Erratum to: Iron isotope variations in spinel peridotite xenoliths from North China Craton: implications for mantle metasomatism

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.     

Combined iron and magnesium isotope geochemistry of pyroxenite xenoliths from Hannuoba,North China Craton: implications for mantle metasomatism

We present high-precision iron and magnesium isotopic data for diverse mantle pyroxenite xenoliths collected from Hannuoba, North China Craton and provide the first combined iron and magnesium isotopic study of such rocks. Compositionally, these xenoliths range from Cr-diopside pyroxenites and Al-augite pyroxenites to garnet-bearing pyroxenites and are taken as physical evidence for different episodes of melt injection. Our results show that both Cr-diopside pyroxenites and Al-augite pyroxenites of cumulate origin display narrow ranges in iron and magnesium isotopic compositions (δ⁵⁷Fe = ?0.01 to 0.09 with an average of 0.03 ± 0.08 (2SD, n = 6); δ²⁶Mg = ? 0.28 to ?0.25 with an average of ?0.26 ± 0.03 (2SD, n = 3), respectively). These values are identical to those in the normal upper mantle and show equilibrium inter-mineral iron and magnesium isotope fractionation between coexisting mantle minerals. In contrast, the garnet-bearing pyroxenites, which are products of reactions between peridotites and silicate melts from an ancient subducted oceanic slab, exhibit larger iron isotopic variations, with δ⁵⁷Fe ranging from 0.12 to 0.30. The δ⁵⁷Fe values of minerals in these garnet-bearing pyroxenites also vary widely (?0.25 to 0.08 in olivines, ?0.04 to 0.25 in orthopyroxenes, ?0.07 to 0.31 in clinopyroxenes, 0.07 to 0.48 in spinels and 0.31–0.42 in garnets). In addition, the garnet-bearing pyroxenite shows light δ²⁶Mg (?0.43) relative to the mantle. The δ²⁶Mg of minerals in the garnet-bearing pyroxenite range from ?0.35 for olivine and orthopyroxene, to ?0.34 for clinopyroxene, 0.04 for spinel and ?0.68 for garnet. These measured values stand in marked contrast to calculated equilibrium iron and magnesium isotope fractionation between coexisting mantle minerals at mantle temperatures derived from theory, indicating disequilibrium isotope fractionation. Notably, one phlogopite clinopyroxenite with an apparent later metasomatic overprint has the heaviest δ⁵⁷Fe (as high as 1.00) but the lightest δ²⁶Mg (as low as ?1.50) values of all investigated samples. Overall, there appears to be a negative co-variation between δ⁵⁷Fe and δ²⁶Mg in the Hannuoba garnet-bearing pyroxenite and in the phlogopite clinopyroxenite xenoliths and minerals therein. These features may reflect kinetic isotopic fractionation due to iron and magnesium inter-diffusion during melt–rock interaction. Such processes play an important role in producing inter-mineral iron and magnesium isotopic disequilibrium and local iron and magnesium isotopic heterogeneity in the subcontinental mantle.     

安徽女山橄榄岩包体的氧同位素比值和微量元素组成：地幔交代作用

1本文的研究对象是9个来自安徽女山新生代玄武岩的橄榄岩包体,利用激光氟化技术系统分析了橄榄石、单斜辉石和斜方辉石的氧同位素比值;利用LA-ICPMS技术分析了单斜辉石的微量元素组成,结果显示,女山橄榄岩存在辉石与橄榄石之间的氧同位素不平衡分馏现象,其中△Cpx-O1^18O（=δ^18OCpx-δ^18OO1））出现低至-0．5‰的负值;合角闪石包体中的单斜辉石明显富集大离子亲石元素（LILE,如Sr、Th、LREE）和亏损高场强元素（HFSE,如Nb、Zr、Ti）,氧同位素的数据表明,女山上地幔曾经经历过俯冲洋壳流体参与的交代作用,根据单斜辉石和橄榄石之间氧同位素分馏值△Cpx-O1^18O与包体形成深度之间的负相关性,提出了一个“上升＋缓冲”的流体交代模型,虽然微量元素特征也表明女山上地慢受到过地壳流体的影响,但是氧同位素的不平衡分馏和LILE的富集／HFSE的亏损并不是同步的,对这种现象的可能解释是：引起氧同位素不平衡分馏的交代作用和造成女山单斜辉石微量元素特征的交代作用不是同一期的,即女山上地幔曾经发生过至少2次交代事件.     

