共查询到20条相似文献,搜索用时 125 毫秒
1.
Tatjana Rehfeldt Karsten Obst Leif Johansson 《International Journal of Earth Sciences》2007,96(3):433-450
Jurassic basanite necks occurring at the junction of two major fault zones in Scania contain ultramafic (peridotites, pyroxenites)
and mafic xenoliths, which together indicate a diversity of upper mantle and lower crustal assemblages beneath this region.
The peridotites can be subdivided into lherzolites, dunites and harzburgites. Most lherzolites are porphyroclastic, containing
orthopyroxene and olivine porphyroclasts. They consist of Mg-rich silicates (Mg# = Mg/(Mg + Fetot) × 100; 88–94) and vermicular spinel. Calculated equilibration temperatures are lower in porphyroclastic lherzolites (975–1,007°C)
than in equigranular lherzolite (1,079°C), indicating an origin from different parts of the upper mantle. According to the
spinel composition the lherzolites represent residues of 8–13% fractional melting. They are similar in texture, mineralogy
and major element composition to mantle xenoliths from Cenozoic Central European volcanic fields. Dunitic and harzburgitic
peridotites are equigranular and only slightly deformed. Silicate minerals have lower to similar Mg# (83–92) as lherzolites
and lack primary spinel. Resorbed patches in dunite and harzburgite xenoliths might be the remnants of metasomatic processes
that changed the upper mantle composition. Pyroxenites are coarse, undeformed and have silicate minerals with partly lower
Mg# than peridotites (70–91). Pyroxenitic oxides are pleonaste spinels. According to two-pyroxene thermometry pyroxenites
show a large range of equilibration temperatures (919–1,280°C). In contrast, mafic xenoliths, which are mostly layered gabbronorites
with pyroxene- and plagioclase-rich layers, have a narrow range of equilibration temperatures (828–890°C). These temperature
ranges, together with geochemical evidence, indicate that pyroxenites and gabbroic xenoliths represent mafic intrusions within
the Fennoscandian crust. 相似文献
2.
Xinmiao Zhao Hongfu Zhang Xiangkun Zhu Suohan Tang Yanjie Tang 《Contributions to Mineralogy and Petrology》2010,160(1):15-14
Iron isotopes, together with mineral elemental compositions of spinel peridotite xenoliths and clinopyroxenites from Hannuoba
and Hebi Cenozoic alkaline basalts, were analyzed to investigate iron isotopic features of the lithospheric mantle beneath
the North China Craton. The results show that the Hannuoba spinel peridotite xenoliths have small but distinguishable Fe isotopic
variations. Overall variations in δ57Fe are in a range of −0.25 to 0.14‰ for olivine, −0.17 to 0.17‰ for orthopyroxene, −0.21 to 0.27‰ for clinopyroxene, and −0.16
to 0.26‰ for spinel, respectively. Clinopyroxene has the heaviest iron isotopic ratio and olivine the lightest within individual
sample. No clear linear relationships between the mineral pairs on “δ-δ” plot suggest that iron isotopes of mineral separates
analyzed have been affected largely by some open system processes. The broadly negative correlations between mineral iron
isotopes and metasomatic indexes such as spinel Cr#, (La/Yb)N ratios of clinopyroxenes suggest that iron isotopic variations in different minerals and peridotites were probably produced
by mantle metasomatism. The Hebi phlogopite-bearing lherzolite, which is significantly modified by metasomatic events, appears
to be much heavier isotopically than clinopyroxene-poor lherzolite. This study further confirms previous conclusions that
the lithospheric mantle has distinguishable and heterogeneous iron isotopic variations at the xenoliths scale. Mantle metasomatism
is the most likely cause for the iron isotope variations in mantle peridotites. 相似文献
3.
The paper discusses the results of mineralogical and petrographic studies of spinel lherzolite xenoliths and clinopyroxene megacrysts in basalt from the Jixia region related to the central zone of Cenozoic basaltic magmatism of southeastern China. Spinel lherzolite is predominantly composed of olivine (Fo89.6–90.4), orthopyroxene (Mg# = 90.6–92.7), clinopyroxene (Mg# = 90.3–91.9), and chrome spinel (Cr# = 6.59–14.0). According to the geochemical characteristics, basalt of the Jixia region is similar to OIB with asthenospheric material as a source. The following equilibrium temperatures and pressures were obtained for spinel peridotite: 890–1269°C and 10.4–14.8 kbar. Mg# of olivine and Cr# of chrome spinel are close to the values in rocks of the enriched mantle. It is evident from analysis of the textural peculiarities of spinel lherzolite that basaltic melt interacted with mantle rocks at the xenolith capture stage. Based on an analysis of the P–T conditions of the formation of spinel peridotite and clinopyroxene megacrysts, we show that mantle xenoliths were captured in the course of basaltic magma intrusion at a significantly lower depth than the area of partial melting. However, capture of mantle xenoliths was preceded by low-degree partial melting at an earlier stage. 相似文献
4.
