Rare earth element partitioning between minerals from anhydrous spinel peridotite xenoliths

REE abundances in minerals from spinel peridotite xenoliths from West Germany, the south-western U.S. and Mongolia decrease in the order clinopyroxene > orthopyroxene > olivine > spinel. While clinopyroxenes are similar in absolute chondrite-normalized concentrations to those known from other studies, orthopyroxenes and olivines are significantly lower in LREE although comparable in HREE. Spinels are much lower in all REE than any previously reported values and are completely negligible for the REE budget of peridotites.Partition coefficients for most orthopyroxene/clinopyroxene pairs increase systematically from La to Lu. Olivine/clinopyroxene and spinel/clinopyroxene partition coefficients increase from the intermediate rare earth elements to Lu and normally are higher for La compared to Sm.The application of Nagasawa's (1966) elastic lattice model suggests that all heavy but only minor amounts of the light REE substitute into structural positions of orthopyroxene and olivine.Significant differences between orthopyroxene/clinopyroxene partition coefficients for various xenoliths may be assigned to dependences upon equilibration temperature and bulk chemistry.Apart from grain surface contaminations, fluid inclusions which are practically always present in mantle minerals, can highly concentrate light rare earth elements and thus may be responsible for unexpectedly high concentrations of incompatible elements frequently reported for mantle olivines or orthopyroxenes.     

Integrated Models of Basalt Petrogenesis: A Study of Quartz Tholeiites to Olivine Melilitites from South Eastern Australia Utilizing Geochemical and Experimental Petrological Data

The Tertiary to Recent basalts of Victoria and Tasmania havemineralogical and major element characteristics of magmas encompassingthe range from quartz tholeiites to olivine melilitites. Abundancesof trace elements such as incompatible elements, including therare earth elements (REE), and the compatible elements Ni, Coand Sc, vary systematically through this compositional spectrum.On the basis of included mantle xenoliths, appropriate 100 Mg/Mg+ Fe₊₂ (68–72) and high Ni contents many of these basaltsrepresent primary magmas (i.e., unmodified partial melts ofmantle peridotite). For fractionated basalts we have derivedmodel primary magma compositions by estimating the compositionalchanges caused by fractional crystallization of olivine andpyroxene at low or moderate pressure. A pyrolite model mantlecomposition has been used to establish and evaluate partialmelting models for these primary magmas. By definition and experimentaltesting the specific pyrolite composition yields parental olivinetholeiite magma similar to that of KilaeauIki, Hawaii (1959–60)and residual harzburgite by 33 per cent melting. It is shownthat a source pyrolite composition differing only in having0.3–0.4 per cent TiO₂ rather than 0.7 per cent TiO₂, isable to yield the spectrum of primary basalts for the Victorian-Tasmanianprovince by 4 per cent to 25 per cent partial melting. The mineralogiesof residual peridotites are consistent with known liquidus phaserelationships of the primary magmas at high pressures and thechemical compositions of residual peridotite are similar tonatural depleted or refractory lherzolites and harzburgites.For low degrees of melting the nature of the liquid and of theresidual peridotite are sensitively dependent on the contentof H₂O, CO₂ and the CO₂/H₂O in the source pyrolite. The melting models have been tested for their ability to accountfor the minor and trace element, particularly the distinctivelyfractionated REE, contents of the primary magmas. A single sourcepyrolite composition can yield the observed minor and traceelement abundances (within at most a factor of 2 and commonlymuch closer) for olivine melilitite (4–6 per cent melt),olivine nephelinite, basanite (5–7 per cent melt), alkaliolivine basalt (11–15 per cent melt), olivine basalt andolivine tholeiite (20–25 per cent melt) provided thatthe source pyrolite was already enriched in strongly incompatibleelements (Ba, Sr, Th, U, LREE) at 6–9 x chondritic abundancesand less enriched (2.5–3 x chondrites) in moderately incompatible(Ti, Zr, Hf, Y, HREE) prior to the partial melting event. Thesources regions for S.E. Australian basalts are similar to thosefor oceanic island basalts (Hawaii, Comores, Iceland, Azores)or for continental and rift-valley basaltic provinces and verydifferent in trace element abundances from the model sourceregions for most mid-ocean ridge basalts. We infer that thismantle heterogeneity has resulted from migration within theupper mantle (LVZ or below the LVZ) of a melt or fluid (H₂O,CO₂-enriched) with incompatible element concentrations similarto those of olivine melilitite, kimberlite or carbonatite. Asa result of this migration, some mantle regions are enrichedin incompatible elements and other areas are depleted. Although it is possible, within the general framework of a lherzolitesource composition, to derive the basanites, olivine nephelinitesand olivine melilitites from a source rock with chondritic relativeREE abundances at 2–5 x chondritic levels, these modelsrequire extremely small degrees of melting (0.4 per cent forolivine melilitite to 1 per cent for basanite). Furthermore,it is not possible to derive the olivine tholeiite magmas fromsource regions with chondritic relative REE abundances withoutconflicting with major element and experimental petrology argumentsrequiring high degrees (15 per cent) of melting and the absenceof residual garnet. If these arguments are disregarded, andpartial melting models are constrained to source regions withchondritic relative REE abundances, then magmas from olivinemelilitites to olivine tholeiites can be modelled if degreesof melting are sufficiently small, e.g., 7 per cent meltingfor olivine tholeiite. However, the source regions must be heterogenousfrom 1 to 5 x chondritic in absolute REE abundances and heterogerieousin other trace elements as well. This model is rejected in favorof the model requiring variation in degree of melting from 4per cent to 25 per cent and mantle source regions ranging fromLREE-enriched to LREE-depleted relative to chondritic REE abundances.     

