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1.
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   

2.
Alcedo volcano is one of six shield volcanoes on Isabela Islandin the western Galpagos Islands. Although Alcedo is dominantiybasaltic, it is unusual in that it also has erupted 1 km3 ofrhyolite. The rhyolitic phase marked a 10-fold decrease in themass-eruption rate of the volcano, and the volcano has returnedto erupting basalt. The basalts are tholeiitic and range fromstrongly to sparsely porphyritic. Olivine and plagiodase arethe liquidus phases in the most primitive basalts. The MgO andNi concentrations in the most primitive basalts indicate thatthey have undergone substantial differentiation since extractionfrom the mantle. The rhyolites contain the assemblage oligoclase-augite-titanomagnetite-fayalite-apatiteand sparse xenoliths of quenched basalt and cumulate gabbros.Intermediate rocks are very rare, but some are apparently basaltrhyolitehybrids, and others resulted from differentiation of tholeiiticmagma. Several modeling approaches and Sr-, Nd-, and O-isotopicdata indicate that the rhyolites resulted from 90% fractionation(by weight) of plagiodase, augite, titanomagnetite, olivine,and apatite from the most primitive olivine tholeiite. The dataare inconsistent with the rhyolites originating by crustal anatexis.The extreme Daly gap may be caused by the large increase inviscosity as the basaltic magma differentiates to intermediateand siliceous compositions; highly evolved magmas are eruptibleonly after they become saturated with volatiles by second boiling.The close association of the hybrid intermediate magmas andmagmatic inclusions with the climactic plinian eruption indicatesmixing between mafic and silicic magmas immediately before eruption.Rhyolite production was favored by the decrease in supply ofbasaltic magma as Alcedo was carried away from the focus ofthe Galpagos hotspot. A three-stage model for the magmaticevolution of a Galpagos volcano is proposed. In the first stage,the supply of basaltic magma is large. Basaltic magma continuallyintrudes the subcaldera magma chamber, buffering the magmas'compositional and thermal evolution. As the volcano is carriedaway from the basaltic source, the magma chamber is allowedto cool and differentiate, as exemplified by Alcedo's rhyoliticphase. Finally, the volcano receives even smaller influx ofbasalt, so a large magma chamber cannot be sustained, and thevolcano shifts to isolated basaltic eruptions. KEY WORDS: Galpagos; oceanic rhyolites; fractional crystallization; Isabela Island *Corresponding author, e-mail: Geist{at}IDUI1.csrv.uidaho.edu. Telephone: 208-885-6491. Fax: 208-885-5724  相似文献   

3.
Many volcanic centers in the Aleutian Islands have erupted lavas that range in composition from high-Mg basalt (MgO>9 wt%) to more fractionated and voluminous high-Al basalts and basaltic andesites. The petrogenetic relationships between these rock types and the composition of primary magmas has been vigorously debated. The phase relations of a typical high-Mg basalt from the Makushin volcanic field on Unalaska Island provide important constraints on petrogenetic models. Results of one-atmosphere and moderate-to high-pressure (5–20 kb) anhydrous experiments are similar to results obtained from primitive MORB. At low pressures olivine is the liquidus phase joined by plagioclase and clinopyroxene at progressively lower temperatures. Clinopyroxene is the second phase to crystallize at pressures greater than 5 kb and replaces olivine on the liquidus at approximately 10 kb. Above 10 kb the liquidus pyroxene is aluminous augite and orthopyroxene is the second phase to crystallize. Glasses in equilibrium with olivine and clinopyroxene at intermediate-pressure (5 to 10 kb) are similar in composition to high-Al basalt. Plagioclase is not involved and most likely does not become a liquidus phase until the liquid has evolved significantly. Although our studies do not confirm the primary nature of high-Mg basalts they do support a model in which high-Al basalts are generated by moderate amounts of crystal fractionation from more primitive (high Mg/Mg+Fe, lower Al2O3) basaltic magmas near the arc crust-mantle boundary.Abbreviations Ol olivine - Cpx Clinopyroxene - Pl plagioclase - L liquid - Sp spinel - Pig pigeonite - Opx Orthopyroxene  相似文献   

