Relationship between the Layered Series and the overlying evolved rocks in the Bjerkreim-Sokndal Intrusion, southern Norway

The Bjerkreim-Sokndal layered intrusion (BKSK) consists of a > 7000-m-thick Layered Series comprising anorthosites, leuconorites, troctolites, norites, gabbronorites and jotunites (hypersthene monzodiorites), overlain by an unknown thickness of massive, evolved rocks: mangerites (hypersthene monzonites; MG), quartz mangerites (QMG) and charnockites (CH). The Layered Series is subdivided into six megacyclic units that represent the crystallisation products of successive major influxes of magma. We have studied a ca. 2000-m-thick section that straddles the sequence from the uppermost part of the Layered Series to the QMG in the northern part of the intrusion. Mineral compositions in 37 samples change continuously in the lower part of the sequence up to the middle of the MG-unit (plagioclase An37-18; olivine Fo40-7; Ca-poor pyroxene Mg#57-15; Ca-rich pyroxene Mg#65-21). Above this compositions are essentially constant in the upper part of the MG-unit and in the QMG (An21-13; Fo6-4; Mg#opx17-13; Mg#cpx25-20). The amount of interstitial quartz and the amount of normative orthoclase, however, both increase systematically upwards through the QMG-unit, implying that these rocks are cumulates. There is no evidence of a compositional break in the MG-QMG sequence that could reflect influx of relatively primitive magma.

Two types of QMG/CH are known in the uppermost part of BKSK. Olivine-bearing types are comagmatic with the underlying Layered Series; the studied stratigraphic sequence belongs to this suite. Two-pyroxene QMG and amphibole CH define a separate compositional lineage related to jotunites. An intrusive unit of dominantly two-pyroxene QMG is discordant to the olivine-bearing jotunite-MG-QMG sequence near Rapstad, confirming the presence of two compositionally distinct suites of QMG and related lithologies in the upper part of BKSK.

A xenolith-rich unit near the olivine-bearing MG-QMG boundary represents a major collapse of the roof of the magma chamber during the final stages of crystallisation.     

Crystallization Orders and Phase Chemistry of Glassy Lavas from the Pillow Sequences, Troodos Ophiolite, Cyprus

Three genetically unrelated magma suites are found in the extrusivesequences of the Troodos ophiolite, Cyprus. A stratigraphicallylower pillow lava suite contains andesite and dacite glassesand shows the crystallization order plagioclase; augite, orthopyroxene;titanomagnetite (with the pyroxenes appearing almost simultaneously).These lavas can in part be correlated chemically and mineralogicallywith the sheeted dikes and the upper part of the gabbro complexof the ophiolite. The second magma suite is represented in astratigraphically upper extrusive suite and contains basalticandesite and andesite glasses with the crystallizaton orderchromite; olivine; Ca-rich pyroxene; plagioclase. This magmasuite can be correlated chemically and mineralogically withparts of the ophiolitic ultramafic and mafic cumulate sequence,which has the crystallization order olivine; Ca-rich pyroxene;orthopyroxene; plagioclase. The third magma suite is representedby basaltic andesite lavas along the Arakapas fault zone andshows a boninitic crystallization order olivine; orthopyroxene;Ca-rich pyroxene; plagioclase. One-atmosphere, anhydrous phaseequilibria experiments on a lava from the second suite indicateplagioclase crystallization from 1225?C, pigeonite from 1200?C,and augite from 1165?C. These experimental data contrast withthe crystallization order suggested by the lavas and the associatedcumulates. The observed crystallization orders and the presenceof magmatic water in the fresh glasses of all suites are consistentwith evolution under relatively high partial water pressures.In particular, high P_H2O (1–3 kb) can explain the lateappearances of plagioclase and Ca-poor pyroxene in the majorityof the basaltic andesite lavas as the effects of suppressedcrystallization temperatures and shifting of cotectic relations.The detailed crystallization orders are probably controlledby relatively minor differences in the normative compositionsof the parental magmas. The basaltic andesite lavas are likelyto reach augite saturation before Ca-poor pyroxene saturation,whereas the Arakapas fault zone lavas, which have relativelyless normative diopside and more quartz, reached the Ca-poorpyroxene-olivine reaction surface and crystallized Ca-poor pyroxeneafter olivine.     