粗粒与剪切结构橄榄岩捕虏体及其单斜辉石微量元素对比

地山西栖霞具不同结构的“干”灾晶石相橄榄岩进行了全岩化学、微量元素,矿物成分和单斜辉石微量元素分析和对比。表明在橄榄岩从粗粒结构向剪切结构的转化中,随着变质变形作用的增强存在着复杂的熔／流体的加入富集和熔体的提取亏损作用;交代介质属具强渗透性的SiO2不饱和的硅酸盐碳酸岩熔体。同时发现不同结构橄榄岩中单斜辉石的REE与其全岩的REE程度有如下的关系;粗粒结构橄榄岩石中矿物与岩石的差别量大,但REE的配合分形可以反映全岩的情况;剪切结构橄榄岩中两者的差别较小。其它高度不相容微量元素可能主要赋存粒间组分或／和矿物流体包裹体中。     

Experimental determination of partition coefficients for rare earth and high-field-strength elements between clinopyroxene,garnet, and basaltic melt at high pressures

Clinopyroxene/melt and garnet/melt partition coefficients have been determined for Ti, Sr, Y, Zr, Nb, Hf, and rare earth elements from 19 doped experiments on 1921 Kilauea basalt. The experiments were carried out from 2.0 to 3.0 GPa and 1310° to 1470 °C. The purpose was to derive a set of partition coefficients for high-field-strength elements (HFSE) and rare earth elements (REE) in a systematic, linked set of experiments at P and T conditions relevant to basalt petrogenesis. These data are used in melting models to understand the development of negative HFSE anomalies observed in many abyssal peridotite clinopyroxenes. It is shown that melting can account for the observed trace element patterns in some residual peridotites, but that other processes may also be needed to account for most residual mantle compositions in mid-ocean ridge systems. It is also shown that REE are more strongly fractionated by garnet at these P-T conditions than previously thought. Received: 1 July 1997 / Accepted: 11 May 1998     

Origin of Ross Island basanitoids and limitations upon the heterogeneity of mantle sources for alkali basalts and nephelinites

