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.     

Strontium isotope geochemistry of megacrysts and host basalts from southeastern Australia

Strontium isotopic data for megacrysts and lavas from six eruptive centers within the Newer Basalts province of southeastern Australia show that megacrysts of clinopyroxene are in isotopic equilibrium with associated basalts, but that megacrysts of kaersutite, ferrokaersutite, orthopyroxene and anorthoclase may exhibit slight disequilibrium with their host basalts. Furthermore, the anorthoclase megacrysts may be either more or less radiogenic than their hosts. The ⁸⁷Sr/⁸⁶Sr ratios for 14 basalts from throughout the province vary from 0.7035 to 0.7045 and it is proposed that anorthoclase, amphibole and orthopyroxene megacrysts which crystallized in isotopic equilibrium with one magma may have been caught up in a pulse of a later magma of a different isotopic composition. The variations in the ⁸⁷Sr/⁸⁶Sr ratios for the basalts are attributed to variations in the isotopic composition of their source regions. Such isotopic heterogeneity is supported by published data for ultramafic xenoliths which occur in the Newer Basalts lavas.     

Clinopyroxene megacrysts from Enmelen melanephelinitic volcanoes (Chukchi Peninsula,Russia): application to composition and evolution of mantle melts

Clinopyroxene megacrysts from young melanephelinitic lavas were divided into Cr-rich and Cr-poor suites. Sr, Nd, and Pb isotopic ratios of leached megacrysts and host lava are indistinguishable from each other and indicate a depleted source. Host lavas do not display chemical evidence for significant fractional crystallization, which is required to explain the compositional range of the megacrysts. This rules out a simple cognate genetic relationship between the two, and strictly defines megacrysts as xenocrysts. The well-defined correlations of trace elements with the Mg-numbers in the megacrysts are interpreted as the result of extensive fractional/equilibrium crystallization of magma over a large temperature range at near isobaric condition in the upper mantle. Trace element variations in megacrysts are consistent with fractional crystallization of clinopyroxene alone for the Cr-rich suite, and clinopyroxene + garnet for the Cr-poor suite from at least two bathes of related melts. Megacrysts parent magma might represent mantle melts, which were never erupted in their initial composition.     

Cr-rich megacrysts of clinopyroxene and garnet from Lac de Gras kimberlites,Slave Craton,Canada – implications for the origin of clinopyroxene and garnet in cratonic lherzolites

Kimberlites from the Diavik and Ekati diamond mines in the Lac de Gras kimberlite field contain abundant large (>1 cm) clinopyroxene (Cr-diopside) and garnet (Cr-pyrope) crystals. We present the first extensive mineral chemical dataset for these megacrysts from Diavik and Ekati and compare their compositions to cratonic peridotites and megacrysts from the Slave and other cratons. The Diavik and Ekati Cr-diopside and Cr-pyrope megacrysts are interpreted to belong to the Cr-rich megacryst suite. Evidence for textural, compositional, and isotopic disequilibrium suggests that they constitute xenocrysts in their host kimberlites. Nevertheless, their formation may be linked to extensive kimberlite magmatism and accompanying mantle metasomatism preceding the eruption of their host kimberlites. It is proposed that the formation of megacrysts may be linked to failed kimberlites. In this scheme, the Cr-rich megacrysts are formed by progressive interaction of percolating melts with the surrounding depleted mantle (originally harzburgite). As these melts percolate outwards, they may contribute to the introduction of clinopyroxene and garnet into the depleted mantle, thereby forming lherzolite. This model hinges on the observation that lherzolitic clinopyroxenes and garnets at Lac de Gras have compositions that are strikingly similar to those of the Cr-rich megacrysts, in terms of major and trace elements, as well as Sr isotopes. As such, the Cr-rich megacrysts may have implications for the origin of clinopyroxene and garnet in cratonic lherzolites worldwide.

     

Quantitative petrological evidence for the origin of K-feldspar megacrysts in dacites from Taapaca volcano,Chile

K-feldspar megacrysts are common in granitoids, but relatively rare in chemically equivalent volcanic rocks. Dacites from Taapaca volcano have euhedral sanidine megacrysts up to 5 cm long. Small crystals, where present, are rounded. Growth of the megacrysts engulfed plagioclase and amphibole crystals. Crystal size distributions (CSD) of sanidine megacrysts are hump shaped. All these data show that megacrysts developed from the host magma by coarsening: this was enabled by the cycling of magma temperature around the sanidine liquidus temperature in response to injections of more mafic magma and subsequent magmatic overturns. Plagioclase crystals enclosed in the megacrysts are small and have short, steep, straight CSDs, which contrasts with the CSDs of plagioclase in the groundmass which are shallower and extend to larger sizes. This shows that plagioclase was also coarsened approximately synchronously with sanidine, in response to the same temperature conditions.     

