Silicate-melt inclusions in magmatic rocks: applications to petrology

Silicate-melt inclusions in igneous rocks provide important information on the composition and evolution of magmatic systems. Such inclusions represent accidentally trapped silicate melt (±immiscible H₂O and/or CO₂ fluids) that allow one to follow the evolution of magmas through snapshots, corresponding to specific evolution steps. This information is available on condition that they remained isolated from the enclosing magma after their entrapment. The following steps of investigation are discussed: (a) detailed petrographic studies to characterise silicate-melt inclusion primary characters and posttrapping evolution, including melt crystallisation; (b) high temperature studies to rehomogenise the inclusion content and select chemically representative inclusions: chemical compositions should be compared to relevant phase diagrams.

Silicate-melt inclusion studies allow us to concentrate on specific topics; inclusion studies in early crystallising phases allow the characterisation of primary magmas, while in more differentiated rocks, they unravel the subsequent chemical evolution. The distribution of volatile species (i.e., H₂O, CO₂, S, Cl) in inclusion glass can provide information on the degassing processes and on recycling of subducted material. In intrusive rocks, silicate melt inclusions may preserve direct evidence of magmatic stage evolution (e.g., immiscibility phenomena). Melt inclusions in mantle xenoliths indicate that high-silica melts can coexist with mantle peridotites and give information on the presence of carbonate melt within the upper mantle. Thus, combining silicate-melt inclusion data with conventional petrological and geochemical information and experimental petrology can increase our ability to model magmatic processes.     

Petrology and geochemistry of potassic and ultrapotassic volcanism in central Italy: petrogenesis and inferences on the evolution of the mantle sources

Major, trace element, Sr isotopic and mineral chemical data are reported for mafic volcanic rocks (Mg-value 65) from the northern-central sector of the potassic volcanic belt of Central Italy. The rocks investigated range from potassic series (KS) and high-K series (HKS) to lamproitic (LMP) and kamafugitic (KAM) through a transitional series (TRANS), thus covering the entire compositional spectrum of potassic and ultrapotassic magmas. KAM rocks are strongly silica undersaturated and, compared with the other rock series, have low SiO₂, Al₂O₃, Na₂O, Sc and V and high CaO, K/Na, (Na + K)/Al. KS and HKS have high Al₂O₃, CaO and variable enrichment in K₂O and incompatible elements. LMP rocks are saturated in silica and have high SiO₂, K₂O, K₂O/Na₂, MgO, Ni and Cr and low Al₂O₃, CaO, Na₂O, Sc and V. TRANS rocks display intermediate compositional characteristics between LMP and KS.

All the rocks under study have fractionated hygromagmaphile element patterns with high LIL/HFS element values and negative anomalies of Ti, Ta, Nb and Ba. Negative Sr anomalies are observed in the LMP and TRANS rocks. LIL elements show overall positive correlations with K₂O, whereas different trends of Sr and HFSE vs. K₂O are defined by LMP-TRANS and KS-HKS-KAM. ⁸⁷Sr/⁸⁶Sr range from about 0.710 to 0.716. KS, HKS and KAM rocks have similar ⁸⁷Sr/⁸⁶Sr values clustering around 0.710. LMP and TRANS rocks have the highest ⁸⁷Sr/⁸⁶Sr values.

Geochemical and isotopic data reported for the most primitive Italian potassic and ultrapotassic rocks support the hypothesis that the interaction between crustal and mantle reservoirs was a main process in the genesis of Italian potassic magmatism. Simple mass balance calculations exclude, however, an important role of crustal assimilation during ascent of subcrustal magmas to the surface and indicate that the sources of Central Italy volcanics underwent contamination with fluids and/or melts released by upper crustal material previously brought into the mantle by subduction processes.

Different trends of incompatible elements vs. K₂O observed in the studied rocks suggest distinct metasomatic processes for the sources of the investigated magmas. Liquids derived by bulk melting of pelitic sediments are believed to be the most likely contaminants of the source of LMP rocks. Fluids or melts rich in Ca, Sr and with high LILE/HFSE value and Sr isotopic composition around 0.710 are the most likely contaminant of the source region of KS, HKS and KAM volcanics. Variations in CaO, Na₂O and ferromagnesian element abundances and ratios suggest that, in some zones, the mantle source of potassic magmas experienced partial melting with extraction of basaltic liquids prior to metasomatism.     

