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Nepheline-silica and the rest of the basalt tetrahedron Bazalt és fázisdiagramjai doktori kurzus Készítette: Patkó Levente 2016.02.09.

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1 Nepheline-silica and the rest of the basalt tetrahedron Bazalt és fázisdiagramjai doktori kurzus Készítette: Patkó Levente

2 Nepheline-silica

3 Bevezetés the appearance of nepheline in the norm is always sufficient evidence of a critically undersaturated rock warranting such names as alkali basalt, basanite, olivine nephelinite, etc. the natural mineral always contains "excess" silica (deficient alkali and aluminum) when it is soda rich, the amount of this "excess" decreasing as the composition of Ne ss varies toward KAISiO 4 (kalsilite)

4 A Ne-SiO 2 rendszer A nefelin (NaAl[SiO 4 ]) képes szilárd oldatot alkotni és ekkor a szerkezetbe Si 2 O épül be. A nefelinnek laboratóriumban előállították a nagy hőmérsékletű változatát, ami a carneigit (Cg) nevet kapta. A Cg 1254 °C és 1526 °C között fordul elő (azért nem hivatalos ásványnév, mert természetes előfordulása nem ismert).

5 The system Ne-SiO 2 is an excellent example of a binary system with an intermediate compound, albite, which melts at a maximum and generates two eutectics (E1, E2), one with each of the end-member phases. Az R pont egy invariáns pont, ahol 3 fázis van jelen (Cg ss – Ne ss – L). A pontban a következő peritektikus reakció játszódik le: Ne ss + L = Cg ss A Ne-SiO 2 rendszer

6 A small univariant phase transition loop originates at the inversion point, 254°C, on the Ne sideline, and is truncated by the melting loop Cg ss +L at 1280°C. This small two-phase field represents the mutual saturation limits of relatively silica poor Cg ss and relatively silica- rich Ne ss at any given temperature in the range °C. As the temperature rises toward 1280°C, both solids become richer in SiO 2, but Ne ss is always the more silica-rich of the two.

7 A Ne-SiO 2 rendszer A Ne ss mező a maximum SiO 2 tartalmát az E1 eutektikus pontban éri el 1068 °C-on. A Ne ss mező határa a mezővel, ahol Ne ss likviddel vagy albittal tart fenn egyensúlyt szaggatott vonalú a kísérleti bizonytalanságok miatt. A szaggatott szakasz egy része szolidusz másik része szolvusz.

8 A Ne-SiO 2 rendszer Az albit szilárd oldatot formál mind a Ne, mind az SiO 2 irányába, ennek azonban geológiai jelentősége nincs.

9 A Ne-SiO 2 rendszer -all liquids in the primary phase field of feldspar will move away from feldspar on crystallization, toward either E 1 or E 2 -the compound albite thus generates a thermal barrier or divide -no liquid can cross the divide -The eutectic E 1 is an approximate model of the natural rock phonolite, which is the residual end member of the commonly observed natural alkali basalt fractionation series. -The eutectic E2 is a synthetic analog of rhyolite, the common acid end member of tholeiitic suites. Thermal barrier

10 A Ne-SiO 2 rendszer „szienit-probléma” The composition of albite itself is a model of the natural rock trachyte, and one of the mysteries of petrology is why some fractionation trends terminate in trachyte (or the intrusive equivalent syenite), at an apparent maximum, instead of sliding off toward natural versions of E 1 or E 2 With complex magma compositions the thermal "barrier" has finite width, like a plateau rather than a sharp peak. A plateau would be more likely to hold residual trachytic liquids than a peak.

11 Arról korábban is meggyőződtünk, hogy az En–Ab–Di kritikus sík egy termális határt is jelent. A kérdés, hogy ugyanerről szó lehet-e a Fo – Ab – SiO 2 kritikus síknál. A kérdés eldöntése céljából vizsgájuk meg a Ne-SiO 2 rendszert kibővítve leucittal (Lc), anortittal (An), forszterittel (Fo), fayalittal (Fa), diopsziddal (Di) és spinellel (Sp) Thermal barrier vizsgálat thermal barrier thermal barrier?

12 Phase diagrams illustrating the nature of the equilibrium thermal divides in relationship to the principal join Ne-SiO 2 The plagioclase join, diagram (b), acts as a thermal barrier in the system Ne-An-SiO 2 driving liquids constantly toward either the Ne-Ab or Ab-Tr eutectic. Although we have not yet discussed potassium-bearing systems, it is possible to conclude from diagram (a) that introduction of a potassium feldspar component in small quantities will not alter the thermal barrier relationship. The orthoclase composition (Sa, KAISi 3 O 8 ) lies on the sideline Lc-SiO 2 The important feature of diagram (a), for all but the very special potassic basalts which need not concern us here, is that the Ab-rich part of the alkali feldspar join is in fact a thermal divide, and therefore the entire ternary feldspar join acts as a thermal barrier.

