A parameterized model for REE distribution between low-Ca pyroxene and basaltic melts with applications to REE partitioning in low-Ca pyroxene along a mantle adiabat and during pyroxenite-derived melt and peridotite interaction     

Lijing Yao  Chenguang Sun  Yan Liang 《Contributions to Mineralogy and Petrology》2012,164(2):261-280

Low-Ca pyroxenes play an important role in mantle melting, melt-rock reaction, and magma differentiation processes. In order to better understand REE fractionation during adiabatic mantle melting and pyroxenite-derived melt and peridotite interaction, we developed a parameterized model for REE partitioning between low-Ca pyroxene and basaltic melts. Our parameterization is based on the lattice strain model and a compilation of published experimental data, supplemented by a new set of trace element partitioning experiments for low-Ca pyroxenes produced by pyroxenite-derived melt and peridotite interaction. To test the validity of the assumptions and simplifications used in the model development, we compared model-derived partition coefficients with measured partition coefficients for REE between orthopyroxene and clinopyroxene in well-equilibrated peridotite xenoliths. REE partition coefficients in low-Ca pyroxene correlate negatively with temperature and positively with both calcium content on the M2 site and aluminum content on the tetrahedral site of pyroxene. The strong competing effect between temperature and major element compositions of low-Ca pyroxene results in very small variations in REE partition coefficients in orthopyroxene during adiabatic mantle melting when diopside is in the residue. REE partition coefficients in orthopyroxene can be treated as constants at a given mantle potential temperature during decompression melting of lherzolite and diopside-bearing harzburgite. In the absence of diopside, partition coefficients of light REE in orthopyroxene vary significantly, and such variations should be taken into consideration in geochemical modeling of REE fractionation in clinopyroxene-free harzburgite. Application of the parameterized model to low-Ca pyroxenes produced by reaction between pyroxenite-derived melt and peridotite revealed large variations in the calculated REE partition coefficients in the low-Ca pyroxenes. Temperature and composition of starting pyroxenite must be considered when selecting REE partition coefficients for pyroxenite-derived melt and peridotite interaction.  相似文献   

Ultramafic inclusions and high-pressure xenocrysts in submarine basanitoid,equatorial Mid-Atlantic Ridge     

John M. Sinton 《Contributions to Mineralogy and Petrology》1979,70(1):49-57

A basanitoid dredged from near St. Paul's Rocks represents the first reported occurrence of an abyssal lava containing ultramafic xenoliths and high-pressure xenocrysts. The xenoliths include three separate populations. A suite of clinopyroxene-poor spinel-harzburgite and spinel-lherzolite inclusions have tectonite textures and highly refractory mineral compositions; these nodules probably represent residua from partial melting events unrelated to the host basanitoid. One harzburgite inclusion apparently accumulated from a tholeiitic magma and is also an accidental inclusion. Ultrabasic mylonites comprise a third xenolith group whose members are comparable to lithologies exposed on St. Paul's Rocks. A diverse xenocryst assemblage including labradoritebytownite, andesine, high-Ca pyroxene, low-Ca pyroxene, aluminous Cr-spinel, Cr-free aluminous spinel and Fe-Ti oxides, suggests at least two petro-genetic affinities. Fe-rich low-Ca pyroxene and Ca-rich plagioclase, together with the cumulate harzburgite xenolith, indicate the presence of a differentiated tholeiitic suite in the area. Most xenocrysts, including Ca-rich pyroxene, spinel and andesine have alkalic affinities consistent with crystallization at elevated pressure over a wide range in temperature. Both spinel populations are interpreted as cognate precipitates with Cr-free Al-spinels crystallizing above about 10 kb and aluminous Cr-spinels co-precipitating with plagioclase at lower pressures. Compared to the olivine-dominated, low-pressure phenocryst assemblage, extraction of the generally basaltic, aluminous clino-pyroxene-dominated assemblage at high pressure should have a minor effect on the major element differentiation of the host basanitoid magma.  相似文献   

Origin of nelsonite and Fe–Ti oxides ore of the Damiao anorthosite complex,NE China: Evidence from trace element geochemistry of apatite,plagioclase, magnetite and ilmenite     

