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Unformatted text preview: Mantle Petrology and the Origin of Basaltic Magma ALL basaltic magmas are partial melts of the MANTLE Lherzolite is probably fertile unaltered mantle Dunite and harzburgite are refractory residuum after basalt has been extracted by partial melting
15 Tholeiitic basalt Lherzolite: A type of peridotite with Olivine > Opx + Cpx
90 Wt.% Al2O3 10 Pa al r ti M e g ltin 40 5 Ha rzb urg ite Peridotites Lherzolite Lherzolite Orthopyroxenite Olivine Websterite
10 0 0.0 Harzburgite Dunite
0.6 10 Websterite Clinopyroxenite lite hr We Pyroxenites Clinopyroxene 0.8 Wt.% TiO2 Fertile mantle = Four-phase lherzolite:
(Ol + Opx + Cpx + Al-rich phase) Al-phase is P sensitive q Plagioclase
3 shallow (<30 km) 30-70 km >70 km q Spinel
3 q Garnet
3 sample/chondrite Depth of melting from REE patterns 10.00 8.00 6.00 4.00 2.00 0.00 60% Ol 15% Opx 15% Cpx 10%Plag La Ce Nd Sm Eu Tb Er Yb L 10.00 sample/chondrite 8.00 6.00 4.00 2.00 0.00 67% Ol 17% Opx 14% Cpx 3% Sp 56 58 La Ce 60 Nd 62 64 66 Sm Eu Tb 68 72 Er 70 Yb Lu 10.00 sample/chondrite 8.00 6.00 4.00 2.00 0.00 57% Ol 14% Opx 14% Cpx 14% Grt 56 58 La Ce 60 Nd 62 64 66 Sm Eu Tb 68 72 Er 70 Yb Lu How does the mantle melt??
1) Increase the temperature 2) Decrease the pressure 3) Add volatiles (H O, CO )
2 2 How does the mantle melt??
1) Increase the temperature Decompression Melting (just at melting temp) (200C below melting temp) 2) Lower the pressure
Adiabatic rise of mantle with no conductive heat loss 3 Decompression melting could melt at least 30%
3 Addition of Fluids Lowers melting T Causes partial melting 3) Add volatiles (especially H O)
2 The Effect of Water on Melting
The dry melting reaction is: Albite = Liquid With water present, the melting reaction is: Albite + H2O = Liquid
The effect of H2O saturation on the melting of albite, from the experiments by Burnham and Davis, 1974, American Journal of Science, v. 274, p.902-940. The "dry" melting curve is from Boyd and England, 1963, Journal of Geophysical Research, v. 68, p. 311-323. Effect of pressure, water, and CO2 on the position of the eutectic in the basalt system
Increased pressure moves the ternary eutectic (first melt) from Water moves the (2 Gpa) eutectic toward higher silica, while CO2 silica-saturated to highly undersaturated alkaline basalts moves it to more alkaline types
Ne Ne Volatile-free
3GPa 2GPa 1GPa
Highly undesaturated (nepheline-bearing) alkali olivine basalts P = 2 GPa
1atm Fo Ab En CO dry
Highly undesaturated (nepheline-bearing) alkali olivine basalts HO Ab te d ura salts sat a deriitic b Unole th Oversaturated (quartz-bearing) tholeiitic basalts ed rat alts a tu s erstic ba d Unoleii th Oversaturated (quartz-bearing) tholeiitic basalts Fo En SiO2 Fo En SiO2 Basaltic melts can be created under realistic circumstances:
q Decompression melting 3 Divergent boundaries plates separate and
mantle rises at mid-ocean ridges, MORB mantle plumes, OIB
3 Intraplate hot spots rise of localized q Fluid fluxing 3 important in subduction zones Primary magmas
q Mantle melts with no subsequent modification by fractional crystallization or assimilation q Rarely observed, "Holy Grail" of basalts, very important for petrologists and geochemists, fingerprint of mantle source! Is the Mantle Chemically Homogeneous?? REE data for oceanic basalts Two Types increasing incompatibility Multi-element diagram for oceanic basalts Two Types increasing incompatibility LREE depleted or unfractionated LREE enriched REE data for mantle xenoliths q Spinel
3 LREE depleted or unfractionated 30-70 km
LREE enriched q Garnet
3 >70 km Is the Mantle Chemically Homogeneous?? NO!!! Mantle Sources and Structure
3 3 Upper depleted mantle = MORB source Lower undepleted & enriched OIB source Two principal types of basalt in the ocean basins
Tholeiitic Basalt and Alkaline Basalt q Tholeiites are lower pressure and larger % of partial melting q Alkaline basalts are higher pressure and smaller % of partial melting 2 principal types of basalt in the ocean basins
Tholeiitic Basalt and Alkaline Basalt
Common petrographic differences between tholeiitic and alkaline basalts MORB (generally)
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 Olivine common OIB (generally)
Usually fairly coarse, intergranular to ophitic 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 Phenocrysts or show reaction rims of orthopyroxene Orthopyroxene uncommon Early plagioclase common Clinopyroxene is pale brown augite after Hughes (1982) and McBirney (1993). ...
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This note was uploaded on 05/02/2010 for the course ESCI 322 taught by Professor Evans during the Spring '10 term at Central Connecticut State University.
- Spring '10