Phlogopite-olivine rocks of the Taukha terrane in southeastern Sikhote Alin

The Paleocene ultraferrous Mn-rich phlogopite-olivine rocks of the Taukha terrane belong to the alkaline ultrabasic rocks of the potassic series. The olivine is represented by hortonolite, while the phlogopite is enriched in Cl. Other minerals are represented by Ti-magnetite, Mn-rich ilmenite, Zn-rich pleonaste, apatite, and zircon. There are also epigenetic serpentine, talc, carbonates, magnetite, breithauptite, nickeline, hedleyite, cobaltite, tsumoite, auricupride, cuproauride, and other minerals. The phlogopite-olivine rocks possibly represent a part of a magmatic complex previously unknown in Sikhote Alin, the rocks of which are associated with fluidolites of a large diatreme. There are grounds to suggest that they were formed by the injection of fluid-rich (mainly, H₂O, Cl, F, and S) deep magmas into the upper lithosphere. Based on these specific features, as well as the sharp K predominance over Na and the enrichment in some incompatible elements (Sn, Ta, Nb, and Zr), the phlogopite-olivine rocks are the most close to lamproites but differ in the high contents of Fe, Mn, Au, Pt, and Pd and in the olivine’s composition. The manifestation of such magmatism in the Taukha terrane records the transition from subduction to transform continental margin settings.     

Average composition of basic magmas and mantle sources of island arcs and active continental margins estimated from the data on melt inclusions and quenched glasses of rocks

Based on the generalization of data on melt inclusions and quenched glasses, the average compositions of subduction (island arc and active continental margin settings) basic magmas were estimated. The main geochemical features of the average composition of these magmas are significant depletion in Nb and Ta, less significant depletion in Ti, Zr, and Sm, and enrichment in Cl, H₂O, F, and P in the primitive mantlenormalized patterns. The average normalized contents of moderately incompatible HREE in these magmas are close to those in the basic magmas of other geodynamic settings. Subduction basic magmas exhibit negative correlation of Li, Y, Dy, Er, Yb, Lu, and Ti contents with MgO content. Most of incompatible elements (Nb, Ta, U, Th, LREE) do not correlate with MgO, but correlate with each other and K₂O. Variations in element contents are related to crystallization differentiation, magma mixing, and possibly, participation of several sources. The water content in the island arc basic magmas varies from almost zero value to more than 6 wt %. Most compositions are characterized by weak negative correlation between H₂O and MgO contents, but some compositions define a negative correlation close to that in magmas of mid-ocean ridges (MOR). Considered magmas demonstrate distinct positive correlation between MgO content and homogenization temperature, practically coinciding with that of MOR magmas. Modeling of phase equilibria revealed widening of crystallization field of olivine in the magmas of subduction zones compared to MOR magmas. This can be related to the high water content in subduction magmas. Simultaneous liquidus crystallization of olivine and clinopyroxene in subduction magmas occurs at pressure approximately 5 kbar higher than that of MOR magmas. Based on the average ratios of trace element to K2O content, we determined the average compositions for subduction magma sources. Relative to depleted mantle, they are enriched in all incompatible elements, with positive anomalies of U, Rb, Ba, B, Pb, Cl, H₂O, F, and S, and negative anomalies of Th, K, Be, Nb, Ta, Li, Nd, Pb, and Ti. A general elevated content of incompatible elements indicates a reworking of the rocks of mantle wedge by fluids and melts that were released from the upper layers of subducted plate.     

Origin of carbonatite magma during the evolution of ultrapotassic basite magma

Clinopyroxene phenocrysts in fergusite from a diatreme in the Dunkel’dyk potassic alkaline complex in the southeastern Pamirs, Tajikistan, and from carbonate veinlets cutting across this rock contain syngenetic carbonate, silicate, and complex melt inclusions. The homogenization of the silicate and carbonate material of the inclusions with the complete dissolution of daughter crystalline phases and fluid in each of them occur simultaneously at 1150?1180°C. The pressures estimated using fluid inclusions and mineral geobarometers were 0.5–0.7 GPa. The behavior of the inclusions during their heating and their geochemistry are in good agreement with the origin of carbonate melts via liquid immiscibility. Carbonatite magma was segregated at the preservation of volatile components (H₂O, CO₂, F, Cl, and S) in the melt, and this resulted in the crystallization of H₂O-rich minerals and carbonates and testifies that the magma was not intensely degassed during its ascent to the surface. The silicate melts are rich in alkalis (up to 4 wt % Na₂O and 12 wt % K₂O), H₂O, F, Cl, and REE (up to 1000 ppm), LREE, Ba, Th, U, Li, B, and Be. The diagrams of the concentrations of incompatible elements of these rocks typically show deep Nb, Ta, and Ti minima, a fact making them similar to the unusual type of ultrapotassic magmas: lamproites of the Mediterranean type. These magmas are thought to be generated in relation to subduction processes, first of all, the fluid transport of various components from a down-going continental crustal slab into overlying levels of the mantle wedge, from which ultrapotassic magmas are presumably derived.     

