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Chapter 13 Mid-ocean Ridge Basalts. The Mid-Ocean Ridge System Figure 13-1. After Minster et al. (1974) Geophys. J. Roy. Astr. Soc., 36, 541-576.

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Presentation on theme: "Chapter 13 Mid-ocean Ridge Basalts. The Mid-Ocean Ridge System Figure 13-1. After Minster et al. (1974) Geophys. J. Roy. Astr. Soc., 36, 541-576."— Presentation transcript:

1 Chapter 13 Mid-ocean Ridge Basalts

2 The Mid-Ocean Ridge System Figure After Minster et al. (1974) Geophys. J. Roy. Astr. Soc., 36,

3 Ridge Segments and Spreading Rates Slow-spreading ridges:Slow-spreading ridges: < 3 cm/a Fast-spreading ridges:Fast-spreading ridges: > 4 cm/a Temporal variations are also knownTemporal variations are also known

4 Oceanic Crust and Upper Mantle Structure 4 layers distinguished via seismic velocities 4 layers distinguished via seismic velocities Sample Sources: Deep Sea Drilling Program Deep Sea Drilling Program Dredging of fracture zone scarps Dredging of fracture zone scarps Ophiolites with subaerial exposure Ophiolites with subaerial exposure

5 Oceanic Crust and Upper Mantle Structure Typical Ophiolite Figure Lithology and thickness of a typical ophiolite sequence, based on the Samial Ophiolite in Oman. After Boudier and Nicolas (1985) Earth Planet. Sci. Lett., 76, Wehrlite: a Peridotite mostly composed of olivine plus clinopyroxene

6 Layer 1 A thin layer of pelagic sediment Oceanic Crust and Upper Mantle Structure Figure Modified after Brown and Mussett (1993) The Inaccessible Earth: An Integrated View of Its Structure and Composition. Chapman & Hall. London.

7 Layer 2 is basaltic Subdivided into two sub-layers Subdivided into two sub-layers Layer 2A & B = pillow basalts Layer 2C = vertical sheeted dikes Oceanic Crust and Upper Mantle Structure Figure Modified after Brown and Mussett (1993) The Inaccessible Earth: An Integrated View of Its Structure and Composition. Chapman & Hall. London.

8 Discontinuous diorite and tonalite (“plagiogranite”) bodies = late differentiated liquids Oceanic Crust and Upper Mantle Structure Figure Lithology and thickness of a typical ophiolite sequence, based on the Samial Ophiolite in Oman. After Boudier and Nicolas (1985) Earth Planet. Sci. Lett., 76, Tonalite is an igneous, plutonic (intrusive) rock, of felsic composition, with phaneritic texture. Similar to Granite except Feldspar is mostly present as plagioclase, with less than 10% alkali feldspar.

9 Layer 3 more complex and controversial Believed to be mostly gabbros, crystallized from a shallow axial magma chamber (feeds the dikes and basalts) Layer 3A = upper isotropic and lower, somewhat foliated (“transitional”) gabbros Layer 3B is more layered, & may exhibit cumulate textures

10 Layer 4 = ultramafic rocks Ophiolites: base of 3B grades into layered cumulate wehrlite & gabbro Wehrlite intruded into layered gabbros Below  cumulate dunite with harzburgite xenoliths The ultramafic igneous rock, harzburgite, is a variety of peridotite consisting mostly of the two minerals, olivine and low-calcium (Ca) pyroxene (enstatite) Below this is a tectonite - harzburgite and dunite: unmelted fraction of the partially melted (depleted) mantle.

11 MORB Petrography and Major Element Chemistry A “typical” MORB is an olivine Tholeiite with low K 2 O (< 0.2%) A “typical” MORB is an olivine Tholeiite with low K 2 O (< 0.2%) and low TiO 2 (< 2.0%) and low TiO 2 (< 2.0%) Lab analyses use glass, which is certain to represent liquid. I’ll explain why below. Lab analyses use glass, which is certain to represent liquid. I’ll explain why below.

