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1.
The exchage equilibrium
has been used to measure activity-composition relations along the olivine join FeSi0.5O2−MgSi0.5O2 at 1400 K and 1 atm pressure. Equilibrium Fe−Mg partitioning between the two phases was determined by reversing the compositions
of olivine coexisting with oxide and matallic iron over the composition range Fo23 to Fo92. A detailed study of the thermodynamic properties of the oxide phase has recently been made by Srečec et al. and we have
confirmed their results in the composition range of interest. Application of the oxide data to the exchange equilibrium enables
the properties of olivine to be determined. Within experimental uncertainly (Fe, Mg)Si0.5O2 olivine can, at 1400 K, be treated as a symmetric solution with W
Fe-Mg
o1
of 3.7±0.8 kJ/mol. The data permit the presence of only very slight asymmetry in the series. The data do not support recent
assertions that olivine is highly non-ideal (W≈10 kJ/mol) under these conditions. 相似文献
2.
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
3.
Peter Thy 《Contributions to Mineralogy and Petrology》1983,82(2-3):232-251
Basaltic glasses from the three alkalic areas of Iceland (Snaefellsnes Volcanic Zone, Sudurland Volcanic Zone and Vestmannaeyjar Volcanic Area) contain plagioclase, olivine, clinopyroxene, chromian spinel and titanomagnetite as phenocryst phases. The glasses are hypersthene to nepheline normative alkali basaltic with FeO/ MgO ratios between 1.4–4.7. Olivine ranges in composition from Fo90 to Fo55, plagioclase from An90 to An50 and clinopyroxene from En45Fs10Wo45 to En40Fs17Wo43. Clinopyroxene reveals systematic Ti:Al metastable crystallization trends related to the composition of the enclosing glass. Two types of phenocryst are present in most glasses and show a bimodality in size and composition. Microphenocryst phases are those most likely to have crystallized from the enclosing glass, while macrophenocrysts may have crystallized from a liquid of slightly less evolved composition. The glasses show complex phenocryst-glass relations which can be related to a polybaric effect. The normative glass compositions are related to 2-phase cotectic surfaces in the basalt tetrahedron and define the position of the 3-phase cotectic line. In general with increasing FeO/MgO in the glass the phenocryst assemblages vary from clinopyroxene, olivine and plagioclase along a clinopyroxene-olivine surface to olivine and plagioclase along an olivine-plagioclase surface. The normative glass compositions show a deflection from clinopyroxene-bearing to clinopyroxene-free glasses. The appearance of plagioclase together with clinopyroxene and olivine can be explained in the light of experimental investigations of the effect of pressure on phase relations. The major element variation of the glasses is interpreted as representing mantle derived magma batches of primary liquids, modified to some degree by high (6 kbar) and intermediate to low pressure (below 3 kbar) crystal fractionation towards equilibrium phase relations during ascent and residence in crustal magma chambers. The observed deflection in normative compositions of the glasses marks the position of the high pressure 3-phase cotectic line. The bimodality in size and composition of plagioclase and olivine phenocrysts can be related to high pressure crystal fractionation in the melt. The Fe-Ti basalt glasses from Sudurland are believed to be quenched high pressure compositions. 相似文献
4.
M. V. Portnyagin L. V. Danyushevsky V. S. Kamenetsky 《Contributions to Mineralogy and Petrology》1997,128(2-3):287-301
We present a detailed mineralogical, petrological and melt inclusion study of unusually fresh, primitive olivine + clinopyroxene
phyric Lower Pillow Lavas (LPL) found near Analiondas village in the northeastern part of the Troodos ophiolite (Cyprus).
