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1.
汇聚板块边缘岩浆中金属和氯的地球化学性质研究   总被引：2，自引：0，他引：2  
总结了铜(Cu)、金(Au)、铼(Re)和氯(Cl)在汇聚板块边缘岩浆中的性质。在岛弧型的火山岩岩浆演化的早期,Cu、Au和Re均表现为中度不相容元素,含量随SiO2含量的增加而增加。在SiO2质量分数为58%时,多数岛弧型火山岩中Au、Cu的含量会突然大幅度下降。这一变化与铁和钛的变化是耦合的,铁和钛均由不相容元素变为相容元素,显示钛磁铁矿开始结晶了。进一步的研究表明,钛磁铁矿的结晶使硫酸根被还原为氢硫酸根,后者与Au、Cu形成氢硫酸根络合物,被萃取到流体相中,从而形成成矿流体。这一过程可以很好地解释Au、Cu矿床广泛分布于汇聚板块边缘的现象。与Au、Cu相反,Re的含量在SiO2质量分数为60%时才开始下降,而且是缓慢下降。这是因为Re通常比Au、Cu更亲石。此外,Re还具有强烈的挥发性。氯在东Manus岩浆中表现为高度不相容的特点。氯的性质主要受压力、初始水含量和岩浆演化分异程度的控制。计算结果显示,由于MORB和OIB含水量低,分异演化程度低,氯在上述岩浆中表现为高度不相容的特点。相比之下,氯在岛弧岩浆中的性质就复杂得多。随着水含量和岩浆房深度的不同,氯的性质可以从相容变到高度不相容。  相似文献   

2.
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.  相似文献   

3.
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.  相似文献   

4.
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.  相似文献   

5.
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.  相似文献   

6.
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.  相似文献   

7.
The dacite pumice erupted from Mt. Pinatubo on June 15, 1991 (whole-rock, rhyolitic groundmass glasses and homogenized melt inclusions) has been analyzed using inductively coupled plasma-mass spectrometry (ICP-MS), nanosecond and femtosecond laser ablation ICP-MS and secondary ion mass spectrometry (SIMS) to evaluate its ore-forming potential. Data suggest that adakite magmas are metal-rich and concentrate ore metals during magmatic differentiation. Sulfides segregate in limited amounts under the hydrous, oxidizing conditions typical of adakitic magmas resulting in incompatible behavior for Au (6-22 ppb), Cu (26-77 ppm), and Pb, Mo, As, and Sb in melts of dacitic to rhyolitic compositions. Metal transfer from this adakite magma to the coexisting aqueous phase was favored by the peraluminous composition of the rhyolitic melt and high aqueous chloride concentrations. Mass balance calculations suggest that the pre-eruptive aqueous phase could have extracted a minimum of 100 t Au and 5 × 105 t Cu from the Mt. Pinatubo magma. Our data suggest that intrusives having adakitic signatures are genetically associated with Au-Cu and Cu-Mo mineralization, auriferous porphyry copper deposits, and epithermal gold veins. High H2O, Cl, Sr/Y, Pb/Ce, Mo/Ce, As/Ce and Sb/Ce in Mt. Pinatubo melts reflect the contribution of deep fluids derived from subducted sediments and altered MORBs in the dacite genesis. The slab-derived fluids carrying mobile elements are likely responsible for the enrichment of adakite magmas in gold, associated metals and H2O, and may explain the exceptional ore-forming potential of adakite magmatism.  相似文献   

8.
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  相似文献   

9.
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).  相似文献   

10.
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.  相似文献   

11.
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).  相似文献   

12.
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.  相似文献   

13.
To determine the pre-eruptive composition of peralkaline magma at Frantale volcano, Ethiopia, we have studied glass inclusions in phenocrysts from a lateceupting, glassy pantelleritic lava flow. Matrix glass and crystal-free glass inclusions in quartz were analyzed for all major and most minor elements by electron microprobe and for H2O and 15 lithophile trace elements by ion microprobe (SIMS). Compositions of inclusions may have been slightly modified by post-trapping quartz crystallization, the average concentrations of all constituents but silica may be artificially high by 10% relative. Glass inclusions contain extreme enrichments in H2O (mean of 4.6 to 4.9 wt%) and several lithophile trace elements, which suggest that the lava erupted from a highly evolved, water-rich fraction of magma. The pre-eruptive concentration of water was much higher than that generally considered to occur in pantellerite magmas. Trends observed for lithophile elements in whole-rock samples from pre-,syn-and post-caldera eruptive units are mimicked in glass inclusions from the studied pantellerite lava; concentrations of Rb, Y, Zr, Nb, and Ce±Cl increase with progressive differentiation. With the exception of Cl and H2O contents, the composition of matrix glass is similar to that of glass inclusions suggesting: that few constituents exsolved from magma or cooling glass; eruption and quench of the lava occurred rapidly; and the matrix glass is, largely, compositionally representative of melt. Higher average abundances of Cl and H2O in glass inclusions suggest that these volatiles exsolved after melt entrapment; degassing could have occurred as either an equilibrium or disequilibrium process.  相似文献   