Crystallization processes in the Bjerkreim-Sokndal layered intrusion,south Norway: evidence from the boundary between two macrocyclic units

The Bjerkreim-Sokndal (BKSK) layered intrusion belongs to the Rogaland anorthosite province in southern Norway. The northwestern part of BKSK consists of a ca. 6 km-thick Layered Series, made up of macrocyclic units (MCU) arranged in a syncline. Each MCU, which resulted from the crystallization of a major-magma influx, can be subdivided into a series of cumulate zones. The MCU III/IV boundary has been studied in seven profiles across its strike length of 24 km. Massive piC¹ at the base of MCU IV overlies laminated and modally layered phimC in the central part of the chamber and phimacC towards the flanks; there is a discordance of between 2 and 6° between the base of MCU IV and phase layering in MCU III. The MCU IV piC is overlain by 75–100 m of massive poiC (the Svaalestad unit of Michot 1960; a similar olivine-bearing unit occurs near the base of MCU III) which has more primitive compositions than the underlying piC. This is followed by laminated and modally layered phiC, phimC and phimacC. The reversal to more primitive mineral assemblages across the MCU III/IV boundary is accompanied by a cryptic reversal; plagioclase and Ca-poor pyroxene have compositions of about An 44/Mg no. 71 at the top MCU III and about An 52/Mg no. 77 near the base of MCU IV. Olivines in the MCU IV poiC vary unsystematically from Fo 66 to 76. Macrocyclic units III and IV crystallized from monzonoritic parental magma. The BKSK magma chamber had a broad saucer-like shape with a small thickness to breadth ratio. The magma in the chamber during crystallization of MCU III was compositionally zoned and crystallized on an inward-sloping floor by down-dip accretion. Just before the major-magma influx at the base of MCU IV, phimC was crystallizing from the basal-magma layer at the centre of the chamber, while phimacC was crystallizing towards the flanks. The new, dense magma fountained into and mixed with the basal-magma layers already in the chamber. This hybrid magma crystallized during continued influx to produce massive piC at the base of MCU IV. This hybrid unit is thickest near the centre of the chamber and smoothed out the floor to an essentially horizontal surface. Continued influx resulted in the dense, primitive magma ponding on the floor; this crystallized fairly rapidly to produce the massive poiC unit. The return of normal fractional crystallization conditions is marked by the overlying sequence of modally and cryptically layered cumulates which duplicate the succession in MCU III. The variation in thickness of the upper part of MCU IV indicates that crystallization of the BKSK Layered Series was accompanied by sinking of the floor at a greater rate near the centre of the chamber than towards the flanks. This was accompanied by compaction of the underlying cumulates, promoting the development of lamination and the expulsion of intercumulus melt to encourage the development of adcumulates. ¹ p plagioclase - i ilmenite - h Ca-poor pyroxene - o olivine - m magnetite - a apatite - c Ca-rich pyroxene - C cumulate     

Slow,dense replenishments of a basic magma chamber: the layered series of the Newark Island layered intrusion,Nain, Labrador

The Newark Island layered intrusion is a composite layered intrusion within the Nain anorthosite complex, Labrador. The intrusion comprises a lower layered series (LS) dominated by troctolites, olivine gabbros and oxide-rich cumulates and an upper hybrid series (HS) characterized by a wide range of mafic, granitic and hybrid cumulates and discontinuous layers of chilled mafic rocks (Wiebe 1988). The HS crystallized from a series of replenishments of both silicic and basic magmas. The LS crystallized from periodically replenished basic magmas. The LS has a lower zone that consists mainly of olivine-plagioclase cumulates and contains minor cryptic reversals in mineral compositions that resulted from replenishments of relatively primitive magma. An upper zone is dominated by olivine-plagioclaseaugite-ilmenite cumulates. Cumulus titanomagnetite and pyrrhotite occur within some oxide-rich cumulates, and the stratigraphically highest layers contain cumulus apatite. At intermediate levels in the sequence, cumulus inverted pigeonite occurs in place of olivine. Several prominent regressions in the stratigraphy of the upper zone are marked by fine-grained troctolitic layers with much higher Mg no. [100 MgO/(MgO+FeO)] and anorthite than underlying cumulates. These layers coarsen upward and grade back to oxide-bearing olivine gabbros within thicknesses ranging from 10 cm to 15 m. Dikes that cut the LS have major- and trace-element compositions that strongly suggest that they are feeders for the replenishments. In the lower zone when olivine and plagioclase were the only cumulus phases, replenishments were less dense than the resident magma and rose as plumes and mixed with it. Precipitation of cumulus oxides in the upper zone lowered the density of resident magma so that subsequent replenishments were more dense than resident magma. Replenishments that occurred after oxides began to precipitate had small injection velocities. These post-oxide injections flowed along the interface between resident magma and the cumulate pile and precipitated flow-banded, fine-grained troctolites.     

