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1.
Data on melt inclusions in minerals provide direct information on the physicochemical petrogenetic parameters of Late Cenozoic basaltic complexes in the Southern Baikal and Southern Khangai Volcanic Areas (SBVA and SKVA, respectively) in Central Asia. Newly obtained data on inclusions in olivine reveal differences between the temperatures of the magmatic systems that produced basalts in SBVA and SKVA. The comparison of the experimentally determined homogenization temperatures and parameters calculated from data on the composition of glasses in the melt inclusions allowed us to realistically evaluate the temperatures of the petrogenetic processes that generated Late Cenozoic basaltic complexes in SBVA (1130–1160°C and 1175–1250°C) and SKVA (1145–1185°C, 1210–1270, and about 1300–1310°C). The analysis of fluid phases in the inclusions testifies that basaltic melts in SBVA were rich in carbon dioxide, which ensured elevated pressures (up to 5–6.6 kbar) during the crystallization of the minerals. Data on the composition of inclusions in the olivine highlight differences between the chemistries of magmatic systems in the two territories: elevated TiO2, Al2O3, and CaO concentrations at relatively low FeO and MgO contents in the SBVA melts as compared to analogous concentrations in the SKVA basaltic magmas. The petrochemical and geochemical parameters of the primary melt inclusions and the composition of the olivine generally testify that deep plume magmatic processes were actively involved in the generation of basalts in both SBVA and SKVA. Data on melt inclusions in olivine and the composition of the clinopyroxene reveal similarities between the geochemistry, mineralogy, and crystallization parameters of Late Cenozoic basalts in both SBVA and SKVA and Cretaceous-Paleogene basalts in the Tien Shan and their certain differences from the plume-related systems of the OIB type. These data suggest that the geodynamic environment of the Cenozoic and Late Mesozoic intraplate plume magmatism in Central Asia were different from the geodynamic environment of typical long-lived mantle plumes like that at Hawaii.  相似文献   

2.
Melt inclusions in olivine Fo83–72 from tephras of 1867, 1971 and 1992 eruptions of Cerro Negro volcano represent a series of basaltic to andesitic melts of narrow range of MgO (5.6–8 wt %) formed by ~46 wt % fractional crystallization of olivine (~6 wt %), plagioclase (~27 wt %), pyroxene (~13 wt %) and magnetite (<1 wt %) from primitive basaltic melt (average SiO2 = 49 wt %, MgO = 7.6 wt %, H2O = 6 wt %) as it ascended to the surface from the depth of about 14 km. The crystallization occurred at increasing liquidus temperature from 1,050 to 1,090 °C in the pressure range from 400 to 50 MPa and was induced by release of mixed H2O–CO2 fluid from the melt at decreasing pressure. Matrix glass compositions fall at the high-Si end of the melt inclusion trend and represent the final stage of melt crystallization during and after eruption. The bulk compositions of erupted Cerro Negro magmas (tephras and lavas) range from high- to low-MgO (3–10 wt %) basalts, which form a compositional array crossing the trend of melt inclusions so that virtually no rock from Cerro Negro has composition akin to true melt represented by the inclusions. The variations of the bulk magma (rocks) and melt (melt inclusions) compositions can be generated in a dyke connecting a deep primitive magma reservoir with the Cerro Negro edifice. While the melt inclusions represent the compositional trend of instantaneous melts along the magma pathway at decreasing pressure and H2O content, occurrence of low-Mg to high-Mg basalts reflects the process of phenocryst re-distribution in progressively evolving melt. The crystallization scenario is anticipated to operate everywhere in dykes feeding basaltic volcanoes and can explain the predominance of plagioclase-rich high-Al basalts in island arc as well as typical compositional variations of magmas during single eruptions.  相似文献   

