1、Chapter 10: Mantle Melting and the Generation of Basaltic Magma,Geology 346- Petrology,2 principal types of basalt in the ocean basins,Table 10.1,Common petrographic differences between tholeiitic and alkaline basalts,Tholeiitic Basalt,Alkaline Basalt,Usually fine-grained, intergranular,Usually fair
2、ly coarse, intergranular to ophitic,Groundmass,No olivine,Olivine common,Clinopyroxene = augite (plus possibly pigeonite),Titaniferous augite (reddish),Orthopyroxene (hypersthene) common, may rim ol.,Orthopyroxene absent,No alkali feldspar,Interstitial alkali feldspar or feldspathoid may occur,Inter
3、stitial glass and/or quartz common,Interstitial glass rare, and quartz absent,Olivine rare, unzoned, and may be partially resorbed,Olivine common and zoned,Phenocrysts,or show reaction rims of orthopyroxene,Orthopyroxene uncommon,Orthopyroxene absent,Early plagioclase common,Plagioclase less common,
4、 and later in sequence,Clinopyroxene is pale brown augite,Clinopyroxene is titaniferous augite, reddish rims,after Hughes (1982) and McBirney (1993).,Tholeiitic Basalt and Alkaline Basalt,Tholeiites are generated at mid-ocean ridges Also generated at oceanic islands, subduction zones Alkaline basalt
5、s generated at ocean islands Also at subduction zones,Each is chemically distinct Evolve via FX as separate series along different paths,Sources of mantle material,Ophiolites Slabs of oceanic crust and upper mantle Thrust at subduction zones onto edge of continent Dredge samples from oceanic crust N
6、odules and xenoliths in some basalts Kimberlite xenoliths Diamond-bearing pipes blasted up from the mantle carrying numerous xenoliths from depth,15,10,5,0,0.0,0.2,0.4,0.6,0.8,Wt.% Al2O3,Wt.% TiO2,Dunite,Harzburgite,Lherzolite,Tholeiitic basalt,Partial Melting,Residuum,Lherzolite is probably fertile
7、 unaltered mantle Dunite and harzburgite are refractory residuum after basalt has been extracted by partial melting,Figure 10-1 Brown and Mussett, A. E. (1993), The Inaccessible Earth: An Integrated View of Its Structure and Composition. Chapman & Hall/Kluwer.,Lherzolite: A type of peridotite with O
8、livine Opx + Cpx,Olivine,Clinopyroxene,Orthopyroxene,Lherzolite,Harzburgite,Wehrlite,Websterite,Orthopyroxenite,Clinopyroxenite,Olivine Websterite,Peridotites,Pyroxenites,90,40,10,10,Dunite,Figure 2.2 C After IUGS,Phase diagram for aluminous 4-phase lherzolite:,Plagioclase shallow ( 400 km,Al-phase
9、=,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,Figure 10.3. Melting by raising the temperature.,2) Lower th
10、e pressure Adiabatic rise of mantle with no conductive heat loss Decompression partial melting could melt at least 30%,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 approxi
11、mate % melting.,3) Add volatiles (especially H2O),Figure 10.4. Dry peridotite solidus compared to several experiments on H2O-saturated peridotites.,Fraction melted is limited by the availability of water,15% 20% 50% 100%,Figure 7.22. Pressure-temperature projection of the melting relationships in th
12、e system albite-H2O. From Burnham and Davis (1974). A J Sci., 274, 902-940.,Heating of amphibole-bearing peridotite 1) Ocean geotherm 2) Shield geotherm,Figure 10.6 Phase diagram (partly schematic) for a hydrous mantle system, including the H2O-saturated lherzolite solidus of Kushiro et al. (1968),
13、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 Annive
14、rsary Perspectives. Princeton University Press, Princeton, N. J, pp. 483-520.,Melts can be created under realistic circumstances,Plates separate and mantle rises at mid-ocean ridges Adibatic rise decompression melting Hot spots localized plumes of melt Fluid fluxing may give LVL Also important in su
15、bduction zones and other settings,Generation of tholeiitic and alkaline basalts from a chemically uniform mantle,Variables (other than X) Temperature Pressure,Figure 10.2 Phase diagram of aluminous lherzolite with melting interval (gray), sub-solidus reactions, and geothermal gradient. After Wyllie,
16、 P. J. (1981). Geol. Rundsch. 70, 128-153.,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 pyr
17、olite,Figure 10.9 After Green and Ringwood (1967). Earth Planet. Sci. Lett. 2, 151-160.,Initial Conclusions:,Tholeiites favored by shallower melting 25% melting at 30 km tholeiite 25% melting at 60 km olivine basalt Tholeiites favored by greater % partial melting (F) 20 % melting at 60 km alkaline b
