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Mantle properties and the MOR process: a new and versatile model for mid-ocean ridges Miles Osmaston, Woking, UK. Why do we need a new and versatile MOR.

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Presentation on theme: "Mantle properties and the MOR process: a new and versatile model for mid-ocean ridges Miles Osmaston, Woking, UK. Why do we need a new and versatile MOR."— Presentation transcript:

1 Mantle properties and the MOR process: a new and versatile model for mid-ocean ridges Miles Osmaston, Woking, UK. Why do we need a new and versatile MOR model - THREE reasons 1.MOR structures change a lot with spreading rate (mm/yr):- Fast (EPR ) - straight axes, orthogonal segmentation, smooth flanks Medium-to-slow (southern MAR) - rift valley and jagged flanks Ultra-slow (Arctic Gakkel 7-13) has curved or oblique axis, variable deep rifting, poor segmentation, hardly any crust and a deep ridge-crest. 2. For a petrophysical reason, known since 1986 &1996, and evident geodynamically, interstitially-melted mantle of the LVZ is actually very stiff and is part of the oceanic plate. So the divergent mantle flow model won’t work. 3. Such thick oceanic plate is thermally buoyant, so our MOR model must push it hard to make it subduct. 1

2 Construction of thick oceanic plate at MORs The narrow mantle crack model for MORs that push themselves apart and generate seismic anisotropy Figure illustrates intermediate-to-slow rate case 2

3 Title Mantle properties and the MOR process: a new and versatile model for mid-ocean ridges Miles F Osmaston Woking, UK EGU2014. Session GD3.5, PICO presentation, 30 th April

4 Introduction THREE reasons why we need a new and versatile MOR model 1.MOR structures change a lot with spreading rate (mm/yr):- Fast (EPR ) - straight axes, orthogonal segmentation, undulating flanks Medium-to-slow (southern MAR) - rift valley and jagged flanks Ultra-slow (Arctic Gakkel 7-13) has curved or oblique axis, variable deep rifting, poor segmentation, hardly any crust and a deep ridge-crest. 2. For a petrophysical reason, Nature1986, EPSL1996, and evident geodynamically, interstitially-melted mantle of the LVZ is actually very stiff and is part of the oceanic plate, so the divergent mantle flow model can’t happen. 3. Such thick oceanic plate is thermally buoyant, so the new MOR model must develop strong push to make it subduct. MORs are a primary source of the geodynamic record so we need to understand how they work 4

5 The divergent mantle flow (DMF) model for MORs Firstly, dynamically. Problems with this flow-shear idea for seismic anisotropy. Note the >70km thickness of the anisotropy especially seen in the fast-spreading Pacific (Ekström & Dziewonski 1998). To get such shearing you need a continuous velocity gradient. But shearing in silicates is typically self-concentrating into a narrow zone. Why not here? Why is the MORB magmatism from the corresponding supposedly wide zone of axial mantle-upwelling actually concentrated along the very narrow crest of the EPR? to provide seismic anisotropy is DOUBLY untenable 5

6 Secondly, due to the properties of LVZ mantle A.Both on seismological (Forsyth 1992) and experimental (Faul 1997) evidence, interstitial melts up to ~3wt% are present in the oceanic Low Velocity Zone (LVZ) and probably DON’T migrate from there. B.Water partitions very strongly into any interstitial melt, so (if the water-weakening of the mineral structure is below a critical level) the water-weakening is virtually removed by its presence and the LVZ fabric will be MUCH MORE CREEP- and SHEAR- RESISTANT (up to 2 orders) (Karato 1986, Hirth & Kohlstedt 1996). [Note that for mechanical disaggregation to arise, ~10wt% melt is required.] So LVZ material is NOT INTRINSICALLY MOBILE, as assumed in the DMF model. I show later that this feature of mantle behaviour, far from being relevant only to present-day MORs, has been a major factor in the geodynamic evolution of the Earth and for the chemical evolution of its atmosphere. 6

7 Construction of thick oceanic plate at MORs The narrow mantle crack model for MORs that push themselves apart and generate seismic anisotropy Figure illustrates intermediate-to-slow rate case (e.g. MAR, SEIR) 7

8 Principles of action - (1) Narrow crack This feature offers 2 special properties (Osmaston 1995); (a) Differential accretion to the walls of a non-straight mantle crack will make the MOR segment become straight (see the later slide, 14); (b) Columnar growth of olivine at the crack walls will, by crystallization, build in seismic anisotropy straightaway. The a-axis of olivine has much its highest seismic velocity and has 10 times higher thermal conductivity than silicate melt (Chai et al 1996; Snyder et al 1994, 1995, 1996) so olivines which crystallize onto the crack walls with their a-axis projecting will conduct latent heat away and grow the fastest, yielding columnar structure with inherent seismic anisotropy. 8

