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1.
FeO*‐Al2O3‐TiO2‐rich rocks are found associated with transitional tholeiitic lava flows in the Tertiary Bana plutono‐volcanic complex in the continental sector of the Cameroon Line. These peculiar rocks consist principally of iron‐titanium oxides, aluminosilicates and phosphates, and occur as layers 1–3 m thick occupying the upper part of lava flows on the southwest (site 1) and northwest (site 2) sites of the complex. Mineral constituents of the rocks include magnetite, ilmenite, hematite, rutile, corundum, andalusite, sillimanite, cordierite, quartz, plagioclase, alkali feldspar, apatite, Fe‐Mn phosphate, Al phosphate, micas and fine mixtures of sericite and silica. Texturally and compositionally, the rocks can be subdivided into globular type, banded type, and Al‐rich fine‐gained massive type. The first two types consist of dark globule or band enriched in Fe‐Ti oxides and apatite and lighter colored groundmass or bands enriched in aluminosilicates and quartz, respectively. The occurrence of andalusite and sillimanite and the compositional relations of magnetite and ilmenite in the FeO*‐Al2O3‐TiO2‐rich rocks suggest temperatures of crystallization in a range of 690–830°C at low pressures. The Bana FeO*‐Al2O3‐TiO2‐rich rocks are characterized by low concentrations of SiO2 (25–54.2 wt%), Na2O + K2O (0–1%), CaO (0–2%) and MgO (0–0.5%), and high concentrations of FeO* (total iron as FeO, 20–42%), Al2O3 (20–42%), TiO2 (3–9.2%), and P2O5 (0.26–1.30%). TiO2 is positively correlated with Al2O3 and inversely correlated with FeO*. The bulk rock compositions cannot be derived from the associated basaltic magma by crystal fractionation or by partial melting of the mantle or lower crustal materials. In ternary diagrams of (Al2O3)?(CaO + Na2O + K2O)?(FeO*+ MnO + MgO) and (SiO2)?(FeO*)?(Al2O3), the compositional field of the rocks is close to that of laterite and is distinct from the common volcanic rocks, suggesting that the rocks are derived from lateritic materials by recrystallization when the materials are heated by the basaltic magmas. A hydrothermal origin is discounted because the rocks contain high‐temperature mineral assemblages and lack sulfide minerals. It is proposed that the FeO*‐Al2O3‐TiO2‐rich rocks of the Bana complex were formed by pyrometamorphism of laterite by the heat of basaltic magmas.  相似文献   

2.
An attempt at a solution of a difficult problem: the prerequisites of the segregation of Fe, Cr, Ti, B as oxides, by decomposition of their silicates, from the view point of the phase rule and by analysis of the mineral parageneses. The passage of Cr from the silicates to the oxides is favored by excesses of FeO and MgO, in a reducing environment, and that the reverse reaction, the fixation of Cr by silicates, is favored by high Ca and, at low temperatures, by Al2O3 and H2O. Existence of Fe3 O4 (magnetite) is inhibited by alkali silicates, CaSiO3, H2S, and (at high temperatures) by TiO2, but is favored by high Al2O3 and SiO2, CO2, and O2, under certain conditions. High-lime, high-alumina environments are conducive to the liberation of Ti from silicates (as Ti magnetite), but a shortage of alumina, with excess of Mg, drive Ti into the silicate phase. Although the - passage of B from borosilicates into borates is encouraged by excesses of Mg and Fe and the reverse, i.e., the fixation of B as a borosilicate, is favored by alkali aluminosilicates, Ca, Fe, alumina, and the rare earths, the many apparent or real contradictions in the parageneses require further investigations. — V.P. Sokoloff  相似文献   

3.
When melts of Ca–Al inclusions in chondrites, which are dominated by the oxides SiO2, MgO, CaO, and Al2O3, evaporate at high temperatures, the SiO2 and MgO fugacities are inverted: SiO2, which is more volatile than MgO, becomes less volatile when melts rich in refractory CaO and Al2O3 evaporate. This fugacity inversion can be realistically explained within the framework of D.S. Korzhinskii’s theory of acid–base interaction between components in silicate melts. According to this theory, an increase in CaO concentration in the melt increases its basicity, and this, in turn, increases the activity (and hence, also fugacity) of MgO and decreases those of SiO2. In the real compositions of the Ca–Al inclusions in chondrites, the MgO/SiO2 ratio systematically decreases with an increase in the CaO concentration under the effect of acid–base interaction.  相似文献   

