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1.
The temperature changes caused by water exsolution from magma have been determined through calorimetric measurements performed on phonolitic and albitic compositions. The enthalpies of mixing of water with these melts have been derived from HF solution calorimetry, made at 323 K on glass samples containing up to 5 wt.% water, together with heat capacity data for the same series of samples. Mixing between aluminosilicate melts and water appears nearly ideal at magmatic temperatures, with small enthalpies of mixing that are negative for both melts at low pressures but can become positive for albite at high pressure. Regardless of the endothermic or exothermic nature of the process, water exsolution is associated with negligible temperature changes of only a few degrees even when 5 wt.% H2O is degassed. However, thermal effects might be greater for more depolymerized melts such as basalts and related compositions.  相似文献   

2.
Although trace element modeling has been used to great advantage for petrogenetic interpretations of basaltic systems, similar studies on igneous rocks of granitic composition have been fewer. In general the mineral/melt distribution coefficients for rare earth elements (REE) in granitic melts are equal to or greater than those for similar minerals in the basaltic system. Thus the effects of these minerals on the REE patterns of granitic melts during partial melting or differentiation are exaggerated as compared to basaltic systems, making detection of residual phases easier. For the K/Rb ratio, if neither a K-feldspar component nor biotitephlogopite is present in the residue, it is difficult to reduce the K/Rb ratio of the melt relative to the parent by a factor of two by either differentiation or partial melting.The petrogenesis of four distinctly different rocks are received: (1) an Archean tonalite presumably derived by partial melting of an Archean tholeiite at mantle depths, leaving a garnet plus clinopyroxene residue; (2) an Archean quartz monzonite presumably derived by partial melting of a short-lived graywacke-argillite sequence at crustal depths; (3) a dacite from Saipan presumably derived by differentiation from a basaltic parent; and (4) a trachyte from Ross Island, Antarctica, presumably derived by differentiation from a basanitoid parent and contaminated by continental crustal components.  相似文献   

3.
Rare earth element abundances have been measured in pyroxenitic (19.6% MgO) to gabbroic (7.7% MgO) rocks from the upper part of a thick, layered komatiite lava flow (Fred's Flow) in Munro Township, Ontario. This flow apparently erupted as a highly basic liquid which subsequently differentiated into layers of ultramafic cumulate rocks and a basaltic residual liquid. The analyzed rocks have compositions and spinifex or equigranular textures interpreted to indicate that they represent the complete range of liquids that were present during the differentiation of the lava.All the analyzed rocks are depleted in light REE, and also exhibit a slight depletion of Yb and Er relative to Gd and Dy. Chondrite-normalized Ce and Yb abundances range from 3.2 to 7.8 and 5.1 to 9.7 respectively. Proportions of fractionating minerals were estimated using a major element petrological mixing program and petrographic data. REE modeling based on these results indicates that the dominant process relating the samples is low-pressure fractional crystallization of olivine, followed at lower temperatures by clinopyroxene and plagioclase. Except for Eu, correspondence between observed and calculated REE abundances obviates any need to appeal to processes of major REE redistribution during diagenesis and low-grade metamorphism. Major differences in REE patterns of other ultramafic and mafic komatiitic lava flows [6,11], therefore, probably reflect different episodes of partial melting and/or differences in mantle source composition. The consistency of the REE in the layered flow, however, supports the concept that mafic komatiites can also be derived from ultrabasic parental magmas by low-pressure fractional crystallization. The light-REE-depleted patterns of these komatiites resemble those of modern MORB, suggesting that the mantle source of the komatiites had undergone a previous melting episode.  相似文献   

4.
Abstract Pliocene–Quaternary adakites and spatially and temporally associated niobium-enriched basalts (the latter thought to be derived by melting of slab melt-metasomatized mantle) from the Philippine island arcs have been selected for analysis of high field strength elements (HFSE). All these samples have nearly constant and chondritic Zr/Hf ratio (36.5) and slightly infrachondritic Nb/Ta ratio (14.7). We interpret adakitic magmas to be derived from the partial melting at approximately 900°C of subducted mid-ocean ridge basalts (MORB) crust, with rutile and/or ilmenite as residual minerals. Melting calculations show that, under these conditions, slab melts should have suprachondritic Nb/Ta ratios. The obvious discrepancy with our data is attributed to insufficient knowledge of rutile-melt partition coefficients for HFSE. Consequently, abnormal Nb/Ta or Zr/Hf ratios cannot be considered as potential markers of slab melting processes in island arcs.  相似文献   

