共查询到20条相似文献,搜索用时 62 毫秒
1.
Throughout the ultrahigh-pressure (UHP) metamorphic unit of the Dora-Maira massif, western Alps, pyrope megablasts contain
the typical assemblage clinochlore–kyanite–talc–rutile ± phlogopite ± ellenbergerite as prograde inclusions. In the upper
part of the UHP unit in Val Gilba, some megablasts (XMg=0.89–0.98) contain in addition polymineralic inclusions consisting
of various combinations of enstatite, gedrite, sapphirine, clinochlore, talc, magnesiostaurolite and rare corundum or spinel.
We present evidence that these assemblages developed from cracks running across the megablasts, and are therefore of late
origin, post-dating the highest-pressure stage. Enstatite (XMg=0.94–0.99) contains 0.7 to, typically, 3 wt% Al2O3, but up to 8.4 wt% in the presence of sapphirine. Sapphirine (XMg=0.96–0.998, Be-free) shows the largest compositional variations,
with Si contents ranging from 1.7 to at least 2.1 atoms pfu, thereby clearly exceeding the 2:2:1 stoichiometry. The late-stage
talc contains up to 4 wt% Al2O3, 0.35 wt% Na2O and 0.6 wt% F; gedrite 1.1–2.9 wt% Na2O and up to 0.36 wt% F. The successive development within pyrope of alternative hydrous assemblages involving first enstatite
plus an Al-rich phase (kyanite, sapphirine, magnesiostaurolite) ± clinochlore, then a gedrite compositionally close to pyrope,
then talc plus an Al-rich phase (sapphirine, corundum), is a clear record of decompression. However, the temperature conditions
implied under the assumption of high H2O activity are 100 to 150 °C higher than, and so inconsistent with existing constraints on the decompression path. These constraints
are in particular the stability of talc + phengite in the matrix assemblage during decompression, and the absence of regional
evidence for a granulite-facies event. This inconsistency can only be alleviated if H2O activity inside the garnet megablast was (or became) considerably reduced with respect to that in the matrix. Fluid influx
into an opening fracture in garnet, sealing of the fracture by breakdown products of pyrope and continued evolution under
closed-system conditions may have led to increasing solute concentration and such low H2O activity within the garnet megablast, driving the microsystem toward fluid-absent conditions. Micrometre-size inclusions of Ca-sulfate and
crandallite-type compounds in minerals of these reactive areas may be evidence for such residual brines and suggest that these
were phosphate- and sulfate- rather than halide-dominated. This finding is additional evidence for the very local control
that fluid composition and H2O activity may have on the occurrence of granulite-facies assemblages, regardless of temperature. It highlights the role of
deformation (here fracturing) in triggering reactions in otherwise unreactive systems. It also shows how carefully inclusion-
to-host relationships have to be considered, post-growth reaction within the host being more common than hitherto reported.
Received: 4 February 1999 / Accepted: 24 August 2000 相似文献
2.
D. I. Rezvukhin V. G. Malkovets I. S. Sharygin D. V. Kuzmin K. D. Litasov A. A. Gibsher N. P. Pokhilenko N. V. Sobolev 《Doklady Earth Sciences》2016,466(2):173-176
The results of study of rutile inclusions in pyrope from the Internatsionalnaya kimberlite pipe are presented. Rutile is characterized by unusually high contents of impurities (up to 25 wt %). The presence of Cr2O3 (up to 9.75 wt %) and Nb2O5 (up to 15.57 wt %) are most typical. Rutile inclusions often occur in assemblage with Ti-rich oxides: picroilmenite and crichtonite group minerals. The Cr-pyropes with inclusions of rutile, picroilmenite, and crichtonite group minerals were formed in the lithospheric mantle beneath the Mirnyi field during their joint crystallization from melts enriched in Fe, Ti, and other incompatible elements as a result of metasomatic enrichment of the depleted lithospheric mantle. 相似文献
3.
Jan C. M. de Hoog Manfred J. van Bergen 《Contributions to Mineralogy and Petrology》2000,139(4):485-502
Potassium-rich calc-alkaline lavas of Lewotolo volcano, situated in the East Sunda Arc, Indonesia, contain the rare mineral
zirconolite (CaZrTi2O7). Samples in which tiny grains of this mineral (3–25 μm in size) were found span the entire range of lava compositions (47–62 wt%
SiO2). To the best of our knowledge, this is the first record of primary zirconolite in juvenile arc volcanics. The mineral forms
part of a vesicle-filling assemblage consisting of a network of quenched feldspar crystals and an SiO2 phase, probably cristobalite. High contents of Th, U and REE (up to 9.3, 4.3 and 15.6 wt% oxide respectively) and very high
Fe contents (up to 13.5 wt% Fe2O3) distinguish these zirconolites from those of other rock types. The extraction of volatile-rich phases with changing compositions
in successive stages is considered to be responsible for the zirconolite formation. We hypothesise that a fluid capable of
transporting HFSE, REE, Th and U was extracted from the magma and (partly) crystallised within voids which had formed earlier
upon saturation of an aqueous fluid. Assuming that zirconolite compositions largely reflect trace metal contents of the coexisting
fluid phase, significant amounts of `immobile' elements must have been transported on a macroscopic scale. Our findings thus
point to a late-stage transfer of HFSE, REE, Th and U between different domains in a cooling magma body. Such a volatile-induced
redistribution of trace elements at shallow levels of high-K volcanic systems may be significant for conventional geochemical
modelling of magma evolution and for Th–U disequilibrium studies.
