排序方式： 共有26条查询结果，搜索用时 91 毫秒
1.
Constraining the potential temperature of the Archaean mantle: A review of the evidence from komatiites 总被引：16，自引：0，他引：16
The maximum potential temperature of the Archaean mantle is poorly known, and is best constrained by the MgO contents of komatiitic liquids, which are directly related to eruptive temperatures. However, most Archaean komatiites are significantly altered and it is difficult to assess the composition of the erupted liquid. Relatively fresh lavas from the SASKMAR suite, Belingwe Greenstone Belt, Zimbabwe (2.7 Ga) include chills of 25.6 wt.% MgO, and olivines ranging to Fo93.6, implying eruption at around 1520°C. A chill sample from Alexo Township, Ontario (also 2.7 Ga) is 28 wt.% MgO, and associated olivines range to Fo94.1, implying eruption at 1560°C. However, inferences of erupted liquids containing 32–33 wt.% MgO, from lavas in the Barberton Greenstone Belt, South Africa (3.45 Ga) and from the Perseverance Complex, Western Australia (2.7 Ga) may be challenged on the grounds that they contain excess (cumulate) olivine, or were enriched in Mg during alteration or metamorphism. Re-interpretation of olivine compositions from these rocks shows that they most likely contained a maximum of 29 wt.% MgO corresponding to an eruption temperature of 1580°C. This composition is the highest liquid MgO content of an erupted lava that can be supported with any confidence. The hottest modern magma, on Gorgona Island (0.155 Ga) contained 18–20% MgO and erupted at circa 1400°C.
If 1580°C is taken as the temperature of the most magnesian known eruption, then the source mantle from which the liquids rose would have been at up to 2200°C at pressures of 18 GPa corresponding to a mantle potential temperature of 1900°C. These temperatures are in excess of the mantle temperatures predicted by secular cooling models, and thus komatiites can only be formed in hot rising convective jets in the mantle. This result requires that Archaean mantle jets may have been 300°C hotter than the Archaean ambient mantle temperature. This temperature difference is similar to the 200–300°C temperature difference between present day jets and ambient mantle temperatures. An important subsidiary result of this study is the confirmation that spinifex rocks may be cumulates and do not necessarily represent liquid compositions. 相似文献
3.
E.G. Nisbet 《Precambrian Research》1982,19(2):111-118
The proposed definition of the Archaean-Proterozoic boundary at 2500 Ma (Sims, 1980) is unsatisfactory as it is difficult to use, contravenes the international stratigraphic guide and does not allow for revisions of isotopic decay constants. Instead it is proposed that the boundary be set at the cooling age of the Great Dyke of Zimbabwe (2.5 Ga), which provides an equivalent definition in the rock. The division of the Archaean at 2.9 Ga and 3.5 Ga (Sims, 1980) is also unsatisfactory and instead reference successions are proposed in the Rhodesian craton, the Pilbara block in Australia and in Greenland, to serve as possible future stratotypes for Archaean eras. 相似文献
4.
The petrology and stable isotope chemistry of cyanobacterial stromatolites of Archaean age (2.7 Ga) from the Cheshire and Manjeri Formations of the Belingwe greenstone belt in Zimbabwe have been examined. Palaeomagnetic data suggest that the stromatolites formed in tropical to subtropical latitudes. The Cheshire Formation shows little evidence of either anion or cation exchange during metamorphism, and the stable carbon and oxygen isotope ratios suggest a formation at temperatures perhaps considerably below 80°C. The Manjeri Formation, only slightly older, but overlain by a thick volcanic sequence, shows a low grade of metamorphism, and isotope ratios that are consistent with a metamorphic temperature of around 200°C. 相似文献
5.
The compositions of liquidus olivines and orthopyroxenes of natural specimens of spinifex- and quench-textured peridotitic komatiites from the Belingwe Greenstone Belt, Rhodesia have been determined for pressures between 10 and 40 kbars. In conjunction with the chemical variation exhibited by these peridotitic komatiites it is concluded that the more magnesian lavas cannot be derived by concentration of olivine phenocrysts, fractional crystallisation or equilibrium partial melting. The peridotitic komatiites could evolve by polybaric assimilation and complete melting of garnet lherzolite into an initial liquid containing about 24% MgO. 相似文献
6.