西克尔橄榄岩包体揭示塔里木西北缘岩石圈地幔经历多期次交代改造

西克尔新生代玄武岩中的二辉橄榄岩包体矿物主量元素、单斜辉石（Cpx）微量元素及Sr同位素被用来揭示塔里木西北缘岩石圈地幔经历的交代改造作用。依据岩相学和矿物化学特征,西克尔二辉橄榄岩可分为A、B、C三个组。A组二辉橄榄岩具有较高的橄榄石含量（57%~65%）和Mg^#（>90）,代表了较为古老且适度难熔的岩石圈地幔,其Cpx富集大离子亲石元素,具有较高的（La/Yb）_N、低的Ti/Eu比值,指示碳酸盐交代。根据Cpx的Sr同位素及高场强元素（HFSE）的异常特征,A组又可以细分为两个亚组（A1和A2）：A1亚组Cpx核部呈现高的⁸⁷Sr/⁸⁶Sr（0.70451~0.70485）和HFSE的强烈负异常,而A2亚组Cpx核部呈现低的⁸⁷Sr/⁸⁶Sr（0.70353~0.70389）和HFSE的轻微负异常,分别代表着俯冲沉积碳酸盐和软流圈来源碳酸盐交代作用。B组和C组二辉橄榄岩具有较低的橄榄石Mg^#（< 90）,代表了经受改造后饱满的岩石圈地幔。其中B组的Cpx具极低的Zr/Hf,以及与A1亚组相似的特征,可能与蚀变洋壳熔融所产生富CO₂硅酸盐熔体交代有关;C组尖晶石高的TiO₂含量和Cpx高的重稀土含量,指示明显高熔/岩比的熔体-橄榄岩反应印迹,Cpx勺型的轻稀土配分、较低的（La/Yb）_N、高的Ti/Eu比值、及HFSE强烈亏损的特征,表明其后期经历小体积富水硅酸盐熔体渗透的影响。此外A组Cpx的海绵边和B、C组包体与寄主岩交界处Cpx的反应边呈现与核部不一致的成分特征,同时B、C组包体与寄主岩交界处的斜方辉石也呈现被替代的现象。这些指示了橄榄岩在被携带至地表前不久或在此过程中熔体渗透引发的Cpx不一致熔融及熔岩反应的改造过程。多期次的地幔交代作用指示塔里木西北缘岩石圈地幔所经历的复杂改造历史。

     

吉林双辽地区橄榄岩包体中熔体-岩石作用特征及其对地幔交代作用的启示

吉林双辽地区古近纪玄武岩中一方辉橄榄岩包体记录了上地幔交代作用的信息。原生斜方辉石被交代成因的单斜辉石和橄榄石所围绕,或形成反应边结构,或斜方辉石残留在次生单斜辉石中。这些反应结构仅出现在尖晶石的周围。电子探针分析表明次生单斜辉石具有高Mg#、Cr#和CaO/Al2O3比值,次生橄榄石高Mg#、CaO和Cr2O3,被交代的尖晶石边部高Cr#、CaO。由于交代作用并未影响橄榄岩体系的Mg#,而且熔体-岩石反应结构指示Opx(斜方辉石) Sp(尖晶石) 熔体(Ⅰ)→Cpx(单斜辉石) Ol(橄榄石) 熔体(Ⅱ),结合实验以及文献资料,认为双辽地区的岩石圈地幔受到了硅酸盐熔体的交代。这种交代导致橄榄岩中斜方辉石逐渐减少,单斜辉石和橄榄石逐渐增加,从而使方辉橄榄岩渐变成易剥橄榄岩。这种交代现象可能发生在软流圈-岩石圈接触带上的熔-岩反应区,暗示了在古近纪双辽地区岩石圈减薄和软流圈上涌导致的软流圈-岩石圈的相互作用仍在继续。     