18、asalt incompatibles (alkalis) initial melts 30 % melting at 60 km tholeiite,Crystal Fractionation of magmas as they rise,Tholeiite alkaline by FX at med to high P Not at low P Thermal divide Al in pyroxenes at Hi P Low-P FX hi-Alshallow magmas(“hi-Al” basalt),Figure 10.10 Schematic representation of
19、 the fractional crystallization scheme of Green and Ringwood (1967) and Green (1969). After Wyllie (1971). The Dynamic Earth: Textbook in Geosciences. John Wiley & Sons.,Figure 10.11 After Kushiro (2001).,Other, more recent experiments on melting of fertile (initially garnet-bearing) lherzolite conf
20、irm that alkaline basalts are favored by high P and low F,Primary magmas,Formed at depth and not subsequently modified by FX or Assimilation Criteria Highest Mg# (100Mg/(Mg+Fe) really parental magma Experimental results of lherzolite melts Mg# = 66-75 Cr 1000 ppm Ni 400-500 ppm Multiply saturated,Mu
21、ltiple saturation,Low P Ol then Plag then Cpx as cool 70oC T range,Figure 10.13 Anhydrous P-T phase relationships for a mid-ocean ridge basalt suspected of being a primary magma. After Fujii and Kushiro (1977). Carnegie Inst. Wash. Yearb., 76, 461-465.,Low P Ol then Plag then Cpx as cool 70oC T rang
22、e High P Cpx then Plag then Ol,Multiple saturation,Figure 10.13 Anhydrous P-T phase relationships for a mid-ocean ridge basalt suspected of being a primary magma. After Fujii and Kushiro (1977). Carnegie Inst. Wash. Yearb., 76, 461-465.,Low P Ol then Plag then Cpx as cool 70oC T range High P Cpx the
23、n Plag then Ol 25 km get all at once,= Multiple saturation Suggests that 25 km is the depth of last eqm with the mantle,Multiple saturation,Summary,A chemically homogeneous mantle can yield a variety of basalt types Alkaline basalts are favored over tholeiites by deeper melting and by low % PM Fract
24、ionation 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,Review of REE,increasing incompatibility,Figure 9.4. Rare Earth concentrations (normalized to chondrite) for melts produced at various v
25、alues 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.,REE data for oceanic basalts,Figure 10.14a. REE diagram for a typical alkaline ocean island basalt (OIB) an
26、d tholeiitic mid-ocean ridge basalt (MORB). From Winter (2001) An Introduction to Igneous and Metamorphic Petrology. Prentice Hall. Data from Sun and McDonough (1989).,increasing incompatibility,Spider diagram for oceanic basalts,increasing incompatibility,Figure 10.14b. Spider diagram for a typical
27、 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 isnt conclusive. Depleted mantle could both MORB a
28、nd OIB.,REE data for UM xenoliths,Figure 10.15 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,87Rb 87Sr l = 1.42 x 10-11 a Rb (parent) conc. in enriched reservoir (incomp
29、atible) Enriched reservoir,Figure 9.13. After Wilson (1989). Igneous Petrogenesis. Unwin Hyman/Kluwer.,develops more 87Sr over time,Review of Nd isotopes,147Sm 143Nd l = 6.54 x 10-13 a Nd (daughter) enriched reservoir Sm Enriched reservoir,develops less 143Nd over time,Figure 9.15. After Wilson (198
30、9). Igneous Petrogenesis. Unwin Hyman/Kluwer.,Nd and Sr isotopes of Ocean Basalts,“Mantle Array”,Figure 10.16a. 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
31、of Kimberlite Xenoliths,Figure 10.16b. 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).,“Whole Mantle” circulation model,Figure 10-17a After Basaltic Volcanism Study Project (1981
32、). Lunar and Planetary Institute.,Upper depleted mantle = MORB source Lower undepleted & enriched OIB source,“Two-Layer” circulation model,Figure 10-17b After Basaltic Volcanism Study Project (1981). Lunar and Planetary Institute.,Experiments on melting enriched vs. depleted mantle samples:,Tholeiit
33、e easily created by 10-30% PM More silica saturated at lower P Grades toward alkalic at higher P,1. Depleted Mantle,Figure 10-18a. Results of partial melting experiments on depleted lherzolites. Dashed lines are contours representing percent partial melt produced. Strongly curved lines are contours
34、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:,Tholeii
35、tes extend to higher P than for DM Alkaline basalt field at higher P yet And lower % PM,2. Enriched Mantle,Figure 10-18b. Results of partial melting experiments on fertile lherzolites. Dashed lines are contours representing percent partial melt produced. Strongly curved lines are contours of the nor
36、mative 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.,