Composition and chemical structure of oceanic mantle plumes

The average compositions (including H₂O, Cl, F, and S contents) and chemical structure of oceanic mantle plumes were estimated on the basis of the ratios of incompatible volatile components, potassium, and some other elements in the basaltic magmas of ocean islands (melt inclusions and quenched glasses). The following average concentrations were estimated for the plume mantle: 510 ppm K₂O, 520 ppm H₂O, 21 ppm Cl, 55 ppm F, and 83 ppm S; these values are significantly higher than those of the depleted mantle (except for S). The abundances of H₂O, Cl, and S are lower than in the primitive mantle. The normalized H₂O content in the plume mantle is similar to the concentrations of similarly incompatible La and Ce but lower than the concentrations of K₂O, Cl, and Sr. This is at odds with the idea of wet mantle plumes. Three types of basaltic magmas corresponding to three types of plume sources (M1, M2, and M3) were distinguished. The concentrations of incompatible elements in these reservoirs were estimated using two models, assuming either an isochemical mantle or a moderately enriched composition of plume material. The latter model gave the following average concentrations of H₂O, Cl, F, and S: 130, 33, 11, and 110 ppm for M1, 110, 12, 65, and 45 ppm for M2; 530, 29, 49, and 110 ppm for M3. The plume mantle is not homogeneous, and its heterogeneity is related to the existence of three main compositions, one of which (M1) is similar to the mantle of mid-ocean ridges, and two others (M2 and M3) are moderately enriched in K₂O, TiO₂, P₂O₅, F, and incompatible trace elements. The compositions of M2 and M3 are strongly different in H₂O, Cl, and S contents. The M2 mantle reservoir is significantly poorer in these components and richer in incompatible trace elements than M3. The plume mantle was formed mainly by the mixing of three sources: ultradepleted mantle, moderately enriched relatively dry mantle, and moderately enriched H₂O-rich mantle. In addition to the three main components of the plume mantle, there are probably minor components enriched in chlorine and depleted in fluorine. It is supposed that all these components are entrained into the plume mantle through the mantle recycling of components of the oceanic and continental crust. The established relationships are in agreement with the zonal model of a mantle plume, which includes a hot central part poor in H₂O, Cl, and S; an outer part enriched in volatile and nonvolatile incompatible elements; and enclosing mantle material interacting with the plume.     

蛇纹石化和俯冲带蛇纹岩变质脱水过程中流体活动性元素的行为

蛇纹石是大洋岩石圈和俯冲带内水和流体活动性元素最重要的载体之一。研究蛇纹石化和蛇纹岩变质脱水过程中流体活动性元素的行为是认识俯冲带元素地球化学循环的关键。蛇纹岩是指主要由蛇纹石类矿物构成的岩石,包括利蛇纹石、纤蛇纹石和叶蛇纹石。蛇纹石化过程中会造成流体活动性元素(B、Li、As、Sb、Pb、Cs、U、Sr和Ba等)的显著富集,并且由于原岩性质、流体成分和氧逸度等条件的不同,大洋岩石圈蛇纹岩和弧前蛇纹岩的特征也略有不同。例如,弧前蛇纹岩具有相对高的As、Sb、B和相对低的U,这反映了俯冲沉积物来源流体的贡献。在俯冲带蛇纹岩的变质脱水过程中,利蛇纹石向叶蛇纹石的转变伴随着矿物内超过50%F和Cl的释放,以及一些流体活动性元素(如B和Li)的迁出;此外,蛇纹石分解形成的变质橄榄石中的流体包裹体指示,蛇纹石脱水分解所产生的流体具有高于原始地幔几个数量级的Cl、Cs、Pb、As、Sb、Ba、Rb、B、Sr、Li和U含量。由于利蛇纹石中的Fe~(3+)含量较叶蛇纹石高,这种矿物相转变过程中也伴随着俯冲通道内的一系列氧化还原过程,从而影响流体性质和新形成的叶蛇纹石的成分。蛇纹岩与岛弧岩浆在流体活动性元素富集规律上的相似性说明蛇纹岩在俯冲带元素循环中扮演着重要的角色。此外,蛇纹石矿物相转变过程中F、Cl、B等元素的释放,可能对于斑岩型金矿、蛇绿岩中的金矿和某些蛇纹岩作为赋矿围岩的硼矿的形成起到重要的作用。     