5.
We have designed and calibrated a piston-cylinder cell assembly suitable for conductingin situ measurements of enthalpies of phase transitions at elevated pressures by heat-flux differential scanning calorimetry (DSC). The high-pressure DSC detector consists of a Pt-Pt13%Rh thermopile wrapped around a frame of fired pyrophyllite. Four thermocouple junctions, arranged radially around the sample capsule, are connected in series, with four reference thermocouple junctions located 3–4 mm above the sample and embedded in thermally inert ceramic. A W-W25%Re control thermocouple is situated directly above the top of the sample; the whole detector assembly is enclosed in a 1.5 mm thick cylindrical ceramic sleeve located at the center of a 8–10 mm long hot-zone in the tapered graphite furnace. Using this detector design and cell assembly, we have observed the thermal signal associated with the fusion of Au at 0.5 and 1.2 GPa, and have calculated a calibration factor (K) for this detector based on the gold melting curve ofMirwald andKennedy (1979). Detector sensitivity decreases by a factor of four over this pressure-temperature interval. The reproducibility of the enthalpy of fusion of gold at 0.5 GPa suggests that detector geometry is reproducible from one experiment to the next, and thus confirms the viability of this particular detector design for quantitative DSC measurements. Subsequent experiments will assess the dependence of (K) on temperature and pressure by measuring the enthalpies of fusion of additional metals (e.g., Ag, Cu, Al, Ge) and salts (e.g., NaCl, CsCl).  相似文献   

6.
Bulk compositions of igneous and microbreccia lithic fragments, glasses, and chondrules from Luna 16 fines as well as compositions of minerals in basaltic lithic fragments were determined with the electron microprobe. Igneous lithic fragments and glasses are divided into two groups, the anorthositic-noritic-troctolitic (hereafter referred to as ANT) and basaltic groups. Chondrules are always of ANT composition and microbreccia lithic fragments are divided into groups 1 and 2. The conclusions reached may be summarized as follows: (1) Luna 16 fines are more similar in composition to Apollo 11 than to Apollo 12 and 14 materials (e.g. Apollo 11 igneous lithic fragments and glasses fall into similar ANT and basaltic groups; abundant norites in Luna 16 and Apollo 11 are not KREEP as in Apollo 12 and 14; Luna 16 basaltic lithic fragments may represent high-K and low-K suites as is the case for Apollo 11; rare colorless to greenish, FeO-rich and TiO2-poor glasses were found in both Apollo 11 and Luna 16; Luna 16 spinels are similar to Apollo 11 spinels but unlike those from Apollo 12). (2) No difference was noted in the composition of lithic fragments, glasses and chondrules from Luna 16 core tube layers A and D. (3) Microbreccia lithic fragments of group 1 originated locally by mixing of high proportions of basaltic with small proportions of ANT materials. (4) Glasses are the compositional analogs to the lithic fragments and not to the microbreccias; most glasses were produced directly from igneous rocks. (5) Glasses show partial loss of Na and K due to vaporization in the vitrification process. (6) Luna 16 chondrules have ANT but not basaltic composition. It is suggested that either liquid droplets of ANT composition are more apt to nucleate from the supercooled state; or basaltic droplets have largely been formed in small and ANT droplets in large impact events (in the latter case, probability for homogeneous and inhomogeneous nucleation is larger. (7) No evidence for ferric iron and water-bearing minerals was found. (8) Occurrence of a great variety of igneous rocks in Luna 16 samples (anorthosite, noritic anorthosite, anorthositic norite, olivine norite, troctolite, and basalt) confirm our earlier conclusion that large-scale melting or partial melting to considerable depth and extensive igneous differentiation must have occurred on the moon.  相似文献   