T. K. Kyser W. E. Cameron E. G. Nisbet 《Contributions to Mineralogy and Petrology》1986,93(2):222-226
18O values of unaltered olivine and pyroxene phenocrysts in boninites from several areas range from 5.8 to 7.4 and indicate that the source for most boninites is more 18O-rich than MORBs and other oceanic basalts. The source for oxygen and other major elements is most likely a refractory portion of the mantle having a
18O value of up to 7.0 to which must be added a small amount of H2O-rich fluid to induce partial melting. This fluid, which is derived from subducted crust, is the vehicle for LREEs including Nd. The variable, normally low
Nd values typical of boninites do not correlate with the
18O values.Post eruptive exchange of oxygen in the glass of boninites with that of sea water at low temperatures (<150° C) produces
18O values of >10 in optically fresh glass. Hydration of the glass has increased the water contents of most boninites from estimated magmatic values of 1–2 wt% to 2–4 wt% and produced D values of < –80, which may be lower than the original magmatic D values. In contrast to most submarine pillow basalts, the magmatic volatile composition of boninite lavas has been extensively modified as a result of post eruptive interaction with seawater. 相似文献
7.
The Re-Os isotopic systematics of two ca. 2.7-Ga komatiite flows from Belingwe, Zimbabwe are examined. Rhenium and Os concentrations in these rocks are similar to concentrations in other Archean, Proterozoic, and Phanerozoic komatiites. Despite the excellent preservation of primary magmatic minerals, the Re-Os systematics of whole-rock samples of the komatiites show open-system behavior. Consistent model ages for several whole-rock samples suggest a disturbance to the system during the Proterozoic. Despite the open-system behavior in the whole rocks, Re-Os systematics for concentrates of primary magmatic olivine and spinel indicate generally closed-system behavior since the magmatic event that produced the rocks. Regression of the data for the mineral concentrates yields an age of 2721 ± 21 Ga, which is consistent with Pb-Pb and Sm-Nd ages that have been previously reported for the komatiites (Chauvel et al., 1993), and an initial 187Os/188Os ratio of 0.11140 ± 84 (γOs = +2.8 ± 0.8).The 2 to 3% enrichment in 187Os/188Os ratio of the mantle source of the komatiites, relative to the chondritic composition of the contemporaneous convecting upper mantle, most likely reflects either the incorporation of substantially older (≥ 4.2 Ga), Re-rich recycled mafic crust into the mantle source of the komatiites or the contribution of suprachondritic Os to the source from the putative 187Os-enriched outer core. The former interpretation would indicate the Hadean formation and recycling of mafic crust. The latter interpretation would require early formation of a substantial inner core followed by upwelling of a mantle plume from the core-mantle boundary, at least as far back as the Late Archean. Either interpretation requires large-scale mantle convection during the first half of Earth history. 相似文献
8.
9.
Pt-Re-Os and Sm-Nd isotope and HSE and REE systematics of the 2.7 Ga Belingwe and Abitibi komatiites
High-precision Pt-Re-Os and Sm-Nd isotope and highly siderophile element (HSE) and rare earth element (REE) abundance data are reported for two 2.7 b.y. old komatiite lava flows, Tony’s flow (TN) from the Belingwe greenstone belt, Zimbabwe, and the PH-II flow (PH) from Munro Township in the Abitibi greenstone belt, Canada. The emplaced lavas are calculated to have contained ∼25% (TN) and ∼28% (PH) MgO. These lavas were derived from mantle sources characterized by strong depletions in highly incompatible lithophile trace elements, such as light REE (Ce/SmN = 0.64 ± 0.02 (TN) and 0.52 ± 0.01 (PH), ε143Nd(T) = +2.9 ± 0.2 in both sources). 190Pt-186Os and 187Re-187Os isochrons generated for each flow yield ages consistent with respective emplacement ages obtained using other chronometers. The calculated precise initial 186Os/188Os = 0.1198318 ± 3 (TN) and 0.1198316 ± 5 (PH) and 187Os/188Os = 0.10875 ± 17 (TN) and 0.10873 ± 15 (PH) require time-integrated 190Pt/188Os and 187Re/188Os of 0.00178 ± 11 and 0.407 ± 8 (TN) and 0.00174 ± 18 and 0.415 ± 5 (PH). These parameters, which by far represent the most precise and accurate estimates of time-integrated Pt/Os and Re/Os of the Archean mantle, are best matched by those of enstatite chondrites. The data also provide evidence for a remarkable similarity in the composition of the sources of these komatiites with respect to both REE and HSE. The calculated absolute HSE abundances in the TN and PH komatiite sources are within or slightly below the range of estimates for the terrestrial Primitive Upper Mantle (PUM). Assuming a chondritic composition of the bulk silicate Earth, the strong depletions in LREE, yet chondritic Re/Os in the komatiite sources are apparently problematic because early Earth processes capable of fractionating the LREE might also be expected to fractionate Re/Os. This apparent discrepancy could be reconciled via a two-stage model, whereby the moderate LREE depletion in the sources of the komatiites initially occurred within the first 100 Ma of Earth’s history as a result of either global magma ocean differentiation or extraction and subsequent long-term isolation of early crust, whereas HSE were largely added subsequently via late accretion. The komatiite formation, preceded by derivation of basaltic magmas, was a result of second-stage, large-degree dynamic melting in mantle plumes. 相似文献
10.
