浓度变化较大,而在同一地区⁴⁰Ar/³⁶Ar、³He/⁴He值比较一致;⁴⁰Ar/³⁶Ar值介于大气饱和水(包括大气降水和海水)的同位素组成（⁴⁰Ar/³⁶Ar≈295.5）和夏威夷热点（⁴⁰Ar/³⁶Ar=350～360）之间,³He/⁴He值大大高于大陆地壳特征值（³He/⁴He≈0.03Ra）,介于地壳特征值与原始地幔特征值（³He/⁴He≈8.57Ra）和夏威夷热点特征值（³He/⁴He≈27.14Ra）之间,表明成矿流体与地幔柱的活动有关,为地幔、地壳和大气水的混合产物,以地幔流体为主。这一结果为南岭地区锡多金属矿床为壳幔相互作用的产物及区域地幔柱的存在提供了新证据。     

Strain partitioning and preservation of Ar/Ar ages during Variscan exhumation of a subducted crust (Malpica–Tui complex, NW Spain)

The Malpica–Tui complex (NW Iberian Massif) consists of a Lower Continental Unit of variably deformed and recrystallized granitoids, metasediments and sparse metabasites, overridden by an upper unit with rocks of oceanic affinities. Metamorphic minerals dated by the ⁴⁰Ar/³⁹Ar method record a coherent temporal history of progressive deformation during Variscan metamorphism and exhumation. The earliest stages of deformation (D1) under high-pressure conditions are recorded in phengitic white micas from eclogite-facies rocks at 365–370 Ma. Following this eclogite-facies peak-metamorphism, the continental slab became attached to the overriding plate at deep-crustal levels at ca. 340–350 Ma (D2). Exhumation was accompanied by pervasive deformation (D3) within the continental slab at ca. 330 Ma and major deformation (D4) in the underlying para-autochthon at 315–325 Ma. Final tectonothermal evolution included late folding, localized shearing and granitic intrusions at 280–310 Ma.

Dating of high-pressure rocks by the ⁴⁰Ar/³⁹Ar method yields ages that are synchronous with published Rb–Sr and Sm–Nd ages obtained for both the Malpica–Tui complex and its correlative, the Champtoceaux complex in the French Armorican Massif. The results indicate that phengitic white mica retains its radiogenic argon despite been subjected to relatively high temperatures (500–600 °C) for a period of 20–30 My corresponding to the time-span from the static, eclogite-facies M1 peak-metamorphism through D1-M2 eclogite-facies deformation to amphibolite-facies D2-M3. Our study provides additional evidence that under certain geological conditions (i.e., strain partitioning, fluid deficiency) argon isotope mobility is limited at high temperatures, and that ⁴⁰Ar/³⁹Ar geochronology can be a reliable method for dating high pressure metamorphism.     

大庙斜长岩同位素地质年龄、稀土地球化学及其地质意义

大庙斜长岩的⁴⁰Ar/³⁹Ar年龄测定呈现出一条典型的马鞍型年龄谱,在中温阶段有二个明显的坪年龄1656±15 Ma和1029±7 Ma,结合其构造位置和全球斜长岩分布来看,它们分别代表了侵位年龄和后期热扰动的时代。密云奥长环斑花岗岩中角闪石的⁴⁰Ar/³⁹Ar坪年龄为1716±21 Ma。两者时空上密切相关,代表了裂谷作用初期非造山环境中双模式岩浆作用产物。斜长岩类和苏长岩之间稀土配分模式的相似性表明,它们明显为同一成因的岩浆分异系列的产物。     

Structural and geochronologic data from the Shin Jinst area, eastern Gobi Altai, Mongolia: Implications for Phanerozoic intracontinental deformation in Asia

The complex geology of central Asia records significant Phanerozoic growth and deformation of continental lithosphere, yet many questions about the timing and mechanisms of these processes remain. The Gobi Altai of southwestern Mongolia preserves much of this history in exposed mountain ranges and sedimentary basins. The Shin Jinst area (near latitude 44 21' to 44° 32', longitude 99°35' to 99° 25') contains important evidence of Paleozoic growth and accretion as well as subsequent Mesozoic and Cenozoic deformation. Detailed mapping and structural data, including data sets on folds and cross-cutting faults, point to repeated episodes of deformation and add to recent work that documents Triassic and Jurassic compression as well as Cenozoic tectonic activity in this region. New ⁴⁰Ar/³⁹Ar data are interpreted to record Permian to Jurassic convergent margin processes. In particular, these data, in combination with previous studies, suggest that southern Mongolia records protracted deformation throughout the Jurassic that likely resulted from compression associated with the subduction zone at the southern margin of Asia at that time.     