辉南新生代玄武岩中捕虏体橄榄岩矿物化学与华北岩石圈地幔演化

内容提要：在对吉林辉南新生代玄武岩中捕虏体橄榄岩详细的岩相学和矿物主量元素研究基础上,重点分析了单斜辉石激光原位微量元素,并讨论了辉南陆下岩石圈地幔的性质及其在华北克拉通破坏过程中的意义。橄榄石、单斜辉石和尖晶石的Mg#和Cr#均表明该区陆下岩石圈地幔主体是饱满的,同时也存在少量过渡和难熔型地幔。单斜辉石REE配分形式包括LREE亏损、倒U字型REE和LREE富集等不同类型。这些橄榄岩是地幔经历不同程度的部分熔融作用(低于10%)和复杂地幔交代作用的产物。交代介质主体是硅酸盐熔体,但个别样品有碳酸岩熔体交代的残留。橄榄石Mg#和平衡温度无相关性,说明辉南陆下岩石圈地幔不存在明显的分层而是交叉的。这些地幔特征和华北东部其他地区新生代的主体地幔性质相似,是熔-岩反应、侵蚀作用和上涌软流圈物质冷却转变置换的综合结果。     

Hf isotope compositions and HREE variations in off-craton garnet and spinel peridotite xenoliths from central Asia

Garnet-facies continental mantle is poorly understood because the vast majority of mantle xenoliths in continental basalts are spinel peridotite. Peridotite xenoliths from Vitim (southern Siberia) and Mongolia provide some of the best samples of garnet and garnet-spinel facies off-craton lithospheric mantle. Garnets in those fertile to moderately depleted lherzolites show a surprisingly broad range of HREE abundances, which poorly correlate with modal and major oxide compositions. Some garnets are zoned and have Lu-rich cores. We argue that these features indicate HREE redistribution after the partial melting, possibly related to spinel-garnet phase transition on isobaric cooling. Most peridotites from Vitim have depleted to ultra-depleted Hf isotope compositions (calculated from mineral analyses: ε_Hf(0) = +17 to +45). HREE-rich garnets have the most radiogenic ε_Hf values and plot above the mantle Hf-Nd isotope array while xenoliths with normal HREE abundances usually fall within or near the depleted end of the MORB field. Model Hf isotope ages for the normal peridotites indicate an origin by ancient partial melt extraction from primitive mantle, most likely in the Proterozoic. By contrast, an HREE-rich peridotite yields a Phanerozoic model age, possibly reflecting overprinting of the ancient partial melting record with that related to a recent enrichment in Lu. Clinopyroxene-garnet Lu-Hf isochron ages (31-84 Ma) are higher than the likely eruption age of the host volcanic rocks (∼16 Ma). Garnet-controlled HREE migration during spinel-garnet and garnet-spinel phase transitions may be one explanation for extremely radiogenic ¹⁷⁶Hf/¹⁷⁷Hf reported for some mantle peridotites; it may also contribute to Hf isotope variations in sub-lithospheric source regions of mantle-derived magmas.     

Refertilization of ancient lithospheric mantle beneath the central North China Craton: Evidence from petrology and geochemistry of peridotite xenoliths