The exsolution of magmatic hydrosaline chloride liquids

Hydrosaline liquid represents the most Cl-enriched volatile phase that occurs in magmas, and the exsolution of this phase has important consequences for processes of hydrothermal mineralization and for volcanic emission of Cl to the atmosphere. To understand the exsolution of hydrosaline liquids in felsic to mafic magmas, the volatile abundances and (Cl/H₂O) ratios of more than 1000 silicate melt inclusions (MI) have been compared with predicted and experimentally determined solubilities of Cl and H₂O and associated (Cl/H₂O) ratios of silicate melts that were saturated in hydrosaline chloride liquid with or without aqueous vapor in hydrothermal experiments. This approach identifies the minimum volatile contents and the values of (Cl/H₂O) at which a hydrosaline chloride liquid exsolves from any CO₂- or SO₂-poor silicate melt. Chlorine solubility is a strong function of melt composition, so it follows that Cl solubility in magmas varies with melt evolution. Computations show that the (Cl/H₂O) ratio of residual melt in evolving silicate magmas either remains constant or increases to a small extent with fractional crystallization. Consequently, the initial (Cl/H₂O) in melt that is established early during partial melting has important consequences for the exsolution of vapor, vapor plus hydrosaline liquid, or hydrosaline liquid later during the final stages of melt ascent, emplacement, and crystallization or eruption. It is demonstrated that the melt (Cl/H₂O) controls the type of volatile phase that exsolves, whereas the volatile abundances in melt control the relative timing of volatile phase exsolution (i.e., the time of earliest volatile exsolution relative to the rate of magma ascent and crystallization history).

Comparing melt inclusion compositions with experimentally determined (Cl/H₂O) ratios and corresponding volatile solubilities of hydrosaline liquid-saturated silicate melts suggests that some fractions of the eruptive, calc-alkaline dacitic magmas of the Bonnin and Izu arcs should have saturated in and exsolved hydrosaline liquid at pressures of 2000 bars. Application of these same melt inclusion data to the predicted volatile solubilities of Cu-, Au-, and Mo-mineralized, calc-alkaline porphyritic magmas suggests that the chemical evolution of dioritic magmas to more-evolved quartz monzonite compositions involves a dramatic reduction in Cl solubility that increases the probability of hydrosaline liquid exsolution. The prediction that quartz monzonite magmas should exsolve a hydrosaline chloride liquid, that is potentially mineralizing, is consistent with the general observation of metal-enriched, hypersaline fluid inclusions in the more felsic plutons of numerous porphyry copper systems. Moreover, comparing the volatile contents of melt inclusions from the potassic, alkaline magmas of Mt. Somma-Vesuvius with the predicted (Cl/H₂O) ratios of hydrosaline liquid-saturated melts having compositions similar to those of the volatile-rich, alkaline magmas associated with the orthomagmatic gold–tellurium deposits of Cripple Creek, Colorado, suggests that hydrosaline chloride liquid should have exsolved at Cripple Creek as the magmas evolved to phonolite compositions. This prediction is consistent with the well-documented role of Cl-enriched, mineralizing hydrothermal fluids at this major gold-mining district.     

Petrological characterization of the source components of potassic magmas: geochemical and experimental constraints

The source mineralogy and conditions of origin of the three main groups of ultrapotassic rocks are outlined by combining experimental constraints and an abstraction of evidence from whole-rock chemistry (including volatiles), tectonic setting and xenolith contents. Lamproites originate from a depleted source rock which was strongly re-enriched at a later stage, thus producing mica-harzburgite. Melting conditions are H₂O-rich and in most cases strongly reducing. Kamafugites originate from a clinopyroxene-reich source, also with abundant mica, in more oxidizing, CO₂-rich conditions. Members of the third group form in a relatively fertile spinel-peridotite also containing abundant clinopyroxene and mica. Contrasting effects of variation in (i) pressure of melting and (ii) oxygen fugacity, emphasize the importance of these parameters in the sources of ultrapotassic rocks.

Currently popular models for the origin of ultrapotassic melts by partial melting of phlogopite-bearing lherzolite are inconsistent with the now extensive array of liquidus experimental results on ultapotassic rock compositions. The discrepancy between partial melting models and liquidus results is attributed to the implicit, invalid assumption in the partial melting models that incompatible elements are homegeneously distributed on a large scale. Non-peridotitic assemblages rich in mica and pyroxenes which may be completely free of olivine must have an important role in the genesis of potassic rocks as spatially restricted components of inhomogeneous source regions.     

Phlogopite in the generation of olivine-melilitites from Namaqualand, South Africa and implications for element fractionation processes in the upper mantle

Major and trace element and Sr, Nd and Pb isotope analyses are presented for thirteen olivine-melilitites from Namaqualand, South Africa. Major element variations are consistent with derivation from carbonated garnet-peridotite at depths of at least 100 km and trace element abundances indicate melt fractions of 4%. Ubiquitous negative K anomalies and low, buffered K₂O concentrations are interpreted to reflect the effect of residual phlogopite during melting. It is suggested that phlogopite stability and low melt potassium saturation concentrations are enhanced by high CO₂/(CO₂ + H₂O) conditions. Residual phlogopite can also account for low measured Rb/Sr, Ba/Sr and Th/U ratios in the melilitites. REE abundances are controlled by residual garnet and hence Sm/Nd ratios are low (0.13–0.18). U/Pb ratios vary from 0.05 to 5 and are a function of Pb concentration which is in turn controlled by residual Pb-rich phase (probably sulphide). Nd and Sr isotopes are comparable with OIB from St. Helena, although two samples extend to higher ⁸⁷Sr/⁸⁶Sr ratios. Present day Pb isotopes are much more variable and partly reflect radiogenic growth since emplacement as a result of the highly variable U/Pb ratios.

