Origin of Basaltic Magma

Today Updates: ? Today: Different basalts from the same source Crystallization order Effect on Trace elements

Isochron method y = b + x m Divide by stable 86Sr: 87Sr/86Sr = (87Sr/86Sr)o + (87Rb/86Sr)(elt -1) l = 1.4 x 10-11 a-1 y = b + x m = equation for a line in 87Sr/86Sr vs. 87Rb/86Sr plot Slope = (elt -1)

( ) Begin with 3 rocks plotting at a b c at time to to 86Sr 87Sr a b c 87Rb

After some time increment (t0 t1) each sample loses some 87Rb and gains an equivalent amount of 87Sr a b c a1 b1 c1 t1 to 86Sr 87Sr 87Rb o ( )

( ) At time t2 each rock system has evolved  new line Again still linear and steeper line a b c a1 b1 c1 a2 b2 c2 t1 to t2 86Sr 87Sr o ( ) 86Sr 87Rb

Isochron Technique t2 t1 to a b c a1 b1 c1 a2 b2 c2 86Sr 87Sr 86Sr 87Rb

Isochron results S r S r/ Sr 0.710 0.715 0.720 0.725 2 4 6 8 10 12 14 Rb-Sr Isochron, Eagle Peak Pluton, Sierra Nevada Batholith 87 Sr/ 86 Sr = 0.00127 ( Rb/ Sr) + 0.70760 Sr x S r/ S r Figure 9-9. After Hill et al. (1988). Amer. J. Sci., 288-A, 213-241.

Making Sr isotope reservoirs Figure 9-13. After Wilson (1989). Igneous Petrogenesis. Unwin Hyman/Kluwer.

Geotherm and solidus: how to melt

Melt creation in ocean basin

2 types in ocean basins Tholeiitic Basalt and Alkaline Basalt Table 10-1 Common petrographic differences between tholeiitic and alkaline basalts Tholeiitic Basalt Alkaline Basalt Usually fine-grained, intergranular Usually fairly 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 Interstitial 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, and later in sequence Clinopyroxene is pale brown augite Clinopyroxene is titaniferous augite, reddish rims after Hughes (1982) and McBirney (1993).

Lherzolite: A type of peridotite with Olivine > Opx + Cpx Dunite 90 Peridotites Wehrlite Harzburgite Lherzolite 40 Olivine Websterite Pyroxenites Orthopyroxenite 10 Websterite 10 Clinopyroxenite Orthopyroxene Clinopyroxene Figure 2-2 C After IUGS

Examples from the mantle/how we know Ophiolites Obducted oceanic crust + upper mantle Dredge samples from oceanic fracture zones Xenoliths in some basalts: Local example? Kimberlite xenoliths Diamond-bearing pipes blasted up from the mantle

Tholeiite/alkaline basalt based on P&T; big difference: Figure 10-2 After Wyllie, P. J. (1981). Geol. Rundsch. 70, 128-153.

Pressure effects on melting Ne Fo En Ab SiO2 Oversaturated (quartz-bearing) tholeiitic basalts Highly undersaturated (nepheline - bearing) alkali basalts Undersaturated E 3GPa 2Gpa 1GPa 1atm Volatile-free Figure 10-8 After Kushiro (1968), J. Geophys. Res., 73, 619-634.

REE plots sample/chondrite sample/chondrite Large F Small F 0.00 2.00 4.00 6.00 8.00 10.00 atomic number sample/chondrite 0.4 0.2 0.05 0.1 La Ce Nd Sm Eu Tb Er Yb Lu Batch Partial Melting values = F (fraction melted) increasing incompatibility 0.6 0.00 2.00 4.00 6.00 8.00 10.00 sample/chondrite La Ce Nd Sm Eu Tb Er Yb Lu Modeling the source for different F values Large F Figure 9-3 From Winter (2001) An Introduction to Igneous and Metamorphic Petrology. Prentice Hall. Small F

REE data for oceanic basalts increasing incompatibility Figure 9-3 From Winter (2001) An Introduction to Igneous and Metamorphic Petrology. Prentice Hall. Data from Sun and McDonough (1989).

Summary Chemically homogeneous mantle can make tholeiites OR alkali basalts Alkaline basalts are favored over tholeiites by deeper melting and by low % melting XL 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 (earlier lecture)