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1.
《地学前缘(英文版)》2022,13(4):101380
Melting experiments on ultramafic rocks rich in the hydrous minerals phlogopite or phlogopite + K-richterite, some including 5% of accessory phases, have been conducted at 15 and 50 kbar. The assemblages represent probable source components that contribute to melts in cratonic regions, but whose melt compositions are poorly known. A main series of starting compositions based on MARID xenoliths consisted of a third each of clinopyroxene (CPX), phlogopite (PHL) and K-richterite (KR) with or without 5% ilmenite, rutile or apatite. Additional experiments were run without KR and with higher proportions of accessory phases. Melt traps were used at near-solidus temperatures to facilitate accurate analysis of well-quenched melts, for which reversal experiments demonstrate equilibrium.Results show that KR melts rapidly and completely within 50 °C of the solidus, so that melts reflect the composition of the amphibole and its melting reaction. Melts have high SiO2 and especially K2O but low CaO and Al2O3 relative to basaltic melts produced from peridotites at similar pressures. They have no counterparts amongst natural rocks, but most closely resemble leucite lamproites at 15 kbar. KR and PHL melt incongruently to form olivine (OL) and CPX at 15 kbar, promoting SiO2 contents of the melt, whereas orthopyroxene OPX is increasingly stable at lower lithosphere pressures, leading to an increase in MgO and decrease in SiO2 in melts, which resemble olivine lamproites. Melts of mica pyroxenites without KR are richer in CaO and Al2O3 and do not resemble lamproites. These experiments show that low CaO and Al2O3 in igneous rocks is not necessarily a sign of a depleted peridotite source. Accessory phases produce melts exceptionally rich in P2O5 or TiO2 depending on the phases present and are unlike any melts seen at the Earth’s surface, but may be important agents of metasomatism seen in xenoliths. The addition of the 5% accessory phases ilmenite, rutile or apatite result in melting temperatures a few ten of degrees lower; at least two of these appear essential to explain the compositions of many alkaline igneous rocks on cratons.Melting temperatures for CPX + PHL + KR mixtures are close to cratonic geotherms at depths > 130 km: minor perturbations of the stable geotherm at >150 km will rapidly lead to 20% melting. Melts of hydrous pyroxenites with a variety of accessory phases will be common initial melts at depth, but will change if reaction with wall-rocks occurs, leading to volcanism that contains chemical components of peridotite even though the temperature in the source region remains well below the melting point of peridotite. At higher temperatures, extensive melting of peridotite will dilute the initial alkaline melts: this is recognizable as alkaline components in basalts and, in extreme cases, alkali picrites. Hydrous pyroxenites are, therefore, components of most mantle-derived igneous rocks: basaltic rocks should not be oversimplified as being purely melts of peridotite or of mixtures of peridotite and dry pyroxenite without hydrous phases. 相似文献
2.
A suggested origin of MARID xenoliths in kimberlites by high pressure crystallization of an ultrapotassic rock such as lamproite 总被引:1,自引:0,他引:1
Frances G. Waters 《Contributions to Mineralogy and Petrology》1987,95(4):523-533
Chemical, mineralogical and isotopic studies have been made on nodules of the MARID (Mica-Amphibole-Rutile-Ilmenite-Diopside) xenolith suite in southern African kimberlites. All are ultramafic and ultrapotassic (MgO= 20–25%, K2O=4–9%), with bulk compositions reflecting the wide variation in relative proportions of the five minerals amongst the nodules. They are comparable in major element compositions to magnesian lamproites, in particular the ultrabasic olivine-lamproites of Western Australia. In a number of high pressure experimental studies on ultra-potassic rocks, the phases produced between 25–30 kbar from compositions comparable to those of MARID rocks (in the presence of additional water), were predominantly phlogopite and diopside (±K-richterite, ±ilmenite, ±rutile). Furthermore the compositions of experimental minerals produced in the synthetic-biotite-mafurite-H2O system by Edgar et al. (1976) are similar to those in MARID rocks.It is suggested on the basis of these observations and the textural appearance of MARID rocks that they are magmatic compositional equivalents of MgO-rich lamproites that crystallized at high pressures. While lamproites have higher average concentrations of incompatible elements, (including REE), some MARID rocks have comparable abundances. It is suggested that late stage vapour-rich melts carrying substantial REE and other incompatible elements escaped from crystallizing MARID magmas into surrounding subcontinental lithosphere, thus resulting in lower levels of these elements in most MARID rocks. In contrast faster crystallization of lamproitic rocks under volcanic/ hypabyssal conditions would prevent similar losses.The MARID proto-magmas are thought to be either partial melts of metasomatised phlogopite peridotite, or small volume asthenospheric melts which are modified and further enriched by incorporation of small partial melts of enriched subcontinental lithosphere during magma ascent. 相似文献
3.