Olivine phenocrysts in these primitive LPL show a wide compositional range (Fo82–92) and have higher CaO contents than those from the Upper Pillow Lavas (UPL). Cr-spinel inclusions in olivine are significantly
less Cr-rich (Cr/Cr + Al = 28–67 mol%) compared to those from the UPL (Cr# = 70–80). These features reflect differences in
melt compositions between primitive LPL and the UPL, namely higher CaO and Al2O3 and lower FeO* compared to the UPL at a given MgO. LPL parental melts (in equilibrium with Fo92) had ∼10.5 wt% MgO and crystallization temperatures ∼1210 °C, which are significantly lower than those previously published
for the UPL (14–15 wt% MgO and ∼1300 °C for Fo92). The fractionation path of LPL parental melts is also different from that of the UPL. It is characterized initially by olivine + clinopyroxene
cotectic crystallization joined by plagioclase at ∼9 wt% MgO, whereas UPL parental melts experienced a substantial interval
of olivine-only crystallization. Primitive LPL melts were formed from a mantle source which was more fertile than that of
tholeiites from well-developed intra-oceanic arcs, but broadly similar in its fertility to that of Mid-Ocean Ridge Basalt
(MORB) and Back Arc Basin Basalts (BABB). The higher degrees of melting during formation of the LPL primary melts compared
to average MORB were caused by the presence of subduction-related components (H2O). Our new data on the LPL coupled with existing data for the UPL support the existing idea that the LPL and UPL primary
melts originated from distinct mantle sources, which cannot be related by progressive source depletion. Temperature differences
between these sources (∼150 °C), their position in the mantle (∼10 kbar for the colder LPL source vs 15–18 kbar for the UPL
source), and temporal succession of Troodos volcanism, all cannot be reconciled in the framework of existing models of mantle
wedge processes, thermal structure and evolution, if a single mantle source is invoked. Possible tectonic settings for the
origin of the Troodos ophiolite (forearc regions of intra-oceanic island arc, propagation of backarc spreading into arc lithosphere)
are discussed.
Received: 20 May 1996 / Accepted: 25 March 1997 相似文献
5.
Evidence for the trapped liquid shift effect in the Mount Ayliff Intrusion,South Africa 总被引:1,自引:2,他引:1
R. Grant Cawthorn Bernhard K. Sander Ian M. Jones 《Contributions to Mineralogy and Petrology》1992,111(2):194-202
Postcumulus trapped liquid shift in layered complexes produces cumulate minerals with more fractionated compositions than the original primary phases. This effect is shown by olivine compositions from the base of the Mount Ayliff Intrusion, where varying proportions of olivine to interstitial liquid produce a suite of rocks which define a tight linear trend on a binary whole-rock plot of MgO versus FeO. Extrapolation of this trend constrains the composition of the primary cumulus olivine to the range Fo84–86, whereas olivine actually present have compositions Fo77–83. The magnitude of the discrepancy between the theoretical and observed olivine compositions correlates directly with the weight fraction of interstitial liquid. These observations are quantitatively predicted by the trapped liquid shift model. They also argue against significant migration of residual liquid. Trapped liquid shift is documented over a vertical interval of 60 m. It occurred in rocks lying only 1 m above the basal contact of the intrusion and hence must be a comparatively rapid process. 相似文献
6.
Susanne M. Straub Ana Lillian Martin-Del Pozzo 《Contributions to Mineralogy and Petrology》2001,140(4):487-510
Tephra lapilli from six explosive eruptions between April 1996 and February 1998 at Popocatepetl volcano (=Popo) in central
Mexico have been studied to investigate the causes of magma diversification in thick-crusted volcanic arcs. The tephra particles
are sparsely porphyritic (≈5 vol%) magnesian andesites (SiO2=58–65 wt%; MgO=2.6–5.9 wt%) that contain phenocrysts of NiO-rich (up to 0.67 wt% NiO) magnesian olivine (Fo89–91 cores) with inclusions of Cr-spinel (cr#=59–70), orthopyroxene (mg#=63–76), clinopyroxene (mg#=68–86), intermediate to sodic
plagioclase (An33–66), and traces of amphibole. Major and trace element systematics indicate magma mixing. The liquid mg#melt ratios inferred from the ferromagnesian phenocrysts suggest the existence of a mafic (mg#melt ≈ 72–76) and an evolved component magma (mg#melt ≈ 35–40). These component magmas form a hybrid magnesian andesite with an intermediate range of mg#melt=50–72. The mafic end member (mg#melt ≈ 72–75) is saturated with olivine and spinel and crystallizes at temperatures ≈1170–1085 °C with oxygen fugacities close
to the fayalite–magnetite–quartz buffer and elevated water contents of several wt% H2O. A likely location of crystallization is at lower crustal levels, possibly at the Moho. Olivine is followed by high-mg#
clinopyroxene which could start to crystallize during magma ascent. At depths of ≈4 to 13 km, the mafic magma mixes with an
evolved composition containing low-mg# clino- and orthopyroxene and plagioclase at a temperature of ≈950 °C. The repetitive
ascent of batches of mafic magmas spaced days to weeks apart implies multiple episodes of crystallization and magma mixing.