The petrology and geochemistry of peridotite xenoliths in the Cenozoic basalts from Fanshi, the central North China Craton (NCC), provide constraints on the evolution of sub-continental lithospheric mantle. These peridotite xenoliths are mainly spinel-facies lherzolites with minor harzburgites. The lherzolites are characterized by low forsterite contents in olivines (Fo < 91) and light rare earth element (LREE) enrichments in clinopyroxenes. In contrast, the harzburgites are typified by high-Fo olivines (> 91), high-Cr# spinels and clinopyroxenes with low abundances of heavy REE (HREE). These features are similar to those from old refractory lithospheric mantle around the world, and thus interpreted to be relics of old lithospheric mantle. The old lithospheric mantle has been chemically modified by the influx of melts, as evidenced by the Sr–Nd isotopic compositions of clinopyroxenes and relatively lower Fo contents than typical Archean lithospheric mantle (Fo > 92.5). The Sr–Nd isotopic compositions of harzburgites are close to EM1-type mantle, and of the lherzolites are similar to bulk silicate earth. The latter could be the result of recent modification of old harzburgites by asthenospheric melt, which is strengthened by fertile compositions of minerals in the lherzolites. Therefore, the isotopic and chemical heterogeneities of the Fanshi peridotite xenoliths reflect the refertilization of ancient refractory lithospheric mantle by massive addition of asthenospheric melts. This may be an important mechanism for the lithospheric evolution beneath the Central NCC.     

Lithium isotopic systematics of peridotite xenoliths from Hannuoba, North China Craton: Implications for melt-rock interaction in the considerably thinned lithospheric mantle

Li concentrations and isotopic compositions of coexisting minerals (ol, opx, and cpx) from peridotite xenoliths entrained in the Hannuoba Tertiary basalts, North China Craton, provide insight into Li isotopic fractionation between mantle minerals during melt-rock interaction in the considerably thinned lithospheric mantle. Bulk analyses of mineral separates show significant enrichment of Li in cpx (2.4-3.6 ppm) relative to olivine (1.2-1.8 ppm), indicating that these peridotites have been affected by mantle metasomatism with mafic silicate melts. Bulk olivine separates (δ⁷Li ∼ +3.3‰ to +6.4‰) are isotopically heavier than coexisting pyroxenes (δ⁷Li ∼ −3.3‰ to −8.2‰ in cpx, and −4.0‰ to −6.7‰ in opx). Such large variation suggests Li elemental and isotopic disequilibrium. This conclusion is supported by results from in situ SIMS analyses of mineral grains where significant Li elemental and isotopic zonations exist. The olivine and opx have lower Li concentrations and heavier Li isotopes in the rims than in the cores. This reverse correlation of δ⁷Li with Li concentrations indicates diffusive fractionation of Li isotopes. However, the zoning patterns in coexisting cpx show isotopically heavier rims with higher Li abundances. This positive correlation between δ⁷Li and Li concentrations suggests a melt mixing trend. We attribute Li concentration and isotope zonation in minerals to the effects of two-stage diffusive fractionation coupled with melt-rock interaction. The earliest melts may have been derived from the subducted oceanic slab with low δ⁷Li values produced by isotopic fractionation during the dehydration of the seawater-altered slab. Melts at later stages were derived from the asthenosphere and interacted with the peridotites, producing the Li elemental and isotopic zoning in mineral grains. These data thus provide evidence for multiple-stage peridotite-melt interaction in the lithospheric mantle beneath the northern North China Craton.     

Effect of melt composition on basalt and peridotite interaction: laboratory dissolution experiments with applications to mineral compositional variations in mantle xenoliths from the North China Craton

Interaction between basaltic melts and peridotites has played an important role in modifying the lithospheric and asthenospheric mantle during magma genesis in a number of tectonic settings. Compositions of basaltic melts vary considerably and may play an important role in controlling the kinetics of melt–peridotite interaction. To better understand the effect of melt composition on melt–peridotite interaction, we conducted spinel lherzolite dissolution experiments at 2 GPa and 1,425 °C using the dissolution couple method. The reacting melts include a basaltic andesite, a ferro-basalt, and an alkali basalt. Dissolution of lherzolite in the basaltic andesite and the ferro-basalt produced harzburgite–lherzolite sequences with a thin orthopyroxenite layer at the melt–harzburgite interface, whereas dissolution of lherzolite in the alkali basalt produced a dunite–harzburgite–lherzolite sequence. Systematic variations in mineral compositions across the lithological units are observed. These mineral compositional variations are attributed to grain-scale processes that involve dissolution, precipitation, and reprecipitation and depend strongly on reacting melt composition. Comparison of mineral compositional variations across the dissolution couples with those observed in mantle xenoliths from the North China Craton (NCC) helps to assess the spatial and temporal variations in the extent of siliceous melt and peridotite interaction in modifying the lithospheric mantle beneath the NCC. We found that such melt–rock interaction mainly took place in Early Cretaceous, and is responsible for the enrichment of pyroxene in the lithospheric mantle. Spatially, siliceous melt–peridotite interaction took place in the ancient orogens with thickened lower crust.     