Dr. J. Barnes J. R. McIntyre Dr. B. W. Nisbet C. R. Williams 《Mineralogy and Petrology》1990,42(1-4):141-164
Summary The late Archaean Munni Munni Complex occupies an elliptical area of 9 by 25 km, the southern half of which is covered unconformably by a 2.7 Ga volcanic sequence. The Complex consists of a lower 1850 m thick Ultramafic Series (UMS) and an upper Gabbroic Series (GS) at least 3600 m thick, and is in the form of an elongate funnel. The UMS is made up of macrorhythmic cycles of dunite, wehrlite and clinopyroxenite, while the GS shows an uninterrupted fractionation trend from pigeonite gabbros through pigeonite-magnetite gabbros to granophyres. The base of the GS is very sharp, and marked by simultaneous appearance of cumulus plagioclase and pigeonite. GS cumulates show a monotonous upward increase in Fe/Mg and an absence of cyclic layering, indicating crystallization in a closed chamber.The top of the UMS is a distinctive 30 m thick layer of bronzite-porphyritic orthocumulate websterite, which continues up the side walls as a marginal zone in contact with progressively more fractionated gabbros. A pyroxenite dyke intersects the sloping floor of the intrusion at a level close to the top of the UMS, and appears to have fed the uppermost layers of the UMS.Cu-rich magmatic sulphides are weakly disseminated throughout the porphyritic websterite layer, increasing in abundance to 1–3% in a semi-continuous augite orthocumulate layer a few metres below the gabbro. This layer extends over 8.2 km, averages 2.5 m in thickness, and has an average grade of 2.9 g/t Pt + Pd + Au, 0.2% Ni and 0.3% Cu with local higher grade zones. In about 40% of intersections, peak PGE, Au, Cu and Ni grades are coincident, while in the remainder peak PGE grades are offset about 1–2 m below the peak Cu and Ni grades.Coincident intersections are probably derived by homogenization of original offset intersections. Peak PGE grades become lower and more widely dispersed farther away from the intrusion walls.PGE-enriched sulphides also occur close to the websterite-gabbro contact where the websterite occupies a marginal position on the side wall. The marginal websterite zone and the porphyritic websterite layer are physically contiguous and petrographically similar, and are probably correlative.Microprobe data on cumulus pyroxenes indicate that the porphyritic websterite layer crystallised from a mixture of a relatively Mg- and Cr-rich M magma, parental to the Ultramafic Series, and an Fe-rich, strongly Cr-depleted gabbroic G magma. Pyroxenes from the PGE horizon are very low in Cr, suggesting that they crystallised from a G-rich hybrid.The websterite formed as a result of an influx of dense G magma which mixed with hotter resident M magma. The upper few metres of the websterite, including the PGE-rich sulphides, accumulated during a period of quiescence at the end of the influx phase. The PGE-rich sulphides formed by fractional segregation of sulphide liquid from a 500 to 1000 m thick layer of silicate magma.Munni Munni PGE mineralisation shows some striking similarities to that of the Great Dyke, particularly in the stratigraphic position of the mineralisation, the vertical distribution of PGE through the sulphide layer, and the lateral distribution of grades.
