from isotropic to layered gabbro -...

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From isotropic to layeredgabbro

Evolution record in the Omanophiolite Moho transition zone

D. Jousselin, L. Morales, M. Nicolle, A. Stephant

[This is a (near) 15 minutes talk (19 slides + spare ones for questions)about the formation of layered gabbros, presented at the 20[12 Omanconference. I have added a few additional information (in brackets)which would make the talk a bit longer, but that a reader might find ofinterest.These results have since been published in an EPSL paper: Gabbrolayering induced by simple shear in the Oman ophiolite Moho transitionzone (2012).]

The formation of gabbro layering is a long standingissue which might help understanding magmaemplacement. I’ll try to convince you that in Oman wehave relics of several stages showing the formation ofgabbro layering.

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Layered gabbro lensesin the MTZ

From isotropic to layered gabbro…Previous models (« sills ») (Boudier & Nicolas, 1996; Kelemen et al, 1997)

Hzb

Gab

bro

sect

ion

Moho

Layered

Several models have been proposed for the formation of gabbrolayeringOne is the gabbro glacier model where layered gabbro originate fromcrystals subsiding on the floor of the melt lens at the top of the gabbrosection. In this case layering is a consequence of deformation andepisodic filling of the melt lens.In another popular model, each layer may correspond to sill injections.This seems particularly adequate in the Moho transition zone wherelayered gabbros are distributed in sill-shaped lenses embedded inresidual dunite, thus seemingly excluding processes related tosubsidence from the magma lens at the top of the crustal section. Onekey argument for this model is that the layered lenses were notdeformed.

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From isotropic to layered gabbro…Evidences against sill origin

Strong S & Lparallel tomantle flow

Strong shearing(simple shear)

In this slide I want to show that, to the contrary of earlier claims, layeredgabbros in the Moho transition zone show evidences of deformation bysimple shear in magmatic state.First, layers commonly show isoclinal folds with an axis parallel to thelineation, as is typical for strongly sheared rocks.Second, layers are affected with series of small normal magmatic faults,with a strike perpendicular to the lineation, which is parallel to the greenruler on picture D.Lastly, the rocks are well foliated, sometimes with crystals oblique tothe layering (giving a sens of shear), and more often parallel to thelayering as expected for strong deformation.

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From isotropic to layered gabbro…Evidences against sill origin

Lens shapes

Also the gabbro lenses do not always have the shape of an intrusivesill, as some have an irregular shape, especially when gabbros are notas strongly deformed as usual.

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From isotropic to layered gabbro…

Diffuse margins Lots of melt not confined withinthe lenses

Evidences against sill origin

Finally, there is no structural evidences for intrusive contacts such aschilled margins or comb structures.(ADDITONAL INFO: …such as chilled margins or comb structures as isreported for true sills in the Bay of Island ophiolite)On the contrary gabbro lenses margins are diffuse, with the host duniteprogressively grading into plagioclase impregnated dunite and intogabbro over a few centimeters to meters.Also the melt impregnated dunite contains plagioclase euhedralcrystals. It suggests that the dunite contained enough melt to locallydisrupt the solid olivine framework and allow plagioclase to freelycrystallize in melt suspension, and that melt was more widelydistributed before parts of it formed the gabbro lenses.

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From isotropic to layered gabbro… Where to observe transient stages?

One problem that hinders insight into the formation of layering is thatwe are left with observations of the final product without any record oftransient stages in the formation of layering. This might be becausetransient stages are relatively short, right after melt extraction from themantle, and it might be difficult to fossilize these snapshots in time.Also they might be limited in space to the ridge axis.The southern massifs of the Oman ophiolite contain a NE-SW dikesystem on their outside limits, and a NW-SE dike system in the centralparts. It is thus inescapable that it contained a center of accretionsomewhere in the central parts.(ADDITIONAL INFO: considering the near 50 Km width of the NW-SEregion, the axis may have been active during a half million year period,an estimate coherent with dating studies of Rioux et al, 2012 in JGR,doi:10.1029/2012JB009273)This is precisely where we mapped mantle diapirs, capped by a thickMoho transition zone. These represent ideal target regions to observeshort term processes that occured at the ridge. The next observationsare made in the region of the Maqsad diapir.