The trapped fluid phase in upper mantle xenoliths from Victoria,Australia: implications for mantle metasomatism

Mantle-derived xenoliths of spinel lherzolite, spinel pyroxenite, garnet pyroxenite and wehrlite from Bullenmerri and Gnotuk maars, southwestern Victoria, Australia contain up to 3 vol.% of fluids trapped at high pressures. The fluid-filled cavities range in size from fluid inclusions (1–100 m) up to vugs 11/2 cm across, lined with euhedral high-pressure phases. The larger cavities form an integral part of the mosaic microstructure. Microthermometry and Raman laser microprobe analysis show that the fluids are dominantly CO₂. Small isolated inclusions may have densities 1.19 g/cm³, but most inclusions show microstructural evidence of partial decrepitation during eruption, and these have lower fluid densities. Mass-spectrometric analysis of gases released by crushing or heating shows the presence of He, N₂, Ar, H₂S, CO_s and SO₂ in small quantities; these may explain the small freezing-point depressions observed in some inclusions. Petrographic, SEM and microprobe studies show that the trapped fluids have reacted with the cavity walls (in clinopyroxene grains) to produce secondary amphiboles and carbonates. The trapped CO₂ thus represents only a small residual proportion of an original volatile phase, which has undergone at least two stages of modification — first by equilibration with spinel lherzolite to form amphibole (±mica±apatite), then by limited reaction with the walls of the fluid inclusions. The inferred original fluid was a CO₂-H₂O mixture, with significant contents of (at least) Cl and sulfur species. Generation of this fluid phase in the garnet-peridotite stability field, followed by its migration to the spinel peridotite stability field, would provide an efficient mechanism for metasomatic enrichment of the upper mantle in LIL elements. This migration could involve either a volatile flux or transport in small volumes of silicate melt that crystallize in the spinel peridotite field. These observations suggest that some portions of the subcontinental upper mantle contain large reservoirs of free fluid CO₂, which may be liberated during episodes of rifting or magmatism, to induce granulite-facies metamorphism of the lower crust.     

Ultra-depleted peridotite xenoliths in the Northern Taihang Mountains: Implications for the nature of the lithospheric mantle beneath the North China Craton

We report the finding of peridotite xenoliths in the Early Cretaceous Longmengou olivine-bearing diabase (138 Ma) in the Northern Taihang Mountains in the central North China Craton. Based on the modal proportions of olivine, clinopyroxene, amphibole and anorthite, these peridotite xenoliths can be divided into three zones: clinopyroxene-bearing olivine zone (COZ), olivine-clinopyroxene zone (OCZ), and amphibole-bearing anorthite-clinopyroxene zone (AACZ). The core of olivine grains in clinopyroxene-bearing olivine zone have higher Mg^# (> 95), SiO₂ (41.80–42.53 wt%) and lower CaO (< 0.07 wt%), FeO (3.91–4.54 wt%) than the rim (Mg^# = 92.5–93.4, SiO₂ = 41.27–41.98 wt%, CaO = 0.20–0.34 wt%, and FeO = 7.02–8.87 wt%), suggesting that rim is reaction product. The core of olivine grains with higher Mg^# (> 95) and lower NiO content (< 0.04 wt%) in the clinopyroxene-bearing olivine zone was derived from ultra-depleted mantle subsequently altered by high Mg^# melts/magma with low Ni. Two generations of olivine grains occur in the OCZ where the first generation shows exsolution of ilmenite and magnetite rods containing up to 0.35 wt% TiO₂, and was likely derived from garnet peridotite hydrated by water. The second generation shows high Mg^# (96.2–97.1) and cataclastic texture, and was possibly formed by decomposition of the COZ. The occurrence of aluminous spinel suggests the role of melts with extremely high Al and Mg. Clinopyroxene in the AACZ shows systematic core-rim compositional variation with CaO and SiO₂ contents increasing towards the rim, and MgO and Fe₂O₃ concentrations decreasing from the core to the rim, indicating that the amphibole-bearing anorthite-clinopyroxene zone is a product of the reaction between mantle xenoliths and mafic magma. Plagioclase with high An value (92.0–99.95, average 97.79) indicates that the metasomatic melts have high Ca/Na and Al/Si ratios, possibly produced by the partial melting of ultra-depleted mantle under “wet” conditions. Combined with the data on other mantle xenoliths discovered in the NCC, our results suggest that the Mesozoic lithospheric mantle beneath the North Taihang Mountains within the central NCC is composed of ultra-depleted Archean and Paleoproterozoic peridotites and dunites modified by complex melts. We also propose that the destruction of eastern part of the NCC mainly occurred during Early Cretaceous, and that the boundary of the lithospheric destruction coincides with the Taihang Mountains.     