4.
Klyuchevskoy volcano, in Kamchatka’s subduction zone, is one of the most active arc volcanoes in the world and contains some of the highest δ18O values for olivines and basalts. We present an oxygen isotope and melt inclusion study of olivine phenocrysts in conjunction with major and trace element analyses of 14C- and tephrochronologically-dated tephra layers and lavas spanning the eruptive history of Klyuchevskoy. Whole-rock and groundmass analyses of tephra layers and lava samples demonstrate that both high-Mg (7–12.5 wt% MgO) and high-Al (17–19 wt% Al2O3, 3–6.5 wt% MgO) basalt and basaltic andesite erupted coevally from the central vent and flank cones. Individual and bulk olivine δ18O range from normal MORB values of 5.1‰ to values as high as 7.6‰. Likewise, tephra and lava matrix glass have high-δ18O values of 5.8–8.1‰. High-Al basalts dominate volumetrically in Klyuchevskoy’s volcanic record and are mostly high in δ18O. High-δ18O olivines and more normal-δ18O olivines occur in both high-Mg and high-Al samples. Most olivines in either high-Al or high-Mg basalts are not in oxygen isotopic equilibrium with their host glasses, and Δ18Oolivine–glass values are out of equilibrium by up to 1.5‰. Olivines are also out of Fe–Mg equilibrium with the host glasses, but to a lesser extent. Water concentrations in olivine-hosted melt inclusions from five tephra samples range from 0.4 to 7.1 wt%. Melt inclusion CO2 concentrations vary from below detection (<50 ppm) to 1,900 ppm. These values indicate depths of crystallization up to ~17 km (5 kbar). The variable H2O and CO2 concentrations likely reflect crystallization of olivine and entrapment of inclusions in ascending and degassing magma. Oxygen isotope and Fe–Mg disequilibria together with melt inclusion data indicate that olivine was mixed and recycled between high-Al and high-Mg basaltic melts and cumulates, and Fe–Mg and δ18O re-equilibration processes were incomplete. Major and trace elements in the variably high-δ18O olivines suggest a peridotite source for the parental magmas. Voluminous, highest in the world with respect to δ18O, and hydrous basic volcanism in Klyuchevskoy and other Central Kamchatka depression volcanoes is explained by a model in which the ascending primitive melts that resulted from the hydrous melt fluxing of mantle wedge peridotite, interacted with the shallow high-δ18O lithospheric mantle that had been extensively hydrated during earlier times when it was part of the Kamchatka forearc. Following accretion of the Eastern Peninsula terrains several million years ago, a trench jump eastward caused the old forearc mantle to be beneath the presently active arc. Variable interaction of ascending flux-melting-derived melts with this older, high-δ18O lithospheric mantle has produced mafic parental magmas with a spectrum of δ18O values. Differentiation of the higher δ18O parental magmas has created the volumetrically dominant high-Al basalt series. Both basalt types incessantly rise and mix between themselves and with variable in δ18O cumulates within dynamic Klyuchevskoy magma plumbing system, causing biannual eruptions and heterogeneous magma products. Electronic supplementary material  The online version of this article (doi:) contains supplementary material, which is available to authorized users.  相似文献   

5.
This paper characterizes late Holocene basalts and basaltic andesites at Medicine Lake volcano that contain high pre-eruptive H2O contents inherited from a subduction related hydrous component in the mantle. The basaltic andesite of Paint Pot Crater and the compositionally zoned basaltic to andesitic lavas of the Callahan flow erupted approximately 1000 14C years Before Present (14C years b.p.). Petrologic, geochemical and isotopic evidence indicates that this late Holocene mafic magmatism was characterized by H2O contents of 3 to 6 wt% H2O and elevated abundances of large ion lithophile elements (LILE). These hydrous mafic inputs contrast with the preceding episodes of mafic magmatism (from 10,600 to ∼3000 14C years b.p.) that was characterized by the eruption of primitive high alumina olivine tholeiite (HAOT) with low H2O (<0.2 wt%), lower LILE abundance and different isotopic characteristics. Thus, the mantle-derived inputs into the Medicine Lake system have not always been low H2O, primitive HAOT, but have alternated between HAOT and hydrous subduction related, calc-alkaline basalt. This influx of hydrous mafic magma coincides temporally and spatially with rhyolite eruption at Glass Mountain and Little Glass Mountain. The rhyolites contain quenched magmatic inclusions similar in character to the mafic lavas at Callahan and Paint Pot Crater. The influence of H2O on fractional crystallization of hydrous mafic magma and melting of pre-existing granite crust beneath the volcano combined to produce the rhyolite. Fractionation under hydrous conditions at upper crustal pressures leads to the early crystallization of Fe-Mg silicates and the suppression of plagioclase as an early crystallizing phase. In addition, H2O lowers the saturation temperature of Fe and Mg silicates, and brings the temperature of oxide crystallization closer to the liquidus. These combined effects generate SiO2-enrichment that leads to rhyodacitic differentiated lavas. In contrast, low H2O HAOT magmas at Medicine Lake differentiate to iron-rich basaltic liquids. When these Fe-enriched basalts mix with melted granitic crust, the result is an andesitic magma. Since mid-Holocene time, mafic volcanism has been dominated primarily by hydrous basaltic andesite and andesite at Medicine Lake Volcano. However, during the late Holocene, H2O-poor mafic magmas continued to be erupted along with hydrous mafic magmas, although in significantly smaller volumes. Received: 4 January 1999 / Accepted: 30 August 1999  相似文献   