Many of the trace element characteristics of the melilitites are distinct from those of within-plate potassic magmas despite both being derived from phlogopite-bearing, enriched mantle source regions. This can be attributed to the depth at which source enrichment occurred and the subsequent control exerted by phlogopite and carbonate during melting. In contrast to melilitites, potassic magmas are derived from shallower depths under low CO₂/(CO₂ + H₂O) conditions and at higher temperatures at which phlogopite melts more readily.

The incompatible element ratios of the melilitites are also similar to those both observed in HIMU ocean island basalts (OIB) and inferred for HIMU OIB source regions from isotope variations (viz, low Sm/Nd, Rb/Sr, K/Nb, Th/U and high U/Pb and Ce/Pb). It is suggested that HIMU OIB's may be derived from sources that have been subject to enrichment by a melt generated in the presence of residual phlogopite.     

Apatite in the mantle: implications for metasomatic processes and high heat production in Phanerozoic mantle

The abundance of apatite in Phanerozoic mantle may be greatly underestimated. This study shows that apatite has a widespread occurrence in Phanerozoic lithospheric mantle and can be divided into two geochemically distinct types using halogen content, presence or absence of structural CO₂, Sr and trace element (especially U, Th, and light rare earth) ratios and abundances, and association with either metasomatised mantle wall-rock peridotites (Apatite A) or high-pressure magmatic crystallisation products (Apatite B). Apatite A is inferred to result from metasomatism by CO₂- and H₂O-rich fluids derived from a primitive mantle source region, while Apatite B compositions are consistent with crystallisation from magmas within the carbonate–silicate compositional spectrum.

The presence of significant apatite in the lithospheric mantle is important not only for the geochemical budget but also for assessing heat production and heat flow in the mantle. The measured U and Th contents of mantle apatite average 60 and 200 ppm, respectively and 0.5% apatite would dominate heat production. Metasomatised mantle may also contain amphibole and mica with K₂O and clinopyroxene with detectable U and Th. In lithospheric mantle with a thickness of 70 km, this abundance of apatite would result in mantle heat flow contribution of about 12 mW/m², a significant proportion of the total “normal” mantle heat flow of about 18 mW/m².     

俯冲带壳-幔相互作用的高温高压实验:对地幔不均一性成因的启示

使用活塞-圆筒式高温高压装置进行一系列榴辉岩部分熔融熔体与橄榄岩反应实验,可以为深入了解俯冲带壳-幔相互作用的影响因素及地幔不均一性的成因提供重要信息.实验使用反应偶的方法,并在0.8~3.0 GPa和1 200~1 425℃条件下进行.实验结果表明,榴辉岩部分熔融熔体-橄榄岩反应的动力学和结果受控于熔体主量元素成分、熔体中的H₂O、温度、压力和橄榄岩的物理状态等因素.大陆俯冲带地幔岩石中斜方辉石的富集是再循环陆壳熔体与上覆地幔反应的结果,地幔岩石中斜方辉石岩脉的形成与含水熔体交代有关,地幔岩石中的石榴辉石岩和石榴石岩可能形成于高压、低温条件下的熔体-橄榄岩反应.      

A-type granites of crustal origin ultimately result from open-system fenitization-type reactions in an extensional environment

The origin of A-type granites and rhyolites are ultimately relatable to mantle-derived melts and fluids in a zone undergoing extension. The basaltic magmas are accompanied by an alkaline fluid phase, dominantly H₂O + CO₂, which will induce alkali metasomatism of the granulitic crust above. The distinctive mineralogy and geochemistry are thus a direct result of the tectonic environment of formation. Metaluminous and peralkaline granites are magmatic compositions that typically contain evidence of crust and mantle in their genetic baggage, but peraluminous A-type granites may well be caused by efficient loss of alkalis during epizonal degassing. A-type granites and rhyolites are members of a vast family of rift-related magmas that include those of syenitic, nepheline syenitic and carbonatitic character. The fluid phase at work is alkaline. It can carry a host of trace elements in solution, in particular the high-field-strength elements and the rare earths. It can fenitize and fertilize a refractory lower crust, and prepare the precursor for near-complete melting. Some examples of A-type granitic magma do arise by efficient fractional crystallization of a mantle-derived basaltic magma, with or without accompanying assimilation, but many arise by partial or complete melting of an alkali-metasomatized crust.     