Petrology and geochemistry of spinel peridotite xenoliths from Hannuoba and Qixia, North China craton

We report mineralogical and chemical compositions of spinel peridotite xenoliths from two Tertiary alkali basalt localities on the Archean North China craton (Hannuoba, located in the central orogenic block, and Qixia, in the eastern block). The two peridotite suites have major element compositions that are indistinguishable from each other and reflect variable degrees (0–25%) of melt extraction from a primitive mantle source. Their compositions are markedly different from typical cratonic lithosphere, consistent with previous suggestions for removal of the Archean mantle lithosphere beneath this craton. Our previously published Os isotopic results for these samples [Earth Planet. Sci. Lett. 198 (2002) 307] show that lithosphere replacement occurred in the Paleoproterozoic beneath Hannuoba, but in the Phanerozoic beneath Qixia. Thus, we see no evidence for a compositional distinction between Proterozoic and Phanerozoic continental lithospheric mantle. The Hannuoba xenoliths equilibrated over a more extensive temperature (hence depth) interval than the Qixia xenoliths. Neither suite shows a correlation between equilibration temperature and major element composition, indicating that the lithosphere is not chemically stratified in either area. Trace element and Sr and Nd isotopic compositions of the Hannuoba xenoliths reflect recent metasomatic overprinting that is not related to the Tertiary magmatism in this area.     

华北中、新生代岩石圈地幔的交代作用：含金云母地幔岩提供的证据

本文对华北克拉通三个不同地区（河北汉诺坝、内蒙古集宁三义堂、河南鹤壁）新发现的含金云母尖晶石二辉橄榄岩和尖晶石橄榄单斜辉石岩捕虏体进行了详细的矿物组成、单斜辉石的微量元素和Sr-Nd同位素研究.通过与相同地区不含金云母尖晶石二辉橄榄岩捕虏体的系统对比发现通常含金云母的地幔橄榄岩比不含金云母的地幔橄榄岩岩富Al2O3、CaO、NaO、K2O、TiO2,但相对贫镁;其单斜辉石的LREE更为富集,但Sr、Nd同位素组成则相对亏损.这说明地幔交代作用不仅能够造成地幔橄榄岩的玄武质组分和稀土元素的富集,而且亦能够造成全岩和橄榄石Mg#的降低和同位素组成的相对亏损.捕虏体的Rb-Sr等时线年龄暗示地幔交代作用发生在中、新生代;其交代熔体来源于软流圈.同时说明华北新生代岩石圈地幔普遍存在的主、微量元素和同位素组成类似于“大洋型”岩石圈地幔的特征很可能是岩石圈地幔橄榄岩与软流圈来源的熔体的大规模反应的结果,而非真正意义上的新增生的岩石圈地幔.     