Received: 3 November 1999 / Accepted: 29 February 2000 相似文献
4.
Orthopyroxene porphyroblasts zoned to interiors abnormally low in Al and Cr and containing numerous inclusions of olivine
occur in some spinel peridotite xenoliths from the Colorado Plateau. Rims of these orthopyroxene grains contain 2.5–3.0 wt%
Al2O3, consistent with equilibration in spinel peridotite at temperatures near 850 °C, but interiors contain as little as 0.20
wt% Al2O3 and 0.04 wt% Cr2O3. The Al-poor compositions are inferred to have equilibrated in chlorite peridotite, before porphyroblast growth during heating
and consequent reactions that eliminated talc, tremolite, and chlorite. The distinctive orthopyroxene textures are inferred
to have formed during reaction of talc and olivine. Rare intergrowths of orthopyroxene plus diopside are attributed to olivine-tremolite
reaction. Al and Cr have gradients at grain rims that appear little modified by diffusion, but divalent elements are almost
homogeneous throughout the porphyroblasts. Judging from the relative gradients, diffusion of Ca was at least 100 times faster
than that of Al and Cr at the temperatures near and below 850 °C. Diffusion of Al and Cr was most effective along subgrain
boundaries, and along these boundaries it appears to have been at least ten times faster than within the lattice: diffusion
along such boundaries may be a dominant mechanism for re-equilibration of orthopyroxene at low mantle temperatures. Orthopyroxene
with similar low Al and Cr occurs in chlorite peridotite xenoliths from the Navajo field, 300 km east of the Grand Canyon
localities, and in spinel peridotite xenoliths from the Sierra Nevada, 500 km west across the extended Basin and Range province.
Chlorite peridotite may therefore have been a significant minor component in much of the mantle lithosphere of western North
America, although evidence for it would be erased at the higher temperatures recorded by xenoliths from the Basin and Range.
Chemical changes during hydration may have been important in the evolution of these mantle volumes, and the case for addition
of Sr is particularly strong. Dehydration reactions during heating could have influenced patterns of extension and crustal
magmatism.
Received: 1 July 1996 / Accepted: 2 December 1996 相似文献
5.
Summary Ti-bearing phlogopite-biotite is dominant in Ugandan kamafugite-carbonatite effusives and their entrained alkali clinopyroxenite
xenoliths. It occurs as xeno/phenocrysts, microphenocrysts and groundmass minerals and also as a major xenolith mineral. Xenocrystic
micas in kamafugites and carbonatites are aluminous (> 12 wt% Al2O3), typically contain significant levels of Cr (up to 1.1 wt% Cr2O3), and are Ba-poor. Microphenocryst and groundmass micas in feldspathoidal rocks extend to Al-poor compositions, are depleted
in Cr, and are generally enriched in Ba. In general, xenocrystic micas occupy the Al2O3 and TiO2 compositional field of the xenolith mica, and on the basis of Mg#, and high P, T experimental evidence they probably crystallised
at mantle pressures. Mica xenocryst Cr contents range from those in Cr-poor megacryst and MARID phlogopite to higher values
found in primary and metasomatic phlogopites in kimberlite-hosted peridotite xenoliths. Such Cr contents in Ugandan mica xenocrysts
are considered consistent with derivation from carbonate-bearing phlogopite wehrlite and phlogopite-clinopyroxenite mantle.
Olivine melilitite xenocryst micas are distinguished by higher Mg# and Cr content than mica in clinopyroxenite xenoliths and
mica in Katwe-Kikorongo mixed melilitite-carbonatite tephra. Higher Al2O3 distinguishes Fort Portal carbonatite xenocrysts and some contain high Cr. It is suggested that the genesis of Katwe-Kikorongo
olivine melilitite and Fort Portal carbonatite involves a carbonate-bearing phlogopite wehrlite source while the source of
the mixed carbonatite-melilitite rocks may be carbonate-bearing phlogopite clinopyroxenite.
Received January 24, 2000; revised version accepted September 27, 2001 相似文献
6.
