1. Origin of Basaltic Magma

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

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

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

5. 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 ( )

6. ( ) 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

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

8. 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 = (
Rb/
Sr) Sr x
S r/
S r
Figure 9-9. After Hill et al. (1988). Amer. J. Sci., 288-A,

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

10. Geotherm and solidus: how to melt

11. Melt creation in ocean basin

12. 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).

13. 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

14. 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

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

16. Pressure effects on meltingNe 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,

17. 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

18. 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).

19. 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)