Ch 10 Magma Generation

Ch 10 Magma Generation - Mantle Melting and the Generation...

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Unformatted text preview: Mantle Melting and the Generation of Basaltic Magmas Geology 320- Petrology Magma generation q q q basalt is the most common volcanic rock type that results from melting in the mantle (oceanic crust) basalts are the dominant volcanic rock type on all of the solid bodies of the inner solar system and differentiated bodies of the asteroid belt most other igneous rock types can be produced from basalts Most Common Basalts Characteristic Plate Margin Series Convergent Divergent Alkaline yes no Tholeiitic yes yes Calc-alkaline yes no Within Plate Oceanic Continental yes yes yes yes no no Table after Wilson (1989). Igneous Petrogenesis. Unwin Hyman Kluwer; image courtesy of USGS Two Principal Types of Basalt in Ocean Basins Tholeiitic Basalt and Alkaline Basalt Table 10-1 Common petrographic differences between tholeiitic and alkaline basalts Tholeiitic Basalt Alkaline Basalt Usually fairly coarse, intergranular to ophitic Olivine common Titaniferous augite (reddish) Orthopyroxene absent Interstitial alkali feldspar or feldspathoid may occur Interstitial glass rare, and quartz absent Olivine common and zoned Orthopyroxene absent Plagioclase less common, and later in sequence Clinopyroxene is titaniferous augite, reddish rims Usually fine-grained, intergranular Groundmass No olivine Clinopyroxene = augite (plus possibly pigeonite) Orthopyroxene (hypersthene) common, may rim ol. No alkali feldspar Interstitial glass and/or quartz common Olivine rare, unzoned, and may be partially resorbed Phenocrysts or show reaction rims of orthopyroxene Orthopyroxene uncommon Early plagioclase common Clinopyroxene is pale brown augite after Hughes (1982) and McBirney (1993). Tholeiite vs. Alkaline Basalt Each is chemically distinct Evolve via FX as separate series along different paths q q Tholeiites are generated at mid-ocean ridges 3 Also generated at oceanic islands, subduction zones Alkaline basalts generated at ocean islands 3 Also at subduction zones How Do We Know What We Know About the Mantle? q seismology tells us about the interior: 3 S-waves are shear waves and cannot propagate through a liquid v v they propogate through the mantle = solid they do not travel through outer core = liquid q q q q outer core material too dense and too deep to reach surface melts must be derived from the mantle or re-melting of crustal material crust is mantle-derived anyway basalt erupts @ 1100-1200C, indicating a mantle source at least 100 km deep How Do We Know What We Know About the Mantle? q q q q Ophiolites 3 Slabs of oceanic crust and upper mantle 3 Thrust at subduction zones onto edge of continent Dredge samples from oceanic fracture zones Nodules and xenoliths in some basalts Kimberlite xenoliths 3 Diamond-bearing pipes blasted up from the mantle carrying numerous xenoliths from depth 15 Geochemical Trends Wt.% Al2O3 10 Tholeiitic basalt cpx, plag fertile, unaltered mantle? 0% 8 57 i un d te s re u du i l tia r m ti n el g M Pa 5 refractory residuum? 2 % 25 0- el m t Lherzolite 40-90% oliv, opx, cpx; minor: chrom, spin, garnets, plag Harzburgite Figure 10-1 Brown and Mussett, A. E. (1993), The Inaccessible Earth: An Integrated View of Its Structure and Composition. Chapman & Hall/ Kluwer. olivine, enstatite < 90% olivine, minor pyx and chromite 0.4 0.6 0.8 0 0.0 Dunite 0.2 Wt.