Primary basanitoids from Ross Island, Antarctica have REE patterns and Pb isotope ratios similar to those for primary alkali basalts and nephelinites on ocean islands. The lead data from all volcanics on Ross Island have a spread of 4% in the 206/204 ratio and give a two-stage model lead age of 1500 m.y. The age is interpreted to be the time since the development of the chemical heterogeneity of the mantle source, presumably during an earlier melting process. Comparison of REE, K, Rb, Sr, Ba and P₂O₅ concentrations for alkali basalts and nephelinites shows that the chondrite normalized mantle source is enriched in light REE with average La/Sm=3.4, Ce/Sm=2.6, Nd/Sm=1.6. Assuming a mantle source with heavy REE abundances of three times chondrites, nephelinites require 3 to 7% partial melting of the mantle source and alkali basalts require 7 to 15% partial melting. The patterns of K, Cu, V and Ti abundances suggest that phlogopite is a residual mineral for most nephelinite, but not alkali basalt mantle sources, and that a sulfide phase and a titanium-rich mineral are in the residual mantle source for both alkali basalts and nephelinites. Small positive Eu anomalies (2–5%) in near primary alkali basalts and nephelinites suggest that the xxx of the mantle sources is 10^?6 to 10^?9 atm. The progressive enrichment of light REE and incompatible elements in the mantle sources for nephelinites and alkali basalts is proposed to result by intrusion of veins of basaltic melt due to very low percentages of melting 1 000 to 3 000 m.y. ago when this part of the deeper mantle was previously involved in convection and partial melting.     

Major Element Compositions and Rare—earth Element Abundaces of Cenozoic Basalts in Eastern China：Implications for a Pressure Control over LREE／HREE Fractionation in Continental Basalt

Major element compositions and rare-earth element (REE) and transition element(Ni,Cr and V) abundances have been determined on 44 basalt samples from eastern China.These basalts have SiO2 contents ranging from 38.63 to 55.24(wt.%),and Na2O K2O from 3.1 to 9.4(wt.%).Ni and Cr abundances are largely variable,respectively falling in ranges 60-605 and 78-1150 ppm.REE abundances,especially light rare-earth elements(LREE), are highly variable.La/Sm and La/Yb ratios vary 2.8 to 7.6 and 1.8 to 8.1. Although the segregation mainly of olivine and clinopyroxene is requested to account for the vari-able and low MgO,CaO/Al2O3,Cr and Ni characteristic of these basalts studied here,the differ-ences in REE composition of the basalts are still related mainly to the partial melting process.Obvious varations in REE abundances could be principally attributed to the partial melting process.Obvious variations in REE abundances could be principally attributed to the partial melting processes that took place at different depths,in spite of some variations caused by the fractional crystallization processes.REE abundances and La/Sm and La/Yb ratios systematically decrease with increasing SiO2,which probably indicated that the basaltic magma derived from a deeper level has higher LREE and LREE/HREE ratios than that from a shallower level.As viewed from the fact that the D^Yb/D^La ratios of clinopyroxenes in the basaltic system increase with increasing pressure,the increase of LREE/HUEE ratios with increasing melting depth can be interpreted as the pressure dependence of bulk D^HREE/D^LREE ratios of silicate minerals,in addition to the pressure control over the melting degree.     

阿尔泰造山带南缘中泥盆世苦橄岩及其大地构造和岩石学意义

阿尔泰造山带南缘中泥盆世苦橄岩位于北塔山组地层的下部, 其上依次为玄武岩和安山岩.3种岩性共同的特点是贫钛、富铁, 具Nb和Ta的负异常以及高场强元素的丰度与MORB相当, 具有典型的岛弧火山岩系的特点, 是准噶尔洋板块向南西俯冲的结果.苦橄岩和玄武岩的Zr/Nb和Sm/Nd比值与MORB相当, 表明其源区为亏损的MORB源.然而玄武岩的Ti/V和Zr/Sm比值均高于苦橄岩, 而且玄武岩的稀土元素配分曲线呈平缓型, 而苦橄岩则显示出低的稀土总量以及弱富集轻稀土型, 指示了玄武岩是被从俯冲的洋壳释放的流体交代的含角闪石的尖晶石橄榄岩的地幔源区低程度部分熔融形成的, 苦橄岩则是在高温条件下被流体交代过的石榴石橄榄岩高程度熔融的产物.安山岩则可能是榴辉岩部分熔融形成的.      