Density changes during the fractional crystallization of basaltic magmas: fluid dynamic implications

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.     

Snowflake troctolite in the Hettasch intrusion,Labrador: Evidence for magma-mixing and supercooling in a plutonic environment

The snowflake troctolite (SFT) in the Hettasch intrusion is a thin (0–10 m) zone of melatroctolite concordant with the normal leucotroctolites on one limb of the Hettasch intrusion. The textures of this unit are strikingly different from the normal cumulate textures of the Hettasch leucotroctolites and include comb-layered plagioclase, skeletal megacrysts of plagioclase, and spherulitic plagioclase 5–15 cm in diameter (snowflakes). These supersaturation plagioclase morphologies are set in a matrix of fine-grained, layered and unlayered melatroctolite. Because the mineral compositions are more primitive in the SFT than in the surrounding Hettasch cumulates, crystallization models involving supersaturation of the basaltic Hettasch magma fail. However, over a short distance below the texturally-defined SFT and a shorter distance above it, the mineral compositions of the surrounding cumulates gradually merge with those of the SFT. It is concluded, therefore, that the SFT formed from a separate magma that irrupted onto the floor of the Hettasch magma chamber and was supercooled by the basaltic Hettasch magma. Thus the SFT magma is inferred to have been more primitive than basalt. Assuming that fractionation in the SFT was minimal or that no fractionated material has escaped the SFT, an estimate of the bulk composition of the unit should approximate the magma composition. A picritic magma is thereby inferred, and a search for comparable rocks or magma types indicates that these rocks have the most similarities with picritic rocks of the Brito-Arctic province which formed during the early opening of the North Alantic. This similarity supports the hypothesis that the anorthositic Nain complex in Labrador also formed in a rifting environment about 1.4 Gyr ago.     

Petrology of the Treknattan intrusion in the Fongen-Hyllingen complex, Trondheim region, Norway: a late intrusion into an evolved layered complex

The basic-ultrabasic Treknattan intrusion is an important example of a late intrusion in a solidified, evolved, layered complex and sheds light on possible mechanisms by which such associations may develop. The Treknattan intrusion, emplaced into the basic Fongen-Hyllingen intrusion shortly after the latter had solidified, consists mainly of massive or weakly layered peridotite (olivine ± Cr-spinel cumulate) and troctolite (plagioclase + olivine ± Cr-spinel cumulate). The mineral compositional range partially overlaps the most primitive end of the much larger variation-interval in the Fongen-Hyllingen intrusion. The margin of the Treknattan intrusion is sometimes outlined by massive feldspathic websterite which appears to have formed by reaction between magma and melts of gabbroic country rock. The parental magma appears to have been a relatively water-rich picritic basalt with a possible genetic relationship to the magma parental to the enveloping Fongen-Hyllingen intrusion, both displaying tholeiitic relationship between olivine and Ca-poor pyroxene, and having crystallized from relatively water-rich magmas with an early crystallization order of olivine ± Cr-spinel-plagioclase-Ca-rich pyroxene. The recognition of the Treknattan intrusion as a separate body suggests that the bulk composition of the Fongen-Hyllingen intrusion is dioritic rather than gabbroic as previously thought.     

Electron-probe studies of the earlier pyroxenes and olivines from the Skaergaard intrusion,East Greenland