3.
Süphan is a 4,050 m high Pleistocene-age stratovolcano in eastern Anatolia, Turkey, with eruptive products consisting of transitional calc-alkaline to mildly alkaline basalts through trachyandesites and trachytes to rhyolites. We investigate the relative contributions of fractional crystallization and magma mixing to compositional diversity at Süphan using a combination of petrology, geothermometry, and melt inclusion analysis. Although major element chemistry shows near-continuous variation from basalt to rhyolite, mineral chemistry and textures indicate that magma mixing played an important role. Intermediate magmas show a wide range of pyroxene, olivine, and plagioclase compositions that are intermediate between those of basalts and rhyolites. Mineral thermometry of the same rocks yields a range of temperatures bracketed by rhyolite (~750°C) and basalt (~1,100°C). The linear chemical trends shown for most major and trace elements are attributed to mixing processes, rather than to liquid lines of descent from a basaltic parent. In contrast, glassy melt inclusions, hosted by a wide range of phenocryst types, display curved trends for most major elements, suggestive of fractional crystallization. Comparison of these trends to experimental data from basalts and trachyandesites of similar composition to those at Süphan indicates that melt inclusions approximate true liquid lines of descent from a common hydrous parent at pressures of ~500 MPa. Thus, the erupted magmas are cogenetic, but were generated at depths below the shallow, pre-eruptive magma storage region. We infer that chemical differentiation of a mantle-derived basalt occurred in the mid- to lower crust beneath Süphan. A variety of more and less evolved melts with ≥55 wt% SiO2 then ascended to shallow level where they interacted. The presence of glomerocrysts in many lavas suggests that cogenetic plutonic rocks were implicated in the interaction process. Blending of diverse, but cogenetic, minerals, and melts served to obscure the true liquid lines of descent in bulk rocks. The fact that chemical variation in melt inclusions preserves deep-seated chemical differentiation indicates that inclusions were trapped in phenocrysts prior to shallow-level blending. Groundmass glasses evolved after mixing and display trends that are distinct from those of melt inclusions.  相似文献   

4.
Mantle xenoliths (lherzolites, clinopyroxene dunites, wehrlites, and clinopyroxenites) in the Early Cretaceous volcanic rocks of Makhtesh Ramon (alkali olivine basalts, basanites, and nephelinites) represent metasomatized mantle, which served as a source of basaltic melts. The xenoliths bear signs of partial melting and previous metasomatic transformations. The latter include the replacement of orthopyroxene by clinopyroxene in the lherzolites and, respectively, the wide development of wehrlites and olivine clinopyoroxenites. Metasomatic alteration of the peridotites is accompanied by a sharp decrease in Mg, Cr, and Ni, and increase of Ti, Al, Ca contents and 3+Fe/2+Fe ratio, as well as the growth of trace V, Sc, Zr, Nb, and Y contents. The compositional features of the rocks such as the growth of 3+Fe/2+Fe and the wide development of Ti-magnetite in combination with the complete absence of sulfides indicate the high oxygen fugacity during metasomatism and the low sulfur concentration, which is a distinctive signature of fluid mode during formation of the Makhtesh Ramon alkali basaltic magma. Partial melting of peridotites and clinopyroxenites is accompanied by the formation of basanite or alkali basaltic melt. Clino- and orthopyroxenes are subjected to melting. The crystallization products of melt preserved in the mantle rock are localized in the interstices and consist mainly of fine-grained clinopyroxene, which together with Ti-magnetite, ilmenite, amphibole, rhenite, feldspar, and nepheline, is cemented by glass corresponding to quartz–orthopyroxene, olivine–orthopyroxene, quartz–feldspar, or nepheline–feldspar mixtures of the corresponding normative minerals. The mineral assemblages of xenoliths correspond to high temperatures. The high-Al and high-Ti clinopyroxene, calcium olivine, feldspar, and feldspathoids, amphibole, Ti-magnetite, and ilmenite are formed at 900–1000°. The study of melt and fluid inclusions in minerals from xenoliths indicate liquidus temperatures of 1200–1250°C, solidus temperatures of 1000–1100°C, and pressure of 5.9–9.5 kbar. Based on the amphibole–plagioclase barometer, amphibole and coexisting plagioclase were crystallized in clinopyroxenites at 6.5–7.0 kbar.  相似文献   

5.
There are many melt and fluid inclusions (mainly CO2-rich) in olivine and pyroxene phenocrysts in basalts from the Ross Island area. The melt inclusions can be classified as follows: (1) crystalline melt inclusions (type I), (2) fluid-melt inclusions (type II) and (3) glass inclusions (type III). The daughter minerals in type I include olivine, plagioclase, ilmenite, etc. Fluid-melt inclusions are a new type which represent the immiscibility of magma and fluid at a particular stage of evolution. Three types of fluid-melt inclusions were examined in this study: a) crystal + liquid + gas, b) inclusions coexisting with glass inclusions and fluid inclusions, and c) crystal + daughter mineral (dissolved salt) + gas. Both primary and secondary melt inclusions are recognizable in the samples. The secondary melt inclusions were formed during healing of fractures in the host minerals in the process of magma rise. The homogenization temperatures (both Leitz 1350 stage and quench method were used) of melt inclusions in basalts range from 1190 to 135°C at high pressure (about 7 kbars), indicating that the basalts may have come from the upper mantle. Melt-fluid immiscibility in basaltic magma shows that the CO2-rich fluids may be the main fluid phase in the upper mantle, which are of significance in understanding the evolution of magma and various processes in the deep levels of the earth. The homogenization temperatures of melt and aqueous fluid inclusions in granites and metamorphic rocks in this area vary from 980 to 1100°C and 279 to 350°C, respectively.  相似文献   