The tephra is similar to the Popo magnesian andesites, suggesting similar generic processes for the common lavas of the volcano.
The advantage of the tephra is that it can be used to reconstruct the composition of the mafic magma. Building on the elemental
systematics of the tephra and a comparison to the near-primary basalts from the surrounding monogenetic fields, we infer that
the Popo mafic end member is a magnesian andesite with variable, but high SiO2 contents of ≈55–62 wt% and near-primary characteristics, such as high-mg#melt of 72–75, FeO*/MgO ratios <1 (if extrapolated to an mg#melt of 72–75), and high Ni contents (=200 ppm Ni). This model implies that the typical elemental signature of the Popo andesites,
such as the low CaO, Al2O3, FeO*, high Na2O contents, and the depletion in high-field strength elements (e.g., P, Zr, Ti), are mantle source phenomena. Thus, determining
the elemental budget of the magnesian andesite, as it is prior to the modifications by crustal differentiation, is central
to quantifying the subcrustal mass fluxes beneath Popo.
Received: 13 December 1999 / Accepted: 11 August 2000 相似文献
7.
George E. Sullivan 《Contributions to Mineralogy and Petrology》1991,106(3):296-308
The chemical compositions of melt inclusions in a primitive and an evolved basalt recovered from the mid-Atlantic ridge south
of the Kane Fracture Zone (23°–24°N) are determined. The melt inclusions are primitive in composition (0.633–0.747 molar Mg/(Mg+Fe2+), 1.01–0.68 wt% TiO2) and are comparable to other proposed parental magmas except in having higher Al2O3 and lower CaO. The primitive melt inclusion compositions indicate that the most primitive magmas erupted in this region are
not near primary magma compositions. Olivine and plagioclase microphenocrysts are close to exchange equilibrium with their
respective basalt glasses, whose compositions are displaced toward olivine from 1 atm three phase saturation. The most primitive
melt inclusion compositions are close to exchange equilibrium with the anorthitic cores of zoned plagioclases (An78.3-An83.1; the hosts for the melt inclusions in plagioclase) and with olivines more forsteritic (Fo89-Fo91) than the olivine microphenocrysts (the hosts for the melt inclusions in olivine). Xenocrystic olivine analyzed is Fo89 but contains no melt inclusions. These observations indicate that olivines have exchanged components with the melt after
melt inclusion entrapment, whereas plagioclase compositions have remained the same since melt inclusion entrapment. Common
denominator element ratio diagrams and oxide versus oxide variation diagrams show that the melt inclusion compositions, which
represent liquids higher along the liquid line of descent, are related to the glass compositions by the fractionation of olivine,
plagioclase and clinopyroxene (absent from the mincral assemblage), probably occurring at elevated pressures. A model is proposed
whereby clinopyroxene segregates from the melt at elevated pressures (to account for its absence in the erupted lavas that
have the chemical imprint of clinopyroxene fractionation). Zoned plagioclases in the erupted lavas are thought to be survivors
of decompressional melting during magma ascent. Since similar primitive melt inclusions occur in olivine microphenocrysts
and in the cores of zoned plagioclases, any model must account for all phases present. 相似文献
8.
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
9.
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
10.
William P. Meurer S. Klaber Alan E. Boudreau 《Contributions to Mineralogy and Petrology》1997,130(1):81-92
Mineralogically zoned and unzoned discordant bodies composed predominately of plagioclase with up to 35% olivine, occur at
three different levels in Olivine-Bearing zones III and IV of the Middle Banded series of the Stillwater complex. The discordant
bodies are elongate perpendicular to the layering of the host cumulates with slender concordant apophyses. Although the host
olivine-gabbros are foliated with tabular plagioclase, the discordant bodies lack a discernible fabric and have blocky plagioclase.
