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1.
The Aurora volcanic field, located along the northeastern margin of Mono Lake in the Western Great Basin, has erupted a diverse
suite of high-K and shoshonitic lava types, with 48 to 76 wt% SiO2, over the last 3.6 million years. There is no correlation between the age and composition of the lavas. Three-quarters of
the volcanic field consists of evolved (<4 wt% MgO) basaltic andesite and andesite lava cones and flows, the majority of which
contain sparse, euhedral phenocrysts that are normally zoned; there is no evidence of mixed, hybrid magmas. The average eruption
rate over this time period was ∼200 m3/km2/year, which is typical of continental arcs and an order of magnitude lower than that for the slow-spreading mid-Atlantic
ridge. All of the Aurora lavas display a trace-element signature common to subduction-related magmas, as exemplified by Ba/Nb
ratios between 52 and 151. Pre-eruptive water contents ranged from 1.5 wt% in plagioclase-rich two-pyroxene andesites to ∼6
wt% in a single hornblende lamprophyre and several biotite-hornblende andesites. Calculated oxygen fugacities fall within
–0.4 and +2.4 log units of the Ni-NiO buffer. The Aurora potassic suite follows a classic, calc-alkaline trend in a plot of
FeOT/MgO vs SiO2 and displays linear decreasing trends in FeOT and TiO2 with SiO2 content, suggesting a prominent role for Fe-Ti oxides during differentiation. However, development of the calc-alkaline trend
through fractional crystallization of titanomagnetite would have caused the residual liquid to become so depleted in ferric
iron that its oxygen fugacity would have fallen several log units below that of the Ni-NiO buffer. Nor can fractionation of
hornblende be invoked, since it has the same effect as titanomagnetite in depleting the residual liquid in ferric iron, together
with a thermal stability limit that is lower than the eruption temperatures of several andesites (∼1040–1080°C; derived from
two-pyroxene thermometry). Unless some progressive oxidation process occurs, fractionation of titanomagnetite or hornblende
cannot explain a calc-alkaline trend in which all erupted lavas have oxygen fugacites ≥ the Ni-NiO buffer. In contrast to
fractional crystallization, closed-system equilibrium crystallization will produce residual liquids with an oxygen fugacity
that is similar to that of the initial melt. However, the eruption of nearly aphryic lavas argues against tapping from a magma
chamber during equilibrium crystallization, a process that requires crystals to remain in contact with the liquid. A preferred
model involves the accumulation of basaltic magmas at the mantle-crust interface, which solidify and are later remelted during
repeated intrusion of basalt. As an end-member case, closed-system equilibrium crystallization of a basalt, followed by equilibrium
partial melting of the gabbro will produce a calc-alkaline evolved liquid (namely, high SiO2 and low FeOT/MgO) with a relative f
O
2 (corrected for the effect of changing temperature) that is similar to that of the initial basalt. Differentiation of the
Aurora magmas by repeated partial melting of previous underplates in the lower crust rather than by crystal fractionation
in large, stable magma chambers is consistent with the low eruption rate at the Aurora volcanic field.
Received: 7 July 1995 / Accepted: 19 April 1996 相似文献
2.
Crystallization experiments have been conducted on compositions along tholeiitic liquid lines of descent to define the compositional space for the development of silicate liquid immiscibility. Starting materials have 46–56 wt% SiO2, 11.7–17.7 wt% FeOtot, and Mg-number between 0.29 and 0.36. These melts fall on the basaltic trends relevant for Mull, Iceland, Snake River Plain lavas and for the Sept Iles layered intrusion, where large-scale liquid immiscibility has been recognized. At one atmosphere under anhydrous conditions, immiscibility develops below 1,000–1,020°C in all of these compositionally diverse lavas. Extreme iron enrichment is not necessary; immiscibility also develops during iron depletion and silica enrichment. Variations in melt composition control the development of silicate liquid immiscibility along the tholeiitic trend. Elevation of Na2O + K2O + P2O5 + TiO2 promotes the development of two immiscible liquids. Increasing melt CaO and Al2O3 stabilizes a single-liquid field. New data and published phase equilibria show that anhydrous, low-pressure fractional crystallization is the most favorable condition for unmixing during differentiation. Pressure inhibits immiscibility because it expands the stability field of high-Ca clinopyroxene, which reduces the proportion of plagioclase in the crystallizing assemblage, thus enhancing early iron depletion. Magma mixing between primitive basalt and Fe–Ti–P-rich ferrobasalts can serve to elevate phosphorous and alkali contents and thereby promote unmixing. Water might decrease the temperature and size of the two-liquid field, potentially shifting the binodal (solvus) below the liquidus, leading the system to evolve as a single-melt phase. 相似文献
3.