Richterite-bearing peridotites and MARID-type inclusions in lavas from North Eastern Morocco: mineralogy and D/H isotopic studies 总被引:4,自引:0,他引:4
C. Wagner Etienne Deloule Abdelkader Mokhtari 《Contributions to Mineralogy and Petrology》1996,124(3-4):406-421
K-richterite/phlogopite-bearing peridotite xenoliths and MARID inclusions have been found in Late Cretaceous (67±0.2 million
years) monchiquites and an olivine nephelinite from North Eastern Morocco. It is the first evidence of MARID rocks and K-richterite/phlogopite-bearing
peridotites outside the kimberlitic context. In the hydrous xenoliths, textural features suggest that K-richterite, phlogopite
and Al-poor diopside are replacement minerals. K-richterites contain 2–5 wt% FeO, 0.1–1.5 wt% TiO2 and <0.5 wt% Cr2O3. Micas contain 5.4–7.4 wt% FeO and 0.3–2.2 wt% TiO2, with Cr and Ni contents <0.2 wt%. Diopsides are Al-poor (<0.2 wt% Al2O3) and contain 0.1–0.2 wt% TiO2, 0.9–1.1 wt% Na2O and 1.3–1.7 wt% Cr2O3. Compared to known K-richterites and micas from metasomatised peridotite nodules (PKP types), the Moroccan minerals are more
Fe rich, K-richterites have higher Ti and micas less Cr and Ni. They are thus closer to MARID than to PKP minerals. K-richterites
and mica from the MARID inclusions show typical characteristics, e.g. high FeO (4.3–4.7 wt% in richterite and 7.2 wt% in mica),
low NiO and/or Cr2O3 and the incomplete filling of the tetrahedral site by Si+Al. Ion probe D/H analyses of amphiboles and micas from both xenolith
types give high δD values ranging from –8 to –73, with large variations within single grains (up to 50‰). Both the D-enrichment
and the δD variations are inherited from the mantle. The similar chemical composition and δD values of K-richterite/phlogopite
from the hydrous peridotites and MARID minerals suggest a genetic link between the two types of xenoliths. The conditions
required for producing MARIDs and K-richterite/phlogopite-bearing peridotites may thus exist in contexts other than stable
cratonic settings. MARID rocks and the associated metasomites may result from a hydrous fluid interaction with a peridotite,
the metasomatic agent being characterised by a high K and low Al signature and a high δD value. A D-rich source is involved
in the metasomatic event producing the hydrous minerals, and the scatter observed in the δD values suggests a mixing between
this source and another one with typical upper mantle D/H composition. As indicated by the low δD (–74) values of micas from
the host lava, metasomatism predated and is unrelated to the alkaline volcanism.
Received: 9 March 1995 / Accepted: 4 April 1996 相似文献
7.
Maya G. Kopylova John J. Gurney Leon R. M. Daniels 《Contributions to Mineralogy and Petrology》1997,129(4):366-384
More than 99% of mineral inclusions in diamonds from the River Ranch pipe in the Late Archean Limpopo Mobile Belt (Zimbabwe),
are phases of harzburgitic paragenesis, namely olivine (Fo92–93), orthopyroxene (Mg# = 93), G10 garnets and chromites. The diamond inclusion (DI) chemistry demonstrates a limited overlap
with River Ranch kimberlite macrocrysts: the DI garnets are more Ca-undersaturated, and DI spinel and garnet are more Mg-rich.
Most River Ranch diamond inclusions were equilibrated at T = 1080–1320 °C, P = 47–61 kbar, and f
O2 between IW and WM buffers. The P/T profile beneath the Limpopo Mobile Belt (LMB) is consistent with a paleo-heat flow of 41–42 mW/m2, similar to calculations for Roberts Victor, but hotter than for the Finsch, Kimberley, Koffiefontein and Premier Mines.
This is ascribed to the younger tectonothermal age of the LMB and its proximity to Late Archean oceans. Like diamond inclusions
from all other kimberlites studied, the River Ranch DI have a lithospheric affinity and therefore indicate that an ancient,
chemically depleted, thick (at least 200 km) mantle root existed beneath the Limpopo Mobile Belt 530–540 Ma ago. The mantle
root might have developed beneath the continental Central Zone of the LMB as early as the Archean, and could be alien to the
overthrust allochthonous sheet of the Limpopo Belt. Oxygen fugacity estimates for diamond inclusions at River Ranch are similar
to other diamondiferous harzburgites beneath the Kaapvaal craton, indicating that the Kaapvaal mantle as a whole was well
buffered and homogeneous with respect to f
O2 at the time of peridotitic diamond crystallization.
Received: 11 January 1995 / Accepted: 10 June 1997 相似文献
8.
Origin of phlogopite-orthopyroxene inclusions in chromites from the Merensky Reef of the Bushveld Complex,South Africa 总被引:1,自引:1,他引:0
About 30% of the chromite grains of variable sizes in a chromitite seam at the base of the Merensky Reef of the Bushveld Complex
on the farm Vlakfontein contain abundant composite mineral inclusions. The inclusions are polygonal to circular with radial
cracks that protrude into the enclosing chromite. They vary from a few microns to several millimeters in diameter and are
concentrated in the cores and mantles of chromite crystals. Electron backscattered patterns indicate that the host chromites
are single crystals and not amalgamations of multiple grains. Na-phlogopite and orthopyroxene are most abundant in the inclusions.
Edenitic hornblende, K-phlogopite, oligoclase and quartz are less abundant. Cl-rich apatite, rutile, zircon and chalcopyrite
are present at trace levels. Na-phlogopite is unique to the inclusions; it has not been found elsewhere in the Bushveld Complex.
Other minerals in the inclusions are also present in the matrix of the chromitite seam, but their compositions are different.