Estimation of the average contents of H2O, Cl, F, and S in the depleted mantle on the basis of the compositions of melt inclusions and quenched glasses of mid-ocean ridge basalts

Using our database of the compositions of melt inclusions and quenched glasses of basaltic magmas from mid-ocean ridges (MORB), the average concentrations and ratios of H₂O, Cl, F, S, K₂O, Ce, and Dy were determined in these magmas. Assuming that the concentration ratios of volatile components to K₂O are constant in the MORB magmas and their sources (depleted mantle, DM), and taking an average K₂O content in the DM of 72 ppm, the following average contents were estimated for the DM: 158 ppm H₂O, 6.6 ppm Cl, and 8.3 ppm F. Using an S/Dy ratio of 212 for MORB melts and a Dy concentration of 0.531 ppm in the DM, the concentration of S in the DM was estimated as 113 ppm. Our value for the average content of Cl is much higher than estimates obtained by other authors. This discrepancy could be due either to the assimilation of crustal (and hydrospheric) Cl by MORB magmas or to the deep mantle recycling of Cl. The latter mechanism is supported by the statistically significant positive correlation of Cl with K₂O, H₂O, and F. Such a correlation is not consistent with the hypothesis of basaltic magma contamination by seawater-derived chloride brines. Similar to other surface processes, the assimilation of crustal material operates within the existing global correlations and disturbs them. Based on the average integrated degree of mantle melting and the average degree of MORB magma differentiation (0.05), the average contents of potassium and volatile components in N-MORB and E-MORB mantle sources were estimated as 39 and 126 ppm K₂O, 103 and 197 ppm H₂O, 4.0 and 10.7 ppm Cl, and 3.9 and 9.1 ppm F, respectively. It is not likely that normal MORB magmas can be derived from depleted mantle that experienced a previous partial melting event (for instance, during the extraction of the primordial continental crust in the Early Precambrian), which was referred to as the ultradepleted mantle. Ordinary (not ultradepleted) MORB magmas can be derived either by the melting of a zone enriched DM (for instance, progressively enriched in incompatible components with depth), which is hardly possible, or by the continuous addition (mixing) of an enriched component to the ultradepleted mantle at the expense of sediments and crustal materials involved in deep recycling.     

Volatiles in basaltic magmas of ocean islands and their mantle sources: II. Estimation of contents in mantle reservoirs

In this paper, we address the average compositions (including the contents of H₂O, Cl, F, and S) and the compositional structure of oceanic mantle plumes on the basis of element contents and ratios in ocean island magmas. The average contents of incompatible volatile and nonvolatile elements were calculated for the material of mantle plumes using a thermal and a more plausible moderately enriched model. The following average contents were estimated for the plume mantle: 510 ppm K₂O, 520 ppm H₂O, 21 ppm Cl, 55 ppm F, and 83 ppm S. These values are significantly higher than those of the depleted mantle (except for S). The primitive mantle normalized average content of water in mantle plumes is similar to those of La and Ce but lower than those of K, Cl, and Sr. This is at odds with the hypothesis of “wet” mantle plumes. Three types of basaltic magmas distinguished in our previous study (Part I) characterize three types of plume sources (MI, MII, and MIII). Using the favored moderately enriched model, the average contents of H₂O, Cl, F, and S were estimated for the three sources (ppm): 130, 33, 11, and 110 for MI; 110, 12, 65, and 45 for MII; and 530, 29, 49, and 110 for MIII, respectively. The plume mantle is heterogeneous and its heterogeneity can be described by the presence of three main types of compositions, one of which (MI) is similar to the composition of the mid-ocean ridge mantle and the other two types (MII and MIII) are moderately enriched in K, Ti, P, F, and incompatible trace elements but depleted in Cl, H₂O, and sometimes S. The compositions of MII and MIII have different H₂O, Cl, and S contents: MII is significantly depleted in these components compared with MIII. The MII component is probably similar to the enriched mantle (EM). In addition to the aforementioned three main components, the plume mantle probably contains high-Cl and low-F materials, which are related to the recycling of the oceanic and continental crust. All the observed characteristics of the mantle plumes are in adequate agreement with the model of a zonal mantle plume including a central part hot and depleted in H₂O, Cl, and S; a periphery enriched in volatile components; and the enclosing mantle interacting with the plume material.     