Jürgen Konzett 《Contributions to Mineralogy and Petrology》1997,128(4):385-404
Experiments have been conducted in a peralkaline Ti-KNCMASH system representative of MARID-type bulk compositions to delimit
the stability field of K-richterite in a Ti-rich hydrous mantle assemblage, to assess the compositional variation of amphibole
and coexisting phases as a function of P and T, and to characterise the composition of partial melts derived from the hydrous assemblage. K-richterite is stable in experiments
from 0.5 to 8.0 GPa coexisting with phlogopite, clinopyroxene and a Ti-phase (titanite, rutile or rutile + perovskite). At
8.0 GPa, garnet appears as an additional phase. The upper T stability limit of K-richterite is 1200–1250 °C at 4.0 GPa and 1300–1400 °C at 8.0 GPa. In the presence of phlogopite, K-richterite
shows a systematic increase in K with increasing P to 1.03 pfu (per formula unit) at 8.0 GPa/1100 °C. In the absence of phlogopite, K-richterite attains a maximum of 1.14 K
pfu at 8.0 GPa/1200 °C. Titanium in both amphibole and mica decreases continuously towards high P with a nearly constant partitioning while Ti in clinopyroxene remains more or less constant. In all experiments below 6.0 GPa
ΣSi + Al in K-richterite is less than 8.0 when normalised to 23 oxygens+stoichiometric OH. Rutiles in the Ti-KNCMASH system
are characterised by minor Al and Mg contents that show a systematic variation in concentration with P(T) and the coexisting assemblage. Partial melts produced in the Ti-KNCMASH system are extremely peralkaline [(K2O+Na2O)/Al2O3 = 1.7–3.7], Si-poor (40–45 wt% SiO2), and Ti-rich (5.6–9.2 wt% TiO2) and are very similar to certain Ti-rich lamproite glasses. At 4.0 GPa, the solidus is thought to coincide with the K-richterite-out
reaction, the first melt is saturated in a phlogopite-rutile-lherzolite assemblage. Both phlogopite and rutile disappear ca.
150 °C above the solidus. At 8.0 GPa, the solidus must be located at T≤1400 °C. At this temperature, a melt is in equilibrium with a garnet- rutile-lherzolite assemblage. As opposed to 4.0 GPa, phlogopite
does not buffer the melt composition at 8.0 GPa. The experimental results suggest that partial melting of MARID-type assemblages
at pressures ≥4.0 GPa can generate Si-poor and partly ultrapotassic melts similar in composition to that of olivine lamproites.
Received: 23 December 1996 / Accepted: 20 March 1997 相似文献
4.
Phase relations have been determined at 20 kbar and primarily under suprasolidus conditions in the Fe−Ti-free F-bearing K-richterite—phlogopite
and K-richterite—apatite systems in order to assess the partitioning of F among phlogopite, K-richterite, apatite, and melt
under upper-mantle conditions. Both systems are pseudoternary because they contain forsterite, enstatite and a diopside-rich
clinopyroxene from the breakdown of the mica and K-richterite. The F-bearing K-richterite systems have lower minimum melting
temperatures than the F-bearing phlogopite —apatite system at the same pressure. However in the systems studied, F in phlogopite
appears the most effective component in altering minimum liquid compositions whereas comparison between the present study
and previous systems suggests that the presence of P2O5 during melting may result in more K-enriched melts. Variations in the compositions of the F-bearing phases are primarily
controlled by the bulk compositions of the end-member minerals and by temperature, although buffering by non-F bearing minerals
(e.g. clinopyroxene) may be effective. Distribution coefficients (as wt% ratios) between F-bearing minerals and coexisting
liquids have been determined as functions of bulk composition and temperature for products of experiments. Distribution coefficients
between K-richterite—liquid, apatite—liquid, and phlogopite—liquid are ≥1 to slightly <1 for most bulk compositions, indicating
thatF is generally a compatible element. This conclusion is in agreement with the sequence ofF distribution for similar phases in ultrapotassic rocks. These results preclude F-bearing mineral reservoirs in the mantle,
at depths corresponding to 20 kbar, being capable of producing F-enrichment in ultrapotassic magmas, or being effective in
redox melting processes.
Editorial responsibility: K. Hodges 相似文献
5.