7.
The densities of silicate liquids with basic, picritic, and ultrabasic compositions have been estimated from the melting curves of minerals at high pressures. Silicate liquids generated by partial melting of the upper mantle are denser than olivine and pyroxenes at pressures higher than 70 kbar, and garnet is the only phase which is denser than the liquid at pressures from 70 kbar to at least 170 kbar. In this pressure range, garnet and some fraction of liquid separate from ascending partially molten diapirs. It is therefore suggested that aluminium-depleted komatiite with a high Ca/OAl2O3 ratio may be derived from diapirs which originated in the deep upper mantle at pressures from 70 kbar to at least 140 kbar (200–400 km in depth), where selective separation of pyropic garnet occurs effectively. On the other hand, aluminium-undepleted komatiite is probably derived from diapirs originating at shallower depths (< 200 km). Enrichment of pyropic garnet is expected at depths greater than 200 km by selective separation of garnet from ascending diapirs. The 200-km discontinuity in the seismic wave velocity profile may be explained by a relatively high concentration of pyropic garnet at depths greater than 200 km.  相似文献   

8.
Carbonatites are rarely igneous rocks distributed on the earth. The rocks usually form ring complexes with alkalic rocks, occurring in the environments of continental rift, collisional oro-genic zone and oceanic island[1, 2]. Numerous facts and experiment…  相似文献   

9.
We present analyses of spheres magnetically extracted from mid-Pacific abyssal clays 0–500,000 years old. The concentration of spheres >200 μm is a few times 10 ppb. The spheres were divided into three groups using their dominant mineralogy, and are named iron, glassy, and silicate. Most spheres were formed from particles that completely melted as they separated from their parent meteoroids during the ablation process. However, some of the silicate spheres contain relict grains of the parent meteoroids that did not experience any melting. Typically, these relict grains are olivine crystals whose cores are Mg-rich (Fo89–99). Commonly the outer rims of these grains were altered during heating. Other relict mineral grains include enstatite, ferrous spinel, chromite, and pentlandite.The three groups of spheres may possibly indicate some genetic significance. It seems reasonable to expect iron-rich spheres to be produced during ablation of iron and metal-rich silicate meteoroids. Metal spheres are probably not produced by ablation of predominantly silicate meteoroids because studies of fusion crusts and laboratory ablated silicate materials have never yielded separate metal spheres, but rather have produced spheres with intergrown iron oxide and silicate phases. The iron spheres possess identical mineralogy with the fusion crusts of Boguslavka, Norfork, and N'Kandhla iron meteorites as well as with the ablation debris created in the laboratory using iron and nickel-iron samples.The glassy spheres are considerably more Fe-rich than the silicate spheres. They consist of magnetite and a Fe-rich glass which is relatively low in Si. Some of these spheres may have experienced pronounced volatile depletion during the ablation process and could have been derived from silicate or metal-rich silicate meteoroids.The silicate spheres are undoubtedly derived from ablation of stony meteoroids. Two of the mineral assemblages occurring in these spheres (olivine-magnetite-glass and sulfide) are identical to those described in the natural fusion crusts of Allende, Orgueil, and Murchison meteorites, laboratory-made ablation debris, and melted interplanetary dust collected from the stratosphere. Bulk compositions and relict grains are useful for determining the parent meteoroid types for the silicate spheres. Bulk analyses of spheres have non-volatile elemental abundances similar to chondritic abundances. Analyses of relict grains identified high-temperature minerals which often occur as larger crystals in a fine-grained matrix that is characterized by voids. These voids were caused by escaping volatiles as minerals decomposed during ablation. Because larger crystals of higher-temperature minerals are associated with fine-grained, low-temperature, volatile-rich matrix, the obvious candidates for parent meteoroids of the silicate spheres containing relict grains are carbonaceous chondrites.  相似文献   

10.
在1.0 GPa压力、343～962 K温度和0.1～106Hz频率的条件下,使用Solartron 1260阻抗-增益/相位分析仪对含角闪石的片麻岩从平行和垂直面理两个不同方向分别进行了电阻抗的测定,并且进一步分析了片麻岩的微观导电机制.高温高压实验结果表明:片麻岩的复阻抗对温度、频率表现出明显的依赖性.片麻岩的电导...  相似文献   

