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1.
Investigations of peridotite xenolith suites have identified a compositional trend from lherzolite to magnesian wehrlite
in which clinopyroxene increases at the expense of orthopyroxene and aluminous spinel, and in which apatite may be a minor
phase. Previous studies have shown that this trend in mineralogy and chemical composition may result from reaction between
sodic dolomitic carbonatite melt and lherzolite at pressures around 1.7 to 2 GPa. This reaction results in decarbonation of
the carbonatite melt, releasing CO 2-rich fluid. In this study, we have experimentally reversed the decarbonation reaction by taking two natural wehrlite compositions
and reacting them with CO 2 at a pressure of 2.2 GPa and temperatures from 900 to 1150° C. Starting materials were pargasite-bearing wehrlites, one with
minor apatite (composition 71001 *) and one without apatite (composition 70965 *). At lower temperatures (900° C) the products were apatite+pargasite+magnesite harzburgite for runs using composition 71001 *, and pargasite+dolomite lherzolite for runs using composition 70965 *. At and above 1000° C, carbonatite melt with harzburgite residue (olivine+orthopyroxene+spinel) and with lherzolite residue
(olivine+orthopyroxene+clinopyroxene+ spinel) were produced respectively. Phase compositions in reactants and products are
consistent with the documented carbonatite/lherzolite reactions, and also permit estimation of the carbonatite melt compositions.
In both cases the melts are sodic dolomitic carbonatites. The study supports the hypothesis of a significant role for ephemeral,
sodic dolomitic melts in causing metasomatic changes in the lithosphere at P≤2 GPa. The compositions of wehrlites imply fluxes of CO 2, released by metasomatic reactions, which are locally very large at around 5 wt% CO 2.
Received: 15 December 1995/Accepted: 14 February 1996 相似文献
2.
Li contents and its isotopes of minerals in mantle peridotite xenoliths from late Cretaceous mafic dikes, analyzed in situ
by Cameca IMS-1280, reveal the existence of melt/rock interaction in remains of refertilized Archean lithospheric mantle in
Qingdao, Jiaodong Peninsula, North China Craton. Two groups of peridotites exist, i.e., low-Mg# lherzolite and high-Mg# harzburgites.
The low-Mg# lherzolite has a relatively homogeneous Li concentration (ol: 2.01–2.11 ppm; opx: 1.77–1.88 ppm; cpx: 1.75–1.93 ppm)
and Li isotopic composition (δ 7Li in ol: 4.2–7.6‰; in opx: 6.0–8.3‰; in cpx: 5.3–8.4‰). The similarity in δ 7Li value to the fresh MORB provides further evidence for the argument that the low-Mg# lherzolite could be the fragment of
the newly accreted lithospheric mantle. The high-Mg# harzburgites have heterogeneous Li abundances (ol: 0.83–2.09 ppm; opx:
0.92–1.94 ppm; cpx: 1.12–4.89 ppm) and Li isotopic compositions (δ 7Li in ol: −0.5 to +11.5‰; in opx: −6.2 to +11.1‰; in cpx: −34.3 to +10.1‰), showing strong disequilibrium in Li partitioning
and Li isotope fractionation between samples. The cores of most minerals in these high-Mg# harzburgites have relatively homogeneous
δ 7Li values, which are higher than those of fresh MORB, but similar to those previously reported for arc lavas. These harzburgites
have enriched trace elemental and Sr–Nd isotopic compositions. These observations indicate that in the early Mesozoic the
lithospheric mantle beneath the southeastern North China Craton was similar to that in arc settings, which is metasomatized
by subducted crustal materials. Extremely low δ 7Li preserved in cpxs requires diffusive fractionation of Li isotopes from later-stage melt into the minerals. Thus, the Li
data provide further evidence that the Archean refractory lithospheric mantle represented by the high-Mg# harzburgites was
refertilized through melt/rock interaction and transformed to the Mesozoic less refractory and incompatible element and Sr–Nd
isotopes enriched lithospheric mantle. 相似文献
3.
The distribution of rare earth elements (REE) between clinopyroxene (cpx) and basaltic melt is important in deciphering the
processes of mantle melting. REE and Y partition coefficients from a given cpx-melt partitioning experiment can be quantitatively
described by the lattice strain model. We analyzed published REE and Y partitioning data between cpx and basaltic melts using
the nonlinear regression method and parameterized key partitioning parameters in the lattice strain model ( D
0, r
0 and E) as functions of pressure, temperature, and compositions of cpx and melt. D
0 is found to positively correlate with Al in tetrahedral site (Al
T
) and Mg in the M2 site (Mg M2) of cpx and negatively correlate with temperature and water content in the melt. r
0 is negatively correlated with Al in M1 site (Al M1) and Mg M2 in cpx. And E is positively correlated with r
0. During adiabatic melting of spinel lherzolite, temperature, Al
T
, and Mg M2 in cpx all decrease systematically as a function of pressure or degree of melting. The competing effects between temperature
and cpx composition result in very small variations in REE partition coefficients along a mantle adiabat. A higher potential
temperature (1,400°C) gives rise to REE partition coefficients slightly lower than those at a lower potential temperature
(1,300°C) because the temperature effect overwhelms the compositional effect. A set of constant REE partition coefficients
therefore may be used to accurately model REE fractionation during partial melting of spinel lherzolite along a mantle adiabat.