Pyroxenes and olivines from the earlier stages of fractionation of the Skaergaard intrusion (Wager and Brown, 1968; Brown, 1957) have been studied using the electron microprobe. The subsolidus trend for both Ca-rich and Ca-poor pyroxenes has been established, from the Mg-rich portion of the quadrilateral to the Hed-Fs join, together with the orientations of the tie-lines joining coexisting pyroxenes. For the Mg-rich Ca-poor pyroxenes, Brown's (1957) solidus trend has been modified slightly. From a study of a previously undescribed drill core, reversals in the cryptic layering have been found in the Lower Zone. The reversals are attributed to existence within the convecting magma chamber of local temperature differences. The Skaergaard magma temperatures are postulated to have passed out of the orthopyroxene stability field into the pigeonite stability field at EnFs ratios of 7228, for Ca-free calculated compositions, and specimen 1849, a perpendicular-feldspar rock, is interpreted as straddling the orthopyroxene-pigeonite transition interval. The cessation of crystallisation of Ca-poor pyroxene and the increase in Wo content of the Ca-rich pyroxene trend have been reexamined, and Muir's (1954) peritectic reaction (pigeonite+liquid=augite) has been confirmed. The composition at which Ca-poor pyroxene starts reacting with the liquid is postulated as Wo₁₀ En_36.7Fs_{53 3}. It is suggested that the cessation of crystallisation of Ca-poor pyroxene is sensitive to the amount of plagioclase crystallising from the liquid.A complete series of accurate olivine compositions for the whole Skaergaard sequence is presented for the first time, including the compositions of the Middle Zone olivine reaction rims.     

四川攀西地区层状侵入体中堆晶岩成因的矿物学约束

火成岩中可以包含多种晶体群这一发现具有重要意义,使得成因矿物学重新成为揭示岩浆系统演化的基本指导思想。但是,这种重要性在许多文献中都没有得到反映,其典型实例就是镁铁质层状侵入体中堆晶岩的成因。争论在于堆晶矿物是循环晶还是母岩浆的液相线相。因此,本文致力于探讨四川攀西地区镁铁质层状侵入体中堆晶岩的形成过程,重申成因矿物学的重要意义。显微镜观察表明,堆晶单斜辉石富含Fe-Ti氧化物出溶叶片(含叶片辉石),表明其形成环境明显不同于与斜长石呈共结关系的单斜辉石(无叶片辉石);无叶片辉石和斜长石中的橄榄石包裹体呈浑圆状,表明了橄榄石与结晶环境间的热力学不平衡。橄榄石与熔体间Fe-Mg分配关系分析表明,根据母岩浆成分推测的橄榄石Fo值远低于岩体中观测橄榄石化学成分变化范围(Fo₆₁-Fo₈₁)的高限,表明至少部分橄榄石不是寄主侵入体的液相线相。橄榄石的Mg^#值(100×Mg/(Mg+Fe))与微量元素(特别是Ni)的相关关系表明存在多种橄榄石晶体群,它们形成于不同的热力学环境中。晶体沉降过程分析表明,寄主岩浆析出的晶体几乎不可能发生快速重力沉降来形成堆晶岩。所有这些证据都表明,形成堆晶岩的矿物主要来自岩浆系统深部不同的岩浆房中,是被岩浆携带输运到终端岩浆房的循环晶。     

Geochemistry of cumulates from the Bjerkreim–Sokndal layered intrusion (S. Norway) Part II. REE and the trapped liquid fraction

Rare earth elements in bulk cumulates and in separated minerals (plagioclase, apatite, Ca-poor and Ca-rich pyroxenes, ilmenite and magnetite) from the Bjerkreim–Sokndal layered intrusion (Rogaland Anorthosite Province, SW Norway) are investigated to better define the proportion of trapped liquid and its influence on bulk cumulate composition. In leuconoritic rocks (made up of plagioclase, Ca-poor pyroxene, ilmenite, ±magnetite, ±olivine), where apatite is an intercumulus phase, even a small fraction of trapped liquid significantly affects the REE pattern of the bulk cumulate, together with cumulus minerals proportion and composition. Contrastingly, in gabbronoritic cumulates characterized by the presence of cumulus Ca-rich pyroxene and apatite, cumulus apatite buffers the REE content. La/Sm and Eu/Eu* vs. P₂O₅ variations in leuconorites display mixing trends between a pure adcumulate and the composition of the trapped liquid, assumed to be similar to the parental magma. Assessment of the trapped liquid fraction in leuconorites ranges from 2 to 25% and is systematically higher in the north-eastern part of the intrusion. The likely reason for this wide range of TLF is different cooling rates in different parts of the intrusion depending on the distance to the gneissic margins. The REE patterns of liquids in equilibrium with primitive cumulates are calculated with mass balance equations. Major elements modelling (Duchesne, J.C., Charlier, B., 2005. Geochemistry of cumulates from the Bjerkreim–Sokndal layered intrusion (S. Norway): Part I. Constraints from major elements on the mechanism of cumulate formation and on the jotunite liquid line of descent. Lithos. 83, 299–254) permits calculation of the REE content of melt in equilibrium with gabbronorites. Partition coefficients for REE between cumulus minerals and a jotunitic liquid are then calculated. Calculated liquids from the most primitive cumulates are similar to a primitive jotunite representing the parental magma of the intrusion, taking into account the trapped liquid fraction calculated from the P₂O₅ content. Consistent results demonstrate the reliability of liquid compositions calculated from bulk cumulates and confirm the hypothesis that the trapped liquid has crystallized as a closed-system without subsequent mobility of REE in a migrating interstitial liquid.     