Average olivine in the host rocks is slightly more magnesian than that of the discordant bodies (Mg#75.8 ± 0.7 versus Mg#74.6 ± 0.3 respectively) but plagioclase compositions are indistinguishable (An77.6 ± 2.0 versus An76.6 ± 4.3– average host and discordant bodies respectively). Whole-rock major- and trace-element compositions of the discordant bodies
are generally indistinguishable from cumulates with similar modal abundance. However, bulk compositions of anorthositic cores
from the discordant bodies are enriched in K, Na, Ba, Sr and P. We conclude that the discordant bodies formed when cooler
volatile fluids or fluid-rich silicate liquids moved upward and encountered a hotter undersaturated solid-plus-liquid assemblage.
Continued liquid/fluid fluxing increased the permeability along the flow path and focused the flow, allowing the original
bulk compositions to be modified and leaving plagioclase-rich troctolites and anorthosites. The shapes of the discordant bodies
suggest that the cumulus pile had anisotropic permeability during late-stage liquid/fluid flow. Chemical and mineralogical
evidence from other parts of Olivine-Bearing zones III and IV suggests that the processes that formed the discordant bodies
may have influenced other cumulates. In fact, it appears that the same processes that formed the discordant bodies operated
within an anorthositic layer, strongly modifying the chemistry of the rock but leaving no mineralogical or textural evidence.
Received: 10 December 1996 / Accepted: 12 August 1997 相似文献
11.
The influence of water on melting of mantle peridotite 总被引:47,自引:8,他引:39
This experimental study examines the effects of variable concentrations of dissolved H2O on the compositions of silicate melts and their coexisting mineral assemblage of olivine + orthopyroxene ± clinopyroxene ± spinel ± garnet.
Experiments were performed at pressures of 1.2 to 2.0 GPa and temperatures of 1100 to 1345 °C, with up to ∼12 wt% H2O dissolved in the liquid. The effects of increasing the concentration of dissolved H2O on the major element compositions of melts in equilibrium with a spinel lherzolite mineral assemblage are to decrease the
concentrations of SiO2, FeO, MgO, and CaO. The concentration of Al2O3 is unaffected. The lower SiO2 contents of the hydrous melts result from an increase in the activity coefficient for SiO2 with increasing dissolved H2O. The lower concentrations of FeO and MgO result from the lower temperatures at which H2O-bearing melts coexist with mantle minerals as compared to anhydrous melts. These compositional changes produce an elevated
SiO2/(MgO + FeO) ratio in hydrous peridotite partial melts, making them relatively SiO2 rich when compared to anhydrous melts on a volatile-free basis. Hydrous peridotite melting reactions are affected primarily
by the lowered mantle solidus. Temperature-induced compositional variations in coexisting pyroxenes lower the proportion of
clinopyroxene entering the melt relative to orthopyroxene. Isobaric batch melting calculations indicate that fluid-undersaturated
peridotite melting is characterized by significantly lower melt productivity than anhydrous peridotite melting, and that the
peridotite melting process in subduction zones is strongly influenced by the composition of the H2O-rich component introduced into the mantle wedge from the subducted slab.
Received: 7 April 1997 / Accepted: 9 January 1998 相似文献
12.
Mineral chemistry of submarine lavas from Hilo Ridge, Hawaii: implications for magmatic processes within Hawaiian rift zones 总被引:10,自引:0,他引:10
Huai-Jen Yang Frederick A. Frey David A. Clague Michael O. Garcia 《Contributions to Mineralogy and Petrology》1999,135(4):355-372
The crustal history of volcanic rocks can be inferred from the mineralogy and compositions of their phenocrysts which record
episodes of magma mixing as well as the pressures and temperatures when magmas cooled. Submarine lavas erupted on the Hilo
Ridge, a rift zone directly east of Mauna Kea volcano, contain olivine, plagioclase, augite ±orthopyroxene phenocrysts. The
compositions of these phenocryst phases provide constraints on the magmatic processes beneath Hawaiian rift zones. In these
samples, olivine phenocrysts are normally zoned with homogeneous cores ranging from ∼ Fo81 to Fo91. In contrast, plagioclase, augite and orthopyroxene phenocrysts display more than one episode of reverse zoning. Within each
sample, plagioclase, augite and orthopyroxene phenocrysts have similar zoning profiles. However, there are significant differences
between samples. In three samples these phases exhibit large compositional contrasts, e.g., Mg# [100 × Mg/(Mg+Fe+2)] of augite varies from 71 in cores to 82 in rims. Some submarine lavas from the Puna Ridge (Kilauea volcano) contain phenocrysts
with similar reverse zonation.