% TiO2 Lherzolite: A type of peridotite with Olivine > Opx + Cpx Olivine Dunite Peridotite dense, coarse-grained igneous rock consisting of oliv and pyx; comprises the Earth's mantle te 90 Peridotites lite hr We r zb urg i 40 Lherzolite Ha Orthopyroxenite Olivine Websterite 10 Pyroxenites Orthopyroxene 10 Websterite Clinopyroxenite Clinopyroxene Figure 2-2 C After IUGS Phase diagram for aluminous 4-phase lherzolite: Al-phase 2MgSiO3 + CaMgSi2O6 + MgAl2O4 opx cpx plag spinel oliv CaAl2Si2O8 + Mg2SiO4 Mg2SiO4 + Mg3Al2Si3O12 olivine garnet MgAl2O4 + 4MgSiO3 spinel opx Figure 10-2 Phase diagram of aluminous lherzolite with melting interval (gray), sub-solidus reactions, and geothermal gradient. After Wyllie, P. J. (1981). Geol. Rundsch. 70, 128-153. How Does the Mantle Melt? 1) Increase the temperature TC radioactive decay insufficient too long to heat up peridotite rock (poor conductor) would still radiate heat away lack of mechanisms exception: hot spots hot spots don't acct for all 100 volcanism 0 1000 2000 5 Depth (km) Figure 10-3. Melting by raising the temperature. P(GPa) 200 10 300 2) Lower the pressure 0 1000 TC 2000 adiabatic rise of mantle w/ no conductive heat loss some heat loss does occur if P is rapid enough, solidus is crossed could melt @ least 30% mantle upwelling occurs @ divergent plate boundaries 100 5 Depth (km) P(GPa) 200 10 300 Figure 10-4. Melting by (adiabatic) pressure reduction. Melting begins when the adiabat crosses the solidus and traverses the shaded melting interval. Dashed lines represent approximate % melting. 3) Add volatiles (especially H2O) 800 0 volatile species relatively mobile H2O present? phlogopite and amphibole in mantle xenoliths fluid inclusions CO2 present? carbonate inclusions H2O lowers the solidus 1000 TC 1200 1400 1600 We tS oli du 1 s Dr P(GPa) yS 2 G eo du oli s 3 t her G mal radie 4 Dry peridotite solidus compared to several experiments on H2O-saturated peridotites. Data from Green, 1972; Ito & Kennedy, 1967; Kushiro et al., 1968; Mysen, 1973 nt Fraction melted is limited by availability of water P(GPa) 0 800 Albite + vapor 1000 TC 1200 0.2 0.3 0.4 0.2 H2O content of saturate d melt 0.5 0.4 Albite Liquid X mw = 0 .6 0.6 f melting or X w 0. 1 Xm w = 0 0. 2 m 0.8 0. 7 = 0.3 0. 4 0. 5 0. 6 0. 7 Figure 7-22. Pressure-temperature projection of the melting relationships in the system albite-H2O. From Burnham and Davis (1974). A J Sci., 274, 902-940. 1.0 q Heating of amphibole-bearing peridotite 1) Ocean geotherm TC 1000 400 1200 1400 600 800 2) Shield geotherm 0 1 sh oce iel d an ge ge ot h ot erm 40 60 2 he rm P(GPa) 3 hib amp ole 100 120 phlogopite 4 Figure 10-6 Phase diagram (partly schematic) for a hydrous mantle system, including the H2O-saturated lherzolite solidus of Kushiro et al. (1968), the dehydration breakdown curves for amphibole (Millhollen et al., 1974) and phlogopite (Modreski and Boettcher, 1973), plus the ocean and shield geotherms of Clark and Ringwood (1964) and Ringwood (1966). After Wyllie (1979). In H. S. Yoder (ed.), The Evolution of the Igneous Rocks. Fiftieth Anniversary Perspectives. Princeton University Press, Princeton, N. J, pp. 483-520. 140 160 180 5 6 Depth (km) 80 H 2O-s at ur at ed s o lidus Melts can be created under realistic circumstances q q q Plates separate and mantle rises at midocean ridges 3 Adibatic rise decompression melting Hot spots localized plumes of melt Fluid fluxing may give LVL 3 Also important in subduction zones and other settings Generation of basalts from a chemically uniform mantle Variables (other than X) 3 Pressure 3 Temperature Figure 10-2 Phase diagram of aluminous lherzolite with melting interval (gray), sub-solidus reactions, and geothermal gradient. After Wyllie, P. J. (1981). Geol. Rundsch. 