Formation of K-feldspar megacrysts in granodioritic plutons by thermal cycling and late-stage textural coarsening

K-feldspar megacrysts in granite and granodiorite plutons are generally inferred to be early crystallizing phases (grown to large sizes when the magma was mostly liquid) owing to their large size, euhedral form, and features that suggest deposition by magmatic sedimentation. However, phase equilibrium experiments and natural examples of crystallization and partial melting demonstrate that K-feldspar is one of the last phases to nucleate and that most crystal growth must occur after the magma has exceeded 50% crystallization and is thus largely incapable of flow and sedimentation. Megacryst size distributions, compositions, and textural relationships from the Cretaceous Tuolumne Intrusive Suite, California, reveal that the gradational transition from equigranular to megacrystic granodiorite likely occurred via textural coarsening caused by thermal cycling. Experimental and theoretical studies demonstrate that rising temperature induces relatively more melting in small crystals than in large ones, whereas linear growth rates during cooling are similar. Thus, during thermal cycling material is transferred from small crystals to larger ones. Megacryst growth via thermal cycling during incremental emplacement is consistent with the required late growth of K-feldspar, explains the presence of megacrysts in the inner parts of theTuolumne Intrusive Suite and elsewhere, and may be a common process in formation of megacrystic granitic rocks.     

The origin of K-feldspar megacrysts hosted in alkaline potassic rocks from central Italy: a track for low-pressure processes in mafic magmas

In situ Sr-isotope and microchemical studies were used to determine the provenance of K-feldspar megacrysts hosted in mafic alkaline potassic, ultrapotassic rocks and in differentiated rocks from two nearby volcanic apparatus in central Italy.

At Monte Cimino volcanic complex, mafic leucite-free ultrapotassic megacryst-bearing rocks of olivine latitic composition are associated with evolved latite and trachyte. Here, latites and trachytes straddle the sub-alkaline field. Age-corrected ⁸⁷Sr/⁸⁶Sr values (Sr_i) of the analysed Cimino olivine latites vary from 0.71330 and 0.71578 and strongly increase at constant Mg value. Latite and trachyte have lower Sr_i than olivine latites ranging between 0.71331 and 0.71361. Sr_i of K-feldspar megacrysts from olivine latites are between 0.71352 and 0.71397, but core and rim ⁸⁷Sr/⁸⁶Sr ratios within individual megacryst are indistinguishable. In all the mafic rocks, the megacrysts are not in isotopic equilibrium with the hosts. K-feldspar megacrysts from both the latite and trachyte have similar Sr-isotope compositions (Sr_i=0.71357–0.71401) to those in the olivine latites. However, Sr_i of megacryst in the trachyte vary significantly from core to rim (Sr_i from 0.71401 to 0.71383). As with the olivine latites, the K-feldspar megacrysts are not in isotopic equilibrium with bulk rock compositions of the latite or trachyte.

At Vico volcano, megacryst-bearing rocks are mafic leucite-free potassic rocks, mafic leucite-bearing ultrapotassic rocks and old trachytic rocks. The mafic leucite-bearing and leucite-free rocks are a tephri-phonolite and an olivine latite, respectively. A megacryst in Vico trachyte is isotopically homogeneous (Sr_i CORE=0.71129, RIM=0.71128) and in equilibrium with the host rock (Sr_i bulk ROCK=0.71125). Sr_i of megacryst from tephri-phonolite is clearly not in isotopic equilibrium with its host (Sr_i bulk ROCK=0.71158), and it increases from core (Sr_i=0.71063) to rim (Sr_i=0.71077). A megacryst in Vico olivine latite is isotopically homogeneous (Sr_i CORE=0.71066, RIM=0.71065), but not in equilibrium with the host rock (Sr_i bulk ROCK=0.71013).