The compositional variations of these phenocrysts can be explained by mixing of a multiphase (plagioclase, augite and orthopyroxene)
saturated, evolved magma with more mafic magma saturated only with olivine. The differences in the compositional ranges of
plagioclase, augite and orthopyroxene crystals between samples indicate that these samples were derived from isolated magma
chambers which had undergone distinct fractionation and mixing histories. The samples containing plagioclase and pyroxene
with small compositional variations reflect magmas that were buffered near the olivine + melt ⇒Low-Ca pyroxene + augite +
plagioclase reaction point by frequent intrusions of mafic olivine-bearing magmas. Samples containing plagioclase and pyroxene
phenocrysts with large compositional ranges reflect magmas that evolved beyond this reaction point when there was no replenishment
with olivine-saturated magma. Two of these samples contain augite cores with Mg# of ∼71, corresponding to Mg# of 36–40 in
equilibrium melts, and augite in another sample has Mg# of 63–65 which is in equilibrium with a very evolved melt with a Mg#
of ∼30. Such highly evolved magmas also exist beneath the Puna Ridge of Kilauea volcano. They are rarely erupted during the
shield building stage, but may commonly form in ephemeral magma pockets in the rift zones.
The compositions of clinopyroxene phenocryst rims and associated glass rinds indicate that most of the samples were last equilibrated
at 2–3 kbar and 1130–1160 °C. However, in one sample, augite and glass rind compositions reflect crystallization at higher
pressures (4–5 kbar). This sample provides evidence for magma mixing at relatively high pressures and perhaps transport of
magma from the summit conduits to the rift zone along the oceanic crust-mantle boundary.
Received: 8 July 1998 / Accepted: 2 January 1999 相似文献
13.
MORB suites display variations in their chemical differentiation trends which are closely related to the incompatible element enrichment of the basalts. We examine suites of primitive to evolved basalts from the Pacific-Nazca Ridge at 28° S (mostly depleted); from the Juan Fernandez microplate region (depleted) and from the Explorer Ridge, northeast Pacific (mostly enriched). Trends for incompatible element enriched MORBs consistently show less depletion of Al2O3 and less enrichment of FeO when plotted on MgO variation diagrams.Least squares modeling indicates that enriched basalts have undergone less plagioclase crystallization than depleted basalts especially in the early stages of differentiation. Using thermodynamic modelling, we show that variations between MORB differentiation trends result largely from differences in the major element chemistry and H2O content of primary magmas. Our chosen enriched and depleted near-primary magmas are similar in major element chemistry but the enriched near-primary magma has higher H2O and lower Al2O3 than the depleted near-primary magma. The MORB crystallization sequence is: olivineolivine+plagioclase olivine+plagioclase+high-Ca pyroxene; and the separate and combined effects of lower Al2O3 and higher H2O are to cause plagioclase to crystallize later (lower temperature), and to make the interval of olivine+plagioclase crystallization shorter. As a result, enriched differentiates have higher Al2O3 and lower FeO than depleted MORBs at a given MgO content, even though their parents' Al2O3 is lower. Crystallization of enriched basalts at higher pressure than depleted basalts is not able to account for differences between the differentiation trends because the proportion of plagioclase is higher during three-phase crystallization at high pressure.The variations in trends do not depend on geographic location and thus are superimposed on any regional variations in MORB chemistry or mantle source. Nor are they related to spreading rate. Depleted basalts from the fast-spreading 28° S and Juan Fernandez ridges have differentiation trends similar to depleted basalts from the medium-spreading Galapagos Spreading Center, whereas differentiation trends for enriched basalts from the medium-spreading Explorer Ridge are quite different. Fe3+/Fetotal is similar (and quite low) for enriched and depleted basalts, indicating that neither oxidation state nor early magnetite crystallization are important. 相似文献
14.