4.
Micas from coarse granular Iherzolites in S. African kimberlites may be separated into two groups; those showing primary textural relationships with coexisting silicates and those with secondary, alteration relationships with other silicates. Primary-textured micas form a tight cluster with a mean composition from 10 coarse garnet lherzolites of: SiO2 41.0, TiO2 0.18, Al2O3 13.5, Cr2O3 0.82, total Fe as FeO 2.60, MnO 0.02, MgO 26.0, NiO 0.22, CaO 0.01, BaO 0.29, Na2O 0.31, K2O 10.0, Rb2O 0.028, Cl 0.08, F 0.43 wt%. Primary-textured micas in aggregates with clinopyroxene have higher TiO2 and four specimens which look similar to the primary group but have textural ambiguities have still higher TiO2Micas with secondary textures have wide ranges of composition which may be correlated with details of the textural parageneses. Micas from kelyphitic rims around garnets tend to be Cr-rich while those from veinlets are Cr-poor. Both groups tend to have higher FeO and TiO2 than the primary group. Micas produced by alteration of, or filling veinlets through, orthopyroxene have a wide compositional range which overlaps that of the primary-textured micas, especially for harzburgite specimens.The primary-textured micas show a positive correlation with coexisting pyroxenes for MgO/(MgO + FeO) and TiO2, but not for Cr2O3. Secondary-textured micas do not show correlations with coexisting pyroxenes for any elements.The ‘primary-metasomatic’ micas described by Harte and Gurney (1975) and metasomatic and other micas described by Boettcher et al. (1979) and Boettcher and O'neil (1979) are richer in FeO and TiO2 than the present primary-textured micas, and are attributed to crystallization from fractionated fluids.  相似文献   

5.
The presence in rocks of coexisting sapphirine + quartz has been widely used to diagnose conditions of ultra‐high‐temperature (UHT) metamorphism (>900 °C), an inference based on the restriction of this assemblage to temperatures >980 °C in the conventionally considered FeO–MgO–Al2O3–SiO2 (FMAS) chemical system. With a new thermodynamic model for sapphirine that includes Fe2O3, phase equilibra modelling using thermocalc software has been undertaken in the FeO–MgO–Al2O3–SiO2–O (FMASO) and FeO–MgO–Al2O3–SiO2– TiO2–O (FMASTO) chemical systems. Using a variety of calculated phase diagrams for quartz‐saturated systems, the effects of Fe2O3 and TiO2 on FMAS phase relations are shown to be considerable. Importantly, the stability field of sapphirine + quartz assemblages extends down temperature to 850 °C in oxidized systems and thus out of the UHT range.  相似文献   

6.
An inversion of SiO2 and MgO volatility occurs during high-temperature melt evaporation in the CaO–MgO–Al2O3–SiO2 (CMAS) system. This results in that SiO2, which is usually more volatile than MgO, becomes less volatile during the evaporation of melts enriched in the refractory oxides CaO and Al2O3. The volatility inversion is adequately explained within the theory of acid–base interaction of silicate melt components developed by D.S. Korzhinskii. The compositions of high-Al2O3 and SiO2-poor glasses (known as HASP glasses) from the lunar regolith show a systematic decrease in MgO/SiO2 with increasing CaO content, which is a direct consequence of the influence of acid–base effects.  相似文献   

7.
Cordierite–orthoamphibole gneisses and rocks of similar composition commonly contain low‐variance mineral assemblages that can provide useful information about the metamorphic evolution of a terrane. New calculated petrogenetic grids and pseudosections are presented in the FeO–MgO–Al2O3–SiO2–H2O (FMASH), Na2O–CaO–K2O–FeO–MgO–Al2O3–SiO2–H2O (NCKFMASH) and Na2O–CaO–K2O–FeO–MgO–Al2O3–SiO2–H2O–TiO2–Fe2O3 (NCKFMASHTO) chemical systems to investigate quantitatively the phase relations in these rocks. Although the bulk compositions of cordierite–orthoamphibole gneisses are close to FMASH, calculations in this system do not adequately account for the observed range of mineral assemblages. Calculations in NCKFMASH and NCKFMASHTO highlight the role of minor constituents such as Ca, Na and Fe3+ in the mineral assemblage evolution of such rocks and these systems are more appropriate for interpreting the evolution of natural examples.  相似文献   