6.
Quaternary basalts, andesites and dacites from the Abu monogenetic volcano group, SW Japan, (composed of more than 40 monogenetic volcanoes) show two distinct chemical trends especially on the FeO*/MgO vs SiO2 diagram. One trend is characterized by FeO*/MgO-enrichment with a slight increase in SiO2 content (Fe-type trend), whereas the other shows a marked SiO2-enrichment with relatively constant FeO*/MgO ratios (Si-type trend). The Fe-type trend is explained by fractional crystallization with subtraction of olivine and augite from a primitive alkali basalt magma. Rocks of the Si-type trend are characterized by partially melted or resorbed quartz and sodic plagioclase phenocrysts and/or fine-grained basaltic inclusions. They are most likely products of mixing of a primitive alkali basalt magma containing olivine phenocrysts with a dacite magma containing quartz, sodic plagioclase and hornblende phenocrysts. Petrographic variation as well as chemical variation from basalt to dacite of the Si-type trend is accounted for by various mixing ratios of basalt and dacite magmas. Pargasitic hornblende and clinopyroxene phenocrysts in andesite and dacite may have crystallized from basaltic magma during magma mixing. Olivine and spinel, and quartz, sodic plagioclase and common hornblende had crystallized in basaltic and dacitic magmas, respectively, before the mixing. Within a lava flow, the abundance of basaltic inclusions decreases from the area near the eruptive vent towards the perimeter of the flow, and the number of resorbed phenocrysts varies inversely, suggesting zonation in the magma chamber.The mode of mixing changes depending on the mixing ratio. In the mafic mixture, basalt and dacite magmas can mix in the liquid state (liquid-liquid mixing). In the silicic mixture, on the other hand, the basalt magma was quenched and formed inclusions (liquid-solid mixing). During mixing, the disaggregated basalt magma and the host dacite magma soon reached thermal equilibrium. Compositional homogenization of the mixed magma can occur only when the equilibrium temperature is sufficiently above the solidus of the basalt magma. The Si-type trend is chemically and petrographically similar to the calc-alkalic trend. Therefore, a calc-alkalic trend which is distinguished from a fractional crystallization trend (e.g. Fe-type trend) may be a product of magma mixing.  相似文献   

7.
Mt. Shasta andesite and dacite lavas contain high MgO (3.5–5 wt.%), very low FeO*/MgO (1–1.5) and 60–66 wt.% SiO2. The range of major and trace element compositions of the Shasta lavas can be explained through fractional crystallization (~50–60 wt.%) with subsequent magma mixing of a parent magma that had the major element composition of an H2O-rich primitive magnesian andesite (PMA). Isotopic and trace element characteristics of the Mt. Shasta stratocone lavas are highly variable and span the same range of compositions that is found in the parental basaltic andesite and PMA lavas. This variability is inherited from compositional variations in the input contributed from melting of mantle wedge peridotite that was fluxed by a slab-derived, fluid-rich component. Evidence preserved in phenocryst assemblages indicates mixing of magmas that experienced variable amounts of fractional crystallization over a range of crustal depths from ~25 to ~4 km beneath Mt. Shasta. Major and trace element evidence is also consistent with magma mixing. Pre-eruptive crystallization extended from shallow crustal levels under degassed conditions (~4 wt.% H2O) to lower crustal depths with magmatic H2O contents of ~10–15 wt.%. Oxygen fugacity varied over 2 log units from one above to one below the Nickel-Nickel Oxide buffer. The input of buoyant H2O-rich magmas containing 10–15 wt.% H2O may have triggered magma mixing and facilitated eruption. Alternatively, vesiculation of oversaturated H2O-rich melts could also play an important role in mixing and eruption.  相似文献   