《Ore Geology Reviews》2016

Nelsonite and Fe–Ti oxides ore are common in Proterozoic massif-type anorthosites and layered intrusions. Their geneses have long been controversial, with existing hypotheses including liquid immiscibility between Si-rich and Fe–Ti–P-rich melts and gravitational fractionation among apatite, magnetite, ilmenite and silicates. In this paper, we report detailed field geology and mineral geochemical studies of the nelsonite and Fe–Ti oxides ore from the Damiao anorthosite complex, NE China. Geological observations indicate that the nelsonite and Fe–Ti oxides ore occur as irregularly inclined stratiform-like or lensoid or veins, and are in sharp contact with the anorthosite and gabbronorite. The widespread veins and lenses structure of the Damiao nelsonite and Fe–Ti oxides ore in the anorthosite indicates their immiscibility-derived origin. The apatite in the nelsonite and gabbronorite shows evolution trends different from that in the gabbronorite in the diagrams of Sr versus REEs and Eu/Eu*, suggesting that petrogenesis of the nelsonite and gabbronorite is different from the gabbronorite. Compared with the gabbronorite, the nelsonite and Fe–Ti oxides ore have magnetite high in Cr, plagioclase high in Sr and low in An, and apatite high in Sr, low in REEs with negative Eu anomaly. The evidence permits us to propose that the Damiao Fe–Ti oxides ore/nelsonite and gabbronorite were derived from different parental magmas. The gabbronorite was formed by solidification of the interstitial ferrodioritic magma in the anorthosite, which was the residual magma after extensive plagioclase and pyroxene crystallization and was carried upward by the plagioclase crystal mesh. In contrast, the Fe–Ti oxides ore and nelsonites and mangerite were produced by crystallization of the Fe–Ti–P-rich and SiO₂-rich magmas, respectively, due to the liquid immiscibility that occurred when the highly evolved ferrodioritic magma mixed with newly replenished magmas. The variation from Fe–Ti oxides ore to nelsonite and gabbro-nelsonite upwards (as apatite content increases with height) in the steeply inclined Fe–Ti oxides orebodies suggest that gravity fractionation may have played important roles during the crystallization of the Fe–Ti–P-rich magma.  相似文献   

Composition and petrogenesis of oxide-, apatite-rich gabbronorites associated with Proterozoic anorthosite massifs: examples from the Adirondack Mountains,New York     

James McLelland  Lewis Ashwal  Laura Moore 《Contributions to Mineralogy and Petrology》1994,116(1-2):225-238

Mafic dikes and sheets rich in Fe, Ti-oxides and apatite are commonly associated with Proterozoic massif anorthosites and are referred to as oxide-apatite gabbronorites (OAGN). Within the Adirondacks, field evidence indicates that during middle to late stages of anorthositic evolution, these bodies were emplaced as magmas with unspecified liquid-crystal ratios. Sixty whole rock analyses of Adirondack OAGN and related rocks define continuous oxide trends on Harker variation diagrams (SiO₂=37–54%). Similar trends exist for Sr, Y, Nb, Zr, and REE and together suggest a common origin via fractional crystallization. A representative parental magma (plagioclase-rich crystal mush) has been chosen from this suite, and successive daughter magmas have been produced by removal of minerals with compositions corresponding to those determined in actual rocks. Least squares, mass balance calculations of major element trends indicate that removal of intermediate plagioclase (An_40–50) plus lesser amounts of pyroxene account for the compositional variation of this suite and produce very low sums of the squares of the residuals (R² _s>0.25). The extracted mineral phases correspond volumetrically and compositionally to those of the anorthositic suite, and the model succeeds in accounting for the observed OAGN trends. The major element model is utilized to calculate trace elejent concentrations for successive magmas, and these agree closely with observation. We conclude that, beginning with a plagioclase-rich crystal mush, the extraction of intermediate plagioclase (An_40–50) drives residual magmas to increasingly Fe-, Ti-, and P-rich and SiO₂-poor conditions characteristic of Fenner-type fractionation. The crystallization sequence is plagioclaseplagioclase+orthopyroxeneplagioclase+orthopyroxene (pigeonite)+augite. Fe, Ti-oxides begin to crystallize near the end of the sequence and are followed by apatite and fayalitic olivine which appears in place of pigeonite. Augitic pyroxene becomes the dominant ferromagnesian phase in late stages of fractionation. Resultant OAGN magmas are injected into congealed anorthosite by filter pressing of liquid-rich interstitial fractions. Varying compositions of the dikes reflect filter pressing at different stages during fractionation and thereby provide information on the fractionation history of Proterozoic massif anorthosites.  相似文献   