High PT experiments were performed in the range 2.5–19 GPa and 800–1,500°C using a synthetic peridotite doped with trace elements
and OH-apatite or with Cl-apatite + phlogopite. The aim of the study was (1) to investigate the stability and phase relations
of apatite and its high PT breakdown products, (2) to study the compositional evolution with P and T of phosphate and coexisting
silicate phases and (3) to measure the Cl-OH partitioning between apatite and coexisting calcic amphibole, phlogopite and
K-richterite. Apatite is stable in a garnet-lherzolite assemblage in the range 2.5–8.7 GPa and 800–1,100°C. The high-P breakdown
product of apatite is tuite γ-Ca3 (PO4)2, which is stable in the range 8–15 GPa and 1,100–1,300°C. Coexisting apatite and tuite were observed at 8 GPa/1,050°C and
8.7 GPa/1,000°C. MgO in apatite increases with P from 0.8 wt% at 2.5 GPa to 3.2 wt% at 8.7 GPa. Both apatite and tuite may
contain significant Na, Sr and REE with a correlation indicating 2 Ca2+=Na+ + REE3+. Tuite has always higher Sr and REE and lower Fe and Mg than apatite. Phosphorus in the peridotite phases decreases in the
order Pmelt ≫ Pgrt ≫ PMg2SiO4 > Pcpx > Popx. The phosphate-saturated P2O5 content of garnet increases from 0.07 wt% at 2.5 GPa to 1.5 wt% at 12.8 GPa. Due to the low bulk Na content of the peridotite,
[8]Na[4]P[8]M2+
−1
[4]Si−1 only plays a minor role in controlling the phosphorus content of garnet. Instead, element correlations indicate a major contribution
of [6]M2+[4]P[6]M3+
−1
[4]Si−1. Pyroxenes contain ~200–500 ppm P and olivine has 0.14–0.23 wt% P2O5 in the P range 4–8.7 GPa without correlation with P, T or XMg. At ≥12.7 GPa, all Mg2SiO4 polymorphs have <200 ppm P. Coexisting olivine and wadsleyite show an equal preference for phosphorus. In case of coexisting
wadsleyite and ringwoodite, the latter fractionates phosphorus. Although garnet shows by far the highest phosphorus concentrations
of any peridotite silicate phase, olivine is no less important as phosphorus carrier and could store the entire bulk phosphorus
budget of primitive mantle. In the Cl-apatite + phlogopite-doped peridotite, apatite contains 0.65–1.35 wt% Cl in the PT range
2.5–8.7 GPa/800–1,000°C. Apatite coexists with calcic amphibole at 2.5 GPa, phlogopite at 2.5–5 GPa and K-richterite at 7 GPa,
and all silicates contain between 0.2 and 0.6 wt% Cl. No solid potassic phase is stable between 5 and 8.7 GPa. Cl strongly
increases the solubility of K in hydrous fluids. This may lead to the breakdown of phlogopite and give rise to the local presence
in the mantle of fluids strongly enriched in K, Cl, P and incompatible trace elements. Such fluids may get trapped as micro-inclusions
in diamonds and provide bulk compositions suitable for the formation of unusual phases such as KCl or hypersilicic Cl-rich
mica. 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.
The Finero phlogopite-peridotite massif: an example of subduction-related metasomatism 总被引:13,自引:0,他引:13
Alberto Zanetti Maurizio Mazzucchelli Giorgio Rivalenti Riccardo Vannucci 《Contributions to Mineralogy and Petrology》1999,134(2-3):107-122
The Finero peridotite massif is a harzburgite that suffered a dramatic metasomatic enrichment resulting in the pervasive
presence of amphibole and phlogopite and in the sporadic occurrence of apatite and carbonate (dolomite)-bearing domains. Pyroxenite
(websterite) dykes also contain phlogopite and amphibole, but are rare. Peridotite bulk-rock composition retained highly depleted
major element characteristics, but was enriched in K, Rb, Ba, Sr, LREE (light rare earth elements) (LaN/YbN = 8–17) and depleted in Nb. It has high radiogenic Sr (87Sr/86Sr(270) = 0.7055–0.7093), low radiogenic Nd (ɛNd(270) = −1 to −3) and EMII-like Pb isotopes. Two pyroxenite – peridotite sections examined in detail show the virtual absence of
major and trace element gradients in the mineral phases. In both rock types, pyroxenes and olivines have the most unfertile
major element composition observed in Ivrea peridotites, spinels are the richest in Cr, and amphibole is pargasite. Clinopyroxenes
exhibit LREE-enriched patterns (LaN/YbN ∼16), negative Ti and Zr and generally positive Sr anomaly. Amphibole has similar characteristics, except a weak negative
Sr anomaly, but incompatible element concentration ∼1.9 (Sr) to ∼7.9 (Ti) times higher than that of coexisting clinopyroxene.
Marked geochemical gradients occur toward apatite and carbonate-bearing domains which are randomly distributed in both the
sections examined. In these regions, pyroxenes and amphibole (edenite) are lower in mg## and higher in Na2O, and spinels and phlogopite are richer in Cr2O3. Both the mineral assemblage and the incompatible trace element characteristics of the mineral phases recall the typical
signatures of “carbonatite” metasomatism (HFSE depletion, Sr, LILE and LREE enrichment). Clinopyroxene has higher REE and
Sr concentrations than amphibole (amph/cpxDREE,Sr = 0.7–0.9) and lower Ti and Zr concentrations. It is proposed that the petrographic and geochemical features observed at
Finero are consistent with a subduction environment. The lack of chemical gradients between pyroxenite and peridotite is explained
by a model where melts derived from an eclogite-facies slab infiltrate the overhanging harzburgitic mantle wedge and, because
of the special thermal structure of subduction zones, become heated to the temperature of the peridotite. If the resulting
temperature is above that of the incipient melting of the hydrous peridotite system, the slab-derived melt equilibrates with
the harzburgite and a crystal mush consisting of harzburgite and a silica saturated, hydrous melt is formed. During cooling,
the crystal mush crystallizes producing the observed sequence of mineral phases and their observed chemical characteristics.