U. Wiechert Dmitri A. Ionov Karl Hans Wedepohl 《Contributions to Mineralogy and Petrology》1997,126(4):345-364
Spinel peridotite xenoliths from the Atsagin-Dush volcanic centre, SE Mongolia range from fertile lherzolites to clinopyroxene(cpx)-bearing
harzburgites. The cpx-poor peridotites typically contain interstitial fine-grained material and silicate glass and abundant
fluid inclusions in minerals, some have large vesicular melt pockets that apparently formed after primary clinopyroxene and
spinel. No volatile-bearing minerals (amphibole, phlogopite, apatite, carbonate) have been found in any of the xenoliths.
Fifteen peridotite xenoliths have been analysed for major and trace elements; whole-rock Sr isotope compositions and O isotope
composition of all minerals were determined for 13 xenoliths. Trace element composition and Sr-Nd isotope compositions were
also determined in 11 clinopyroxene and melt pocket separates. Regular variations of major and moderately incompatible trace
elements (e.g. heavy-rare-earth elements) in the peridotite series are consistent with its formation as a result of variable
degrees of melt extraction from a fertile lherzolite protolith. The Nd isotope compositions of LREE (light-rare-earth elements)-depleted
clinopyroxenes indicate an old (≥ 1 billion years) depletion event. Clinopyroxene-rich lherzolites are commonly depleted in
LREE and other incompatible trace elements whereas cpx-poor peridotites show metasomatic enrichment that can be related to
the abundance of fine-grained interstitial material, glass and fluid inclusions in minerals. The absence of hydrous minerals,
ubiquitous CO2-rich microinclusions in the enriched samples and negative anomalies of Nb, Hf, Zr, and Ti in primitive mantle-normalized
trace element patterns of whole rocks and clinopyroxenes indicate that carbonate melts may have been responsible for the metasomatic
enrichment. Low Cu and S contents and high δ34S values in whole-rock peridotites could be explained by interaction with oxidized fluids that may have been derived from
subducted oceanic crust. The Sr-Nd isotope compositions of LREE-depleted clinopyroxenes plot either in the MORB (mid-ocean-ridge
basalt) field or to the right of the mantle array, the latter may be due to enrichment in radiogenic Sr. The LREE-enriched
clinopyroxenes and melt pockets plot in the ocean island-basalt field and have Sr-Nd isotope signatures consistent with derivation
from a mixture of the DMM (depleted MORB mantle) and EM (enriched mantle) II sources.
Received: 18 January 1996 / Accepted: 23 August 1996 相似文献
5.
Jian Wang Kéiko H. Hattori Rolf Kilian Charles R. Stern 《Contributions to Mineralogy and Petrology》2007,153(5):607-624
Quaternary basalts in the Cerro del Fraile area contain two types of mantle xenoliths; coarse-grained (2–5 mm) C-type spinel
harzburgites and lherzolites, and fine-grained (0.5–2 mm) intensely metasomatized F-type spinel lherzolites. C-type xenoliths
have high Mg in olivine (Fo = 90–91) and a range in Cr# [Cr/ (Cr + Al) = 0.17–0.34] in spinel. Two C-type samples contain
websterite veinlets and solidified patches of melt that is now composed of minute quenched grains of plagioclase + Cr-spinel + clinopyroxene + olivine.
These patches of quenched melts are formed by decompression melting of pargasitic amphibole. High Ti contents and common occurrence
of relic Cr-spinel in the quenched melts indicate that the amphibole is formed from spinel by interaction with the Ti-rich
parental magma of the websterite veinlets. The fO2 values of these two C-type xenoliths range from ΔFMQ −0.2 to −0.4, which is consistent with their metasomatism by an asthenospheric
mantle-derived melt. The rest of the C-type samples are free of “melt,” but show cryptic metasomatism by slab-derived aqueous
fluids, which produced high concentrations of fluid-mobile elements in clinopyroxenes, and higher fO2 ranging from ΔFMQ +0.1 to +0.3. F-type lherzolites are intensely metasomatized to form spinel with low Cr# (∼0.13) and silicate
minerals with low MgO, olivine (Fo = ∼84), orthpyroxene [Mg# = Mg/(Mg + ΣFe) = ∼0.86] and clinopyroxene (Mg# = ∼0.88). Patches
of “melt” are common in all F-type samples and their compositions are similar to pargasitic amphibole with low TiO2 (<0.56 wt%), Cr2O3 (<0.55 wt%) and MgO (<16.3 wt%). Low Mg# values of silicate minerals, including the amphibole, suggest that the metasomatic
agent is most likely a slab melt. This is supported by high ratios of Sr/Y and light rare earth elements (REE)/heavy REE in
clinopyroxenes. F-type xenoliths show relatively low fO2 (ΔFMQ −0.9 to −1.1) compared to C-type xenoliths and this is explained by the fusion of organic-rich sediments overlying
the slab during the slab melt. Trench-fill sediments in the area are high in organic matter. The fusion of such wet sediments
likely produced CH4-rich fluids and reduced melts that mixed with the slab melt. High U and Th in bulk rocks and clinopyroxene in F-type xenoliths
support the proposed interpretation. 相似文献
6.
