G EOL 2312 I GNEOUS AND M ETAMORPHIC P ETROLOGY Lecture 10 Mantle Melting and the Generation of Basaltic Magma February 16, 2009

M ELTING THE M ANTLE M AKES M AFIC M AGMA ALWAYS! How does this happen? But aren’t there different types of “mafic” magmas? What is this made of?

C OMPOSITION OF THE M ANTLE L HERZOLITE Evidence: Ophiolites Slabs of oceanic crust and upper mantle Obducted onto edge of continent at convergent zones Dredge samples from oceanic fracture zones Nodules and xenoliths in some basalts Kimberlite xenoliths Pipe-like intrusions quickly intruded from the deep mantle carrying numerous xenoliths Olivine Clinopyroxene Orthopyroxene Lherzolite Harzburgite Wehrlite Websterite Orthopyroxenite Clinopyroxenite Olivine Websterite Peridotites Pyroxenites Dunite Wt.% Al 2 O 3 Wt.% TiO 2 Dunite Harzburgite Lherzolite Tholeiitic basalt 20% Partial Melting Residuum + Al-bearing Phase Plagioclase <30km Spinel km Garnet >80km Mantle Melt

P HASE D IAGRAM OF N ORMAL M ANTLE Winter (2001) Figure 10-2 Phase diagram of aluminous lherzolite with melting interval (gray), sub-solidus reactions, and geothermal gradient. After Wyllie, P. J. (1981). Geol. Rundsch. 70, Mantle should not melt under”normal” geothermal conditions How to get it to melt?

M ELTING THE M ANTLE I NCREASING T EMPERATURE – M ANTLE P LUMES Zone of Melting Normal Geotherm Plume- influenced Geotherm

M ELTING THE M ANTLE A DIABATIC D ECOMPRESSION (R ISE OF THE M ANTLE WITH NO C ONDUCTIVE H EAT L OSS ) Adiabatic Geotherm

M ELTING THE M ANTLE R OLE OF V OLATILES DRAMATICALLY LOWERS THE LIQUIDUS T OF THE M ANTLE “Dry” curve has a positive slope because increased P favors lower V phase (solid), increased T favors S phase (liquid) H 2 O-saturated curve has negative slope because V of liq+vapor (Liq aq ) is less than V of solid+vapor (or fluid); change is most extreme at low overall pressures.

M ELTING A H YDRATED M ANTLE Ocean Geothermal Gradient

M ELTING A H YDRATED M ANTLE Problem: Water content of the mantle typically <0.2% (far from saturated) and it is structurally locked into hydrous mineral phases like amphibole and phlogopite (biotite) Exceeding the melting points of amphibole (b) and phlogopite (c) will release H 2 O and allow small degrees (<1%) of melting if saturation is achieved.

C REATING C OMPOSITIONAL T YPES OF M AFIC M AGMAS IN N ON -S UBDUCTION S ETTINGS A LKALINE AND S UBALKALINE (T HOLEIITIC ) Ne Ab Q Ab + Tr Tr + L Ab + L Ne + L Liquid Ab + L Ne + Ab Thermal Divide Qtz- saturated Qtz-under saturated Incompatible Elements in the Mantle

C REATING C OMPOSITIONAL T YPES OF M AFIC M AGMAS IN N ON -S UBDUCTION S ETTINGS C HANGING P RESSURE (96 km) (65 km) (34 km)

C REATING C OMPOSITIONAL T YPES OF M AFIC M AGMAS IN N ON -S UBDUCTION S ETTINGS C HANGING V OLATILE C ONTENT Ne FoEn Ab SiO 2 Oversaturated (quartz-bearing) tholeiitic basalts Highly undesaturated (nepheline-bearing) alkali olivine basalts Undersaturated tholeiitic basalts CO 2 H2OH2O dry P = 2 GPa Not really applicable to non-subduction settings

C REATING C OMPOSITIONAL T YPES OF M AFIC M AGMAS IN N ON -S UBDUCTION S ETTINGS C HANGING D EGREE OF I NCONGRUENT P ARTIAL M ELTING Winter (2001) Figure 10.9, After Green and Ringwood (1967). Earth Planet. Sci. Lett. 2,

I NCONGRUENT P ARTIAL M ELTING OF THE M ANTLE From Yoder (1976) Mantle Gt Lherzolite Fo 65 Py 20 Di 15 (Fo 57 En 17 Py 14 Di 12 ) Melt – Fo 5 Py 45 Di 50 +En Batch Melting Experiments of Natural Lherzolites

C REATING C OMPOSITIONAL T YPES OF M AFIC M AGMAS IN N ON -S UBDUCTION S ETTINGS C HANGING D EGREE OF P ARTIAL M ELTING AND D EPTH OF M ELTING Winter (2010), Figure After Kushiro (2001).