Fe-rich tholeiitic liquids and their cumulate products in the Pleasant Bay layered intrusion, coastal Maine 总被引:4,自引:0,他引:4
R. A. Wiebe 《Contributions to Mineralogy and Petrology》1997,129(4):255-267
Fe-rich tholeiitic liquids are preserved as chilled pillows and as the chilled base of a 27 meter thick macrorhythmic layer
in the Pleasant Bay mafic-silicic layered intrusion. The compositions of olivine (Fo1) and plagioclase (An13−8) in these extremely fine grained rocks suggest that they represent nearly end stage liquids that formed by fractionation
of tholeiitic basalt. Their major element compositions (∼17.5 wt% FeOT and 54 wt%SiO2) closely resemble highly evolved glasses in the Loch Ba ring dike and some recent estimates of end-stage liquids related
to the Skaergaard layered intrusion, and are consistent with recent experimental studies of tholeiite fractionation. Their
trace element compositions are consistent with extensive earlier fractionation of plagioclase, olivine, clinopyroxene, ilmenite,
magnetite and apatite. The mineral assemblage of the chilled rocks (olivine, clinopyroxene, quartz, ilmenite and magnetite),
apatite saturation temperatures, and very low Fe3+/Fe2+indicate conditions of crystallization at temperatures of about 950 °C and f
O
2
about two log units below FMQ. Cumulates that lie about 3 meters above the chilled base of the macrorhythmic layer contain
cumulus plagioclase, olivine, clinopyroxene, ilmenite, apatite and zircon. This mineral assemblage and the Fe-Mg ratio in
clinopyroxene cores suggest that this cumulate was in equilibrium with a liquid having a composition identical to that of
the chilled margin which lies directly beneath it. The high FeOT and low SiO2 concentrations of this cumulate (23.3 and 45.8 wt%, respectively) are comparable to those in late stage cumulates of the
Skaergaard and Kiglapait intrusions. This association of a chilled liquid and cumulate in the Pleasant Bay intrusion suggests
that late stage liquids in tholeiitic layered intrusions may have been more SiO2-rich than field-based models suggest and lends support to recent experimental studies of tholeiite fractionation at low f
O2 which indicate that saturation of an Fe-Ti oxide phase should cause FeOT to decrease in the remaining liquid.
Received: 17 January 1997 / Accepted: 10 June 1997 相似文献
4.
特克斯哈拉达拉辉长岩体是西南天山出露规模最大的层状基性-超基性侵入杂岩体,其富含Fe-Ti氧化物(含量高达15%)。哈拉达拉辉长岩中的橄榄石具有较低的Fo值(64~75),斜长石主要为中-拉长石,单斜辉石相对低Mg#(0.66~0.87),属于次透辉石或普通辉石,总体上显示出较高的岩浆演化程度。Fe-Ti氧化物以钒钛磁铁矿为主(TiO2=0.8%~20.6%;V2O3=0.10%~0.83%),常与钛铁矿呈矿物对共生或出溶钛铁矿。晚期的云母矿物均围绕他形的Fe-Ti氧化物生长,主要为富MgO的金云母,少量属于黑云母;大部分云母成分富含F、Cl(F+Cl高达3.14%),指示岩浆形成于贫水环境。哈拉达拉辉长岩体的主量元素成分变化很大(Mg#=0.48~0.73),橄榄辉长岩主要受橄榄石和斜长石的结晶分异/堆晶作用影响,而辉长岩的成分变化主要受控于斜长石和Fe-Ti氧化物的堆晶作用。哈拉达拉岩体的Fe-Ti氧化物主要为岩浆正常结晶分异作用的结果,该体系具有较低氧逸度、贫水的特点,其地幔源区的熔融程度较高,可能是塔里木地幔柱在天山造山带这一构造薄弱带的早期岩浆活动。 相似文献
5.