Carbonatite metasomatized peridotite xenoliths from southern Patagonia: implications for lithospheric processes and Neogene plateau magmatism 总被引:10,自引:0,他引:10
The mineral chemistry, major and trace element, and Sr–Nd isotopic composition of Cr-diopside, spinel peridotite xenoliths
from the Estancia Lote 17 locality in southern Patagonia document a strong carbonatitic metasomatism of the backarc continental
lithosphere. The Lote 17 peridotite xenolith suite consists of hydrous spinel lherzolite, wehrlite, and olivine websterite,
and anhydrous harzburgite and lherzolite. Two-pyroxene thermometry indicates equilibration temperatures ranging from 870 to
1015 °C and the lack of plagioclase or garnet suggests the xenoliths originated from between ˜40 and 60 km depth. All of the
xenoliths are LILE- and LREE-enriched, but have relatively low 87Sr/86Sr (0.70294 to 0.70342) and high ɛNd (+3.0 to +6.6), indicating recent trace element enrichment (∼25 Ma, based on the low 87Sr/86Sr and high Rb concentrations of phlogopite separates) in the long-term, melt-depleted Patagonian lithosphere. Lote 17 peridotite
xenoliths are divided into two basic groups. Group 1 xenoliths consist of fertile peridotites that contain hydrous phases
(amphibole ± phlogopite ± apatite). Group 1 xenoliths are further subdivided into three groups (a, b, and c) based on distinctive
textures and whole-rock chemistry. Group 1 xenolith mineralogy and chemistry are consistent with a complex metasomatic history
involving variable extents of recent carbonatite metasomatism (high Ca/Al, Nb/La, Zr/Hf, low Ti/Eu) that has overprinted earlier
metasomatic events. Group 2 xenoliths consist of infertile, anhydrous harzburgites and record cryptic metasomatism that is
attributed to CO2-rich fluids liberated from Group 1 carbonatite metasomatic reactions. Extremely variable incompatible trace element ratios
and depleted Sr–Nd isotopic compositions of Lote 17 peridotite xenoliths indicate that the continental lithosphere was neither
the primary source nor an enriched lithospheric contaminant for Neogene Patagonian plateau lavas. Neogene plateau magmatism
associated with formation of asthenospheric slab windows may have triggered this occurrence of “intraplate-type” carbonatite
metasomatism in an active continental backarc setting.
Received: 26 January 2000 / Accepted: 1 March 2000 相似文献
7.
Dexter Perkins Elizabeth Y. Anthony 《Contributions to Mineralogy and Petrology》2011,162(6):1139-1157
In peridotites, olivine, clinopyroxene, and orthopyroxene are complex solid solutions with wide stability fields. Depending
mostly on bulk composition and pressure, these minerals may be accompanied by plagioclase (low pressure), spinel (moderate
pressure), or garnet (high pressure), resulting in 4-phase and rarer 5-phase assemblages. Although a particular mineral assemblage
is stable over a range of P–T, the compositions of the individual minerals vary with changing P–T conditions. Application
of standard geothermobarometers to olivine–clinopyroxene–orthopyroxene–spinel peridotites is problematic. An alternative approach
is to use a bulk rock composition to calculate equilibrium phase diagrams to determine the conditions under which a particular
assemblage is stable. This requires consideration of the 7-component system SiO2–Al2O3–Cr2O3–FeO–MgO–CaO–Na2O, internally consistent thermodynamic data for end members, and reliable mixing models for all mineral solutions. Experimental
studies in simpler systems, and solution models from the literature, permit derivation of multicomponent thermodynamic mixing
models for the key minerals. The models, when applied to xenoliths from Kilbourne Hole, constrain P and T of equilibration
and are less sensitive to mineral compositional variations, or uncertainty in activity models, than standard thermobarometry.
Our modeling provides the first tightly constrained pressure estimates for Kilbourne Hole, placing the xenoliths in the spinel
stability field at depths (30–45 km) that correspond to the uppermost mantle beneath the Rio Grande Rift. The fine-grained
equigranular lherzolite, porphyroclastic lherzolite, and some harzburgite-dunite specimens equilibrated at average conditions
of 11.5 Kbar-930°C, 12 Kbar-990°C, and 13 Kbar-1,080°C, respectively. The mantle beneath the Rio Grande Rift is layered; the
fine-grained equigranular lherzolite derives from relatively shallow depth (35 km average), and the porphyroclastic lherzolite
from slightly deeper levels. Lying 5–10 km beneath both lherzolites, the harzburgite-dunite represents a depth where melt
extraction has significantly altered mantle chemistry and where local thermodynamic equilibrium has not been maintained. 相似文献
8.