Coexisting clinopyroxene,garnet and amphibole from an “eclogite”, Kakanui,New Zealand     

A. J. R. White  B. W. Chappell  P. Jakeš 《Contributions to Mineralogy and Petrology》1972,34(3):185-191

Analyses of major and rare earth elements are presented for co-existing garnet, clinopyroxene, and amphibole from a Kakanui eclogite.New and previously published analyses of garnets suggest a gradual increase of Fe and decrease of Mg from xenocrysts through garnet pyroxene eclogitic rocks to amphibole-rich eclogitic rocks. Clinopyroxenes show a parallel increase in Fe/Mg ratio and an increase in Jadeite component and decrease in Tschermak's component. These data indicate crystallization of garnet and clinopyroxene from an alkali-rich undersaturated magma and are consistent with the concept of eclogite fractionation, but rare earth data allow severe constraints to be placed on this process. The eclogites are considered to be deep-seated crystallization products of nephelinite, but eclogite fractionation is small and cannot account for the association of alkali basalt, basanite and nephelinite.  相似文献   

Lunar Magma Ocean crystallization revisited: Bulk composition, early cumulate mineralogy, and the source regions of the highlands Mg-suite     

Stephen M. Elardo  David S. Draper  Charles K. Shearer Jr. 《Geochimica et cosmochimica acta》2011,75(11):3024-5093

Crystallization of the Lunar Magma Ocean (LMO) has been numerically modeled and its products inferred from sample observations, but it has never been fully tested experimentally. This study is a reexamination of the LMO hypothesis by means of the first experimental simulation of lunar differentiation. Two end-member bulk Moon compositions are considered: one enriched in refractory lithophile elements relative to Earth and one with no such enrichment. A “two-stage” model of magma ocean crystallization based on geophysical constraints is simulated and features early crystal suspension and equilibrium crystallization followed by fractional crystallization of the residual magma ocean. An initially entirely molten Moon is assumed. Part 1 of this study, presented here, focuses on stage 1 of this model and considers the early cumulates formed by equilibrium crystallization, differences in mantle mineralogy resulting from different bulk Moon compositions, and implications for the source regions of the highlands Mg-suite.Refractory element enriched bulk Moon compositions produce a deep mantle that contains garnet and trace Cr-spinel in addition to low-Ca pyroxene and olivine. In contrast, compositions without refractory element enrichment produce a deep dunitic mantle with low-Ca pyroxene but without an aluminous phase. The differences in bulk composition are magnified in the residual melt; the residual LMO from the refractory element enriched composition will likely produce plagioclase and ilmenite earlier and in greater quantities. Both compositions produce Mg-rich early cumulate piles that extend from the core-mantle boundary to ∼355 km depth, if 50% equilibrium crystallization and whole Moon melting are assumed. These early LMO cumulates provide good fits for the source regions for a component of the high-Mg^∗, Ni- and Co-poor parental magmas of the Mg-suite cumulates, if certain conditions are called upon. The olivine in early LMO cumulates produced by either bulk Moon composition is far too rich in Cr to be reasonable for the source regions of the Mg-suite, meaning either core formation in the presence of S and/or C must be invoked to deplete the LMO and the crystallizing olivine in Cr, or that current estimates of the bulk lunar Cr content are too high. We infer that melts meeting the criteria of the Mg-suite parents could be produced from early LMO cumulates by solid state KREEP and plagioclase hybridization near the base of the crust and subsequent partial melting. Additionally, we propose a revised model for Mg-suite petrogenesis.  相似文献   

A comparative study of olivine and clinopyroxene spinifex flows from Alexo,abitibi greenstone Belt,Ontario, Canada     

Sarah-Jane Barnes  M. P. Gorton  A. J. Naldrett 《Contributions to Mineralogy and Petrology》1983,83(3-4):293-308