P. M. E. Tollan I. Bindeman J. D. Blundy 《Contributions to Mineralogy and Petrology》2012,163(2):189-208
In order to shed light on upper crustal differentiation of mantle-derived basaltic magmas in a subduction zone setting, we
have determined the mineral chemistry and oxygen and hydrogen isotope composition of individual cumulus minerals in plutonic
blocks from St. Vincent, Lesser Antilles. Plutonic rock types display great variation in mineralogy, from olivine–gabbros
to troctolites and hornblendites, with a corresponding variety of cumulate textures. Mineral compositions differ from those
in erupted basaltic lavas from St. Vincent and in published high-pressure (4–10 kb) experimental run products of a St. Vincent
high-Mg basalt in having higher An plagioclase coexisting with lower Fo olivine. The oxygen isotope compositions (δ18O) of cumulus olivine (4.89–5.18‰), plagioclase (5.84–6.28‰), clinopyroxene (5.17–5.47‰) and hornblende (5.48–5.61‰) and hydrogen
isotope composition of hornblende (δD = −35.5 to −49.9‰) are all consistent with closed system magmatic differentiation of
a mantle-derived basaltic melt. We employed a number of modelling exercises to constrain the origin of the chemical and isotopic
compositions reported. δ18OOlivine is up to 0.2‰ higher than modelled values for closed system fractional crystallisation of a primary melt. We attribute this
to isotopic disequilibria between cumulus minerals crystallising at different temperatures, with equilibration retarded by
slow oxygen diffusion in olivine during prolonged crustal storage. We used melt inclusion and plagioclase compositions to
determine parental magmatic water contents (water saturated, 4.6 ± 0.5 wt% H2O) and crystallisation pressures (173 ± 50 MPa). Applying these values to previously reported basaltic and basaltic andesite
lava compositions, we can reproduce the cumulus plagioclase and olivine compositions and their associated trend. We conclude
that differentiation of primitive hydrous basalts on St. Vincent involves crystallisation of olivine and Cr-rich spinel at
depth within the crust, lowering MgO and Cr2O3 and raising Al2O3 and CaO of residual melt due to suppression of plagioclase. Low density, hydrous basaltic and basaltic andesite melts then
ascend rapidly through the crust, stalling at shallow depth upon water saturation where crystallisation of the chemically
distinct cumulus phases observed in this study can occur. Deposited crystals armour the shallow magma chamber where oxygen
isotope equilibration between minerals is slowly approached, before remobilisation and entrainment by later injections of
magma. 相似文献
6.
Segregation veins are common in lava sheets and result from internal differentiation during lava emplacement and degassing.
They consist of evolved liquid, most likely replaced by gas-filter pressing from a ∼50% crystallised host lava. Pairs of samples,
host lavas and associated segregation veins from the Reykjanes Peninsula (Iceland), Lanzarote (Canary Islands) and the Masaya
volcano (Nicaragua) show extreme mineralogical and compositional variations (MgO in host lava, segregation veins and interstitial
glass ranges from 8–10 wt%, 3–6 wt%, and to less than 0.01 wt%, respectively). These samples allow the assessment of the internal
lava flow differentiation mechanism, since both the parental and derived liquid are known in addition to the last magma drops
in the form of late interstitial glasses. The mineralogical variation, mass-balance calculated from major- and trace element
composition, and transitional metal partition between crystals and melts are all consistent with fractional crystallisation
as the dominant differentiation mechanism. The interstitial glasses are highly silicic (SiO2 = 70–80 wt%) and represent a final product of high-degree (75–97%) fractional crystallisation of olivine tholeiite at a pressure
close to one atmosphere. The tholeiitic liquid-line-of-decent and the composition of the residual melts are governed by the
K2O/Na2O of the initial basaltic magma. The granitic minimum is reached if the initial liquid has a high K2O/Na2O whereas trondhjemitic composition is the final product of magma with low initial K2O/Na2O.
Electronic supplementary material The online version of this article (doi:) contains supplementary material, which is available to authorized users. 相似文献
7.
S. K. Pandey J. P. Shrivastava G. S. Roonwal 《Journal of the Geological Society of India》2009,74(5):559-572
Mid-Oceanic Ridge Basalt (MORB) samples collected from southern East Pacific Rise (SEPR) have been investigated. These highly
phyric plagioclase basalts (HPPB) and moderately phyric plagioclase basalts (MOPB) show rare cumulate and vitrophyric textures
with plagioclase (>10% as phenocryst) and abundant glass (>72%). Electron Probe Micro Analysis (EPMA) showed large compositional
variations in the megacrysts as well as microcrysts of plagioclase (An62 to An82), olivine (Fo78 to Fo87), pyroxene (ferroaugite to augite) and iron oxides, mostly titaniferous magnetite. Olivine grains show high Mg# (>80%) and
distinctly low in NiO (0.01–0.2%). Ferroan trevorite (NiO =16.22 and FeO(t) =83.06) a characteristic meteoritic mineral has been identified from the olivine megacrysts of MORB, possibly attributed
to Ni-enrichment, resulted from heterogeneity of the lower mantle. Wide range of An composition in plagioclase is indicative
of large pressure range of crystal nucleation under decompression at a depth of ∼70 km (An82) up to the ocean spreading centre. Absence of zoning observed in all the minerals present in the MORB samples, possibly attributed
to unmixing and dominant fractionation process. 相似文献
8.