The behavior of chlorine and sulfur during differentiation of the Mt. Somma-Vesuvius magmatic system

Summary Reheated silicate melt inclusions in volcanic rock samples from Mt. Somma-Vesuvius, Italy, have been analyzed for 29 constituents including H₂O, S, Cl, F, B, and P₂O₅. This composite volcano consists of the older Mt. Somma caldera, formed between 14 and 3.55 ka before present, and the younger Vesuvius cone. The melt inclusion compositions provide important constraints on pre-eruptive magma geochemistry, identify relationships that relate to eruption behavior and magma evolution, and provide extensive evidence for magmatic fluid exsolution well before eruption. The melt inclusion data have been categorized by groups that reflect magma compositions, age, and style of eruptions. The data show distinct differences in composition for eruptive products older than 14.0 ka (pre-caldera rocks) versus eruptive products younger than 3.55 ka. Moreover, pre-caldera eruptions were associated with magmas relatively enriched in SiO₂, whereas eruptions younger than 3.55 ka (i.e., the syn- and post-caldera magmas which generated the Somma caldera and the Vesuvius cone) were derived from magmas comparatively enriched in S, Cl, CaO, MgO, P₂O₅, F, and many lithophile trace elements. Melt inclusion data indicate that eruptive behavior at Vesuvius correlates with pre-eruptive volatile enrichments. Most magmas associated with explosive plinian and subplinian events younger than 3.55 ka contained more H₂O, contained significantly more S, and exhibited higher (S/Cl) ratios than syn- and post-caldera magmas which erupted during relatively passive interplinian volcanic phenomena. Received January 10, 2000 Revised version accepted July 17, 2000     

Chlorine in submarine volcanic glasses from the eastern manus basin

Submarine volcanic glasses from the eastern Manus Basin of Papua New Guinea, ranging from basalt to rhyodacite, clarify the geochemical behavior of Cl in arc-type magmas. For the Manus samples, Cl is well correlated with non-volatile highly incompatible trace elements, suggesting it was not highly volatile and discounting significant seawater contamination. The Cl partition coefficient is close to but slightly lower than that of Nb and K₂O, a behavior similar to that in mid-ocean ridge basalts (MORB) and ocean island basalts (OIB). The similar incompatibilities of Cl and Nb imply that the Cl/Nb values of the eastern Manus Basin glasses reflect their magma source. For glasses from other west Pacific back-arc basins, Cl/Nb, Ba/Nb, and U/Nb increase towards the subduction trench, indicating increased contribution of a component enriched in Cl, Ba, and U, likely from subduction-released slab fluids. It is estimate that ∼80% of the Cl in the Manus arc-type glasses was added directly from subducted slab-derived fluids. We have also modeled Cl behavior during magma evolution in general. Our results show that the behavior of Cl in magma is strongly influenced by pressure, initial H₂O content, and the degree of magmatic fractionation. At early stages of magmatic evolution, for magmas with initial H₂O content of <4.0 wt%, Cl is highly incompatible under all pressures. By contrast, for more evolved magmas at moderately high pressure and high H₂O contents, considerable amounts of Cl can be extracted from the magma once H₂O saturation is reached. Accordingly, Cl is usually highly incompatible in MORB and OIB because of their low H₂O contents and relatively low degrees of fractional crystallization. The behavior of Cl in arc magmas is more complicated, ranging from highly incompatible to compatible depending on H₂O content and depth of magma chambers. The behavior of Cl in the eastern Manus Basin magmas is consistent with low H₂O contents (1.1-1.7 wt%) and evolution at low pressures (<0.1 GPa). Modeling results also indicate that Cl will behave differently in intrusive rocks compared to volcanic rocks because of the different pressures involved. This may have a strong influence on the mechanisms of ore genesis in these two tectonic settings.     

The systematics of chlorine, fluorine, and water in Izu arc front volcanic rocks: Implications for volatile recycling in subduction zones