A petrological and geochemical study of an olivine and of a clinipyroxene spinifex textured flow, from Alexo, indicates that the initial liquid in both flows probably came from the same mantle melting event and that the source was incompatible element depleted. The starting liquid of the clinopyroxene flow had experienced more olivine fractionation (10%) prior to its emplacement at Alexo, than the initial liquid of the olivine spinifex flow. The development of each of the textural and compositional zones in the flows can be modelled by means of crystal fractionation. In the case of the clinopyroxene flow the B-zone is formed by the fractionation of olivine, low-Ca pyroxene and chromite. An unusual feature of the Alexo clinopyroxene flow is presence of a peridotitic komatiite above the pyroxene cumulate layer, where a basaltic komatiite would usually be present. The presence of the peridotitic komatiite suggests an influx of new magma and hence a dynamic model for the flow. The composition of the clinopyroxene spinifex zone represents a mixture of clinopyroxene plus liquid, rather than simply a frozen liquid. This could happen if the clinopyroxene needles grew stalactitelike from the chilled upper surface of the flow into a flowing basaltic liquid. In the olivine spinifex flow the zones can be modelled as frozen liquids in the A₂-zone, as initial liquid which has fractionated 30% olivine in the A₃-zone and as liquid plus 50% olivine in the B-zone. But, if the clinopyroxene spinifex developed by stalactite growth of clinopyroxene needles into the a flowing liquid, the possibility that the olivine spinifex represent fractionated liquid plus stalactite olivines arises.  相似文献   

Density changes during the fractional crystallization of basaltic magmas: fluid dynamic implications   总被引：3，自引：1，他引：3  

R. Stephen J. Sparks  Herbert E. Huppert 《Contributions to Mineralogy and Petrology》1984,85(3):300-309

The dynamical behaviour of basaltic magma chambers is fundamentally controlled by the changes that occur in the density of magma as it crystallizes. In this paper the term fractionation density is introduced and defined as the ratio of the gram formula weight to molar volume of the chemical components in the liquid phase that are being removed by fractional crystallization. Removal of olivine and pyroxene, whose values of fractionation density are larger than the density of the magma, causes the density of residual liquid to decrease. Removal of plagioclase, with fractionation density less than the magma density, can cause the density of residual liquid to increase. During the progressive differentiation of basaltic magma, density decreases during fractionation of olivine, olivine-pyroxene, and pyroxene assemblages. When plagioclase joins these mafic phases magma density can sometimes increase leading to a density minimum. Calculations of melt density changes during fractionation show that compositional effects on density are usually greater than associated thermal effects.In the closed-system evolution of basaltic magma, several stages of distinctive fluid dynamical behaviour can be recognised that depend on the density changes which accompany crystallization, as well as on the geometry of the chamber. In an early stage of the evolution, where olivine and/or pyroxenes are the fractionating phases, compositional stratification can occur due to side-wall crystallization and replenishment by new magma, with the most differentiated magma tending to accumulate at the roof of the chamber. When plagioclase becomes a fractionating phase a zone of well-mixed magma with a composition close to the density minimum of the system can form in the chamber. The growth of a zone of constant composition destroys the stratification in the chamber. A chamber of well-mixed magma is maintained while further differentiation occurs, unless the walls of the chamber slope inwards, in which case dense boundary layer flows can lead to stable stratification of cool, differentiated magma at the floor of the chamber.In a basaltic magma chamber replenished by primitive magma, the new magma ponds at the base and evolves until it reaches the same density and composition as overlying magma. Successive cycles of replenishment of primitive magma can also form compositional zonation if successive cycles occur before internal thermal equilibrium is reached in a chamber. In a chamber containing well-mixed, plagioclase — saturated magma, the primitive magma can be either denser or lighter than the resident magma. In the first case, the new magma ponds at the base and fractionates until it reaches the same density as the evolved magma. Mixing then occurs between magmas of different temperatures and compositions. In the second case a turbulent plume is generated that causes the new magma to mix immediately with the resident magma.  相似文献   

赛马-柏林川碱性杂岩常量组分变异的分离结晶制约     

谭东娟  林景仟  单玄龙 《吉林大学学报(地球科学版)》1998,(4)

