Lecture 8: Igneous Petrogenesis. Igneous rock classification Phase relations Mantle melting Trace element geochemistry

Transcription

1 Lecture 8: Igneous Petrogenesis Igneous rock classification Phase relations Mantle melting Trace element geochemistry

2 >70% of Earth s annual volcanic budget is erupted in the oceans.

3 Igneous Rock Classification: Texture The first distinction is between volcanic and plutonic rocks. Volcanic rocks are erupted at the Earth s surface and cool very quickly. There is insufficient time to grow large crystals. This leads to formation of glass or very fine-grained rocks, or to phenocrysts (crystals that grew before eruption) in a fine groundmass. Plutonic rocks crystallize at some depth, and therefore lose heat relatively slowly. Crystals have time to grow after nucleation, and the resulting rocks generally have individual crystals large enough to see unaided. Rocks of exactly the same composition and mineralogy get different names in their volcanic and plutonic forms, because they look different.

4 Basalt Gabbro 5 mm 5 mm

5 Aphanitic: mineral grains or groundmass that are smaller than mm (need microscope or hand lens to see). Porphyritic: larger mineral grains within an aphanitic or phaneritic matrix. Phaneritic: mineral grains easily seen with naked eye >3 mm.

6 Mineral groups ) Silicates (SiO 4 ) make up 96% of minerals, e.g., olivine 2) Carbonates (CO 3 ): e.g, calcite CaCO 3 3) Oxides: metal and oxygen (e.g., hematite, magnetite) 4) Sulfides: element + S 2 (pyrite FeS) 5) Sulfates: element + SO 4 (gypsum CaSO 4 2H 2 O) 6) Halides: element + halide (salt - NaCl) 7) Native elements: e.g., Cu, Au, Ag Key minerals in mafic igneous rocks: Olivine: (Mg,Fe) 2 SiO 4 -- Forsterite (Mg), Fayalite (Fe) Pyroxene: (Mg,Fe,Ca) 2 Si 2 O 6 -- Mg-, Fe-, Ca-, (Clinopyroxene); Na-, Al- (Orthopyroxene) Feldspar: (K,Na,Ca)AlSi 3 O 8 -- Albite (Na-),Anorthite (Ca-), Plagioclase (Na+Ca), Alkali (K-) gypsum

7 Igneous Rock Classification: Mineralogy The standard classification scheme uses the mineralogy of the rock (how much quartz, how much plagioclase, etc.) There is one important twist for volcanic rocks you usually cannot measure the actual minerals present (or it may be a glass and there are no minerals present). In this case, instead of the actual minerals, you classify based on normative mineralogy The norm is a calculation based on the bulk composition of a volcanic rock, for what minerals would be present if it were fully crystallized. The standard norm calculation is called the CIPW norm, after Cross, Iddings, Pirsson, and Washington (902).

8 Quartz: SiO 2 Orthoclase: KAlSi 3 O 8 Plagioclase: NaAlSi 3 O 8 Feldspathoid: feldspar with Al:Si =.

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10 peridotite pyroxenite Dunite Lherzolite Pyroxenite

11 Wt.% Al2O3. Ophiolites 2. Dredge samples from oceanic fracture zones 3. Xenoliths in basalts 4. Kimberlites Tholeiitic basalt 5 l tia r Pa ing % lt 20 Me 5 Lherzolite Harzburgite Residuum Dunite 0.2 Dunite 0.4 Wt.% TiO Lherzolite Pyroxenite

12 By silica percentage: Other igneous rock classifications %SiO 2 Designation %Dark Minerals Designation Examples >66 Acid <40 Felsic Granite, rhyolite Intermediate Intermediate Diorite, andesite Basic Mafic Gabbro, basalt <45 Ultrabasic >90 Ultramafic Dunite, komatiite By alumina saturation (which dark minerals show up): Chemistry Designation %Dark Minerals Al 2 O 3 >Na 2 O+K 2 O+CaO Na 2 O+K 2 O+CaO>Al 2 O 3 & Al 2 O 3 > Na 2 O+K 2 O Peraluminous Metaluminous Muscovite, biotite, topaz, corundum, garnet, tourmaline Melilite, biotite, pyroxene, hornblende, epidote Al 2 O 3 ~ Na 2 O+K2O Subaluminous Olivine, pyroxenes Al 2 O 3 < Na 2 O + K 2 O Peralkaline Sodic pyroxenes & amphiboles