8.
Roca Redonda volcano is a mostly submarine shield volcano that rises nearly 3 km from the adjacent seafloor. Over twenty lava flows and palagonite tuff are exposed in a 60 meter high oblong outcrop above sea level, and several other flows are exposed in the shallow water surrounding the islet. Thick, slightly alkaline picritic flows form the base of the section. Thinner picrites interbedded with sparsely porphyritic alkali-olivine basaltic pahoehoe toes characterize the upper section. The subaerial section probably records the filling of a palagonite tuff cone with younger lavas. Numerous fumaroles that may have a magmatic component are present in the shallow (<30 m) submarine zone and indicate that the volcano is probably still active. Three lava types are exposed: the basal picrites with 19% > MgO > 14%, high-Mg basalts with MgO of about 9%, and low-Mg basalts with MgO of about 6%. The Sr and Nd isotopic ratios of the three lava types are within analytical uncertainty. Olivine compositions indicate that the picrites are basaltic liquids that have accumulated olivine whose composition is in equilibrium with the host basaltic liquid. Apparently, basaltic magmas percolated through dunite and troctolite that had crystallized from slightly older Roca Redonda basaltic magma. Lavas from Roca Redonda have enriched trace element contents and isotopic ratios relative to nearby Wolf volcano, but they are quite similar to lavas from Cerro Azul and Ecuador volcanoes. The common characteristic of these volcanoes is that they lie on the periphery of the archipelago and are in a stage of subaerial growth. This suggests that Galápagos volcanoes may go through a juvenile alkaline stage before a mature tholeiitic stage, analogous to the Loihi stage of Hawaiian volcanism. A low 3He/4He ratio in olivine from one of the picrites indicates a small contribution by the Galápagos mantle plume. Received: 15 December 1997 / Accepted: 6 May 1998  相似文献   

9.
Lavas from the 1967 Halemaumau eruption and picrites eruptedin August 1968 at Hiiaka Crater and along the east-rift zonehave the same incompatible element ratios, a feature consistentwith a comagmatic origin. Competing hypotheses for the relationnshipbetween the picrites and basalts are accumulation of olivinein a basaltic melt similar in composition to the Halemaumaubasalts or fractionation of olivine from a primitive picriticmelt to produce the basalts. The results of thermodynamic modelingand major element trends on element ratio diagrams disprovethe first hypothesis but are consistent with the second, fractionation.Thermodynamic modeling provides further tests of the fractionationhypothesis in the form of predicted phase compositions. Predictedand observed phase compositions do not differ significantly.As predicted by modeling, late crystallizing augite and plagioclasein the east-rift picrites are similar to early crystallizingaugite and plagioclase in the Halemaumau basalts. These resultsare all consistent with the hypothesis that primitive picriticmelts with more than 15% MgO fractionated along a liquid lineof descent from the Hiiaka picrites to the Halemaumau basalts.The original magma entered the volcano, probably near its baseon the ocean floor. Part ascended and differentiated under Halemaumauand part erupted at Hiika, 5 km down the east-rift with littlechemical modification. More extensively crystallized picritesthat represent fractionation stages between the Hiiaka Craterpicrites and the Halemaumau basalts erupted another 4?5 and19?5 km down rift from Hiiaka, ending the August, 1968 eruption.  相似文献   

10.
Late Quaternary, porphyritic basalts erupted in the Kaikohe-Bay of Islands area, New Zealand, provide an opportunity to explore the crystallization and ascent history of small volume magmas in an intra-continental monogenetic volcano field. The plagioclase phenocrysts represent a diverse crystal cargo. Most of the crystals have a rim growth that is compositionally similar to groundmass plagioclase (~?An65) and is in equilibrium with the host basalt rock. The rims surround a resorbed core that is either less calcic (~?An20–45) or more calcic (>?An70), having crystallized in more differentiated or more primitive melts, respectively. The relic cores, particularly those that are less calcic (<?~?An45), have 87Sr/86Sr ratios that are either mantle-like (~?0.7030) or crustal-like (~?0.7040 to 0.7060), indicating some are antecrysts formed in melts fractionated from plutonic basaltic forerunners, while others are true xenocrysts from greywacke basement and/or Miocene arc volcanics. It is envisaged that intrusive basaltic forerunners produced a zone where various degrees of crustal assimilation and fractional crystallization occurred. The erupted basalts represent mafic recharge of this system, as indicated by the final crystal rim growths around the entrained antecrystic and xenocrystic cargo. The recharge also entrained cognate gabbros that occur as inclusions, and produced mingled groundmasses. Multi-stage magmatic ascent and interaction is indicated, and is consistent with the presence of a partial melt body in the lower crust detected by geophysical methods. This crystallization history contrasts with traditional concepts of low-flux basaltic systems where rapid ascent from the mantle is inferred. From a hazards perspective, the magmatic system inferred here increases the likelihood of detecting eruption precursor phenomena such as seismicity, degassing and surface deformation.  相似文献   