Eucrites are extraterrestrial plagioclase-pigeonite basalts. Experimental studies suggest that they were produced by partial melting of an olivine (Fo65)-pigeonite (Wo5En65)-plagioclase (An94)-spinel-metal source region. Quantitative modeling of the evolution of REE abundances in the eucrites indicates that the main group of eucrites (e.g. Juvinas) may be produced by approximately 10% equilibrium partial melting of a source region with initial REE abundances which were chondritic relative and absolute. Other eucrites appear to represent greater (e.g. Sioux County—15%) or smaller (e.g. Stannern—4%) degrees of melting. Moore County and Serra de Magé appear to be cumulates of pyroxene and plagioclase produced by fractional crystallization of a Juvinas-like melt. Nuevo Laredo may represent a residual liquid after such fractional crystallization. Our calculations are consistent with the conclusion that the eucrites were derived from a single type of source region. The close correspondence of the age of the eucrites (? 4.6 AE) to the age of the solar system appears to preclude the possibility of extensive chemical differentiation of the eucrite parent body prior to the event which produced the eucritic melts. Thus our calculations have yielded not only the mode of the source region but, assuming homogeneous accretion, the mode and hence the bulk composition of the eucrite parent body as well. We are unable to estimate quantitatively the ratio of metal to olivine in the parent body. If no metal is present, the bulk composition (in oxide wt%) is Na2O—0.04, MgO—29.7, Al2O3—1.8, SiO2—39.0, CaO—1.2, FeO—28.3. If, in contrast, the parent body contained 30% metal, the bulk composition of the silicate portion of the eucrite parent body is Na2O—0.06, MgO—28.0, Al2O3—2.6, SiO2—41.3, CaO—1.9, FeO—26.3. Relative abundances of the meteorites suggest that the eucrite parent body is still intact. The solar system object most closely resembling the eucrites is asteroid 4 Vesta. Because Vesta is unique among the asteroids, we have license to conclude that it is the source of the eucrites and its bulk composition is close to the analyses given above. 相似文献
15.
The origin of island arc high-alumina basalts 总被引:5,自引:1,他引:5
A. J. Crawford T. J. Falloon S. Eggins 《Contributions to Mineralogy and Petrology》1987,97(3):417-430
A detailed examination of the hypothesis that high-alumina basalts (HAB) in island arcs are primary magmas derived by 50–60% partial melting of subducted ocean crust eclogite shows that this model is unlikely to be viable. Evidence suggests that the overwhelming majority of arc HAB are porphyritic lavas, enriched in Al2O3 either by protracted prior crystallization of olivine and clinopyroxene, or by plagioclase phenocryst accumulation in magmas of basaltic andesite to dacite composition. Experimentally-determined phase relationships of such plagioclase-enriched (non-liquid) compositions have little bearing on the petrogenesis of arc magmas, and do not rule out the possibility that arc HAB can be derived by fractionation of more primitive arc lavas. Although models invoking eclogite-melting can match typical arc HAB REE patterns, calculations indicate that the Ni and Cr contents of proposed Aleutian primary HAB are many times lower than such models predict. In contrast, Ni vs Sc and Cr vs Sc trends for arc HAB are readily explained by olivine (+Cr-sp) and clinopyroxene-dominated fractionation from more primitive arc magmas. GENMIX major element modelling of several HAB compositions as partial melts of MORB eclogite, using appropriate experimentally (26–34 kb)-determined garnet and omphacite compositions yields exceptionally poor matches, especially for CaO, Na2O, MgO and Al2O3. These mismatches are easily explained if the HAB are plagioclase-accumulative. Groundmasses of arc HAB are shown to vary from basaltic andesite to dacite in composition. Crystal fractionation driving liquid compositions toward dacite involves important plagioclase separation, resulting in development of significant negative Eu anomalies in more evolved lavas. Plagioclase accumulation in such evolved liquids tends to diminish or eliminate negative Eu anomalies. Therefore, the absence of positive Eu anomaly in a plagioclase-phyric HAB does not indicate that plagioclase has not accumulated in that lava. In addition, we show that plagioclase phenocrysts in arc HAB are not in equilibrium with the liquids in which they were carried (groundmass). Exceptional volumes of picrite and olivine basalt occur in the Solomons and Vanuatu arcs; the presence in lavas from these and other arcs (Aleutian, Tonga) of olivine phenocrysts to Fo94, finds no ready explanation in the primary eclogite-derived HAB model. We suggest that most lavas in intra-oceanic arcs are derived from parental magmas with >10% MgO; fractionation of olivine (+Cr-sp) and clinopyroxene drives liquids to basalt compositions with <7% MgO, but plagioclase nucleation is delayed by their low but significant (<1%?) H2O contents. Thus evolved liquid compositions in the basaltic andesite—andesite range may achieve relatively high Al2O3 contents (<17.5%). The majority of arc basalts, however, have Al2O3 contents in excess of 18%, reflecting plagioclase accumulation. We give new experimental data to show that HAB liquids may be generated by anhydrous, low-degree (<10%) partial melting of peridotite at P<18 kb. Relative to arc HAB, these experimental melts have notably higher Mg#(69–72) and are in equilibrium with olivine Fo87–89. Only further detailed trace element modelling will show if they might be parental magmas for some arc HAB. 相似文献
16.