6.
The study of clinopyroxenes and melt inclusions provided direct (independent on secondary alteration) information on the petrogenesis of the island arc complexes of the Chara zone, East Kazakhstan. It was shown that magmatism of this zone evolved from primitive island-arc systems with boninites to mature island arc with calc-alkaline melts. In terms of trace and rare-earth element distribution, the melt inclusions in the clinopyroxenes of the Chara zone differ from mid-ocean ridge basalts, being closer to the island-arc calcalkaline series. Based on inclusion composition, the parental melts of the considered complexes crystallized within 1150–1190°C with decreasing iron, magnesium, calcium, and sodium contents. Simulation based on melt inclusion data in clinopyroxenes indicates that the melts contained up to 1 wt % water, which was confirmed by direct ion-microprobe determination of 0.84 wt % H2O in the inclusions. Calculated liquidus temperatures are consistent with homogenization temperatures of the inclusions. Our calculations on the basis of inclusion data testify that the primary melts of the studied basaltic series of the Chara zone were generated from the mantle protolith within temperatures of 1350–1530°C at depths of 50–95 km. Similar parameters are typical of the generation of the tholeiitic and boninitic island-arc magmas in the modern ocean-continent transition zones of the Pacific type. In general, the study of clinopyroxenes and melt inclusions suggests that the considered complexes of the Chara zone were formed with the participation of tholeiitic and calcalkaline volcanogenic systems of basaltic, basaltic andesite, and, possibly, boninitic composition in the paleogeodynamic setting of evolving ancient island arc.  相似文献   

7.
Melt inclusions were investigated in olivine phenocrysts from the New Caledonia boninites depleted in CaO and TiO2 and enriched in SiO2 and MgO. The rocks are composed of olivine and pyroxene phenocrysts in a glassy groundmass. The olivine phenocrysts contain melt inclusions consisting of glass, a fluid vesicle, and daughter olivine and orthopyroxene crystals. The daughter minerals are completely resorbed in the melt at 1200?C1300°C, whereas the complete dissolution of the fluid phase was not attained in our heating experiments. The compositions of reheated and naturally quenched melt inclusions, as well as groundmass glasses were determined by electron microprobe analysis and secondary ion mass spectrometry. Partly homogenized melts (with gas) contain 12?C16 wt % MgO. The glasses of inclusions and groundmass are significantly different in H2O content: up to 2 wt % in the glasses of reheated inclusions, up to 4 wt % in naturally quenched inclusions, and 6?C8 wt % in groundmass glasses. A detailed investigation revealed a peculiar zoning in olivine: its Mg/(Mg + Fe) ratio increased in a zone directly adjacent to the glass of inclusions. This effect is probably related to partial water (hydrogen) loss and Fe oxidation after inclusion entrapment. The numerical modeling of such a process showed that the water loss was no higher than a few tenths of percent and could not be responsible for the considerable difference between the compositions of inclusions and groundmass glasses. It is suggested that the latter were enriched in H2O after the complete solidification of the rock owing to interaction with seawater. Based on the obtained data, the compositions of primary boninite magmas were estimated, and it was supposed that variations in melt composition were related not only to olivine and pyroxene fractionation from a single primary melt but also to different degrees and (or) depths of magma derivation.  相似文献   

8.
A wide variety of rock types are present in the O'Leary Peak and Strawberry Crater volcanics of the Pliocene to Recent San Francisco Volcanic Field (SFVF), AZ. The O'Leary Peak flows range from andesite to rhyolite (56–72 wt % SiO2) and the Strawberry Crater flows range from basalt to dacite (49–64 wt % SiO2). Our interpretation of the chemical data is that both magma mixing and crustal melting are important in the genesis of the intermediate composition lavas of both suites. Observed chemical variations in major and trace elements can be modeled as binary mixtures between a crustal melt similar to the O'Leary dome rhyolite and two different mafic end-members. The mafic end-member of the Strawberry suite may be a primary mantle-derived melt. Similar basalts have also been erupted from many other vents in the SFVF. In the O'Leary Peak suite, the mafic end-member is an evolved (low Mg/(Mg+ Fe)) basalt that is chemically distinct from the Strawberry Crater and other vent basalts as it is richer in total Fe, TiO2, Al2O3, MnO, Na2O, K2O, and Zr and poorer in MgO, CaO, P2O5, Ni, Sc, Cr, and V. The derivative basalt probably results from fractional crystallization of the more primitive, vent basalt type of magma. This evolved basalt occurs as xenolithic (but originally magmatic) inclusions in the O'Leary domes and andesite porphyry flow. The most mafic xenolith may represent melt that mixed with the O'Leary dome rhyolite resulting in andesite preserved as other xenoliths, a pyroclastic unit (Qoap), porphyry flow (Qoaf) and dacite (Darton Dome) magmas. Thermal constraints on the capacity of a melt to assimilate (and melt) a volume of solid material require that melt mixing and not assimilation has produced the observed intermediate lavas at both Strawberry Crater and O'Leary Peak. Textures, petrography, and mineral chemistry support the magma mixing model. Some of the inclusions have quenched rims where in contact with the host. The intermediate rocks, including the andesite xenoliths, contain xenocrysts of quartz, olivine and oligoclase, together with reversely zoned plagioclase and pyroxene phenocrysts. The abundance of intermediate volcanic rocks in the SFVF, as observed in detail at O'Leary Peak and Strawberry Crater, is due in part to crustal recycling, the result of basalt-driven crustal melting and the subsequent mixing of the silicic melts with basalts and derivative magmas.  相似文献   