12 The low-P crystallization sequence is: olivine (  Mg-Cr Spinel), olivine + plagioclase (  Mg-Cr Spinel), olivine + plagioclase + clinopyroxene The low-P crystallization sequence is: olivine (  Mg-Cr Spinel), olivine + plagioclase (  Mg-Cr Spinel), olivine + plagioclase + clinopyroxene Figure 7-2. After Bowen (1915), A. J. Sci., and Morse (1994), Basalts and Phase Diagrams. Krieger Publishers. Why low pressure?

13 In MORBS, Fe-Ti oxides are restricted to the groundmass, and thus form late in the MORB sequence In MORBS, Fe-Ti oxides are restricted to the groundmass, and thus form late in the MORB sequence odp.tamu.edu/publications/176_SR/chap_08/ c8_f4.htm Ulvöspinel - TiFe 2 O 4 Hence the early Fe-enrichment characteristic of the tholeiite trend on an ACF diagram – the iron doesn’t precipitate out until late, so it becomes relatively more abundant in early glass as Mg++ is used up.

14 The major element chemistry of MORBs Originally considered to be extremely uniform, interpreted as a simple petrogenesis Originally considered to be extremely uniform, interpreted as a simple petrogenesis More extensive sampling has shown that they display a (restricted) range of compositions More extensive sampling has shown that they display a (restricted) range of compositions

15 The major element chemistry of MORBs MAR : Mid-Atlantic Ridge EPR : East-Pacific Rise IOR: Indian Ocean Ridge Normative minerals: q Quartz, or Orthoclase, ab Albite, an Anorthite, di Diopside, hy Hyperthene, ol Olivine, mt Magnetite, il Ilmenite, ap Apatite MORBs vary a little in composition EPR the most different

16 Figure “Fenner-type” variation diagrams for basaltic glasses of the MAR. Note different ordinate scales. From Stakes et al. (1984) J. Geophys. Res., 89, MORBs cannot all be primary magmas; most are derivative magmas resulting from fractional crystallization

17 Figure Data from Schilling et al. (1983) Amer. J. Sci., 283, Even when we compare for constant Mg# considerable variation is still apparent. Fig shows the variation in K 2 O with Mg# for the MAR data set of Schilling et al. (1983) Recall Mg# = 100 Mg ++ / Mg ++ + Fe ++

18 Conclusions about MORBs, and the processes beneath mid-ocean ridges –MORBs are not the completely uniform magmas that they were once considered to be  They show chemical trends consistent with fractional crystallization of olivine, plagioclase, and perhaps clinopyroxene  As early forming crystals remove elements from the melt, new chemical compositions become frequent.

19 Fast ridge segments (EPR)  a broader range of compositions and a larger proportion of evolved liquids Fast ridge segments (EPR)  a broader range of compositions and a larger proportion of evolved liquids Magmas erupted slightly off the axis of ridges are more evolved than those at the axis itself. Magmas erupted slightly off the axis of ridges are more evolved than those at the axis itself. Magma chamber processes may be different at fast- spreading ridges compared to slow ones Fast ridge segments (EPR) display a broader range of compositions, and produce a larger proportion of evolved liquids than do slow segments Also magmas erupted slightly off the axis of ridges are more evolved than those at the axis itself

20 Depleted mantle is the residue that remains after a given element has been removed from Peridotite to form a basaltic melt. The incompatible elements (e.g. K, Sr, Rb, U, and rare-earth elements) are preferentially partitioned into a melt, and during ocean crustal formation these elements in particular have been removed from the mantle, leaving the mantle depleted in incompatibles. Incompatibles present in a MORB melt generally solidify in late fractionation minerals derived from the basaltic melt. WE SHOULD USE ANALYSES OF GLASS (no crystal structure) at any stage if we want melt compositions. The depleted mantle can still partially melt and form MORBs, all you need is low pressure IDEA IDEA later MORBs will have less incompatibles such as LILE K +, as some were already removed by earlier MORB formation.