8.
Electron-probe microanalysis of a series of garnets in metapelitic rocks of the chloritoid staurolite, kyanite and sillimanite metamorphic zones, eastern area of the Sierra de Guadarrama, Sistema Central, Spain, manifest the well-known cryptozonation commonly observed in these minerals, with MgO and FeO increasing and MnO and CaO decreasing from the center to the outer rim of the crystals.The differences in composition of the garnets, from one metamorphic zone to another, is mainly a result of small differences in composition of the host-rock, since: (1) the amounts of MnO in the garnet are controlled by the amounts of SiO2, Al2O3 and FeO present in the host-rock; and (2) the percentages of MnO and MgO of the parent-rock influence in some way the concentration of CaO in the garnet, and those of MnO, Al2O3 and CaO influence the concentration of FeO. Nevertheless, the amount of FeO in the garnet is finally controlled, due to the diadochy, by the concentration of MnO + CaO in this mineral.  相似文献   

9.
The content of major element oxides (Al2O3, SiO2, CaO, MgO, Fe2O3, P2O5, K2O, TiO2, and MnO) in soil surrounding the light metal factory (TLM), which was producing aluminium from 1937 till 1991, was investigated by EDXRF and XRD. The evaluation of a possible aluminium pollution linked to the industrial activity was studied by a comparison of soil around the TLM with control soil samples. Taking into account natural variations caused by bedrock lithology, control samples were taken in the soil developed upon the Upper Cretaceous limestones and Middle Eocene flysch deposits, while the TLM soil was sampled over the Upper Cretaceous and Early to Middle Eocene limestones; that way the samples constituted four groups, each containing five to seven samples. Control soil overlying flysch deposits differed strikingly from the other three groups by its increased CaO values, and decreased content of Al, Fe, Mn, and Ti-oxides, confirming the overall geochemical imprint of bedrock lithology on the analysed soils. However, a specific mineral composition (presence of alumina) of the TLM soil, together with peculiar correlations among Al2O3 and other oxides indicates a certain extent of aluminium pollution of the investigated area.  相似文献   

10.
The Jálama pluton (JP) is a Variscan peraluminous granitoid that intruded into low-grade metasediments from the Central Iberian Zone (CIZ). It comprises a sillimanite-bearing two-mica monzogranite in the inner zone, followed by a tourmaline-bearing two-mica monzogranite, and a marginal tourmaline-muscovite leucogranite in the northern half of the pluton. Microgranitoid enclaves and metasedimentary xenoliths occur locally in monzogranites. The change in rock type from the central monzogranite to the marginal leucogranite corresponds to decreasing TiO2, MgO, FeO, CaO, Sr, Ba, Zr, and ΣREE, and increasing SiO2, Na2O, P2O5, Rb, Li, Cs, Ta, Sn, and W. Fe/(Fe+Mg) ratios in biotite, muscovite and tourmaline increase with increasing Fe/(Fe+Mg) in bulk rock, suggesting an important control of the bulk-rock composition on mineral chemistry. The high peraluminosity, the low CaO and high P contents, as well as the similarity of ε(Nd)300 values in both the granites and metasediments of the southern CIZ constitute strong evidences for a crustal origin of the granite suite, probably by melting of these metasedimentary rocks. Field and petrographic observations, together with mineralogical and geochemical data, suggest that assimilation and mingling/mixing acted in concert with fractional crystallization during the formation of the JP. These processes may also have been important in the evolution of other granitoids from this region.  相似文献   