峨眉山玄武岩的辉石研究

本文研究的峨眉山玄武岩的辉石均为单斜辉石。东川碱性火山岩中以透辉石为主,显示碱性岩系辉石的特征,但相对贫 LREE、Ti 等不相容元素,结晶时的 fO₂较高;攀枝花岩带主要是透辉石质普通辉石;其他剖面和地区多为普通辉石,且具拉斑质岩系富钙辉石的成分和演化特点。攀枝花带和西岩区辉石在化学成分上比较原始,而东岩区和中岩区辉石的成分演化程度相对较高。含 Ti 的“其他”阳离子对在各岩区（带）的辉石中均限重要,暗示它们的形成与火山弧环境无关。     

Cenozoic alkali basaltic magmas of western Germany and their products of differentiation

Analytical data on major elements and 31 trace elements in olivine nephelinites, nepheline basanites, basanitic alkali olivine basalts and their differentiates (tephrites, hawaiites, mugearites, benmoreites, latites, phonolites and trachytes) from Hegau, Kaiserstuhl, Rhön, Hessian Depression, Vogelsberg, Westerwald, Siebengebirge, E Eifel and Hocheifel are evaluated. They were based on 400 samples with new or unpublished data on about one third of the rocks. The Sr–Nd isotopic compositions for 78 rocks are included. The alkali basaltic volcanism is caused by adiabatic decompression of asthenospheric mantle updomed to a minimum depth of 50 km in connection with the Alpine continent collision. The chemical compositions of the primary basaltic melts from the different areas are similar containing about one hundred-fold enrichment of highly incompatible elements relative to the primitive mantle from partial melting of depleted and secondarily enriched peridotite. The elements Cs, K, Pb and Ti are specifically depleted in the basalts partly because of phlogopite being residual at partial melting. The Tertiary alkali basalts range in Nd-isotopic composition from 0.51288 to 0.51273 and in Sr-isotopic ratios from 0.7032 to 0.7042. These ranges indicate mixtures of HIMU, depleted and enriched mantle components in the metasomatically altered peridotite source which resembles that of certain ocean islands. The Nd-Sr-isotopic compositions of the Quaternary E Eifel are close to bulk Earth ratios. East and W Eifel plots differ distinctly from the Tertiary Hocheifel which is geographically intermediate. This isotopic difference, beside specific K/Na ratios, is probably caused by separate metasomatic pulses that immediately preceded the respective periods of volcanism. The metasomatically altered mantle had partly primitive mantle signatures (Nb/Ta, Zr/Sm and Th/U ratios) and partly ocean island (or MORB) source properties (Rb/Cs). A MORB source can be excluded because of the low K/Rb and high Th/U ratios. A correlation of D with ⁸⁷Sr/⁸⁶Sr in amphibole and phlogopite and a slightly larger ¹⁸O than in MORB is conformable with a seawater and crustal impact on the source of alkali basalts. Slightly higher than average water concentrations in the source of certain primary basaltic melts (indicated by amphibole phenocrysts in their basalts) are required for differentiation of these basalts in magma chambers of the upper crust. Model calculations are presented to explain compositions of differentiates which range from about 60% to about 20% residual melt. The latter are represented by phonolites and trachytes. The Nd- and Sr-isotopic signatures of the majority of differentiates indicate contamination by a granitic partial melt from the wall rocks of magma chambers. Olivine nephelinite magma was the common source of contaminated differentiates.     