Heterogeneous distribution of phosphorus in olivine from otherwise well-equilibrated spinel peridotite xenoliths and its implications for the mantle geochemistry of lithium

The major- and trace-element abundances of the coexisting phases of four metasomatized spinel peridotite xenoliths from the Anakies locality (SE Australia) were determined by electron microprobe and laser-ablation ICP-MS. The compositions of all phases are remarkably homogeneous, with the exception of phosphorus (P), lithium (Li) and sodium (Na) in olivine. These three elements are enriched in large parts of most olivine crystals due to a second metasomatic episode. Apart from these elements, all phases are in mutual equilibrium with respect to both their major- and trace-element compositions. Li and Na show a strong correlation with P in olivine, although molar Li + Na are an order of magnitude less than molar P, indicating that the substitution mechanism of these elements is more complex than the simple charge-balanced coupled exchange ^IVSi⁴⁺ + ^VI(FeMg)²⁺ = ^IVP⁵⁺ + ^VI(LiNa)⁺. We suggest that Li and Na are decorating octahedral-site cation vacancies formed by the original incorporation of P. Elemental maps revealed that the P zoning patterns are concentric in a few large olivine porphyroblasts, but form irregular patches in most crystals. This distribution of P is proposed to be the result of a two-stage process, whereby the initial concentric zoning, caused by its exceptionally sluggish diffusion after metasomatic influx, is broken up by extensive sub-solidus deformation and recrystallization, attesting to large grain-scale strains even within the lithosphere. Such strains must be an efficient means of ensuring trace-element equilibrium during partial melting. The association of Li with P in olivine may help to explain the variability of Li abundances in mantle minerals and to interpret Li diffusion experiments and Li isotopic fractionation.     

Multi-stage metasomatism of lithospheric mantle by asthenosphere-derived melts: evidence from mantle xenoliths in daxizhuang at the eastern North China craton

Mineralogy and Petrology - A detailed study on petrology and mineral chemistry of 12 mantle xenoliths from Late Cretaceous basaltic lava flows at Daxizhuang has been conducted to constrain the...     

Zoned minerals in garnet peridotite nodules from the Colorado Plateau: implications for mantle metasomatism and kinetics

Some garnet peridotite nodules from The Thumb, a minette neck on the Colorado Plateau in the southwestern United States, contain zoned minerals. Zoning does not exceed 1.5 wt.% for any oxide, but some relative changes are large: in one garnet TiO₂ and Cr₂O₃ ranges are 0.05–0.65 and 3.5–5.0 wt.%, respectively. In two porphyroclastic nodules, garnet rims are depleted in Mg and enriched in Fe, Ti, and Na compared to cores, and one garnet is irregularly zoned in Ti and Cr. Olivine crystals in these rocks are unzoned, and pyroxene zoning is slight, yet matrix olivine and pyroxene contain more Fe and Ti and less Mg and Cr than inclusions of these phases in garnet. In three coarse nodules, garnet rims are Ti-rich compared to cores, and Ca, Fe, Mg, and Cr zoning patterns are complex. Several nodules appear to have partially equilibrated near 1200° C and 35 kb, and under these conditions cation mobility in pyroxene was greater than in garnet. The zoning partly reflects Fe and Ti metasomatism in the mantle. Calculations indicate that Fe-Mg gradients in garnet could have persisted for only a short time in the mantle, perhaps thousands of years or less, so the metasomatism occurred shortly before eruption. The minette host, a likely source of the Fe and Ti, is rich in light rare earth elements: since the nodules are much poorer in these elements, little or no infiltrated minette was trapped in them. Diffusion is a possible mechanism for nodule metasomatism. Some fertile peridotite nodules from kimberlites may have been affected by similar events. Compositional differences between inclusions in garnet and matrix phases are intriguingly similar to some of the differences between most peridotite inclusions in diamonds and common lherzolite phases.     