The Mg/(Mg+Fe2+) ratios of orthopyroxene in the inclusions are slightly higher than those of orthopyroxene in the matrix. K-phlogopite in
the inclusions contains more Na than in the matrix. The average compositions of the inclusions are characterized by high MgO
(26 wt%), Na2O (2.4 wt%) and H2O (2.6 wt%), and low CaO (1.1 wt%) and FeO (4.4 wt%). The δ18O value of the trapped melt, estimated by analysis of inclusion-rich and inclusion-poor chromites, is ∼7‰. This value is consistent
with the previous estimates for the Bushveld magma and with the δ18O values of silicate minerals throughout the reef. The textural features and peculiar chemical compositions are consistent
with entrapment of orthopyroxene with variable amounts of volatile-rich melts during chromite crystallization. The volatile-rich
melts are thought to have resulted from variable degrees of mixing between the magma on the floor of the chamber and Na-K-rich
fluids expelled from the underlying crystal pile. The addition of fluid to the magma is thought to have caused dissolution
of orthpyroxene, leaving the system saturated only in chromite. Both oxygen and hydrogen isotopic values are consistent with
the involvement of a magmatic fluid in the process of fluid addition and orthopyroxene dissolution. Most of the Cr and Al
in the inclusions was contributed through wall dissolution of the host chromite. Dissolution of minor rutile trapped along
with orthopyroxene provided most of the Ti in the inclusions. The Na- and K-rich hydrous silicate minerals in the inclusions
were formed during cooling by reaction between pyroxene and the trapped volatile-rich melts. 相似文献
9.
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
10.
Trace element distribution in Central Dabie eclogites 总被引:16,自引:0,他引:16
R. Sassi B. Harte D. A. Carswell H. Yujing 《Contributions to Mineralogy and Petrology》2000,139(3):298-315
Coesite-bearing eclogites from Dabieshan (central China) have been studied by ion microprobe to provide information on trace
element distributions in meta-basaltic mineral assemblages during high-pressure metamorphism. The primary mineralogy (eclogite
facies) appears to have been garnet and omphacite, usually with coesite, phengite and dolomite, together with high-alumina
titanite or rutile, or both titanite and rutile; kyanite also occurs occasionally as an apparently primary phase. It is probable
that there was some development of quartz, epidote and apatite whilst the rock remained in the eclogite facies. A later amphibolite
facies overprint led to partial replacement of some minerals and particularly symplectitic development after omphacite. They
vary from very fine-grained dusty-looking to coarser grained Am + Di + Pl symplectites. The eclogite facies minerals show
consistent trace element compositions and partition coefficients indicative of mutual equilibrium. Titanite, epidote and apatite
all show high concentrations of REE relative to clinopyroxene. The compositions of secondary (amphibolite facies) minerals
are clearly controlled by local rather than whole-rock equilibrium, with the composition of amphibole in particular depending
on whether it is replacing clinopyroxene or garnet. REE partition coefficients for Cpx/Grt show a dependence on the Ca content
of the host phases, with D
REE
Cpx/Grt
decreasing with decreasing D
Ca
. This behaviour is very similar to that seen in mantle eclogites, despite differences in estimated temperatures of formation
of 650–850 °C (Dabieshan) and 1000–1200 °C (mantle eclogites). With the exception of HREE in garnet, trace elements in the
eclogites are strongly distributed in favour of minor or accessory phases. In particular, titanite and rutile strongly concentrate
Nb and Zr, whilst LREE–MREE go largely into epidote, titanite and apatite. If these minor/accessory minerals behave in a refractory
manner during melting or fluid mobilisation events and do not contribute to the melt/fluid, then the resultant melts and fluids
will be strongly depleted in LREE–MREE.
Received: 11 February 1999 / Accepted: 31 January 2000 相似文献
11.
Thomas Stachel Gerhard P. Brey Jeff W. Harris 《Contributions to Mineralogy and Petrology》2000,140(1):1-15
Peridotitic inclusions in alluvial diamonds from the Kankan region of Guinea in West Africa are mainly of lherzolitic paragenesis.
Nevertheless, extreme Cr2O3 contents (max. 17 wt%) in some of the exclusively lherzolitic garnets document that the diamond source experienced a previous
stage of melt extraction in the spinel stability field. This initial depletion was followed by at least two metasomatic stages:
(1) enrichment of LREE and Sr and (2) introduction mainly of MREE–HREE and other HFSE (Ti, Y, Zr, Hf). The Ti- and HFSE-poor
character of stage (1) points towards a CHO-rich fluid or carbonatitic melt, the high HFSE in stage (2) favour silicate melts
as enriching agent. Eclogitic inclusions are derived from a large depth interval ranging from the lithosphere through the
asthenosphere into the transition zone. The occurrence of negative Eu anomalies in garnet and clinopyroxene from both lithosphere
and transition zone suggests a possible relationship to subducted oceanic crust. Lithospheric eclogitic inclusions are derived
from heterogeneous sources, that may broadly be divided into a low-Ca group with LREE depleted trace element patterns and
a high-Ca group representing a source with negative LREE–HREE slope that is moderately enriched in incompatible elements relative
to primitive mantle. High-Ca inclusions of majoritic paragenesis are significantly more enriched in incompatible elements,
such as in Sr and LREE. Calculated whole rock compositions require metasomatic enrichment even if a derivation from MORB is
assumed.
Received: 26 January 2000 / Accepted: 18 May 2000 相似文献
12.