The Role of Lithospheric Mantle Heterogeneity in the Generation of Plio-Pleistocene Alkali Basaltic Suites from NW Harrat Ash Shaam (Israel)

Plio-Pleistocene volcanism in the Golan and Galilee (northeasternIsrael) shows systematic variability with time and location:alkali basalts were erupted in the south during the Early Pliocene,whereas enriched basanitic lavas erupted in the north duringthe Late Pliocene (Galilee) and Pleistocene (Golan). The basaltsshow positive correlations in plots of ratios of highly to moderatelyincompatible elements versus the concentration of the highlyincompatible element (e.g. Nb/Zr vs Nb, La/Sm vs La) and indiagrams of REE/HFSE (rare earth elements/high field strengthelements) vs REE concentration (e.g. La/Nb vs La). Some of thesecorrelations are not linear but upward convex. ⁸⁷Sr/⁸⁶Sr ratiosvary between 0·7031 and 0·7034 and correlate negativelywith incompatible element concentrations and positively withRb/Sr ratios. We interpret these observations as an indicationthat the main control on magma composition is binary mixingof melts derived from two end-member mantle source components.Based on the high Sr/Ba ratios and negative Rb anomalies inprimitive mantle normalized trace element diagrams and the moderateslopes of MREE–HREE (middle REE–heavy REE) in chondrite-normalizeddiagrams, we suggest that the source for the alkali basalticend-member was a garnet-bearing amphibole peridotite that hadexperienced partial dehydration. The very high incompatibleelement concentrations, low K content, very low Rb contentsand steep MREE–HREE patterns in the basanites are attributedto derivation from amphibole- and garnet-bearing pyroxeniteveins. It is suggested that the veins were produced via partialmelting of amphibole peridotites, followed by complete solidificationand dehydration that effectively removed Rb and K. The requirementfor the presence of amphibole limits both sources to lithosphericdepths. The spatial geochemical variability of the basalts indicatesthat the lithosphere beneath the region is heterogeneous, composedof vein-rich and vein-poor domains. The relatively uniform ¹⁴³Nd/¹⁴⁴Nd(Nd = 4·0–5·2) suggests that the two mantlesources were formed by dehydration and partial melting of anoriginally isotopically uniform reservoir, probably as a resultof a Paleozoic thermal event. KEY WORDS: basanites; lithospheric heterogeneity; magma mixing; amphibole peridotite; pyroxenites     

Petrological and geochemical relationships between pyroxene megacrysts and associated alkali-basalts from Massif Central (France)

Major, trace element, and REE analyses, as well as Sr isotopic ratios, have been obtained on twelve clinopyene megacrysts and phenocrysts and their alkali-basalt hosts from the French Massif Central. Equilibrium between crystals and host was examined based on petrographic and geochemical data.Two types of pyroxenes are recognized: the acmite-bearing clinopyroxenes, rich in incompatible elements and the salitic clinopyroxenes, poor in incompatible elements. ⁸⁷Sr/ ⁸⁶Sr isotopic data reveal no significant difference between clinopyroxenes and host lavas: they are in apparent isotopic equilibrium. The Sr isotopic ratios of the two types of pyroxenes are also quite similar. However pyroxene crystals from the first group are not in equilibrium with their host; they have crystallized at high-pressure from differentiated alkali-lavas and have been incorporated in a more primitive magma. Pyroxene crystals from the second group are in apparent equilibrium with their host lava; they have crystallized at various pressures. For the latter, distribution coefficients are proposed for compatible elements, trace elements and REE.     

An Example of Consequent Mantle Metasomatism in Peridotite Inclusions from Nunivak Island, Alaska

Many of the coarse-grained peridotite inclusions in basanitesfrom Nunivak Island, Alaska, contain amphibole and a smallerfraction also contain phlogopite and apatite. All of these peridotiteshave light REE/heavy REE abundance ratios greater than chondritesand many have abundances of K, Rb, Sr, Ba and light REE whichexceed estimates for primitive mantle. On the basis of mineraltextures and compositions we infer that the clinopyroxene, amphibole,phlogopite and apatite equilibrated with a metasomatic fluid.Isotopic (Sr and Nd) ratios and parent-daughter abundance datafor the coarse-grained peridotites constrain the age of themetasomatism to be less than 200 million years. Associated amphibole pyroxenite inclusions are not metasomatized;these inclusions probably formed as crystal segregates froman alkalic magma. Both pyroxenites and coarse-grained peridotitesare isotopically similar to basalts from Nunivak Island. Usingthese data, we propose a model in which the metasomatized peridotiteswere wallrocks located adjacent to the pyroxenites, and thatmetasomatism of these peridotites was caused by the infiltrationof a residual silicate melt or volatile-rich fluid derived fromthe parental magma of the pyroxenites; i.e. the metasomatismwas a consequence of basaltic magmatism. Furthermore, the parentalmagma of the pyroxenites was probably petrogenetically relatedto the Nunivak volcanism. REE modelling of fluids in equilibriumwith clinopyroxenes from the coarse-grained peridotites is consistentwith this model.     