D. J. Smith M. G. Petterson A. D. Saunders I. L. Millar G. R. T. Jenkin T. Toba J. Naden J. M. Cook 《Contributions to Mineralogy and Petrology》2009,158(6):785-801
Savo, Solomon Islands, is a historically active volcano dominated by sodic, alkaline lavas, and pyroclastic rocks with up
to 7.5 wt% Na2O, and high Sr, arc-like trace element chemistry. The suite is dominated by mugearites (plagioclase–clinopyroxene–magnetite ± amphibole ± olivine)
and trachytes (plagioclase–amphibole–magnetite ± biotite). The presence of hydrous minerals (amphibole, biotite) indicates
relatively wet magmas. In such melts, plagioclase is relatively unstable relative to iron oxides and ferromagnesian silicates;
it is the latter minerals (particularly hornblende) that dominate cumulate nodules at Savo and drive the chemical differentiation
of the suite, with a limited role for plagioclase. This is potentially occurring in a crustal “hot zone”, with major chemical
differentiation occurring at depth. Batches of magma ascend periodically, where they are subject to decompression, water saturation
and further cooling, resulting in closed-system crystallisation of plagioclase, and ultimately the production of sodic, crystal
and feldspar-rich, high-Sr rocks. The sodic and hydrous nature of the parental magmas is interpreted to be the result of partial
melting of metasomatised mantle, but radiogenic isotope data (Pb, Sr, Nd) cannot uniquely identify the source of the metasomatic
agent. 相似文献
9.
H2O-undersaturated melting experiments of synthesized basalt (SiO2 = 50.7 wt.%, MgO = 8.3 wt.%, Mg# = 60) were conducted at fO2 corresponding to NNO+1 and NNO−1 to clarify the effects of pressure (2–7 kbar) and H2O on fractional crystallization in island arcs. H2O content was ranged from nominally anhydrous to 4.4 wt.%. Differentiation trends, namely the liquid lines of descent, change
sensitively according to pressure-H2O relations. Tholeiitic differentiation trends are reproduced with H2O ≤ ∼2 wt.% in primary magma. With such quantities of H2O, fractional crystallization is controlled by olivine + plagioclase at 2 kbar. Increasing the pressure from 2 to ≥4 kbar
induces early crystallization of orthopyroxene instead of olivine and therefore SiO2 enrichment in the residual melts is suppressed. Increasing H2O (≥ ∼2 wt.% in primary magma) stabilizes clinopyroxene relative to orthopyroxene and/or magnetite. Although the phase relations
and proportions strongly depend on fO2 and H2O content, differentiation trends are always calc-alkaline. 相似文献
10.
Michael J. Toplis William L. Brown Elsa Pupier 《Contributions to Mineralogy and Petrology》2008,155(3):329-340
The anorthite content of plagioclase grains (XAn) in 12 rocks from the layered series of the Skaergaard intrusion has been studied by electron microprobe (typically ∼30 core
and ∼70 rim analyses per thin section). Mean core compositions vary continuously from An66 at the base of the layered series (LZa) to An32–30 at the top. On the other hand, crystal rims are of approximately constant composition (An50 ± 1) from the LZa to the lower Middle Zone (MZ). Above the MZ, core and rim compositions generally overlap. Profiles across individual
plagioclase grains from the lower zone show that most crystals have an external zone buffered at XAn ∼50 ± 1. The simplest explanation for these features is that during postcumulus crystallization in the lower zone, interstitial
liquids passed through a density maximum. This interpretation is consistent with proposed liquid lines of descent that predict
silica enrichment of the liquid associated with the appearance of cumulus magnetite. 相似文献
11.
Chris Harris H. Stuart Smith Anton P. le Roex 《Contributions to Mineralogy and Petrology》2000,138(2):164-175
We have measured the δ18O values of the major phenocrysts (olivine, clinopyroxene and plagioclase) present in lavas from Tristan da Cunha and Gough
Island. These islands, which result from the same mantle plume, have enriched radiogenic isotope ratios and are, therefore,
prime candidates for an oxygen isotope signature that is distinct from that of MORB. Consistent differences between the δ18O values of olivine, pyroxene and feldspar in the Gough lavas show that the phenocrysts in the mafic Gough Island lavas are
in oxygen isotope equilibrium. The olivines in lavas with SiO2 <50 wt% have a mean δ18O value of 5.19‰, consistent with crystallization from a magma having the same oxygen isotope composition as MORB. Phenocrysts
in all the Gough lavas show a systematic increase in δ18O value as silica content increases, which is consistent with closed-system fractional crystallization. The lack of enrichment
in δ18O of the Gough magmas suggests that the mantle source contained <2% recycled sediment. In contrast, the Tristan lavas with
SiO2 >48 wt% contain phenocrysts which have δ18O values that are systematically ∼0.3‰ lower than their counterparts from Gough. We suggest that the parental mafic Tristan
magmas were contaminated by material from the volcanic edifice that acquired low δ18O values by interaction with water at high temperatures. The highly porphyritic SiO2-poor lavas show a negative correlation between olivine δ18O value and whole-rock silica content rather than the expected positive correlation. The minimum δ18O value occurs at an SiO2 content of about 45 wt%. Below 45 wt% SiO2, magmas evolved via a combination of assimilation, fractionational crystallization and crystal accumulation; above 45 wt%
SiO2, magmas appeared to have evolved via closed-system fractional crystallization.
