Cr-rich Mg-chloritoid, a first record in high-pressure metagabbros from Monviso (Cottian Alps), Italy

1987 ◽  
Vol 51 (363) ◽  
pp. 681-687 ◽  
Author(s):  
J. R. Kienast ◽  
B. Messiga
Keyword(s):  
High Pressure ◽  
Bulk Composition ◽  
First Record ◽  
Alpine Metamorphism ◽  
End Members ◽  
Host Rocks ◽  
Primary Minerals

AbstractMetamorphosed troctolite cumulates occur interbedded with Mg-rich metagabbros in the Monviso ophiolitic massif; they have developed chloritoid, omphacite, talc and garnet during the eclogitic stage of the Eo-alpine metamorphism. The lack of penetrative deformation in the rocks has made it possible to recognize different microstructural sites of chloritoid growth, in which the chloritoid composition may vary widely and is controlled by the specific inherited chemical domain.The chloritoid compositions are unusually rich in Cr and Mg with large variations in Fe2+/Mg and Cr/AlVI ratio. The Cr/Al ratio in chloritoid is linked to both the Cr value of the primary minerals (i.e. Cr-rich spinels, Cr-end-members in the clinopyroxenes) and a limited redistribution of Cr during metamorphism. The Fe2+/Mg ratio, while being partly affected by bulk composition of the host rocks, also varies between different microstructural sites; the highest ratio is recorded in coronas between clinopyroxene and plagioclase, lower values occurring in coronas between plagioclase and olivine.

Exemples de mélange de magmas en contexte plutonique: les enclaves des tonalites–granodiorites du massif de Bono (Sardaigne septentrionale)

1989 ◽  
Vol 26 (6) ◽  
pp. 1264-1281 ◽  
Author(s):  
C. Cocirta ◽  
J. B. Orsini ◽  
C. Coulon
Keyword(s):  
Geochemical Data ◽  
Mixing Process ◽  
Calc Alkaline ◽  
Specific Group ◽  
Mineralogical Analysis ◽  
Magmatic Origin ◽  
Interaction Mechanisms ◽  
Compositional Variations ◽  
End Members ◽  
Host Rocks

In calc-alkaline orogenic plutons, the dark xenoliths and their host rocks must be considered the expression of partial mixing of magma.Three associations of this type have been investigated and are illustrated by the Bono pluton (northern Sardinia)— a composite pluton including three intrusives of different nature (tonalitic to granodioritic) and containing a very large number of basaltic xenoliths of magmatic origin. Detailed mineralogical analysis of the two end members in each association, coupled with geochemical data, has determined the major petrogenetic mechanisms intervening in the mixing process in a plutonic setting: temperature equilibration, mechanical exchanges of crystals, chemical exchanges, etc. The most important result of this article, however, is to show that each intrusion is related to a specific group of xenoliths that is characterized by constant FeOt/MgO. The latter reflects the different composition of basaltic components, and it is concluded that each intrusive event is associated with a unique mixing episode. As in volcanic settings, the mixing process may have initiated the intrusion.The extreme compositional variations in the magmatic xenoliths, recognized in several series of orogenic plutons, is explained here by different initial basaltic end members and by variation in the intensity of the interaction mechanisms. [Journal Translation]

Empirical ferric iron corrections: necessity, assumptions, and effects on selected geothermobarometers

1991 ◽  
Vol 55 (378) ◽  
pp. 3-18 ◽  
Author(s):  
John C. Schumacher
Keyword(s):  
Ferrous Iron ◽  
Ferric Iron ◽  
Bulk Composition ◽  
Total Iron ◽  
Chemical Analyses ◽  
Treatment Results ◽  
Ideal Stoichiometry ◽  
End Members ◽  
Excess Cations ◽  
Mineral Analyses