70, 128-153. Pressure effects An increase in P alters the phase equilibria of natural systems Eutectic points, liquidus, and solidus shifts Figure 7-16. Effect of lithostatic pressure on the liquidus and eutectic composition in the diopsideanorthite system. 1 GPa data from Presnall et al. (1978). Contr. Min. Pet., 66, 203-220. Can affect the eutectic minimum in ternary systems Changes in final mineral assemblage In this phase diagram, a shift from tholeiitic to alkaline basalts Tholeiite basalts = shallow melting Alkaline basalts = deeper melting Pressure effects Figure 10-8 Change in the eutectic (first melt) composition with increasing pressure from 1 to 3 GPa projected onto the base of the basalt tetrahedron. After Kushiro (1968), J. Geophys. Res., 73, 619-634. Liquids and residuum of melted pyrolite Figure 10-9. After Green and Ringwood (1967). Earth Planet. Sci. Lett. 2, 151-160. Initial Conclusions q q Tholeiites favored by shallower melting 3 25% melting at <30 km tholeiite 3 25% melting at 60 km olivine basalt Tholeiites favored by greater % partial melting 3 20 % melting at 60 km alkaline basalt v incompatibles (alkalis) initial melts 3 30 % melting at 60 km tholeiite Crystal Fractionation of magmas as they rise q q q Tholeiite alkaline by FX at med to high P Not at low P 3 Thermal divide Al in pyroxenes at Hi P 3 Low-P FX hi-Al shallow magmas ("hi-Al" basalt) Figure 10-10 Schematic representation of the fractional crystallization scheme of Green and Ringwood (1967) and Green (1969). After Wyllie (1971). The Dynamic Earth: Textbook in Geosciences. John Wiley & Sons. Primary magmas q q Formed at depth and not subsequently modified by FX or assimilation Criteria 3 Highest Mg# (100Mg/(Mg+Fe)) really parental magma 3 Experimental results of lherzolite melts Mg# = 66-75 v Cr > 1000 ppm v Ni > 400-500 ppm v Multiply saturated v Multiple saturation q Low P 3 Ol then Plag then Cpx as cool 3 ~70oC T range Figure 10-12. Anhydrous P-T phase relationships for a midocean ridge basalt suspected of being a primary magma. After Fujii and Kushiro (1977). Carnegie Inst. Wash. Yearb., 76, 461465. Multiple saturation q Low P 3 Ol then Plag then Cpx as cool 3 70oC T range q High P 3 Cpx then Plag then Ol Figure 10-12 Anhydrous P-T phase relationships for a midocean ridge basalt suspected of being a primary magma. After Fujii and Kushiro (1977). Carnegie Inst. Wash. Yearb., 76, 461465. Multiple saturation q Low P 3 Ol then Plag then Cpx as cool 3 70oC T range q High P 3 Cpx then Plag then Ol q 25=km get all at 3 Multiple saturation once 3 Suggests that 25 km is the depth of last eqm with the Summary q q q q A chemically homogeneous mantle can yield a variety of basalt types Alkaline basalts are favored over tholeiites by deeper melting and by low % PM Fractionation at moderate to high depths can also create alkaline basalts from tholeiites At low P there is a thermal divide that separates the two series Review of REE 10.00 8.00 6.00 4.00 2.00 0.00 sample/chondrite La Ce Nd Sm Eu Tb Er Yb Lu atomic number increasing incompatibility Review of REE Figure 9-4. Rare Earth concentrations (normalized to chondrite) for melts produced at various values of F via melting of a hypothetical garnet lherzolite using the batch melting model (equation 9-5). From Winter (2001) An Introduction to Igneous and Metamorphic Petrology. Prentice Hall. increasing incompatibility REE data for oceanic basalts increasing incompatibility Figure 10-13a. REE diagram for a typical alkaline ocean island basalt (OIB) and tholeiitic mid-ocean ridge basalt (MORB). From Winter (2001) An Introduction to Igneous and Metamorphic Petrology. Prentice Hall. Data from Sun and