11.
Kahoolawe Island, Hawaii (18×11 km), is a basaltic shield volcano with caldera-filling lavas, seven identified postshield vents, and at least two occurrences of apparent rejuvenated-stage eruptive. We examined 42 samples that represent all stages of Kahoolawe volcano stratigraphy for their petrography, whole-rock major-and trace-element contents, mineral compositions, and K–Ar ages. The two oldest shield samples have an average age of 1.34±0.08 Ma, and four postshield samples (3 are alkalic) average 1.15±0.03 Ma; ages of 1.08 and 0.99 Ma for two additional tholeiitic samples probably are minimum ages. Whole-rock major- and trace-element and mineral compositions of Kahoolawe shield and caldera-fill laves are generally similar to the lavas forming Kilauea and Mauna Loa tholeiitic shields, but in detail, Kahoolawe shield lavas have distinctive compositions. An unusual aspect of many postshield Ka-hoolawe lavas is anomalously high REE and Y abundances (up to 200 ppm La and 175 ppm Y) and negative Ce anomalies. These enrichments reflect surficial processes, where weathering and soil development promoted REE-Y transport at the weathering front. Major element abundances (MgO, 10–6 wt.%) for shield and caldera-fill basalts are consistent with fractionation of ol+px+pl in frequently replenished magma reservoirs. In general, tholeiitic basalts erupted from late vents are higher in SiO2 than the shield lavas, and temporal differences in parental magma compositions are the likely explanation. Alkalic basalts that erupted from vents are comparable in composition to those at other Hawaiian volcanoes. Trace-element abundance ratios indicate that alkalic basalts represent either relatively lower degrees of melting of the shield source or a distinct source. Apparent rejuvenated-stage basalts (i.e., emplaced after substantial Kahoolawe erosion) are tholeiitic, unlike the rejuvenated-stages at other Hawaiian volcanoes (alkalic). Kahoolawe, like several other Hawaiian volcanoes, has intercalated tholeiitic and alkalic basalts in the postshield stage, but it is the only volcano that appears to have produced tholeiitic rejuvenated-stage lavas.  相似文献   

12.
Summary Augite and diopside crystals from the Oligo-Miocene high-Mg and high-Al basalts, basaltic andesites and andesites of Montresta (Sardinia, Italy) have been analysed by means of X-ray single crystal diffraction and electron microprobe. It has been found that crystallization or equilibration processes took place within the crust at pressures below 5 kbar. In particular, samples from high-Al basalts, basaltic andesites and andesites crystallised at pressures below one kbar while those from high-Mg basalts crystallised in a range from about 1.5 to 4.5 kbar. The intracrystalline temperatures range between about 550 °C for high-Mg basalts and 750 °C for andesites. The M2′ site is occupied in each of the studied crystals, which implies slow cooling rates.  相似文献   

13.
Basaltic lava flows and high-silica rhyolite domes form the Pleistocene part of the Coso volcanic field in southeastern California. The distribution of vents maps the areal zonation inferred for the upper parts of the Coso magmatic system. Subalkalic basalts (<50% SiO2) were erupted well away from the rhyolite field at any given time. Compositional variation among these basalts can be ascribed to crystal fractionation. Erupted volumes of these basalts decrease with increasing differentiation. Mafic lavas containing up to 58% SiO2, erupted adjacent to the rhyolite field, formed by mixing of basaltic and silicic magma. Basaltic magma interacted with crustal rocks to form other SiO2-rich mafic lavas erupted near the Sierra Nevada fault zone.Several rhyolite domes in the Coso volcanic field contain sparse andesitic inclusions (55–61% SiO2). Pillow-like forms, intricate commingling and local diffusive mixing of andesite and rhyolite at contacts, concentric vesicle distribution, and crystal morphologies indicative of undercooling show that inclusions were incorporated in their rhyolitic hosts as blobs of magma. Inclusions were probably dispersed throughout small volumes of rhyolitic magma by convective (mechanical) mixing. Inclusion magma was formed by mixing (hybridization) at the interface between basaltic and rhyolitic magmas that coexisted in vertically zoned igneous systems. Relict phenocrysts and the bulk compositions of inclusions suggest that silicic endmembers were less differentiated than erupted high-silica rhyolite. Changes in inferred endmembers of magma mixtures with time suggest that the steepness of chemical gradients near the silicic/mafic interface in the zoned reservoir may have decreased as the system matured, although a high-silica rhyolitic cap persisted.The Coso example is an extreme case of large thermal and compositional contrast between inclusion and host magmas; lesser differences between intermediate composition magmas and inclusions lead to undercooling phenomena that suggest smaller T. Vertical compositional zonation in magma chambers has been documented through study of products of voluminous pyroclastic eruptions. Magmatic inclusions in volcanic rocks provide evidence for compositional zonation and mixing processes in igneous systems when only lava is erupted.  相似文献   