As cpx has low Al and Mg abundances at high temperature during melting in the garnet stability field, REE are more incompatible
in cpx. Heavy REE depletion in the melt may imply deep melting of a hydrous garnet lherzolite. Water-dependent cpx partition
coefficients need to be considered for modeling low-degree hydrous melting. 相似文献
4.
We performed modified iterative sandwich experiments (MISE) to determine the composition of carbonatitic melt generated near
the solidus of natural, fertile peridotite + CO 2 at 1,200–1,245°C and 6.6 GPa. Six iterations were performed with natural peridotite (MixKLB-1: Mg# = 89.7) and ∼10 wt% added
carbonate to achieve the equilibrium carbonatite composition. Compositions of melts and coexisting minerals converged to a
constant composition after the fourth iteration, with the silicate mineral compositions matching those expected at the solidus
of carbonated peridotite at 6.6 GPa and 1,230°C, as determined from a sub-solidus experiment with MixKLB-1 peridotite. Partial
melts expected from a carbonated lherzolite at a melt fraction of 0.01–0.05% at 6.6 GPa have the composition of sodic iron-bearing
dolomitic carbonatite, with molar Ca/(Ca + Mg) of 0.413 ± 0.001, Ca# [100 × molar Ca/(Ca + Mg + Fe*)] of 37.1 ± 0.1, and Mg#
of 83.7 ± 0.6. SiO 2, TiO 2 and Al 2O 3 concentrations are 4.1 ± 0.1, 1.0 ± 0.1, and 0.30 ± 0.02 wt%, whereas the Na 2O concentration is 4.0 ± 0.2 wt%. Comparison of our results with other iterative sandwich experiments at lower pressures indicate
that near-solidus carbonatite derived from mantle lherzolite become less calcic with increasing pressure. Thus carbonatitic
melt percolating through the deep mantle must dissolve cpx from surrounding peridotite and precipitate opx. Significant FeO*
and Na 2O concentrations in near solidus carbonatitic partial melt likely account for the ∼150°C lower solidus temperature of natural
carbonated peridotite compared to the solidus of synthetic peridotite in the system CMAS + CO 2. The experiments demonstrate that the MISE method can determine the composition of partial melts at very low melt fraction
after a small number of iterations. 相似文献
5.
Mantle xenoliths hosted by the Historic Volcan de San Antonio, La Palma, Canary Islands, fall into two main group. Group I consists of spinel harzburgites, rare spinel lherzolites and spinel dunites, whereas group II comprises spinel wehrlites, amphibole wehrlites, and amphibole clinopyroxenites. We here present data on group I xenoliths,
including veined harzburgites and dunites which provide an excellent basis for detailed studies of metasomatic processes.
The spinel harzburgite and lherzolite xenoliths have modal ol−opx−cpx ratios and mineral and whole rock major element chemistry
similar to those found in Lanzarote and Hierro, and are interpreted as highly refractory, old oceanic lithospheric mantle.
Spinel dunites are interpreted as old oceanic peridotite which has been relatively enriched in olivine and clinopyroxene (and
highly incompatible elements) through reactions with basaltic Canarian magmas, with relatively high melt/peridotite ratio.
Group I xenoliths from La Palma differ from the Hierro and Lanzarote ones by a frequent presence of minor amounts of phlogopite (and
amphibole). Metasomatic processes are also reflected in a marked enrichment of strongly incompatible relative to moderately
incompatible trace elements, and in a tendency for Fe−Ti enrichment along grain boundaries in some samples. The veins in the
veined xenoliths show a gradual change in phase assemblage and composition of each phase, from Fe−Ti-rich amphibole+augite+Fe−Ti-oxides+apatite+basaltic
glass, to Ti-poor phlogopite+Cr-diopside±chromite+ Si−Na−K-rich glass+fluid. Complex reaction zones between veins and peridotite
include formation of clinopyroxene±olivine+glass at the expense of orthopyroxene in harzburgite, and clinopyroxene+spinel±amphibole±glass
at the expense of olivine in dunite. The dramatic change in glass composition from the broadest to the narrowest veins includes
increasing SiO 2 from 44 to 67 wt%, decreasing TiO 2/Al 2O 3 ratio from >0.24 to about 0.02, and increasing K 2O and Na 2O from 1.8 to >7.0 wt% and 3.8 to 6.7 wt%, respectively. The petrographic observations supported by petrographic mixing calculations
indicate that the most silicic melts in the veined xenoliths formed as the result of reaction between infiltrating basaltic
melt and peridotite wall-rock. The highly silicic, alkaline melt may represent an important metasomatic agent. Pervasive metasomatism
by highly silicic melts (and possibly fluids unmixed from these) may account for the enriched trace element patterns and frequent
presence of phlogopite in the upper mantle under La Palma.