赛马－柏林川碱性杂岩体为侵位于古元古宙与新元古宙之间的缓倾斜岩席状岩体,缓倾的张裂隙制约了分异岩浆的侵位和含矿溶液的流通,交代富集的上地幔产生的富碱岩浆与富集的地壳产生的岩浆以不同比例混合,形成了正长质岩浆和霞石正长质岩浆。含地壳组分较高的正长质岩浆,受富钙辉石、角闪石和镁质较高黑云母的结晶分离制约向硅酸过饱和方向演化;霞石正长质岩浆受富钙辉石—霓辉石、白榴石、含Ｆｅ较高黑云母、黑榴石的分离结晶制约发生分异。稀土元素矿床是霞石正长质岩浆分异残余熔体的产物,侵位于岩体最高层位。  相似文献   

Pyroxene in enclaves and syenites of the Red Hill complex,New Hampshire: an ion and electron microprobe study     

Michael J. Dorais 《Contributions to Mineralogy and Petrology》1993,114(1):130-138

The Red Hill complex, New Hampshire, contains both silica-undersaturated and silica-saturated to oversaturated syenites. Ion microprobe analyses of pyroxene from the Nepheline Sodalite Syenite (NSS) and its enclaves reveal that the REE abundances increase in a systematic manner from low values in the enclave pyroxenes to higher values in the NSS host rock pyroxenes. This variation is interpreted to have resulted from differentiation and coupled with previously published mineral and bulk-rock compositions, suggests that the enclaves represent samples of NSS parental liquids that intruded into residual, syenitic liquids in a zoned magma chamber. Pyroxene analyses of the Garland Peak Syenite (GPS) and its enclaves indicate that the enclaves are of several populations: some may be related to the GPS, others are not. The GPS itself is heterogeneous and pyroxene trace element zoning is difficult to explain by fractionation processes. The silica-oversaturated Outer Coarse Syenite (OCS) contains pyroxenes with trace element characteristics that are distinct from the NSS. The low V concentrations suggest that the OCS exprienced magnetite fractionation prior to pyroxene growth. It is proposed that high f O₂in the OCS magma caused Fe-Ti oxide crystallization, which in turn, influenced magmatic silica activity. The chondrite normalized REE patterns of OCS pyroxenes are also suggestive of titanite crystallization, another indication of high f O₂. In contrast to the influence of magnetite crystallization, the high and rimwardly increasing Y and Yb concentrations in both the NSS and the OCS pyroxenes suggest that amphibole fractionation was not the major influence on silica activity. Therefore, it is unlikely that the OCS was derived by amphibole fractionation from a NSS precursor magma.  相似文献   

Geochemistry of fractionation of coherent elements (Zr and Hf) during the profound differentiation of peralkaline magmatic systems: A case study of the Lovozero Complex     

L. N. Kogarko 《Geochemistry International》2016,54(1):1-6

The paper presents pioneering data on Hf distribution in peralkaline rocks, ores, and rock-forming and accessory minerals of the Lovozero Complex. Variations in the Zr/Hf ratio are determined in all rocks of the Lovozero alkaline massif. This ratio is proved to increase in the course of evolution of alkaline magma because of fractionation of alkaline pyroxene. The Hf distribution coefficient is evaluated for alkali-rich pyroxene, whose crystallization controls Zr and Hf fractionation during the differentiation of alkaline magma. These data and the equation of equilibrium and fractional crystallization are utilized in a model suggested for Zr and Hf fractionation in the course of evolution of the Lovozero intrusion.  相似文献   

Late Neoproterozoic A-type granites in the northernmost Arabian-Nubian Shield formed by fractionation of basaltic melts     

G.H. Jarrar  W.I. Manton  D. Zachmann 《Chemie der Erde / Geochemistry》2008,68(3):295-312