We studied the systematics of Cl, F and H₂O in Izu arc front volcanic rocks using basaltic through rhyolitic glass shards and melt inclusions (Izu glasses) from Oligocene to Quaternary distal fallout tephra. These glasses are low-K basalts to rhyolites that are equivalent to the Quaternary lavas of the Izu arc front (Izu VF). Most of the Izu glasses have Cl ∼400-4000 ppm and F ∼70-400 ppm (normal-group glasses). Rare andesitic melt inclusions (halogen-rich andesites; HRA) have very high abundances of Cl (∼6600-8600 ppm) and F (∼780-910 ppm), but their contents of incompatible large ion lithophile elements (LILE) are similar to the normal-group glasses. The preeruptive H₂O of basalt to andesite melt inclusions in plagioclase is estimated to range from ∼2 to ∼10 wt% H₂O. The Izu magmas should be undersaturated in H₂O and the halogens at their preferred levels of crystallization in the middle to lower crust (∼3 to ∼11 kbar, ∼820° to ∼1200°C). A substantial portion of the original H₂O is lost due to degassing during the final ascent to surface. By contrast, halogen loss is minor, except for loss of Cl from siliceous dacitic and rhyolitic compositions. The behavior of Cl, F and H₂O in undegassed melts resembles the fluid mobile LILE (e.g.; K, Rb, Cs, Ba, U, Pb, Li). Most of the Cl (>99%), H₂O (>95%) and F (>53%) in the Izu VF melts appear to originate from the subducting slab. At arc front depths, the slab fluid contains Cl = 0.94 ± 0.25 wt%, F = 990 ± 270 ppm and H₂O = 25 ± 7 wt%. If the subducting sediment and the altered basaltic crust were the only slab sources, then the subducted Cl appears to be almost entirely recycled at the Izu arc (∼77-129%). Conversely, H₂O (∼13-22% recycled at arc) and F (∼4-6% recycled) must be either lost during shallow subduction or retained in the slab to greater depths. If a seawater-impregnated serpentinite layer below the basaltic crust were an additional source of Cl and H₂O, the calculated percentage of Cl and H₂O recycled at arc would be lower. Extrapolating the Izu data to the total length of global arcs (∼37,000 km), the global arc outflux of fluid-recycled Cl and H₂O at subduction zones amounts to Cl ∼2.9-3.8 × 10¹² g/yr and H₂O ∼0.7-1.0 × 10¹⁴ g/yr, respectively—comparable to previous estimates. Further, we obtain a first estimate of global arc outflux of fluid-recycled F of ∼0.3-0.4 × 10¹²g/yr. Despite the inherent uncertainties, our results support models suggesting that the slab becomes strongly depleted in Cl and H₂O in subduction zones. In contrast, much of the subducted F appears to be returned to the deep mantle, implying efficient fractionation of Cl and H₂O from F during the subduction process. However, if slab devolatilization produces slab fluids with high Cl/F (∼9.5), slab melting will still produce components with low Cl/F ratios (∼0.9), similar to those characteristic of the upper continental crust (Cl/F ∼0.3-0.9).     

Volatiles in basaltic magmas of ocean islands and their mantle sources: I. Melt compositions deduced from melt inclusions and glasses in the rocks

Statistical analysis of a data bank of the compositions of glasses and melt inclusions in minerals from ocean-island basalts. The initial database contains more than 45 000 published analyses of ocean-island igneous rocks from around the world. Much attention was given to the contents of volatiles (H₂O, Cl, F, and S) and their ratios to one another and to nonvolatile components of close incompatibility (Ti, P, K, and Ce). The average compositions of melt inclusions are similar to those of glasses of the rocks, including volatiles, with consideration for a somewhat higher degree (by approximately 20%) of the differentiation of glasses. The average compositions of ocean-island melts differ from those of mid-ocean basalts in having wider variations and elevated contents of some of the most incompatible elements (Sr, Nb, Ta, Ba, U, Th, and others), as well as H₂O, F, and Cl. Based on the correlation of volatiles to one another and to incompatible elements, three groups of ocean-island basalts are distinguished: (I) low-K, P, Ti magma compositions approximating mid-ocean ridge magmas, (II) high-K, Ce, P, and Ti magmas that resemble continental rift magmas but differ from them in low H₂O content, and (III) high-K, H₂O, Ce, P, and Ti magmas close to continental rift magma. All three types of the melts were found only in the Hawaiian Archipelago, whereas other ocean islands are dominated by any one of these types. The distinguished melt types presumably reflect the differences (heterogeneity) in the compositions of the sources.     

Subduction cycling of volatiles and trace elements through the Central American volcanic arc: evidence from melt inclusions

Compositions of melt inclusions in olivine (Fo_90-64) from 11 localities in Guatemala, Nicaragua and Cost Rica along the Central American Volcanic Arc are used to constrain combined systematics of major and trace elements and volatile components (H₂O, S, Cl, F) in parental melts and to estimate volcanic fluxes of volatile elements. The melt inclusions cover the entire range of compositions reported for whole rocks from Central America. They point to large heterogeneity of magma sources on local and regional scales, related to variable contributions of diverse crustal (from the subducting and overriding plates) and mantle (from the wedge and incoming plate) components involved in magma genesis. Water in parental melts correlates inversely with Ti, Y and Na and positively with Ba/La and B/La (with the exception of Irazú Volcano), which indicates mantle melting fluxed by Ba-, B- and H₂O-rich, possibly, serpentinite-derived fluid beneath most parts of the arc. Different components with melt-like characteristics (high LREE, La/Nb and probably also Cl, S and F and low Ba/La) control the geochemical peculiarities of Guatemalan and Costa Rican magmas. The composition of parental magmas together with published data on volcanic volumes and total SO₂ flux from satellite measurements are used to constrain fluxes of volatile components and to estimate total magmatic flux in Central America. We found that volcanic flux accounts for only 13% of total magmatic and volatile fluxes. The remaining 87% of magmas remained in the lithosphere to form cumulates (∼39%) and intrusives (∼48%). The intrusive fraction of magmatic flux may be significantly larger beneath Nicaragua compared to Costa Rica. Interestingly, total fluxes of magmas and volatiles in Central America are quite similar to the global average estimates. Electronic supplementary material The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