9 Principles of action - (2) Mantle phase changes Thermodynamic calcs show that mantle phase changes (gt-sp perid. at 70-90km depth; sp-plag perid. at shallow depth) each generate >50 X more volume increase, per joule, than pure expansivity. So eruption-heated solid-state phase-change makes the walls bulge inward and make contact at the phase-change level, causing push-apart of the walls elsewhere. This, alternating along strike, intermittently induces the flow into the crack and repeats the process. This solid-state push-apart mechanism gives this thick-plate MOR model much greater (>10-fold?) ridge-push than the divergent-flow models, and is needed:- (a) for driving the subduction of the buoyant plates created by this MOR model; (b) for the progressive foreland-directed thrusting across the flat-slab Andes (e.g. Jordan et al 1983 GSAB); (c) for horizontally compressing the oceanic plate sufficiently for a M = >9 subduction earthquake. 9

10 Principles of action - (3) The log-jam segregation of magma Magmatic evolution of an induced diapir in a deep mantle crack. The mantle induced into the bottom of the crack undergoes pressure-relief melting, which gives it buoyancy but, as it moves higher, wall-cooling affects the flow and the restite solids grow again with cumulate intergrowths. At some level in the crack these become big enough to form a ‘log-jam’ through which the melt is forced. The depth of the jam decides the major-element composition of the magma. At MORs, the walls are hot, so the jam is shallow, giving tholeiite as observed. Nature of the log-jam mechanism Empirical basis (engineers; various situations, well known to engineers, especially hard-rock grouting of fractures). Also former UK code of practice CP110 for design of reinforced concrete – “min spacing of bars = 4 times stones size”. A jam is INEVITABLE (except at uninterestingly low flow rates and high viscosity) if the size of the lumps >20-25% of the crack width. Shape effect is minor. No theoretical treatment is known. 10

11 Spreading-rate variants The fast EPR has a small hot zone of plag peridotite around the crest; so the sp-to-plag peridotite boundary is very shallow and push-apart there produces only trivial rifting. The flank topography is relatively smooth, where the high shrinkage upon return to sp peridotite produces abyssal hills and the nipple-like crestal profile. At slightly lower spreading rate (SW EPR ~70mm/yr), the plag perid zone may vanish intermittently along strike, reverting to the deeper gt-to-sp push-apart and rift valley, illustrated here. At ultra-slow MORs, the induced melting is too low for oriented wall crystallization or for log-jam-mode segregation, so crust production is minimal (Gakkel) and seismic anisotropy probably absent. Instead, the split is filled by a wide intrusion, due to the high viscosity of the induced diapir. The (now very long) push-apart cycle then involves squeezing that intrusion instead of having first to close the ‘crack’. Push-apart force is therefore maximised, and so is the related suction below it, as recorded by the ~150m dent in the geoid around southern India (GRACE/GOCE data), the mantle flow to supply the Arctic being sucked between the Baltica and Angara cratonic keels. 11

12 Effects of a wet mantle – (1) Reykjanes Ridge (RR). Water-rich glasses have been dredged from the RR and all the way N to part-way along Gakkel. By lowering the solidus this is surely the cause of much-increased mantle melting, without the extra heat supposed for the so-called ‘Icelandic plume’. Note first its effects on the MOR process at the RR. The axis is markedly non-orthogonal to the spreading direction. At 20mm/y, this borders on ‘Ultraslow’, yet there is no axial rift and crust appears thick, both presumably due to high-volume magmatism that fills the rift and may conceal multiple offsets. Source of this wet mantle is likely the mantle Transition Zone (TZ) at km depth. Both the Earth’s orbital a.m. (Osmaston 2010) and its mantle Fe isotope ratio (Halliday 2013, Craddock et al 2013) show that Earth’s core cannot have been made by percolation of molten Fe. This necessitates reversion to the Ringwood model for this, which reacts hot volcanic FeO with nebular H to make Fe and lots of water. That gave the early Earth a high fO 2 mantle with a water-weakened mineral structure, well able to convect away the early radioactive heating without plumes. 12