黑龙江省嫩江—黑河构造混杂岩带科洛金矿床成因:来自黄铁矿化学成分及He-Ar、S、Pb同位素证据

科洛金矿床位于黑龙江省嫩江—黑河构造混杂岩带中,为一正在勘查的中型金矿床。前人研究认为该矿床为一中温、富水、低盐度的韧性剪切带型金矿床。但随着勘查工作的持续开展,新发现了大量含多金属硫化物的石英脉型矿石。为进一步明确该矿床成因,对科洛金矿床内韧性变形和浸染状-团块状两种不同产状黄铁矿进行了系统的成分标型及稳定同位素特征研究。结果表明:韧性变形黄铁矿较浸染-团块状黄铁矿的S/Fe值低,Au、Ag、Co含量高,As含量低,总体显示韧性变形黄铁矿相对贫硫;黄铁矿微量元素显示两类黄铁矿REE具有相似的地球化学特征,整体呈轻稀土富集、重稀土亏损的“右倾”式稀土配分模式。稀土元素总量为17.00×10^-6~66.95×10^-6,韧性变形黄铁矿含量明显偏低,LREE/HREE值为5.25~12.50,相对较稳定。Y/Ho值与地幔和地壳重合范围较多,Zr/Hf值、Nb/Ta值变化范围较大,显示成矿环境不稳定;黄铁矿³He含量为2.405×10^-13~10.811×10^-13,⁴He含量为3.35×10^-7~4.99×10^-7,³He-⁴He图和R/R_a-⁴⁰Ar/³⁶Ar图显示成矿流体有以地幔为主的壳幔混源特征,成矿过程应有大气降水的参与;黄铁矿δ³⁴S变化范围小,集中在+2.3‰~+7.6‰(韧性变形黄铁矿δ³⁴S平均值偏低),稍正向偏离陨石硫特征;黄铁矿的²⁰⁶Pb/²⁰⁴Pb=18.157~18.211;²⁰⁷Pb/²⁰⁴Pb=15.542~15.594;²⁰⁸Pb/²⁰⁴Pb=38.032~38.218(韧性变形黄铁矿的²⁰⁶Pb/²⁰⁴Pb、²⁰⁷Pb/²⁰⁴Pb、²⁰⁸Pb/²⁰⁴Pb平均值偏低),均显示与造山作用关系密切。利用黄铁矿化学成分特征元素Co/Ni值及Co-Ni成因判别图解认为科洛金矿有热液成因矿床特征;利用w(Fe+S)-w(As)图解和Co-Ni-As含量图解显示石英脉型矿石反映浅成低温热液型金矿特征。综合区域变质变形事件、典型矿床特征及矿区内已取得的年龄、流体包裹体等相关数据认为,科洛金矿床应存在两期矿化:三叠纪形成的韧性剪切带型金矿床和白垩纪形成的浅成低温热液型金矿床。建议在今后的矿产勘查中注意寻找真正的韧性剪切带型金矿。     

雅鲁藏布江缝合带造山型金矿床成矿流体特征:来自He-A r同位素的约束

雅鲁藏布江缝合带造山型金成矿带是近些年来在青藏高原新确立的碰撞造山型金成矿带,成矿流体是一套CO₂-H₂O-NaCl-CH₄-N₂体系,以变质水为主。为探讨地幔组分对成矿的影响,本文选取雅鲁藏布江缝合带内邦布矿床和念扎矿床两个大型的造山型金矿为研究对象,对与成矿相关的黄铁矿及胶黄铁矿进行He-Ar同位素分析。结果显示,邦布矿床含金石英脉中黄铁矿中流体包裹体的He和Ar的浓度变化范围较窄,³He/⁴He为0.59~0.70 Ra,40Ar/36Ar为365.1~423.1;2件胶黄铁矿样品的³He/⁴He为0.11 Ra及0.19 Ra,⁴⁰Ar/³⁶Ar为302.1和305.9;念扎矿床石英-多金属硫化物脉中黄铁矿的³He/⁴He为0.27~0.60 Ra,⁴⁰Ar/³⁶Ar为370.2~1777.9。雅鲁藏布江造山型金成矿带中造山型金矿成矿流体为壳幔源混合流体,地壳端元主要为变质水,地幔组分在邦布金矿和念扎金矿中的贡献率分别为12.8%~15.2%和5.7%~13.0%;这与藏南拆离系中的金锑成矿作用明显不用,藏南拆离系中的金锑矿床成矿流体主要表现为改造型饱和大气水的特征,未有地幔组分的贡献。     