C REATING C OMPOSITIONAL T YPES OF M AFIC M AGMAS IN N ON -S UBDUCTION S ETTINGS F RACTIONAL C RYSTALLIZATION DURING A SCENT Winter (2001) Figure Schematic representation of the fractional crystallization scheme of Green and Ringwood (1967) and Green (1969). After Wyllie (1971). The Dynamic Earth: Textbook in Geosciences. John Wiley & Sons.

C REATING C OMPOSITIONAL T YPES OF M AFIC M AGMAS IN N ON -S UBDUCTION S ETTINGS W HAT IS A P RIMARY M AGMA ? Magma that last equilibrated with the mantle and was not subsequently modified by fractional crystallization or assimilation Modified magmas are termed derivative or evolve d. Criteria - Most Primitive Composition Highest mg# (MgO/(MgO+FeO) = Highest compatible element concentrations (Cr > 1000 ppm, Ni > ppm) Lowest incompatible element abundances Lowest radiogenic isotope ratios Multiply saturated

Winter (2010) Figure 10.13), Anhydrous P-T phase relationships for a mid-ocean ridge basalt suspected of being a primary magma. After Fujii and Kushiro (1977). Carnegie Inst. Wash. Yearb., 76, D ETERMINING THE DEPTH OF P RIMARY M AGMA F ORMATION BY THE INVERSE METHOD OF FINDING MULTIPLE SATURATION Involves melting a rock suspected of corresponding to a primary magma and finding the P&T of multiple saturation. This corresponds to “eutectic” point of the multi-component magma.

C REATING C OMPOSITIONAL T YPES OF M AFIC M AGMAS IN N ON -S UBDUCTION S ETTINGS C OMPOSITIONALLY H ETEROGENEOUS M ANTLE Upper deplete mantle = MORB source Lower undepleted mantle = enriched OIB source Whole Mantle Convection Model Two Layer Mantle Convection Model From Courtillot et al. (2003). Winter (2010) Figure 10-17b After Basaltic Volcanism Study Project (1981). Lunar and Planetary Institute.

C REATING C OMPOSITIONAL T YPES OF M AFIC M AGMAS IN N ON -S UBDUCTION S ETTINGS C OMPOSITIONALLY H ETEROGENEOUS M ANTLE increasing incompatibility Ocean Island Basalt (plume-influenced) Mid-ocean Ridge Basalt (normal upper mantle)

C REATING C OMPOSITIONAL T YPES OF M AFIC M AGMAS IN N ON -S UBDUCTION S ETTINGS C OMPOSITIONALLY H ETEROGENEOUS M ANTLE increasing incompatibility Winter (2010) Figure 10.14b. Spider diagram for a typical alkaline ocean island basalt (OIB) and tholeiitic mid-ocean ridge basalt (MORB). From Winter (2001) An Introduction to Igneous and Metamorphic Petrology. Prentice Hall. Data from Sun and McDonough (1989).

C REATING C OMPOSITIONAL T YPES OF M AFIC M AGMAS IN N ON -S UBDUCTION S ETTINGS C OMPOSITIONALLY H ETEROGENEOUS M ANTLE Melting “Fertile” Mantle Melting “Infertile” (previously melted) Mantle – MORB?

Winter (2010) Figure Chondrite-normalized REE diagrams for spinel (a) and garnet (b) lherzolites. After Basaltic Volcanism Study Project (1981). LREE enriched LREE depleted or unfractionated LREE depleted or unfractionated LREE enriched From Rollinson (1996)

C REATING C OMPOSITIONAL T YPES OF M AFIC M AGMAS IN N ON -S UBDUCTION S ETTINGS C OMPOSITIONALLY H ETEROGENEOUS M ANTLE Winter (2010) Figure 10-18a. Results of partial melting experiments on depleted and enriched lherzolites. Dashed lines are contours representing percent partial melt produced. Strongly curved lines are contours of the normative olivine content of the melt. “Opx out” and “Cpx out” represent the degree of melting at which these phases are completely consumed in the melt. After Jaques and Green (1980). Contrib. Mineral. Petrol., 73, % normative olivine % partial melting = MORB = EMORB/OIB