9.
Melt and fluid inclusions have been studied in olivine phenocrysts (Fo 81–79) from trachybasalts of the Southern Baikal volcanic area, Dzhida field. The melt inclusions were homogenized, quenched, and analyzed on an electron and ion microprobe. The study of homogenized glasses of nine inclusions showed that basaltic melts (SiO2 = 47.1–50.3 wt %, MgO = 5.0–7.7 wt %, CaO = 7.1–11.1 wt %) have high contents of Al2O3 (17.1–19.6 wt %), Na2O (4.1–6.2 wt %), K2O (2.2–3.3 wt %), and P2O5 (0.6–1.1 wt %). The volatile contents are low (in wt %): 0.24–0.31 H2O, 0.08 F, 0.03 Cl, and 0.02 S. Primary fluid inclusions in olivines from four trachybasalt samples contain high-density CO2 (0.73–0.87 g/cm3), indicating a CO2 fluid pressure of 4.3–6.6 kbar at 1200–1300°C and olivine crystallization depths of 16–24 km. Ion microprobe analyses of 20 glasses from melt inclusions for trace elements showed that the magmas of the Baikal rift were enriched in incompatible elements, thus differing from oceanic rift basalts and resembling oceanic island basalts. A comparison of our data on melt and fluid inclusions in olivine from trachybasalts of the Dzhida field with preexisting data on the Eastern Tuva volcanic highland in the Southern Baikal volcanic area showed that they had similar contents of volatiles, major, and trace elements.  相似文献   

10.
A great volume of original information on the formation of the ultrabasic rocks of the Siberian Platform has been accumulated owing to the study of melt inclusions in Cr-spinels. The inclusions show the general tendencies in the behavior of the magmatic systems during the formation of the ultrabasic massifs of the Siberian Platform, tracing the main evolution trend of decreasing Mg number with SiO2 increase in the melts with subsequent transition from picrites through picrobasalts to basalts. The compositions of the melt inclusions indicate that the crystallization conditions of the rocks of the concentrically zoned massifs (Konder, Inagli, Chad) sharply differ from those of the Guli massif. Numerical modeling using the PETROLOG and PLUTON softwares and data on the composition of inclusions in Cr-spinels yielded maximum crystallization temperatures of the olivines from the dunites of the Konder (1545–1430°C), Inagli (1530–1430°C), Chad (1460–1420°C), and Guli (1520–1420°C) massifs, and those of Cr-spinels from the Konder (1420–1380°C), Inagli (up to 1430°C), Chad (1430–1330°C), and Guli (1410–1370°C) massifs. Modeling of the Guli massif with the PLUTON software using the compositions of the melt inclusions revealed the possible formation of the alkaline rocks at the final reverse stage of the evolution of the picritic magmas (with decrease of SiO2 and alkali accumulation) after termination of olivine crystallization with temperature decrease from 1240–1230°C to 1200–1090°C. Modeling with the PLUTON software showed that the dunites of the Guli massif coexisted with Fe-rich (with moderate TiO2 contents) melts, the crystallization of which led (beginning from 1210°C) to the formation of pyroxenes between cumulate olivine. Further temperature decrease (from 1125°C) with decreasing FeO and TiO2 contents provided the formation of clinopyroxenes of pyroxenites. For the Konder massif, modeling with the PLUTON software indicates the possible formation of kosvites from picrobasaltic magmas beginning from 1350°C and the formation of clinopyroxenites and olivine–diopside rocks from olivine basaltic melts from 1250°C.  相似文献   