14.
Noble gas abundances in basaltic glasses from ocean islands (OIBs) are generally lower than those of mid-oceanic ridge basalts (MORBs), contrary to most geodynamic models which usually require that the source of OIBs is less degassed (resulting in higher primordial noble gas abundances) and more trace element enriched (resulting in higher radiogenic noble gas abundances) than the MORB source. Therefore, noble gas abundances in OIBs are often thought to have been reduced by extensive gas loss from the magma before eruption.The extent of magmatic degassing can be tested as it will cause characteristic changes in the composition of the volatiles; notably the 4He/40Ar* ratio (where 40Ar* is 40Ar corrected for atmospheric contamination) will increase in residual volatiles due to the higher solubility of He relative to Ar. The degree of He-Ar fractionation for a given fraction of gas loss depends on the ratio of the solubilities, SHe/SAr, which is sensitive to (among other things) the CO2 and H2O content of the basalt at the time of degassing.From a global database of OIB and MORB glasses, we show that 4He/40Ar* ratios of MORB glasses are broadly consistent with degassing of a magma with an initial 40Ar of ≈1.5 × 10−5 ccSTP/g, i.e., similar to that of the “popping rock.” However, OIB glasses generally have lower 40Ar* concentration for a given 4He/40Ar*. While this would appear to require lower 40Ar* abundances in the undegassed OIB magmas, the higher volatile contents of OIBs will reduce SHe/SAr (relative to MORBs) during degassing. By modeling SHe/SAr in OIBs, it is possible to show that extensive degassing of OIBs can occur without dramatically increasing the 4He/40Ar* ratio. We show that undegassed 40Ar concentrations of OIB magmas were probably similar to those of MORBs.  相似文献   

15.
WALLACE  PAUL J. 《Journal of Petrology》2002,43(7):1311-1326
Submarine pillow basalts (34 Ma) recovered from the NorthernKerguelen Plateau at ODP Site 1140 contain abundant unalteredglass, providing the first opportunity to measure the volatilecontents of tholeiitic basaltic magmas related to the Kerguelenmantle plume. The glasses have La/Sm and Nb/Zr ratios that varyfrom values similar to Southeast Indian Ridge (SEIR) MORB (Unit1), to slightly more enriched (Unit 6), to values transitionalbetween SEIR MORB and basaltic magmas formed by melting of theKerguelen plume (Units 2 and 3). Volatile contents for glassesin Units 1 and 6 are similar to depleted mid-ocean ridge basalt(MORB) values (0·25–0·27 wt % H2O, 1240–1450ppm S, 42–54 ppm Cl). In contrast, H2O contents are higherfor the enriched glasses (Unit 2, 0·44 wt % H2O; Unit3, 0·69 wt %), as are S (1500 ppm) and Cl (146–206ppm). Cl/K ratios for all glasses are relatively low (0·03–0·04),indicating that assimilation of hydrothermally altered materialdid not occur during shallow-level crystallization. H2O/Ce forthe enriched glasses (Units 2 and 3) is significantly lowerthan Pacific and South Atlantic MORB values, suggesting thatlow H2O/Ce may be an inherent characteristic of the Kerguelenplume source. Vapor saturation pressures calculated using theH2O and CO2 contents of the glasses indicate that  相似文献   

16.
The evolution of volatiles in the AD 79 magma chamber at Vesuvius (Italy) was investigated through the study of melt inclusions (MI) in crystals of different origins. FTIR spectroscopy and EMPA were used to measure H2O, CO2, S and Cl of the different melts. This allowed us to define the volatile content of the most evolved, phonolitic portion of the magma chamber and of the mafic melts feeding the chamber. MI in sanidine from phonolitic and tephri-phonolitic pumices show systematic differences in composition and volatile content, which can be explained by resorption of the host mineral during syn-eruptive mixing. The pre-eruption content of phonolitic magma appears to have been dominated by H2O and Cl (respectively 6.0 to 6.5 wt% and 6700 ppm), while magma chamber refilling occurred through the repeated injection of H2O, CO2 and S-rich tephritic magmas (respectively 3%, 1500 ppm and 1400 ppm). Strong CO2 degassing probably occurred during the decompressional path of mafic batches towards the magma chamber, while sulphur was probably released by the magma following crystallization and mixing processes. Water and chlorine strongly accumulated in the magma and reached their solubility limits only during the eruption. Chlorine solubility appears to have been strongly compositionally controlled, and Cl release was inhibited by groundmass crystallization of leucite, which shifted the composition of the residual liquid towards higher Cl solubilities. Received: 28 October 1999 / Accepted: 21 April 2000  相似文献   