11.
Manganese oxides, which are widespread and of great practical importance, are formed and transformed by the active role of microorganisms. Manganese aggregates occur as both crystallized varieties and disordered fine-grained phases with significant ore grade and up to 50–60 vol % of X-ray amorphous components. X-ray amorphous nanosized Mn oxides in Fe-Mn nodules from the Pacific Ocean floor were examined from the standpoint of their biogenic origin. SEM examination showed abundant mineralized biofilms on the studied samples. The chemical composition of bacterial mass is as follows (wt %): 28.34 MnO, 17.14 Fe2O3, 7.11 SiO2, 2.41 CaO, 17.90 TiO2, 1.74 Na2O, 1.73 Al2O3, 1.30 MgO, 1.25 P2O5, 1.25 SO3, 0.68 CoO, 0.54 CuO, 0.53 NiO, and 0.50 K2O. The chemical composition of fossilized cyanobacterial mats within the interlayer space of nodulesis as follows (wt %): 48.35 MnO, 6.23 Fe2O3, 8.76 MgO, 5.05 Al2O3, 4.45 SiO2, 3.63 NiO, 2.30 Na2O, 2.19 CuO, 1.31 CaO, and 0.68 K2O is direct evidence for participation of bacteria in Mn oxide formation. This phase consists of mineralized glycocalix consisting of nanosized flakes of todorokite. Native metals (Cu, Fe, and Zn) as inclusions 10–20 μm in size were identified in ferromanganese nodules as well. The formation of nativemetals can be explained by their crystallization at highly reducing conditions maintained by organic matter.  相似文献   

12.
Sixteen gabbro (hyperite)/amphibolite pairs were collected from geological situations where it seems clear that the amphibolite formed from material similar in composition to that of the adjacent hyperite.Major element analyses show that the hyperite to amphibolite transition was not an isochemical one. K2O, H2O, P2O5 and Fe2O3 were increased strongly. CaO and FeO were diminished; SiO2 may have been also. TiO2, total iron, MgO, MnO and Na2O were static; so probably was Al1O3.  相似文献   

13.
Two Holocene sediment cores were retrieved respectively from the enclosed Lake Daihai in the monsoon/arid transition zone of North China and the Taihu Lake coast in the monsoonal area of the Yangtze delta, Eastern China. Distribution of major geochemical elements and their ratios were employed to reveal the characteristics of Holocene climate and associated environmental implications in the two regions. It is suggested that the temporal distribution of major elements serve as a useful indicator to denote the variations of monsoon effective precipitation for the enclosed lake area. High values of resistant elements such as Al2O3, SiO2, TiO2, (FeO + Fe2O3), MnO in the lake sediments correspond to the depressed chemical weathering and weakened mon-soon effective precipitation, while the highs of mobile and easy soluble elements such as MgO, CaO, Na2O reflect the enhanced chemical weathering and increased monsoon effective precipitation in the lake basin. In comparison, the behaviors of the major elements in sediments of the Taihu Lake coast were largely controlled by the changes both in sea transgression in the different Holocene time periods and the monsoon precipitation. The relatively highs of Al2O3, TiO2, (FeO + Fe2O3), in marine-influenced sediments suggest relatively strong coastal hydrodynamics and chemical weathering, and vice versa. Meanwhile, the lows of SiO2, Na2O and CaO in the non-marine-influenced sediments also denote relatively strong hydrodynamics and chemical weathering due to enhanced monsoon precipitation, and vice versa. Sedimentary environment should be taken into account when achieving a full understanding of their climate implications.  相似文献   

14.
The equilibrium (Mg, Fe, Zn)3Al2Si3O12+2Al2SiO5=3(Mg, Fe, Zn)Al2O4+5SiO2 garnet + sillimanite/kyanitc = spinel + quartz was calibrated in the piston-cylinder apparatus between 11 and 30 kbar, and over the temperature range of 950 to 1200°C. Three experimental mixes of Mg no. [100*MgO/(MgO+FeO)] 40, 47 and 60, in the FeO –MgO–Al2O3–SiO2–ZnO (FMASZn) system were used under low oxygen fugacities and anhydrous conditions. We derive a ternary Fe–Mg–Zn symmetric mixing model for aluminous spinels in equilibrium with garnet, to quantify the increase in gahnitic end-member of spinel with increasing pressure and descreasing temperature. Further experiments in the spinel-cordieritequartz-sillimanite field were combined with garnet-cordierite data from the literature to produce a consistent set of equations describing the exchange reactions in FMASHZn relevant to quartz-sillimanite bearing rocks at granulite facies conditions. As spinel is an important mineral participating in many rocks of aluminous composition at granulite-facies conditions, and as zinc contributes to an enlargement of spinel's stability field towards higher pressures and lower temperatures, the thermobarometric calibrations presented here will be most significant in delineating the prograde and retrograde trajectory of P-T paths.  相似文献   