The Sr isotope microdrilling technique reveals that Cimino megacrysts were crystallised in a Cimino trachytic magma and were subsequently incorporated by mixing/mingling processes in the latitic and olivine latitic melts. A model invoking the presence of a mafic sub-alkaline magma, which was mixed with the olivine latite, is proposed to justify the lack of simple geochemical mixing relation between Cimino trachytes and olivine latites. This magmatological model is able to explain the geochemical characteristics of Cimino olivine latites, otherwise ascribed to mantle heterogeneity.

The similarity of core Sr_i of megacrysts hosted in Vico tephri-phonolite and olivine latite suggests that the K-feldspar megacrysts are co-genetic. Isotopic equilibrium between megacryst and Vico host trachyte indicates that the trachyte is the parent of this megacryst. On the contrary, the megacrysts hosted in tephri-phonolite and olivine latite do not derive from the old trachytic magma because no diffusion process may explain the core to rim Sr isotope increase of the xenocryst hosted in the tephri-phonolite. The megacrysts hosted in the Vico mafic rocks might derive from a trachytic melt similar in composition to the old Vico trachytes.     

Sr and Nd isotopic investigations towards the origin of feldspar megacrysts in microgranular enclaves in two I‐type plutons of the Lachlan Fold Belt,southeast Australia

New Sr and Nd isotopic data are presented for several large feldspar crystals occurring in microgranular enclaves in the Swifts Creek and Bridle Track plutons, along with analyses of their host enclave groundmass and adjacent granitoid. In the Swifts Creek Pluton several previous studies have concluded that the microgranular enclaves represent admixed, more mafic and more primitive magmas, and new data presented here confirm that. Feldspar megacrysts in the microgranular enclaves have Sr and Nd isotopic signatures that are distinct from the surrounding enclave groundmass and from other enclaves in the pluton and therefore cannot have crystallised in situ. Isotopic compositions of these feldspars are more consistent with their having crystallised in a reservoir similar in composition to the most primitive granitoid analyses. The crystals were then physically transferred from the granitoid magma into the enclave while the latter was still partially liquid, thus invalidating arguments for a porphyroblastic origin. Field, petrographic and geochemical data are consistent with microgranular enclaves in the Bridle Track pluton also originating as admixed, more mafic magmas. However, Sr isotopic compositions of the enclaves are identical, within error, to the host granite and indicate that significant Sr isotopic equilibration has occurred. Nd isotopic compositions of the enclaves extend to slightly higher ¹⁴³Nd/¹⁴⁴Nd_(i) and are consistent with a mingled magma origin followed by major isotopic equilibration. Feldspar phenocrysts in the studied enclave have isotopic compositions indistinguishable from both the enclave groundmass and host granite, preventing an interpretation of their origin using isotopic evidence alone.     

汉诺坝幔源单斜辉石巨晶氢同位素组成的离子探针分析:微尺度不均一性

运用离子探针技术测定了河北汉诺坝新生代玄武岩中3个单斜辉石巨晶的氢同位素组成,结果显示同一颗粒内部表现出微尺度的不均一性,2mm范围内δD的变化达到60‰。δD和氢含量之间不存在同步的变化,巨晶内部的化学成分均一,因此我们认为巨晶的氢同位素不均一性继承自母岩浆。母岩浆的氢同位素变化可能是去气过程中气相与熔体之间的分馏引起的。单斜辉石巨晶形成后很短的时间内即被寄主岩浆带至地表并经历了快速淬火。     

Trace Element and Strontium Isotope Constraints on the Origin and Evolution of Paran? Continental Flood Basalts of Santa Catarina State (Southern Brazil)

Forty-five selected samples from two different sequences, andfrom different sites spread over the Parana Basin have beenanalysed for major and trace elements and Sr isotope ratios.Results demonstrate the existence of two geochemically and isotopicallydistinct groups, namely a high phosphorus and titanium (HPT)and a low phosphorus and titanium (LPT) group. HPT rocks are relatively rich in LIL elements and have morehomogeneous and markedly lower isotopic Sr ratios than LPT rocks.Since no relationship between the two groups was found, theymay derive from different mantle sources, the HPT source beingmore enriched in most incompatible trace elements except Th,U and Rb than the LPT one. The LPT group, comprising basalts to rhyodacites, displays alarge range in Sr isotopic ratios. This is interpreted as aresult of a fractional crystallization process occurring athigh crustal levels associated with assimilation of crustalmaterial.     