Experimental investigation of the effect of metasomatism by carbonatic melt on the composition and structure of the deep mantle

The compositions of various transition-zone and lower-mantle phases and coexisting carbonatic melts were determined by exploratory melting experiments in chemically complex CO₂-bearing systems at 20–24.5 GPa and 1600–2000 °C. The melts are highly ultramafic, enriched in K, Na, Ca, Fe, and Mg, and depleted in Al and Si. Melting experiments were also carried out with the compositions on the join Mg₂SiO₄–Na₂CO₃ at 3.7 GPa and 1200–1600 °C. The solidus assemblage of MgCO₃ and Na₂MgSiO₄ melts incongruently to produce forsterite and Na-rich melt. The new results and other recent studies in CO₂-bearing systems suggest that carbonatic melt could be present, either transiently or permanently, in the whole Earth's upper mantle and at least the uppermost lower mantle. Carbonate-melt metasomatism is recognized as a process that could have a major effect on the composition and structure of the deep mantle, and thus play an important role in its evolution. Due to the unique properties of the carbonatic melt, its circulation in an otherwise static mantle could be a more efficient process than the solid-state convection for maintaining equilibrium in most of the mantle not involved directly in plate tectonics.     

吉林蛟河上地幔岩碎块内熔融微区矿物学及其地质意义

吉林蛟河地幔岩碎块是被碱性橄榄玄武岩岩浆喷发携带至地壳浅部或地表的。碱性橄榄玄武岩中地幔岩碎块含量40%~55%,局部达60%以上;碎块大小不等,一般直径以5~10 cm居多,大者达20~35 cm,故定名为地幔岩集块熔岩(岩流)。地幔岩碎块以尖晶石二辉橄榄岩和尖晶石斜辉橄榄岩碎块为主,纯橄榄岩次之,未发现石榴石橄榄岩;胶结物为碱性橄榄玄武岩岩浆。本次研究发现地幔岩内存在丰富的、不同成分和形态的熔融微区。熔融微区类型以其形状可分为滴状、扇状、球状、不规则状、短脉状和环边状,以其特征新生矿物分为OL型、K型、Na+Chl型、PL型、OL+SP型、C+SP型和SP+Chl+Ser型。熔融微区结构为玻基间隐结构或放射状结构;矿物呈骸晶状、中空为玻璃质;残余玻璃脱玻化,产生少量针状和不透明黑色雏晶。熔融微区的形状、结构、物质组成及矿物结晶等特征具有标型性,表征这些熔融体是在上地幔深度保存的幔源岩熔融交代的产物,幔源结晶岩是固相残留。该幔源岩经历强火山喷发使其发生爆炸的地质事件,导致K、Na、Al、Ca易熔组分和H₂O、CO₂等挥发分开始熔融和气体释放,营造快速固化结晶和淬火的环境。这些少量的熔融物择优占据矿物间隙、裂隙、位错或晶体缺陷处汇聚并熔融交代相邻矿物,不断扩展空间,遂形成滴状等特征形状的“微区”。由于熔融程度不同,产生的熔融物的化学成分和结晶程度也有差异,所代表的初始岩浆性质也不一样,可以是超基性或碱性橄榄玄武质,抑或碧玄岩质岩浆。从检测出的这些信息证实,蛟河地幔岩是被不一致熔融抽取后的地幔残留,即岩石圈地幔。     

Chemical transfer between mantle xenoliths and basic magmas: Evidence from oceanic magma chambers. The trinity ophiolite (northern California)

The Trinity ophiolite consists of small magma chambers inside a large mantle body. Xenoliths of mantle peridotite occur both in gabbroic cumulates along the walls and in the matrices of ultrabasic breccias on the floors of the magma chambers. Field relationships and petrographic data suggest that these fragments of original mantle peridotite were modified by contact with basic magmas by modal metasomatism. Quantitative elemental mass transfers determined from the composition, volume and density variations of reacting minerals demonstrate both closed and open system conditions for the major (Si, Al, Ti, Na, Ca, Fe and Mg) and trace elements (Cr, Ni). In the open system, material gains and losses provide information on the composition of the fluid taking part in the metasomatic reaction.

During a first stage of metasomatism the mantle xenoliths were affected by high-temperature reactions at 600 to 925°C. They resulted from the interaction between solid mantle lherzolites and basic melts. The reactions are:

1. (1)those forming orthopyroxene-magnetite simplectite

2. (2)those forming plagioclase-magnetite corona

3. (3)clinopyroxene+spinel I→pargasitic hornblende+spinel II.