13 Phase diagrams illustrating the nature of the equilibrium thermal divides in relationship to the principal join Ne-SiO 2 -the incongruent melting of En (labelled Pr) is observed on the sideline Fo-SiO 2 and inside the ternary system as well -the field of Fo overwhelms this diagram, and covers most of the join Pr-Ab as well as most of the join Fo-Ab -the cotectic boundary L (Fo-Ab) crosses the Fo-Ab join very close to the Ab composition Note that arrows point away from the line Fo-Ab on either side, showing that liquids tend either to the isobaric ternary eutectic L (Fo,Ab,Ne) or to its silica-saturated counterpart, L (Pr,Ab,Tr). The join Fo-Ab is a thermal barrier. The iron end member olivine, fayalite, shows a similar relationship (fig. d).

14 Phase diagrams illustrating the nature of the equilibrium thermal divides in relationship to the principal join Ne-SiO 2 -Diagram (e) shows that liquids move away from the join Di-Ab toward eutectics L (Di, PI, Ne) and L (Di, PI, Tr). -Evidently there is a thermal barrier in this system, although it does not precisely coincide with Di-Ab.

15 We may therefore conclude from the diagrams that the plagioclase feldspars (including varieties as K-rich as anorthoclase, or calcic sanidine) constitute a thermal divide between merely undersaturated and critically undersaturated liquids, that this divide lies very near the critical plane in the basalt tetrahedron, and that the properties of the divide persist in the presence of iron-bearing silicates such as fayalite. Kivétel, amikor a barrier átjárható: This mechanism is oxidation or reduction of iron in the magma. The effect of oxidation on silica saturation may be simply stated, however, by noting that if the ferrous iron normally incorporated in silicates (e.g., fayalite) is oxidized enough to cause formation of magnetite (FeO· Fe 2 O 3 ), the crystallization of this non-silicate mineral will "release" silica to the residual liquid which otherwise would have been bound up in ferrous silicates. Apart from the effects of pressure and oxidation-reduction reactions, the concept of the thermal barrier appears to be a durable one. We may therefore state that at low pressures and within certain ranges of oxygen partial pressures, no normal basaltic liquid can make a transit of the natural critical plane of silica undersaturation; alkali basalts cannot be parental to olivine tholeiites, and vice versa. Thermal barrier vizsgálat

16 The system Fo-Di-An-Ab, the expanded critical plane of the basalt tetrahedron The critical plane in the basalt tetrahedron is the ternary system Fo-Di-Ab. Although pure Ab generates the thermal barrier discussed above, it is an unrealistic component by itself for basalt discussions; intermediate plagioclase would be far better. The discussion of critically undersaturated basalts can be improved by adding the component An to form the critically undersaturated basalt system. This is the system Fo-Di-An- Ab, containing the principal silicate phases of basalt: pl, cpx, and ol.

17 The system Fo-Di-An-Ab, the expanded critical plane of the basalt tetrahedron -the four-phase curve L (PI,Fo,Di) extends across the system with a temperature range from 1270°C to 1135°C. Yoder and Tilley used this system as a basis for discussing experimental studies on natural basalts. They had hoped to deduce parent-daughter relationships from the relative temperatures at which the natural rocks when melted and then crystallized, reached the four-phase L (PI, Fo, Di) saturation condition. However, they found that all the basalts studied reached this condition in a very small range of temperature (1155° to 1170° C), implying that they all lie very near the same fractionation path, or a set of equivalent fractionation paths in the several primary phase regions.

18 Felmerülő bazalt nevezéktani megjegyzés Pikrites bazalt: az a speciális eset, amikor a gyorsan felszín felé hatoló magmában gyakorlatilag nem zajlik frakcionációs kristályosodás útja során, ami az ultramafikus összetétel megtartását eredményezi. Afíros bazalt: aprószemcsés bazalt, ahol vagy nem történt a bazalt útja során frakcionációs kristályosodás, vagy a frakcionálódó termékek visszamaradtak.

19 Fontos megállapítások The critical plane divides the tetrahedron into three fundamentally different regions, the undersaturated basalts, which fractionate away from the plane and toward Ne, the critically undersaturated basalts which always remain critically undersaturated in the trachyte trend, and the tholeiitic basalts which fractionate away from the plane and toward hypersthene or silica saturation. Alkali basalt cannot, at low pressures, fractionate to produce tholeiitic basalt (unless oxidation is involved), nor can tholeiitic basalt fractionate to produce alkali basalt. Within the tholeiitic volume to the silica-rich side of the critical plane, olivine tholeiites may fractionate to produce hypersthene basalt or quartz basalt; the reverse process cannot occur. Olivine tholeiites can be and no doubt often are parental to any of the suite of rocks having higher degrees of silica saturation, up to and including rhyolite (in small amounts controlled by the amount of alkalies, Na 2 O and K 2 O, in the initial magma).

20 Forrás Az összes ábra forrása a Morse-könyv. Morse, S. A. (1980). Basalts and phase diagrams: an introduction to the quantitative use of phase diagrams in igneous petrology. Springer Verlag


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