Received: 15 January 1996 / Accepted 30 May 1996 相似文献
6.
Despite the growing interest for Li and B as geochemical tracers, especially for material transfer from subducting slabs to
overlying peridotites, little is known about the behaviour of these two elements during partial melting of mantle sources.
In particular, mineral/melt partition coefficients for B and to a lesser extent Li are still a matter of debate. In this work,
we re-equilibrated a synthetic basalt doped with ~10 ppm B and ~6 ppm Li with an olivine powder from a spinel lherzolite xenolith
at 1 GPa–1,330°C, and we analyzed Li and B in the run products by secondary ion mass spectrometry (SIMS). In our experiment,
B behaved as a highly incompatible element with mineral/melt partition coefficients of the order of 10 −2 ( D
ol/melt = 0.008 (0.004–0.013); D
opx/melt = 0.024 (0.015–0.033); D
cpx/melt = 0.041 (0.021–0.061)), and Li as a moderately incompatible element ( D
ol/melt = 0.427 (0.418–0.436); D
opx/melt = 0.211 (0.167–0.256); D
cpx/melt = 0.246 (0.229–0.264)). Our partition coefficients for Li are in good agreement with previous determinations. In the case
of B, our partition coefficients are equal within error to those reported by Brenan et al. ( 1998) for all the mineral phases analyzed, but are lower than other coefficients from literature for some of the phases (up to
5 times for cpx). Our measurements complement the data set of Ds for modelling partial melting of the upper mantle and basalt generation, and confirm that, in this context, B is more incompatible
than previously anticipated. 相似文献
7.
Using various methods of melt inclusion investigation, including electron and ion microprobe techniques, we estimated the
composition, evolution, and formation conditions of melts producing the trachydacites and pantellerites of the Late Paleozoic
bimodal volcanic association of Dzarta-Khuduk, Central Mongolia. Primary crystalline and melt inclusions were detected in
anorthoclase from trachydacites and quartz from pantellerites and pantelleritic tuffs. Among the crystalline inclusions, we
identified hedenbergite, fluorapatite, and pyrrhotite in the trachydacites and F-arfvedsonite, fluorite, ilmenite, and the
rare REE diorthosilicate chevkinite in the pantellerites. Melt inclusions in anorthoclase from the trachydacites are composed
of glass, a gas phase, and daughter minerals (F-arfvedsonite, fluorite, villiaumite, and anorthoclase rim on the inclusion
wall). Melt inclusions in quartz from the pantellerites are composed of glass, a gas phase, and a fine-grained salt aggregate
consisting of Li, Na, and Ca fluorides (griceite, villiaumite, and fluorite). Melt inclusions in quartz crystalloclasts from
the pantelleritic tuffs are composed of homogeneous silicate glasses. The phenocrysts of the trachydacites and pantellerites
crystallized at temperatures of 1060–1000°C. During thermometric experiments with quartz-hosted melt inclusions from the pantellerites,
the formation of immiscible silicate and salt (fluoride) melts was observed at a temperature of 800°C. Homogeneous melt inclusions
in anorthoclase from the trachydacites have both trachydacite and rhyolite compositions (wt %): 68–70 SiO 2, 12–13 Al 2O 3, 0.34–0.74 TiO 2, 5–7 FeO, 0.4–0.9 CaO, and 9–12 Na 2O + K 2O. The agpaitic index ranges from 0.92 to 1.24. The glasses of homogenized melt inclusions in quartz from the pantellerites
and pantelleritic tuffs have rhyolitic compositions. Compared with the homogeneous glasses trapped in anorthoclase of the
trachydacites, quartz-hosted inclusions from the pantellerites show higher SiO 2 (72–78 wt %) and lower Al 2O 3 contents (7.8–10.0 wt %). They also contain 0.14–0.26 wt % TiO 2, 2.5–4.9 wt % FeO, 9–11 wt % Na 2O + K 2O, and 0.9–0.15 wt % CaO and show an agpaitic index of 1.2–2.05. Homogeneous melt inclusions in quartz from the pantelleritic
tuffs contain 69–72 wt % SiO 2. The contents of other major components, including TiO 2, Al 2O 3, FeO, and CaO, are close to those in the homogeneous glasses of quartzhosted melt inclusions in the pantellerites. The contents
of Na 2O + K 2O are 4–10 wt %, and the agpaitic index is 1.0–1.6. The glasses of melt inclusions from each rock group show distinctive volatile
compositions. The H 2O content is up to 0.08 wt % in anorthoclase of the trachydacites, 0.4–1.4 wt % in quartz of the pantellerites, and up to
5 wt % in quartz of the pantelleritic tuffs. The content of F in the glasses of melt inclusions in the phenocrysts of the
trachydacites is no higher than 0.67 wt %, and up to 1.4–2.8 wt % in quartz from the pantellerites. The Cl content is up to
0.2 wt % in the glasses of melt inclusions in the minerals of the trachydacites and up to 0.5 wt % in the glasses of quartz-hosted
melt inclusions from the pantellerites. The investigation of trace elements in the homogenized glasses of melt inclusions
in minerals showed that the trachydacites and pantellerites were formed from strongly evolved rare-metal alkaline silicate
melts with high contents of Li, Zr, Rb, Y, Hf, Th, U, and REE. The analysis of the composition of homogeneous melt inclusions
in the minerals of the above rocks allowed us to distinguish magmatic processes resulting in the enrichment of these rocks
in trace and rare earth elements. The most important processes are the crystallization differentiation and immiscible separation
of silicate and fluoride salt melts. It was also shown that all the melts studied evolved in spatially separated magma chambers.