Chemical interactions between the upper mantle and oceanic magma chambers occurred as soon as the basic magmas had ascended through the upper mantle. The chemically modified magmas, within oceanic magma chambers, were depleted in Ti, Fe and Na. This could partly explain regional variations of the chemical compositions of primary magmas produced beneath slow-spreading ridges. The breakdown of olivine to orthopyroxene and magnetite participates in the control of the partition of magnetic Fe---Ti oxides between oceanic crust and mantle.

During the second stage, the serpentinization of olivine and the production of talc were superimposed on the products of the first stage. These reactions require large amounts of H₂O. The hydrothermal fluid was probably seawater. It circulated in the brecciated area along the walls and floors of the magma chambers located at shallow depths. Such structural discontinuities thus played the role of penetration channels favoring seawater circulation in the oceanic crust.

All the chemical reactions examined suggest a significant open-system element transfer by infiltrating melts or circulating fluids. The results of this study suggest that caution is required in the interpretation of mineralogical and chemical information provided by mantle xenoliths carried to the surface by ascending magmas.     

Mineralogy and petrology of cretaceous subsurface lamproite sills, southeastern Kansas, USA

Cores and cuttings of lamproite sills and host sedimentary country rocks in southeastern Kansas from up to 312 m depth were analyzed for major elements in whole rocks and minerals, certain trace elements in whole rocks (including the REE) and Sr isotopic composition of the whole rocks. The lamproites are ultrapotassic (K₂O/Na₂O = 2.0–19.9), alkalic [molecular (K₂O/Na₂O)/Al₂O₃ = 1.3-2.8], enriched in mantle-incompatible elements (light REE, Ba, Rb, Sr, Th, Hf, Ta) and have nearly homogeneous initial Sr isotopic compositions (0.707764-0.708114).

These lamproites could have formed by variable degrees of partial melting of harzburgite country rock and cross-cutting veins composed of phlogopite, K-Ti richterite, titanite, diopside, K-Ti silicates, or K-Ba-phosphate under high H₂O/CO₂ ratios and reducing conditions. Variability in melting of veins and wall rock and variable composition of the metasomatized veins could explain the significantly different composition of the Kansas lamproites.

Least squares fractionation models preclude the derivation of the Kansas lamproites by fractional crystallization from magmas similar in composition to higher silica phlogopite-sanidine lamproites some believe to be primary lamproite melts found elsewhere. In all but one case, least squares fractionation models also preclude the derivation of magmas similar in composition to any of the Kansas lamproites from one another. A magma similar in composition to the average composition of the higher SiO₂ Ecco Ranch lamproite (237.5–247.5 m depth) could, however, have marginally crystallized about 12% richterite, 12% sanidine, 7% diopside and 6% phlogopite to produce the average composition of the Guess lamproite (305–312 m depth).

Lamproite from the Ecco Ranch core is internally fractionated in K₂O, Al₂O₃, Ba, MgO, Fe₂O₃, Co and Cr most likely by crystal accumulation-removal of ferromagnesian minerals and sanidine. In contrast, the Guess core (305–312 m depth) has little fractionation throughout most of the sill except in several narrow zones. Lamproite in the Guess core has large enrichments in TiO₂, Ba, REE, Th, Ta and Sc and depletions in MgO, Cr, Co and Rb possibly concentrated in these narrow zones during the last dregs of crystallization of this magma.

The Ecco Ranch sill did not show any evidence of loss of volatiles or soluble elements into the country rock. This contrasts to the previously studied, shallow Silver City lamproite which did apparently lose H₂O-rich fluid to the country rock. Perhaps a greater confining pressure and lesser amount of H₂O-rich fluid prevented it from escaping.     

Time-dependent geochemistry of clinopyroxene from the Alban Hills (Central Italy): Clues to the source and evolution of ultrapotassic magmas

We investigated chemical and isotopic compositions of clinopyroxene crystals from well age-constrained juvenile scoria clasts, lava flows, and hypoabyssal magmatic ejecta representative of the whole eruptive history of the Alban Hills Volcanic District. The Alban Hills is a Quaternary ultra-potassic district that was emplaced into thick limestone units along the Tyrrhenian margin of Italy. Alban Hills volcanic products, even the most differentiated, are characterised by low SiO₂ content. We suggest that the low silica activity in evolving magmas can be ultimately due to a decarbonation process occurring at the magma/limestone interface. According to the liquid line of descent we propose, the differentiation process is driven by crystallisation of clinopyroxene + leucite ± apatite ± magnetite coupled with assimilation of a small amount of calcite and/or with interaction with crustal CO₂. By combining age, chemical data, strontium and oxygen isotopic compositions, and REE content of clinopyroxene, we give insights into the evolution of primitive ultrapotassic magmas of the Alban Hills Volcanic District over an elapsed period of about 600 kyr. Geochemical features of clinopyroxene crystals, consistent with data coming from other Italian ultrapotassic magmas, indicate that Alban Hills primary magmas were generated from a metasomatized lithospheric mantle source. In addition, our study shows that the ⁸⁷Sr / ⁸⁶Sr and LREE/HREE of Alban Hills magmas continuously diminished during the 600–35 ka time interval of the Alban Hills eruptive history, possibly reflecting the progressive depletion of the metasomatized mantle source of magmas.     