In this context pyroxenites are regions of higher concentration of the melt in equilibrium with the harzburgite and not passage-ways
through which exotic melts percolated. Only negligible chemical gradients can appear as an effect of the crystallization process,
which also accounts for the high amphibole/clinopyroxene incompatible trace element ratios. The major element refractory composition
is explained by an initially high peridotite/melt ratio. The apatite, carbonate-bearing domains are the result of the presence
of some CO2 in the slab-derived melt. The CO2/H2O ratio in the peridotite mush increased by crystallization of hydrous phases (amphibole and phlogopite) locally resulting
in the unmixing of a late carbonate fluid. The proposed scenario is consistent with subduction of probably Variscan age and
with the occurrence of modal metasomatism before peridotite incorporation in the crust.
Received: 20 July 1998 / Accepted: 28 October 1998 相似文献
8.
Célia Dalou Kenneth T. Koga Nobumichi Shimizu Julien Boulon Jean-Luc Devidal 《Contributions to Mineralogy and Petrology》2012,163(4):591-609
We experimentally determined F and Cl partition coefficients together with that of 19 trace elements (including REE, U-Th,
HFSE and LILE) between basaltic melt and lherzolite minerals: olivine, orthopyroxene, clinopyroxene, plagioclase and garnet.
Under conditions from 8 to 25 kbars and from 1,265 to 1,430°C, compatibilities of F and Cl are globally ordered as D
Cpx/melt > D
Opx/melt > D
Grt/melt > D
Ol/melt > D
Plag/melt, and D
F
mineral/melt is larger than D
Clmineral/melt. Four other major results were brought to light. (1) Chlorine partition coefficients positively correlate with the jadeite
component in orthopyroxene, and increase of the CaTs component promotes Cl incorporation in clinopyroxene. (2) Variations
of fluorine partition coefficients correlate strongly with melt viscosity. (3) F and Cl partition coefficients correlate with
the Young’s modulus (E
0) of pyroxene octahedral sites (M sites) and with Raman vibrational modes of pyroxenes. This demonstrates a fundamental interaction
between the M site of pyroxenes and the incorporation of F and Cl. (4) We also determined the parameters of the lattice-strain
model applied to 3+ cation trace elements for the two M sites in orthopyroxene and clinopyroxene: D
0M1, D
0M2, r
0M1, r
0M2, E
0M1 and E
0M2. 相似文献
9.
Numerous lamproite dykes are hosted by the Eastern Dharwar Craton, southern India, particularly towards the northwestern margin of the Cuddapah Basin. We present here a comprehensive mineralogical and geochemical (including Sr and Nd isotopic) study on the lamproites from the Vattikod Field, exposed in the vicinity of the well-studied Ramadugu lamproite field. The Vattikod lamproites trend WNW–ESE to NW–SE and reveal effects of low-temperature post-magmatic alteration. The studied lamproites show porphyritic texture with carbonated and serpentinized olivine, diopside, fluorine-rich phlogopite, amphibole, apatite, chromite, allanite, and calcite. The trace-element geochemistry (elevated Sr and HFSE) reveals their mixed affinity to orogenic as well as anorogenic lamproites. Higher fluorine content of the hydrous phases coupled with higher whole-rock K2O highlights the role of metasomatic phlogopite and apatite in the mantle source regions. Trace-element ratios such as Zr/Hf and Ti/Eu reveal carbonate metasomatism of mantle previously enriched by ancient subduction processes. The initial 87Sr/86Sr-isotopic ratios (calculated for an assumed emplacement age of 1350 Ma) vary from 0.7037 to 0.7087 and ?Nd range from ??10.6 to ??9.3, consistent with data on global lamproites and ultrapotassic rocks. We attribute the mixed orogenic–anorogenic character for the lamproites under study to multi-stage metasomatism. We relate the (1) earlier subduction-related enrichment to the Paleoproterozoic amalgamation of the Columbia supercontinent and the (2) second episode of carbonate metasomatism to the Mesoproterozoic rift-related asthenospheric upwelling associated with the Columbia breakup. This study highlights the association of lamproites with supercontinent amalgamation and fragmentation in the Earth history. 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.