This caused the differences in the character of melt evolution between the trachydacites and pantellerites. During the final
stages of differentiation, when the magmatic system was saturated with respect to ore elements, Na-Ca fluoride melts were
separated and extracted considerable amounts of Li. 相似文献
8.
Peridotite xenoliths found in Cenozoic alkali basalts of northern Victoria Land, Antarctica, vary from fertile spinel-lherzolite
to harzburgite. They often contain glass-bearing pockets formed after primary pyroxenes and spinel. Few samples are composite
and consist of depleted spinel lherzolite crosscut by amphibole veins and/or lherzolite in contact with poikilitic wehrlite.
Peridotite xenoliths are characterized by negative Al 2O 3–Mg# and TiO 2–Mg# covariations of clino- and orthopyroxenes, low to intermediate HREE concentrations in clinopyroxene, negative Cr–Al trend
in spinel, suggesting variable degrees of partial melting. Metasomatic overprint is evidenced by trace element enrichment
in clinopyroxene and sporadic increase of Ti–Fe tot. Preferential Nb, Zr, Sr enrichments in clinopyroxene associated with high Ti–Fe tot contents constrain the metasomatic agent to be an alkaline basic melt. In composite xenoliths, clinopyroxene REE contents
increase next to the veins suggesting metasomatic diffusion of incompatible element. Oxygen isotope data indicate disequilibrium
conditions among clinopyroxene, olivine and orthopyroxene. The highest δ 18O values are observed in minerals of the amphibole-bearing xenolith. The δ 18O cpx correlations with clinopyroxene modal abundance and geochemical parameters (e.g. Mg# and Cr#) suggest a possible influence
of partial melting on oxygen isotope composition. Thermobarometric estimates define a geotherm of 80°C/GPa for the refractory
lithosphere of NVL, in a pressure range between 1 and 2.5 GPa. Clinopyroxene microlites of melt pockets provide P–T data close
to the anhydrous peridotite solidus and confirm that they originated from heating and decompression during transport in the
host magma. All these geothermometric data constrain the mantle potential temperature to values of 1250–1350°C, consistent
with the occurrence of mantle decompressional melting in a transtensive tectonic regime for the Ross Sea region. 相似文献
9.
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% H 2O 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 H 2O in depressing plagioclase crystallization leads to a direct relation between the Al 2O 3 content of the residual melt and its H 2O concentration, calibrated as a geohygrometer. The most primitive phenocryst assemblage in the Soufriere suite (Fo 89.6 olivine, Mg-, Al- and Ti-rich clinopyroxene, Cr–Al spinel) crystallized from near-primary (Mg# = 73.5), hydrous (∼5 wt% H 2O) 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 H 2O. Primitive basaltic liquids at Soufriere, St Vincent, have a wide range of H 2O concentrations (2–5 wt%). 相似文献
10.
Ophiolite complexes, formed in a suprasubduction zone environment during Neoproterozoic time, are widely distributed in the
Eastern Desert of Egypt. Their mantle sections provide important information on the origin and tectonic history of ocean basins
these complexes represent. The geochemistry and mineralogy of the mantle section of the Wizer ophiolite complex, represented
by serpentinites after harzburgite containing minor dunite bodies, are presented. Presence of antigorite together with the
incipient alteration of chromite and absence of chlorite suggests that serpentinization occurred in the mantle wedge above
a Neoproterozoic subduction zone. Wizer peridotites have a wide range of spinel compositions. Spinel Cr# [100Cr/(Cr + Al)]
decrease gradually from dunite bodies (Cr# = 81–87) and their host highly depleted harzburgites (Cr# = 67–79) to the less
depleted harzburgites (Cr# = 57–63). Such decreases in mantle refractory character are accompanied by higher Al and Ti contents
in bulk compositions. Estimated parental melt compositions point to an equilibration with melts of boninitic composition for
the dunite bodies (TiO 2 = ~<0.07–0.22 wt%; Al 2O 3 = 9.4–10.6 wt%), boninitic-arc tholeiite for the highly depleted harzburgites (TiO 2 = <0.09–0.28 wt%; Al 2O 3 = 11.2–14.1 wt%) and more MORB-like affinities for the less depleted harzburgites (TiO 2 = ~<0.38–0.51 wt%; Al 2O 3 = 14.5–15.3 wt%). Estimated equilibrium melts are found in the overlying volcanic sequence, which shows a transitional MORB–island
arc geochemical signature with a few boninitic samples. Enrichment of some chromites in TiO 2 and identification of sulfides in highly depleted peridotites imply interaction with an impregnating melt. A two-stage partial
melting/melt–rock reaction model is advocated, whereby, melting of a depleted mantle source by reaction with MORB-like melts
is followed by a second stage melting by interaction with melts of IAT–boninitic affinities in a suprasubduction zone environment
to generate the highly depleted harzburgites and dunite bodies. The shift from MORB to island arc/boninitic affinities within
the mantle lithosphere of the Wizer ophiolite sequence suggests generation in a protoarc-forearc environment. This, together
with the systematic latitudinal change in composition of ophiolitic lavas in the Central Eastern Desert (CED) of Egypt from
IAT–boninitic affinities to more MORB-like signature, implies that the CED could represent a disrupted forearc-arc-backarc
system above a southeast-dipping subduction zone. 相似文献
11.