AbstractThe ferromagnesian silicate minerals, such as garnets, pyroxenes, micas, and amphiboles, appear in a variety of geothermometers and geobarometers. Where complete chemical analyses are available and regardless of bulk composition (metamorphosed pelitic or mafic), the aforementioned minerals commonly contain ferric iron. In mineral analyses using the electron microprobe, ferric and ferrous iron are not distinguished, and all the iron is treated as FeO. In ferric Fe-bearing minerals, this treatment results in (1) low analytical sums and (2) excess cations in the mineral formulae. Assuming ideal stoichiometry (ideal formula cations and oxygens) allows direct ferric estimates in garnets and pyroxenes; amphiboles require additional assumptions concerning site occupancies, and, for micas, no acceptable constraint exists for a ferric estimate. Based on ferric iron determinations for some metamorphic ferromagnesian silicates, the proportion of ferric to total iron increases at higher XMg values. The influence of ferric estimates on T and P calculations depends on the model used and on the extent the ferric estimate alters the relative proportions of end-members. Several examples suggest that, in general, if ferric estimates (or determinations) are made, they should be made for all the relevant minerals.

2D thermo-mechanical-chemical coupled numerical models of interactions between a cooling magma chamber and a visco-elastic host rock

2020 ◽  
Author(s):  
Dániel Kiss ◽  
Evangelos Moulas ◽  
Lisa Rummel ◽  
Boris Kaus
Keyword(s):  
Magma Chamber ◽  
Host Rock ◽  
Numerical Models ◽  
Bulk Composition ◽  
Inertial Forces ◽  
Volume Changes ◽  
Consistent System ◽  
Host Rocks ◽  
2D Numerical Model ◽  
Crustal Magma

<p>A recent focus of studies in geodynamic modeling and magmatic petrology is to understand the coupled behavior between deformation and magmatic processes. Here, we present a 2D numerical model of an upper crustal magma (or mush) chamber in a visco-elastic host rock, with coupled thermal, mechanical and chemical (TMC) processes. The magma chamber is isolated from deeper sources of magma and it is cooling, and thus shrinking. We quantify the mechanical interaction between the shrinking magma chamber and the surrounding host rock, using a compressible visco-elastic formulation, considering several geometries of the magma chamber.</p><p>We present a self-consistent system of the conservation equations for coupled TMC processes, under the assumptions of slow (negligible inertial forces), visco-elastic deformation and constant chemical bulk composition. The thermodynamic melting/crystallization model is based on a pelitic melting model calculated with Perple_X, assuming a granitic composition and is incorporated as a look-up table. We will discuss the numerical implementation, show the results of systematic numerical simulations, and illustrate the effect of volume changes due to crystallization on stresses in the host rocks.</p>

Origin of anomalous iron meteorites

1979 ◽  
Vol 43 (327) ◽  
pp. 415-421 ◽  
Author(s):  
Edward R. D. Scott
Keyword(s):  
Fractional Crystallization ◽  
Genetic Relationships ◽  
Bulk Composition ◽  
Iron Meteorites ◽  
End Members ◽  
Chemical Groups ◽  
Chemical Trends

SummaryAnomalous iron meteorites are those which do not have Ni, Ga, and Ge contents appropriate to one of the twelve chemical groups; they account for 14% of all irons. The chemistry of irons in the twelve groups can be largely understood in terms of primary fractionation in the nebula, which established the bulk composition of the groups, and secondary fractionation in the parent bodies (probably fractional crystallization), which produced the chemical trends within groups. Logarithmic element-Ga graphs containing data for groups and anomalous irons reveal that anomalous irons experienced the same primary and secondary fractionations as affected the groups.The uniformity of chemical trends within groups allows possible genetic relationships between anomalous irons and groups and among anomalous irons to be tested. It is concluded that the sixty-nine anomalous irons are samples from fifty-odd additional groups, which had similar histories to the twelve groups. Less than five of the anomalous irons could be compositional end- members or reprocessed irons from the groups.Because ‘anomalous’ means abnormal, some other term for the irons which do not belong to the twelve groups would be a useful reminder that these irons formed in a similar way to irons in the major groups. They could be called members of minor groups or grouplets.