Metamorphic K-Feldspar in Low-grade Metasediments from the Ogcheon Metamorphic Belt in South Korea

Metamorphic K-feldspar was found in the low-grade pelitic rocks from the Ogcheon metamorphic belt in South Korea. It occurs as extremely fine-grained crystals (5-15 mm in width and 15-25 mm in length) closely associated with fine-grained muscovite and biotite. Their micro structural relations by X-ray mapping analyses using an electron-probe microanalyzer strongly suggest that K-feldspar has grown directly from the matrix phases as a stable phase coexisting with muscovite and biotite during the Ogcheon metamorphism. The phengite component in muscovite indicates about 4.2 kbar at 400°C, suggesting intermediate P/T type of metamorphism. Muscovite separates of two size fractions, 2-4 and 4-8 mm, give K-Ar ages of 153.4±3.3 and 156.7±3.4 Ma, respectively. The biotite separate is 156.5±3.3 Ma in age. Coarse-grained biotite crystal (ca. 0.5 mm) often occurs and it was analyzed by ⁴⁰Ar/³⁹Ar method using a laser probe step heating technique. It gives a plateau age of 158.2±0.5 Ma that is the same as the K-Ar muscovite and biotite ages. These data, including the previous works, suggest that intermediate P/T type of Ogcheon metamorphic belt exhumed in Middle Jurassic with the dextral strike-slip fault movement.     

Ar/Ar Single Grain Dating and Mineral Chemistry of Hornblendes South of the Red River Shear Zone (Vietnam): New Evidence for Early Proterozoic Tectonothermal Event

The pre-Mesozoic metamorphic belt runs parallel to the Day Nui Con Voi - Red River shear zone in Vietnam to the south. The belt is mainly composed of hornblende gneisses, amphibolite lenses and mica-schists. Five hornblendes from a gneiss and an amphibolite were analyzed chemically and chronologically by Electron Probe Micro Analysis (EPMA) and ⁴⁰Ar/³⁹Ar methods. EPMA analyses show that hornblendes in the gneiss and the amphibolite have significant amount of edenite component and similar average composition. However, the recalculated Fe³⁺ content is significantly heterogeneous in a thin section while total Fe is nearly the same among the analyses. The rim of each crystal is higher in Fe³⁺/(Fe³⁺ + Fe²⁺) than the core. These chemical and petrological features suggest that the hornblendes have suffered significant oxidation, in particular, largely in the gneiss.

⁴⁰Ar/³⁹Ar analyses showed that the gneiss has a significant variation of plateau ages (2089±14, 1977±19 and 1873±13 Ma) among three hornblende grains, whereas the amphibolite gives the same plateau ages (2056±14 and 2044±21 Ma) for two grains. All grains of both samples have excess ages in the first few fractions at low temperatures and partial-loss ages between the excess and plateau spectra. The Ca/K ratios indicate some disturbed phases for the lower temperature spectra but the partial-loss ages are also derived from hornblende phase. These facts suggest that hornblende in the gneiss has experienced partial argon loss by oxidation and/or thermally activated argon diffusion process. However, the gneiss and the associated amphibolite have preserved the early Proterozoic tectono-metamorphic event in the hornblende crystals except for their rims, giving new evidence for the early Proterozoic event within the pre-Mesozoic metamorphic belt (northern Vietnam) south of the Red River shear zone in Indochina.     