With 7 Figures and 1 Plate 相似文献
Platin-Mineralisation im Munni Munni-Komplex, West-Australien
Zusammenfassung Der spät-archaische Munni Munni-Komplex bedeckt eine elliptische Fläche von 9 × 25 km, deren südliche Hälfte diskordant von einer 2.7 Ga alten vulkanischen Abfolge überlagert wird. Der Komplex besteht aus einer unteren, 1850 m mächtigen ultramafischen Serie (UMS) und einer oberen gabbroischen Serie, die mindestens 3600 m mächtig ist und die Form eines länglichen Trichters hat. Die UMS besteht aus makrorhytmischen Zyklen von Dunit, Wehrlit und Klinopyroxenit, während die GS einen ununterbrochenen Fraktionierungs-Trend von Pigeonit-Gabbros über Pigeonit Magnetit-Gabbros zu Granophyren zeigt. Die Basis der GS ist scharf und wird durch das gleichzeitige Erscheinen von Cumulus-Plagioklas und Pigeonit definiert. GS Cumulate zeigen gegen das Hangende zu eine monotone Zunahme von Fe/Mg und ein Fehlen zyklischen Lagenbaues, was auf Kristallisation in einer geschlossenen Kammer hinweist.Der oberste Teil der UMS ist eine deutlich ausgebildete, 30 m mächtige Lage von Bronzit-porphyritischem Orthokumulat-Websterit, welche sich an den Seitenwänden als randliche Zone fortsetzt, die in Kontakt mit zunehmend mehr fraktionierten Gabbros ist. Ein Pyroxenet-Gang durchschlägt den geneigten Boden der Intrusion im Bereich der obersten UMS, und dürfte als Zufuhrkanal für die obersten Lagen der UMS gedient haben.Eine schwache Dissemination von Cu-reichen magmatischen Sulfiden ist im Gesamtbereich der porphyritischen Websterit-Lage zu beobachten; in einer Augit-Orthocumulat-Lage wenige Meter unterhalb des Gabbros steigt diese auf 1–3% Cu-Sulfide an. Diese Lage erstreckt sich über 8.2 km, ist im Durchschnitt 2.5 m mächtig, und hat einen Durchschnittsgehalt von 2.9 g/t Pt + Pd + Au, 0.2% Ni und 0.3% Cu, mit lokal reicheren Zonen. In etwa 40% der untersuchten Bohrkerne fallen maximale Gehalte an PGE, Au, Ni und Cu zusammen, während sonst maximale PGE-Gehalte etwa 1–2 m unterhalb der Cu- und Ni-Maxima auftreten.Zusammenfallende Maxima dürften durch Homogenisation ursprünglich separater Maxima entstanden sein. Mit zunehmender Entfernung von den Rändern der Intrusion nehmen PGE Gehalte ab und werden unregelmäsiger.PGE-reiche Sulfide kommen auch nahe am Websterit-Gabbro-Kontakt vor, wo der Websterit eine randliche Position einnimmt. Die randliche Websterit-Zone und die porphyritische Websterit-Lage hängen zusammen, sind petrographisch ähnlich, und sind wahrscheinlich zu korrellieren.Mikrosonden-Analysen von Kumulus-Pyroxenen zeigen dass die porphyritische Websterit-Lage aus einer Mischung von relativ Mg- und Cr-reichem M-Magma dem die ultramafische Serie zuzuordnen ist, und einem Fe-reichen, Cr-armen gabbroischen G-Magma entstanden ist. Pyroxene aus der PGE-Lage führen sehr niedrige Cr-Gehalte; dies dürfte auf Kristallisation aus einem G-reichen Hybrid-Magma zurück gehen.Der Websterit wurde als das Resultat der Zufuhr von dichtem G-Magma das sich mit höher temperiertem M-Magma mischte, gebildet. Die obersten Meter der Websterit Abfolge, mit den PGE-reichen Sulfiden, bildeten sich während einer ruhigen Periode am Ende der Influx-Phase. Die PGE-reichen Sulfide sind das Produkt fraktionierter Segregation von sulfidischer Schmelze aus einer 500 bis 1000 m mächtigen Lage silikatischen Magmas.Die PGE-Mineralisation des Munni Munni-Komplexes ist der des Great Dyke von Zimbabwe in vieler Hinsicht ähnlich, besonders was die stratigraphische Position, die vertikale Verteilung der PGE in der Sulfid-Lage, und die laterale Verteilung der Gehalte betrifft.
With 7 Figures and 1 Plate 相似文献