11.
Yoga A.  Sendjaja  Jun-Ichi  Kimura  Edy  Sunardi 《Island Arc》2009,18(1):201-224
The Sunda Arc of Indonesia developed along the convergent margin between the Eurasian and the Australian Plates. More than 100 Quaternary volcanic centers occur along the arc. The West Java Arc is a segment of the Sunda Arc in which more than 10 volcanic centers are located, corresponding to the 120 to 200 km depth contours of the Wadati–Benioff zone. The geochemistry of 207 Quaternary lavas from six centers across the arc was investigated. The lavas range from basalt to dacite. Incompatible element abundances increase from the volcanic front to the rear‐arc in response to a change from low‐K to high‐K suites. Nd–Sr isotope compositions of the basalts scatter between mid‐ocean ridge basalt (MORB) source mantle and Indian Ocean sediment (SED) compositions, with volcanic front low‐K basalts having more radiogenic Nd than the rear‐arc basalts. It is suggested that mixing between slab‐derived fluids mainly from the SED and melt from MORB source mantle played a significant role in determining the geochemistry of the West Java basalts. Incompatible element patterns in primitive mantle normalized multi‐element plots are almost identical across the arc, except for greater inclination and weaker positive Sr spikes in the rear‐arc basalts. This suggests a lower degree of partial melting in the rear‐arc mantle, accompanied by change in SED fluid composition between the volcanic front and the rear‐arc. The latter is confirmed by fluid‐fluxed melting model calculations using multiple trace elements and Nd and Sr isotopes. All the West Java Arc lavas require deficit of Sr from the slab SED. This may occur due to selective breakdown of Sr‐rich hydrous silicate minerals, such as zoisite, at shallower depths before the SED component reaches the depth of dehydration effective for magma genesis. The rear‐arc basalts need further Sr deficits along with lesser fluid. These features are commonly observed in many arc basalts, and are likely attributable to the same mechanism.  相似文献   

12.
Loihi Seamount is the southeasternmost active volcano of the Emperor-Hawaii linear volcanic chain. It comprises a spectrum of basalt compositional varieties including basanite, alkali basalt, transitional basalt and tholeiite. Samples from four dredge collections made on Scripps Institution of Oceanography Benthic Expedition in October 1982 are tholeiite. The samples include highly vesicular, olivine-rich basalt and dense glass-rich pillow fragments containing olivine and augite phenocrysts. Both quartz-normative and olivine-normative tholeiites are present. Minor and trace element data indicate relatively high abundances of low partition coefficient elements (e.g., Ti, K, P. Rb, Ba, Zr) and suggest that the samples were derived by relatively small to moderate extent of partial melting, of an undepleted mantle source. Olivine composition, MgO, Cr and Ni abundances, and Mg/(Mg+Fe), are typical of moderately fractionated to relatively unfractionated “primary” magmas. The variations in chemistry between samples cannot be adequately explained by low-pressure fractional crystallization but can be satisfied by minor variations in extent of melting if a homogeneous source is postulated. Alternatively, a heterogeneous source with variable abundances of certain trace elements, or mixing of liquids, may have been involved. Data for 3He/4He, presented in a separate paper, implies a mantle plume origin for the helium composition of the Loihi samples. There is little variation in the helium isotope ratio for samples having different compositions and textures. The helium data are not distinctive enough to unequivocally separate the magma sources for the tholeiitic rocks from the other rock types such as Loihi alkalic basalts and the whole source region for Loihi may have a nearly uniform helium compositions even though other element abundances may be variable. Complex petrologic processes including variable melting, fractional crystallization and magma mixing may have blurred original helium isotopic signatures.  相似文献   

13.
The application of the Sr/Ca-Ba/Ca systematics to volcanic rocks of the Andean Southern Volcanic Zone (33°S–46°S) has revealed a good correlation between the estimated degree of partial melting required to generate primary magmas and the projected extensions of the oceanic Nazca plate fracture zones under the continental South American plate. Magmas erupted at volcanic centers situated above these projections are thought to have been derived from primary magmas generated by relatively high degrees of melting, whereas those erupted at other centers are thought to have evolved from magmas produced by comparatively low degree of fusion. We interpret this relationship to reflect the facilitation of heat and mass transfer from the asthenosphere below the subducted oceanic lithosphere to the subarc mantle by the fracture zones. This contribution enhances the degree of melting of the subarc mantle source as well as the fraction of material derived from the subducted oceanic crust. This model predicts the predominance of basalts depleted in incompatible trace elements in centers located above the Nazca plate fracture zone extensions and of basalts enriched in incompatible trace elements in centers situated between boundaries of fracture extensions.  相似文献   