Equilibrium volumes and expansivities of three liquids in the system anorthite (CaAl2Si2O8)–diopside (CaMgSi2O6) have been derived from dilatometric measurements of the equilibrium length of samples in the glass transition range. The
typical temperature range of 40 K for the measurements is limited at low temperature by the very long times necessary to reach
structural equilibrium and at high temperature by the penetration of the rod used to measure sample dilatation. Despite such
narrow intervals, the expansivities are determined to better than 3% thanks to the high precision with which length changes
are measured. The coefficient of volume thermal expansion (1/V dV/dT) of the fully relaxed liquid just above the glass transition
is found to decrease linearly from diopside composition (139 ± 4 × 10−6 K−1) to anorthite composition (59 ± 2 × 10−6 K−1). These values are greater than those determined for the same liquids at superliquidus temperatures, demonstrating that expansivities
of silicate melts may decrease markedly with increasing temperature. A predictive model based upon partial molar volumes which
vary as a linear function of the logarithm of temperature is proposed.
Received: 25 February 2000 / Accepted: 29 May 2000 相似文献
17.
Summary Mineral compositions in leucite-bearing and leucite-free rocks from Vico volcano are reported. FeO/MgO partitioning (Kdol/liq) between olivine and latite (0.14–0.22), and between olivine and trachyte (0.06–0.10) indicates a lack of equilibrium between
mineral and host rock. This suggests that mingling and/or mixing between magmas was a leading process during magmatic differentiation.
In addition, a phono-tephrite olivine population with high (0.84) and equilibrium (0.23–0.29) Kdol/liq values has been produced by the interaction of differently evolved magmas. Zoning in clinopyroxene and plagioclase from these
rocks recorded the same processes. In addition, resorbed quartz xenocrysts with coronas of clinopyroxene microlites indicate
that digestion of crustal rocks occurred during the residence of magma in a shallow level reservoir. Increasing Fe coupled
with decreasing Ca in diopside crystals from some phonolites, together with the petrographic and trace element data, indicate
that polybaric fractional crystallisation also may be involved in the genesis of magmas of the second period of Vico activity.
Leucite-free trachybasalts erupted in a late stage contain highly forsteritic olivine phenocrysts (forsterite 84–88 mol.%)
in-equilibrium (Kdol/liq = 0.24–0.35) with the host rock, which indicate that they did not suffer chemical modification at low pressure.
Received November 28, 2000; revised version accepted September 27, 2001 相似文献
18.