McDonough (1989). Spider diagram for oceanic basalts increasing incompatibility Figure 10-13b. Spider diagram for a typical alkaline ocean island basalt (OIB) and tholeiitic mid-ocean ridge basalt (MORB). From Winter (2001) An Introduction to Igneous and Metamorphic Petrology. Prentice Hall. Data from Sun and McDonough (1989). Suggests different mantle source types, but isn't conclusive. Depleted mantle could both MORB and OIB. LREE depleted or unfractionated LREE enriched REE data for UM xenoliths LREE depleted or unfractionated LREE enriched Figure 10-14 Chondrite-normalized REE diagrams for spinel (a) and garnet (b) lherzolites. After Basaltic Volcanism Study Project (1981). Lunar and Planetary Institute. Review of Sr isotopes q q q q 87Rb 87Sr l = 1.42 x 10-11 a Rb (parent) conc. in enriched reservoir (incompatible) Enriched reservoir develops more 87 Sr over time Depleted reservoir (less Rb) develops less 87 Sr over time Figure 9-13. After Wilson (1989). Igneous Petrogenesis. Unwin Hyman/Kluwer. Review of Nd isotopes q 147 q q REE diagram q Sm 143Nd l = 6.54 x 10-13 a Nd Nd (daughter) enriched reservoir > Sm Enriched res. develops less 143 Nd over time Depleted res. (higher Sm/Nd) develops higher 143 Nd/144Nd over time Sm Figure 9-15. After Wilson (1989). Igneous Petrogenesis. Unwin Hyman/Kluwer. Nd & Sr isotopes of Ocean Basalts "Mantle Array" Figure 10-15 (a) Initial 143Nd/144Nd vs. 87Sr/86Sr for oceanic basalts. From Wilson (1989). Igneous Petrogenesis. Unwin Hyman/Kluwer. Data from Zindler et al. (1982) and Menzies (1983). Nd and Sr isotopes of Kimberlite Xenoliths Figure 10-15 (b) Initial 143Nd/144Nd vs. 87Sr/86Sr for mantle xenoliths. From Wilson (1989). Igneous Petrogenesis. Unwin Hyman/Kluwer. Data from Zindler et al. (1982) and Menzies (1983). Mantle model circa 1975 Figure 10-16a After Basaltic Volcanism Study Project (1981). Lunar and Planetary Institute. Newer mantle model 3 3 Upper depleted mantle = MORB source Lower undepleted & enriched OIB source Figure 10-16b After Basaltic Volcanism Study Project (1981). Lunar and Planetary Institute. Experiments on melting enriched vs. depleted mantle samples: 1. Depleted Mantle q Tholeiite easily created by 10-30% PM q More silica saturated at lower P q Grades toward alkalic at higher P Figure 10-17a. Results of partial melting experiments on depleted lherzolites. Dashed lines are contours representing percent partial melt produced. Strongly curved lines are contours of the normative olivine content of the melt. "Opx out" and "Cpx out" represent the degree of melting at which these phases are completely consumed in the melt. After Jaques and Green (1980). Contrib. Mineral. Petrol., 73, 287-310. Experiments on melting enriched vs. depleted mantle samples: 2. Enriched Mantle q q Tholeiites extend to higher P than for DM Alkaline basalt field at higher P yet 3 And lower % PM Figure 10-17b. Results of partial melting experiments on fertile lherzolites. Dashed lines are contours representing percent partial melt produced. Strongly curved lines are contours of the normative olivine content of the melt. "Opx out" and "Cpx out" represent the degree of melting at which these phases are completely consumed in the melt. The shaded area represents the conditions required for the generation of alkaline basaltic magmas. After Jaques and Green (1980). Contrib. Mineral. Petrol., 73, 287-310. ...
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This note was uploaded on 04/07/2008 for the course GEOL 320 taught by Professor Milam during the Winter '08 term at Ohio University- Athens.

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