11.
Plagioclase-bearing peridotites are commonly associated with gabbroic rocks sampled around the Moho Transition Zone. Based on mineral chemistry, texture, and spatial relations, the formation of plagioclase-bearing peridotites has been attributed to impregnation of basalt into residual peridotites. We conducted reactive dissolution and crystallization experiments to test this hypothesis by reacting a primitive mid-ocean ridge basalt with a melt-impregnated lherzolite at 1,300 °C and 1 GPa and then cooling to 1,050 °C as pressure decreased to 0.7 GPa. Crystallization during cooling produced lithologic sequences of gabbro–wehrlite or gabbro–wehrlite–peridotite, depending on reaction time. Wehrlitic and peridotitic sections contain significant amounts of plagioclase interstitial to olivine and clinopyroxene and plagioclase compositions are spatially homogeneous. Clinopyroxene in the wehrlite–peridotite section is reprecipitated from the melt and exhibits poikilitic texture with small rounded olivine chadacrysts. Mineral composition in olivine and clinopyroxene varies spatially, both at the scale of the sample and within individual grains. Olivine grains that crystallized close to the melt–peridotite interface are enriched in iron due to their proximity to the basaltic melt reservoir. Consistent with many field studies, we observed gradual spatial variation in olivine and clinopyroxene composition across a lithologically sharp boundary between the gabbro and wehrlite–peridotite. Plagioclase compositions show no obvious dependence on distance from the melt–rock interface and were precipitated from late-stage trapped melts. Compositional trends of olivine, pyroxene, and plagioclase are consistent with previous experimental results and natural observations of the Moho Transition Zone. Different lithological sequences form based primarily on the melt–rock ratio, composition of the melt and host peridotite, and thermochemical conditions, but are expected to grade from gabbro to wehrlite or troctolite to peridotite. Plagioclase-bearing peridotite represents the low melt–rock ratio end member where pyroxene is only partially replaced by olivine and melt, whereas dunite is expected to form where melts overwhelm and consume all other phases. This study confirms that under nominally anhydrous conditions, the gabbro–wehrlite–plagioclase-peridotite sequence can be formed by reaction between basalt and lherzolite and subsequent crystallization at intermediate to low pressures. Melt–rock reaction is a fundamental process in the formation of new crust at the shallowest part of the melting column where pyroxene-undersaturated melts percolate through depleted peridotite.  相似文献   

12.
St. Kitts lies in the northern Lesser Antilles, a subduction-related intraoceanic volcanic arc known for its magmatic diversity and unusually abundant cognate xenoliths. We combine the geochemistry of xenoliths, melt inclusions and lavas with high pressure–temperature experiments to explore magma differentiation processes beneath St. Kitts. Lavas range from basalt to rhyolite, with predominant andesites and basaltic andesites. Xenoliths, dominated by calcic plagioclase and amphibole, typically in reaction relationship with pyroxenes and olivine, can be divided into plutonic and cumulate varieties based on mineral textures and compositions. Cumulate varieties, formed primarily by the accumulation of liquidus phases, comprise ensembles that represent instantaneous solid compositions from one or more magma batches; plutonic varieties have mineralogy and textures consistent with protracted solidification of magmatic mush. Mineral chemistry in lavas and xenoliths is subtly different. For example, plagioclase with unusually high anorthite content (An≤100) occurs in some plutonic xenoliths, whereas the most calcic plagioclase in cumulate xenoliths and lavas are An97 and An95, respectively. Fluid-saturated, equilibrium crystallisation experiments were performed on a St. Kitts basaltic andesite, with three different fluid compositions (XH2O = 1.0, 0.66 and 0.33) at 2.4 kbar, 950–1025 °C, and fO2 = NNO ? 0.6 to NNO + 1.2 log units. Experiments reproduce lava liquid lines of descent and many xenolith assemblages, but fail to match xenolith and lava phenocryst mineral compositions, notably the very An-rich plagioclase. The strong positive correlation between experimentally determined plagioclase-melt KdCa–Na and dissolved H2O in the melt, together with the occurrence of Al-rich mafic lavas, suggests that parental magmas were water-rich (> 9 wt% H2O) basaltic andesites that crystallised over a wide pressure range (1.5–6 kbar). Comparison of experimental and natural (lava, xenolith) mafic mineral composition reveals that whereas olivine in lavas is predominantly primocrysts precipitated at low-pressure, pyroxenes and spinel are predominantly xenocrysts formed by disaggregation of plutonic mushes. Overall, St. Kitts xenoliths and lavas testify to mid-crustal differentiation of low-MgO basalt and basaltic andesite magmas within a trans-crustal, magmatic mush system. Lower crustal ultramafic cumulates that relate parental low-MgO basalts to primary, mantle -derived melts are absent on St. Kitts.  相似文献   