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

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.     

Geochemistry of cumulates from the Bjerkreim–Sokndal layered intrusion (S. Norway). Part I: Constraints from major elements on the mechanism of cumulate formation and on the jotunite liquid line of descent

Whole-rock major element compositions are investigated in 99 cumulates from the Proterozoic Bjerkreim–Sokndal layered intrusion (Rogaland Anorthosite Province, SW Norway), which results from the crystallization of a jotunite (Fe–Ti–P-rich hypersthene monzodiorite) parental magma. The scattering of cumulate compositions covers three types of cumulates: (1) ilmenite–leuconorite with plagioclase, ilmenite and Ca-poor pyroxene as cumulus minerals, (2) magnetite–leuconorite with the same minerals plus magnetite, and (3) gabbronorite made up of plagioclase, Ca-poor and Ca-rich pyroxenes, ilmenite, Ti-magnetite and apatite. Each type of cumulate displays a linear trend in variation diagrams. One pole of the linear trends is represented by plagioclase, and the other by a mixture of the mafic minerals in constant proportion. The mafic minerals were not sorted during cumulate formation though they display large density differences. This suggests that crystal settling did not operate during cumulate formation, and that in situ crystallization with variable nucleation rate for plagioclase was the dominant formation mechanism. The trapped liquid fraction of the cumulate plays a negligible role for the cumulate major element composition. Each linear trend is a locus for the cotectic composition of the cumulates. This property permits reconstruction by graphical mass balance calculation of the first two stages of the liquid line of descent, starting from a primitive jotunite, the Tjörn parental magma. Another type of cumulate, called jotunite cumulate and defined by the mineral association from the Transition Zone of the intrusion, has to be subtracted to simulate the most evolved part of the liquid line of descent. The proposed model demonstrates that average cumulate compositions represent cotectic compositions when the number of samples is large (> 40). The model, however, does not account for the K₂O evolution, suggesting that the system was open to contamination by roof melts. The liquid line of descent corresponding to the Bjerkreim–Sokndal cumulates differs slightly from that obtained for jotunitic dykes in that the most Ti-, P- and Fe-rich melts (evolved jotunite) are lacking. The constant composition of the mafic poles during intervals where cryptic layering is conspicuous is explained by a compositional balance between the Fe–Ti oxide minerals, which decrease in Fe content in favour of Ti, and the pyroxenes which increase in Fe.     

Factors effecting the stability field of Ca-poor pyroxene and the origin of the Ca-poor minimum in Ca-rich pyroxenes from tholeiitic intrusions

Ca-poor pyroxene ceases to crystallise towards the end of fractionation in tholeiitic intrusions and is usually replaced by Fe-rich olivine. Using the data of Nicholls et al. (1971), the \(a_{{\text{SiO}}_2 }\) at which olivine and pyroxene can coexist has been calculated at different temperatures and pressures. From these calculations it is clear that the Fe/Mg ratio of the last Ca-poor pyroxene to crystallise from a melt is increased by raising the temperature or pressure of crystallisation. The Ca-poor pyroxene-Fe-rich olivine relationship is also dependent on the \(a_{{\text{SiO}}_2 }\) of the melt. In magmas which crystallise Fe-rich olivine before quartz, inicreasing their \(a_{{\text{SiO}}_2 }\) will raise the Fe/Mg ratio of the last Ca-poor pyroxene to crystallise. If the \(a_{{\text{SiO}}_2 }\) of the magma is so high that SiO₂ saturation is reached before the appearance of cumulus Fe-rich olivine, any further increase in the \(a_{{\text{SiO}}_2 }\) of the melt will not influence the stability field of Ca-poor pyroxene. The replacement of Ca-poor pyroxene by Fe-rich olivine requires the magma to reach a high level of a _FeO late in its fractionation. If a magma fractionates with an FeO depletion trend, Ca-poor pyroxene is replaced by Ca-rich pyroxene. The reaction is initiated by the appearance of cumulus K-feldspar which results in a marked reduction in the amount of anorthite crystallising from the magma. This increases the a _CaO of the melt so that Ca-poor pyroxene is replaced by Ca-rich pyroxene.     