Volatiles in glasses from Mauna Loa Volcano,Hawai'i: implications for magma degassing and contamination,and growth of Hawaiian volcanoes

Glasses from Mauna Loa pillow basalts, recent subaerial vents, and inclusions in olivine were analyzed for S, Cl, F, and major elements by electron microprobe. Select submarine glasses were also analyzed for H₂O and CO₂ by infrared spectroscopy. The compositional variation of these tholeiitic glasses is dominantly controlled by crystal fractionation and they indicate quenching temperatures of 1,115-1,196 °C. Submarine rift zone glasses have higher volatile abundances (except F) than nearly all other submarine and subaerial glasses with the maximum concentrations increasing with water depth. The overwhelming dominance of degassed glasses on the submarine flanks of Mauna Loa implies that much of volcano's recent submarine growth involved subaerially erupted lava that reached great water depths (up to 3.1 km) via lava tubes. Anomalously high F and Cl in some submarine glasses and glass inclusions indicate contamination possibly by fumarolic deposits in ephemeral rift zone magma chambers. The relatively high CO₂ but variable H₂O/K₂O and S/K₂O in some submarine rift zone glasses indicates pre-eruptive mixing between degassed and undegassed magma within Mauna Loa's rift system. Volatile compositions for Mauna Loa magmas are similar to other active Hawaiian volcanoes in S and F, but are less Cl-rich than Ll'ihi glasses. However, Cl/K2O ratios are similar. Mauna Loa and Ll'ihi magmas have comparable, but lower H₂O than those from Kilauea. Thus, Kilauea's source may be more H₂O-rich. The dissimilar volatile distribution in glasses from active Hawaiian volcanoes is inconsistent with predictions for a simple, concentrically zoned plume model.     

The importance of talc and chlorite “hybrid” rocks for volatile recycling through subduction zones; evidence from the high-pressure subduction mélange of New Caledonia

The transfer of fluid and trace elements from the slab to the mantle wedge cannot be adequately explained by simple models of slab devolatilization. The eclogite-facies mélange belt of northern New Caledonia represents previously subducted oceanic crust and contains a significant proportion of talc and chlorite schists associated with serpentinite. These rocks host large quantities of H₂O and CO₂ and may transport volatiles to deep levels in subduction zones. The bulk-rock and stable isotope compositions of talc and chlorite schist and serpentinite indicate that the serpentinite was formed by seawater alteration of oceanic lithosphere prior to subduction, whereas the talc and chlorite schists were formed by fluid-induced metasomatism of a mélange of mafic, ultramafic and metasedimentary rocks during subduction. In subduction zones, dehydration of talc and chlorite schists should occur at sub-arc depths and at significantly higher temperatures (∼ 800°C) than other lithologies (400–650°C). Fluids released under these conditions could carry high trace-element contents and may trigger partial melting of adjacent pelitic and mafic rocks, and hence may be vital for transferring volatile and trace elements to the source regions of arc magmas. In contrast, these hybrid rocks are unlikely to undergo significant decarbonation during subduction and so may be important for recycling carbon into the deep mantle. Electronic supplementary material The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

Average compositions of magmas and mantle sources of mid-ocean ridges and intraplate oceanic and continental settings estimated from the data on melt inclusions and quenched glasses of basalts