13.
Melt inclusions in olivine and pyroxene phenocrysts in kersantite and camptonite at Chhaktalao in Madhya Pradesh, India are mainly of the evolved type forming daughter minerals of olivine, pyroxene, plagioclase, spinel, mica, titanomagnetite and sulphides. Heating studies exhibit a temperature range from 1215° to 1245°C for the melt inclusions in olivine in camptonite and 1220–1245°C for olivine in kersantite. The temperature for melt inclusions in pyroxene ranged from 1000° to 1150°C in camptonite and 850–1100°C for pyroxene in kersantite. The bubble inside these melt inclusions is mainly CO2. The Th°C of CO2 into liquid phase occurred between 26° and 31°C in olivine and 25–30°C in pyroxene from kersantite and camptonite. The maximum density estimated is 0.72 g/cm3 and the minimum is 0.45 g/cm3. The depth of entrapment of the melt inclusion is estimated between 10–15 km. The pressure of entrapment of melt inclusion in olvine is 4.6 kbar where as that in pyroxene is 3.7 kbar. The lamprophyres in the Chhaktalao area are considered to be derived from low depth and low pressure region, possibly within spinel lherzolite zone.  相似文献   

14.
 A variety of cognate basalt to basaltic andesite inclusions and dacite pumices occur in the 7-Ma Rattlesnake Tuff of eastern Oregon. The tuff represents ∼280 km3 of high-silica rhyolite magma zoned from highly differentiated rhyolite near the roof to less evolved rhyolite at deeper levels. The mafic inclusions provide a window into the processes acting beneath a large silicic chamber. Quenched basaltic andesite inclusions are substantially enriched in incompatible trace elements compared to regional primitive high-alumina olivine tholeiite (HAOT) lavas, but continuous chemical and mineralogical trends indicate a genetic relationship between them. Basaltic andesite evolved from primitive basalt mainly through protracted crystal fractionation and multiple cycles (≥10) of mafic recharge, which enriched incompatible elements while maintaining a mafic bulk composition. The crystal fractionation history is partially preserved in the mineralogy of crystal-rich inclusions (olivine, plagioclase ± clinopyroxene) and the recharge history is supported by the presence of mafic inclusions containing olivines of Fo80. Small amounts of assimilation (∼2%) of high-silica rhyolite magma improves the calculated fit between observed and modeled enrichments in basaltic andesite and reduces the number of fractionation and recharge cycles needed. The composition of dacite pumices is consistent with mixing of equal proportions of basaltic andesite and least-evolved, high-silica rhyolite. In support of the mixing model, most dacite pumices have a bimodal mineral assemblage with crystals of rhyolitic and basaltic parentage. Equilibrium dacite phenocrysts are rare. Dacites are mainly the product of mingling of basaltic andesite and rhyolite before or during eruption and to a lesser extent of equilibration between the two. The Rattlesnake magma column illustrates the feedback between mafic and silicic magmas that drives differentiation in both. Low-density rhyolite traps basalts and induces extensive fractionation and recharge that causes incompatible element enrichment relative to the primitive input. The basaltic root zone, in turn, thermally maintains the rhyolitic magma chamber and promotes compositional zonation. Received: 1 June 1998 / Accepted: 5 February 1999  相似文献   

15.
Melt inclusions were examined in phenocrysts in basalt, andesite, dacite, and rhyodacite from the Karymskii volcanic center in Kamchatka and dacite form Golovnina volcano in Kunashir Island, Kuriles. The inclusions were examined by homogenization and by analyzing glasses in more than 80 inclusions on an electron microscope and ion microprobe. The SiO2 concentrations in the melt inclusions in plagioclase phenocrysts from basalts from the Karymskii volcanic center vary from 47.4 to 57.1 wt %, these values for inclusions in plagioclase phenocrysts from andesites are 55.7–67.1 wt %, in plagioclase phenocrysts from the dacites and rhyodacites are 65.9–73.1 wt %, and those in quartz in the rhyodacites are 72.2–75.7 wt %. The SiO2 concentrations in melt inclusions in quartz from dacites from Golovnina volcano range from 70.2 to 77.0 wt %. The basaltic melts are characterized by usual concentrations of major components (wt %): TiO2 = 0.7–1.3, FeO = 6.8–11.4, MgO = 2.3–6.1, CaO = 6.7–10.8, and K2O = 0.4–1.7; but these rocks are notably enriched in Na2O (2.9–7.4 wt % at an average of 5.1 wt %, with the highest Na2O concentration detected in the most basic melts: SiO2 = 47.4–52.0 wt %. The concentrations of volatiles in the basic melts are 1.6 wt % for H2O, 0.14 wt % for S, 0.09 wt % for Cl, and 50 ppm for F. The andesite melts are characterized by high concentrations (wt %) of FeO (6.5 on average), CaO (5.2), and Cl (0.26) at usual concentrations of Na2O (4.5), K2O (2.1), and S (0.07). High water concentrations were determined in the dacite and rhyodacite melts: from 0.9 to 7.3 wt % (average of 15 analyses equals 4.5 wt %). The Cl concentration in these melts is 0.15 wt %, and those of F and S are 0.06 and 0.01 wt %, respectively. Melt inclusions in quartz from the dacites of Golovnina volcano are also rich in water: they contain from 5.0 to 6.7 wt % (average 5.6 wt %). The comparison of melt compositions from the Karymskii volcanic center and previously studied melts from Bezymyannyi and Shiveluch volcanoes revealed their significant differences. The former are more basic, are enriched in Ti, Fe, Mg, Ca, Na, and P but significantly depleted in K. The melts of the Karymskii volcanic center are most probably less differentiated than the melts of Bezymyannyi and Shiveluch volcanoes. The concentrations of water and 20 trace elements were measured in the glasses of 22 melt inclusions in plagioclase and quartz from our samples. Unusually high values were obtained for Li concentrations (along with high Na concentrations) in the basaltic melts from the Karymskii volcanic center: from 118 to 1750 ppm, whereas the dacite and rhyolite melts contain 25 ppm Li on average. The rhyolite melts of Golovnina volcano are much poorer in Li: 1.4 ppm on average. The melts of the Karymskii volcanic center are characterized by relative minima at Nb and Ti and maxima at B and K, as is typical of arc magmas.  相似文献   