17.
Halogens show a range from moderate (F) to highly (Cl, Br, I) volatile and incompatible behavior, which makes them excellent tracers for volatile transport processes in the Earth’s mantle. Experimentally determined fluorine and chlorine partitioning data between mantle minerals and silicate melt enable us to estimate Mid Ocean Ridge Basalt (MORB) and Ocean Island Basalt (OIB) source region concentrations for these elements. This study investigates the effect of varying small amounts of water on the fluorine and chlorine partitioning behavior at 1280?°C and 0.3 GPa between olivine and silicate melt in the Fe-free CMAS+F–Cl–Br–I–H2O model system. Results show that, within the uncertainty of the analyses, water has no effect on the chlorine partitioning behavior for bulk water contents ranging from 0.03 (2) wt% H2O (DCl ol/melt = 1.6?±?0.9 × 10?4) to 0.33 (6) wt% H2O (DCl ol/melt = 2.2?±?1.1 × 10?4). Consequently, with the effect of pressure being negligible in the uppermost mantle (Joachim et al. Chem Geol 416:65–78, 2015), temperature is the only parameter that needs to be considered for the determination of chlorine partition coefficients between olivine and melt at least in the simplified iron-free CMAS+F–Cl–Br–I–H2O system. In contrast, the fluorine partition coefficient increases linearly in this range and may be described at 1280?°C and 0.3 GPa with (R 2?=?0.99): \(D_{F}^{\text{ol/melt}}\ =\ 3.6\pm 0.4\ \times \ {{10}^{-3}}\ \times \ {{X}_{{{\text{H}}_{\text{2}}}\text{O}}}\left( \text{wt }\!\!\%\!\!\text{ } \right)\ +\ 6\ \pm \ 0.4\times \,{{10}^{-4}}\). The observed fluorine partitioning behavior supports the theory suggested by Crépisson et al. (Earth Planet Sci Lett 390:287–295, 2014) that fluorine and water are incorporated as clumped OH/F defects in the olivine structure. Results of this study further suggest that fluorine concentration estimates in OIB source regions are at least 10% lower than previously expected (Joachim et al. Chem Geol 416:65–78, 2015), implying that consideration of the effect of water on the fluorine partitioning behavior between Earth’s mantle minerals and silicate melt is vital for a correct estimation of fluorine abundances in OIB source regions. Estimates for MORB source fluorine concentrations as well as chlorine abundances in both mantle source regions are within uncertainty not affected by the presence of water.  相似文献   

18.
Degassing processes in basaltic magmas rich in both water andcarbon dioxide can be modeled using the solubilities of theendmember systems and the assumption of Henry's law. Suitesof vapor-saturated basaltic melts having a range of initialCO2/H2O ratios and erupted over a narrow depth interval willdefine negatively sloped arrays on an H2O vs CO2 plot. It isimportant that all of the major volatile species be consideredsimultaneously when interpreting trends in dissolved volatilespecies concentrations in magmas. Based on measured concentrations of water and carbon dioxidein basaltic glasses, the composition of the vapor phase at 1200°Cthat could coexist with a basaltic melt and the pressure atwhich it would be vapor saturated can be calculated. The rangein vapor compositions in equilibrium with submarine basaltsreflects the range in water contents in the melts characteristicof each environment. The ranges in the molar proportion of CO2in vapor phases (XCO2) calculated to be in equilibrium withsubmarine tholeiitic glasses are 0•93–1•00 formid-ocean ridge basalts (MORB), 0•60–0•99 forglasses from Kilauea [representative of ocean island basalts(OIB)] and 0–0•94 for glasses from back-arc basins(BABB). MORB glasses from spreading centers ranging from slow(e.g. the Mid-Atlantic Ridge) to fast (e.g. East Pacific Rise,9–13°N) are commonly supersaturated with respect toCO2-rich vapor, resulting from magma ascent rates so rapid thatmagmas erupt on the sea-floor without having been fully degassedby bubble nucleation and growth during ascent. In contrast tothe MORB glasses, volatile contents in submarine glasses fromKilauea are consistent with having been in equilibrium witha vapor phase containing 60–100 mol% CO2 at the pressureof eruption, reflecting differences in average magma transportrates during eruptions at mid-ocean ridges and hotspot volcanoes. Degassing during decompression of tholeiitic basaltic magmais characterized by strong partitioning of CO2 into the vaporphase. During open system degassing, CO2 is rapidly removedfrom the melt with negligible loss of water, until a pressureis reached at which the melt is in equilibrium with nearly purewater vapor. From this pressure downward, the water contentof the melt follows the water solubility curve. During closedsystem degassing, water and CO2 contents in vapor-saturatedbasaltic magmas will depend strongly on the vapor compositionas determined by the initial volatile concentrations. Deviationfrom open system behavior, toward lower dissolved H2O and CO2saturation concentrations at a given pressure, will be greatestin melts having high total volatile concentrations and highCO2:H2O ratios. Closed system degassing of basaltic melts havingthe low initial H2O and CO2 contents typical of MORB and OIB,however, are similar to the open system case. KEY WORDS: mid-ocean ridge basalts; water and carbon dioxide solubility; degassing  相似文献   