14.
Geochemical and mineralogical characteristics of the Eocene volcanic succession in Tafresh area of the Urumieh–Dokhtar Magmatic Assemblage (UDMA) are unique in the 2000‐km‐length assemblage. Demonstrating rather steep rare earth element (REE) patterns and the widespread presence of amphibole (+biotite) phenocrysts are two distinct characters that dominate the Eocene volcanic succession of mainly andesitic composition. Coincidence of the geochemical and mineralogical characteristics of the whole volcanic succession with adakites, rather amphibole‐ (+biotite) rich dacitic (with 61–64 wt% SiO2) stocks and dykes, is considered as the key in unraveling the role of ‘slab‐derived melt contribution’ in petrogenesis of the volcanic succession. Slab‐derived melting has been an ongoing process that metasomatized some parts of the mantle wedge from which hybrid rocks (andesites) are derived. Basalts with distinct signatures of slab melt metasomatism are yet another support for the occurrence of slab melting. Interlayering of normal, island‐arc‐type calc‐alkaline volcanic rocks with the slab‐melt metasomatized basalts and hybrid andesites suggests that the slab melting has been motivated by the subduction. Formation of the Tafresh Caldera, the likely consequence of an explosive eruption, is compatible with the volatile‐bearing nature of the adakitic volcanism in the study area. It is indicated by the ubiquitous presence of the hydrous minerals. Beneath the Tafresh area, in Eocene time, the subducting slab seems to have reached a critical high depth that is enough for the development of amphibolite–eclogite. The slab deformation, motivated by the geometry of subduction and/or the underlying mantle's steeper geotherms, is suggested to have resulted in the slab melting that helped develop a rock assemblage unique to the UDMA.  相似文献   

15.
Fractional crystallization behaviour of a magma ocean extending to lower mantle depths was deduced from estimations of melting relations for the deep mantle and the density relationships between ultrabasic liquid and mantle minerals. The accretional growth of the Earth necessarily involves a molten zone (magma ocean) in the outer layer of the growing Earth. The fractionation by melting during accretion results in primary stratification composed of a molten ultrabasic upper mantle (magma ocean), a perovskite-rich lower mantle, and an iron core. A certain amount of Al2O3 and CaO was removed from the magma ocean and retained in the lower mantle due to eclogite fractionation in the early stage of accretion and the perovskite fractionation in the later stage of accretion. Models of the stratification of the upper mantle arising from fractional crystallization of the magma ocean and subsequent convective disturbance were deduced on the basis of estimations of melting relations for the deep mantle and the density relationships between the ultrabasic liquid and mantle minerals. The stratification of the mantle, which is consistent with geophysical constraints is as follows; the upper mantle is composed of two layers, the upper olivine-rich layer and the lower garnet-rich layer with a thickness around 200 km, and the lower mantle with a perovskite-rich composition. In this model, both the 400 and 650 km discontinuities are the chemical boundaries.  相似文献   

16.
The compositions of liquidus olivines and orthopyroxenes of natural specimens of spinifex- and quench-textured peridotitic komatiites from the Belingwe Greenstone Belt, Rhodesia have been determined for pressures between 10 and 40 kbars. In conjunction with the chemical variation exhibited by these peridotitic komatiites it is concluded that the more magnesian lavas cannot be derived by concentration of olivine phenocrysts, fractional crystallisation or equilibrium partial melting. The peridotitic komatiites could evolve by polybaric assimilation and complete melting of garnet lherzolite into an initial liquid containing about 24% MgO.  相似文献   