The basaltic maar of Youkou, situated in the Adamawa Volcanic Massif in the eastern branch of the continental segment of the Cameroon Volcanic Line, contains mantle-derived xenoliths of various types in pyroclastites. Spinel-bearing lherzolite xenoliths from the Youkou volcano generally exhibit protogranular textures with olivine (Fo89.4?90.5), enstatite (En89???91Fs8.7?9.8Wo0.82?1.13), clinopyroxene, spinel (Cr#Sp?=?9.4–13.8), and in some cases amphibole (Mg#?=?88.5–89.1). Mineral equilibration temperatures in the lherzolite xenoliths have been estimated from three–two pyroxene thermometers and range between 835 and 937 °C at pressures of 10–18 kbar, consistent with shallow mantle depths of around 32–58 km. Trends displayed by bulk-rock MgO correlate with Al2O3, indicating that the xenoliths are refractory mantle residues after partial melting. The degree of partial melting estimated from spinel compositions is less than 10%: evidences for much higher degrees of depletion are preserved in one sample, but overprinted by refertilization in others. Trace element compositions of the xenoliths are enriched in highly incompatible elements (LREE, Sr, Ba, and U), indicating that the spinel lherzolites underwent later cryptic metasomatic enrichment induced by plume-related hydrous silicate melts. The extreme fertility (Al2O3?=?6.07–6.56 wt% in clinopyroxene) and the low CaO/Al2O3 ratios in the spinel lherzolites suggest that they could not be a simple residue of partial melting of primitive mantle and must have experienced refertilization processes driven by the infiltration of carbonatite or carbonated silicate melts. 相似文献
9.
Ultra-depleted, shallow cratonic mantle beneath West Greenland: dunitic xenoliths from Ubekendt Ejland 总被引:1,自引:0,他引:1
Stefan Bernstein Karen Hanghøj Peter B. Kelemen C. Kent Brooks 《Contributions to Mineralogy and Petrology》2006,152(3):335-347
Dunitic xenoliths from late Palaeogene, alkaline basalt flows on Ubekendt Ejland, West Greenland contain olivine with 100 × Mg/(Mg + Fe), or Mg#, between 92.0 and 93.7. Orthopyroxene has very low Al2O3 and CaO contents (0.024–1.639 and 0.062–0.275 wt%, respectively). Spinel has 100 × Cr/(Cr + Al), or Cr#, between 46.98 and 95.67. Clinopyroxene is absent. The osmium isotopic composition of olivine and spinel mineral separates shows a considerable span of 187Os/188Os values. The most unradiogenic 187Os/188Os value of 0.1046 corresponds to a Re-depletion age of ca. 3.3 Gy, while the most radiogenic value of 0.1336 is higher than present-day chondrite. The Os isotopic composition of the xenoliths is consistent with their origin as restites from a melt extraction event in the Archaean, followed by one or more subsequent metasomatic event(s). The high Cr# in spinel and low modal pyroxene of the Ubekendt Ejland xenoliths are similar to values of some highly depleted mantle peridotites from arc settings. However, highly depleted, arc-related peridotites have higher Cr# in spinel for a given proportion of modal olivine, compared to cratonic xenolith suites from Greenland, which instead form coherent trends with abyssal peridotites, dredged from modern mid-ocean ridges. This suggests that depleted cratonic harzburgites and dunites from shallow lithospheric mantle represent the residue from dry melting in the Archaean. 相似文献
10.
Five mafic lava flows located on the southern flank of Mount Baker are among the most primitive in the volcanic field. A comprehensive
dataset of whole rock and mineral chemistry reveals the diversity of these mafic lavas that come from distinct sources and
have been variably affected by ascent through the crust. Disequilibrium textures present in all of the lavas indicate that
crustal processes have affected the magmas. Despite this evidence, mantle source characteristics have been retained and three
primitive endmember lava types are represented. These include (1) modified low-K tholeiitic basalt (LKOT-like), (2) typical
calc-alkaline (CA) lavas, and (3) high-Mg basaltic andesite and andesite (HMBA and HMA). The Type 1 endmember, the basalt
of Park Butte (49.3–50.3 wt% SiO2, Mg# 64–65), has major element chemistry similar to LKOT found elsewhere in the Cascades. Park Butte also has the lowest
overall abundances of trace elements (with the exception of the HREE), indicating it is either derived from the most depleted
mantle source or has undergone the largest degree of partial melting. The Type 2 endmember is represented by the basalts of
Lake Shannon (50.7–52.6 wt% SiO2, Mg# 58–62) and Sulphur Creek (51.2–54.6 wt% SiO2, Mg# 56–57). These two lavas are comparable to calc-alkaline rocks found in arcs worldwide and have similar trace element
patterns; however, they differ from each other in abundances of REE, indicating variation in degree of partial melting or
fractionation. The Type 3 endmember is represented by the HMBA of Tarn Plateau (51.8–54.0 wt% SiO2, Mg# 68–70) and the HMA of Glacier Creek (58.3–58.7 wt% SiO2, Mg# 63–64). The strongly depleted HREE nature of these Type 3 units and their decreasing Mg# with increasing SiO2 suggests fractionation from a high-Mg basaltic parent derived from a source with residual garnet. Another basaltic andesite
unit, Cathedral Crag (52.2–52.6 wt% SiO2, Mg# 55–58), is an Mg-poor differentiate of the Type 3 endmember. The calc-alkaline lavas are least enriched in a subduction
component (lowest H2O, Sr/PN, and Ba/Nb), the LKOT-like lavas are intermediate (moderate Sr/PN and Ba/Nb), and the HMBA are most enriched (highest H2O, Sr/PN and Ba/Nb). The generation of the LKOT-like and calc-alkaline lavas can be successfully modeled by partial melting of a spinel
lherzolite with variability in composition of slab flux and/or mantle source depletion. The HMBA lavas can be successfully
modeled by partial melting of a garnet lherzolite with slab flux compositionally similar to the other lava types, or less
likely by partial melting of a spinel lherzolite with a distinctly different, HREE-depleted slab flux. 相似文献
11.
