Origin of Basaltic MagmaOrigin of Basaltic Magma

Updates:Updates:

??

Today:Today: Different basalts from the same sourceDifferent basalts from the same source Crystallization orderCrystallization order Effect on Trace elementsEffect on Trace elements

TodayToday

y = b + x m y = b + x m

= equation for a line in = equation for a line in 8787Sr/Sr/8686Sr vs. Sr vs. 8787Rb/Rb/8686Sr plotSr plot

Slope = (eSlope = (ett -1) -1)

Divide by stable Divide by stable 8686Sr:Sr:

8787Sr/Sr/8686Sr = (Sr = (8787Sr/Sr/8686Sr)Sr)oo + ( + (8787Rb/Rb/8686Sr)(eSr)(ett -1) -1)

= 1.4 x 10= 1.4 x 10-11-11 a a-1-1

Isochron methodIsochron method

a b c to86Sr

87Sr

o( )

86Sr

87Sr

86Sr

87Rb

Begin with 3 rocks plotting at Begin with 3 rocks plotting at a b ca b c at time at time ttoo

After some time increment (tAfter some time increment (t00 tt11) each sample loses ) each sample loses

some some 8787Rb and gains an equivalent amount of Rb and gains an equivalent amount of 8787SrSr

a b c

a1b1

c1t1

to

86Sr

87Sr

86Sr

87Rb

86Sr

87Sr

o( )

At timeAt time t t22 each rock system has evolved each rock system has evolved new line new line

Again still linear and steeper lineAgain still linear and steeper line

a b c

a1b1

c1a2

b2

c2t1

to

t2

86Sr

87Sr

86Sr

87Sr

o( )

86Sr

87Rb

Isochron TechniqueIsochron Technique

a b ca1

b1c1a2

b2c2 t1

to

t286Sr87Sr

86Sr87Rb

Isochron resultsIsochron results

0.710

0.715

0.720

0.725

0 2 4 6 8 10 12 14

Rb-Sr Isochron, Eagle Peak Pluton, Sierra Nevada Batholith

87Sr/86Sr = 0.00127 (87Rb/86Sr) + 0.70760

87Rb/86Sr

x

87 S

r/8

6 S r

Figure 9-9. After Hill et al. (1988). Amer. J. Sci., 288-A, 213-241.

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

Making Sr isotope reservoirsMaking Sr isotope reservoirs

Geotherm and solidus: how to meltGeotherm and solidus: how to melt

Melt creation in ocean basinMelt creation in ocean basin

2 types in ocean basins 2 types in ocean basins

Table 10-1Table 10-1 Common petrographic differences between tholeiitic and alkaline basaltsCommon petrographic differences between tholeiitic and alkaline basalts

Tholeiitic Basalt Alkaline BasaltUsually 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).

Tholeiitic Basalt and Alkaline Basalt

Lherzolite: A type of peridotite Lherzolite: A type of peridotite with Olivine > Opx + Cpxwith Olivine > Opx + Cpx

OlivineOlivine

ClinopyroxeneClinopyroxeneOrthopyroxeneOrthopyroxene

LherzoliteLherzoliteH

arzb

urgi

teW

ehrlite

Websterite

OrthopyroxeniteOrthopyroxenite

ClinopyroxeniteClinopyroxenite

Olivine Websterite

PeridotitesPeridotites

PyroxenitesPyroxenites

90

40

10

10

DuniteDunite

Figure 2-2 C After IUGSFigure 2-2 C After IUGS

Examples from the mantle/how we knowExamples from the mantle/how we know

OphiolitesOphiolites Obducted oceanic crust + upper mantleObducted oceanic crust + upper mantle

Dredge samples from oceanic fracture zonesDredge samples from oceanic fracture zones Xenoliths in some basalts: Local example?Xenoliths in some basalts: Local example? Kimberlite xenolithsKimberlite xenoliths

Diamond-bearing pipes blasted up from the Diamond-bearing pipes blasted up from the mantlemantle

Tholeiite/alkaline basalt based Tholeiite/alkaline basalt based on P&T; big difference:on P&T; big difference:

Figure 10-2 After Figure 10-2 After Wyllie, P. J. (1981). Geol. Rundsch. Wyllie, P. J. (1981). Geol. Rundsch. 7070, 128-153., 128-153.

Pressure effects on Pressure effects on meltingmelting

Figure 10-8 Figure 10-8 After Kushiro (1968), After Kushiro (1968), J. Geophys. Res.J. Geophys. Res., , 7373, 619-634., 619-634.

NeNe

FoFo EnEn

AbAb

SiOSiO22

Oversaturated(quartz-bearing)tholeiitic basalts

Highly undersaturated

(nepheline - b

earing)

alkali basalts

Undersaturated

tholeiitic basalts

EE3GPa3GPa

EE 2Gpa2Gpa

EE1GPa1GPa

EE 1atm1atm

Volatile-freeVolatile-free

REE plotsREE plots

0.000.00

2.002.00

4.004.00

6.006.00

8.008.00

10.0010.00

atomic numberatomic number

sam

ple/

chon

drite

sam

ple/

chon

drite

0.4

0.2

0.05

0.1

La Ce Nd Sm Eu Tb Er Yb LuLa Ce Nd Sm Eu Tb Er Yb Lu

Batch Partial Meltingvalues = F (fraction melted)

increasing incompatibilityincreasing incompatibility

0.6

Figure 9-3 From Winter Figure 9-3 From Winter (2001) An Introduction to (2001) An Introduction to Igneous and Metamorphic Igneous and Metamorphic Petrology. Prentice Hall.Petrology. Prentice Hall.

0.000.00

2.002.00

4.004.00

6.006.00

8.008.00

10.0010.00

sam

ple/

chon

drite

sam

ple/

chon

drite

La Ce Nd Sm Eu Tb Er Yb LuLa Ce Nd Sm Eu Tb Er Yb Lu

Modeling the source fordifferent F values

Small F

Large F

REE data for oceanic basaltsREE data for oceanic basalts

Figure 9-3 From Winter (2001) An Introduction to Igneous and Metamorphic Figure 9-3 From Winter (2001) An Introduction to Igneous and Metamorphic Petrology. Prentice Hall. Petrology. Prentice Hall. Data from Sun and McDonough (1989).Data from Sun and McDonough (1989).

increasing incompatibilityincreasing incompatibility

SummarySummary Chemically homogeneous mantle can make Chemically homogeneous mantle can make

tholeiites OR alkali basaltstholeiites OR alkali basalts Alkaline basalts are favored over tholeiites Alkaline basalts are favored over tholeiites

by deeper melting and by low % meltingby deeper melting and by low % melting XL fractionation at moderate to high depths XL fractionation at moderate to high depths

can also create alkaline basalts from can also create alkaline basalts from tholeiitestholeiites

At low P there is a thermal divide that At low P there is a thermal divide that separates the two series (earlier lecture)separates the two series (earlier lecture)