The nature of the sub-continental mantle: constraints from the major-element composition of continental flood basalts

Many continental flood basalts (CFB) have isotope and trace-element signatures that differ from those of oceanic basalts and much interest concerns the extent to which these reflect differences in their upper mantle source regions. A review of selected data sets from the Mesozoic and Tertiary CFB confirms significant differences in their major- and trace-element compositions compared with those of basalts erupted through oceanic lithosphere. In general, those CFB suites characterised by low Nb/La, high (⁸⁷Sr/⁸⁶Sr)_i and low εNd_i tend to exhibit relatively low TiO₂, CaO/Al₂O₃, Na₂O and/or Fe₂O₃, and relatively high SiO₂. In contrast, those which have high Nb/La, low (⁸⁷Sr/⁸⁶Sr)_i and high εNd_i ratios, like the upper units in the Deccan Traps, have major- and trace-element compositions similar to oceanic basalts. It would appear that those CFB that have distinctive isotope and trace-element ratios also exhibit distinctive major-element contents, suggesting that major and trace elements have not been decoupled significantly during magma generation and differentiation.

When compared (at 8% MgO) with oceanic basalt trends, the displacement of many CFB to lower Na₂O, Fe₂O₃^*, TiO₂ and CaO/Al₂O₃, but higher SiO₂, at similar Mg#, is not readily explicable by crustal contamination. Rather, it reflects source composition and/or the effects of the melting processes. The model compositions of melts produced by decompression of mantle plumes beneath continental lithosphere have relatively low SiO₂ and high Fe₂O₃^*. In contrast, the available experimental data indicate that partial melts of peridotite have low TiO₂, Na₂O and Fe₂O₃^*CaO/Al₂O₃, if the peridotite has been previously depleted by melt extraction. Moreover, melting of hydrated, depleted peridotite yields SiO₂-rich, Fe₂O₃- and CaO-poor melts. Since anhydrous, depleted peridotite has a high-temperature solidus, it is argued that the source of these CFB was variably melt depleted and hydrated mantle, inferred to be within the lithosphere. Isotope data suggest these source regions were often old and relatively enriched in incompatible trace elements, and it is envisaged that H₂O±CO₂ were added at the same time as the incompatible elements. An implication is that a significant proportion of the new continental crust generated since the Permian reflected multistage processes involving mobilization of continental mantle lithosphere that was enriched in minor and trace elements during the Proterozoic.     

俯冲隧道内不同深度的壳幔相互作用:地幔楔超镁铁质岩的镁同位素记录

在不同的俯冲深度,俯冲板片会释放出不同来源和组成的熔/流体进入俯冲隧道中,并进而影响上覆地幔楔及衍生岛弧岩浆的地球化学组成.然而,如何识别俯冲隧道中不同深度熔/流体组分的来源一直是俯冲带研究中的难点.对不同深度来源的地幔楔超基性岩进行了Mg同位素研究,发现了Mg同位素具有示踪俯冲板块熔/流体来源的能力.首先,研究了美国加州Franciscan杂岩中一套经历了多期次流体交代作用的浅部来源（< ~60 km）的变质超基性岩.这些部分蛇纹石化的地幔楔超基性岩在蛇纹石脱水形成滑石的过程中会释放轻Mg同位素进入流体,而重Mg同位素更多地残留在滑石相中;随后进一步受俯冲板块来源流体的交代形成具有高CaO和轻Mg同位素组成的透闪石化变橄榄岩,暗示流体中含有源自俯冲板片的、富集轻Mg同位素的碳酸盐,说明在弧前~60 km深度,部分含Mg碳酸盐（方解石）可以在俯冲隧道中发生溶解并迁移交代上覆地幔楔橄榄岩.对深部地幔楔来源（~160 km）的大别造山带毛屋地区超镁铁质岩体岩相学和元素地球化学研究结果证实了其交代成因.结合多相包裹体、元素地球化学以及前人估计的温-压条件,推测交代介质更接近超临界流体.锆石U-Pb年代学研究揭示,交代作用主要发生在古生代洋壳俯冲阶段（454±58 Ma）,超高压变质作用则发生在三叠纪陆壳俯冲阶段（232.8±7.9 Ma）.古生代锆石中大量的碳酸盐矿物包裹体和重O同位素特征说明古生代洋壳俯冲交代过程中有沉积碳酸盐组分加入.全岩和单矿物的Mg同位素组成均显著低于地幔值以及大别新元古代榴辉岩,说明交代的碳酸盐组分来源应为循环的沉积富Mg碳酸盐,暗示了在俯冲带深部富Mg沉积碳酸盐在超临界流体中会发生溶解迁移.由于沉积碳酸盐具有独特的、显著富集轻Mg同位素组成的特征,这种交代作用会造成地幔楔局部具有异常的Mg同位素组成,从而解释目前观察到的岛弧火山岩的Mg同位素特征.因此,Mg同位素是示踪俯冲碳酸盐与上覆地幔楔相互作用的有效工具.      