21 An incompatible element is an element that is unsuitable in size and/or charge to fit in the cation sites of the possible minerals. Elements that have difficulty in entering cation sites of the early high temperature crystallization minerals (Olivine, Ca- Plagioclase, Pyroxenes) are concentrated in the melt phase of magma (liquid phase), and remain there until late in the solidification of the magma. Another way to classify incompatible elements is by mass: light rare earth elements are La - Sm, and heavy rare earth elements (HREE) are Eu - Lu. Rocks or magmas rich in light rare earth elements (LREE) are referred to as fertile, and those with strong depletions in LREE are referred to as depleted.

22 We see two types of MORBs with Rare Earths: REE diagram for MORBs REE diagram for MORBs Figure Data from Schilling et al. (1983) Amer. J. Sci., 283, An incompatible element is an element that is unsuitable in size and/or charge to fit in the cation sites of the possible minerals. Elements that have difficulty in entering cation sites of the minerals are concentrated in the melt phase of magma (liquid phase). Another way to classify incompatible elements is by mass: light rare earth elements are La - Sm, and heavy rare earth elements (HREE) are Eu - Lu. Rocks or magmas rich, or only slightly depleted in light rare earth elements (LREE) are referred to as fertile, and those with strong depletions in LREE are referred to as depleted.

23 There are incompatible-rich and incompatible-poor mantle source regions for MORB magmas –N-MORB (normal MORB) taps the depleted upper mantle source  Mg# > 65: K 2 O 65: K 2 O < 0.10 TiO 2 < 1.0  Depleted in LREE, Low LILE e.g. K+ –E-MORB (enriched MORB, also called P-MORB for plume) taps the (deeper) fertile mantle  Mg# > 65: K 2 O > 0.10 TiO 2 > 1.0  Rich in LREE, higher in LILE e.g. K+ An incompatible element is an element that is unsuitable in size and/or charge to fit in the cation sites of the possible minerals. Elements that have difficulty in entering cation sites of the minerals are concentrated in the melt phase of magma (liquid phase). Another way to classify incompatible elements is by mass: light rare earth elements are La - Sm, and heavy rare earth elements (HREE) are Eu - Lu. Rocks or magmas rich, or only slightly depleted in light rare earth elements (LREE) are referred to as fertile, and those with strong depletions in LREE are referred to as depleted.

24 E-MORBs (squares) enriched in LREE over N-MORBs (red triangles): regardless of Mg# Lack of distinct break suggests three MORB types Lack of distinct break suggests three MORB types –E-MORBs La/Sm > 1.8 –N-MORBs La/Sm < 0.7 –T-MORBs (transitional) intermediate values Figure Data from Schilling et al. (1983) Amer. J. Sci., 283,

25 N-MORBs: 87 Sr/ 86 Sr ,  depleted mantle source N-MORBs: 87 Sr/ 86 Sr ,  depleted mantle source E-MORBs extend to more enriched values  stronger support distinct mantle reservoirs for N-type and E-type MORBs E-MORBs extend to more enriched values  stronger support distinct mantle reservoirs for N-type and E-type MORBs Figure Data from Ito et al. (1987) Chemical Geology, 62, ; and LeRoex et al. (1983) J. Petrol., 24,

26 Conclusions: MORBs have multiple source regions MORBs have multiple source regions The mantle beneath the ocean basins is not homogeneous The mantle beneath the ocean basins is not homogeneous –N-MORBs tap an upper, depleted mantle –E-MORBs tap a deeper enriched source Idea: –T-MORBs = mixing of N- and E- magmas during ascent and/or in shallow chambers

27 MORB Petrogenesis Separation of the plates Separation of the plates Upward motion of mantle material into extended zone Upward motion of mantle material into extended zone Decompression partial melting associated with near- adiabatic rise Decompression partial melting associated with near- adiabatic rise N-MORB melting initiated ~ km depth in upper depleted mantle where it inherits depleted trace element and isotopic char. N-MORB melting initiated ~ km depth in upper depleted mantle where it inherits depleted trace element and isotopic char. Generation Figure After Zindler et al. (1984) Earth Planet. Sci. Lett., 70, and Wilson (1989) Igneous Petrogenesis, Kluwer.