Based on the generalization of the compositions of melt inclusions and quenched glasses from basaltic rocks, the average compositions of magmas were estimated for mid-ocean ridges (MOR), intraplate continental environments (CR), and ocean islands and plateaus (OI). These compositions were used to constrain the average contents of trace and volatile elements in mantle sources. A procedure was developed for the estimation of the average contents of incompatible elements, including volatiles (H₂O, Cl, F, and S), in the mantle. A comparison of the obtained average contents for the depleted mantle (DM) with the available published estimates showed that the contents of most incompatible trace elements (H₂O, Cl, F, Be, B, Rb, Sr, Zr, Ba, La, Ce, Nd, Sm, Eu, Hf, Ta, Th, and U) can be reliably estimated from the ratio of K to the desired trace element in the MOR magmas and the average content of K in the DM. For Nb, Ti, P, S, Li, Y, and heavy REE, we used the ratios of their contents to an element with a similar degree of incompatibility in MOR magmas (U for Nb and Dy for the other elements). This approach was used to determine the average contents of incompatible elements in oceanic plume mantle (OPM) and the subcontinental mantle of intraplate settings or continental plume mantle (CPM). It was shown that the average composition of both suboceanic and subcontinental mantle plumes is moderately enriched compared with the DM in the most incompatible elements (K, U, Ba, and La) and volatile components (H₂O, Cl, and F). The extent of volatile component enrichment in the plume mantle (500–1500 ppm H₂O) is insufficient for a significant depression of the mantle solidus. Therefore, mantle plumes must be hotter than the ambient depleted mantle. The average contents of incompatible trace elements in the OPM are similar to those of the primitive mantle, which could be related either to the retention of primitive mantle material in the regions of plume generation or to DM fertilization at the expense of the deep mantle recycling of crustal materials. In the latter case, the negative anomaly of water in the trace-element distribution patterns of the OPM is explained by the participation of dehydrated crust in its formation. Variations in the compositions of magmas and their sources were considered for various geodynamic settings, and it was shown that the sources are heterogeneous with respect to trace and volatile components. The chemical heterogeneity of the magma sources and gradual transitions between them suggest that the mantle reservoirs interact with each other. Chemical variations in continental and oceanic plume magmas can be attributed to the existence of several interacting sources, including one depleted and at least two enriched reservoirs with different contents of volatiles. These variations are in agreement with the zoned structure of mantle plumes, which consist of a hot and relatively dry core, a colder outer shell with high contents of volatile components, and a zone of interaction between the plume and depleted mantle.     

Deep pre-eruptive storage of silicic magmas feeding Plinian and dome-forming eruptions of central and northern Dominica (Lesser Antilles) inferred from volatile contents of melt inclusions

Volatiles contribute to magma ascent through the sub-volcanic plumbing system. Here, we investigate melt inclusion compositions in terms of major and trace elements, as well as volatiles (H₂O, CO₂, SO₂, F, Cl, Br, S) for Quaternary Plinian and dome-forming dacite and andesite eruptions in the central and the northern part of Dominica (Lesser Antilles arc). Melt inclusions, hosted in orthopyroxene, clinopyroxene and plagioclase are consistently rhyolitic. Post-entrapment crystallisation effects are limited, and negligible in orthopyroxene-hosted inclusions. Melt inclusions are among the most water-rich yet recorded (≤?8 wt% H₂O). CO₂ contents are generally low (<?650 ppm), although in general the highest pressure melt inclusion contain the highest CO₂. Some low-pressure (<?3 kbars) inclusions have elevated CO₂ (up to 1100–1150 ppm), suggestive of fluxing of shallow magmas with CO₂-rich fluids. CO₂-trace element systematics indicate that melts were volatile-saturated at the time of entrapment and can be used for volatile-saturation barometry. The calculated pressure range (0.8–7.5 kbars) indicates that magmas originate from a vertically-extensive (3–27 km depth) storage zone within the crust that may extend to the sub-Dominica Moho (28 km). The vertically-extensive crustal system is consistent with mush models for sub-volcanic arc crust wherein mantle-derived mafic magmas undergo differentiation over a range of crustal depths. The other volatile range of composition for melt inclusions from the central part is F (75–557 ppm), Cl (1525–3137 ppm), Br (6.1–15.4 ppm) and SO₂ (<?140 ppm), and for the northern part it’s F (92–798 ppm), Cl (1506–4428 ppm), Br (not determined) and SO₂ (<?569; one value at 1015 ppm). All MIs, regardless of provenance, describe the same Cl/F correlation (8.3?±?2.7), indicating that the magma source at depth is similar. The high H₂O content of Dominica magmas has implications for hazard assessment.     

Volatiles in submarine volcanic rocks from the Mariana Island arc and trough

High temperature mass spectrometric analyses of glasses from quenched pillow rims of andesites dredged from 1170 m water depth in the northern portion of the Mariana Island arc indicate substantially less H₂O (~ 1 wt.%) and more CO₂ (~ 0.24 wt.%) than previously reported for volcanic arc rocks. Glass-vapor inclusions within plagioclase phenocrysts from quenched rims have $\text{CO}{}_2\text{H}{}_2\text{O}$ ratios of 1:1. These results are similar to analyses of basaltic samples from the Mariana Trough (a back-arc basin). Generally, F and Cl contents are higher and S lower in the arc rocks compared to the samples from the back-arc basin. These results favor models for the production of island arc magmas which involve melting of the subducted slab, rather than just melting of the overlying mantle wedge because of the high volatile content needed to produce island arc magmas from peridotite (10–15 wt.%). The trough samples, although similar in non-volatile composition to mid-ocean ridge rocks, have much higher H₂O. somewhat higher CO₂ and lower S contents. Either near surface addition of voiatiles has enriched the magmas or H₂O must be a more important component in the generation and evolution of back-arc basin lavas than in the genesis of mid-ocean ridge basalts.     