Experimental reconstruction of sodic dolomitic carbonatite melts from metasomatised lithosphere 总被引:3,自引:0,他引:3
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 CO2-rich fluid. In this study, we have experimentally reversed the decarbonation reaction by taking two natural wehrlite compositions
and reacting them with CO2 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 CO2, released by metasomatic reactions, which are locally very large at around 5 wt% CO2.
Received: 15 December 1995/Accepted: 14 February 1996 相似文献
12.
Melting and metasomatism in the continental lithosphere: laser ablation ICPMS analysis of minerals in spinel lherzolites from eastern Australia 总被引:28,自引:0,他引:28
Marc D. Norman 《Contributions to Mineralogy and Petrology》1998,130(3-4):240-255
Olivine, low-Ca pyroxene, diopside, and spinel from a suite of protogranular lherzolite xenoliths from southeastern Australia
have been analysed for their major and trace element compositions using electron microprobe and laser ablation ICPMS. Bulk
compositions of the lherzolites range from fertile (12–13% modal diopside) to depleted (2–3% modal diopside), with equilibration
temperatures of 850–900 °C indicating entrainment of these lherzolites from relatively shallow depths (probably ≤ 35 km) within
the lithosphere. Mineral compositions and abundances indicate a primary control by partial melting, with decreasing abundance
of modal diopside accompanied by increasing Mg# of olivine and pyroxene, decreasing Al and Ti contents of diopside, increasing
Ni contents of olivine, and increasing Cr/Al of spinel. HREE, Y, and Ga in diopside also follow melting trends, decreasing
in concentration with increasing Mg#. In contrast, highly incompatible elements such as LREE, Nb, and Th reveal divergent
behaviour that cannot be ascribed entirely to partial melting. Diopsides from the fertile lherzolites have mantle-normalized
patterns that are depleted in Th, Nb, and the LREE relative to Y and the HREE, whereas, diopsides from the cpx-poor samples
are strongly enriched in Th, Nb and the LREE, and have elevated Sm/Hf and Zr/Hf, and low Ti/Nb. All diopsides have strongly
negative Nb anomalies relative to Th and the LREE. Trace element patterns of diopside in the fertile lherzolites can be reproduced
by ≤ 5% batch melting of a primitive source. The negative Nb anomalies are a consequence of this melting, and do not require
special conditions or tectonic environments. The low concentrations of Y and HREE in diopside from the cpx-poor lherzolites
cannot be produced by realistic degrees of batch melting, but can be accomplished by up to ∼20% fractional melting, suggesting
multiple episodes of melt depletion. Os isotopic compositions of these lherzolites show that the melt depletion events occurred
in the middle and late Proterozoic, demonstrating the long-term stability of lithospheric mantle beneath regions of eastern
Australia. The LREE-enriched diopsides are well equilibrated and record metasomatic enrichment events that pre-date the magmatism
that entrained these xenoliths. Trace element patterns of these pyroxenes suggest a carbonatitic melt as the metasomatic agent.
Received: 24 September 1996 / Accepted: 12 August 1997 相似文献
13.
Carbonate-bearing mantle peridotite xenoliths from Spitsbergen: phase relationships,mineral compositions and trace-element residence 总被引:11,自引:0,他引:11
D. A. Ionov Suzanne Y. O’Reilly Yuri S. Genshaft Maya G. Kopylova 《Contributions to Mineralogy and Petrology》1996,125(4):375-392
Carbonates of mantle origin have been found in xenoliths from Quaternary basaltic volcanoes in NW Spitsbergen. The carbonates
range from dolomite to Mg-bearing calcite and have high Mg-numbers [Mg/(Mg+Fe)=(0.92–0.99)]. In some samples they occur interstitially,
e.g. at triple junctions of silicate minerals and appear to be in textural and chemical equilibrium with host lherzolite.
Most commonly, however, the carbonates make up fine-grained aggregates together with (Ca,Mg)-rich olivine and (Al,Cr,Ti)-rich
clinopyroxene that typically replace spinel, amphibole, and orthopyroxene as well as primary clinopyroxene and olivine. Some
lherzolites contain amphibole and apatite that appear to have formed before precipitation of the carbonates. In situ analyses by proton microprobe show very high contents of Sr in the clinopyroxene, carbonates and apatite; the apatite is
also very rich in LREE, U, Th, Cl, Br. Disseminated amphibole in carbonate-bearing rocks is very poor in Nb and Zr, in contrast
to vein amphibole and mica from carbonate-free rocks that are rich in Nb and Zr. Overall, the Spitsbergen xenoliths provide
evidence both for the occurrence of primary carbonate in apparent equilibrium with the spinel lherzolites (regardless of the
nature of events that emplaced them) and for the formation of carbonate-bearing pockets consistent with metasomatism by carbonate
melts. Calcite and amorphous carbonate-rich materials occur in com- posite carbonate-fluid inclusions, veins and partial melting
zones that appear to be related to fluid action in the mantle, heating of the xenoliths during their entrainment in basaltic
magma, and to decompression melting of the carbonates. Magnesite is a product of secondary, post-eruption alteration of the
xenoliths.