Andesites and high-alumina basalts from the central-south Chile high Andes: Geochemical evidence bearing on their petrogenesis

High-alumina basalts from seven High-Andean stratovolcanoes (37 °30′S to 41 °S) have major and trace element (including rare earth elements, REE) that are consistent with derivation by partial melting (typically 10–15%) of garnet-free peridotite followed by fractional crystallization of olivine and pyroxene. High-alumina basalts from two stratovolconoes require significantly lower degrees of melting (<5%) or melting of an incompatible, element-enriched source. However, a poorly understood feature of all of these basalts-and calc-alkaline rocks in general-is the mechanism for causing their low TiO₂ and heavy REE content relative to oceanic basalts. Further north in Chile (33 °–34 °S and 21 °–22 °S) amphibole-bearing andesites have REE abundances consistent with derivation from a garnet-bearing source such as incompatible, element-rich eclogite (e.g., Franciscan eclogites) or garnet peridotite. The marked petrological and geochemical changes along strike of the Andes are probably related to the varying nature of the subduction process; e.g., dips of the downgoing slab varying from 10 to 30 °.     

地幔橄榄岩中碳酸盐熔体交代作用及其鉴定特征

碳酸盐熔体交代作用是指在地幔碳酸盐熔体与橄榄岩之间的相互作用,是改造地幔的重要方式之一.碳酸盐熔体交代会显著改变地幔橄榄岩的岩石学和地球化学特征.首先,碳酸盐熔体交代作用会改变地幔橄榄岩中的矿物组成和比例.尽管碳酸盐熔体与橄榄岩的反应结果受控于初始反应物成分和反应的温压条件,但多数反应会导致橄榄岩中辉石的比例增加,而且有时还会出现磷灰石、独居石等副矿物.另外,在有些受碳酸盐熔体交代显著的橄榄岩的矿物中不仅可发现大量CO₂流体包裹体和碳酸盐熔体包裹体,也会出现特殊的反应边结构和熔体囊.其次,碳酸盐熔体在改造地幔橄榄岩过程中,会在地幔矿物中留下明显的地球化学指纹.在主量元素特征上,受到碳酸盐熔体交代的橄榄岩中的单斜辉石往往具有偏高的Mg#和Ca/Al比值（>5）;而在微量元素组成特征上的变化更为显著,包括单斜辉石具有高的（La/Yb）_N、Eu/Ti、Zr/Hf、Y/Ho比值,并显著亏损HFSE等.另外,值得注意的是,碳酸盐熔体与地幔橄榄岩反应的程度不同也会导致这些地球化学特征存在差异,因此在判别碳酸盐熔体交代作用时要采用岩石学和地球化学特征相结合,多方面对比分析.对于引起地幔碳酸盐熔体交代作用的交代介质来源的识别主要用Mg-Zn-Ca-Sr等多种同位素体系进行示踪研究,尤其是近年来微区Sr同位素分析方法的建立为地幔碳酸盐熔体交代作用研究提供了重要手段.      

The Characteristics of Spinels in Mantle-derivedXenoliths in Cenozoic Basalts from Eastern Chinaand Their Thermal States

The colours and chemical composition variations of 160 spinels in peridotite and pyroxenite xenoliths from Cenozoic basalts in eastern China and their petrogeneses have been studied in detail. The relationships between major elements of spinels are discussed. The equilibrium temperatures, pressures and oxygen fugacities of spinels and their coexisting olivines, orthopyroxenes and clinopyroxenes have been determined using the Brey-kohler' s T-P calculation methods (1990) and Ballhaus' fo2 calculation method (1991). The relationships betweenthe composition and the equilibrium temperatures, pressures or oxygen fugacities of spinels in peridotite xenoliths from the basalts and the stable field of the spinels in the upper mantle have been shown.     