14.
The conditions under which rear-arc magmas are generated were estimated using primary basalts from the Sannome-gata volcano, located in the rear of the NE Japan arc. Scoriae from the volcano occur with abundant crustal and mantle xenoliths, suggesting that the magma ascended rapidly from the upper mantle. The scoriae show significant variations in their whole-rock compositions (7.9–11.1 wt% MgO). High-MgO scoriae (MgO > ~9.5 wt%) have mostly homogeneous 87Sr/86Sr ratios (0.70318–0.70320), whereas low-MgO scoriae (MgO < ~9 wt%) have higher 87Sr/86Sr ratios (>0.70327); ratios tend to increase with decreasing MgO content. The high-MgO scoriae are aphyric, containing ~5 vol% olivine microphenocrysts with Mg# [100 × Mg/(Mg + Fe2+)] of up to 90. In contrast, the low-MgO scoriae have crustal xenocrysts of plagioclase, alkali feldspar, and quartz, and the mineralogic modes correlate negatively with whole-rock MgO content. On the basis of these observations, it is inferred that the high-MgO scoriae represent primary or near-primary melts, while the low-MgO scoriae underwent considerable interaction with the crust. Using thermodynamic analysis of the observed petrological features of the high-MgO scoriae, the eruption temperature of the magmas was constrained to 1,160–1,220 °C. Given that the source mantle was depleted MORB-source mantle, the primary magma was plausibly generated by ~7 % melting of a garnet-bearing spinel peridotite; taking this into consideration, and considering the constraints of multi-component thermodynamics, we estimated that the primary Sannome-gata magma was generated in the source mantle with 0.5–0.6 wt% H2O at 1,220–1,230 °C and at ~1.8 GPa, and that the H2O content of the primary magma was 6–7 wt%. The rear-arc Sannome-gata magma was generated by a lower degree of melting of the mantle at greater depths and lower temperatures than the frontal-arc magma from the Iwate volcano, which was also estimated to be generated by ~15 % melting of the source mantle with 0.6–0.7 wt% H2O at ~1,250 °C and at ~1.3 GPa.  相似文献   

15.
Aleutian tholeiitic and calc-alkaline magma series I: The mafic phenocrysts   总被引：1，自引：0，他引：1  
Diagnostic mafic silicate assemblages in a continuous spectrum of Aleutian volcanic rocks provide evidence for contrasts in magmatic processes in the Aleutian arc crust. Tectonic segmentation of the arc exerts a primary control on the variable mixing, fractional crystallization and possible assimilation undergone by the magmas. End members of the continuum are termed calc-alkaline (CA) and tholeiitic (TH). CA volcanic rocks (e.g., Buldir and Moffett volcanoes) have low FeO/MgO ratios and contain compositionally diverse phenocryst populations, indicating magma mixing. Their Ni and Cr-rich magnesian olivine and clinopyroxene come from mantle-derived mafic olivine basalts that have mixed with more fractionated magmas at mid-to lower-crustal levels immediately preceding eruption. High-Al amphibole is associated with the mafic end member. In contrast, TH lavas (e.g., Okmok and Westdahl volcanoes) have high FeO/MgO ratios and contain little evidence for mixing. Evolved lavas represent advanced stages of low pressure crystallization from a basaltic magma. These lavas contain groundmass olivine (FO 40–50) and lack Ca-poor pyroxene. Aleutian volcanic rocks with intermediate FeO/MgO ratios are termed transitional tholeiitic (TTH) and calc-alkaline (TCA). TCA magmas are common (e.g., Moffett, Adagdak, Great Sitkin, and Kasatochi volcanoes) and have resulted from mixing of high-Al basalt with more evolved magmas. They contain amphibole (high and low-Al) or orthopyroxene or both and are similar to the Japanese hypersthene-series. TTH magmas (e.g., Okmok and Westdahl) contain orthopyroxene or pigeonite or both, and show some indication of upper crustal mixing. They are mineralogically similar to the Japanese pigeonite-series. High-Al basalt lacks Mg-rich mafic phases and is a derivative magma produced by high pressure fractionation of an olivine tholeiite. The low pressure mineral assemblage of high-Al basalt results from crystallization at higher crustal levels.  相似文献   