16.
Extrusive and intrusive igneous rocks represent different parts of a magmatic system and ultimately provide complementary information about the processes operating beneath volcanoes. To shed light on such processes, we have examined and quantified the textures and mineral compositions of plutonic and cumulate xenoliths and lavas from Bequia, Lesser Antilles arc. Both suites contain assemblages of iddingsitized olivine, plagioclase, clinopyroxene and spinel with rare orthopyroxene and ilmenite. Mineral zoning is widespread, but more protracted in lavas than xenoliths. Plagioclase cores and olivine have high anorthite (An?≤?98) and low forsterite (Fo?≤?84) compositions respectively, implying crystallisation from a hydrous mafic melt that was already fractionated. Xenolith textures range from adcumulate to orthocumulate with variable mineral crystallisation sequences. Textural criteria are used to organize the xenoliths into six groups. Amphibole, notably absent from lavas, is a common feature of xenoliths, together with minor biotite and apatite. Bulk compositions of xenoliths deviate from the liquid line of descent of lavas supporting a cumulate origin with varying degrees of reactive infiltration by evolved hydrous melts, preserved as melt inclusions in xenolith crystals. Volatile saturation pressures in melt inclusions indicate cumulate crystallization over a 162–571 MPa pressure range under conditions of high dissolved water contents (up to 7.8 wt% H2O), consistent with a variety of other thermobarometric estimates. Phase assemblages of xenoliths are consistent with published experimental data on volatile-saturated low-magnesium and high-alumina basalts and basaltic andesite from the Lesser Antilles at pressures of 200–1000 MPa, temperatures of 950–1050 °C and dissolved H2O contents of 4–7 wt%. Once extracted from mid-crustal mushes, residual melts ascend to higher levels and undergo H2O-saturated crystallization in shallow, pre-eruptive reservoirs to form phenocrysts and glomerocrysts. The absence of amphibole from lavas reflects instability at low pressures, whereas its abundance in xenoliths testifies to its importance in mid-crustal differentiation processes. A complex, vertically extensive (6 to at least 21 km depth) magmatic system is inferred beneath Bequia. Xenoliths represent fragments of the mush incorporated into ascending magmas. The widespread occurrence of evolved melts in the mush, but the absence of erupted evolved magmas, in contrast to islands in the northern Lesser Antilles, may reflect the relative immaturity of the Bequia magmatic system.  相似文献   

17.
 The use of ocean floor basalt chemistry as a tool to investigate mantle composition and processes requires that we work with basalts that have been modified little since leaving the mantle. One source of such basalts is melt inclusions trapped in primitive crystals. However, obtaining information from these melt inclusions is complicated by the fact that melt inclusions in natural basalts are essentially always altered by post-entrapment crystallization. This requires that we develop techniques for reconstructing the original trapped liquid compositions. We conducted a series of experiments to reverse the effects of post-entrapment crystallization by re-heating the host crystals to their crystallization temperature. For these experiments we used plagioclase crystals separated from a single Gorda Ridge lava. The crystallization temperature for these crystals was determined by a set of incremental re-heating experiments to be ∼1240–1260° C. The inclusions are primitive, high Ca-Al basaltic melts, saturated with plagioclase, olivine and Al-rich chromite at low pressure. The inclusion analyses can be linked to the host lava composition by low pressure fractionation. The major element composition of the re-homogenized melt inclusions within each crystal is relatively constant. However, the incompatible element analyses have extremely wide ranges. The range of La/Sm and Ti/Zr from inclusions analyzed from a single sample from the Gorda Ridge exceeds the range reported for lavas sampled from the entire ridge. The pyroxene compositions predicted to be in equilibrium with the melt inclusion trace element signature cover much of the range represented by pyroxenes from abyssal peridotites. The volumetric proportions of the magmas entering the base of the crust can be evaluated using frequency distribution of melt inclusion compositions. This distribution suggests that the array of magmas was skewed towards the more depleted compositions, with little evidence for an enriched component in this system. This pattern is more consistent with a dynamic flow model of the mantle or of a passive flow model where the melts produced in the peripheral areas of the melting regime were not focused to the ridge. Received: 5 January 1995 / Accepted: 13 June 1995  相似文献   

