Ch 10 Magma Generation

Origin of Basaltic MagmaOrigin of Basaltic Magma

Geology 346- PetrologyGeology 346- Petrology

2 principal types of basalt in 2 principal types of basalt in the ocean basins the 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

Tholeiites are generated at mid-ocean ridgesTholeiites are generated at mid-ocean ridges Also generated at oceanic islands, Also generated at oceanic islands,

subduction zonessubduction zones Alkaline basalts generated at ocean islandsAlkaline basalts generated at ocean islands

Also at subduction zonesAlso at subduction zones

Each is chemically distinctEach is chemically distinct

Evolve via FX as separate series Evolve via FX as separate series along different pathsalong different paths

Sources of mantle materialSources of mantle material OphiolitesOphiolites

Slabs of oceanic crust and upper mantleSlabs of oceanic crust and upper mantle Thrust at subduction zones onto edge of continentThrust at subduction zones onto edge of continent

Dredge samples from oceanic fracture zonesDredge samples from oceanic fracture zones Nodules and Nodules and xenolithsxenoliths in some basalts in some basalts Kimberlite xenolithsKimberlite xenoliths

Diamond-bearing pipes blasted up from the Diamond-bearing pipes blasted up from the mantle carrying numerous xenoliths from depthmantle carrying numerous xenoliths from depth

15

10

5

00.0 0.2 0.4 0.6 0.8

Wt.%

Al 2O

3

Wt.% TiO2

DuniteHarzburgite

Lherzolite

Tholeiitic basalt

Partial

Melt

ing

Residuum

LherzoliteLherzolite is probably fertile unaltered mantle is probably fertile unaltered mantleDuniteDunite and and harzburgiteharzburgite are refractory residuum after basalt has been are refractory residuum after basalt has been

extracted by partial meltingextracted by partial melting

Figure 10-1 Figure 10-1 Brown and Mussett, Brown and Mussett, A. E. (1993), A. E. (1993), The Inaccessible The Inaccessible Earth: An Integrated View of Its Earth: An Integrated View of Its Structure and Composition. Structure and Composition. Chapman & Hall/Kluwer.Chapman & Hall/Kluwer.

LherzoliteLherzolite: A type of : A type of peridotiteperidotite with Olivine > Opx + Cpxwith Olivine > Opx + Cpx

OlivineOlivine

ClinopyroxeneClinopyroxeneOrthopyroxeneOrthopyroxene

LherzoliteLherzoliteHa

rzbu

rgite

Wehrlite

Websterite

OrthopyroxeniteOrthopyroxenite

ClinopyroxeniteClinopyroxenite

Olivine Websterite

PeridotitesPeridotites

PyroxenitesPyroxenites

90

40

10

10

DuniteDunite

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

Phase diagram for aluminous Phase diagram for aluminous 4-phase lherzolite:4-phase lherzolite:

PlagioclasePlagioclase shallow (< 50 km)shallow (< 50 km)

SpinelSpinel 50-80 km50-80 km

GarnetGarnet 80-400 km80-400 km

Si Si VI coord. VI coord. > 400 km> 400 km

Al-phase =Al-phase =

Figure 10-2 Figure 10-2 Phase diagram of aluminous lherzolite with melting interval (gray), sub-solidus Phase diagram of aluminous lherzolite with melting interval (gray), sub-solidus reactions, and geothermal gradient. After reactions, and geothermal gradient. After Wyllie, P. J. (1981). Geol. Rundsch. 70, 128-153.Wyllie, P. J. (1981). Geol. Rundsch. 70, 128-153.

How does the mantle melt??How does the mantle melt??1) 1) Increase the temperatureIncrease the temperature

Figure 10-3. Figure 10-3. Melting by raising the temperature.Melting by raising the temperature.

2) 2) Lower the pressureLower the pressure AdiabaticAdiabatic rise of mantle with no conductive heat loss rise of mantle with no conductive heat loss Decompression meltingDecompression melting could melt at least 30% could melt at least 30%

Figure 10-4. Figure 10-4. Melting by (adiabatic) pressure reduction. Melting begins when the adiabat crosses the solidus and traverses the shaded melting interval. Dashed lines represent approximate % melting.