Models of corundum origin from alkali basaltic terrains: a reappraisal 总被引:10,自引:0,他引:10
F. Lin Sutherland Paul W. O. Hoskin C. Mark Fanning Robert R. Coenraads 《Contributions to Mineralogy and Petrology》1998,133(4):356-372
Corundums from basalt fields, particularly in Australia and Asia, include a dominant blue-green-yellow zoned “magmatic” suite
(BGY suite) and subsidiary vari-coloured “metamorphic” suites. The BGY corundums have distinctive trace element contents (up
to 0.04 wt% Ga2O3 and low Cr/Ga and Ti/Ga ratios <1). Different melt origins for BGY corundums are considered here from their inclusion and
intergrowth mineralogy, petrologic associations and tectonic setting. Analysed primary inclusion minerals (over 100 inclusions)
cover typical feldspars, zircon and Nb-Ta oxides and also include hercynite-magnetite, gahnospinel, rutile-ilmenite solid
solution, calcic plagioclase, Ni-rich pyrrhotite, thorite and low-Si and Fe-rich glassy inclusions. This widens a previous
inclusion survey; New England, East Australia corundums contain the most diverse inclusion suite known from basalt fields
(20 phases). Zircon inclusion, intergrowth and megacryst rare earth element data show similar patterns, except for Eu which
shows variable depletion. Temperature estimates from magnetite exsolution, feldspar compositions and fluid inclusion homogenization
suggest that some corundums crystallized between 685–900 °C. Overlap of inclusion Nb, Ta oxide compositions with new comparative
data from niobium-yttrium-fluorine enriched granitic pegmatites favour a silicate melt origin for the corundums. The feasibility
of crystallizing corundum from low-volume initial melting of amphibole-bearing mantle assemblages was tested using the MELTS
program on amphibole-pyroxenite xenolith chemistry from basalts. Corundum appears in the calculations at 720–880 °C and 0.7–1.1 GPa
with residual feldspathic assemblages that match mineral compositions found in corundums and their related xenoliths. A model
that generates melts from amphibole-bearing lithospheric mantle during magmatic plume activity is proposed for BGY corundum
formation.
Received: 3 January 1997 / Accepted: 8 July 1998 相似文献
13.
T. F. D. Nielsen I. P. Solovova I. V. Veksler 《Contributions to Mineralogy and Petrology》1997,126(4):331-344
Perovskite and melilite crystals from melilitolites of the ultramafic alkaline Gardiner complex (East Greenland) contain
crystallised melt inclusions derived from: (1) melilitite; (2) low-alkali carbonatite; (3) natrocarbonatite. The melilitite
inclusion (1) homogenisation temperature of 1060 °C is similar to liquidus temperatures of experimentally investigated natural
melilitites. The compositions are peralkaline, low in MgO (ca.␣5 wt%), Ni and Cr, and they are low-pressure fractionates of
more magnesian larnite-normative ultramafic lamprophyre-type melts of primary mantle origin. Low-alkali carbonatite compositions
(2) homogenise at 1060–1030 °C and are compositionally similar to immiscible calcite carbonatite dykes derived from the melilitolite
magma. Natrocarbonatite inclusions (3) homogenise between 1030 and 900 °C and are compositionally similar to natrocarbonatite
lava from Oldoinyo Lengai. Nephelinitic to phonolitic dykes which are related to the calcite carbonatite dykes, are very Zr-rich
and agpaitic (molecular Na2O + K2O/Al2O3 > 1.2) and resemble nephelinites of Oldoinyo Lengai. The petrographic, geochemical and temporal relationships indicate unmixing
of carbonatite compositions (ca. 10% alkalies) from evolving melilitite melt and continued fractionation of melilitite to
nephelinite. It is suggested that the natrocarbonatite compositions represent degassed supercritical high temperature fluid
formed in a cooling body of strongly larnite-normative nephelinite or evolved melilitite. The Gardiner complex and similar
melilitolite and carbonatite-bearing ultramafic alkaline complexes are believed to represent subvolcanic complexes formed
beneath volcanoes comparable to Oldoinyo Lengai and that the suggested origin of natrocarbonatite may be applied to natrocarbonatites
of Oldoinyo Lengai.
Received: 18 January 1996 / Accepted: 2 September 1996 相似文献
14.
Fluids in low-pressure migmatites: a fluid inclusion study of rocks from the Gennargentu Igneous Complex (Sardinia, Italy) 总被引:1,自引:0,他引:1
Summary The low-pressure emplacement of a quartz diorite body in the metapelitic rocks of the Gennargentu Igneous Complex (Sardinia,
Italy) produced a contact metamorphic aureole and resulted in migmatisation of part of the aureole through partial melting.