Geochemical, isotopic and age constraints support a comagmatic origin for Ghuweir Mafics and the Feinan A-type granites. The two rocks types, named collectively in this paper as the Feinan Ghuweir Magmatic Suite (FGMS), formed between 556 and 572 Ma ago according to Rb-Sr whole-rock dating. FGMS has low Sr initial ratios, which preclude a significant contribution of much older crust in the magma genesis.The FGMS has a wide range of silica contents, with a gap at 55-65 wt% SiO₂. It has a transalkaline to alkaline character; belongs to the medium to high K calc-alkaline series; it ranges from metaluminous to mildly peraluminous character and belongs to the alkali and alkali-calcic series. The Feinan granites and the Ghuweir rhyolites and rhyodacites are classified as A-type granites and belong to group A2 of Eby [Eby, N.G., 1992. Chemical subdivision of the A-type granitoids: petrogenetic and tectonic iplications. Geology 20, 641-644].According to geochemical modeling the Ghuweir Mafics were derived from a subduction modified lithospheric mantle by 10% batch modal partial melting of a phlogopite-bearing spinel lherzolite. The intra-suite geochemical variations can be ascribed to fractional crystallization of olivine, pyroxene, and plagioclase. The accumulation of apatite in the most evolved samples is responsible for the high concentrations of REE.The Feinan granites and the Ghuweir rhyolites and rhyodacites were derived from the mafic magma by the fractional crystallization of ≈78% of the original magma to the mineral assemblage olivine+pyroxene+plagioclase+magnetite. The intra-suite geochemical variations in the Feinan A-type granites are due to the fractional crystallization of the mineral phases: amphibole +Na and K-feldspar+apatite +magnetite+zircon+allanite.The FGMS correlates with time-equivalent rocks in many parts of the Arabian-Nubian Shield and the surrounding areas.  相似文献   

Early Cretaceous Sung Valley ultramafic-alkaline-carbonatite complex, Shillong Plateau, Northeastern India: petrological and genetic significance     

Rajesh K. Srivastava  A. K. Sinha 《Mineralogy and Petrology》2004,80(3-4):241-263

Summary The Shillong Plateau of northeastern India hosts four Early Cretaceous (105–107Ma) ultramafic-alkaline-carbonatite complexes (UACC), which have been associated with the Kerguelen plume igneous activity. Petrological and geochemical characteristics of one of these UACC, the Sung Valley, are presented. The Sung Valley UACC was emplaced in to the Proterozoic Shillong Group of rocks and consists of ultramafics (serpentinized peridotite, pyroxenite, and melilitolite), alkaline rocks (ijolite and nepheline syenite), and carbonatites. Serpentinized peridotite, pyroxenite, and ijolitic rocks form the major part of the complex, the others constitute less than 5% of the total volume. Ijolite and melilitolite intrude peridotite and pyroxenite, while nepheline syenite and carbonatite intrude the ultramafic rocks as well as ijolite. Mineralogically, the carbonatites are classified as calcite carbonatite with minor apatite, phlogopite, pyrochlore and ilmenite. The serpentinized peridotites are wehrlitic. Chemical compositions of the silicate rocks do not show a distinct co-genetic relationship amongst them, nor do they show any geochemical relationships with the carbonatites. No noticeable fractionation trend is observed on the chemical variation diagrams of these rocks. It is difficult to establish the genetic evolution of the Sung Valley UACC through fractional crystallization of nephelinitic magma or through immiscible liquids. On the basis of petrological and geochemical data and previously published isotopic results from these rocks, it is suggested that they have been derived from a primary carbonate magma generated by the low-degree melting of a metasomatized mantle peridotite.  相似文献   

Cyclicity in the Main and Upper Zones of the Bushveld Complex, South Africa: Crystallization from a Zoned Magma Sheet   总被引：9，自引：0，他引：9  

TEGNER  CHRISTIAN; CAWTHORN  R. GRANT; KRUGER  F. JOHAN 《Journal of Petrology》2006,47(11):2257-2279