15.
The equilibrium coexistence of sapphirine + quartz is inferred to record temperatures in excess of 980 °C, based on the stability of this assemblage in the simplified chemical system FeO–MgO–Al2O3–SiO2 (FMAS) system. However, the potential for sapphirine to contain significant Fe3+ suggests that the stability of sapphirine + quartz could extend to lower temperatures than those constrained in this ideal system. The Wilson Lake terrane in the Grenville Province of central Labrador preserves sapphirine + quartz‐bearing assemblages in highly oxidized bulk compositions, and provides an opportunity to explore the stability of sapphirine + quartz in such rock compositions within the Na2O–CaO–K2O–FeO–MgO–Al2O3–SiO2–H2O–TiO2–O (NCKFMASHTO) chemical system. Starting with the phase equilibria in FeO–MgO–Al2O3–SiO2–TiO2–O (FMASTO), expansion into K2O–FeO–MgO–Al2O3–SiO2–H2O–TiO2–O (KFMASHTO) allows the effect of the stability of the additional phases, biotite, K‐feldspar and melt, on the stability of sapphirine + quartz to be assessed. These phase relations are evaluated generally using P–T projections, and the ultimate extension into NCKFMASHTO is done with pseudosections. Conditions of peak metamorphism in the Wilson Lake terrane are constrained using P–T pseudosections, and the appropriate H2O and O contents to use in the modelled compositions are investigated using T–MH2O and T–MO pseudosections. The peak P–T estimates from a sapphirine + quartz‐bearing sample are ～960 to 935 °C at ～10 to 8.6 kbar, similar to estimates from orthopyroxene + sillimanite + quartz ± garnet‐bearing samples. Whereas the sapphirine + quartz‐bearing sample is more Fe‐rich than the orthopyroxene + sillimanite‐bearing sample on an all‐Fe‐as‐FeO basis, once the oxidation state is taken into account, the former is effectively more magnesian than the latter, accounting for the sapphirine occurrence.  相似文献   

16.
Titaniferous chromite (up to 8 wt% TiO2) and magnesian ilmenite (up to 10 wt% MgO) coexist at the base of the differentiated tholeiitic Mount Ayliff Intrusion in the Karoo Province of southern Africa, suggesting that the original magma was TiO2-rich. Picritic lavas with 3% TiO2 from the Lebombo monocline of the Karoo Province also contain microphenocrysts of magnesian ilmenite (up to 6 wt% mgO) and armalcolite (up to 7 wt% MgO). These oxide mineral associations and compositions are atypical of tholeiitic magmas, in which chromite usually has less than 1 wt% TiO2, ilmenite less than 3 wt% MgO and armalcolite is rarely a primary mineral. Experiments have been conducted at one atmosphere pressure on a range of compositions to determine the effect of TiO2 on the crystallization and composition of chromite, ilmenite and armalcolite. The results indicate that increasing the TiO2 content of picritic magmas increases the TiO2 content of the spinel, mainly at the expense of Al2O3, whereas Cr2O3 is not affected. Spinel compositions in the Mount Ayliff Intrusion (with over 45 wt% Cr2O3, less than 10 wt% Al2O3 and 8 wt% TiO2) were duplicated in experiments on a picrite at temperatures of about 1,200°C at the Ni/NiO buffer. Increasing fO2 from fayalite-magnetite-quartz to Ni/NiO buffer is shown to increase the crystallization temperature of armalcolite and to decrease that of ilmenite. The total FeO content of the liquid has little influence on the crystallization temperature of these phases. The TiO2 content of the liquid, when either ilmenite or armalcolite crystallizes, varies inversely with SiO2 content. The MgO content of the liquid at which ilmenite or armalcolite crystallizes depends upon the TiO2 content of the starting composition, with naturally occurring and experimetally determined saturation being demonstrated for liquids with 5 wt% MgO and 5.5 wt% TiO2. The partition coefficent for MgO between armalcolite or ilmenite and liquid is about 1.5. Observed magnesian armalcolite and ilmenite compositions in picrite lavas (both minerals) and in the Mount Ayliff Intrusion (ilmenite only) are consistent with crystallization from a TiO2-rich magma with approximately 5 wt% MgO. The Fe 2 3+ TiO5 component of armalcolite in the picrite lavas matches those formed experimentally at temperatures of 1,150–1,110°C and fO2 of the Ni/NiO to Ni/NiO+1 log unit. Similarities also exist between the compositions of chromite, ilmenite and armalcolite and liquid fraction-ation trends of some Hawaiian high-TiO2 lavas and the experimental studies presented here.  相似文献   