3) 3) Add volatilesAdd volatiles (especially (especially HH22OO))

Figure 10-4. Figure 10-4. Dry peridotite solidus compared to several experiments on H2O-saturated peridotites.

Fraction melted is Fraction melted is limited by limited by availability of wateravailability of water

15% 20% 50% 100%15% 20% 50% 100%

Figure 7-22. Pressure-temperature projection of the melting relationships in the system albite-H2O. From Burnham and Davis (1974). A J Sci., 274, 902-940.

Heating of Heating of amphibole-bearing peridotiteamphibole-bearing peridotite1) Ocean geotherm 1) Ocean geotherm 2) Shield geotherm2) Shield geotherm

Figure 10-6 Figure 10-6 Phase diagram (partly Phase diagram (partly schematic) for a hydrous mantle system, schematic) for a hydrous mantle system, including the H2O-saturated lherzolite including the H2O-saturated lherzolite solidus of Kushiro et al. (1968), the solidus of Kushiro et al. (1968), the dehydration breakdown curves for dehydration breakdown curves for amphibole (Millhollen et al., 1974) and amphibole (Millhollen et al., 1974) and phlogopite (Modreski and Boettcher, 1973), phlogopite (Modreski and Boettcher, 1973), plus the ocean and shield geotherms of plus the ocean and shield geotherms of Clark and Ringwood (1964) and Ringwood Clark and Ringwood (1964) and Ringwood (1966). (1966). After Wyllie (1979).After Wyllie (1979). In H. S. Yoder In H. S. Yoder (ed.), (ed.), The Evolution of the Igneous Rocks. The Evolution of the Igneous Rocks. Fiftieth Anniversary Perspectives. Fiftieth Anniversary Perspectives. Princeton University Press, Princeton, N. J, Princeton University Press, Princeton, N. J, pp. 483-520.pp. 483-520.

Melts Melts cancan be created under be created under realistic circumstancesrealistic circumstances

Plates separatePlates separate and mantle rises at mid- and mantle rises at mid-ocean ridgesocean ridges Adibatic rise Adibatic rise decompression melting decompression melting

Hot spotsHot spots localized plumes of melt localized plumes of melt Fluid fluxingFluid fluxing may give LVL may give LVL

Also important in Also important in subduction zonessubduction zones and and other settingsother settings

Generation of tholeiitic and Generation of tholeiitic and alkaline basalts from a alkaline basalts from a chemicallychemically uniform uniform mantle mantle

Variables (other than X)Variables (other than X) TemperatureTemperature PressurePressure

Figure 10-2 Figure 10-2 Phase diagram of aluminous lherzolite Phase diagram of aluminous lherzolite with melting interval (gray), sub-solidus reactions, and with melting interval (gray), sub-solidus reactions, and geothermal gradient. After geothermal gradient. After Wyllie, P. J. (1981). Geol. Wyllie, P. J. (1981). Geol. Rundsch. 70, 128-153.Rundsch. 70, 128-153.

Pressure effects:Pressure effects:

Figure 10-8 Figure 10-8 Change in the eutectic Change in the eutectic (first melt) composition with (first melt) composition with increasing pressure from 1 to 3 GPa increasing pressure from 1 to 3 GPa projected onto the base of the basalt projected onto the base of the basalt tetrahedron. tetrahedron. After Kushiro (1968), After Kushiro (1968), J. J. Geophys. Res.Geophys. Res., 73, 619-634., 73, 619-634.

Liquids and residuum of melted pyroliteLiquids and residuum of melted pyrolite

Figure 10-9 Figure 10-9 After Green and Ringwood (1967).After Green and Ringwood (1967). Earth Planet. Sci. Lett.Earth Planet. Sci. Lett. 2, 151-160. 2, 151-160.