High-pressure crystallography and compression behavior of the alkali-scandium-germanate end-members LiScGe2O6 and NaScGe2O6

2015 ◽  
Vol 229 ◽  
pp. 188-196 ◽  
Author(s):  
Gregor Hofer ◽  
Johann Kuzel ◽  
Katharina S. Scheidl ◽  
Günther Redhammer ◽  
Ronald Miletich
Keyword(s):  
High Pressure ◽  
Compression Behavior ◽  
High Pressure Crystallography ◽  
End Members

Crystal chemistry of danalite from Daba Shabeli Complex (N Somalia)

1996 ◽  
Vol 60 (399) ◽  
pp. 375-379 ◽  
Author(s):  
P. Nimis ◽  
G. Molin ◽  
D. Visonà
Keyword(s):  
General Formula ◽  
Crystal Chemistry ◽  
The Other ◽  
The World ◽  
Alkali Granites ◽  
End Members ◽  
Host Rocks

Danalite is the Fe2+ end-member of the minerals of the helvite group, which have the general formula M8(BeSiO4)6S2, with M = (Mn,Fe2+,Zn). These minerals are relatively uncommon, although limited amounts are known at many localities round the world (e.g. Ragu, 1994; Larsen, 1988; Kwak and Jackson, 1986, and references therein). Their typical host rocks are skams, but occurrences in mineralized veins pegmatites, and altered alkali granites have also been reported. Danalite is rarer than helvite (Mn endmember) and genthelvite (Zn end-member) and, unlike the other two end-members, has never been found nor synthesized as a pure mineral (Mel'nikov et al., 1968).

scholarly journals Donwilhelmsite, [CaAl4Si2O11], a new lunar high-pressure Ca-Al-silicate with relevance for subducted terrestrial sediments

2020 ◽  
Vol 105 (11) ◽  
pp. 1704-1711
Author(s):  
Jörg Fritz ◽  
Ansgar Greshake ◽  
Mariana Klementova ◽  
Richard Wirth ◽  
Lukas Palatinus ◽  
...  
Keyword(s):  
High Pressure ◽  
High Pressures ◽  
Bulk Composition ◽  
Hexagonal Structure ◽  
Aluminum Silicate ◽  
New Mineral ◽  
International Mineralogical Association ◽  
Earth’S Mantle ◽  
Lunar Meteorite ◽  
Melt Pockets

Abstract We report on the occurrence of a new high-pressure Ca-Al-silicate in localized shock melt pockets found in the feldspatic lunar meteorite Oued Awlitis 001 and discuss the implications of our discovery. The new mineral crystallized as tiny, micrometer-sized, acicular grains in shock melt pockets of roughly anorthitic bulk composition. Transmission electron microscopy based three-dimensional electron diffraction (3D ED) reveals that the CaAl4Si2O11 crystals are identical to the calcium aluminum silicate (CAS) phase first reported from static pressure experiments. The new mineral has a hexagonal structure, with a space group of P63/mmc and lattice parameters of a = 5.42(1) Å; c = 12.70(3) Å; V = 323(4) Å3; Z = 2. This is the first time 3D ED was applied to structure determination of an extraterrestrial mineral. The International Mineralogical Association (IMA) has approved this naturally formed CAS phase as the new mineral “donwilhelmsite” [CaAl4Si2O11], honoring the U.S. lunar geologist Don E. Wilhelms. On the Moon, donwilhelmsite can form from the primordial feldspathic crust during impact cratering events. In the feldspatic lunar meteorite Oued Awlitis 001, needles of donwilhelmsite crystallized in ~200 mm sized shock melt pockets of anorthositic-like chemical composition. These melt pockets quenched within milliseconds during declining shock pressures. Shock melt pockets in meteorites serve as natural crucibles mimicking the conditions expected in the Earth's mantle. Donwilhelmsite forms in the Earth's mantle during deep recycling of aluminous crustal materials, and is a key host for Al and Ca of subducted sediments in most of the transition zone and the uppermost lower mantle (460–700 km). Donwilhelmsite bridges the gap between kyanite and the Ca-component of clinopyroxene at low pressures and the Al-rich Ca-ferrite phase and Ca-perovskite at high-pressures. In ascending buoyant mantle plumes, at about 460 km depth, donwilhelmsite is expected to break down into minerals such as garnet, kyanite, and clinopyroxene. This process may trigger minor partial melting, releasing a range of incompatible minor and trace elements and contributing to the enriched mantle (EM1 and EM2) components associated with subducted sedimentary lithologies.

Sign in / Sign up

Export Citation Format

Share Document