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From isotropic to layered gabbro…New observations

Type 1: unlayered gabbro

A new important observation is occurrences of isotropic gabbro withindunite, with one outcrop extending over a 500 m wide area.

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From isotropic to layered gabbro…New observations

Type 1: unlayered gabbro

There is a progressive passage from the host dunite to the gabbro, withimpregnated dunite progressively grading into gabbro over a fewmeters, as summarized by the blocks on this picture. Also the isotropicgabbro contain patches of dunite. This all suggests that the gabbro is infact a dunite heavilly impregnated in plagioclase, and a fewclinopyroxene crystals.

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From isotropic to layered gabbro… New observations

At the limit between type 1 gabbro and dunite

In support with this idea we find that when plagioclase andclinopyroxene reach 25%, they contour and isolate olivine grains,fossilizing a stage where dunite switch from solid state flow to anisotropic magma mush where solid olivine grains were disconnected.

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Type 2: weakly defined layers

Within the same outcrop, we find weakly defined layers which can befollowed only over a few centimeters. We distinguish those as « type 2gabbros ».

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Type 3: <2 m long layers

Type 3 gabbros show more strongly defined layers, but with diffuse orwavy limits which can be followed over less than 2 meters.All of those gabbros, and the more classic well layered gabbros, showmagmatic textures (i.e. with plagioclase euhedral crystals, devoid ofintracrystalline deformation)

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New observations and 30 petrofabric data

ISOTROPIC

PLANAR (no L) = compaction

In an effort to understand the state of deformation for the different typeof gabbros, we measured the crystallographic fabric for a selection of30 samples.The unlayered gabbro show an isotropic olivine fabric, showing that it isnot deformed.The type 2 gabbros show a planar fabric, with a clear [010] preferredorientation that define the plane of flattening. Other axis define a girdlein the (010) plane, without a clear lineation, orare dispersed. We infer that type 2 gabbros were affected by flattening.

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L defined by olivine long axis= shape fabric= magmatic flow

L defined by [100] (010) glidingsystem (plastic deformation)

Type 3 samples show a [001] lineation superimposed on the foliationplane. This axis correspond to the longest shape axis of olivine, whichwould align with flow. We infer that as melt fraction decreases aftercompaction, more transient solid chains form and interact, thuscoupling with the simple shear imposed by the surrounding mantle flowis possible, and leads to the development of a magmatic lineation in theolivine CPO.Type 4 samples show a [100] (010) olivine CPO which corresponds tothe classic high-temperature gliding system.We argue that as a critical threshold of olivine connectivity is reached,olivine deformation rapidly switches from magmatic to plastic.In this laststage of deformation, where the melt fraction is greatly reduced,deformation must concentrate in the weakest layers, which are theolivine rich layers. The decoupling between layers may certainlycontribute to further define the layering.

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From isotropic to layered gabbro…Physical modeling constrains

Any type of difference in the particle species lead to shear-inducedsegregation

We have seen that the Moho transition zone contains magmaticmushes at different stages of deformation by simple shear. Modelingand geological studies show that shearing leads to layering. Two mainfactors may play a roleFirst, the magma contains different crystals with differentcharacteristics. Numerical and analogical modeling of shearing forgranular material, slurries or suspended particles show that any type ofdifference in the particle species lead to mechanical sorting. The aboveexample is a numerical study for particles of different sizes. Others dealwith surface tension, geometry, etc…

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Tectonic transposition of heterogeneities


Ramsay & Graham, 1970

We have also seen that the initial isotropic gabbro contains primaryheterogeneities (And other heterogeneities due to later melt venue mayalso arise).This is the second factor: any type of heterogeneity, if it is soft enough,will be tectonically transposed by the flow, and will form elongatedlayers.Here is a beautifull example of transposed xenoliths, deformed byplastic flow in a shear zone.