R. G. Trønnes 《Mineralogy and Petrology》2002,74(2-4):129-148
Summary The phase relations of K-richterite, KNaCaMg5Si8O22(OH)2, and phlogopite, K3Mg6 Al2Si6O20(OH)2, have been investigated at pressures of 5–15 GPa and temperatures of 1000–1500 °C. K-richterite is stable to about 1450 °C
at 9–10 GPa, where the dp/dT-slope of the decomposition curve changes from positive to negative. At 1000 °C the alkali-rich,
low-Al amphibole is stable to more than 14 GPa. Phlogopite has a more limited stability range with a maximum thermal stability
limit of 1350 °C at 4–5 GPa and a pressure stability limit of 9–10 GPa at 1000 °C. The high-pressure decomposition reactions
for both of the phases produce relatively small amounts of highly alkaline water-dominated fluids, in combination with mineral
assemblages that are relatively close to the decomposing hydrous phase in bulk composition. In contrast, the incongruent melting
of K-richterite and phlogopite in the 1–3 GPa range involves a larger proportion of hydrous silicate melts.
The K-richterite breakdown produces high-Ca pyroxene and orthoenstatite or clinoenstatite at all pressures above 4 GPa. At
higher pressures additional phases are: wadeite-structured K2SiVISiIV
3O9 at 10 GPa and 1500 °C, wadeite-structured K2SiVISiIV
3O9 and phase X at 15 GPa and 1500 °C, and stishovite at 15 GPa and 1100 °C. The solid breakdown phases of phlogopite are dominated
by pyrope and forsterite. At 9–10 GPa and 1100–1400 °C phase X is an additional phase, partly accompanied by clinoenstatite
close to the decomposition curve. Phase X has variable composition. In the KCMSH-system (K2CaMg5Si8O22(OH)2) investigated by Inoue et al. (1998) and in the KMASH-system investigated in this report the compositions are approximately K4Mg8Si8O25(OH)2 and K3.7Mg7.4Al0.6Si8.0O25(OH)2, respectively.
Observations from natural compositions and from the phlogopite-diopside system indicate that phlogopite-clinopyroxene assemblages
are stable along common geothermal gradients (including subduction zones) to 8–9 GPa and are replaced by K-richterite at higher
pressures. The stability relations of the pure end member phases of K-richterite and phlogopite are consistent with these
observations, suggesting that K-richterite may be stable into the mantle transition zone, at least along colder slab geotherms.
The breakdown of moderate proportions of K-richterite in peridotite in the upper part of the transition zone may be accompanied
by the formation of the potassic and hydrous phase X. Additional hydrogen released by this breakdown may dissolve in wadsleyite.
Therefore, very small amounts of hydrous fluids may be released during such a decomposition.
Received April 10, 2000; revised version accepted November 6, 2000 相似文献
12.
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
13.
In this study we experimentally determine phlogopite/melt partition coefficients of Ra and other trace elements in a lamproitic system. This work was achieved using an analytical technique (LA-ICP-MS) with low detection limits (~ 0.01 fg) permitting the measurement of the very low Ra concentrations feasible in experiments (~ 1 ppb). DRaphlogopite/melt was determined to 2.28 ± 0.44 and 2.84 ± 0.47 in two experiments, the ratio DRa/DBa is around 1.6. The compatibility of Ra in phlogopite results from an ionic radius being close to the apex of the lattice strain parabola for earth alkalis in the large XII-coordinated interlayer site of phlogopite. A re-evaluation of DRa and DRa/DBa for magmatic minerals containing appreciable Ra, yields DRamineral/melt ranging from ~ 2.6 for phlogopite down to 2–3 ? 10? 5 for pyroxenes, and DRa/DBamineral/melt from ~ 4 for leucite to 2 ? 10? 2 for orthopyroxene. The influence of melt composition on DRa/DBa is less than 10%. All investigated minerals have different DRa/DBa, strongly fractionating Ra from Ba. Thus, for magmatic systems, (226Ra)/Ba in the various minerals is not constant, these minerals do not form a straight line in the (226Ra)/Ba–(230Th)/Ba system at the time of crystallization and thus, there is no (226Ra)/Ba–(230Th)/Ba isochron at t0. 226Ra–230Th–Ba mineral dating is thus applicable only to model ages calculated from mineral–glass pairs with known DRa. 相似文献
14.