Received: 23 November 1998 / Accepted: 27 September 1999 相似文献
12.
Near-liquidus crystallization experiments have been carried out on two basalts (12.5 and 7.8 wt% MgO) from Soufriere, St Vincent
(Lesser Antilles arc) to document the early stages of differentiation in calc-alkaline magmas. The water-undersaturated experiments
were performed mostly at 4 kbar, with 1.6 to 7.7 wt% H2O in the melt, and under oxidizing conditions (ΔNNO = −0.8 to +2.4). A few 10 kbar experiments were also performed. Early
differentiation of primitive, hydrous, high-magnesia basalts (HMB) is controlled by ol + cpx + sp fractionation. Residual
melts of typical high-alumina basalt (HAB) composition are obtained after 30–40% crystallization. The role of H2O in depressing plagioclase crystallization leads to a direct relation between the Al2O3 content of the residual melt and its H2O concentration, calibrated as a geohygrometer. The most primitive phenocryst assemblage in the Soufriere suite (Fo89.6 olivine, Mg-, Al- and Ti-rich clinopyroxene, Cr–Al spinel) crystallized from near-primary (Mg# = 73.5), hydrous (∼5 wt% H2O) and very oxidized (ΔNNO = +1.5–2.0) HMB liquids at middle crustal pressures and temperatures from ∼1,160 to ∼1,060°C. Hornblende
played no role in the early petrogenetic evolution. Derivative HAB melts may contain up to 7–8 wt% dissolved H2O. Primitive basaltic liquids at Soufriere, St Vincent, have a wide range of H2O concentrations (2–5 wt%). 相似文献
13.
One-atmosphere, melting experiments, controlled at the fayalite-magnetite-quartz oxygen buffer, on mildly alkalic and transitional basalts from Iceland show that these begin to crystallize Fe-Ti oxide minerals (magnetite and/or ilmenite) at 1105±5°C, apparently independently of bulk composition and the order of silicate and oxide mineral crystallization. Most samples crystalline plagioclase and olivine as the first two crystalline phases, augite as the third phase, and an Fe-Ti oxide mineral as the fourth phase. The main effects of Fe-Ti oxide crystallization are a marked decrease in FeO and TiO2 in the liquid, and a notable increase in SiO2 and Al2O3, and the minor oxides K2O and P2O5, with decreasing temperature. The most silicic glasses are compositionally mugearitic and shoshonitic basaltic andesites. Because the smallest amount of glass that could be analyzed with the microprobe represents 20–55 percent liquid remaining, it can be expected that more silicic liquids will occur at lower temperatures. On normative, pseudoternary projections, the general effect of Fe-Ti oxide crystallization for mildly alkalic and transitional basalts is a marked increase in normative quartz. This is caused by a strong systematic convergence, with the appearance of Fe-Ti oxides, of the bulk solid precipitates toward the liquid compositions, as projected on the triangle plagioclase-diopside-olivine. For alkalic basalts, the bulk solid precipitate shows an increase in normative diopside with falling temperature and Fe-Ti oxide crystallization. This causes the liquids to move toward decreasing normative diopside and relatively little variation in nepheline. The experimental observations imply that mildly alkalic and transitional magmas, without stabilizing a Fe-Ti oxide mineral, will not evolve toward early silica saturation. 相似文献
14.
Twenty nine samples of silica sinter, ranging in age from modern to Miocene, record temporal changes in both mineralogy and
texture. When first deposited, sinters consist largely of noncrystalline spheres (<1–8 μm diameter) of opal-A exhibiting varying
degrees of close-packing. Particle densities range from 1.5 to 2.1 g cm−3, total water 4–10 wt%, and porosities 35–60%. Changes over ∼10,000 years following deposition are slight although the spheres
may be invested by an additional film of secondary silica. For the next 10,000 to ∼50,000 years, the silica incrementally
crystallises to become poorly crystalline opal-CT and/or opal-C; spherical particles of thin-bladed crystals (lepispheres)
replace opal-A particles and coalesce in microbotryoidal aggregates (∼10–30 μm diameter). Amygdaloidal fibrous clusters occur
with lepispheres. As silica lattice ordering becomes enhanced, total water content drops to <7 wt%, particle density increases
to ∼2.3 g cm−3, and porosity reduces to <30%. The change from opal-A to opal-C takes place over a briefer periods (∼50 years) in silica
sinters that contain other materials (e.g. calcite, sulfur, alunite, plant remains). Sinters older than ∼50,000 years have
recrystallised to microcrystalline quartz. With the onset of quartz crystallisation at ∼20,000 years, total water is <0.2 wt%,
particle density approximates quartz (2.65 g cm−3), and porosity is <4%. The progressive changes in silica species and texture yield ageing profiles for sinters that may serve
as guides to the paleohydrology of geothermal systems and/or epithermal ore deposits in areas where surface thermal activity
has declined or ceased.