Distinct brief major events in the Karoo large igneous province clarified by new Ar/Ar ages on the Lesotho basalts

Recent mineral separate ages obtained on the Karoo large igneous province (southern Africa) suggest that the province was built by several distinct magmatic pulses over a rather long period on the order of 5–6 Ma concerning the main erupted volume [Jourdan, F., Féraud, G., Bertrand, H., Kampunzu, A.B., Tshoso, G., Watkeys, M.K., Le Gall., B., 2005. The Karoo large igneous province: Brevity, origin, and relation with mass extinction questioned by new ⁴⁰Ar/³⁹Ar age data, Geology 33, 745–748]. Although this apparently atypical province is dated in more detail compared to many other large igneous provinces, volumetrically important areas still lack sufficient high-quality data. The timing of the Karoo province is crucial as this event is correlated with the breakup activity of the Gondwana supercontinent. The Lesotho basalts represent a major lava sequence of the province, but have not yet been precisely dated by systematic analysis of mineral separates. We analyzed plagioclase separates from five lava flows encompassing the complete 1.4-km-thick Lesotho sequence from top to bottom using the ⁴⁰Ar/³⁹Ar method. We obtained five plateau and mini-plateau ages statistically indistinguishable and ranging from 182.3 ± 1.6 to 181.0 ± 2.0 Ma (2σ). We derived an apparent maximum duration for this event of  0.8 Ma by neglecting correlated errors embedded in the age uncertainties.

A critical review of previous ages obtained on the Lesotho sequence [Duncan R.A., Hooper, P.R., Rehacek, J., Marsh, J.S., Duncan, A.R., 1997. The timing and duration of the Karoo igneous event, southern Gondwana. Journal of Geophysical Research 102, 18127–18138] shows that groundmass analyses are unreliable for high-resolution geochronology, due to alteration and ³⁹Ar recoil effects. Discrepancy between our ages and a previous plagioclase age at  184 Ma obtained by the later workers is tentatively attributed to the heterogeneity of the monitor used and/or cryptic excess ⁴⁰Ar. The current age database suggests that at least three temporally and spatially distinct brief major events (the Lesotho and southern Botswana lava piles and the Okavango dyke swarm) are so far recognized in the Karoo province. Identification of brief and volumetrically important Karoo magmatic events allows detecting the migration of the Karoo magmatism and potentially the stress regime that affected the southern African lithosphere at this time. A filtered compilation of 60 ages obtained with homogeneous intercalibrated standards suggests a shorter duration for the main pulses of the magmatism between 3 and 4.5 Ma, compared to a whole province duration of  10 Ma, between  182 and  172 Ma.     

高压-超高压变质岩外来40Ar成因探讨

高压—超高压变质带是地球上重要的地质单元,特殊的形成环境使该类变质岩的矿物中常因含有外来⁴⁰Ar而⁴⁰Ar／³⁹Ar年龄偏老,制约了⁴⁰Ar／³⁹Ar年代学在此类岩石中的应用。本文探讨了氩在不同相中的溶解度与分配系数的差异,并着重论述了扩散参数与压力的关系及压力对岩体封闭性的影响,系统总结了高压-超高压变质矿物中外来⁴⁰Ar的形成机制。此外,随Fe/Mg、Si/Al、K等组分不同,氩在矿物中保存能力的某种规律性变化、离子孔隙度研究等也从理论上为研究氩的扩散和封闭行为提供了新的视角。在高压—超高压变质过程中多种因素共同作用,氩在这类矿物中的扩散更为复杂。对外来⁴⁰Ar形成机制的探讨对揭示高压—超高压变质结晶环境、岩体开放与否、不同矿物对氩保存能力等有重要的指示意义。     

祁漫塔格韧性剪切带中绢云母40Ar-39Ar定年及地质意义

祁漫塔格韧性剪切带是祁漫塔格蛇绿混杂岩带与北昆仑岩浆弧的区域主构造边界,对剪切带内花岗质糜棱岩中绢云母40Ar-39 Ar法年龄测定,获得了(271.1±2.2) Ma的坪年龄,相应的36Ar/40 Ar-39 Ar/40 Ar反等时线年龄为(270.5±3.7)Ma (MSWD=2.7),39Ar/36 Ar-40 Ar/35 Ar正等时线年龄为(270.9±4.7) Ma (MSWD=0.22);坪年龄(271.1±2.2) Ma接近于绢云母矿物的形成年龄,也代表了祁漫塔格韧性剪切带的形成年龄.通过韧性剪切变形带内运动学特征的研究,表明剪切带具有右旋斜冲的性质.祁漫塔格韧性剪切带的形成与古特提斯洋的俯冲作用有关,是古特提斯洋向北俯冲造山过程的远程效应.     