13 Total alkalis + silica (TAS) classification Rhyolite Basalt

14 Geodynamic setting of igneous rocks Igneous rocks are formed today at plate margins or in continental or oceanic plate interiors (but most of the action is at plate boundaries).

15 Mantle melting terminology Geotherm Vertical temperature profile in the earth Solidus Temperature at which a rock will first start to melt Liquidus Temperature at which a rock will be fully molten. Adiabat A packet of the mantle that moves up/down without gaining or losing heat.

16 Another explanation of the adiabat Imagine the Earth with its present distribution of material but without gravity. The material is uncompressed and there is no pressure increase with depth. Set the initial temperature everywhere to the Earth's surface temperature. Now turn gravity back on. The gravitational pressure causes the material to contract, with material compressing more at greater depths because of the greater pressure. The temperature will also increase because of the compression. If this is done such that no heat is gained or lost by any given piece of the material, the temperature increase for any parcel of matter will be adiabatic, and the temperature increase with depth will thus be adiabatic. Temperature Depth No gravity, isothermal Gravity turned on, adiabatic

17 Another explanation of the adiabat Another way to achieve an approximate adiabatic temperature distribution is to have material convect heat from the hotter interior to the cooler exterior. The heat is carried upwards by the upwards movement or flow of material, while material cooled near the surface descends. If the temperature gradient (increase of temperature with depth) is adiabatic, then upwards movement of a parcel of material will not result in a temperature difference of the parcel with respect to the surrounding material. However, if the temperature gradient is greater than adiabatic (super-adiabatic), the temperature of an upwards moving parcel will only decrease by the adiabatic gradient, and so will be greater than that of its surroundings. Temp. excess of parcel at shallower depth Adiabatic gradient Depth In situ geotherm (super-adiabatic temperature gradient) Upward movement of material along adiabatic gradient

18 Mantle Plumes Mantle melts between ~ ºC due to: Increase in temperature Decrease in pressure Addition of volatile phases

19 Mid-Ocean Ridges (and Plumes) Mantle melts between ~ ºC due to: Increase in temperature Decrease in pressure Addition of volatile phases

20 Subduction Zones Mantle melts between ~ ºC due to: Increase in temperature Decrease in pressure Addition of volatile phases

21 Percentage of melting (F) The pressure (or depth) versus temperature (P-T) path of upwelling mantle beneath a mid-ocean ridge leads to a maximum of ~25% melt.

22 Phase Diagrams A phase diagram is common way to represent the system state at specific pressure (P) and temperature (T) conditions. Lines on the diagram represent conditions under which a phase change is at equilibrium. At a point on a line, it is possible for two or more phases to coexist at equilibrium. In other regions, only one phase exists at equilibrium. Phase diagram for water Triple point: where 3 phases coexist

23 Phase diagram for Olivine solid solution Forsterite (Fo) Fayallite (Fa) Solidus: the temperature below which the substance is stable in the solid state Liquidus: the temperature above which the substance is stable in the liquid state Lever Rule: to determine quantitatively the relative composition of a mixture in a two-phase region in a phase diagram

24 Equilibrium Melting Equilibrium melting occurs when the solid and liquid phases are kept together as melting progresses.

25 Lever Rule S solid composition L liquid composition A system composition We can write fraction x of solid as xs + (-x)l = A which can also be written as x (A S) = (-x)(l-a) We can solve the above equations to get the proportion of solid x = (A L) / (S L)

26 Fractional Melting Fractional melting occurs if the liquid is immediately removed from the solid as the solid melts.

27 Equilibrium Solidification Equilibrium solidification occurs when the solid and liquid phases are kept together as solidifications progresses.