M ELTING THE M ANTLE MAKES M AFIC M AGMA Partially Melting the Heterogeneous Mantle makes Various Types of Mafic Magma o A chemically homogenous mantle can yield a variety of basalt types o Alkaline basalts are favored over tholeiites by deeper melting and by lower % partial melting o Crystal fractionation at moderate to great depths in the mantle can also create alkaline basalts from tholeiites o At mod to high P, there is a thermal divide that separates the two series o Mantle varies in bulk composition and fertility due to prior melting events (upper – depleted; lower undepleted)

P RESENT -D AY M ID -O CEAN R IDGES B IRTHPLACES OF MORB

O CEANIC C RUST AND U PPER M ANTLE S TRUCTURE EVIDENCE l Seismic Velocities l Deep Sea Drilling Program l Ophiolites l Dredging of Fracture Zone Scarps

O CEANIC C RUST AND U PPER M ANTLE FORMATION Winter (2010) Figure After Langmuir et al. (1992). AGU.

C OMPOSITION OF MORB Primitive Magma Fractional Crystallization

C OMPOSITIONAL V ARIABILITY IN MORB Fractional crystallization appears to play a minor role. Suggests instead, incompatible element-rich and incompatible element-poor mantle source regions for MORB magmas  N-MORB (normal MORB) taps the depleted upper mantle source  Mg# > 65: K 2 O < 0.10 TiO 2 < 1.0  E-MORB (enriched MORB) taps undepleted (deeper?) mantle  Mg# > 65: K 2 O > 0.10 TiO 2 > 1.0 FC

O RIGIN OF NMORB AND EMORB

X-Sectional View Longitudinal View D ISTRIBUTION OF NMORB & EMORB AT F AST S PREADING R IDGES P ROMOTES DEVELOPMENT OF LARGER MAGMA CHAMBERS  MORE FRACTIONATED BASALT COMPOSITIONS Winter (2001) Figure 13-15, After Perfit et al. (1994) Geology, 22, Winter (2001) Figure ; After Sinton and Detrick (1992) J. Geophys. Res., 97,

S LOW - SPREADING R IDGES Winter (2001) Figure After Sinton and Detrick (1992) J. Geophys. Res., 97, Smaller, crystal-rich, dike-like magma bodies results is less fractionation  less evolved MORB

O CEAN I SLAND B ASALTS (OIB) P LUME - INFLUENCED V OLCANISM Figure Map of relatively well-established hotspots and selected hotspot trails (island chains or aseismic ridges). Hotspots and trails from Crough (1983) with selected more recent hotspots from Anderson and Schramm (2005). Also shown are the geoid anomaly contours of Crough and Jurdy (1980, in meters). Note the preponderance of hotspots in the two major geoid highs (superswells).

H AWAIIAN V OLCANISM S TAGING OF A LKALINE AND T HOLEIITIC M AGMA S ERIES

M AJOR E LEMENT C HEMISTRY OF OIB Winter (2001) Figure After Wilson (1989) Igneous Petrogenesis. Kluwer. Variable alkalinity likely reflects variable depths and degrees of partial melting in the plume and variable degrees of mixing and re- equilbration as magma rises through the mantle plume to the ocean crust.

REE C OMPOSITIONS OF OIB Similar to E-MORB, but stronger depletion of HREE for both tholeiitic and alkali magmas series  deep garnet-bearing source for both with re-equilibration at shallower depths Lack of Eu anomaly indicates no significant fractionation of plagioclase Alkali basalts strongly LREE-enriched  low degrees of partial melting

T RACE E LEMENTS OIB / MORB Winter (2001) Figure An Introduction to Igneous and Metamorphic Petrology. Prentice Hall. Data from Sun and McDonough (1989).

A M ODEL FOR O CEAN M AGMATISM Winter (2001) Figure Nomenclature from Zindler and Hart (1986). After Wilson (1989) and Rollinson (1993). Mantle Reservoirs (based on radiogenic isotopes) Chondritic (undepleted) mantle Previously melted mantle U-enriched mantle Chondritic (undepleted) mantle Low Sr 87 /Sr 86 mantle Low Nd 143 /Nd 144 mantle From recycled ocean crust