Seven alkali basalt centers in the southern Canadian Cordilleracontain mantle xenolith suites that comprise spinel Cr-diopsideperidotites, spinel augite-bearing wehrlites and orthopyroxene-poorlherzolites, and minor pyroxenites. The Cr-diopside peridotitesappear to be residues of the extraction of Mg-rich basalts byup to 15% partial melting (median 510%) of a pyrolite-likesource in the spinel stability field. The xenoliths are similarto other mantle xenolith suites derived from beneath convergentcontinental margins, but are less depleted, less oxidized, andhave lower spinel mg-number than peridotites found in fore-arcsettings. Their dominant high field strength element depletedcharacter, however, is typical of arc lavas, and may suggestthat fluids or melts circulating through the Canadian Cordilleralithosphere were subduction related. Modeling using MELTS isconsistent with the augite-bearing xenoliths being formed byinteraction between crystallizing alkaline melts and peridotite.Assimilationfractional crystallization modeling suggeststhat the trace element patterns of liquids in equilibrium withthe augite xenoliths may represent the initial melts that reactedwith the peridotite. Moreover, the compositions of these meltsare similar to those of some glasses observed in the mantlexenoliths. Meltrock interaction may thus be a viablemechanism for the formation of Si- and alkali-rich glass inperidotites. KEY WORDS: Canadian Cordillera; mantle xenolith; peridotite; wehrlite; meltrock reaction 相似文献
12.
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 (Fo 89–92) 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 SiO 2 contents; about 36 wt.% MgO and 50 wt.% SiO 2. They may have formed as segregates from a SiO 2-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
13.
We have studied the Sr isotopic composition of partial melts of biotite granite generated experimentally and by intrusion
of basalt into the Sierra Nevada Batholith. The experiments employed large, 3-cm cubes of granite to duplicate natural grain-boundary
textures and were performed in air over the temperature interval 1000–1250 °C, to simulate basalt-induced wall rock and xenolith
melting in the shallow crust. In both the experimental and natural analogs, fusion of plagioclase + alkali-feldspar ± quartz
and biotite + plagioclase ± quartz results in the formation of colorless and brown melt (quenched to glass) respectively.
Accordant with this melting behavior, brown glasses are enriched in radiogenic Sr and MgO, FeO, CaO, and TiO 2 relative to colorless glasses. These results support recent studies indicating that the isotopic compositions of crustal
melts can reflect the relative contributions of mineral phases entering the melt, rather than the isotopic composition of
the bulk source rock. In addition, we show that at shallow-crustal conditions preferential breakdown of biotite leads to initial
high- 87Sr/ 86Sr, low-Sr concentration melts. However, as the degree of melting increases, melts become less radiogenic yet are more enriched
in elemental Sr due to loss of biotite from the restite and increased consumption of feldspars. Our results therefore suggest,
if partial melts of granitic crust segregate rapidly during episodic magmatic underplating, successive melt batches can evolve
from high- 87Sr/ 86Sr to low- 87Sr/ 86Sr liquids as melting progresses.
Received: 25 August 1998 / Accepted: 10 March 1999 相似文献
14.
The paper presents data on naturally quenched melt inclusions in olivine (Fo 69–84) from Late Pleistocene pyroclastic rocks
of Zhupanovsky volcano in the frontal zone of the Eastern Volcanic Belt of Kamchatka. The composition of the melt inclusions
provides insight into the latest crystallization stages (∼70% crystallization) of the parental melt (∼46.4 wt % SiO 2, ∼2.5 wt % H 2O, ∼0.3 wt % S), which proceeded at decompression and started at a depth of approximately 10 km from the surface. The crystallization
temperature was estimated at 1100 ± 20°C at an oxygen fugacity of ΔFMQ = 0.9–1.7. The melts evolved due to the simultaneous
crystallization of olivine, plagioclase, pyroxene, chromite, and magnetite ( Ol: Pl: Cpx: ( Crt-Mt) ∼ 13: 54: 24: 4) along the tholeiite evolutionary trend and became progressively enriched in FeO, SiO 2, Na 2O, and K 2O and depleted in MgO, CaO, and Al 2O 3. Melt crystallization was associated with the segregation of fluid rich in S-bearing compounds and, to a lesser extent, in
H 2O and Cl. The primary melt of Zhupanovsky volcano (whose composition was estimated from data on the most primitive melt inclusions)
had a composition of low-Si (∼45 wt % SiO 2) picrobasalt (∼14 wt % MgO), as is typical of parental melts in Kamchatka and other island arcs, and was different from MORB.