17.
The processes of differentiation in the magmatic chamber of the Ioko-Dovyren layered dunite-troctolite-gabbro-gabbronorite massif were simulated using the COMAGMAT-3.5 software package, which is based on the convection-accumulation model for the crystallization of magmatic intrusions. The initial magma composition was assumed to be equal to the weighted mean composition of the rocks composing the intrusion (wt %: 43.92 SiO2, 9.72 Al2O3, 10.53 FeO, 27.88 MgO, 6.99 CaO, 0.59 Na2O, 0.07 K2O, and 0.11 TiO2). The results obtained by simulating the crystallization of this composition within a pressure range of 0–10 kbar indicate that the crystallization sequence determined for the rocks Ol + Chr → Ol+ Pl+ Chr → Ol + Pl+ CPx → ± Ol + Pl+ CPx + LowCaPx in an anhydrous system takes place under pressures of 0–2 kbar. A series of simulations for a system closed with respect to oxygen yielded estimates for the phase and chemical composition of the emplaced magma and the parameters of the optimum model, which reproduces accurately enough the geochemical structure of the Ioko-Dovyren intrusion: the naturally occurring distributions of minerals and components in its vertical section. The correlation coefficients between the concentrations of oxides determined in the rocks and calculated within the model are \(r_{MgO,Al_2 O_3 ,CaO} \) ≥ 0.9 and \(r_{FeO,SiO_2 ,Na_2 O} \) ≥ 0.6. The simulated phase composition of the magma during its emplacement corresponded to melt + olivine (Fo 89). The crystallinity of the parental magma was determined to have been equal to approximately 40 vol % at an assumed cumulus density of 90% near the lower contact and 70% near the upper one. The temperature of the magma during its emplacement was close to 1340°C at a pressure of 1 kbar. In the model, plagioclase and clinopyroxene appear on the liquidus at T?1255°C at T?1210°C, respectively, and the crystallization sequence of cumulus minerals corresponds to that observed in nature. The liquid phase (melt) of the parental magma during its emplacement had the following composition (wt %): 45.95 SiO2, 15.93 Al2 O3, 14.49 MgO, 10.88 FeO, 11.46 CaO, 0.97 Na2O, 0.11 K2O, and 0.18 TiO2. Our results confirm the plausibility of the hypothesis that the inner structure of the Ioko-Dovyren intrusion was formed by the emplacement and differentiation of a single magma portion with no less than 40 vol % crystallinity.  相似文献   

18.
The carbon isotopic composition of 66 inclusion-containing diamonds from the Premier kimberlite, South Africa, 93 inclusion-containing diamonds and four diamonds of two diamond-bearing peridotite xenoliths from the Finsch kimberlite, South Africa was measured. The data suggest a relationship between the carbon isotopic composition of the diamonds and the chemical composition of the associated silicates. For both kimberlites similar trends are noted for diamonds containing peridotite-suite inclusions (P-type) and for diamonds containing eclogite-suite inclusions (E-type): Higher δ13C P-type diamonds tend to have inclusions lower in SiO2 (ol), Al2O3 (opx, gt), Cr2O3, MgO, Mg(Mg + Fe) (ol, opx, gt) and higher in FeO (ol, opx, gt) and CaO (gt). Higher δ13C E-type diamonds tend to have inclusions lower in SiO2, Al2O3 (gt, cpx), MgO, Mg(Mg + Fe) (gt), Na2O, K2O, TiO2 (cpx) and higher in CaO, Ca(Ca + Mg) (gt, cpx).Consideration of a number of different models that have been proposed for the genesis of kimberlites, their xenoliths and diamonds shows that they are all consistent with the conclusion that in the mantle, regions exist that are characterized by different mean carbon isotopic compositions.  相似文献   