The Neoproterozoic Korab Kansi mafic-ultramafic intrusion is one of the largest (100 km2) intrusions in the Southern Eastern Desert of Egypt. The intrusion consists of Fe-Ti-bearing dunite layers, amphibole peridotites, pyroxenites, troctolites, olivine gabbros, gabbronorites, pyroxene gabbros and pyroxene-hornblende gabbros, and also hosts significant Fe-Ti deposits, mainly as titanomagnetite-ilmenite. These lithologies show rhythmic layers and intrusive contacts against the surrounding granites and ophiolitic-island arc assemblages. The wide ranges of olivine forsterite contents (Fo67.9-85.7), clinopyroxene Mg# (0.57–0.95), amphibole Mg# (0.47–0.88), and plagioclase compositions (An85.8-40.9) indicate the role of fractional crystallization in the evolution from ultramafic to mafic rock types. Clinopyroxene (Cpx) has high REE contents (2–30 times chondrite) with depleted LREE relative to HREE, like those crystallized from ferropicritic melts generated in an island-arc setting. Melts in equilibrium with Cpx also resemble ferropicrites crystallized from olivine-rich mantle melts. Cpx chemistry and its host rock compositions have affinities to tholeiitic and calc-alkaline magma types. Compositions of mafic-ultramafic rocks are depleted in HFSE (e.g. Nb, Ta, Zr, Th and U) relative to LILE (e.g. Li, Rb, Ba, Pb and Sr) due to the addition of subduction-related hydrous fluids (rich in LILE) to the mantle source, suggesting an island-arc setting. Fine-grained olivine gabbros may represent quenched melts approximating the primary magma compositions because they are typically similar in assemblage and chemistry as well as in whole-rock chemistry to ferropicrites. We suggest that the Korab Kansi intrusion crystallized at temperatures ranging from ~700 to 1100 °C from ferropicritic magma derived from melting of metasomatized mantle at <5 Kbar. These hydrous ferropicritic melts were generated in the deep mantle and evolved by fractional crystallization under high ƒO2 at relatively shallow depth. Fractionation formed calc-alkaline magmas during the maturation of an island arc system, reflecting the role of subduction-related fluids. The interaction of metasomatized lithosphere with upwelling asthenospheric melts produced the Fe and Ti-rich ferropicritic parental melts that are responsible for precipitating large quantities of Fe-Ti oxide layers in the Korab Kansi mafic-ultramafic intrusion. The other factors controlling these economic Fe-Ti deposits beside parental melts are high oxygen fugacity, water content and increasing degrees of mantle partial melting. The generation of Ti-rich melts and formation of Fe-Ti deposits in few layered intrusions in Egypt possibly reflect the Neoproterozoic mantle heterogeneity in the Nubian Shield. We suggest that Cryogenian-Tonian mafic intrusions in SE Egypt can be subdivided into Alaskan-type intrusions that are enriched in PGEs whereas Korab Kansi-type layered intrusions are enriched in Fe-Ti-V deposits. 相似文献
15.
Mineral water contents, together with the elements and isotopes of minerals and whole‐rock, were determined for garnet pyroxenites enclosed by ultrahigh‐pressure (UHP) metamorphic gneiss at Hujialin in the Sulu orogen. The results suggest that the garnet pyroxenites were generated in the Triassic by metasomatic reaction of the mantle wedge peridotite with hydrous felsic melts derived from partial melting of the deeply subducted continental crust. Measured water contents vary from 523 to 1213 ppm for clinopyroxene, and 55 to 1476 ppm for garnet. These mineral water contents are not only correlated with mineral major and trace element abundances but also relatively homogenous within single mineral grains. Such features preclude significant disturbance of the mineral water contents during pyroxenite exhumation from the mantle depth to the surface and thus indicate preservation of the primary water contents for the UHP metasomatites. The garnet pyroxenites are estimated to have bulk water contents of 424–660 ppm, which are higher than those for the MORB source, similar to or higher than those for the OIB sources and close to the lower limit for the arc magma source. The relationships between contents of mineral water and some elements suggest that the high water contents of garnet pyroxenites are primarily determined by the abundance of water‐rich clinopyroxene. Calculated whole‐rock H2O/Ce ratios are 63–145, higher than those for Hawaiian garnet pyroxenites and SWIR abyssal pyroxenites. These observations suggest that metasomatic pyroxene‐rich lithologies have the capacity to contribute high H2O concentrations and variable H2O/Ce ratios to the mantle. This lends support to the interpretation that the source of some intraplate basalts may be a heterogeneous mixture of peridotite and pyroxenite. On the other hand, the high water contents of garnet pyroxenites suggest that the ultramafic metasomatites are an important water reservoir in the mantle wedge. 相似文献
16.
I. P. Solovova A. V. Girnis I. D. Ryabchikov N. N. Kononkova 《Geochemistry International》2009,47(6):578-591
Carbonatite veinlets in fergusite from the Dunkeldyk potassium-rich basaltoid complex (southeastern Pamirs) are composed of clinopyroxene, phlogopite, and apatite phenocrysts embedded in a crystallized calcite-bearing groundmass. The examination of back-scattered electron images revealed areas of significantly different compositions in fluorapatite and fluorphlogopite. The content of BaO in the phlogopite ranges from 0.68 to 10.9 wt %. There are also variations in MgO and F contents. The maximum BaO content corresponds to high mole fractions of the Ba end member kinoshitalite (up to 0.24) in the phlogopite. The zoned fluorapatite phenocrysts are rich in SrO (0.77–25.4 wt %). An increase in SrO content is accompanied by an increase in Ce2O3, La2O3, and BaO and a distinct decrease in CaO. Most of the apatite grains are rimmed by elongated colorless crystals showing the maximum SrO contents. Based on the experimentally determined Ba and Sr partition coefficients between these minerals, silicate and carbonate melts, and fluid, a model was proposed for the enrichment of phases in these trace elements. It was shown that the mineral-forming media of the Ba-rich phlogopites was a residual melt enriched in volatiles (including F) and fluid-mobile elements. During that stage, the decomposition reactions of early Ba-bearing feldspars with subsequent incorporation of BaO in Ba-rich phlogopites played an important role. The mechanism of formation of Sr-rich apatites is fundamentally different: early apatite grains with moderate Sr contents recrystallized under the influence of Sr-rich fluids released during the late magmatic stage. Thus, despite their close association in a single rock, the Ba-bearing phlogopite and Sr-rich apatite were formed by significantly different mechanisms. Our previous investigations of melt and fluid inclusions in minerals from the rocks of the Dunkeldyk complex and the results obtained in this study allowed us to suggest that the barium, fluorite-carbonatite, and rare metal mineralization occurring in the region developed owing to the prolonged evolution of primary magmas, resulting in the formation of melt-solutions (brines) and hydrothermal systems. 相似文献
17.