Dawnika L. Blatter Ian S. E. Carmichael 《Contributions to Mineralogy and Petrology》1998,132(2):121-138
Approximately 150 km west of Mexico City in the central part of the Mexican Volcanic Belt (MVB) near Zitácuaro, Mexico, young
volcanism has produced shield volcanoes, large volume silicic deposits, and fault-related basalt and andesite lava flows and
cinder cones. This paper concerns a small cluster of Pleistocene andesite cones and flows which can be separated into two
distinct groups: high-magnesium andesites (>6% MgO, 57–59% SiO2), conveniently called basaltic andesites, with phenocrysts of orthopyroxene and augite, or augite and olivine; and andesites
(60–62% SiO2, <4.6% MgO), which have phenocrysts of orthopyroxene and augite, and ghosts of relict hornblende. Remarkably, plagioclase
phenocrysts are absent, and evenly distributed but sparse (0.5–3.5%) quartz xenocrysts are present in all the lavas. In order
to establish the conditions under which early crystallizing plagioclase is suppressed in these lavas, water saturated experiments
up to 3 kbars were performed on one of the basaltic andesites. The conditions required to reproduce the phenocryst assemblages
(either olivine + augite or opx + augite) are temperatures in excess of 1000 °C, with water saturated liquids (>3 wt%) at
pressures of about 1 kbar. Compared to basaltic andesites of western Mexico, the Zitácuaro basaltic andesites have ∼2 wt%
lower Al2O3 concentrations, which causes plagioclase to precipitate at significantly lower temperatures, and it therefore follows the
crystallization sequence: olivine, augite, and orthopyroxene. Based on ubiquitous quartz xenocrysts, with glassy rhyolitic
inclusions, a reasonable conclusion is that substantial mixing of a quartz-bearing rhyolitic magma with a parental basaltic
andesite has occurred at low pressure (shallow depth), and this would account for the low Al2O3 concentrations in the Zitácuaro basaltic andesites. Whatever the mechanism of incorporation, the quartz xenocrysts are evidence
of contamination of basaltic magma with more siliceous material, thus making it difficult to use these magmas as indicators
of mantle melting processes.
Received: 29 July 1997 / Accepted: 29 January 1998 相似文献
19.
Dr. Norman J. Page 《Contributions to Mineralogy and Petrology》1967,14(4):321-342
The Burro Mountain ultramafic complex, Monterey County, California, consists of dunites and peridotites which are partially or wholly serpentinized. Primary minerals in both rock types are olivine, enstatite, diopside, and picotite which upon alteration yield chrysotile, lizardite, brucite, magnetite, talc, tremolite, and carbonate. Electron microprobe analyses show that enstatite, En85.8 to En90.8, alters to “bastite” composed only of lizardite (5.0–12.0 weight percent FeO), whereas olivine, Fo90.8 to Fo91.6, forms lizardite+chrysotile+brucite with or without magnetite. The chrysotile ranges from 3.0 to 5.0 weight percent FeO, the brucite from 16.0 to 43.0 weight percent FeO. As Serpentinization proceeds, the alteration products are enriched in FeO relative to MgO. Serpentinization probably originates in a changing \(P_{O_2 }\)-T environment by two different reactions:
- (a)Olivine+enstatite+H2O+O2?Mg, Fe+2 chrysotile+Mg, Fe+3, Fe+2 lizardite with or without magnetite. 相似文献
20.
S. Maaløe 《Mineralogy and Petrology》2005,85(3-4):163-204
Summary The investigated mantle section of the Leka ophiolite complex extends 1.4 km from and 1.1 km along the exposed Moho. The foliated
peridotite contains numerous tabular and elongated dunite bodies, orthopyroxenite dikes, websterite veins, and dikes. The
foliation of the peridotite is inclined by about 45° to the Moho. The dunite bodies and the dikes cut the foliation at low
angles. The dunite bodies vary in width from 0.1 to 50 m and in length from 10 m to more than 1 km. Wider dunite bodies are
commonly surrounded by 0 to 1.0 m wide margins of dunitized peridotite. Websterite veins may be present outside these margins.
Apart from sporadic chromite layers the dunite is very homogenous. The dunite bodies are considered to have formed by deposition
of olivine along the walls of dikes originally containing tholeiitic melt. The tholeiitic melt at first heated the peridotitic
sidewalls so that they became partially molten and dunitized. The ascending magma then eroded the sidewalls and removed olivine
as xenocrysts. When the ascent rate decreased, the temperature of the sidewalls decreased, so that olivine (Fo89–92) began to crystallize along the dike walls. There is also evidence for percolative melt migration along foliation planes,
however, the largest proportion of the melts intruded along dikes.
The websterite dikes are mostly 1 to 4 cm wide and 3 to 20 m long and dispersed with mutual distances of 20–50 m. The websterite
veins and dikes probably originated from melts that were generated along the heated sidewalls of the dunite bodies. The 0.02
to 10 m wide orthopyroxenite dikes have exceptionally high MgO contents for their SiO2 contents; about 36 wt.% MgO and 50 wt.% SiO2. They may have formed as segregates from a SiO2-rich magma, although the parent magma does not appear to have been boninitic. The parent magma may instead have formed by
second stage partial melting of depleted lherzolite. 相似文献