18.
It is generally believed that the lithospheric mantle and the mantle transition zone are important carbon reservoirs. However, the location of carbon storage in Earth’s interior and the reasons for carbon enrichment remain unclear. In this study, we report CO2-rich olivine-hosted melt inclusions in the mantle xenoliths of late Cenozoic basalts from the Penglai area, Hainan Province, which may shed some light on the carbon enrichment process in the lithospheric mantle. We also present ...  相似文献   

19.
We present a comprehensive mineral chemical dataset (～400 analyses) on subalkaline meimechitic (Mg-number?=?74–80) and ferropicritic (Mg-number?=?67–69) dike samples from the Antarctic extension of the Karoo large igneous province (LIP) in Vestfjella, western Dronning Maud Land. Some of the meimechites, previously considered to be cumulates from ferropicritic magmas, are characterized by forsteritic olivine (with core composition up to Fo92) that is in, or close to Fe-Mg equilibrium with the host rock. The olivines are subhedral to euhedral, contain Ti-rich (volcanic) spinel inclusions, have a high CaO content (≥0.19 wt. %), and are thus unlikely to represent xenocrysts from mantle peridotite. Igneous amphibole is found in olivine-hosted, crystallized melt inclusions, indicating that the parental magmas had a H2O content of 1–2 wt. %. The olivine data suggests generation of extremely MgO-rich (up to 25 wt. %) melts during the Karoo magmatism. Based on our petrogenetic modeling, such melts are likely to have originated from the partial melting of garnet peridotite at high pressures (5–6 GPa) and mantle potential temperatures (>1,600°C) that are compatible with the involvement of a mantle plume in the generation of the Karoo LIP. A geochemical comparison of the Vestfjella meimechites with meimechites from the Siberian Traps LIP and the assumed komatiitic parental melts of the Horingbai picrites (Paraná-Etendeka LIP) reveals key similarities, suggesting that all these suites were generated from broadly similar sources and/or by similar melting processes in anomalously hot subcontinental mantle.  相似文献   

20.
The Khopoli intrusion, exposed at the base of the Thakurvadi Formation of the Deccan Traps in the Western Ghats, India, is composed of olivine gabbro with 50–55 % modal olivine, 20–25 % plagioclase, 10–15 % clinopyroxene, 5–10 % low-Ca pyroxene, and <5 % Fe-Ti oxides. It represents a cumulate rock from which trapped interstitial liquid was almost completely expelled. The Khopoli olivine gabbros have high MgO (23.5–26.9 wt.%), Ni (733–883 ppm) and Cr (1,432–1,048 ppm), and low concentrations of incompatible elements including the rare earth elements (REE). The compositions of the most primitive cumulus olivine and clinopyroxene indicate that the parental magma of the Khopoli intrusion was an evolved basaltic melt (Mg# 49–58). Calculated parental melt compositions in equilibrium with clinopyroxene are moderately enriched in the light REE and show many similarities with Deccan tholeiitic basalts of the Bushe, Khandala and Thakurvadi Formations. Nd-Sr isotopic compositions of Khopoli olivine gabbros (εNdt?=??9.0 to ?12.7; 87Sr/86Sr?=?0.7088–0.7285) indicate crustal contamination. AFC modelling suggests that the Khopoli olivine gabbros were derived from a Thakurvadi or Khandala-like basaltic melt with variable degrees of crustal contamination. Unlike the commonly alkalic, pre- and post-volcanic intrusions known in the Deccan Traps, the Khopoli intrusion provides a window to the shallow subvolcanic architecture and magmatic processes associated with the main tholeiitic flood basalt sequence. Measured true density values of the Khopoli olivine gabbros are as high as 3.06 g/cm3, and such high-level olivine-rich intrusions in flood basalt provinces can also explain geophysical observations such as high gravity anomalies and high seismic velocity crustal horizons.  相似文献   

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