28 Region of melting Region of melting Melt blobs separate at about km Melt blobs separate at about km Generation Figure After Zindler et al. (1984) Earth Planet. Sci. Lett., 70, and Wilson (1989) Igneous Petrogenesis, Kluwer.

29 Idea: Idea: Convective flow that caused the divergence at MOR runs to Boundary Layer. Convective flow that caused the divergence at MOR runs to Boundary Layer. Lower enriched mantle reservoir may also be drawn upward and an E-MORB plume initiated Lower enriched mantle reservoir may also be drawn upward and an E-MORB plume initiated Figure After Zindler et al. (1984) Earth Planet. Sci. Lett., 70, and Wilson (1989) Igneous Petrogenesis, Kluwer.

30 The Axial Magma Chamber Original Model Semi-permanent, large Semi-permanent, large Fractional crystallization  derivative MORB magmas Fractional crystallization  derivative MORB magmas Periodic reinjection of fresh, primitive MORB from below Periodic reinjection of fresh, primitive MORB from below Dikes upward through the extending and faulting roof Dikes upward through the extending and faulting roof Differentiated melts on sides Differentiated melts on sides Figure From Byran and Moore (1977) Geol. Soc. Amer. Bull., 88, The infinite onion

31 The crystal mush zone contains perhaps 30% melt and constitutes an excellent boundary layer for the in situ crystallization process proposed by Langmuir Langmuir’s idea: crystallization is nearly complete along the cold wall rock, so the liquid there is more evolved than in the interior of the chamber. Figure From Winter (2001) An Introduction to Igneous and Metamorphic Petrology. Prentice Hall

32 Recent seismic work has failed to detect any chambers of this size at ridges. Modern View: completely liquid body is a thin (tens to hundreds of meters thick) and narrow (< 2 km wide) sill-like lens 1-2 km beneath the seafloor Provides reflector noticed in detailed seismic profiles shot along and across sections of the EPR Melt surrounded by a wider mush and transition zone of low seismic velocity Transition zone transmits shear waves, but may still have a minor amount of melt) “Magma chamber” = melt + mush zone (the liquid portion is continuous through them) Lens maintained by reinjection.

33 Figure After Perfit et al. (1994) Geology, 22, A modern concept of the axial magma chamber beneath a FAST ridge Completely liquid body is a thin (tens to hundreds of meters thick) and narrow (< 2 km wide) sill-like lens 1-2 km beneath the seafloor Provides reflector noticed in detailed seismic profiles shot along and across sections of the EPR Melt surrounded by a wider mush and transition zone of low seismic velocity Transition zone transmits shear waves, but may still have a minor amount of melt) “Magma chamber” = melt + mush zone (the liquid portion is continuous through them) Lens maintained by reinjection.

34 Melt body  continuous reflector up to several kilometers along the ridge crest, with gaps at fracture zones, small deviations in alignment (devals) and offset spreading centers ( OSCs ). Melt body  continuous reflector up to several kilometers along the ridge crest, with gaps at fracture zones, small deviations in alignment (devals) and offset spreading centers ( OSCs ).OSCs Large-scale chemical variations indicate poor mixing along axis, and/or intermittent liquid magma lenses, each fed by a source conduit Large-scale chemical variations indicate poor mixing along axis, and/or intermittent liquid magma lenses, each fed by a source conduit Figure After Sinton and Detrick (1992) J. Geophys. Res., 97, Devals: subtle bends or tiny offsets less than 500 meters in size.

35 Model for magma chamber beneath a slow-spreading ridge, such as the Mid-Atlantic Ridge Sinton and Detrick (1992) Model for magma chamber beneath a slow-spreading ridge, such as the Mid-Atlantic Ridge dike-like mush zone and a smaller transition zone are beneath a well-developed rift valley –Model: With a reduced heat and magma supply, a steady-state eruptible melt lens is absent. Instead a dike-like mush zone and a smaller transition zone are beneath a well-developed rift valley –Model assumption: Most of body well below the liquidus temperature. –Prediction: convection and mixing is far less likely than at fast ridges. Figure After Sinton and Detrick (1992) J. Geophys. Res., 97,


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