Received: 18 November 1998 / Accepted: 6 July 1999 相似文献
15.
We introduce a novel scheme that enables natural silicic glasses to be projected into the synthetic system Qz-Ab-Or-H2O in order to relate variations in volcanic glass chemistry to changing pressure (P) and temperature (T) conditions in the sub-volcanic magma system. By this means an important distinction can be made between ascent-driven and cooling-driven crystallisation under water-saturated or undersaturated conditions. In samples containing feldspar and a silica phase (quartz or tridymite), quantitative P-T estimates of the conditions of last equilibrium between crystals and melt can be made. Formation of highly silicic melts (i.e. >77 wt% SiO2) is a simple consequence of the contraction of the silica phase volume with decreasing pressure, such that high silica glasses can only form by crystallisation at low pressure. Resorption of quartz crystals appears to be a further diagnostic feature of decompression crystallisation. Groundmass and inclusion glasses in dacites from the 1980-1986 eruption of Mount St Helens volcano (WA) span a wide range in SiO2 (68-80 wt%, anhydrous). The compositions of the least evolved (SiO2-poor) inclusions in amphibole phenocrysts record entrapment of silicic liquids with Е.4 wt% water, corresponding to a water saturation pressure of ~200 MPa at 900 °C. The compositions of more evolved (higher SiO2) plagioclase-hosted inclusions and groundmass glasses are consistent with extensive ascent-driven fractional crystallisation of plagioclase, oxide and orthopyroxene phenocrysts and microlites to low pressures. During this polybaric crystallisation, plagioclase phenocrysts trapped melts with a wide range of dissolved water contents (3.5-5.7 wt%). Magmas erupted during the Plinian phase of the 18 May 1980 eruption were derived from a large reservoir at depths of ̈́ km. Subsequent magmas ascended to varying depths within the sub-volcanic system prior to extraction. From glass chemistry and groundmass texture two arrest levels have been identified, at depths of 0.5-1 and 2-4 km. A single dome sample from February 1983 contains groundmass plagioclase, tridymite and quartz, testifying to temperatures of at least 885 °C at 11 MPa. These shallow storage conditions are comparable to those in the cryptodome formed during spring 1980. The corresponding thermal gradient, А.2 °C MPa-1, is consistent with near-adiabatic magma ascent from ~8 km. We argue that the crystallisation history of Mount St Helens dacite magma was largely a consequence of decompression crystallisation of hot magma beyond the point of water saturation. This challenges the conventional view that phenocryst crystallisation occurred by cooling in a large magma chamber prior to the 1980-1986 eruption. Because the crystallisation process is both polybaric and fractional, it cannot be simulated directly using isobaric equilibrium crystallisation experiments. However, calculation of the phase proportions in water-saturated 910ᆣ °C experiments by Rutherford et al. (1985) over the pressure range 220-125 MPa reproduces the crystallisation sequence and phenocryst modes of Mount St Helens dacites from 18 May 1980. By allowing for the effects of fractional versus equilibrium crystallisation, entrained residual source material, and small temperature differences between nature and experiment, phase compositions can also be matched to the natural samples. We conclude that decompression of water-saturated magma may be the dominant driving force for crystallisation at many other silicic volcanic centres. 相似文献
16.
The 150 m thick late Miocene Graveyard Point sill (GPS) is situated at the Idaho-Oregon border near the southwestern edge
of the western Snake River Plain. It records from bottom to top continuous fractional crystallization of a tholeiitic parent
magma (lower chilled border, FeO/(FeO+MgO) = 0.59, Ni = 90 ppm) towards granophyres (late pods and dikes, FeO/(FeO+MgO) = 0.98,
78 wt% SiO2 3.5 wt% K2O, <4 ppm Ni) showing a typical trend of Fe and P enrichment. Fractionating minerals are olivine (Fo79-Fo2), augite (X
Fe = 0.18−0.95), feldspars (An80Or1-An1Or62), Fe-Ti oxides (Ti-rich magnetite and ilmenite), apatite and in two samples super-calcic pigeonite (Wo18–28 Fs41–54). The granophyres may bear some quartz. Compositionally zoned minerals record a large interval of the fractionation process
in every single sample, but this interval changes with stratigraphic height. In super-calcic pigeonite-bearing samples, olivine
is scarce or lacking and because super-calcic pigeonite occurs as characteristic overgrowths on augite, its formation is interpreted
to be related to the schematic reaction: augite + olivine (component in melt) + SiO2 (in melt) = pigeonite, that defines the cotectic between augite and pigeonite in olivine-saturated basaltic systems. Line
measurements with the electron microprobe reveal that the transition from augite to super-calcic pigeonite is continuous.
