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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 model - THREE reasons
1. MOR structures change a lot with spreading rate (mm/yr):Fast (EPR 70-150) - 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
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
Title
Mantle properties and
the MOR process: a new
and versatile model for
mid-ocean ridges
Miles F Osmaston
Woking, UK
[email protected]
http://osmaston.org.uk
EGU2014. Session GD3.5, PICO presentation, 30th April 2014
3
Introduction
MORs are a primary source of the geodynamic
record so we need to understand how they work
THREE reasons why we need a new and versatile MOR model
1. MOR structures change a lot with spreading rate (mm/yr):Fast (EPR 70-150) - 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.
4
The divergent mantle flow (DMF) model for MORs
to provide
seismic
anisotropy
is DOUBLY
untenable
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?
5
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 SHEARRESISTANT (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
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
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.
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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.
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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
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
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
410-660km 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 fO2 mantle with a water-weakened mineral structure,
well able to convect away the early radioactive heating without plumes.
12
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
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
Some more features of MORs on which the
new model bears
15
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
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.
Ryukyu
IzuBonin
(old plate)
Lumps mostly
too little stish
to descend on
their own
Fukao et al 2001
Transition
Zone (TZ)
Slope is due to W-moving
source-point. Lumps fall
vertically. Big ones fall
faster so the trace widens
vertically 7-fold
Blue = High Vp
17
Izudeq17
18
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 excrustal lumps – to
build D”
‘Shower’ finishes
up as a layer on
D”, giving D” the
observed seismic
anisotropy.
Stishovite, the high-P polymorph of SiO2 , is uniquely capable
of carrying H2O 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.
19
upper
History of upper mantle depletion
It’s clearly unreasonable to interpret this as showing that crustal production has tripled since the Archaean
20
A 2-layer mantle now? So when and how did
it change from a whole-mantle mode?
Answer – During the well-established (Windley,
Condie) 2.45-2.2Ga gap in zircon dates for
orogenic granitoids and greenstone belts.
Details -- In the 2.8-2.45Ga 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 CO2 in
weathering, giving the first global glaciations 2.45-2.3Ga.
Before all this, oxygenic life had been confined to the top 50-200m 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 ironformation) 2.8-2.3Ga from the acid ocean’s Fe2+.
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