19.
We compiled a database containing more than 480000 determinations for 73 elements in melt inclusions in minerals and quenched glasses of volcanic rocks. These data were used to estimate the mean contents of major, volatile, and trace elements in igneous melts from main geodynamic settings. The following settings were distinguished: (I) oceanic spreading zones (mid-ocean ridges); (II) zones of mantle plume activity on oceanic plates (oceanic islands and plateaus); (III) and (IV) settings related to subduction processes, including (III) zones of island-arc magmatism generated on the oceanic crust and (IV) magmatic zones of active continental margins involving the continental crust into magma generation processes; (V) intracontinental rifts and continental hot spots; and (VI) back-arc spreading centers. The histogram of SiO2 contents in the natural igneous melts of all geodynamic settings exhibits a bimodal distribution with two maxima at SiO2 contents of 50–52 wt % and 72–74 wt %. The range 62–64 wt % SiO2 comprises the minimum number of determinations. Primitive mantle-normalized spidergrams were constructed for average contents of elements in the igneous melts of basic, intermediate, and acidic compositions from settings I–V. The diagrams reflect the characteristic features of melt compositions for each geodynamic setting. On the basis of the analysis of data on the composition of melt inclusions and glasses of rocks, average ratios of incompatible trace and volatile components (H2O/Ce, K2O/Cl, Nb/U, Ba/Rb, Ce/Pb, etc.) were estimated for the igneous melts of all of the settings. Variations of these ratios were determined, and it was shown that, in most cases, the ratios of incompatible elements are significantly different between settings. The difference is especially pronounced for the ratios of elements with different degrees of incompatibility (e.g., Nb/Yb) and for some ratios with volatile components (e.g., K2O/H2O).  相似文献   

20.
The distribution of H2O, F, Cl and S in the Campanian Ignimbrite (CI) magma chamber was investigated through study of primary glass inclusions and matrix glasses from pumices of the Plinian fall deposit. The eruption, fed by trachytic to phono-trachytic magmas, mainly produced a trachytic non-welded to partially welded tuff, underlain by a minor cogenetic fallout deposit. The entire chemical variability of the eruptive products is well represented in the pumices of the Plinian fall deposit, which we divide into a basal Lower Fall Unit (LFU) and an overlying Upper Fall Unit (UFU). Primary glass inclusions were only found in clinopyroxenes associated with the LFU pumice and contain a mean of 1.60ǂ.32 wt% H2O (analysed by FTIR), 0.11ǂ.08 wt% F, 0.37ǂ.03 wt% Cl and 0.08ǂ.04 wt% SO3 (EMP analysis); CO2 concentrations were below the FTIR detection limit (10-20 ppm). The coexisting matrix glasses contain similar amounts of halogens and sulfur but less water (~0.60 wt%). Partially degassed matrix glasses from UFU pumices contain a mean of 0.30ǂ.02 H2O, 0.28ǂ.10 F, 0.04ǂ.02 SO3 and 0.80ǂ.04 wt% Cl. To reconstruct the total amount of volatiles dissolved in the most evolved trachytes we have used experimental solubility data and mass balance calculations concerning the amount of crystal fractionation required to produce the most evolved trachyte from the least evolved trachyte; these yield an estimated pre-eruptive magma volatile content (H2O + Cl + F) of ~5.5 wt% for the most evolved magmas. On the basis of new determinations of Cl solubility limits in hydrous trachytic melts coexisting with an aqueous fluid phase + hydrosaline melt (brine), we suggest that the upper part of the magma chamber which fed the CI eruption was fluid(s) saturated and at a minimum depth of ~2 km. Variations in eruptive style (Plinian fallout, pyroclastic flows) do not appear to be related to significant variations in pre-eruptive volatile contents.  相似文献   

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