However, some crystals show an abrupt “reversal” towards augite after super-calcic pigeonite growth. Two processes compete
with each other in the GPS: fractional crystallization of the bulk liquid (the bulk melt separates from solids and interstitial
liquids in the solidification front) and fractional crystallization of interstitial melt in the solidification front itself.
Interplay between those two processes is proposed to account for the observed variations in mineral chemistry and mineral
textures.
Received: 25 November 1998 / Accepted: 14 June 1999 相似文献
17.
Compositional variation and primary water contents of differentiated interstitial and included glasses in boninites 总被引:2,自引:0,他引:2
Interstitial glasses and glasses in small inclusions in Mg-rich phenocrysts of 14 boninites from New Caledonia, the Mariana
Trench, Cape Vogel and Chichi-jima were analysed by electron microprobe and the water contents measured in situ by ion microprobe.
The glasses are remarkably fresh and abundant (∼30–90 vol.%), and the phenocrysts are often skeletal with glass inclusions.
Broad-beam analyses (∼1030) of interstitial glasses and ∼180 point analyses of glass inclusions were carried out, as well
as ∼100 hydrogen analyses. Most glasses have low water-free totals, high water contents, very low MgO, and low total iron;
they are almost entirely quartzofeldspathic and with few exceptions (Q+or+ab+ an+C) lies in the range 83–96. The interstitial
glasses from New Caledonia, the Marianas and most of the glasses from Chichi-jima are dacitic, those from Cape Vogel straddle
dacitic and andesitic compositions, whereas the glasses in a highly glassy sample from Chichi-jima are high-Mg andesitic or
boninitic with up ∼9 wt% MgO, and are, with the exception of a few high-Ca boninites from Tonga, the most Mg-rich interstitial
glasses so far described in boninites. Glasses included in orthopyroxene, olivine or clinoenstatite are boninitic or high-Mg
andesitic in the highly glassy rock and dacitic to high-silica dacitic in the others. They are in general slightly more differentiated
than the interstitial glasses, because of more-extensive crystallization on the host crystal in small inclusions. The interstitial
glass compositions show a direct relationship between silica and Al2O3 and, for most glasses, alkalis, and inverse relationships between silica and CaO, FeO and MgO; alkalis and TiO2 show, however, a broad spread in values in glasses from the Marianas and New Caledonia. Included glasses show similar variations.
Water contents in interstitial glasses are ∼2 wt% for the highly glassy high-Mg andesitic glasses from Chichi-jima, ∼5.4 wt%
for the more differentiated andesitic to dacitic glasses from Cape Vogel, and ∼6.7–7.0 wt% in the most differentiated dacitic
ones from the Marianas and New Caledonia. Water contents in glass inclusions in olivine, orthopyroxene and clinoenstatite
are in the range ∼1.9–3.3 wt%. The interstitial glasses are black and not vesicular, showing that the liquids did not reach
supersaturation after eruption on or intrusion near the sea floor, or were insufficiently so to allow nucleation of water
vapour bubbles. The water is inferred to be primary and to increase strongly with crystallization in the residual liquid down
to the glass-transition T.
Received: 19 December 1994 / Accepted: 5 October 1995 相似文献
18.
Heather Katherine Handley Colin G. Macpherson Jon P. Davidson 《Contributions to Mineralogy and Petrology》2010,159(6):885-908
The Gede Volcanic Complex (GVC) of the Sunda island arc (West Java, Indonesia) consists of multiple volcanic centres and eruptive
groups with complex magmatic histories. We present new petrological, mineralogical, whole-rock major and trace element and
Sr–O isotopic data to provide constraints on the relative importance of fractional crystallisation and magma mixing in petrogenesis,
as well as on the role and nature of the arc crust. Banded juvenile scoria from Young and Old Gede provide unequivocal evidence
for the (late-stage) interaction of distinct magmas at Gede volcano. However, the relatively small-degree compositional zoning
observed in plagioclase phenocrysts of all eruptive groups (up to ~20 mol% An) may be attributed to physical changes in magma
properties (e.g. P, T, and PH2O) rather than changes in melt composition. Major element and trace element variations within each eruptive series are inconsistent
with magmatic evolution through simple mixing processes. Instead, mixing of variably fractionated magma batches is suggested
to account for the significant scatter in some element variation diagrams. No correlation is observed between textural complexity
and/or mineral disequilibrium and whole-rock geochemistry. REE data and geochemical modelling indicate that fractional crystallisation
involving amphibole in the mid- to lower crust, and fractionation of plagioclase, clinopyroxene, Fe–Ti oxide ± olivine ± orthopyroxene
provide strong control on the geochemical evolution of GVC rocks. Two-pyroxene geothermobarometry provides pre-eruption crystallisation
temperatures of 891–1,046°C and pressures of 3.4–6.5 kbar, equivalent to ~13–24 km depth beneath the volcanoes (mid- to lower
crust). Low, mantle-like clinopyroxene δ18O values of GVC lavas and poor correlation of Sr isotope ratios with indices of differentiation precludes significant assimilation
of isotopically distinct crust during magmatic differentiation. Therefore, we suggest that the geochemical character of the
moderately thick West Javan arc crust is relatively immature compared to typical continental crust. Trace element ratios and
strontium isotopes show that the magmatic source composition of the older geographical units, Gegerbentang and Older Quaternary,
is distinct from the other GVC groups. 相似文献
19.