Fluid inclusions in mantle xenoliths

Fluid inclusions in olivine and pyroxene in mantle-derived ultramafic xenoliths in volcanic rocks contain abundant CO₂-rich fluid inclusions, as well as inclusions of silicate glass, solidified metal sulphide melt and carbonates. Such inclusions represent accidentally trapped samples of fluid- and melt phases present in the upper mantle, and are as such of unique importance for the understanding of mineral–fluid–melt interaction processes in the mantle. Minor volatile species in CO₂-rich fluid inclusions include N₂, CO, SO₂, H₂O and noble gases. In some xenoliths sampled from hydrated mantle-wedges above active subduction zones, water may actually be a dominant fluid species. The distribution of minor volatile species in inclusion fluids can provide information on the oxidation state of the upper mantle, on mantle degassing processes and on recycling of subducted material to the mantle. Melt inclusions in ultramafic xenoliths give information on silicate–sulphide–carbonatite immiscibility relationships within the upper mantle. Recent melt-inclusion studies have indicated that highly silicic melts can coexist with mantle peridotite mineral assemblages. Although trapping-pressures up to 1.4 GPa can be derived from fluid inclusion data, few CO₂-rich fluid inclusions preserve a density representing their initial trapping in the upper mantle, because of leakage or stretching during transport to the surface. However, the distribution of fluid density in populations of modified inclusions may preserve information on volcanic plumbing systems not easily available from their host minerals. As fluid and melt inclusions are integral parts of the phase assemblages of their host xenoliths, and thus of the upper mantle itself, the authors of this review strongly recommend that their study is included in any research project relating to mantle xenoliths.     

Experimental abiotic synthesis of methanol in seafloor hydrothermal systems during diking events

The abiotic synthesis of organic compounds in seafloor hydrothermal systems is one mechanism through which the subsurface environment could be supplied with reduced carbon. A flow-through, fixed-bed laboratory reactor vessel, the Catalytic Reactor Vessel (CRV) system, has been developed to investigate mineral–surface promoted organic synthesis at temperatures up to 400°C and pressures up to 30 MPa, conditions relevant to seafloor hydrothermal systems. Here we present evidence that metastable methanol can be directly synthesized from a gas-rich CO₂–H₂–H₂O mixture in the presence of a mineral substrate. Experiments have been performed without a substrate, with quartz, and with a mixture of quartz and magnetite. Temperatures and pressures in the experiments ranged from 200°C to 350°C and from 15 to 18 MPa, respectively. Maximum conversion of 5.8×10⁻⁴% CO₂ to CH₃OH per hour was measured using a mixture of magnetite and quartz in the reactor. After passivation of the stainless steel reactor vessel, experiments demonstrate that methanol is formed at temperatures up to 350°C in the presence of magnetite, and that the formation rate decreases over time. The experiments also show a loss of surface reactivity at 310°C and a regeneration of surface reactivity with increased temperature up to 350°C. Concentrations of CO₂ and H₂ used in the experiments simulate periodic, localized and dynamic conditions occurring within the seafloor during and immediately following magmatic diking events. High concentrations of CO₂ and H₂ may be generated by dike injection accompanied by exsolution of CO₂ and reaction of dissolved H₂O with FeO in the magma to form H₂. The experiments described here examine how the ephemeral formation of an H₂–CO₂-rich vapor phase within seafloor hydrothermal systems may supply reactants for abiotic organic synthesis reactions. These experiments show that the presence of specific minerals can promote the abiotic synthesis of simple organic molecules from common inorganic reactants such H₂O, CO₂ and H₂ under geologically realistic conditions.     

Evidence for low-temperature ultrapotassic siliceous fluids in subduction zone environments from experiments in the system K2O---MgO---Al2O3---SiO2---H2O (KMASH)

Experiments in the system K₂O---MgO---Al₂O₃---SiO₂---H₂O (KMASH) were undertaken with the piston-cylinder-apparatus to study the reactions:

1. (1) phengite±quartz+K,Mg-rich siliceous fluid=feldspar+phologopite+H₂O

2. (2) phengite+talc+K,Mg-rich siliceous fluid=phlogopite+quartz/coesite+H₂O

at temperatures between 400 and 700°C. The ultrapotassic fluid appearing at pressures above 15 kbar on the low-temperature sides of the corresponding reaction curves, which show positive dP/dT slopes, is probably supercritical. The P-T positions of the reactions are compatible with KMASH mineral reactions studied previously and with melting investigations in the KMASH system undertaken at temperatures higher than 700°C.