Received: 6 October 1995/Accepted: 17 June 1996 相似文献
14.
Petrologic, geochemical and isotopic investigations on two ultramafic xenoliths with metasomatic veins from the TUBAF Seamount in the Bismarck Archipelago NE of Papua New Guinea reveal different styles of metasomatic overprinting. The first xenolith, a clinopyroxene–poor spinel lherzolite, was part of the depleted upper mantle. It contains an orthopyroxene-rich vein that formed by hydrous metasomatism at ~ 980 °C and ~ 1.5 GPa. The second xenolith is a clinopyroxene-dominated spinel olivine websterite that formed as a magmatic cumulate at the transition of the upper mantle to the oceanic crust. The websterite contains a vein with orthopyroxenes and clinopyroxenes, which give evidence for high-temperature crystallization at ~ 1300 °C and < 0.36 GPa. Both xenoliths were transported to the seafloor by a Quaternary trachybasalt in a fore-arc position. The vein minerals show a strong affinity to a supra-subduction zone or island arc setting. The REE pattern of the vein in the clinopyroxene–poor lherzolite strongly resembles the one from the host trachybasalt, with a high enrichment of the LREE and a strong to moderate enrichment of the MREE and HREE. Although broadly similar in shape, the REE pattern of the vein in the websterite shows a much weaker enrichment. The same applies to the trace-element patterns, although there are significant differences in the Eu, Zr, Hf and Nb concentrations. The isotope signatures of both veins suggest a derivation from a subducted slab that had been hydrothermally altered by seawater (high 87Sr/86Sr values).The contrasting crystallization temperatures of the vein minerals as well as their overall geochemical differences indicate that the metasomatic agents responsible for the vein in the websterite were mobilized from a previously depleted source at a much deeper mantle level than those forming the vein of the clinopyroxene–poor lherzolite. The metasomatic agents may also have been mobilized at different times and from different plates, i.e., the deeply subducted Solomon Sea Microplate (for the veins in the websterite) and the shallow dehydrating Pacific Plate (for the veins in the clinopyroxene–poor lherzolite).Metasomatic agents responsible for similar petrologic phenomena, i.e., modal or cryptic metasomatism, may have distinctly different origins and show contrasting histories. A strongly depleted lherzolite may totally lose its initial geochemical signature by the influence of an enriched metasomatic agent, whereas a primarily enriched ultramafic rock, e.g., a websterite, may strongly obscure the trace-element pattern of a less enriched metasomatic vein. Furthermore, the geochemistry of the ultramafic xenoliths may reflect polyphase cryptic and modal metasomatism related to veining and later transport by the hosting melt to the seafloor. 相似文献
15.
Crystallization and Breakdown of Metasomatic Phases in Graphite-bearing Peridotite Xenoliths from Marsabit (Kenya) 总被引:1,自引:0,他引:1
Mantle-derived xenoliths from the Marsabit shield volcano (easternflank of the Kenya rift) include porphyroclastic spinel peridotitescharacterized by variable styles of metasomatism. The petrographyof the xenoliths indicates a transition from primary clinopyroxene-bearingcryptically metasomatized harzburgite (light rare earth element,U, and Th enrichment in clinopyroxene) to modally metasomatizedclinopyroxene-free harzburgite and dunite. The metasomatic phasesinclude amphibole (low-Ti Mg-katophorite), Na-rich phlogopite,apatite, graphite and metasomatic low-Al orthopyroxene. Transitionalsamples show that metasomatism led to replacement of clinopyroxeneby amphibole. In all modally metasomatized xenoliths melt pockets(silicate glass containing silicate and oxide micro-phenocrysts,carbonates and empty vugs) occur in close textural relationshipwith the earlier metasomatic phases. The petrography, majorand trace element data, together with constraints from thermobarometryand fO2 calculations, indicate that the cryptic and modal metasomatismare the result of a single event of interaction between peridotiteand an orthopyroxene-saturated volatile-rich silicate melt.The unusual style of metasomatism (composition of amphibole,presence of graphite, formation of orthopyroxene) reflects lowP –T conditions (850–1000°C at < 1·5GPa) in the wall-rocks during impregnation and locally low oxygenfugacities. The latter allowed the precipitation of graphitefrom CO2. The inferred melt was possibly derived from alkalinebasic melts by melt–rock reaction during the developmentof the Tertiary–Quaternary Kenya rift. Glass-bearing meltpockets formed at the expense of the early phases, mainly throughincongruent melting of amphibole and orthopyroxene, triggeredby infiltration of a CO2-rich fluid and heating related to themagmatic activity that ultimately sampled and transported thexenoliths to the surface. KEY WORDS: graphite; peridotite xenoliths; Kenya Rift; modal metasomatism; silicate glass 相似文献
16.