19.
Progress relating to calculation of partial melting equilibria for metapelites   总被引：36，自引：4，他引：32  
Improved activity–composition relationships for biotite, garnet and silicate liquid are used to construct updated P–T grids and pseudosections for high‐grade metapelites. The biotite model involves Ti charge‐balanced by hydrogen deprotonation on the hydroxyl site, following the substitution , where HD represents the hydroxyl site. Relative to equivalent biotite‐breakdown melting reactions in P–T grids in K2O–FeO–MgO–Al2O3–SiO2–H2O (KFMASH), those in K2O–FeO–MgO–Al2O3–SiO2–H2O–TiO2–O2 (KFMASHTO) occur at temperatures close to 50 °C higher. A further consequence of the updated activity models is that spinel‐bearing equilibria occur to higher temperature and higher pressure. In contrast, the addition of Na2O and CaO to KFMASH to make the Na2O–CaO–K2O–FeO–MgO–Al2O3–SiO2–H2O (NCKFMASH) system lowers key biotite‐breakdown melting reactions in P–T space relative to KFMASH. Combination of the KFMASHTO and NCKFMASH systems to make Na2O–CaO–K2O–FeO–MgO–Al2O3–SiO2–H2O–TiO2–O2 (NCKFMASHTO) results in key biotite‐breakdown melting reactions occurring at temperatures intermediate between those in KFMASHTO and those in NCKFMASH. Given such differences, the choice of model system will be critical to inferred P–T conditions in the application of mineral equilibria modelling to rocks. Further, pseudosections constructed in KFMASH, NCKFMASH and NCKFMASHTO for several representative rock compositions show substantial differences not only in the P–T conditions of key metamorphic assemblages but also overall topology, with the calculations in NCKFMASHTO more reliably reflecting equilibria in rocks. Application of mineral equilibria modelling to rocks should be undertaken in the most comprehensive system possible, if reliable quantitative P–T information is to be derived.  相似文献   

20.
The Losevka pluton of rare-metal albite granite, which was explored as a possible source of columbite-zircon-malacon ore, is composed of quartz, sodic plagioclase, potassium feldspar, annite, protolithionite, lepidomelane, and Li-muscovite. The average chemical composition of this rock is as follows, wt %: 74.14 SiO2, 0.04 TiO2, 14.07 Al2O3, 1.05 Fe2O3, 0.78 FeO, 0.15 MnO, 0.09 MgO, 0.47 CaO, 4.65 Na2O, 4.11 K2O, and 0.03 P2O5. The accessory minerals are zircon, malacon, and cyrtolite (874 ppm); apatite (18 ppm); ilmenite (114 ppm); xenotime and monazite (119 ppm); and Nb-columbite (463 ppm). The black inclusions up to 15 cm in size, which are observed in this granite and called “birthmarks” by local geologists, consist of the same rock-forming minerals as the surrounding granite, but are enriched in MnO, MgO, CaO, TiO2, and F and depleted in SiO2 relative to the light granite. The black granite is also distinguished by much higher Sr and Ba contents and lower La, Rb, Y, Nb, REE, Cs, Ta, Th, and U contents. The black color is caused by enrichment in manganese oxides, manganoilmenite, and Mn-annite. All rock-forming minerals are pervaded by thin veinlets of Mn-oxides. In addition, bastnaesite, Y-and Th-fluorides, zircon, and malacon have been identified. Aggregates of black-colored minerals are not the products of the fractionation of the initial magma or immiscibility effects, because the structure of the albite-potassium feldspar-quartz-mica matrix is the same both in black and light granites. The percolation of a deep-sourced fluid enriched in Mn and F into a granitic melt might be a more probable origin.  相似文献   

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