The leucosome, formed by dehydration melting involving biotite, is characterised by granophyric intergrowth and abundant magnetite
crystals. A large portion of the high temperature contact aureole shows petrographic features that are intermediate between
quartz diorite and migmatite s.s. (i.e. hybrid rocks). A fluid inclusion study has been performed on quartz crystals from the quartz diorite and related contact
aureole rocks, i.e. migmatite sensu stricto (s.s.) and hybrid rocks. Three types of fluid inclusions have been identified: I) monophase V inclusions, II) L + V, either L-rich
or V-rich aqueous saline inclusions and III) multiphase V + L + S inclusions. Microthermometric data characterised the trapped
fluid as a complex aqueous system varying from H2O–NaCl–CaCl2 in the quartz diorite to H2O–NaCl–CaCl2–FeCl2 in the migmatite and hybrid rocks. Fluid salinities range from high saline fluids (50 wt% NaCl eq.) to almost pure aqueous
fluid. Liquid-vapour homogenisation temperatures range from 100 to over 400 °C with an average peak around 300 °C. Temperatures
of melting of daughter minerals are between 300 and 500 °C. Highly saline liquid- and vapour-rich inclusions coexist with
melt inclusions and have been interpreted as brine exsolved from the crystallising magma. Fluid inclusion data indicate the
formation of fluid of high iron activity during the low-pressure partial melting and a fluid mixing process in the hybrid
rocks. 相似文献
15.
Melt inclusions in pegmatite quartz: complete miscibility between silicate melts and hydrous fluids at low pressure 总被引:10,自引:1,他引:9
Fluorine-, boron- and phosphorus-rich pegmatites of the Variscan Ehrenfriedersdorf complex crystallized over a temperature
range from about 700 to 500 °C at a pressure of about 1 kbar. Pegmatite quartz crystals continuously trapped two different
types of melt inclusions during cooling and growth: a silicate-rich H2O-poor melt and a silicate-poor H2O-rich melt. Both melts were simultaneously trapped on the solvus boundaries of the silicate (+ fluorine + boron + phosphorus) − water
system. The partially crystallized melt inclusions were rehomogenized at 1 kbar between 500 and 712 °C in steps of 50 °C by
conventional rapid-quench hydrothermal experiments. Glasses of completely rehomogenized inclusions were analyzed for H2O by Raman spectroscopy, and for major and some trace elements by EMP (electron microprobe). Both types of melt inclusions
define a solvus boundary in an XH2O–T pseudobinary system. At 500 °C, the silicate-rich melt contains about 2.5 wt% H2O, and the conjugate water-rich melt about 47 wt% H2O. The solvus closes rapidly with increasing temperature. At 650 °C, the water contents are about 10 and 32 wt%, respectively.
Complete miscibility is attained at the critical point: 712 °C and 21.5 wt% H2O. Many pegmatites show high concentrations of F, B, and P, this is particularly true for those pegmatites associated with
highly evolved peraluminous granites. The presence of these elements dramatically reduces the critical pressure for fluid–melt
systems. At shallow intrusion levels, at T ≥ 720 °C, water is infinitely soluble in a F-, B-, and P-rich melt. Simple cooling
induces a separation into two coexisting melts, accompanied with strong element fractionation. On the water-rich side of the
solvus, very volatile-rich melts are produced that have vastly different physical properties as compared to “normal” silicate
melts. The density, viscosity, diffusivity, and mobility of such hyper-aqueous melts under these conditions are more comparable
to an aqueous fluid.
Received: 15 September 1999 / Accepted: 10 December 1999 相似文献
16.
Water solubility in pyrope to 100 kbar 总被引:14,自引:0,他引:14
The solubility and incorporation mechanism of water in natural, almost pure pyrope from Dora Maira, Western Alps was investigated.
The infrared spectrum of the natural, untreated sample (58 ppm water) shows several exceptionally sharp bands in the OH-stretching
region, including a single band at 3601.9 cm−1 and a band system with main components at 3640.5, 3650.8 and 3660.6 cm−1. High-temperature and high-pressure infrared spectra suggest that the two absorption features arise from almost free OH groups
in sites with different compressibility and thermal expansivity, with the site causing the 3601.9 cm−1 band being much stiffer. Pyrope samples were annealed in a piston-cylinder or multi-anvil apparatus for several days in the
presence of excess water, excess SiO2 and excess Al2SiO5 to determine the equilibrium solubility of water in pyrope to 100 kbar. Total solubility increases with pressure, however,
this is exclusively due to the high-frequency band system, while the intensity of the low-frequency band decreases with pressure.
At 1000 °C and the oxygen fugacity of the Ni-NiO buffer, the bulk solubility can be described by the equation c
OH
=Af
H2O
0.5exp(−PΔV/RT) with A = 0.679 ppm/bar0.5 and ΔV = 5.71 cm3/mol. This equation implies the incorporation of water in the crystal as isolated OH groups. With increasing temperature,
solubility appears to decrease with ΔH = − 14 kJ/mol. At Fe-FeO buffer conditions, solubility is 30 to 50% lower than with the Ni-NiO buffer, suggesting that the
incorporation of OH is not coupled to the reduction of Fe3+. Possibly, the 3601.9 cm−1 band is associated with the tetrahedral OH B defect and the high-frequency system with the dodecahedral OH Li defect. Based
on the experimentally established solubility model, it is estimated that garnet in a hot subducted slab will transport 170 ppm
of water into the mantle beyond the breakdown limit of amphibole. In a cold slab, 470 ppm of water can be incorporated into
garnet at the breakdown limit of phengite. These numbers imply that a significant fraction of the total water in the hydrosphere
has been recycled into the mantle since the Proterozoic.
Received: 6 January 1997 / Accepted: 27 March 1997 相似文献
17.