Mineral chemistry and thermobarometry of inclusions from De Beers Pool diamonds, Kimberley, South Africa

Silicate and oxide mineral inclusions in diamonds from the geologically and historically important De Beers Pool kimberlites in Kimberley, South Africa, are characterised by harzburgitic compositions (>90%), with lesser abundances from eclogitic and websteritic parageneses. The De Beers Pool diamonds contain unusually high numbers of inclusion intergrowths, with garnet+orthopyroxene±chromite±olivine and chromite+olivine assemblages dominant. More unusual intergrowths include garnet+olivine+magnesite and an eclogitic assemblage comprising garnet+clinopyroxene+rutile. The mineral chemistry of the De Beers Pool inclusions overlaps that of most worldwide localities. Peridotitic garnet inclusions exhibit variable CaO (<5.8 wt.%) and Cr₂O₃ contents (3.0–15.0 wt.%), although the majority are harzburgitic with very low calcium concentrations (<2 wt.% CaO). Eclogitic garnet inclusions are characterised by a wide range in CaO (3.3–21.1 wt.%) with low Cr₂O₃ (<1 wt.%). Websteritic garnets exhibit intermediate compositions. Most chromite inclusions contain 63–67 wt.% Cr₂O₃ and <0.5 wt.% TiO₂. Olivine and orthopyroxene inclusions are magnesium-rich with Mg-numbers of 93–97. Olivine inclusions in chromite exhibit the highest Mg-numbers and also contain elevated Cr₂O₃ contents up to 1.0 wt.%. Peridotitic clinopyroxene inclusions are Cr-diopsides with up to 0.8 wt.% K₂O. Eclogitic and websteritic clinopyroxene inclusions exhibit overlapping compositions with a wide range in Mg-numbers (66–86).

Calculated temperatures for non-touching inclusion pairs from individual diamonds range from 1082 to 1320 °C (average=1197 °C), whereas pressures vary from 4.6 to 7.7 GPa (average=6.3 GPa). Touching inclusion assemblages are characterised by equilibration temperatures of 995 to 1182 °C (average=1079 °C) and pressures of 4.2–6.8 GPa (average=5.4 GPa). Provided that the non-touching inclusions represent equilibrium assemblages, it is suggested that these inclusions record the conditions at the time of diamond crystallisation (1200 °C; 3.0 Ga). The lower average temperatures for touching inclusions are attributed to re-equilibration in a cooling mantle (1050 °C) prior to kimberlite eruption at 85 Ma. Pressure estimates for touching garnet–orthopyroxene inclusions are also skewed towards lower values than most non-touching inclusions. This apparent difference may be an artefact of the Al-exchange geobarometer and/or the result of sampling bias, due to limited numbers of non-touching garnet–orthopyroxene inclusions. Alternatively pressure differences could be caused by differential uplift in the mantle or possibly variations in thermal compressibility between diamond and silicate inclusions. However, thermodynamic modelling suggests that thermal compressibility differences would cause only minor changes in internal inclusion pressures (<0.2 GPa/100 °C).     

New insights into the occurrence of C-depleted carbon in the mantle from two closely associated kimberlites: Letlhakane and Orapa, Botswana

Carbon isotope measurements on diamonds from the Letlhakane kimberlite, and the analyses of their inclusions, permit the examination of km-scale mantle-composition variations by comparing the results with those for the nearby Orapa kimberlite. Diamonds from Letlhakane have a wide range in carbon isotopic composition (−3‰ to −21‰); however, the relative abundance of diamonds depleted in ¹³C is significantly lower than in the Orapa kimberlite. Most of the ¹³C-depleted diamonds belong to the eclogictic or websteritic paragenesis. The relative abundance of inclusions in diamonds and their composition indicate that there are significant differences in petrology in the mantle below the two locations. At Letlhakane, peridotitic compositions are more prevalent than at Orapa and the protolith of P-Type inclusions in diamonds may have experienced a higher degree of partial melting at Letlhakane compared to Orapa. P/T estimates for both W- and E-Type diamonds indicate that a region of ¹³C-depletion may exist beneath the two kimberlites. The relationships between carbon isotopic composition of the host diamond and the Al₂O₃/Cr₂O₃ ratios of their websteritic and eclogitic garnet inclusions indicate that the low δ¹³C regions may represent a primary mantle feature, unrelated to a crustal component.