Origin of megacrysts in volcanic rocks of the Cameroon volcanic chain – constraints on magma genesis and crustal contamination

Lavas of the Biu and Jos Plateaus, Northern Cameroon Volcanic Line (CVL), contain abundant genetically related megacrysts of clinopyroxene, garnet and subordinately plagioclase, ilmenite and amphibole. P, T-estimates of crystallization for the primitive group of cpx and gnt megacrysts are 1.7–2.3 GPa and ~1,400 °C. Because crustal thickness in these areas is only ~30 km (~0.9 GPa), megacrysts must have formed within the lithospheric mantle. Primitive Biu and Jos lavas are isotopically heterogeneous in Sr-Nd isotope space (⁸⁷Sr/⁸⁶Sr=0.70285–0.70360 and _Nd=7.5–4.6). Biu Plateau megacrysts overlap the range of Biu lavas in Sr-Nd isotope composition, indicating that crustal contamination of Biu lavas was minor. Jos Plateau lavas are isotopically more enriched than their associated megacrysts. Therefore an additional contamination of Jos lavas due to assimilation of continental crust (~5%) or enriched shallow lithospheric mantle is indicated. Lavas of Biu and Jos Plateau do not reflect simple fractionation or equilibrium crystallization products, but instead reflect mixing between primary melts and their fractionated derivatives.Editorial Responsibility: I. Carmichael     

微区-微量样品Rb-Sr同位素分析技术及其应用前景

利用微钻取样技术和微量样品Rb-Sr同位素分析方法,本文对出露在东秦岭造山带的中生代合峪花岗岩的自形钾长石巨晶进行微区-微量样品Rb-Sr同位素组成分析。分析结果表明,钾长石斑晶具有显著的Rb/Sr比值和Sr同位素组成变化,斑晶和基质钾长石均构成年龄为132~133Ma的Rb-Sr等时线,代表岩浆的后期冷却时代。钾长石晶体的初始⁸⁷Sr/⁸⁶Sr比值由边缘相到中心相没有明显的变化,代表花岗质岩浆结晶阶段的Sr同位素组成,暗示合峪花岗岩的钾长石巨晶为原生成因。以高空间分辨率为特征,微区取样技术已经广泛地应用在变质岩和深成岩浆岩的同位素年代学和成因研究。结合微量样品同位素分析技术,微区-微量样品Rb-Sr同位素方法有望在火山岩的成因和年代学方面得到应用。     

华北晚中生代和新生代玄武岩中单斜辉石巨晶来源及其对寄主岩岩浆过程的制约:以莒南和鹤壁为例

本文对华北克拉通晚中生代和新生代碱性玄武质岩石中的单斜辉石巨晶进行了主、微量元素和Sr-Nd同位素的综合研究,发现晚中生代和新生代单斜辉石巨晶存在明显的主、微量元素和同位素组成上的差异.新生代单斜辉石巨晶有Al-普通辉石和次透辉石两类;而中生代单斜辉石巨晶只有Al-普通辉石.新生代单斜辉石SiO2含量高、REE配分型式为上凸型、LILE和放射性元素含量高,并具有比寄主碱性玄武岩更亏损的Sr和Nd同位素组成;而中生代单斜辉石SiO2含量低、REE配分型式为LREE富集型、LILE和部分HFSE以及放射性元素含量低,并具有比寄主碱性玄武岩稍富集的Sr和Nd同位素组成;巨晶的结构、矿物成分和地球化学特征,以及Mg-Fe在熔体与单斜辉石间的分配状况皆说明,新生代碱性玄武岩中单斜辉石巨晶是碱性玄武岩浆在高压下结晶的,因此二者是同源的;而中生代单斜辉石巨晶是被寄主岩浆偶然捕获的捕虏晶,是不同源的.华北新生代单斜辉石巨晶存在于碱性玄武岩和拉斑玄武岩中,它们具有比寄主碱性玄武岩更亏损的Sr和Nd同位素组成,说明即使是碱性玄武岩也不能完全代表软流圈来源的原始岩浆,其在上升过程中或多或少存在同位素组成富集的物质的混入.同时,拉斑玄武岩不是碱性玄武质岩浆直接结晶分异的产物,亦不是完全由部分熔融程度的不同造成的.拉斑玄武岩中存在岩石圈地幔物质的贡献或是岩浆房内碱性玄武质岩浆受地壳混染作用的结果.     