Initial Conclusions:Initial Conclusions: Tholeiites favored by shallower meltingTholeiites favored by shallower melting

25% melting at 25% melting at <<30 km 30 km tholeiite tholeiite 25% melting at 60 km 25% melting at 60 km olivine basalt olivine basalt

Tholeiites favored by greater % partial meltingTholeiites favored by greater % partial melting 20 % melting at 60 km 20 % melting at 60 km alkaline basalt alkaline basalt

incompatibles (alkalis) incompatibles (alkalis) initial melts initial melts 30 % melting at 60 km 30 % melting at 60 km tholeiite tholeiite

Crystal Fractionation of magmas Crystal Fractionation of magmas as they riseas they rise

Tholeiite Tholeiite alkaline alkalineby FX by FX at med to high Pat med to high P

Not at low PNot at low P Thermal divideThermal divide

Al in pyroxenes at Hi PAl in pyroxenes at Hi P Low-P FX Low-P FX hi-Al hi-Al shallow magmasshallow magmas (“hi-Al” basalt)(“hi-Al” basalt)

Figure 10-10 Figure 10-10 Schematic representation of the Schematic representation of the fractional crystallization scheme of Green and fractional crystallization scheme of Green and Ringwood (1967) and Green (1969). Ringwood (1967) and Green (1969). After Wyllie After Wyllie (1971).(1971). The Dynamic Earth: Textbook in The Dynamic Earth: Textbook in GeosciencesGeosciences. John Wiley & Sons.. John Wiley & Sons.

Primary magmasPrimary magmas Formed at depth and not subsequently modified Formed at depth and not subsequently modified

by FX or Assimilationby FX or Assimilation CriteriaCriteria

Highest Mg# Highest Mg# (100Mg/(Mg+Fe))(100Mg/(Mg+Fe)) really really parentalparental magmamagma

Experimental results of lherzolite meltsExperimental results of lherzolite melts Mg# = 66-75Mg# = 66-75 Cr > 1000 ppmCr > 1000 ppm Ni > 400-500 ppmNi > 400-500 ppm Multiply saturatedMultiply saturated

Multiple saturationMultiple saturation Low PLow P

Ol then Plag then Ol then Plag then Cpx as coolCpx as cool

~70~70ooC T rangeC T range

Figure 10-12 Figure 10-12 Anhydrous P-T phase relationships Anhydrous P-T phase relationships for a mid-ocean ridge basalt suspected of being a for a mid-ocean ridge basalt suspected of being a primary magma. primary magma. After Fujii and Kushiro (1977).After Fujii and Kushiro (1977). Carnegie Inst. Wash. Yearb.Carnegie Inst. Wash. Yearb., 76, 461-465., 76, 461-465.


Ol then Plag then Cpx Ol then Plag then Cpx as coolas cool

7070ooC T rangeC T range High PHigh P

Cpx then Plag then OlCpx then Plag then Ol

Figure 10-12 Figure 10-12 Anhydrous P-T phase relationships Anhydrous P-T phase relationships for a mid-ocean ridge basalt suspected of being a for a mid-ocean ridge basalt suspected of being a primary magma. primary magma. After Fujii and Kushiro (1977).After Fujii and Kushiro (1977). Carnegie Inst. Wash. Yearb.Carnegie Inst. Wash. Yearb., 76, 461-465., 76, 461-465.


Ol then Plag then Cpx Ol then Plag then Cpx as coolas cool

7070ooC T rangeC T range High PHigh P

Cpx then Plag then OlCpx then Plag then Ol 25 km get all at once25 km get all at once

= = Multiple saturationMultiple saturation Suggests that 25 km is Suggests that 25 km is

the depth of the depth of last eqlast eqmm withwith the mantle the mantle

SummarySummary A chemically homogeneous mantle can A chemically homogeneous mantle can

yield a variety of basalt typesyield a variety of basalt types Alkaline basalts are favored over tholeiites Alkaline basalts are favored over tholeiites

by deeper melting and by low % PMby deeper melting and by low % PM Fractionation at moderate to high depths can Fractionation at moderate to high depths can

also create alkaline basalts from tholeiitesalso create alkaline basalts from tholeiites At low P there is a thermal divide that At low P there is a thermal divide that