The major element composition of plagioclase, pyroxene, olivine,and magnetite, and whole-rock ⁸⁷Sr/⁸⁶Sr data are presented forthe uppermost 2·1 km of the layered mafic rocks (upperMain Zone and Upper Zone) at Bierkraal in the western BushveldComplex. Initial ⁸⁷Sr/⁸⁶Sr ratios are near-constant (0·7073± 0·0001) for 24 samples and imply crystallizationfrom a homogeneous magma sheet without major magma rechargeor assimilation. The 2125 m thick section investigated in drillcore comprises 26 magnetitite and six nelsonite (magnetite–ilmenite–apatite)layers and changes up-section from gabbronorite (An₇₂ plagioclase;Mg# 74 clinopyroxene) to magnetite–ilmenite–apatite–fayaliteferrodiorite (An₄₃; Mg# 5 clinopyroxene; Fo₁ olivine). The overallfractionation trend is, however, interrupted by reversals characterizedby higher An% of plagioclase, higher Mg# of pyroxene and olivine,and higher V₂O₅ of magnetite. In the upper half of the successionthere is also the intermittent presence of cumulus olivine andapatite. These reversals in normal fractionation trends definethe bases of at least nine major cycles. We have calculateda plausible composition for the magma from which this entiresuccession formed. Forward fractional crystallization modelingof this composition predicts an initial increase in total iron,near-constant SiO₂ and an increasing density of the residualmagma before magnetite crystallizes. After magnetite beginsto crystallize the residual magma shows a near-constant totaliron, an increase in SiO₂ and decrease in density. We explainthe observed cyclicity by bottom crystallization. Initiallymagma stratification developed during crystallization of thebasal gabbronorites. Once magnetite began to crystallize, periodicdensity inversion led to mixing with the overlying magma layer,producing mineralogical breaks between fractionation cycles.The magnetitite and nelsonite layers mainly occur within fractionationcycles, not at their bases. In at least two cases, crystallizationof thick magnetitite layers may have lowered the density ofthe basal layer of melt dramatically, and triggered the proposeddensity inversion, resulting in close, but not perfect, coincidenceof mineralogical breaks and packages of magnetitite layers. KEY WORDS: layered intrusion; mineral chemistry; isotopes; magma; convection; differentiation  相似文献   

Geochemical effects of decoupled fractional crystallization and crustal assimilation   总被引：2，自引：0，他引：2  

J.W. Cribb  M. Barton 《Lithos》1996,37(4):293-307

Most models of crustal assimilation assume that the amount of assimilant added to the magma is proportional to each infinitesimally small amount of solid removed during crystallization (AFC). In some magmatic systems, however, assimilation and crystallization are not strictly related and the mass assimilated is decoupled from, and therefore varies independently of, the mass crystallized (FCA). The geochemical consequences of FCA are examined and compared to those of AFC. The behavior of incompatible elements is identical during AFC and FCA, and ratios of these elements do not allow discrimination between the two processes. Major-oxide least-squares mass-balance models do not discriminate between AFC and FCA at F ≥ 0.7 (F = fraction of melt remaining). However, FCA yields magmas richer in compatible elements and with higher Sr-isotopic ratios than AFC at a given value of F. Repeated cycles of FCA and AFC combined with magma mixing (FAM) may result in unusual geochemical trends, such as the evolution of a calc-alkaline basaltic parent to a tholeiitic daughter magma, or the evolution of low- and medium-K calc-alkaline basalts to high-K andesites, dacites, trachyandesites or trachydacites. Lavas erupted by the volcano Micro Profitis Ilias on Santorini, Hellenic arc, Greece, provide an example of magmas which evolved by combined fractionation, assimilation by FCA and mixing.  相似文献   

Oxygen isotopic constraints on the origin of magnesian chondrules and on the gaseous reservoirs in the early Solar System     

Marc Chaussidon  Guy Libourel  Alexander N. Krot 《Geochimica et cosmochimica acta》2008,72(7):1924-1938

We report in situ ion microprobe analyses of the oxygen isotopic composition of the major silicate phases (olivine, low-Ca pyroxene, silica, and mesostasis) of 37 magnesian porphyritic (type I) chondrules from CV (Vigarano USNM 477-2, Vigarano UH5, Mokoia, and Efremovka) and CR (EET 92042, EET 92147, EET 87770, El Djouf 001, MAC 87320, and GRA 95229) carbonaceous chondrites. In spite of significant variations of the modal proportions of major mineral phases in CR and CV chondrules, the same isotopic characteristics are observed: (i) olivines are isotopically homogeneous at the ‰ level within a chondrule although they may vary significantly from one chondrule to another, (ii) low-Ca pyroxenes are also isotopically homogeneous but systematically ¹⁶O-depleted relative to olivines of the same chondrule, and (iii) all chondrule minerals analyzed show ¹⁶O-enrichments relative to the terrestrial mass fractionation line, enrichments that decrease from olivine (±spinel) to low-Ca pyroxene and to silica and mesostasis. The observation that, in most of the type I chondrules studied, the coexisting olivine and pyroxene crystals and glassy mesostasis have different oxygen isotopic compositions implies that the olivine and pyroxene grains are not co-magmatic and that the glassy mesostasis is not the parent liquid of the olivine. The δ¹⁸O and δ¹⁷O values of pyroxene and olivine appear to be strongly correlated for all the studied CR and CV chondrules according to: