# Why does a subduction zone produce a serpentinization diapir rather than volcanism?

The classic Troodos Ophiolite in Cyprus has been uplifted by a 'serpentinization event'. Upper mantle (peridotite) has been serpentinized creating a buoyant diapir. This has uplifted the ocean crust sufficiently for it to be thrust over continental crust (underlying continental crust has been inferred from gravitational surveys).

The last sentence of the abstract of "Tertiary uplift history of the Troodos massif, Cyprus", AHF Robertson says:

The dominant driving force may have involved the liberation of water from a subduction zone dipping northward beneath Cyprus.

This makes sense - serpentinization is the alteration process of olivine -> serpentine in the presence of water. And, as we know, subduction slabs release fluids (primarily water) into the overlying mantle wedge.

These fluids usually promote partial melting leading to andesite arc volcanism. However, Cyprus has this big serpentinite diapir instead of volcanism. Why?

What causes the fluids from the subducting slab to occasionally result in serpentinization of the mantle wedge instead of partial melting?

• This is a good question, but I am confused; Don't both serpentinization and partial melting both happen in the mantle wedge? IE they aren't mutually exclusive. The lack of arc volcanism could occur from a clinopyroxene depleted mantle which would make the partial melting in the area lower. I am not confident enough in this theory to place it as an answer. If this makes sense, I could write out a more elaborate explanation. – Neo Apr 18 '14 at 22:01
• That makes sense - that it is a matter of degree. So Cyprus has more serpentinization (enough to form a diapir) and less partial melting (not enough to reach the surface). Then the question becomes 'why' - I'm not too well up on CPX depletion, but it makes sense. – winwaed Apr 18 '14 at 22:07
• I wish I knew too, the area I study, Tonga, has rapid mantle flow that causes a great deal of melting in the south but further north less so. It could have to do with not only the dip of the slab but also its orientation perpendicular to the dip (if the slab is descending northward how is the slab oriented from east to west). Another thing to not that in this region there are tons of complex tectonic margins; it may have nothing to do with the subduction zone at all but what happened before the overriding plate decoupled from the descending slab. Again, this is speculation. – Neo Apr 18 '14 at 23:06
• If you can have both one controlling factor may be in the prevailing shallower stress regime? I recall a diagram from structural geology relating the magnitude of differential stress Svertical - Shorizontal to "volcanic domains". With Shorizontal on the y axis and Svertical on the x axis, areal volcanism was close to the origin (e.g. both Svertical and Shorizontal realtively low), sills were dominant at moderate to high Shorizontal and low Svertical, and dikes at moderate to high Svertical and low Shorizontal. When both stresses are moderate to high you get diapirs, or no flow at all. – MyCarta Apr 21 '14 at 14:39

First of all, your statement implies that volcanism didn't occur in Troodos. That is not true. Troodos was even referred to as "Troodos Volcano" once in Miyashiro's 1973 article about Troodos.

A geological map of Cyprus clearly shows that a large portion of the ophiolite is composed of lavas (volcanic) and dykes (sub-volcanic):

(source)

Note the red, brown and pink colors. This is however not the arc volcanism you were referring to but rather ocean floor volcanism in a spreading center setting.

As for your question, why there isn't any arc volcanism in Troodos, the answer is simply that it wasn't hot enough. Let's put the question of where the water comes from aside, and concentrate on what happens when water meets an ultramafic rock, which is what mantle rocks are.

(source)

You can see that serpentine is stable at temperatures lower than 600°C, and even lower at (the more relevant for our case) low pressures. At higher temperatures serpentine is not stable anymore and the stable mineral is olivine (a rock forming mineral in lherzolite and harzburgite). Olivine is a dry mineral, so what happens to all the water? The water acts to lower the temperature needed to melt the mantle rocks.

You can see the dry versus the wet solidus (the temperature in which a rock begins to melt) in the following diagram.

So to sum it up, serpentinite forms when water reacts with ultramafic rocks at low temperature, for example when seawater infiltrates the mantle rocks, or when fluids from the subducted dehydrated slab reach the colder shallow mantle rocks. Melting and consequently arc volcanism occurs for example when fluids from the subducted dehydrated slab rise to deep and hot mantle rocks and suppress the melting temperature to below the ambient temperature.

There are several reasons that the mantle rocks in Troodos were "cold" enough for this to occur. First of all, you are talking about very shallow rocks, very close to the plutonic section of the ophiolite.

(source)

You can see that shallow mantle rocks are in the 300-500°C range. The arc volcanoes that you see in that figure are situated above the areas where fluids from the subducting slab can infiltrate hot (1000ish°C) mantle rocks. So you can use the Japan Sea as an analogue for Troodos in this case. Now, it is true that temperatures were likely higher because magmatic activity did occur in Troodos, but as Troodos was a slow-spreading center, the magmatic activity was rather sporadic and localized. The Nuriel et al. (2009) paper that Lanzafame refers to actually advocates the idea that Troodos was a core-complex. That is, the mantle rocks were directly exposed to seawater due to faulting, which both cooled them considerably and facilitated serpentinization.

The Troodos ophiolite is indeed a supra-subduction zone ophiolite. And it is reasonable to think that arc volcanism occurred somewhere, but the record is absent from the Troodos ophiolite itself. If you are interested, look up "a h f robertson" on Google Scholar and read some of his newer work. The article you cited is from the 70s and much research has been conducted since then.

• Yes the volcanism is seafloor spreading - predating the emplacement. Temperature being the controlling factor makes sense - but then the question becomes: Why was this section cooler? – winwaed Oct 6 '14 at 22:21
After some brief research, serpentinite diapirism occurs due to deep penetration of seawater at temperature around 100-200$^o$C [1]. To the south of the rift that was forming the ophiolite sequence there was an oceanic plate subducting as African plate moved north. This subducting oceanic plate provided the seawater that serpentinized the harzburgite in the ophiolite sequence. From my understanding this removed the water from the system before it reached depths to form partial melt. The subducting plate also assisted in pushing the diapir to the surface through the detachment faults present from the rifting.[1]:http://earth.huji.ac.il/data/pics/Nuriel%20et%20al%202009.pdf [image]:http://www.moa.gov.cy/moa/gsd/gsd.nsf/dmlTectonic_en/dmlTectonic_en?OpenDocument