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NO DEFORMATION = NO LAYERING

SHEARING IS AN EFFICIENT SORTING PROCESS

STRONGER DEFORMATION=

BETTER DEFINED LAYERING

Summary

In summary, gabbros do not contain any layer prior to deformation.However, undeformed gabbros are not in a steady situation, they arebound to be affected by simple shear after their emplacement in theMoho transition zone.We have seen that shearing is an efficient process to produce layeringby mechanical sorting and tectonic transposition.And better defined layering correlate with stronger deformation.

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From isotropic to layered gabbro… Conclusion 1/2

isotropic planar S-L magmatic S-L plastic

This lead to our first conclusion:Contrary to previous assertions, usual type 4 gabbros are stronglydeformed by simple shear. They seem to derive from successiveephemeral stages which have been fossilized because we had accessto the ridge itself.In the first stage melt accumulates in the Moho transition zone to thepoint that it disrupts the solid framework of the host dunite. In thesecond stage, the resulting isotropic magmatic mush is compacted,which produces a planar fabric with poorly defined layers.With less melt in the matrix (but still enough for a magmatic state),shearing imposed by the host mantle flow produces a magmaticlineation, with better defined layers. And finally, with even less melt,shearing is concentrated in olivine rich layers which deforms at solidstate. The result is a switch from a magmatic to plastic olivine fabric.

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A consequence on melt extraction in the shallow mantle:

Melt accumulation must be faster than layering formation

Melt extracted by dikes rather than porous flow

From isotropic to layered gabbro… Conclusion 2/2

From this model, we derive a consequence on melt extraction.The distance from isotropic gabbro to well layered lenses is 100meters. With a mantle flow velocity of the order of 10 cm/y, thiscorresponds to a 1000 years time laps.The isotropic gabbro extends on an estimated volume of about 18.10^6cubic meters. A minimum of 30% melt in the matrix is necessarry tokeep solid particles disconnected, which reduces the melt volume to6.10^6 cubic meters (this is probably a minimum, as we have seen thatthe melt distribution may largely encompass the area where gabbro iscurrently present).A 100 meters wide porous flow conduit, with an ascending velocity of1m/y for the melt, and a porosity of 1%, would take about 250 000years to fill this volume. This means that as the last portion of melt isdelivered, the first portion is already gone some place else. In otherwords a porous flow channel is not fast enough to produce isotropicgabbro.

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On the other hand, a 10 cm wide and 100 m long dike, with a melt flowvelocity of 1.5 cm/s would take about a year to fill the same volume. Inthis case melt can accumulate fast enough to not be affected byshearing, and can produce isotropic gabbro in the small prerequisitetime window.

To recap, we show that modal layering in gabbro lenses of the MTZdoes not predate shearing but is strongly linked to simple sheardeformation imposed by the surrounding mantle flow, and the discoveryof isotropic gabbro is more coherent with melt extracted by dikes ratherthan porous flow conduits.

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Spare slides for questions

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From isotropic to layered gabbro…Physical modeling constrains in suspensions, slurries and granular material

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From isotropic to layered gabbro… Conclusion

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Consequences of off-axis melt delivery at the Moho: Sr, Nd andOs isotopic results from the Oman ophiolite

Marie Nicolle, Delphine Bosch, Laurie Reisberg, David Jousselinand Aurore Stephant

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Previous models (« glacier »)

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Dike: yes Porous flow channel: not working

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At 1m/yr: 250 000 years to fill the meltvolume

At 1cm/s: less than 2 yearsto fill the melt volume

Dike: yes Porous flow channel: not working

Isotropic gabbro readyto be sheared

Too little, too late

Melt can’t accumulate

Isotropic gabbroNo isotropic gabbro

from isotropic to layered gabbro -...

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