13 Effects of a wet mantle – (2) On abundant geodynamical grounds I have shown (2006,2007,2008, 2009,2012) that cratons possess rigid mantle keels which extend well into the TZ, the rigidity being for the same fluid-content Hirth & Kohlstedt 1996 reason as I have applied here to the LVZ at MORs. So, when a craton splits or two Archaean cratons separate, as they have done in the NE Atlantic, they draw up TZ mantle between them, causing it to undergo pressure-relief melting, much increased due to lowered solidus. Hence what has been called the ‘Icelandic plume’. Finally it remains to show why the TZ material is wet. In a set of 5 Appendix slides I show how it is that we now, since ~2.3Ga, have had a two-layer mantle despite the seismological appearance that ‘slabs’ are entering the lower mantle. The small volume actually entering the Lower Mantle will only need to be compensated by the upward diffusion of LM material into the TZ at the global rate of about 5mm/cy (Osmaston 2000). That LM material is predictably wet, partly inherited from early Earth but also augmented by water carried into it from the upper mantle since 2.3Ga. Therefore much of the TZ is wet, seen in water-rich perovskite in a kimberlite from there (Pearson et at 2014). 13

14 Straightness and orthogonal segmentation of MORs Straightening action of thermal accretion to the walls of a narrow mantle crack Wall accretion rate depends on the lateral extraction of latent heat of crystallization from the flow, so is enhanced at the wall on the outside of a bend, and diminished on the inside wall. This asymmetry progressively results in the straightening of the crack and aligning it perpendicular to the age-gradient-oriented horizontal temperature gradient in the plate (Osmaston 1995 IUGG). An initial separation line that happens to be oblique, not orthogonal, to the separation direction, or becomes oblique due to changes in plate motion, will thus rapidly develop orthogonal segmentation of that line, as seen at some ocean margins. To work like this the crack must be narrow enough (20cm?) that the wall temperature is not dominated by the heat resource in the core of the flow. 14

15 Some more features of MORs on which the new model bears 15

16 Concluding comment This MOR model is a powerful heat engine. But it is NOT THERMAL CONVECTION. The crack width will likely widen out downwards; the proposed 20cm nominal is envisaged at the level of the log-jam and based on kimberlite xenolith sizes from the rupture of jams, but needs careful consideration. To model it, and its behaviour, could need ~5cm across-strike resolution. This compares with the 5km resolution hitherto used for MOR process studies. Appendix (mantle dynamics) 16

17 Resolving the paradox of subduction tomographic sections The world’s most prolonged young-plate subduction (Central America) yields the world’s biggest TZ-and-lower-mantle high-Vp signature, whereas the oldest-plate zone (IzuBonin – 130Ma plate) sends hardly any into the lower mantle. This is the OPPOSITE of what ‘cold slabs’ would do. So what are we looking at? Answer: ex-LVZ heat reaches and melts the oceanic crust at the interface. This, under TZ conditions, yields high density, high-Vp stishovitic residues, lumps of which shower into the lower mantle. So the lower mantle signature is NOT mantle material. Old plates have less heat for melting their crust – so generate less residue. Fukao et al 2001 Ryukyu IzuBonin 17 Blue = High Vp Slope is due to W-moving source-point. Lumps fall vertically. Big ones fall faster so the trace widens vertically 7-fold Transition Zone (TZ) (old plate) Lumps mostly too little stish to descend on their own

18 Izudeq17 18

19 The two-layer mantle Reconciliation with seismology and mantle structure No seismicity below 660 km because no slab penetration; it’s only showers of stishovitic ex- crustal lumps – to build D” Stishovite, the high-P polymorph of SiO 2, is uniquely capable of carrying H 2 O into the lower mantle (Litasov et al 2007 EPSL), thus drying out the mobile part of the upper mantle from its former (early-Earth) wet state. ‘Shower’ finishes up as a layer on D”, giving D” the observed seismic anisotropy. 19

20 History of upper mantle depletion upper 20 It’s clearly unreasonable to interpret this as showing that crustal production has tripled since the Archaean

21 A 2-layer mantle now? So when and how did it change from a whole-mantle mode? Answer – During the well-established (Windley, Condie) Ga gap in zircon dates for orogenic granitoids and greenstone belts. Details -- In the Ga run-up to this Post-Archaean Hiatus, MOR crests deepened, finally lowering sea-level by >3km during the Hiatus. The ~10km erosion of cratons unroofed TTG and used up CO 2 in weathering, giving the first global glaciations Ga. Before all this, oxygenic life had been confined to the top m of oceans, fighting the acidification (pH=4.5) from MORs (CO2, H2S, etc) at a chemocline. Shut-down of MORs enabled it to win its battle, and deposit most of the world’s oxide-BIF (banded iron- formation) Ga from the acid ocean’s Fe 2+. Completing the job at ~2.25Ga, atmospheric O2 finally began to rise, and that’s why we are here. So in this sense we are the living proof that we have a 2-layer mantle. The critical loss of water-weakening from those parts of the Upper Mantle from which the Archaean ocean had evolved was the main trigger for that change. Its action is still evident in the mantle of our new MOR model, 21


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