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We discussed mantle composition and found some differences in magmas may depend on the depth of the source. Last time • Your text, p. 185, suggests the following reactions for transitions in the upper mantle: • From Plagioclase to Spinel Peridotite CaAl2Si2O8 +Mg2SiO4 =2 MgSiO3 +CaMgSi2O6+MgAl2O4 An + Fo = 2 En + Di + Spinel BTW not all of the Olivine is consumed, and we saw earlier that Mg2SiO4 has a Spinel STRUCTURE at depth. • From Spinel to Garnet Peridotites MgSiO3 + MgAl2O4 = Mg2SiO4 +Mg3Al2Si3O12 En + Spinel = Fo + Garnet
Ch 11. Magmatic Differentiation • In Chapter 10 we created a primary magma by partial melting of the mantle • It is a basalt • Can we get the diversity of igneous rocks that we find at the surface from this parent? • Magmatic Differentiation: any process by which a magma is able to diversify and produce a magma or rock of different composition
Magmatic Differentiation • Two essential processes • 1. Creates a compositional difference in one or more phases • 2. Preserves the chemical difference by segregating (or fractionating) the chemically distinct portions
Segregation Separation of a partially melted liquid from the solid residue
Incongruent melting • Many minerals do not melt uniformly. Instead they decompose as they melt, becoming melt plus a new solid mineral species. One example is solid Forsterite (Mg2SiO4), which decomposes to solid Enstatite (MgSiO3) plus liquid silica (SiO2) in the melt. • We say Forsterite is chemically incompatible with quartz, because the reaction ensures Enstatite forms from Olivine and silica. Forsterite reacts with Quartz as follows: Forsterite (Mg2SiO4) (s) + Quartz (SiO2) (l) = 2 Enstatite (MgSiO3) (s)
Incongruent Solidification of a Mantle partial melt: considering only components Mg++ and (SiO4)-4 • We start with a mantle melt between Forsterite Olivine Mg2SiO4 and Enstatite MgSiO3 in composition. • At a, the melt begins cooling. Diagram courtesy of Steve Dutch
Incongruent Solidification of a Mantle partial melt: considering just components Mg++ and (SiO4)-4 • At T= b, the melt has reached the liquidus temperature and solid Forsterite begins to form
Incongruent Solidification of a Mantle partial melt: considering just components Mg++ and (SiO4)-4 • At c, a bit more than half the melt has solidified as Forsterite. The melt has passed the composition of Enstatite, but is still too hot for it to crystallize out.
Incongruent Solidification of a Mantle partial melt: considering just components Mg++ and (SiO4)-4 • At d, we have reached the freezing/melting point of Enstatite. We are on the boundaries of fields containing both Forsterite and Enstatite. Therefore we must have both solid phases present, and Enstatite begins to form. When Enstatite cools, some Enstatite forms directly from the melt, but some forms at the expense of Forsterite.
Incongruent Solidification of a Mantle partial melt: considering just components Mg++ and (SiO4)-4 • Once solid Enstatite begins to form at d, the Temperature remains constant for the phase change, and the solidus moves horizontally as the proportion of En increases in the En + Fo mush. • For example at e, Enstatite is forming and the solid composition moves toward Enstatite. When it reaches the original system composition, the system is completely solidified.
Separation of a partially melted liquid from the solid residue requires a critical melt % • Sufficient melt must be produced for it to • Form a continuous, interconnected film • Have enough interior volume that not all of it is adsorbed to the crystal surfaces
The ability to form an interconnected film is dependent upon the dihedral angle () a property of the melt: easier with smaller angle
Liquid separation motivated by density effects (more buoyant liquid rises and escapes)
Filter pressing, or compaction, in which a crystal mush is squeezed like a sponge by weight of crystals above.