This primary melt could be derived by ∼8% melting of mantle peridotite of composition close to the MORB source, under pressures
of 1.5 ± 0.2 GPa and temperatures 20–30°C lower than the solidus temperature of “dry” peridotite (1230–1240°C). Melting was
induced by the interaction of the hot peridotite with a hydrous component that was brought to the mantle from the subducted
slab and was also responsible for the enrichment of the Zhupanovsky magmas in LREE, LILE, B, Cl, Th, U, and Pb. The hydrous
component in the magma source of Zhupanovsky volcano was produced by the partial slab melting under water-saturated conditions
at temperatures of 760–810°C and pressures of ∼3.5 GPa. As the depth of the subducted slab beneath Kamchatkan volcanoes varies
from 100 to 125 km, the composition of the hydrous component drastically changes from relatively low-temperature H 2O-rich fluid to higher temperature H 2O-bearing melt. The geothermal gradient at the surface of the slab within the depth range of 100–125 km beneath Kamchatka
was estimated at 4°C/km. 相似文献
15.
A suite of spinel lherzolite and wehrlite xenoliths from a Devonian kimberlite dyke near Kandalaksha, Kola Peninsula, Russia, has been studied to determine the nature of the lithospheric mantle beneath the northern Baltic Shield. Olivine modal estimates and Fo content in the spinel lherzolite xenoliths reveal that the lithosphere beneath the Archaean–Proterozoic crust has some similarities to Phanerozoic lithospheric mantle elsewhere. Modal metasomatism is indicated by the presence of Ti-rich and Ti-poor phlogopite, pargasite, apatite and picroilmenite in the xenoliths. Wehrlite xenoliths are considered to represent localised high-pressure cumulates from mafic–ultramafic melts trapped within the mantle as veins or lenses. Equilibration temperatures range from 775 to 969 °C for the spinel lherzolite xenoliths and from 817 to 904 °C for the wehrlites. Laser ablation ICP-MS data for incompatible trace elements in primary clinopyroxenes and metasomatic amphiboles from the spinel lherzolites show moderate levels of LREE enrichment. Replacement clinopyroxenes in the wehrlites are less enriched in LREE but richer in TiO2. Fractional melt modelling for Y and Yb concentrations in clinopyroxenes from the spinel lherzolites indicates 7–8% partial melting of a primitive source. Such a volume of partial melt could be related to the 2.4–2.5 Ga intrusion of basaltic magmas (now metamorphosed to garnet granulites) in the lower crust of the northern Baltic Shield. The lithosphere beneath the Kola Peninsula has undergone several episodes of metasomatism. Both the spinel lherzolites and wehrlites were subjected to an incomplete carbonatitic metasomatic event, probably related to an early carbonatitic phase associated with the 360–380 Ma Devonian alkaline magmatism. This resulted in crystallisation of secondary clinopyroxene rims at the expense of primary orthopyroxenes, with development of secondary forsteritic olivine and apatite. Two separate metasomatic events resulted in the crystallisation of the Ti–Fe-rich amphibole, phlogopite and ilmenite in the wehrlites and the low Ti–Fe amphibole and phlogopite in the spinel lherzolites. Alternatively, a single metasomatic event with a chemically evolving melt may have produced the significant compositional differences seen in the amphibole and phlogopite between the spinel lherzolites and wehrlites. The calculated REE pattern of a melt in equilibrium with clinopyroxenes from a cpx-rich pocket is identical to that of the kimberlite host, indicating a close petrological relationship. 相似文献
16.