The Mineral Chemistry of Ultramafic Xenoliths of Eastern China: Implications for Upper Mantle Composition and the Paleogeotherms 总被引:21,自引:0,他引:21
Ultramafic xenoliths of garnet lherzolite (?rare spinel), spinellherzolites, spinel harzburgites, clinopyroxenites, and clinopyroxenemegacrysts were collected from Cenozoic basalts in all partsof eastern China. From their modal composition and mineral chemistryall the xenoliths may be placed into three types representing:a fertile or more primitive mantle (garnet lherzolite and spinellherzolite), a refractory or more depleted mantle (spinel harzburgiteand dunite), and inclusions cognate with the host alkali basaltsat mantle pressures (pyroxenite and megacrysts). There are systematicdifferences between the mineral compositions of each type. Spinelshows a wide compositional range and the spinel cr-number [100Cr/(Cr + Al)] is a significant indicator of the xenolithtype. Spinel cr-number and Al2O3 of coexisting minerals (spinel,clinopyroxene, and orthopyroxene) are useful as refractory indicatorsfor spinel peridotite in that the cr-number increases and thepercentage of Al2O3 decreases with increasing degrees of melting.In garnet peridotite, however, the same functions vary withpressure, not degree of melting. According to PT estimates,the various xenoliths were derived from a large range of depthsin the upper mantle: spinel peridotite from approximately 11to 22 kb (3766 km), spinel/garnet lherzolite from 19to 24 kb (6280 km), and garnet lherzolite from 24 to25 kb (7983 km). We conclude that the uppermost mantlebeneath eastern China is heterogeneous, with a north-northeastzone of more depleted mantle lying beneath the continental marginand a more primitive mantle occurring towards the continentalinterior. 相似文献
17.
Iron isotope variations in spinel peridotite xenoliths from North China Craton: implications for mantle metasomatism 总被引:2,自引:1,他引:1
Xinmiao Zhao Hongfu Zhang Xiangkun Zhu Suohan Tang Yanjie Tang 《Contributions to Mineralogy and Petrology》2010,160(1):1-14
Iron isotopes, together with mineral elemental compositions of spinel peridotite xenoliths and clinopyroxenites from Hannuoba and Hebi Cenozoic alkaline basalts, were analyzed to investigate iron isotopic features of the lithospheric mantle beneath the North China Craton. The results show that the Hannuoba spinel peridotite xenoliths have small but distinguishable Fe isotopic variations. Overall variations in δ57Fe are in a range of ?0.25 to 0.14‰ for olivine, ?0.17 to 0.17‰ for orthopyroxene, ?0.21 to 0.27‰ for clinopyroxene, and ?0.16 to 0.26‰ for spinel, respectively. Clinopyroxene has the heaviest iron isotopic ratio and olivine the lightest within individual sample. No clear linear relationships between the mineral pairs on “δ-δ” plot suggest that iron isotopes of mineral separates analyzed have been affected largely by some open system processes. The broadly negative correlations between mineral iron isotopes and metasomatic indexes such as spinel Cr#, (La/Yb)N ratios of clinopyroxenes suggest that iron isotopic variations in different minerals and peridotites were probably produced by mantle metasomatism. The Hebi phlogopite-bearing lherzolite, which is significantly modified by metasomatic events, appears to be much heavier isotopically than clinopyroxene-poor lherzolite. This study further confirms previous conclusions that the lithospheric mantle has distinguishable and heterogeneous iron isotopic variations at the xenoliths scale. Mantle metasomatism is the most likely cause for the iron isotope variations in mantle peridotites. 相似文献
18.
Na Metasomatism in the Island-Arc Mantle by Slab Melt--Peridotite Interaction: Evidence from Mantle Xenoliths in the North Kamchatka Arc 总被引:20,自引:0,他引:20
The Pliocene (7 Ma) Nb-enriched arc basalts of the ValovayamVolcanic Field (VVF) in the northern segment of the Kamchatkaarc, Russia, host abundant mantle xenoliths, including spinelIherzolites. Textural and microstructural evidence for high-temperature,multi-stage, creep-related deformations in spinel Iherzolitessupports a sub-arc mantle derivation. Pyroxene chemistry indicatesthe existence of two compositional suites: (1) a Cr-diopsidesuite with low-Tt, moderate-Al clinopyroxene compositions, and(2) an Al-augite suite with high Al and Tt, and low Cr concentrationsin clinopyroxene. Some spinel lherzolite xenoliths contain metasomaticAl-augite-type clinopyroxene, Al-Tt spinel, and felsic veinssimilar to trondhjemite melt. The Al-augite series xenolithstypically contain high-Na plagioclase, Cr-poor, Al-Fe-Mg andAl-Tt-Fe spinels, with occasional almandine-grossularite garnetand high-Al and -Na pargasitic amphibole. Pyroxene and spinel compositional trends suggest that the Crdiopsideseries xenoliths from the VVF Nb-enriched arc basalts representan island-arc mantle affected by a metasomatic event. Occurrenceof high-Na plagioclase and trondhjemitic veins favors the additionof a metasomatic component with high Na, Al and Si to the northernKamchatka arc mantle. Trondhjemitic veins, representing siliceousslab melts, compositionally exemplify the metasomatic component.Na metasomatism by peridotite-slab melt interaction is an importantmantle hybridization process responsible for arc-related alkalinemagma generation from a veined sub-arc mantle. KEY WORDS: metasomatism; island arc; mantle xenoliths; Kamchatka; mantle 相似文献
19.