Depleted and enriched mantle processes under the Rio Grande rift: spinel peridotite xenoliths

Upper mantle xenoliths from the southern Rio Grande rift axis (Potrillo and Elephant Butte) and flank (Adam’s Diggings) have been investigated to determine chemical depletion and enrichment processes. The variation of modal, whole rock, and mineral compositions reflect melt extraction. Fractional melting is the likely process. Fractional melting calculations show that most spinel peridotites from rift axis locations have undergone <5% melting versus 7–14% melting for xenoliths from the rift shoulder, although the total range of fractional melting overlaps at all three locations. In the rift axis, deformed (equigranular and porphyroclastic texture) spinel peridotites are generally characterized by significantly less fractional melting (2–5%) than undeformed (protogranular) xenoliths (up to 16%). This difference may reflect undeformed xenoliths being derived from greater depths and higher temperatures than deformed rocks. Spinel peridotites from the axis and shoulder of the Rio Grande rift have undergone mantle metasomatism subsequent to melt extraction. Under the rift shoulder spinel peridotites have undergone both cryptic and patent (modal) metasomatism, possibly during separate events, whereas the upper mantle under the rift axis has undergone only cryptic metasomatism by alkali basaltic magma.     

华北地幔捕虏体Fe-Mg-Ca同位素研究进展

近年来,得益于同位素分析技术和质谱仪器性能的提高,使得铁（Fe）、镁（Mg）和钙（Ca）等非传统稳定同位素的高精度测量成为可能,并很快在地球化学、天体化学和生物地球化学等研究领域取得了丰硕的成果。本文通过对比分析来自华北克拉通不同地区不同类型地幔捕虏体的Fe、Mg和Ca位素组成特征,揭示华北克拉通岩石圈地幔Fe、Mg和Ca同位素组成不均一性的成因,并在此基础上,探讨华北大陆岩石圈地幔演化过程如部分熔融、橄榄岩-熔体反应过程、熔体的性质和来源等科学问题,为华北克拉通岩石圈的演化过程提供新证据。

     

Mapping lithospheric boundaries using Os isotopes of mantle xenoliths: An example from the North China Craton

The petrology, mineral compositions, whole rock major/trace element concentrations, including highly siderophile elements, and Re-Os isotopes of 99 peridotite xenoliths from the central North China Craton were determined in order to constrain the structure and evolution of the deep lithosphere. Samples from seven Early Cretaceous-Tertiary volcanic centers display distinct geochemical characteristics from north to south. Peridotites from the northern section are generally more fertile (e.g., Al₂O₃ = 0.9-4.0%) than those from the south (e.g., Al₂O₃ = 0.2-2.2%), and have maximum whole-rock Re-depletion Os model ages (T_RD) of ∼1.8 Ga suggesting their coeval formation in the latest Paleoproterozoic. By contrast, peridotites from the south have maximum T_RD model ages that span the Archean-Proterozoic boundary (2.1-2.5 Ga). Peridotites with model ages from both groups are found at Fansi, the southernmost locality in the northern group, which likely marks a lithospheric boundary. The Neoarchean age of the lithospheric mantle in the southern section matches that of the overlying crust and likely reflects the time of amalgamation of the North China Craton via collision between the Eastern and Western blocks. The Late Paleoproterozoic (∼1.8 Ga) lithospheric mantle beneath the northern section is significantly younger than the overlying Archean crust, indicating that the original lithospheric mantle was replaced in this region, either during a major north-south continent-continent collision that occurred during assembly of the Columbia supercontinent at ∼1.8-1.9 Ga, or from extrusion of ∼1.9 Ga lithosphere from the Khondalite Belt beneath the northern Trans-North China Orogen, during the ∼1.85 Ga continental collision between Eastern and Western blocks. Post-Cretaceous heating of the southern section is indicated by high temperatures (>1000 °C) recorded in peridotites from the 4 Ma Hebi suite, which are significantly higher than the temperatures recorded in peridotites from the nearby Early Cretaceous Fushan suite (<720 °C), and likely reflects significant lithospheric thinning after the Early Cretaceous. Combining previous Os isotope results on mantle xenoliths from the eastern North China Craton with our new data, it appears that lithospheric thinning and replacement may have evolved from east to west with time, commencing before the Triassic on the eastern edge of the craton, occurring during the Jurassic-Cretaceous within the interior, and post-dating 125 Ma on the westernmost boundary.