28 Fractional Solidification Fractional solidification occurs if the solid is immediately removed from the liquid as it crystallizes.

29 Diopside (Clinopyroxene) Anorthite (Plagioclase) Diopside (CaMgSi2O6) Dark mineral Gabbro (coarse grained equivalent of basalt) Oceanic Crust Anorthite (CaAl2Si2O8) Light mineral

30 Diopside-Anorthite phase diagram 600 liquidus Liquid 500 Temperature, C Di + liquid eutectic An + liquid 200 Diopside + Anorthite solidus CaMgSi (Diopside) Anorthite content, mol% CaAl 2 Si (Anorthite) Eutectic: mixture that has the lowest freezing point (composition/ temperature of the last solids formed when freezing, first melt formed)

31 Diopside-Anorthite phase diagram 600 liquidus Liquid 500 Temperature, C Di + liquid eutectic An + liquid 200 Diopside + Anorthite solidus CaMgSi (Diopside) Anorthite content, mol% CaAl 2 Si (Anorthite) batch melting

32 Diopside-Anorthite phase diagram 600 liquidus Liquid 500 Temperature, C Di + liquid eutectic An + liquid 200 Diopside + Anorthite solidus CaMgSi (Diopside) Anorthite content, mol% CaAl 2 Si (Anorthite) fractional melting

33 Back to mantle melting Gar OL Cpx Opx OL Olivine Gar Cpx Opx O OL Incr. melt G OL C O OL G O OL C O OL OL OL OL OL Melting of garnet lherzolite begins at opx-cpx-garnet triple junctions (eutectic) in response to a reduction in pressure. Olivine is not involved in melting at early stages. As the extent of melting (F) increases, melt migrates along grain boundaries forming an interconnected network that allows the melt to segregate from the unmelted crystal residue.

34 Wenlu Zhu Melting of garnet lherzolite begins at opx-cpx-garnet triple junctions (eutectic) in response to a reduction in pressure. Olivine is not involved in melting at early stages. As the extent of melting (F) increases, melt migrates along grain boundaries forming an interconnected network that allows the melt to segregate from the unmelted crystal residue.

35 Trace elements in mantle melting Incompatible elements: preferentially partition into the melt phase (D<) Compatible elements: preferentially partition into the solid phase (D>) Partition or distribution coefficient (D) = C solid /C liquid Concentrations normalized to bulk earth, C chondrites, or primitive mantle Most incompatible Less incompatible

36 Partition coefficients Partition coefficients are determined for an element between a unique mineral phase in a unique lattice site and melt, and are determined by three primary factors Size (ionic radius) is fairly intuitive control, since the substituting ion needs to fit into a mineral lattice: Too big or too small a won't be energetically stable. Charge (ionic charge) is also intuitive, since charge must be balance within a lattice and if a charge imbalance is generated by a substitution, a second substitution must occur to correct for this. Electronegativity is harder to visualize, but the disruption to the mineral lattice of replacing a greedy element with a giving element or vice versa is too much for a lattice to take.

37 Partition coefficients Rock Type Mineral Z Elem Value Kd Type Reference Basalt Garnet 4 Nb Experimental Jenner et al. 994 Basalt Garnet 4 Nb 0.0 Phenocryst-Matrix, Keleman & Dunn 992 Basalt Ilmenite 4 Nb 0.8 Experimental Experimental McCallum & Charette 978 Basalt Low Calcium 4 Nb Phenocryst-Matrix, Keleman & Dunn 992 Basalt Pyroxene Magnetite 4 Nb Experimental Calculated Nielsen 992 Basalt Olivine 4 Nb 0.0 Calculated McKenzie & O'Nions 99 Basalt Plagioclase 4 Nb 0.0 Calculated McKenzie & O'Nions 99 Basalt Plagioclase 4 Nb Experimental McCallum & Charette 978 Basalt Plagioclase 4 Nb Experimental Bindeman et al. 998 Basalt Plagioclase 4 Nb Experimental Aignertorres et al Basalt Rutile 4 Nb 6 Experimental McCallum & Charette 978 Basalt Rutile 4 Nb 36 Experimental Foley et al