Recycled crustal melt injection into lithospheric mantle: implication from cumulative composite and pyroxenite xenoliths 总被引:3,自引:0,他引:3
Hong-Fu Zhang Eizo Nakamura Katsura Kobayashi Ji-Feng Ying Yan-Jie Tang 《International Journal of Earth Sciences》2010,99(6):1167-1186
A rare composite xenolith and abundant cumulative pyroxenites obtained from the Mesozoic Fangcheng basalts on the eastern
North China Craton record a complex history of melt percolation and circulation in the subcontinental lithospheric mantle.
The composite xenolith has a dunite core and an olivine clinopyroxenite rim. The dunite is of cumulative origin and has a
granular recrystallized texture and extremely low Mg# [100 Mg/(Mg + Fe) = 81–82] contents in olivines. The olivine clinopyroxenite
contains larger clinopyroxene and/or orthopyroxene with a few fine-grained olivine and tiny phlogopite, feldspar, and/or carbonate
minerals interstitial to clinopyroxene. The clinopyroxene has low Mg# (83–85). Compositional similarity between dunitic olivine
and pyroxenitic one indicates a sequential crystallization of dunite and pyroxenite from a precursor melt. Pyroxenite xenoliths
include olivine websterites and clinopyroxenites, both are of cumulative origin. Estimation of the melt from major oxides
in olivines and REE concentrations in clinopyroxenes in these composite and pyroxenite xenoliths suggests a derivation from
subducted crustal materials, consistent with the highly enriched EMII-like Sr and Nd isotopic ratios observed in the pyroxenites.
Occurrence of phlogopite, feldspar and carbonate minerals in some xenoliths requires the melt rich in alkalis (K, Na), silica
and volatiles (water and CO2) at the latest stage as well, similar to highly silicic and potassic melts. Thus, the occurrence of these composite and pyroxenite
xenoliths provides an evidence for voluminous injection of recycled crustal melts into the lithosphere beneath the southeastern
North China Craton at the Late Mesozoic, a reason for the rapid lithospheric enrichment in both elemental and isotopic compositions. 相似文献
18.
Alkaline lamprophyre dykes from Taourirt (North Morocco) containnumerous xenoliths, ranging from alkaline pyroxenites, kaersutitites,gabbros and nepheline syenites to a calcite carbonatite. Thesilicate xenoliths and the host rocks consist of Al- and Ti-richdiopsidesalite, mica or kaersutitite, ± nepheline,± plagioclase and K-feldspar, and ubiquitous apatite.Both the xenoliths and the lamprophyres are enriched in incompatibleelements. The chemical composition of the lamprophyres cannotbe accounted for by fractional crystallization alone. Moreover,the clinopyroxenes exhibit complex zoning, which requires repeatedmixing of pulses of more or less fractionated melts. The carbonatiteis a sövite cumulate with Sr-rich calcite, pyrochlore,fluorapatite, and rare salite. The SrNd isotopic compositionsof the Taourirt rocks indicate a depleted mantle source, thecarbonatite having the most depleted composition, and definea linear trend similar to that of the East African carbonatites.The different rocks thus represent unrelated magmas, and thetrend is interpreted as mixing between two components with HIMUand EM1 mantle end-member signatures. An EM2 mantle componentcould also be involved for a few samples; it may correspondto hydrous metasomatized mantle of the PPPKP (phlogopiteand phlogopite K-richterite peridotite) and MARID (mica, amphibole,rutile, ilmenite and diopside) type. KEY WORDS: alkaline magmatism; carbonatite; Morocco; REE; SrNd isotopes 相似文献
19.
The Lherz orogenic lherzolite massif (Eastern French Pyrenees) displays one of the best exposures of subcontinental lithospheric
mantle containing veins of amphibole pyroxenites and hornblendites. A reappraisal of the petrogenesis of these rocks has been
attempted from a comprehensive study of their mutual structural relationships, their petrography and their mineral compositions.
Amphibole pyroxenites comprise clinopyroxene, orthopyroxene and spinel as early cumulus phases, with garnet and late-magmatic
K2O-poor pargasite replacing clinopyroxene, and subsolidus exsolution products (olivine, spinel II, garnet II, plagioclase).