17.
The wide variety of basalt types, tholeiitic to basanite, dredged from Loihi Seamount have minor and trace element abundances that are characteristic of subaerial Hawaiian basalts, thereby confirming that Loihi Seamount is a manifestation of the Hawaiian “hot spot”. Within the Loihi sample suite there are well-defined positive correlations among abundances of highly incompatible elements (P, K, Rb, Ba, Nb, light REE and Ta) and moderately incompatible elements (Sr, Ti, Zr and Hf) and between MgO, Ni and Cr. However, within the Loihi suite abundance ratios of geochemically similar elements (Zr/Hf, Nb/Ta and La/Ce) vary by factors of 1.2–1.5 and abundance ratios of highly incompatible elements such as P/Ce, P/Th, K/Rb, Ba/Th and La/Nb vary by factors of 1.2–2.5. These abundance ratios are not readily changed by different degrees of fractionation and melting. Therefore, we conclude that these samples are not genetically related by different degrees of melting of a compositionally homogeneous source.On the basis of K/P, K/Ti, P/Ce, Zr/Nb, Th/P and La/Sm abundance ratios, the twelve samples studied in detail can be divided into six geochemical groups. Samples within each group are similar in 87Sr/86Sr [1], and intra-group compositional variations may reflect low-pressure fractionation and different degrees of melting. In addition, crossing chondrite-normalized REE patterns within the alkalic basalt groups reflect equilibration of the magmas with garnet. In ratio-ratio plots involving abundance ratios of highly incompatible elements, e.g., La/P, Nb/P, K/P, Rb/P, Ba/P and Th/P, the geochemical groups define linear arrays suggestive of mixing. However, these data combined with the isotopic data are not consistent with two-component mixing.  相似文献   

18.
GHODRAT TORABI 《Island Arc》2012,21(3):215-229
Late Permian trondhjemites in the Anarak area occur as stocks and dykes, which cross cut the Anarak ophiolite and its overlying metasedimentary rocks, and are exposed along the northern Anarak east–west main faults. These leucocratic intrusive bodies have enclaves of all ophiolitic units and metamorphic rocks. They are composed of amphibole, plagioclase (oligoclase), quartz, zircon and muscovite. Secondary minerals are chlorite (pycnochlorite), epidote, albite, magnetite and calcite. Whole‐rock major‐ and trace‐element analyses reveal that they are characterized by high SiO 2 (67.8–71.0 wt%), Al 2 O 3 (14.9–17.1 wt%) and Na 2 O (5.3–8.6 wt%), low K 2 O (0.1–1.5 wt%; average: 0.8 wt%), low Rb/Sr ratio (0.01–0.40; average: 0.09), low Y (3–6 ppm), negative Ti, Nb and Ta anomalies, slightly negative or positive Eu anomaly, LREE enrichment and fractionated HREE. These rocks present 2 to 40 times enrichment in inclined chondrite‐normalized REE patterns. Geochemical characteristics of the Anarak trondhjemites all reflect melting of a mafic protolith at more than 10 kbar. The field evidence and whole‐rock chemistry reveal that these rocks have been crystallized from magmas derived from melting of subducted Anarak oceanic crust. This study reveals that melting of garnet amphibolite was an important element of continent formation in the study area.  相似文献   

19.
The Allende meteorite has been examined with a view to applying thermoluminescence (TL) to the study of a meteorite's passage through the atmosphere. At least three kinds of TL-bearing minerals are present. A strong peak at 140°C is due to forsterite, and one at 200°C is probably caused by cordierite. By far the most intense TL comes from an alteration product associated with gehlenite.In the 4-cm diameter meteorite examined the 200°C TL varied in intensity across the stone, showing it to be produced by fragmentation. Temperature gradients induced by atmospheric heating can also be derived, and indicate the orientation of the meteorite. Together with fusion crust measurements these results enable the final phase of the meteorite's passage through the atmosphere to be delineated.  相似文献   

20.
Green clinopyroxenes, commonly rounded and anhedral and richer in Fe, Na and Mn than the pyroxenes of the surrounding groundmass are a common feature of mafic alkaline volcanic rocks (e.g. basanites, monchiquites, leucitites). Some are accompanied by one or more of the following phases: Fe-rich kaersutite and biotite, anorthoclase, sodic plagioclase, apatite, magnetite, sphene, which are believed to be cognate with the green pyroxenes. We review evidence that these minerals have crystallized from mugearite, trachyte or phonolite magmas, and their presence in mafic alkaline rocks is due to magma mixing. The intermediate and salic magmas may sometimes be generated at mantle depths, possibly by melting of mantle material enriched in Fe, Na and volatiles.  相似文献   

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