Middle Devonian Picrites of the Southern Margin of Altay Orogenic Belt and Implications for the Tectonic Setting and Petrogenesis

INTRODUCTION Inrecentyears,greatprogressonthegeologic tec tonicevolutionandmineralresourcesofXinjianghas beenachieved.However,manyissuesarestilldebated, suchasancienttectonicpatternsandtheclosuretimeof theancientoceanicbasin(LiandXu,2004).Theseis sueshavelimitedourknowledgeoftheformationande volutionofAsiancontinents,aswellastheexploration anddevelopmentofmineralresources. Recently,theHilaketehalasuporphyrycopperde positwasdiscoveredinthestrataoftheMiddleDevoni anBeitashanFormatio…     

Zn/Fe systematics in mafic and ultramafic systems: Implications for detecting major element heterogeneities in the Earth’s mantle

Oceanic basalts, such as mid-ocean ridge basalts (MORB) and ocean island basalts (OIB), are characterized by large isotopic and trace element variability that is hard to reconcile with partial melting of a peridotitic mantle alone. Their variability has been attributed to the presence of heterogeneities within the mantle, such as recycled crust, metasomatized material or outer core contribution. There have been few attempts to constrain the major element composition of those heterogeneities, most studies focusing on incompatible trace elements and radiogenic isotopes. Here, we report Zn, Mn and Fe systematics in mafic and ultramafic systems (whole-rocks and minerals) and we explore their use for detecting lithological heterogeneities that deviate from peridotitic mantle dominated by olivine and orthopyroxene. We suggest that Zn/Fe ratio is a particularly promising proxy. Zn/Fe fractionates equally between olivine, orthopyroxene and melt (e.g. the inter-mineral exchange coefficients  ∼  is ∼0.9-1), and the distribution of Zn/Fe between minerals appears to be temperature-independent within error. In contrast, clinopyroxene and garnet are characterized by low Zn/Fe ratios compared to co-existing melt, olivine and orthopyroxene, that is, and are both <<1. These partitioning behaviors imply that Zn/Fe ratios are minimally fractionated during partial melting of peridotite and differentiation of primitive basalts, if differentiation is dominated by olivine control. Thus, the Zn/Fe ratios of primitive basalts preserve the Zn/Fe ratio of the primary parental magma, providing insight into the signature of the mantle source region. We also infer that Zn/Fe ratios in melts are unlikely to be fractionated by modal variations in peridotitic material but are highly fractionated if garnet and/or clinopyroxene are the main phases in the source during melting. Similar Zn/Fe ratios between MORB and average upper mantle confirm the lack of fractionation during peridotite melting. However, high Zn/Fe ratios of some OIB cannot be explained by peridotite melting alone, but instead require the presence of high Zn/Fe lithologies or lithologies that have bulk exchange coefficients  < 1. All garnet-bearing or clinopyroxene-bearing lithologies, such as eclogites and garnet pyroxenites, fit the latter requirement.     