16.
A remarkably diverse suite of lavas erupted during the late-Pliocene at the volcanic front of the western Mexican Volcanic Belt near the town of Los Volcanes, Jalisco. This region is much closer to the Middle America Trench than the main axis of Quaternary andesite-dacite stratovolcanoes, and volcanism occurred in a complex tectonic regime involving both subduction of the young Rivera Plate and transverse crustal extension of the Jalisco structural block. The variety of lava types covers a wide spectrum from highly potassic minettes and leucitites to calc-alkaline basalts and andesites which are compositionally similar to those erupted elsewhere in the Mexican Volcanic Belt. Other alkaline varieties intermediate between these extremes include absarokites, trachybasalts and trachyandesites. Phlogopite, amphibole and apatite are common phenocryst phases; whole-rock compositions show a wide range of alkali contents (e.g. K2O of 1.0–8.6 wt.%), and typically contain >5 wt.% MgO. MgO, Ni, V and Cr show little systematic decrease with increasing SiO2, suggesting that these lavas have evolved from primitive, mantle derived magmas with a wide range of SiO2 contents. Strong enrichments in incompatible trace elements are observed in all of the lavas (Sr 700–5100 ppm, Ba 470–4800, Ce 22–325 ppm, Zr 90–700 ppm), as is the relative enrichment of large ion lithophile (Ba, Sr, Rb) and light rare-earth elements (La, Ce) over the high field strength elements (Ti, Zr) which is typical of magmas in volcanic arcs. This enrichment pattern suggests that these magmas come from source regions which contain incompatible element-rich phases such as phlogopite, amphibole and apatite. The petrological and geochemical features of the lavas which occur in the Los Volcanes region provide direct evidence of the extreme heterogeneity which may exist in magma source regions at convergent margins. The complex tectonic regime in western Mexico further suggests that rifting and crustal extension play an important role in the generation and successful ascent of melts from enriched regions of the sub-arc mantle.  相似文献   

17.
Pre-eruption processes are investigated for magmas erupted in1983 from Miyake-jima volcano, which is one of the most activevolcanoes in Japan. The whole-rock compositional trends of theeruptive products are principally smooth and linear. Magmaserupted from some fissures have compositions that deviate fromthe main linear trend. Phenocryst contents of samples displacedfrom the linear compositional trends are significantly lowerthan those of samples on the main trends. Anorthite-rich plagioclasephenocrysts, present throughout the 1983 products, are too calcicto have crystallized from the erupted magma composition, andwere derived from a basaltic magma through magma mixing. Althoughthe linear whole-rock composition trends favor simple two-componentmagma mixing, this cannot explain the presence of samples thatdeviate from the main trend. Instead, the observed compositiontrends were formed by mixing of a homogeneous basaltic magmawith andesitic magmas exhibiting compositional diversity. Theoriginal linear composition trends of the andesitic end-membermagma were rotated and shifted to the direction of the basalticend-member magma by magma mixing. The samples out of the maintrends represent magmas with less basaltic component than thoseon the trend. The density and viscosity of the basaltic end-membermagma were comparable with those of the andesitic end-membermagmas. The basaltic magma, discharged from one magma chamberat 2 kbar pressure, was injected into a magma chamber at lowerpressure occupied by the chemically zoned andesite magma (1kbar), and possibly as a fountain. To establish the characteristicmixing trend of the 1983 magma, the basaltic component musthave been distributed systematically in the zoned andesite magma.A requirement is that the basaltic magma spread laterally andmixed with the andesite magma at various levels of ascent ofthe fountain in the host andesite magma. Analysis of compositionalzoning in titanomagnetite crystals revealed that the eruptionof the 1983 magmas was initiated soon after the replenishmentof the basaltic magma in the 1 kbar magma chamber. KEY WORDS: compositional trend; liquid–liquid blending; magma chamber; magma mixing; Miyake-jima Volcano  相似文献   

18.
We report here major, trace element and Sr–Nd–Pb isotopic data for a new set of basaltic lavas and melt inclusions hosted in Mg-rich olivines (Fo86–91) from Mota Lava, in the Banks islands of the Vanuatu island arc. The results reveal the small-scale coexistence of typical island-arc basalts (IAB) and a distinct type of Nb-enriched basalts (NEB) characterized by primitive mantle-normalized trace element patterns without high-field-strength element (HFSE) depletion. The IAB show trace element patterns with prominent negative HFSE anomalies acquired during melting of mantle sources enriched with slab-derived, H2O-rich components during subduction. In contrast, the NEB display trace element features that compare favourably with enriched-mid-ocean ridge basalt (MORB) and the most enriched basalts from the Vanuatu back-arc troughs. Both their trace element and Nd–Sr isotopic compositions require partial melting of an enriched-MORB-type mantle source, almost negligibly contaminated by slab-derived fluids (~0.2 wt%). The coexistence of these two distinct types of primitive magma, at the scale of one volcanic island and within a relatively short span of time, would reflect a heterogeneous mantle source and/or tapping of distinct mantle sources. Direct ascent of such distinct magmas could be favoured by the extensive tectonic setting of Mota Lava Island, allowing decompression melting and sampling of variable mantle sources. Significantly, this island is located at the junction of the N–S back-arc troughs and the E–W Hazel Home extensional zone, where the plate motion diverges in both direction and rate. More broadly, this study indicates that crustal faulting in arc contexts would permit basaltic magmas to reach Earth’s surface, while preserving the geochemical heterogeneity of their mantle sources.  相似文献   