Origin of clinopyroxene megacrysts in volcanic rocks from the North China Craton: a comparison study with megacrysts worldwide

ABSTRACT

Clinopyroxene megacrysts in volcanic rocks can provide substantial information on the evolution of the magmatic system at depth. Although considerable attention has been paid to these crystals, their origin is not yet completely resolved. The clinopyroxene megacrysts worldwide can be divided into two major types in general: the green Cr-diopside type and the black Al-augite type. There is a consensus view that the Cr-diopside megacrysts are mantle xenocrysts, whereas two contrasting opinions exist regarding the origin of the black Al-augite megacrysts. One favours a cognate origin, viewing them as crystallization products of the host magmas under high-pressure; while the other argues that they are xenocrysts crystallized from previous alkali basalts or fragments of mantle peridotites, pyroxenites or pegmatite veins. A review study on the clinopyroxene megacrysts in Meso-Cenozoic volcanic rocks from the North China Craton (NCC) and their comparison with those worldwide provides new constraints on their origin, namely, the Cr-diopside megacrysts, as previously thought, are all xenocrysts, representing disaggregated clinopyroxene crystals from clinopyroxene-rich mantle rocks. Contrary to the formerly proposed cognate origin, the Al-augite megacrysts are also xenocrysts, having no direct genetic link to their host rocks. They crystallized from melts that have formed earlier than the host magmas, and probably accumulated in a magma chamber or occurred as sheets or veins filling a fracture network surrounding a magma chamber in the upper mantle. During the subsequent eruption of the host lavas, these previously formed crystals were incorporated into the magma and were brought up to the surface.     

How late are K-feldspar megacrysts in granites?

Various petrologists have suggested that K-feldspar megacrysts grow in granites that are extensively crystallized, even at subsolidus conditions. However, experimental evidence indicates that, though K-feldspar nucleates relatively late in the crystallization history, abundant liquid is available for development of large crystals. A great deal of evidence, involving many different factors, favours a magmatic/phenocrystic origin for K-feldspar megacrysts in granites, namely simple twinning, oscillatory zoning, euhedral plagioclase inclusions, and concentric, crystallographically controlled arrangements of inclusions. In addition, abundant evidence has been presented of (1) mechanical accumulation of K-feldspar megacrysts in granites, (2) alignment of megacrysts and megacryst concentrations in magmatic flow foliations, (3) involvement of megacrysts in zones of magma mixing in granite plutons, and (4) occurrence of megacrysts in some volcanic rocks, implying that the megacrysts were suspended in enough liquid to be moved without fracturing or plastic deformation. Detailed trace element and isotopic data also indicate that megacrysts can move between coexisting felsic and more mafic magmas. Irregular overgrowths on megacrysts are consistent with continued magmatic growth after euhedral megacrystic growth ceased, the overgrowths being impeded by simultaneously crystallizing quartz and feldspar grains.     

Genesis of zircon megacrysts in Cenozoic alkali basalts and the heterogeneity of subcontinental lithospheric mantle, eastern China

Zircon megacrysts are found in alluvial deposits associated with Cenozoic basalts from Changle in Shandong Province, Mingxi in Fujian Province and Penglai in Hainan Province within the coastal area of eastern China. They are colourless, transparent to light brown–maroon, and some of them are up to 16 mm long. U–Pb ages of zircon megacrysts from Changle, Mingxi and Penglai are 19.2?±?0.7 Ma, 1.2?±?0.1 Ma and 4.1?±?0.2 Ma respectively, slightly older than the eruption ages of their corresponding host rocks (16.05–18.87 Ma, 0.9–2.2 Ma, 3 Ma). ε_Hf(t) values of zircon megacrysts are 9.02?±?0.49, 6.83?±?0.47, 4.46?±?0.48 for Changle, Mingxi and Penglai, respectively, which indicates their mantle origin. We suggest that the zircon megacrysts originated from metasomatised lithospheric mantle and were later brought up quickly by the host basaltic magma. The euhedral forms, uniform internal structure and chemical homogeneity within a single grain suggest crystallization under stable conditions. Pronounced positive Ce anomalies and negligible Eu anomalies suggest oxidizing conditions and little or no fractional crystallization of plagioclase. The differences in Hf-isotope compositions among the zircon megacrysts from different localities are consistent with the Sr-Nd-Pb isotopic compositions of their respective host basalts. This indicates that the host basalts acquired their isotopic signatures from the lithospheric mantle from which the zircon megacrysts derived. These data document the lateral compositional heterogeneity in the upper mantle beneath eastern China. Like mantle xenoliths, zircon megacrysts also have the potential to fingerprint the composition and evolution of the subcontinental lithospheric mantle.     