High pressure experimental studies of the melting of lherzolitic upper mantle in the absence of carbon and hydrogen have shown
that the lherzolite solidus has a positive d P/d T and that the percentage melting increases quite rapidly above the solidus. In contrast, the presence of carbon and hydrogen
in the mantle results in a region of ‘incipient’ melting at temperatures below the C,H-free solidus. In this region the presence
or absence of melt and the composition of the melt are dependent on the amount and nature of volatiles, particularly the CO 2, H 2O, and CH 4 contents of the potential C-H-O fluid. Under conditions of low
(IW to IW + 1 log unit at P ∼ 20–35kb), fluids such as CH 4+H 2O and CH 4+H 2 inhibit melting, having a low solubility in silicate melts. Under these conditions, carbon and hydrogen are mobile elements
in the upper mantle. At slightly higher oxygen fugacity (IW+2 log units, P∼20–35 kb) fluids in equilibrium with graphite or diamond in peridotite C-H-O are extremely water-rich. Carbon is thus not
mobile in the mantle in this
range and the melting and phase relations for the upper mantle lherzolite approximate closely to the peridotite-H 2O system. Pargasitic amphibole is stable to solidus temperatures in fertile lherzolite compositions and causes a distinctive
peridotite solidus, the ‘dehydration solidus’, with a marked change in slope (a ‘back bend’) at 29–30kb due to instability
of pargasite at high pressure. Intersections of geothermal gradients with the peridotite-H 2O solidi define the boundary between lithosphere (subsolidus) and asthenosphere (incipient melt region). This boundary is
thus sensitive to changes in
[affecting CH 4:H 2O:CO 2 ratios] and to the amount of H 2O and carbon (CO 2, CH 4) present. At higher
conditions (IW + 3 log units), CO 2-rich fluids occur at low pressures but there is a marked depression of the solidus at 20–21 kb due to intersection with the
carbonation reaction, producing the low temperature solidus for dolomite amphibole lherzolite ( T∼925°C, 21 to >31kb). Melting of dolomite (or magnesite) amphibole lherzolite yields primary sodic dolomitic carbonatite melt
with low H 2O content, in equilibrium with amphibole garnet lherzolite.
The complexity of melting in peridotite-C-H-O provides possible explanations for a wide range of observations on lithosphere/asthenosphere
relations, on mantle melt and fluid compositions, and on processes of mantle metasomatism and magma genesis in the upper mantle. 相似文献
17.
Impact cratering on the Moon’s surface was accompanied by the high-temperature melting of rocks, melt evaporation, and silicate
vapor condensation. Evidence for the extensive evaporative fractionation of melts was found in HASP (High-Alumina Silica-Poor)
glasses from the lunar regolith. Numerous objects of condensation origin were found in the Apollo 14 regolith breccia. They
are referred to as GASP (Gas-Associated Spheroidal Precipitates). With respect to chemical characteristics, namely FeO and
SiO 2 contents, GASP were subdivided into Fe-rich (FeGASP) and Si-rich (SiGASP) condensates. Based on experimental data on the
evaporation of aluminous basalt sample 68415.40 from the Apollo 16 collection and the calculated compositions of residual
melts and complementary vapors at various temperatures, we compared the obtained compositions with the chemical analyses of
the HASP glasses and GASP condensates. The comparison was aimed at estimating the temperature conditions of HASP and GASP
formation. The comparison showed that the compositions of the HASP glasses and GASP condensates are consistent with the compositions
obtained in the equilibrium experiment. In accordance with the experiment, the temperature range of the evaporation of HASP
glasses was estimated as ∼1750–1870°C. The temperature interval of condensation, with allowance for the effect of vapor supercooling,
is ∼1700–1500°C for FeGASP and no higher than 1700–1750°C for SiGASP. This paper discusses the problems of establishing interphase
thermodynamic equilibrium during the dispersion of a vapor-melt cloud, vapor supercooling during its condensation, and the
influence of the curvature of melt and condensate particles on the character of evaporation and condensation. 相似文献
18.
The anhydrous melting behaviour of two synthetic peridotite compositions has been studied experimentally at temperatures ranging from near the solidus to about 200° C above the solidus within the pressure range 0–15 kb. The peridotite compositions studied are equivalent to Hawaiian pyrolite and a more depleted spinel lherzolite (Tinaquillo peridotite) and in both cases the experimental studies used peridotite – 40% olivine compositions. Equilibrium melting results in progressive elimination of phases with increasing temperature. Four main melting fields are recognized; from the solidus these are: olivine (ol)+orthopyroxene (opx)+clinopyroxene (cpx)+Al-rich phase (plagioclase at low pressure, spinel at moderate pressure, garnet at high pressure)+liquid (L); ol+opx+cpx+Cr-spinel+L; ol+opx+Cr-spinel +L: ol±Cr-spinel+L. Microprobe analyses of the residual phases show progressive changes to more refractory compositions with increasing proportion of coexisting melt i.e. increasing Mg/(Mg+Fe) and Cr/(Cr+Al) ratios, decreasing Al 2O 3, CaO in pyroxene.The degree of melting, established by modal analysis, increases rapidly immediately above the solidus (up to 10% melting occurs within 25°–30° C of the solidus), and then increases in roughly linear form with increasing temperature.Equilibrium melt compositions have been calculated by mass balance using the compositions and proportions of residual phases to overcome the problems of iron loss and quench modification of the glass. Compositions from the melting of pyrolite within the spinel peridotite field (i.e. 15 kb) range from alkali olivine basalt (<15% melting) through olivine tholeiite (20–30% melting) and picrite to komatiite (40–60% melting). Melting in the plagioclase peridotite field produces magnesian quartz tholeiite and olivine-poor tholeiite and, at higher degrees of melting (30–40%), basaltic or pyroxenitic komatiite. Melts from Tinaquillo lherzolite are more silica saturated than those from pyrolite for similar degrees of partial melting, and range from olivine tholeiite through tholeiitic picrite to komatiite for melting in the spinel peridotite field.The equilibrium melts are compared with inferred primary magma compositions and integrated with previous melting studies on basalts. The data obtained here and complementary basalt melting studies do not support models of formation of oceanic crust in which the parental magmas of common mid-ocean ridge basalts (MORB) are attributed to segregation from source peridotite at shallow depths ( 25 km) to leave residual harzburgite. Liquids segregating from peridotite at these depths are more silica-rich than common MORB. 相似文献
19.
