Origin of Basaltic Magma
Today
Updates:
?
Today:
 Different basalts from the same source
 Crystallization order
 Effect on Trace elements
Isochron method
Divide by stable 86Sr:
= (87Sr/86Sr)o + (87Rb/86Sr)(elt -1)
l = 1.4 x 10-11 a-1
87Sr/86Sr
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
87Sr
86Sr
( )
87Sr
86Sr
o
a
b
87Rb
86Sr
c
to
After some time increment (t0 t1) each sample loses
some 87Rb and gains an equivalent amount of 87Sr
87Sr
86Sr
t1
c1
( )
b1
a1
87Sr
86Sr
o
a
b
87Rb
86Sr
c
to
At time t2 each rock system has evolved  new line
Again still linear and steeper line
t2
87Sr
c2
86Sr
b2
a2
( )
t1
c1
b1
a1
87Sr
86Sr
o
a
b
c
to
87Rb
86Sr
Isochron Technique
87Sr
86Sr
t2
c2
b2
c1
a2
a1 b1
a
b
c
t1
87Rb
to 86
Sr
Isochron results
0.725
Rb-Sr Isochron, Eagle Peak Pluton, Sierra Nevada Batholith
Sr/86Sr = 0.00127x 87(Rb/86Sr) + 0.70760
87
S r/86S r
87
0.720
0.715
0.710
0
2
4
6
8
87
Rb/86Sr
Figure 9-9. After Hill et al. (1988). Amer. J. Sci., 288-A, 213-241.
10
12
14
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
Common petrographic differences between tholeiitic and alkaline basalts
Table 10-1
Tholeiitic Basalt
Groundmass
Usually fine-grained, intergranular
Usually fairly coarse, intergranular to ophitic
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
Phenocrysts
Alkaline Basalt
Olivine common and zoned
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
Olivine
Dunite
90
Peridotites
Lherzolite
40
Pyroxenites
Olivine Websterite
Orthopyroxenite
10
10
Orthopyroxene
Websterite
Clinopyroxenite
Figure 2-2 C After IUGS
Clinopyroxene
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
mantle
pipes blasted up from the
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
E 3GPa
Volatile-free
E 2Gpa
E 1GPa
Ab
E 1atm
Oversaturated
(quartz-bearing)
tholeiitic basalts
Fo
En
Figure 10-8 After Kushiro (1968), J. Geophys. Res., 73, 619-634.
SiO2
sample/chondrite
REE plots
10.00
8.00
sample/chondrite
Figure 9-3 From Winter
(2001) An Introduction to
Igneous and Metamorphic
Petrology. Prentice Hall.
0.05
6.00
0.1
4.00
0.2
0.4
2.00
0.00
0.6
La Ce
Nd
10.00
Sm Eu
Tb
Er
Yb Lu
atomic number
Modeling the source for
different F values
increasing incompatibility
8.00
6.00
Batch Partial Melting
values = F (fraction melted)
Large F
4.00
2.00
0.00
Small F
La Ce
Nd
Sm Eu
Tb
Er
Yb Lu
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)