Petrogenesis of Hawaiian tholeiites: 2, aspects of dynamic melt segregation

To Hawaiian magma genesis, dynamic melt segregation offers a potential resolution of conflict arising between trace-element evidence and phase-equilibria evidence, for deep garnet-present melting versus shallow garnet-absent melting. In this study comprehensive dynamic melting models, which incorporate phase-equilibria constraints and variable partition coefficients, have been applied in efforts to simulate decompression melting of a mantle plume. These models specifically endeavour to reproduce Hawaiian REE (rare-earth-element) patterns from a peridotitic upper mantle source with chondritic relative abundances of middle and HREE (heavy REE). If the flow of both melt and solid mantle is vertical through the partially molten source region, and melting proceeds beyond the stability limit of garnet in peridotite, dynamic melting processes are unable to produce the fractionated REE patterns of Hawaiian tholeiites. Instead, three-dimensional dynamic melting modles need to be invoked, in which lateral migration of the melt relative to the residual matrix also takes place. This enables the derivation of small garnet-equilibrated melt fractions from a larger source volume than that supplying more extensive melt fractions from shallower garnet-absent levels of melting (i.e melting shapes with a mean degree of melting smaller than the maximum extent of melting). This can be achieved by either drawing small-degree melt fractions, formed in the presence of garnet at the plume peripheries, toward the plume centre, or by advecting the mantle residue away from the plume centre as it ascends. Fluid dynamic theory supports a plume model incorporating the latter, with melt flow occurring vertically through a matrix flow which is deflected by the lithosphere and diverges away from the plume centre. In this framework, the generation of melting shapes dominated by small-degree garnetpresent melt fractions, requires a decrease in the rate of melting with progressive melting and height along melt-flow paths within the plume. This is consistent with a decrease in vertical velocity of the matrix (and thus decompression melting rate) upwards through the plume and, with diminishing melting rates upon exhaustion of garnet and clinopyroxene as melting progresses. Providing melt segregation occurs by percolation, equilibrium between the segregating melt and residual peridotite matrix may be maintained throughout the plume. In this way, primary melts extracted from the Hawaiian plume have their bulk compositions determined by phase equilibrium with the extensively melted matrix residue (harzburgite) at the plume top and shallowest level of melting (2.0 GPa), and their incompatible-trace-element characteristics determined by smaller-degree melt fractions derived from deeper, garnet-present levels of melting (3.0 GPa). Simple unidimensional models for melt segregation by percolation or via channels are shown to produce incompatible-trace-element abundances and ratios which are similar to those generated by equivalent degrees of batch melting. Moreover, contrary to a common belief held for dynamic melting, the enrichment of more-incompatible elements over less-incompatible elements is not always greater than that produced by an equivalent amount of batch melting.     

Hf–Nd isotopic decoupling in continental mantle lithosphere beneath Northeast China: Effects of pervasive mantle metasomatism

Nd–Hf isotopic decoupling has frequently been observed in the continental and oceanic mantle, but its origin remains controversial. Here we present combined elemental and Sr–Nd–Hf isotopic study on peridotite xenoliths entrained in Cenozoic basalts from Shuangliao and Jiaohe in Northeast China, which provides insight into this issue. The data reveal a heterogeneous lithospheric mantle beneath Northeastern China, consisting of fertile (type I) to strongly refractory (type II) peridotites. Type I peridotites are largely shielded from late metasomatism, thus preserving information of depletion events. Nd model age suggests a Proterozoic lithospheric mantle beneath NE China. Type II peridotites are mostly refractory harzburgites and show ubiquitous enrichment of incompatible elements. They are further divided into two sub-groups. Clinopyroxenes from type IIa samples have high and wide Lu/Hf (0.34–1.3) and very radiogenic Hf isotopic ratios (ε_Hf = 44.4–63.8). Hf concentration is generally low (0.12–0.43 ppm) and plots along or slightly above the modeled partial melting depletion trend. In contrast, Nd content in type IIa clinopyroxenes is significantly higher than the modeled concentrations in residues at a given degree of melt depletion. The difference in enrichment of Hf and Nd translates to decoupling of Lu/Hf–Sm/Nd ratios and of Nd–Hf isotopes (ε_Nd = −1.3 to 8.4). Clinopyroxenes from most of type IIb peridotites have relatively low Lu/Hf ratios (0.04–0.24) and coupled Nd–Hf isotopes. Both Hf and Nd plot significantly above the depletion trend; their concentrations are governed by the equilibrium partitioning between percolating melt and peridotites. The distinct geochemical characteristics of type IIa and type IIb clinopyroxenes may have resulted from chromatographic percolation of small volumes of silicate melts, in which percolation fronts of incompatible elements are dependent on their relative incompatibilities.