Phenocryst-bulk rock composition relations of abyssal tholeiites and their petrogenetic significance

In order to infer equilibrium phase relations of abyssal tholeiites, olivine, plagioclase, augite, and pigeonite tholeiites from the ocean floor are plotted in terms of the CIPW norm proportions in the tetrahedron olivine-plagioclase-diopside-quartz. The phase relations of abyssal tholeiites have a general similarity in form to those of the experimentally studied relevant systems. Experimental studies on natural basalts allow the pressure of crystallization for abyssal tholeiitic magmas to be evaluated approximately. It appears that the pressure at which the phenocryst-stage crystallization of abyssal tholeiites takes place is as high as 2 or 3 kbar, provided that abyssal tholeiitic magmas are ‘dry’.Abyssal tholeiites could be derived from liquids that are in equilibrium with Ca-poor pyroxene in the pressure range of about 5–8 kbar. Major element chemistry of abyssal tholeiites is incompatible with the view that these tholeiitic basalts are derived from picritic magma by olivine fractionation.     

Relict silicate inclusions in extraterrestrial chromite and their use in the classification of fossil chondritic material

Chromite is the only common meteoritic mineral surviving long-term exposure on Earth, however, the present study of relict chromite from numerous Ordovician (470 Ma) fossil meteorites and micrometeorites from Sweden, reveals that when encapsulated in chromite, other minerals can survive for hundreds of millions of years maintaining their primary composition. The most common minerals identified, in the form of small (<1-10 μm) anhedral inclusions, are olivine and pyroxene. In addition, sporadic merrillite and plagioclase were found.Analyses of recent meteorites, holding both inclusions in chromite and corresponding matrix minerals, show that for olivine and pyroxene inclusions, sub-solidus re-equilibration between inclusion and host chromite during entrapment has led to an increase in chromium in the former. In the case of olivine, the re-equilibration has also affected the fayalite (Fa) content, lowering it with an average of 14% in inclusions. For Ca-poor pyroxene the ferrosilite (Fs) content is more or less identical in inclusions and matrix. By these studies an analogue to the commonly applied classification system for ordinary chondritic matrix, based on Fa in olivine and Fs in Ca-poor pyroxene, can be established also for inclusions in chromite. All olivine and Ca-poor pyroxene inclusions (>1.5 μm) in chromite from the Ordovician fossil chondritic material plot within the L-chondrite field, which is in accordance with previous classifications. The concordance in classification together with the fact that inclusions are relatively common makes them an accurate and useful tool in the classification of extraterrestrial material that lacks matrix silicates, such as fossil meteorites and sediment-dispersed chromite grains originating primarily from decomposed micrometeorites but also from larger impacts.     

Origin of ultramafic xenoliths containing exsolved pyroxenes from Hualalai Volcano,Hawaii

Hualalai Volcano, Hawaii, is best known for the abundant and varied xenoliths included in the historic 1800 Kaupulehu alkalic basalt flow. Xenoliths, which range in composition from dunite to anorthosite, are concentrated at 915-m elevation in the flow. Rare cumulate ultramafic xenoliths, which include websterite, olivine websterite, wehrlite, and clinopyroxenite, display complex pyroxene exsolution textures that indicate slow cooling. Websterite, olivine websterite, and one wehrlite are spinel-bearing orthopyroxene +olivine cumulates with intercumulus clinopyroxene +plagioclase. Two wehrlite samples and clinopyroxenite are spinel-bearing olivine cumulates with intercumulus clinopyroxene+orthopyroxene + plagioclase. Two-pyroxene geothermometry calculations, based on reconstructed pyroxene compositions, indicate that crystallization temperatures range from 1225° to 1350° C. Migration or unmixing of clinopyroxene and orthopyroxene stopped between 1045° and 1090° C. Comparisons of the abundance of K₂O in plagioclase and the abundances of TiO₂ and Fe₂O₃in spinel of xenoliths and mid-ocean ridge basalt, and a single ⁸⁷Sr/ ⁸⁶Sr determination, indicate that these Hualalai xenoliths are unrelated to mid-ocean ridge basalt. Similarity between the crystallization sequence of these xenoliths and the experimental crystallization sequence of a Hawaiian olivine tholeiite suggest that the parental magma of the xenoliths is Hualalai tholeiitic basalt. Xenoliths probably crystallized between about 4.5 and 9 kb. The 155°–230° C of cooling which took place over about 120 ka — the age of the youngest Hualalai tholeiitic basalt — yield maximum cooling rates of 1.3×10^–3–1.91×10^–3 °C/yr. Hualalai ultramafic xenoliths with exsolved pyroxenes crystallized from Hualalai tholeiitic basalt and accumulated in a magma reservoir located between 13 and 28 km below sealevel. We suspect that this reservoir occurs just below the base of the oceanic crust at about 19 km below sealevel.     