Summary Crust-derived xenoliths hosted by Miocene basaltic diatremes in the Hyblean Plateau (south-eastern Sicily, Italy) provide
new information regarding the nature of a portion of the central Mediterranean lower crust. These xenoliths can be divided
into three groups: gabbros (plagioclase + clinopyroxene + Fe–Ti oxides ± apatite ± amphibole ± Fe-rich green spinel), diorites
(An-poor plagioclase, clinopyroxene ± Fe–Ti oxides ± orthopyroxene) and mafic granulites (plagioclase + clinopyroxene + green
spinel ± orthopyroxene ± Fe–Ti oxides). Gabbros form the main subject of this paper. They represent cumulates whose igneous
texture has been locally obliterated by metamorphic recrystallization and shearing. They were permeated by Fe–Ti-rich melts
related to tholeiitic-type fractional crystallisation. Incompatible element ratios (Zr/Nb = 5–26; Y/Nb = 1.4–11) indicate
that these cumulate gabbros derived from MORB liquids. Late-stage and hydrothermal fluids caused diverse, sometimes important,
metasomatic trasformations. Petrographic and geochemical comparison with gabbroids from well-known geodynamic settings show
that the Hyblean lower crustal xenoliths were probably formed in an oceanic or oceanic-continent transition environment. 相似文献
20.
Anorthositic rocks compose 3540% of the Middle Proterozoic(Keweenawan; 1?1 Ga) Duluth Complexa large, compositemafic body in northeastern Minnesota that was intruded beneatha comagmatic volcanic edifice during the formation of the Midcontinentrift system. Anorthositic rocks, of which six general lithologictypes occur in one area of the complex, are common in an earlyseries of intrusions. They are characterized on a local scale(meters to kilometers) by nonstratiform distribution of rocktypes, variably oriented plagioclase lamination, and compositeintrusive relationships. Variably zoned, subhedral plagioclaseof nearly constant average An (60) makes up 8298% ofthe anorthositic rocks. Other phases include granular to poikiliticolivine (Fo6638), poikilitic clinopyrox-ene (En'7337),subpoikilitic Fe-Ti oxides, and various late-stage and secondaryminerals. Whole-rock compositions of anorthositic rocks are modelled bymass balance to consist of three components: cumulus plagioclase(7095 wt.%), minor cumulus olivine (05%), anda gabbroic postcumulus assemblage (527%) representinga trapped liquid. The postcumulus assemblage has textural andcompositional characteristics which are consistent with crystallizationfrom basaltic magma ranging from moderately evolved olivinetholeiite to highly evolved tholeiite (mg=6025). Sympatheticvariations of mg in plagioclase and in mafic minerals suggestthat cumulus plagioclase, though constant in An, was in approximateequilibrium with the variety of basaltic magma compositionswhich produced the postcumulus assemblages. Standard models of mafic cumulate formation by fractional crystallizationof basaltic magmas in Duluth Complex chambers, although ableto explain the petrogenesis of younger stratiform troctoliticto gabbroic intrusions, are inadequate to account for the field,petrographic, and geochemical characteristics of the anorthositicrocks. Rather, we suggest an origin by multiple intrusions ofplagioclase crystal mushesbasaltic magmas charged withas much as 60% intratelluric plagioclase. The high concentrationsof cumulus plagioclase (7095%) estimated to compose theanorthositic rocks may reflect expulsion of some of the transportingmagma during emplacement or early postcumulus crystallizationof only plagioclase from evolved hyperfeldspathic magma. Althoughthe evolved compositions of anorthositic rocks require significantfractionation of mafic minerals, geophysical evidence indicatesthat ultramafic rocks are, as exposure implies, rare in theDuluth Complex and implies that plagioclase crystal mushes werederived from deeper staging chambers. This is consistent withinterpretations of olivine habit and plagioclase zoning. Moreover,plagioclase could have been segregated from coprecipitatingmafic phases in such lower crustal chambers because of the buoyancyof plagioclase in basaltic magmas at high pressure. The geochemicaleffects of plagioclase suspension in basaltic magmas are consistentwith observed compositions of cumulus plagioclase in the anorthositicrocks and with the geochemical characteristics of many comagmaticbasalts. The petrogenesis of the anorthositic rocks and theoverall evolution of Keweenawan magmas can be related to thedynamics of intracontinental rift formation. 相似文献