separates the two seriesseparates the two series

Review of REEReview of REE

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

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

increasing incompatibilityincreasing incompatibility

Review of REEReview of REE


Figure 9-4.Figure 9-4. Rare Earth Rare Earth concentrations (normalized to concentrations (normalized to chondrite) for melts produced at chondrite) for melts produced at various values of F via melting various values of F via melting of a hypothetical garnet of a hypothetical garnet lherzolite using the batch lherzolite using the batch melting model (equation 9-5). melting model (equation 9-5). From Winter (2001) An From Winter (2001) An Introduction to Igneous and Introduction to Igneous and Metamorphic Petrology. Metamorphic Petrology. Prentice Hall.Prentice Hall.

REE data for oceanic basaltsREE data for oceanic basalts

Figure 10-13a. Figure 10-13a. REE diagram for a typical alkaline ocean island basalt (OIB) and tholeiitic REE diagram for a typical alkaline ocean island basalt (OIB) and tholeiitic mid-ocean ridge basalt (MORB). From Winter (2001) An Introduction to Igneous and mid-ocean ridge basalt (MORB). From Winter (2001) An Introduction to Igneous and Metamorphic Petrology. Prentice Hall. Metamorphic Petrology. Prentice Hall. Data from Sun and McDonough (1989).Data from Sun and McDonough (1989).


Spider diagram for oceanic basaltsSpider diagram for oceanic basalts

increasing incompatibilityincreasing incompatibilityFigure 10-13b. Figure 10-13b. Spider diagram for a typical alkaline ocean island basalt (OIB) and Spider diagram for a typical alkaline ocean island basalt (OIB) and tholeiitic mid-ocean ridge basalt (MORB). From Winter (2001) An Introduction to Igneous tholeiitic mid-ocean ridge basalt (MORB). From Winter (2001) An Introduction to Igneous and Metamorphic Petrology. Prentice Hall. and Metamorphic Petrology. Prentice Hall. Data from Sun and McDonough (1989).Data from Sun and McDonough (1989).

REE data REE data for UM for UM xenolithsxenoliths

Figure 10-14Figure 10-14 Chondrite-normalized REE diagrams Chondrite-normalized REE diagrams for spinel for spinel (a) (a) and garnet and garnet (b)(b) lherzolites. lherzolites. After After Basaltic Volcanism Study Project (1981).Basaltic Volcanism Study Project (1981). Lunar Lunar and Planetary Institute.and Planetary Institute.

LREE enriched

LREE depletedor unfractionated

LREE depletedor unfractionated

LREE enriched

Review of Sr isotopesReview of Sr isotopes 8787Rb Rb 8787Sr Sr = 1.42 x 10 = 1.42 x 10-11-11 a a Rb (parent) conc. in enriched reservoir (incompatible)Rb (parent) conc. in enriched reservoir (incompatible) Enriched reservoirEnriched reservoir

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

develops more87Sr over time

Depleted reservoirDepleted reservoir(less Rb)develops less87Sr over time

Review of Nd isotopesReview of Nd isotopes 147147Sm Sm 143143Nd Nd = 6.54 x 10 = 6.54 x 10-13-13 a a Nd (daughter) Nd (daughter) enriched reservoir > Sm enriched reservoir > Sm Enriched res.Enriched res.

develops lessdevelops less143143Nd over timeNd over time

Depleted res.Depleted res.(higher Sm/Nd)(higher Sm/Nd)develops higherdevelops higher143143Nd/Nd/144144NdNdover timeover time

Nd SmNd Sm

REE diagram

Figure 9-15. Figure 9-15. After Wilson (1989). Igneous Petrogenesis. Unwin Hyman/Kluwer.After Wilson (1989). Igneous Petrogenesis. Unwin Hyman/Kluwer.