西南天山哈拉达拉岩体的锆石SHRIMP年代学及地球化学研究

西南天山哈拉达拉侵入体由橄长岩、橄榄辉长岩和辉长岩组成,橄长岩和橄榄辉长岩具有典型的堆晶结构,堆晶矿物以斜长石和橄榄石为主。辉石、角闪石和金云母主要为堆晶间隙矿物。辉长岩发育辉长—辉绿结构。结晶分异作用在岩浆演化过程中起重要作用。对从辉长岩中分选出来的锆石进行的SHRIMP年代学研究表明,辉长岩形成于308.3±1.8Ma （MSWD=0.86,n=15）。哈拉达拉岩体稀土元素配分模式与E-MORB相似,具有高Rb、Cs、Ba及Sr的特点,⁸⁷Sr/⁸⁶Sr初始比值0.7040~0.7050。这些特征表明,岩浆源区具有富集地幔的特征(古南天山洋俯冲流体交代形成了富集地幔)。根据平坦的稀土元素配分模式以及Gd、Sm、Nb、Zr等微量元素的地球化学行为判别,岩浆源区岩石为含角闪石的尖晶石二辉橄榄岩。批式熔融模拟计算显示,地幔岩10%~15%的部分熔融能够形成哈拉达拉岩体的母岩浆。母岩浆通过48%~50%的结晶分异作用则能够形成哈拉达拉岩体。早期结晶的橄榄石和斜长石通过堆晶作用形成橄长岩和橄榄辉长岩,剩余岩浆结晶形成辉长岩。     

The Munni Munni Complex, Western Australia: Stratigraphy, Structure and Petrogenesis

