THE BASIC CONCEPTS OF ENVIRONMENTAL GEOLOGY AND ITS ROLE IN THE ...
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- cours - matière potentielle : various activities
- rational use
- proposals of rational means of the geoenvironment exploitation
- geoenvironment
- sphere objects
- techno
- environmental geology
- environment
- protection
Igneous Petrology
Properties of Melts Physical nature of melts What compositions do they have Where do they come from How do they change
Start with Basalts
Igneous Petrography
Physical description of rocks-essential first step Mineralogy and texture Modal % essential minerals Varietal and accessory minerals Relations between phases Crystallinity (extent of crystals vs. glass) Sizes, shapes, habits of crystals Specific textures
These will be covered in your laboratory exercises Hand specimens, Thin sections
Nature and Properties of Magma
Magma is a complex mixture Melt(liquid) Crystals(solid) Dissolved vapor(gas) Possibly xenoliths or xenocrysts (solid)torn from conduit walls at depth Magmas behavior controlled by: Composition Viscosity Density
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Chemical Composition of Magma
Nearly all magmas are silicate(rare carbonatites) •Contain between 40 and 75 weight % SiO 2 •9 oxides comprise 99% of most magmas(Box 5.1, p. 90)
•Elements present at ppb or ppm levels =trace elements •Very important in tracking melting or crystallization
Four broad groups (see next slide): •Ultramafic – mafic – intermediate – felsic(Sen Table 9.2) •Bulk chemical distinctions govern mineralogy
Structure of Silicate Magma-1
-4 Networks of [SiO ] tetrahedral units 4 Si, Al = network formers Mg, Fe, Na, K, Ca, etc. cations = network modifiers Bridging vs. non-bridging oxygen
Magma Type
Ultramafic (mafic mins >90%)
Olivinegenerally Mineralogydominant, followed by pyroxene
Wt% SiO 2
MgO + Fe O + 2 3 FeO
Na O + 2 K O 2
>43-49
35-46
<1
Mafic (mafics 60-65%)
Pyroxene and plagioclase dominant
46-53
15-28
2-3.5
Intermediate (mafics 30-60%
Plagioclase and pyroxene and/or amphibole dominant
60-65
10-21
3-6
Structure of Silicate Magma-2
Si-O bonds largely unbroken by melting Bonds between non-bridging O and network modifiers do break Higher SiO = higher polymerization 2 of melt Higher network modifiers = lower polymerization H O is powerful network modifier 2 Thus SiO and H O content strongly 2 2 control viscosity and work against each other.
Felsic (mafics <30%)
Alkali feldspar and quartz dominant; biotite and amphibole variable
>65
<3
5-10
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Viscosity of Silicate Melts-1
Viscosity = internal resistance to flow: η=σ/ε σis applied shear stress εis rate of shear strain Range of behavior:(Sen, Fig. 9.3) Newtonian =linear change in strain rate w/stress - (crystal-free basalt magmas) Bingham= finite yield strength, then linear (crystal-rich basaltic magmas) Pseudoplastic=nonlinear response (rhyolite magmas) Extent of polymerization controls viscosity Temperature increase lowers viscosity(Sen, Fig. 9.4) H O lowers viscosity(Sen, Fig. 9.5) 2
Density (rho,ρ) of Magmas
3 At low P,ρ(Fig. 9.6)between 2.2 and 3.1 g/cm Directly related to Mg + Fe content Magmas rise by buoyancy 3 Mantle peridotite ~ 3.3 g/cm • 3 •Basalt magma ~ 2.8 g/cm 3 Continental crust ~ 2.7 g/cm • •Whats going to happen when the magma gets to the Moho? Compressibility of liquids (magmas)
Viscosity of Silicate Melts-2
Temperature increase lowers viscosity
H O lowers viscosity 2 (Sen, Fig. 9.5)
Where are we going with this?
Basalts Are all dark fine-grained igneous rocks the same? Of course not, that would be too easy. Which are more important; equilibrium or fractional processes? Both, uhh neither, uhh it depends. What other processes and variables must we consider to begin to understand basalt magma generation, migration and crystallization? Assimilation, mixing, immiscibility, filter pressing,
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Are all Basalts the same?
Tremendous range of compositions Between areas and even over time from a single eruptive center Major and minor and trace element differences Observed in nature and as a result of melting natural rocks in the lab The temporal and spatial variations provide vital clues to the internal workings of the Earth.
Basalt Tetrahedron
Basalt Tetrahedron
Take a deep breath, exhale slowly, and lets see what we can do with this diagram. [1] Define the term ‘normative [2] Consider the corners of the tetrahedron [3] Consider the 3 pie-shaped volumes of the tetrahedron
‘Normative mineralogy
[See end of Sen: Chap 8 - p. 252-256] A way to compare the chemistry of coarse- and fine-grained rocks and melts using ‘fake or ‘ideal minerals. CIPW norms are calculated from weight percent analyses following rules and assumptions that simplify the mineralogy and get around the vagaries of real crystallization and alteration processes.
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Norm calculation assumptions
Water is ignored - dry melts, no hydrous phases All Al is in feldspars, not mafic minerals No Fe-Mg variations among minerals Quartz (obviously a ‘silica saturated mineral) and a variety of ‘silica under-saturated (feldspathoids) are mutually incompatible under equilibrium conditions -do not occur together - either in nature. (NaAlSi O - SiO = (NaAlSi O ) - SiO = NaAlSiO ) 3 8 2 2 6 2 4 (KAlSi O - SiO = (KAlSi O ) - SiO = KAlSiO ) 3 8 2 2 6 2 4
System Fo-En-SiO 2
Fixed pressure Peritectic point = Reaction point 3 different cases shown Equilibrium vs Fractional processes
Basalt Tetrahedron - 2
Quartz, Olivine, Diopside, Nepheline corners Quartz- and Olivine-tholeiite volumes Remember the peritectic T-X diagram (next slide) Alkali basalt volume Separated from the rest of the tetrahedron by a ‘thermal divide that magmas cant get over at low pressure by equilibrium OR fractional crystallization Sen: Fig. 7.12, p. 197 for 2D representation of this
Why do we care?
For a hundred plus years igneous rock series have been recognized - genetically related but compositionally diverse suites of rocks. Alkaline, subalkaline and calc-alkaline series recognized - the experimental work has helped explain this. We now recognize an integrated plate tectonic explanation for all the observations. What are the processes that muck with a magma between its formation and crystallization?
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Magma Differentiation
Fractional crystallization (Sen: Fig 10.2) Crystal settling (olivine), flotation (feldspar) In-situand Convective crystallization (Fig 10.3) Side, roof, floor; crystal-rich layers pealing off and settling or flowing down - sedimentary features Probably chief mechanism of basalt differentiation Flowage differentiation Crystal concentrations formed by laminar flow in dikes or sills (Fig 10.4)
In-situCrystallization, Convective Crystallization
Wallrocks
Sidewall cooling xtlln
Convecting Basaltic magma
Magma Differentiation
Filter pressing (Fig 9.11) Squeezing residual melt from crystal-melt mixture Assimilation (Fig 10.5) Reaction or dissolution of wall rock or inclusions Enthalpy (heat) budget; viscosity, convection Magma mixing Liquid immiscibility - important but minor Silicate-sulfide, silicate-carbonatitic
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