38 Trace Elements in mantle melting Incompatible elements: preferentially partition into the melt phase (D<) Compatible elements: preferentially partition into the solid phase (D>) Partition or distribution coefficient (D) = C solid /C liquid

39 Relating trace element concentrations to melt fraction (F) - batch (equilibrium) melting!!"#$! =!!"#!! +!!!!!!"#!! =!!!!"#!! +!!! 0 Melts 0 Solids D=0.0 5!!"#$!!!"# !!"#!!!"# F F F

40 Relating trace element concentrations to melt fraction (F) - fractional melting!!"#$! =!!!!!!!!!!!"#!!"#! =!!!!!!!!!"# 0 Instantaneous Melt Fraction 0 Solid!!"#$!!!"# !!"#!!!"# D= F F

41 Relating trace element concentrations to melt fraction (F) - fractional melting!!"#$!!!"# 0!!"#$ =!!!!!!!!!"# Aggregate Melt F!!"#!!!"# !!"#! =!!!!!!!!!"# 0. F 0.5 Solid 5

42 0 Aggregate Melt 0 Solid!!"#$!!!"# !!"#!!!"# Fractional 0 Equilibrium 0 Melts 0 Solids!!"#$!!!"# D=0.0!!"#!!!"# F F F 0.0

43 Trace element partitioning evidence for differentiation of the Earth D Rb (olivine) = ~0.003 D Rb (pyroxene) = ~0.002 D Rb (garnet) = ~ Continetal Crust Rb K Ba Sr 0 Ca La Y RE E Th Mn Lu U 80 V Li Fe Co Sc Hf Mg 0.2 Ni 60 Cr Ga Zr Ta, N b > Ti 0.0 P Be Ionic Charge Figure 7.. Ionic radius (picometers) vs. ionic charge contoured for clinopyroxene/liquid partition coefficients. Cations normally present in clinopyroxene are Ca, Mg, and Fe, shown by symbols. Elements whose charge and ionic radius most closely match that of the major elements have the highest partition coefficients. modified from White, Geochemistry Pb 87 Sr/ 86 Sr Bulk Earth Billion yrs Depleted Earth s Mantle Time Oceanic Crust Pres

44 00 0 Cont. Crust Plume Melts and crust normalized to primitive mantle MORB Cs Rb Ba Th U Nb La Ce Pr Sr Nd Zr Hf Sm Gd Tb Dy Ho Y Er Yb Lu

45 00 0 Cont. Crust Plume Melts and crust normalized to primitive mantle MORB Cs Rb Ba Th U Nb La Ce Pr Sr Nd Zr Hf Sm Gd Tb Dy Ho Y Er Yb Lu

46 00 0 Cont. Crust Plume Melts and crust normalized to primitive mantle MORB Cs Rb Ba Th U Nb La Ce Pr Sr Nd Zr Hf Sm Gd Tb Dy Ho Y Er Yb Lu

47 Reading for next class OCEANOGRAPHY Enhanced East Pacific Rise hydrothermal activity during the last two glacial terminations D. C. Lund, * P. D. Asimow, 2 K. A. Farley, 2 T. O. Rooney, 3 E. Seeley, E. W. Jackson, 4 Z. M. Durham 4 Mid-ocean ridge magmatism is driven by seafloor spreading and decompression melting of the upper mantle. Melt production is apparently modulated by glacial-interglacial changes in sea level, raising the possibility that magmatic flux acts as a negative feedback on ice-sheet size. The timing of melt variability is poorly constrained, however, precluding a clear link between ridge magmatism and Pleistocene climate transitions. Here we present well-dated sedimentary records from the East Pacific Rise that show evidence of enhanced hydrothermal activity during the last two glacial terminations. We suggest that glacial maxima and lowering of sea level caused anomalous melting in the upper mantle and that the subsequent magmatic anomalies promoted deglaciation through the release of mantle heat and carbon at mid-ocean ridges.