19.
The Valley of Mexico and surrounding regions of Mexico and Morelos states in central Mexico contain more than 250 Quaternary eruptive vents in addition to the large, composite volcanoes of Popocatépetl, Iztaccíhuatl, and Nevado de Toluca. The eruptive vents include cinder and lava cones, shield volcanoes, and isolated andesitic and dacitic lava flows, and are most numerous in the Sierra Chichináutzin that forms the southern terminus of the Valley of Mexico. The Chichináutzin volcanic field (CVF) is part of the E-W-trending Mexican Volcanic Belt (MVB), a subduction-related volcanic arc that extends across Mexico. The crustal thickness beneath the CVF (∼50 km) is the greatest of any region in the MVB and one of the greatest found in any arc worldwide. Lavas and scoriae erupted from vents in the CVF include alkaline basalts and calc-alkaline basaltic andesites, andesites, and dacites. Both alkaline and calc-alkaline groups contain primitive varieties that have whole rock Mg#, MgO, and Ni contents, and liquidus olivine compositions (≤Fo90) that are close to those expected of partial melts from mantle peridotite. Primitive varieties also show a wide range of incompatible trace element abundances (e.g. Ba 210–1080 ppm; Ce 25–100 ppm; Zr 130–280 ppm). Data for primitive calc-alkaline rocks from both the CVF and other regions of the MVB to the west are consistent with magma generation in an underlying mantle wedge that is depleted in Ti, Zr, and Nb and enriched in large ion lithophile (K, Ba, Rb) and light rare earth (La, Ce) elements. Extents of partial melting estimated from Ti and Zr data are lower for primitive calc-alkaline magmas in the CVF than for those from the regions of the MVB to the west where the crust is thinner. The distinctive major element compositions (low CaO and Al2O3, high SiO2) of the primitive calc-alkaline magmas in the CVF indicate a more refractory mantle source beneath this region of thick crust. In contrast, primitive alkaline magmas from the CVF and other regions of the MVB show compositional similarities to intraplate-type alkali basalts erupted behind the arc in the Mexican Basin and Range province. These similarities are consistent with the hypothesis that slab-induced convection in the mantle wedge beneath the MVB causes advection of asthenospheric mantle from behind the arc to the region of magma generation. Trace element systematics of primitive magmas in the MVB reveal substantial variability in both the extent of mantle wedge enrichment by subduction processes and in the composition of mantle heterogeneities that are related to previous extraction of alkaline to sub-alkaline basaltic melts. Received: 23 June 1998 / Accepted: 23 December 1998  相似文献   

20.
 Lavas erupted in the Tuxtla Volcanic Field (TVF) over the last 7 Ma include primitive basanites and alkali basalts, mildly alkaline Hy-normative mugearites and benmoreites, and calc-alkaline basalts and basaltic andesites. The primitive lavas are silica-undersaturated, with high concentrations of both incompatible and compatible trace elements, variable La/Yb with constant Yb at 6 to 8 times chondritic, and low Sr and O and variable Pb and Nd isotopic ratios. The primitive magmas originated by increasing degrees of melting with pressure decreasing from greater than 30 kbar to 20 kbar, in the garnet stability field. Another group of alkali basalts and hawaiites has lower Ni and Cr concentrations and higher Fe/Mg ratios, and was derived from the primitive group by crystal fractionation at pressures of several kbar. Incompatible trace elements in these silica undersaturated lavas show depletion in high field strength elements (HFSE) relative to large ion lithophile elements, similar to subduction-related basalts. Ba/Nb ratios are nearly constant and thus the HFSE depletion cannot be the result of a residual HFSE-bearing phase in the source, but could be the result of generation from a source contaminated by fluids or melts from the subducted lithosphere. The silica-saturated mugearites and benmoreites, and the calc-alkaline basalts and basaltic andesites, were erupted only between 3.3 and 1.0 Ma. These have incompatible element concentrations generally lower than in the silica-undersaturated lavas, and thus could not have been derived by crystal fractionation from the silica-undersaturated alkaline magmas. Magmas parental to the silica-saturated magmas originated by higher degrees of melting at lower pressures than the primitive magmas. Melting may have been promoted by an influx of fluid from the subducted lithosphere. Trace element and Sr, Nd, Pb and O isotopic data suggest that three components are involved in the generation of TVF magmas: the mantle, a fluid from the subducted lithosphere, and continental crust. TVF alkaline lavas are similar to those erupted in the back-arc region of the MVB and Japan, and show characteristics similar to alkaline magmas erupted in the southern Andean volcanic arc. These low degree melts reach the surface along with calc-alkaline lavas in the TVF due to an extensional stress field that allows their passage to the surface. Received: 15 September 1994/Accepted: 14 February 1995  相似文献   

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