The late Archaean Munni Munni Complex is a layered mafic-ultramaficintrusion emplaced into granitic rocks of the west Pilbara Block.It consists of a lower Ultramafic Zone with a maximum thicknessof 1850 m and an overlying Gabbroic Zone at least 3600 m thick.There are strong geometrical and stratigraphic similaritiesto the Great Dyke of Zimbabwe. The Ultramafic Zone comprises multiple macrorhythmic cyclesof olivine-clinopyroxene adcumulates and mesocumulates. Layeringdips towards the centre of the intrusion and trends laterallyinto a narrow and variably contaminated chilled margin. Higherlayers extend progressively further up the sloping floor ofthe intrusion. Cryptic layering is defined by rapid fluctuationsin Cr content of cumulus clinopyroxene, accompanied by relativelysmall variation in Fe/Mg ratio. The base of the Gabbroic Zone is marked by the first appearanceof cumulus plagioclase and the simultaneous appearance of pigeoniteas a persistent cumulus phase. Magnetite appears as a cumulusphase 400–600 m above this. Gabbroic Zone cumulates showa gradual linear upward increase in Fe/Mg and an absence ofcyclic layering, suggesting crystallization in a closed chamber. Chilled margin samples show evidence of in situ contamination,but indicate that the parent magma to the ultramafic portionof the intrusion was a high-Mg, low-Ti basalt with similaritiesto typical Archaean siliceous high-Mg basalts. Partial meltingof granitic wall rocks occurred along steep side walls but wasless extensive along the shallow-dipping floor. A pyroxenitedyke, the Cadgerina Dyke, intersects the floor of the intrusionat a level close to the top of the Ultramafic Zone, and appearsto have acted as a feeder conduit to the Gabbroic Zone and theuppermost layers of the Ultramafic Zone. The contact zone between the Ultramafic Zone and the GabbroicZone is a distinctive 30–50 m thick pyroxenite layer,the Porphyritic Websterite Layer, which also exlends laterallyup the side walls of the intrusion to form a 200 m thick marginalborder zone separating Gabbroic Zone cumulates from countryrock granites. A distinctive suite of bronzite-rich xenoliths,some containing Al-rich, Cr-poor spinel seams, occurs withinand just above the Porphyritic Websterite Layer in the centralpart of the intrusion. There is a steep gradient of decreasing Cr and increasing Fe/Mgin cumulus clinopyroxenes across the upper 100 m of the UltramaficZone. A sharp downward step in Cr occurs a few metres belowthe base of the Gabbroic Zone, immediately beneath a stronglyorthocumulate layer of augite cumulate containing disseminatedplatinum-group element (PGE)-rich sulphides. Lateral pyroxenecomposition trends within the Porphyritic Websterite Layer canbe accounted for by an increase in cumulus porosity as thislayer approaches the floor of the intrusion. Quantitative modelling of pyroxene composition trends indicatesthat Ultramafic Zone cumulates crystallized from relativelysmall volumes of magma, an order of magnitude less than thesize of the magma body inferred from trends in the GabbroicZone. This conclusion, together with the geometry of the PorphyriticWebsterite Layer, implies that the Porphyritic Websterite Layermarks a level at which the chamber expanded as a result of amajor new influx of magma. Pyroxene composition trends indicatethat this influx was of a distinetly different and more fractionatedcomposition than that parental to the Ultramafic Zone. Injection of fractionated tholeiitic magma into more primitivehigh-Mg basalt resident magma formed a turbulent fountain, whichentrained the resident magma and formed a cool, dense basalhybrid layer. Crystallization of the Porphyritic WebsteriteLayer occurred where the top of this hybrid layer impinged onthe sloping floor. Continuing injection of tholeiitic magmaexpanded the thickness of the hybrid layer, causing the PorphyriticWebsterite Layer to accrete progressively up the sloping floorand the walls. After the conclusion of the influx phase, thehybrid layer became homogenized to a final tholeiite-rich composition,which eventually crystallized to form the Gabbroic Zone. Thexenolithic rocks within and above the Porphyritic WebsteriteLayer were probably derived initially by crystallization ofa contaminated silica-enriched melt layer at the roof of theintrusion, followed by detachment and sinking or slumping tothe floor. Orthopyroxene phenocrysts within the PorphyriticWebsterite Layer may also have originated within this roof zone.     

http://www.sciencedirect.com/science/article/pii/S1674987112001053

Massif anorthosites form when basaltic magma differentiates in crustal magma chambers to form low-density plagioclase and a residual liquid whose density was greater than that of enclosing crustal rocks. The plagioclase and minor pyroxene crystallized in-situ on the floor of the magma chamber to produce the anorthosite complex, and the residual liquid migrated downwards, eventually to solidify as dense Fe-rich cumulates some of which were removed to the mantle. These movements were facilitated by high temperatures in Proterozoic continental crust, thus explaining the restriction of large anorthosite massifs to this period in Earth history.     

The Somerset Dam Layered basic intrusion,Southeastern Queensland

The Somerset Dam Layered Basic Intrusion is probably a sub‐volcanic magma chamber, and it consists of 20 saucer‐shaped layers composed of troctolites, olivine gabbros, ferrigabbros, and leucogabbros. The layered sequence is 1650 ft (500 m) thick, and comprises several repetitions of a standard pattern termed a zone. Each zone is generally composed of four layers, and successive mineral assemblages from the base upwards are: plagioclase‐olivine, plagioclase‐augite‐olivine, plagioclase‐augite‐magnetite‐ilmenite, and plagioclase‐uralite. Pronounced modal and textural changes define the boundaries between these layers. Within a zone, systematic variation in the proportions and compositions of minerals is thought to be the result of a slight decrease in temperature, and an increase in the partial pressures of water vapour and oxygen from the base to the top.

Repetition of zones is explained by a mechanism involving periodic renewals of magma. Lack of progressive changes in mineral compositions and proportions from the base to the top of the layered sequence is also a consequence of the magma composition, the control of water vapour pressure, and the limited range of crystallisation temperatures. Gravitational settling of early minerals does not explain the variation within a zone, or the small‐scale rhythmic layering that is locally developed, and it is concluded that diffusion has been an important control.