Fluorine geochemistry of basaltic rocks from continental and oceanic regions and petrogenetic application

Fluorine contents in about 300 samples of various types of basalts and related rocks from continental (southwestern U.S.A.; Zaire; Deccan and South Africa) and oceanic regions (Hawaii and Mid-Atlantic Ridge between 23° N and 40° N) were determined by a selective ion-electrode method.Of all of the major components in these basaltic rocks, F shows good correlation only with K₂O. It increases regularly from tholeiite to perpotassic basalt on continents, and from tholeiite to nephelinite on Hawaii. In the F-K₂O diagram all the basaltic rocks from continents and Hawaii plot between the origin of the coordinate axes and the field of phlogopite in peridotite xenoliths in South African kimberlites. Accordingly, the major proportions of F, K₂O and also H₂O in these basaltic magmas are derived from phlogopite at the source regions in the upper mantle. On the other hand, F in abyssal tholeiites is relatively higher than that of the other tholeiites at equal K₂O content, and it is suggested that most of F, K₂O and H₂O are derived from pargasites.When it is assumed that the upper mantle phlogopite contains about 10% K₂O, 0.4% (0.3–0.5%) F and 4% H₂O, H₂O content in basaltic magmas from continental including island arc and oceanic island regions can be qualitatively estimated based on their proportions of K₂OFH₂O. Similarly, H₂O content in abyssal basaltic rocks is also estimated on the basis of FH₂O in pargasites (Table 2).A suite of Deccan tholeiites shows remarkable F enrichment with increasing K₂O due to separation of anhydrous and K-free minerals during fractionation. F in tholeiitic and alkali basalt magmas in Hawaii also increases regularly with K₂O during progressive fractionation until the later stages, where rhyodacite and trachyte exhibit a relative decrease owing to the effective subtraction of F-bearing amphibole and apatite in addition to anhydrous minerals.     

Apatite Composition of Representative Magnetite‐series and Ilmenite‐series Granitoids in Japan

Apatites of representative magnetite‐series and ilmenite‐series granitoids were studied in the Japanese Islands. Concentrations of the volatile components F, Cl and SO₃ are differently distributed in apatites of these granitoid series. Apatites are always fluoroapatite. They have weakly higher F content in the ilmenite series than in the magnetite series. In contrast, Cl and SO₃, are significantly concentrated in apatites of the magnetite series compared to the ilmenite series. These characteristics reflect the original concentrations of these components in the host granitic magmas. A high fO₂ seems most important for the S‐concentration as sulfate in apatite of the magnetite series. REE and Y are only erratically high in the studied apatites.     

Volatiles in pillow rim glasses from Loihi and Kilauea volcanoes,Hawaii

Volatiles and major elements in submarine glasses from Loihi seamount and Kilauea volcano. Hawaii were analyzed by high temperature mass spectrometry and the electron microprobe. Loihi glasses are subdivided into three groups: tholeiitic, transitional and alkali basalts. The glasses are evolved: Mg numbers range from 48–58. The alkalic lavas are the most evolved.Total volatiles range from 0.73 to 1.40 wt.%. H₂O shows a positive linear correlation with K₂O content [H₂O = 0.83 (± .09) K₂O + 0.08 (± .06)]. Concentrations of H₂O are higher in the alkalic lavas, but Cl and F abundances are highly variable. Variations in ratios of incompatible elements (K₂O, P₂O₅, H₂O) indicate that each group was derived from a distinct source. CO₂ contents range from 0.05 to 0.19 wt.% but show no systematic correlation with rock type or Mg #. A well-defined decrease in glass CO₂ content with increasing vesicularity is shown by the alkalic lavas. CO₂ may have been outgassed from the tholeiitic and transitional magmas prior to eruption during storage in a shallow magma chamber. Reduced carbon species (CO and CH₄) were found in small amounts in most of the alkalic samples. Although the redox histories of Hawaiian lavas are poorly known, these new data indicate the presence of a reduced source for Loihi magmas.The Kilauea tholeiitic glasses are evolved (Mg # 48.3 to 55) and have higher H₂O contents (av. 0.54 wt.%) than Loihi tholeiites (av. 0.42 wt.%) at the same Mg # (~55). Cl is distinctly lower in Kilauea glasses (0.01 wt.%) compared to Loihi glasses (0.09 wt.%). The data indicate significant source differences for the two volcanoes, consistent with results of other geochemical studies.Loihi tholeiites have distinctly higher ³He/⁴He ratios than Kilauea tholeiites and are the highest measured in submarine basalts (KURZ et al., 1983). These high ratios have been used to invoke a primitive source for Loihi basalts. The high Cl content of these basalts, the highest we have ever measured in submarine basalts, may be a fingerprint of this primitive source, as previously noted for Icelandic basalts (Schillinget al. 1980).