We have determined the near-solidus melt compositions for peridotiteMM-3, a suitable composition for the production of mid-oceanridge basalt (MORB) by decompression partial melting, at 1 and1·5 GPa. At 1 GPa the MM-3 composition has a subsolidusplagioclase-bearing spinel lherzolite assemblage, and a solidusat 1270°C. At only 5°C above the solidus, 4% meltis present as a result of almost complete melting of plagioclase.This melting behaviour in plagioclase lherzolite is predictedfrom simple systems and previous experimental work. The persistenceof plagioclase to > 0·8 GPa is strongly dependenton bulk-rock CaO/Na 2O and normative plagioclase content in theperidotite. At 1·5 GPa the MM-3 composition has a subsolidusspinel lherzolite assemblage, and a solidus at 1350°C.We have determined a near-solidus melt composition at 2% meltingwithin 10°C of the solidus. Near-solidus melts at both 1and 1·5 GPa are nepheline normative, and have low normativediopside contents; also they have the highest TiO 2, Al 2O 3 andNa 2O, and the lowest FeO and Cr 2O 3 contents compared with higherdegree partial melts. Comparison of these near-solidus meltswith primitive MORB glasses, which lie in the olivine-only fieldof crystallization at low pressure, indicate that petrogeneticmodels involving aggregation of near-fractional melts formedduring melting at pressures of 1·5 GPa or less are unlikelyto be correct. In this study we use an experimental approachthat utilizes sintered oxide mix starting materials and peridotitereaction experiments. We also examine some recent studies usingan alternative approach of melt migration into, and entrapmentwithin melt traps (olivine, diamond, vitreouscarbon) and discuss optimal procedures for this method. KEY WORDS: experimental petrology; mantle melting; near-solidus; fertile peridotite; MORB 相似文献
20.
We have investigated new samples from the Gees mantle xenolith suite (West Eifel), for which metasomatism by carbonatite
melt has been suggested. The major metasomatic change is transformation of harzburgites into phlogopite-rich wehrlites. Silicate
glasses are associated with all stages of transformation, and can be resolved into two major groups: a strongly undersaturated
alkaline basanite similar to the host magma which infiltrated the xenoliths during ascent, and Si-Al-enriched, variably alkaline
glass present exclusively within the xenoliths. Si-Al-rich glasses (up to 72 wt% SiO 2 when associated with orthopyroxene (Opx) are usually interpreted in mantle xenoliths as products of decompressional breakdown
of hydrous phases like amphibole. In the Gees suite, however, amphibole is not present, nor can the glass be related to phlogopite
breakdown. The Si-Al-rich glass is compositionally similar to glasses occurring in many other xenolith suites including those
related to carbonatite metasomatism. Petrographically the silicate glass is intimately associated with the metasomatic reactions
in Gees, mainly conversion of harzburgite orthopyroxene to olivine + clinopyroxene. Both phases crystallize as microlites
from the glass. The chemical composition of the Si-Al-enriched glass shows that it cannot be derived from decompressional
melting of the Gees xenoliths, but must have been present prior to their entrainment in the host magma. Simple mass-balance
calculations, based on modal analyses, yield a possible composition of the melt prior to ascent of the xenoliths, during which
glass + microlite patches were modified by dissolution of olivine, orthopyroxene and spinel. This parental melt is a calc-alkaline
andesite (55–60 wt% SiO 2), characterized by high Al 2O 3 (ca. 18 wt%). The obtained composition is very similar to high-alumina, calc-alkaline melts that should form by AFC-type
reactions between basalt and harzburgite wall rock according to the model of Kelemen (1990). Thus, we suggest that the Si-Al-enriched
glasses of Gees, and possibly of other suites as well, are remnants of upper mantle hybrid melts, and that the Gees suite
was metasomatized by silicate and not carbonatite melts. High-Mg, high-Ca composition of metasomatic olivine and clinopyroxene
in mantle xenoliths have been explained by carbonatite metasomatism. As these features are also present in the Gees suite,
we have calculated the equilibrium Ca contents of olivine and clinopyroxene using the QUI1F thermodynamical model, to show
that they are a simple function of silica activity. High-Ca compositions are attained at low a SiO 2 and can thus be produced during metasomatism by any melt that is Opx-undersaturated, irrespective of whether it is a carbonatite
or a silicate melt. Such low a SiO 2 is recorded by the microlites in the Gees Si-Al-rich glasses. Our results imply that xenolith suites cannot confidently be
related to carbonatite metasomatism if the significance of silicate glasses, when present, is not investigated.
Received: 2 March 1995 / Accepted: 12 June 1995 相似文献
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