Tatjana Rehfeldt Stephen F. Foley Richard W. Carlson 《Geochimica et cosmochimica acta》2008,72(23):5722-5756
Dunite, wehrlite and websterite are rare members of the mantle xenolith suite in the Kimberley kimberlites of the Kaapvaal Craton in southern Africa. All three types were originally residues of extensive melt extraction and experienced varying amounts and types of melt re-enrichment. The melt depletion event, dated by Re-Os isotope systematics at 2.9 Ga or older, is evidenced by the high Mg# (Mg/(Mg + Fe)) of silicate minerals (olivine (0.89-0.93); pyroxene (0.88-0.93); garnet (0.72-0.85)), high Cr# (Cr/(Cr + Al)) of spinel (0.53-0.84) and mostly low whole-rock SiO2, CaO and Al2O3 contents. Shortly after melt depletion, websterites were formed by reaction between depleted peridotites and silica-rich melt (>60 wt% SiO2) derived by partial melting of eclogite before or during cratonization. The melt-peridotite interaction converted olivine into orthopyroxene.All three xenolith types have secondary metasomatic clinopyroxene and garnet, which occur along olivine grain boundaries and have an amoeboid texture. As indicated by the preservation of oxygen isotope disequilibrium in the minerals and trace-element concentrations in clinopyroxene and garnet, this metasomatic event is probably of Mesozoic age and was caused by percolating alkaline basaltic melts. This melt metasomatism enriched the xenoliths in CaO, Al2O3, FeO and high-field-strength-elements, and might correspond to the Karoo magmatism at 200 Ma. The websterite xenoliths experienced both the orthoyproxene-enrichment and clinopyroxene-garnet metasomatic events, whereas dunite and wehrlite xenoliths only saw the later basaltic melt event, and may have been situated further away from the source of melt migration channels. 相似文献
20.
K-rich glass-bearing wehrlite xenoliths from Yitong,Northeastern China: petrological and chemical evidence for mantle metasomatism 总被引:9,自引:1,他引:9
Y. Xu J.-C. C. Mercier Chuanyong Lin Lanbin Shi M. A. Menzies J. V. Ross B. Harte 《Contributions to Mineralogy and Petrology》1996,125(4):406-420
Ultramafic xenoliths in Cenozoic alkali basalts from Yitong, northeast China comprise three types in terms of their modal
mineralogy: lherzolite, pyroxenite and wehrlite. The wehrlite suite always contains interstitial pale/brown glass which occupies
several per cent by volume of the whole rock. The texture of the wehrlites is porphyroclastic with some large strained grains
of olivine (0.5–1 mm) scattered in a very fine grained matrix (0.1 mm), implying a metamorphic origin for the protolith rather
than an igneous origin. The host minerals are compositionally zoned, showing evidence of reaction with a melt. Petrological
evidence for resorption of spinel (lherzolite) and orthopyroxene (wehrlite) by infiltrating melt further supports the hypothesis
that the wehrlites result from interaction between a partial melting residue and a melt, which preferentially replaced primary
spinel, Cr-diopside and enstatite to produce secondary clinopyroxene (cpx) + olivine (ol) ± chromite ± feldspar (fd). The
composition of the mineral phases supports this inference and, further indicates that, prior to melt impregnation, the protoliths
of these wehrlites must have been subjected to at least one earlier Fe-enrichment event. This explanation is consistent with
the restricted occurrence of glasses in the wehrlite suite. The glass is generally associated with fine-grained (0.1 mm) minerals
(cpx+ol+chromite ±fd). Electron microprobe analyses of these glasses show them to have high SiO2 content (54–60 wt%), a high content of alkalis (Na2O, 5.6–8.0%; K2O, 6.3–9.0%), high Al2O3 (20–24%), and a depletion in CaO (0.13–2.83%), FeO (0.89–4.42%) and MgO (0.29–1.18%). Ion probe analyses reveal a light rare
earth element-enrichment in these glasses with chondrite normalised (La)n = 268–480. The high K2O contents in these glasses and their mode of occurrence argue against an origin by in-situ melting of pre-existent phases. Petrographic characteristics and trace element data also exclude the possibility of percolation
of host-basalt related melts for the origin of these glasses. Thus the glasses must have resulted from local penetration of
mantle metasomatic melts which may have been produced by partial melting of peridotites with involvement of deep-seated fluids.
Such melts may have been significantly modified by subsequent fractional crystallization of ol, cpx and sp, extensive reaction
with the mantle conduit and the xenolith transport process.
Received: 1 August 1995 / Accepted: 19 June 1996 相似文献