Trace element abundances in megacrysts and their host basalts: Constraints on partition coefficients and megacryst genesis

In an effort to obtain information about mineral/melt trace element partitioning during the high pressure petrogenesis of basic rocks, we determined rare earth and other trace element abundances in megacrysts of clinopyroxene, orthopyroxene, amphibole, mica, anorthoclase, apatite and zircon, and in their host basalts. In general, the ranges of mineral/melt partition coefficients established from experimental partitioning studies and phenocryst/matrix measurements overlap with the ranges of megacryst/host abundance ratios. Our data for Hf, Sc, Ta and Th partitioning represent some of the only estimates available. Consideration of phase equilibria, major element partitioning and isotopic ratios indicate that most of the pyroxene and amphibole megacrysts may have been in equilibrium with their host magmas at high pressures (mostly 10–25 kb). In contrast, it is unlikely that mica, anorthoclase, apatite and zircon megacrysts formed in equilibrium with their host basalts; instead, we conclude that they were precipitated from more evolved magmas and have been mixed into their present host magmas. Consequently, the trace element abundance ratios for megacryst/host should not be interpreted as partition coefficients, but only as guides for understanding trace element partitioning during high pressure petrogenesis. With this caveat, we conclude that the megacryst/ host trace element abundance data indicate that mineral/melt partition coefficients in basaltic systems during high pressure fractionation are not drastically different from partition coefficients valid for low pressure fractionation.     

REE Abundaces in Megacrysts and Host Basalts：REE Behavior of Magma at High Pressures

Based on REE abundances in megacrysts and host basalts and their equilibrium conditions,it has proved that megacrysts may have been produced from the magma derived from the host rocks or of more basic composition.The REE ratios of megacrysts to host rocks may be taken as partition coefficients when both are equilibrium with each other.The crystal fractionation of megacrysts has caused the evolution of REE in the magma.It is obvious that some host basalts are the product of magma evolution after crystal fractionation.According to REE abundances in the host rocks and the partition coefficients between crystal and liquid,the history of crystal fractionation of magma can be traced.     

A mathematical model of trace element and isotopic behavior during simultaneous assimilation and imperfect fractional crystallization

The process of coupled assimilation and fractional crystallization (AFC) is one of the most popular petrogenetic concepts that explains magmatic differentiation. Conventional geochemical models for this process assume that crystals are removed instantaneously from the magma body as they are produced; however, recent advances in isotopic microanalysis have clarified that the crystals are suspended within the magma body for a certain period, affecting the whole-rock composition in response to the intra-grain isotopic zoning. This study develops a mass balance model for simultaneous assimilation and imperfect fractional crystallization (AIFC) to describe the effects of suspended crystals on the path of magma evolution. The mass balance differential equations for the liquid and suspended crystals are solved simultaneously. The analytical solution of the AIFC equations gives a quantitative account of the evolution paths of trace elements and isotopes within bulk crystals, liquid, and magma (crystals plus liquid). The chemical path of the magma differs markedly from that predicted by the conventional AFC model.     

Petrologic implications of trace element variation in clinopyroxene megacrysts from the Nógrád volcanic province, north Hungary: a study by laser ablation microprobe-inductively coupled plasma-mass spectrometry

Clinopyroxene megacrysts in alkali basalts are an important source of information about the evolution of magmatic systems at depth. In this study, we have undertaken a detailed examination of the trace element contents in a suite of megacrysts from 2.5 Ma old alkali basalts in the Nógrád volcanic province of Hungary and Slovakia. The megacrysts range in composition from Mg-rich and in equilibrium with their host magmas, to those that are Fe-rich and must have evolved in more fractionated magmas. The conditions of crystallization of these megacrysts, as calculated from the Al^VI/Al^IV ratios, suggests they all formed at about 30 km, or the crust–mantle boundary in this area. Using the most magnesian megacrysts and compositions of the host lavas, we have calculated the partition coefficients for a range of trace elements. However, the trace element contents in the megacrysts show a systematic variation with major element composition. Moreover, the rate of increase or change in the trace element concentrations is not consistent with models involving constant or steady state partition coefficients. Using a series of assumptions and models, we hypothesize that the partition coefficients between clinopyroxene and melt change substantially during the magmatic evolution of the system. This change is not constant for each element group, with the high field strength elements showing the most substantial increases. Electrostatic charge balance may have been the most important factor in controlling the mineral/melt partitioning.