Crystal Fractionation • Dominant mechanism by which most magmas, once formed, differentiate? Gravity settling • The differential motion of crystals and liquid under the influence of gravity due to their differences in density
Gravity settling • Cool point a olivine layer at base of pluton if first olivine sinks • Next get ol+cpx layer • finally get ol+cpx+plag Figure 7-2. After Bowen (1915), A. J. Sci., and Morse (1994), Basalts and Phase Diagrams. Krieger Publishers. • Cumulate texture: • Mutually touching phenocrysts with interstitial crystallized residual melt
r - r 2 2gr ( ) = s l V h 9 Stoke’s Law V = the settling velocity (cm/sec) g = the acceleration due to gravity (980 cm/sec2) r = the radius of a spherical particle (cm) rs= the density of the solid spherical particle (g/cm3) rl = the density of the liquid (g/cm3) h = the viscosity of the liquid (1 c/cm sec = 1 poise)
Olivine in basalt • Olivine (rs = 3.3 g/cm3, r = 0.1 cm) • Basaltic liquid (rl = 2.65 g/cm3, h = 1000 poise) • Use Stoke’s Law: • V = 2·980·0.12 (3.3-2.65)/9·1000 = 0.0013 cm/sec
Rhyolitic melt • h = 107 poise and rl = 2.3 g/cm3 • hornblende crystal (rs = 3.2 g/cm3, r = 0.1 cm) • V = 2 x 10-7 cm/sec, or 6 cm/year • feldspars (rl = 2.7 g/cm3) • V = 2 cm/year • = 200 m in the 104 years that a stock might cool • If 0.5 cm in radius (1 cm diameter) settle at 0.65 meters/year, or 6.5 km in 104 year cooling of stock
Stokes’ Law is overly simplified • 1. Crystals are not spherical • 2.Only basaltic magmas very near their liquidus temperatures behave as Newtonian fluids
Hi-P Low-P Ol Pyx liquid bulk High-P (upper tie-line) has liq > ol Low-P (lower tie-line) has ol > liquid c a b f e d all solids all solids Expansion of olivine field at low pressure causes an increase in the quantity of crystallized olivine Thus, the amount of olivine that crystallizes with a rising basaltic magma will be greater that the amount that forms during isobaric crystallization See Lever Principle, Figs. 6-8 and 6-9 For example, the lower tie line has amount liquid = ef ~ 1/2 there is about twice as much solid Olivine as melt amount solid de
Two other mechanisms that facilitate the separation of crystals and liquid • 1. Flow segregation Idea: The motion of the magma past the stationary walls of the country rock creates shear in the viscous liquid Magma must flow around phenocrysts, thereby exerting pressure on them at constrictions where phenocrysts are near one another or the contact grain dispersive pressure, forcing the grains apart and away from the contact This is probably a relatively minor effect Figure 11-4 Drever and Johnston (1958). Royal Soc. Edinburgh Trans., 63, 459-499.
Volatile Transport • 2. As a volatile-bearing (but undersaturated) magma rises and pressure is reduced, the magma may eventually become saturated in the vapor, and a free vapor phase will be released Figure 7-22. From Burnham and Davis (1974). A J Sci., 274, 902-940.
Fractional crystallization enriches late melt in non-rock-forming (non-lithophile) elements • Particularly enriched with resurgent boiling (melt already evolved when vapor phase released) • Get a silicate-saturated vapor + a vapor-saturated late derivative silicate liquid 3. Late-stage Fractional Crystallization
8 cm tourmaline crystals from pegmatite 5 mm gold from a hydrothermal deposit
Liquid Immiscibility • Liquid immiscibility in the Fo-En-SiO2 system Figure 6-12.Isobaric T-X phase diagram of the system Fo-Silica at 0.1 MPa. After Bowen and Anderson (1914) and Grieg (1927). Amer. J. Sci.
Walker and DeLong (1982) subjected two basalts to thermal gradients of nearly 50oC/mm! Found that: • Samples reached a steady state in a few days • Heavier elements ® cooler end and the lighter ® hot end • The chemical concentration is similar to that expected from fractional crystallization Si at top, Fe Mg Ti Ca on bottom Figure 7-4. AfterWalker, D. C. and S. E. DeLong (1982). Contrib. Mineral. Petrol., 79, 231-240.
Magma Mixing Comingled basalt-Rhyolite Mt. McLoughlin, Oregon Figure 11-8From Winter (2001) An Introduction to Igneous and Metamorphic Petrology. Prentice Hall Basalt pillows accumulating at the bottom of a granitic magma chamber, Vinalhaven Island, Maine
Assimilation • Incorporation of wall rocks (diffusion, xenoliths) • Assimilation by melting is limited by the heat available in the magma • Xenolith melts if the melting point of the country rock is (much) less than the temperature of the magma
Detecting and assessing assimilation Isotopes are generally the best • Continental crust becomes progressively enriched in 87Sr/86Sr and depleted in 143Nd/144Nd • Some trace elements are much more abundant in the continental crust than in mantle-derived magmas. • The assimilation of a modest amount of crustal material rich in that element may have a considerable effect on a magma that initially contained very little of it. • During the fractional crystallization of magma, and magma generation by the partial melting of the Earth's mantle and crust, elements that have difficulty in entering cation sites of the minerals are concentrated in the melt phase of magma (liquid phase). An incompatible element is an element that is unsuitable in size and/or charge to the cation sites of the minerals
Detecting and assessing assimilation • 9-22 238U 234U 206Pb (l = 1.5512 x 10-10 a-1) • 9-23 235U 207Pb (l = 9.8485 x 10-10 a-1) • 9-24 232Th 208Pb (l = 4.9475 x 10-11 a-1) • U-Th-Pb system as an indicator of continental contamination is particularly useful • All incompatibles similar to Zr+4, so they concentrate strongly into the continental crust because they are not removed during early fractionation.
Mixed Processes • May be more than coincidence: two processes may operate in conjunction. • E.g. fractional crystallization + recharge of more primitive magma • As we shall see next time.