Two mantle sources, two plumbing systems: tholeiitic and alkaline magmatism of the Maymecha River basin, Siberian flood volcanic province 总被引:2,自引:0,他引:2
Nicholas Arndt Catherine Chauvel Gerald Czamanske Valeri Fedorenko 《Contributions to Mineralogy and Petrology》1998,133(3):297-313
Rocks of two distinctly different magma series are found in a ∼4000-m-thick sequence of lavas and tuffs in the Maymecha River
basin which is part of the Siberian flood-volcanic province. The tholeiites are typical low-Ti continental flood basalts with remarkably restricted, petrologically evolved compositions. They have basaltic
MgO contents, moderate concentrations of incompatible trace elements, moderate fractionation of incompatible from compatible
elements, distinct negative Ta(Nb) anomalies, and Nd values of 0 to +2. The primary magmas were derived from a relatively shallow mantle source, and evolved in large crustal
magma chambers where they acquired their relatively uniform compositions and became contaminated with continental crust. An
alkaline series, in contrast, contains a wide range of rock types, from meymechite and picrite to trachytes, with a wide range of compositions
(MgO from 0.7 to 38 wt%, SiO2 from 40 to 69 wt%, Ce from 14 to 320 ppm), high concentrations of incompatible elements and extreme fractionation of incompatible
from compatible elements (Al2O3/TiO2∼1; Sm/Yb up to 11). These rocks lack Ta(Nb) anomalies and have a broad range of Nd values, from −2 to +5. The parental magmas are believed to have formed by low-degree melting at extreme mantle depths (>200 km).
They bypassed the large crustal magma chambers and ascended rapidly to the surface, a consequence, perhaps, of high volatile
contents in the primary magmas. The tholeiitic series dominates the lower part of the sequence and the alkaline series the
upper part; at the interface, the two types are interlayered. The succession thus provides evidence of a radical change in
the site of mantle melting, and the simultaneous operation of two very different crustal plumbing systems, during the evolution
of this flood-volcanic province.
Received: 6 January 1998 / Accepted: 29 June 1998 相似文献
18.
N. V. Zubkova D. Yu. Pushcharovsky G. Giester E. Tillmanns I. V. Pekov D. A. Kleimenov 《Mineralogy and Petrology》2002,75(1-2):79-88
Summary
The crystal structure of arsentsumebite, ideally, Pb2Cu[(As, S)O4]2(OH), monoclinic, space group P21/m, a = 7.804(8), b = 5.890(6), c = 8.964(8) ?, β = 112.29(6)°, V = 381.2 ?3, Z = 2, dcalc. = 6.481 has been refined to R = 0.053 for 898 unique reflections with I> 2σ(I). Arsentsumebite belongs to the brackebuschite group of lead minerals with the general formula Pb2
Me(XO4)2(Z) where Me = Cu2+, Mn2+, Zn2+, Fe2+, Fe3+; X = S, Cr, V, As, P; Z = OH, H2O. Members of this group include tsumebite, Pb2Cu(SO4)(PO4)(OH), vauquelinite, Pb2Cu(CrO4)(PO4)(OH), brackebuschite, Pb2 (Mn, Fe)(VO4)2(OH), arsenbracke buschite, Pb2(Fe, Zn)(AsO4)2(OH, H2O), fornacite, Pb2Cu(AsO4)(CrO4)(OH), and feinglosite, Pb2(Zn, Fe)[(As, S)O4]2(H2O). Arsentsumebite and all other group members contain M = M–T chains where M = M means edge-sharing between MO6 octahedra and M–T represents corner sharing between octahedra and XO4 tetrahedra. A structural relationship exists to tsumcorite, Pb(Zn, Fe)2(AsO4)2 (OH, H2O)2 and tsumcorite-group minerals Me(1)Me(2)2(XO4)2(OH, H2O)2.
Received June 24, 2000; revised version accepted February 8, 2001 相似文献
19.