The original magmatic mineralogy and rock compositions were partly obscured by late-intrusive hornblendites and over a few
centimetres by vein–wallrock exchange reactions which continued down to subsolidus temperatures for Mg–Fe. Thermobarometric
data and liquidus parageneses indicate that amphibole pyroxenites started to crystallize at P ≥ 13 kbar and recrystallized at P < 12 kbar. The high AlVI/AlIV ratio (>1) of clinopyroxenes, the early precipitation of orthopyroxene and the late-magmatic amphibole are arguments for
parental melts richer in silica but poorer in water than alkali basalts. Their modelled major element compositions are similar
to transitional alkali basalt with about 1–3 wt% H2O. In contrast to amphibole pyroxenites, hornblendites only show kaersutite as liquidus phase, and phlogopite as intercumulus
phase. They are interpreted as crystalline segregates from primary basanitic magmas (mg=0.6; 4–6 wt% H2O). These latter cannot be related to the parental liquids of amphibole pyroxenites by a fractional crystallization process.
Rather, basanitic liquids mostly reused pre-existing pyroxenite vein conduits at a higher structural level (P ≤ 10 kbar). A continuous process of redox melting and/or alkali melt/peridotite interaction in a veined lithospheric mantle
is proposed to account for the origin of the Lherz hydrous veins. The transitional basalt composition is interpreted in terms
of extensive dissolution of olivine and orthopyroxene from wallrock peridotite by alkaline melts produced at the mechanical
boundary layer/thermal boundary layer transition (about 45–50 km deep). Continuous fluid ingress allowed remelting of the
deeper veined mantle to produce the basanitic, strongly volatiles enriched, melts that precipitated hornblendites. A similar
model could be valid for the few orthopyroxene-rich hydrous pyroxenites described in basalt-hosted mantle xenoliths.
Received: 15 September 1999 / Accepted: 31 January 2000 相似文献
20.
《地学前缘(英文版)》2022,13(5):101429
Here we present new data on the major and trace element compositions of silicate and oxide minerals from mantle xenoliths brought to the surface by the Carolina kimberlite, Pimenta Bueno Kimberlitic Field, which is located on the southwestern border of the Amazonian Craton. We also present Sr-Nd isotopic data of garnet xenocrysts and whole-rocks from the Carolina kimberlite. Mantle xenoliths are mainly clinopyroxenites and garnetites. Some of the clinopyroxenites were classified as GPP–PP–PKP (garnet-phlogopite peridotite, phlogopite-peridotite, phlogopite-K-richterite peridotite) suites, and two clinopyroxenites (eclogites) and two garnetites are relicts of an ancient subducted slab. Temperature and pressure estimates yield 855–1102 °C and 3.6–7.0 GPa, respectively. Clinopyroxenes are enriched in light rare earth elements (LREE) (LaN/YbN = 5–62; CeN/SmN = 1–3; where N = primitive mantle normalized values), they have high Ca/Al ratios (10–410), low to medium Ti/Eu ratios (742–2840), and low Zr/Hf ratios (13–26), which suggest they were formed by metasomatic reactions with CO2-rich silicate melts. Phlogopite with high TiO2 (>2.0 wt.%), Al2O3 (>12.0 wt.%), and FeOt (5.0–13.0 wt.%) resemble those found in the groundmass of kimberlites, lamproites and lamprophyres. Conversely, phlogopite with low TiO2 (<1.0 wt.%) and lower Al2O3 (<12.0 wt.%) are similar to those present in GPP-PP-PKP, and in MARID (mica-amphibole-rutile-ilmenite-diopside) and PIC (phlogopite-ilmenite-clinopyorxene) xenoliths. The GPP-PP-PKP suite of xenoliths, together with the clinopyroxene and phlogopite major and trace element signatures suggests that an intense proto-kimberlite melt metasomatism occurred in the deep cratonic lithosphere beneath the Amazonian Craton. The Sr-Nd isotopic ratios of pyrope xenocrysts (G3, G9 and G11) from the Carolina kimberlite are characterized by high 143Nd/144Nd (0.51287–0.51371) and εNd (+4.55 to +20.85) accompanied with enriched 87Sr/86Sr (0.70405–0.71098). These results suggest interaction with a proto-kimberlite melt compositionally similar with worldwide kimberlites. Based on Sr-Nd whole-rock compositions, the Carolina kimberlite has affinity with Group 1 kimberlites. The Sm-Nd isochron age calculated with selected eclogitic garnets yielded an age of 291.9 ± 5.4 Ma (2 σ), which represents the cooling age after the proto-kimberlite melt metasomatism. Therefore, we propose that the lithospheric mantle beneath the Amazonian Craton records the Paleozoic subduction with the attachment of an eclogitic slab into the cratonic mantle (garnetites and eclogites); with a later metasomatic event caused by proto-kimberlite melts shortly before the Carolina kimberlite erupted. 相似文献