Nd and Sr isotopes of Nd and Sr isotopes of Ocean BasaltsOcean Basalts““Mantle Array”Mantle Array”

Figure 10-15 (a)Figure 10-15 (a) Initial Initial 143143Nd/Nd/144144Nd vs. Nd vs. 8787Sr/Sr/8686Sr for oceanic basaltsSr for oceanic basalts. . From Wilson (1989). Igneous From Wilson (1989). Igneous Petrogenesis. Unwin Hyman/Kluwer. Data from Zindler Petrogenesis. Unwin Hyman/Kluwer. Data from Zindler et alet al. (1982) and Menzies (1983).. (1982) and Menzies (1983).

Nd and Sr isotopes of Nd and Sr isotopes of Kimberlite XenolithsKimberlite Xenoliths

Figure 10-15 (b)Figure 10-15 (b) Initial Initial 143143Nd/Nd/144144Nd vs. Nd vs. 8787Sr/Sr/8686Sr for mantle xenolithsSr for mantle xenoliths. . From Wilson (1989). Igneous From Wilson (1989). Igneous Petrogenesis. Unwin Hyman/Kluwer. Data from Zindler Petrogenesis. Unwin Hyman/Kluwer. Data from Zindler et alet al. (1982) and Menzies (1983).. (1982) and Menzies (1983).

Mantle model circa 1975Mantle model circa 1975

Figure 10-16aFigure 10-16a After Basaltic Volcanism Study Project (1981).After Basaltic Volcanism Study Project (1981). Lunar and Planetary Institute. Lunar and Planetary Institute.

Newer mantle modelNewer mantle model Upper depleted mantle = MORB sourceUpper depleted mantle = MORB source Lower undepleted & enriched OIB sourceLower undepleted & enriched OIB source

Figure 10-16bFigure 10-16b After Basaltic Volcanism Study Project (1981).After Basaltic Volcanism Study Project (1981). Lunar and Planetary Institute. Lunar and Planetary Institute.

Experiments on melting enriched Experiments on melting enriched vs. depleted mantle samples:vs. depleted mantle samples:

Tholeiite easily createdTholeiite easily createdby 10-30% PMby 10-30% PM

More silica saturatedMore silica saturatedat lower Pat lower P

Grades toward alkalicGrades toward alkalicat higher Pat higher P

1. Depleted Mantle1. Depleted Mantle

Figure 10-17a. Figure 10-17a. Results of partial melting experiments on Results of partial melting experiments on depleteddepleted lherzolites. Dashed lines are contours representing lherzolites. Dashed lines are contours representing percent partial melt produced. Strongly curved lines are percent partial melt produced. Strongly curved lines are contours of the normative olivine content of the melt. “Opx contours of the normative olivine content of the melt. “Opx out” and “Cpx out” represent the degree of melting at which out” and “Cpx out” represent the degree of melting at which these phases are completely consumed in the melt. these phases are completely consumed in the melt. After After Jaques and Green (1980).Jaques and Green (1980). Contrib. Mineral. Petrol., 73, 287-310.Contrib. Mineral. Petrol., 73, 287-310.

Experiments on melting enriched Experiments on melting enriched vs. depleted mantle samples:vs. depleted mantle samples:

Tholeiites extend toTholeiites extend tohigher P than for DMhigher P than for DM

Alkaline basalt fieldAlkaline basalt fieldat higher P yetat higher P yet And lower % PMAnd lower % PM

2. Enriched Mantle2. Enriched Mantle

Figure 10-17b. Figure 10-17b. Results of partial melting experiments on Results of partial melting experiments on fertilefertile lherzolites. Dashed lines are contours representing percent lherzolites. Dashed lines are contours representing percent partial melt produced. Strongly curved lines are contours of partial melt produced. Strongly curved lines are contours of the normative olivine content of the melt. “Opx out” and “Cpx the normative olivine content of the melt. “Opx out” and “Cpx out” represent the degree of melting at which these phases are out” represent the degree of melting at which these phases are completely consumed in the melt. The shaded area represents completely consumed in the melt. The shaded area represents the conditions required for the generation of alkaline basaltic the conditions required for the generation of alkaline basaltic magmas. magmas. After Jaques and Green (1980).After Jaques and Green (1980). Contrib. Mineral. Contrib. Mineral. Petrol., 73, 287-310.Petrol., 73, 287-310.

Ch 10 Magma Generation

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