Hydroxyl defects in garnets from mantle xenoliths in kimberlites of the Siberian platform 总被引:12,自引:0,他引:12
A suite of more than 200 garnet single crystals, extracted from 150 xenoliths, covering the whole range of types of garnet
parageneses in mantle xenoliths so far known from kimberlites of the Siberian platform and collected from nearly all the kimberlite
pipes known in that tectonic unit, as well as some garnets found as inclusions in diamonds and olivine megacrysts from such
kimberlites, were studied by means of electron microprobe analysis and single-crystal IR absorption spectroscopy in the v
OH vibrational range in search of the occurrence, energy and intensity of the v
OH bands of hydroxyl defects in such garnets and its potential use in an elucidation of the nature of the fluid phase in the
mantle beneath the Siberian platform. The v
OH single-crystal spectra show either one or a combination of two or more of the following major v
OH bands, I 3645–3662 cm−1, II 3561–3583 cm−1, III 3515–3527 cm−1, and minor bands, Ia 3623–3631 cm−1, IIa 3593–3607 cm−1. The type of combination of such bands in the spectrum of a specific garnet depends on the type of the rock series of the
host xenolith, Mg, Mg-Ca, Ca, Mg-Fe, or alkremite, on the xenolith type as well as on the chemical composition of the respective
garnet. Nearly all garnets contain band systems I and II. Band system III occurs in Ti-rich garnets, with wt% TiO2 > ca. 0.4, from xenoliths of the Mg-Ca and Mg-Fe series, only. The v
OH spectra do not correspond to those of OH− defects in synthetic pyropes or natural ultra-high pressure garnets from diamondiferous metamorphics. There were no indications
of v
OH from inclusions of other minerals within the selected 60 × 60 μm measuring areas in the garnets. The v
OH spectra of pyrope-knorringite- and pyrope-knorringite-uvarovite-rich garnets included in diamonds do not show band systems
I to III. Instead, they exhibit one weak, broad band (Δv
OH 200–460 cm−1) near 3570 cm−1, a result that was also obtained on pyrope-knorringite-rich garnets extracted from two olivine megacrysts. The quantitative
evaluation, on the basis of relevant existing calibrational data (Bell et al. 1995), of the sum of integral intensities of
all v
OH bonds of the garnets studied yielded a wide range of “water” concentrations within the set of the different garnets, between
values below the detection limit of our single-crystal IR method, near 2 × 10−4 wt%, up to 163 × 10−4 wt%. The “water” contents vary in a complex manner in garnets from different xenolith types, obviously depending on a large
number of constraints, inherent in the crystal chemistry as well as the formation conditions of the garnets during the crystallization
of their mantle host rocks. Secondary alteration effects during uplift of the kimberlite, play, if any, only a minor role.
Despite the very complex pattern of the “water” contents of the garnets, preventing an evaluation of a straightforward correlation
between “water” contents of the garnets and the composition of the mantle's fluid phase during garnet formation, at least
two general conclusions could be drawn: (1) the wide variation of “water” contents in garnets is not indicative of regional
or local differences in the composition of the mantle's fluid phase; (2) garnets formed in the high-pressure/high-temperature
diamond-pyrope facies invariably contain significantly lower amounts of “water” than garnets formed under the conditions of
the graphite-pyrope facies. This latter result (2) may point to significantly lower f
H2O and f
O2 in the former as compared to the latter facies.
Received: 25 November 1997 / Accepted: 9 March 1998 相似文献
20.
Geochemical syntheses among the cratonic,off-cratonic and orogenic garnet peridotites and their tectonic implications 总被引:2,自引:0,他引:2
Benxun Su Hongfu Zhang Yanjie Tang Benny Chisonga Kezhang Qin Jifeng Ying Patrick Asamoah Sakyi 《International Journal of Earth Sciences》2011,100(4):695-715
Garnet-bearing mantle peridotites, occurring as either xenoliths in volcanic rocks or lenses/massifs in high-pressure and
ultrahigh-pressure terrenes within orogens, preserve a record of deep lithospheric mantle processes. The garnet peridotite
xenoliths record chemical equilibrium conditions of garnet-bearing mineral assemblage at temperatures (T) ranging from ~700 to 1,400°C and pressures (P) > 1.6–8.9 GPa, corresponding to depths of ~52–270 km. A characteristic mineral paragenesis includes Cr-bearing pyropic garnet
(64–86 mol% pyrope; 0–10 wt% Cr2O3), Cr-rich diopside (0.5–3.5 wt% Cr2O3), Al-poor orthopyroxene (0–5 wt% Al2O3), high-Cr spinel (Cr/(Cr + Al) × 100 atomic ratio = 2–86) and olivine (88–94 mol% forsterite). In some cases, partial melting,
re-equilibration involving garnet-breakdown, deformation, and mantle metasomatism by kimberlitic and/or carbonatitic melt
percolations are documented. Isotope model ages of Archean and Proterozoic are ubiquitous, but Phanerozoic model ages are
less common. In contrast, the orogenic peridotites were subjected to ultrahigh-pressure (UHP) metamorphism at temperature
ranging from ~700 to 950°C and pressure >3.5–5.0 GPa, corresponding to depths of >110–150 km. The petrologic comparisons between
231 garnet peridotite xenoliths and 198 orogenic garnet peridotites revealed that (1) bulk-rock REE (rare earth element) concentrations
in xenoliths are relatively high, (2) clinopyroxene and garnet in orogenic garnet peridotites show a highly fractionated REE
pattern and Ce-negative anomaly, respectively, (3) Fo contents of olivines for off-cratonic xenolith are in turn lower than
those of orogenic garnet and cratonic xenolith but mg-number of garnet for orogenic is less than that of off-cratonic and
on-cratonic xenolith, (4) Al2O3, Cr2O3, CaO and Cr# of pyroxenes and chemical compositions of whole rocks are very different between these garnet peridotites, (5)
orogenic garnet peridotites are characterized by low T and high P, off-cratonic by high T and low P, and cratonic by medium T and high P and (6) garnet peridotite xenoliths are of Archean or Proterozoic origin, whereas most of orogenic garnet peridotites are
of Phanerozoic origin. Taking account of tectonic settings, a new orogenic garnet peridotite exhumation model, crust-mantle
material mixing process, is proposed. The composition of lithospheric mantle is additionally constrained by comparisons and
compiling of the off-cratonic, on-cratonic and orogenic garnet peridotite. 相似文献