It is possible that natural rocks, chiefly K-rich metasediments subducted as minor portions of the oceanic crust, could give rise to low-temperature ultrapotassic fluids, mainly at temperatures between 300° to 600°C and pressures between 15 and 30 kbar. The ascending K-rich fluids would penetrate the overlying mantle to metasomatize it. After termination of the subduction process, heating of this mantle material, previously cooled by the subducted lithosphere, could lead to the formation of high-temperature K-rich magmas.     

Experimental synthesis and phase relations of phengitic muscovite from 6.5 to 11 GPa in a calcareous metapelite from the Dabie Mountains, China

The stability and phase relations of phengitic muscovite in a metapelitic bulk composition containing a mixed H₂O+CO₂ fluid were investigated at 6.5–11 GPa, 750–1050°C in synthesis experiments performed in a multianvil apparatus. Starting material consisted of a natural calcareous metapelite from the coesite zone of the Dabie Mountains, China, ultrahigh-pressure metamorphic complex that had experienced peak metamorphic pressures greater than 3 GPa. The sample contains a total of 2.1 wt.% H₂O and 6.3 wt.% CO₂ bound in hydrous and carbonate minerals. No additional fluid was added to the starting material. Phengite is stable in this bulk composition from 6.5 to 9 GPa at 900°C and coexists with an eclogitic phase assemblage consisting of garnet, omphacite, coesite, rutile, and fluid. Phengite dehydrates to produce K-hollandite between 8 and 11 GPa, 750–900°C. Phengite melting/dissolution occurs between 900°C and 975°C at 6.5–8 GPa and is associated with the appearance of kyanite in the phase assemblage. The formation of K-hollandite is accompanied by the appearance of magnesite and topaz-OH in the phase assemblage as well as by significant increases in the grossular content of garnet (average X_grs=0.52, X_py=0.19) and the jadeite content of omphacite (X_jd=0.92). Mass balance indicates that the volatile content of the fluid phase changes markedly at the phengite/K-hollandite phase boundary. At P≤8 GPa, fluid coexisting with phengite appears to be relatively CO₂-rich (XCO₂/XH₂O=2.2), whereas fluid coexisting with K-hollandite and magnesite at 11 GPa is rich in H₂O (XCO₂/XH₂O=0.2). Analysis of quench material and mass balance calculations indicate that fluids at all pressures and temperatures examined contain an abundance of dissolved solutes (approximately 40 mol% at 8 GPa, 60 mol% at 11 GPa) that act to dilute the volatile content of the fluid phase. The average phengite content of muscovite is positively correlated with pressure and ranges from 3.62 Si per formula unit (pfu) at 6.5 GPa to 3.80 Si pfu at 9 GPa. The extent of the phengite substitution in muscovite in this bulk composition appears to be limited to a maximum of 3.80–3.85 Si pfu at P=9 GPa. These experiments show that phengite should be stable in metasediments in mature subduction zones to depths of up to 300 km even under conditions in which aH₂O1. Other high-pressure hydrous phases such as lawsonite, MgMgAl-pumpellyite, and topaz-OH that may form in subducted sediments do not occur within the phengite stability field in this system, and may require more H₂O-rich fluid compositions in order to form. The wide range of conditions under which phengite occurs and its participation in mixed volatile reactions that may buffer the composition of the fluid phase suggest that phengite may significantly influence the nature of metasomatic fluids released from deeply subducted sediments at depths of up to 300 km at convergent plate boundaries.     

Origin of potassic (C-type) adakite magmas: Experimental and field constraints

The postcollisional, Mesozoic, C-type or high-K adakitic intrusions (K-adakites) of China lack either temporal or spatial association with subduction and have K₂O/Na₂O around unity. Otherwise, their geochemistry is very similar to ordinary adakites. New experimental data, geological observations and theoretical considerations lead us to suggest that these K-adakites were high-T (> 1050 and probably > 1075 °C), rather hydrous magmas ( 6 wt.% H₂O) produced by fluid-absent partial melting of metatonalites, meta-andesites, or possibly potassic metabasalts at pressures exceeding 2 GPa. Their peculiar chemistry is a consequence of both the elevated K₂O/Na₂O in the protolith and the very high pressure of partial melting. The most likely tectonic setting is one of extreme crustal thickening followed by delamination of the eclogitic keel and partial melting of this continental crustal material at mantle depths, with high mantle heat flow, during orogenic collapse.