Hi Gerald,

Using a typical Laacher See phonolite composition, and a Zr content of 2600 ppm, I get a zircon saturation temperature of 950 degrees C. This is just using the simplest description of the model, without taking water contents into account. Essentially, (per)alkaline magmas do not crystallise zircon, and zirconium can reach really high concentrations. That's why they can then start crystallising oddbal minerals like eudialyte in agpaitic rocks.

Hi Marlina,

Thank you for sharing this. So, in the absence of dissolved water, zircon should not crystallize at 760°C (eg. Harms et al 2004) in LLST magma. As this LLST magma was arguably water-saturated, and as Zircon did form, does this indicate that it is dissolved water abundance that causes Zircon to crystallise in the present case ?

(The highest estimated temperatures were 880°C for ULST magma (with low water content), so Zircon should not be observed anywhere in the Laacher See system then.)

This is what intrigues me. Zircon should not crystallize and it does at Laacher See.

Besides, it should not crystallize in a number of other localities around Laacher See, eg. in Ettringer Bellerberg Tephrite Lavas, where temperatures should also have been lower than 950°C for such magma composition.

So I would still like to understand why it does crystallize ?

If anyone has additional insights or references to suggest I should read ?

Thank you Marlina and all the very best for 2016,

Best wishes,

Gerald

Hi Gerald, I obviously didn't make my point very clearly:

If temperatures are lower than the zircon saturation temperature, then zircon would crystallize. If temperatures are higher, then zircon should not form. So if the zircon saturation temperature is 950 ˚C, and the magma is 760 ˚C, then you can expect zircon - unless the zirconium is at the temperature already taken up in other minerals (you get some weird magma compositions where zirconium substitutes in the pyroxenes or amphiboles).

Hi Marlina,

Thank you for clarifying.

So then as Zr melt concentrations are very high (up to 3000 ppm) and temperature is estimated at 760ppm in LLST one may expect quite a lot of zircon to form (and this is not so) unless Zr substitutes in px and amph or other minerals.

It remains intriguing to me that there isn't altogether a lot more Zircon in Laacher See pumices or cumulates. The highest temperature estimate for Laacher See is 880°C and Zr levels, even though they vary, do seem high to very high.

Thank you Marlina.

Kindest regards,

Gerald

I had a quick look at Gerhard Wörner's paper from 1983 (attached), and it seems that the sphene takes up quite some Zr, so that may be the reason that we don't see too much zircon.

Yes, indeed. This seems to explain it. Where sphene crystallizes here, it seems to take up most of the Zr together with lesser amounts in amph and even lesser in cpx, you are right. Then it seems it is only when conditions for sphene crystallization are not met that Zircon may actually form. Sphene has been estimated (HU Schmincke and colleagues) to be more abundant in ULST and much less abundant in MLST and LLST, so this explains that Zircon, when it is found, occurs in MLST and LLST. This seems to work. Thanks a lot Marlina.

Hi Gerald and Marlina,

I am not familiar with the Laacher See magmas, but if I understand correctly, we are talking about phonolitic tephra deposits. I would think that the limited amount of zircon crystallization could be due rather to the low silica content of the magma possibly combined with the rapid cooling during eruption (e.g. nucleation inhibited), rather than zircon saturation. The latter would be coherent with your description of the zircons that are present being very small.

As Marlina states: "(per)alkaline magmas do not crystallise zircon, and zirconium can reach really high concentrations" I think the fact that these magmas are undersaturated in silica could limit the amount of zircon crystallization, rather than the zirconium concentration. Hence the high zirconium concentrations.

I am quite sure that a phonolitic composition falls outside of the calibration range of the experiments of Harrison, Watson and coworkers. I am not familiar enough with the liquid line of decent of phonolitic magmas to be able to say whether during crystallization and subsequent fractionation the melt composition would indeed change enough to fall within the calibration range.

Regardless of whether their model is applicable, a key point would be whether the eruption temperature of the magma was above or below the zircon saturation temperature. As I think considering zircon crystallization in terms of the zircon saturation model during the eruption is unlikely to yield meaningful results. With the rapid changes in pressure, temperature, degassing etc. going on I do not believe we can consider the system as being in equilibrium.

Due to the low viscosity of phonolitic melts I suppose it may be possible for these magma to erupt below their zircon saturation temperature, in which case some of the zircons could have crystallized prior to rather than during eruption. Are the temperatures you mention crystallization or eruption temperatures? If you could distinguish between pre- and post eruption zircons based on their occurrence (within the pumice vs. cumulate nodules), It would be interesting to see whether, in these young rocks, the time difference between pre- and post eruption could be resolved using U-Pb-TIMS geochronolgy.

Best,

Roel

Hi Roel, hi Marlina, hi everyone,

Thank you for adding your thoughts to try and understand what may have happened here. The work by coworkers of Hans Schmincke (eg. Gerhard Wörner, E Harms...) has constrained pre-eruptive (ie. crystallization) temperatures for the zones of the pre-eruptive MC: top (760°C, erupted first as LLST) and lower MC (880°C, erupted last as ULST). The magmas as you say are silica-undersaturated and these workers have constrained a liquid line of descent from basanite (not erupted), tephrite, tephritic phonolite, phonolite. As you are hinting at, there is also extensive evidence of magma mingling and disequilibrium parageneses in at least some of the erupted products (eg. work by G Wörner et al). As the chamber is modelled as stably stratified with 3 main zones, mixing is thought to have occurred within any of these single broad layers with little mixing in between (evidence for compositional gaps), whereas withdrawal upon eruption appears to have extensively mingled some of these components together with each other. It is thus not surprising that they found extensive evidence for desequilibrium.

What you are suggesting last, seems to have been at least partially carried out by Axel Schmitt (GSA 34, 597-600, 2006): high-spatial resolution (ion microprobe analyses) 230Th/238U disequilibrium dating revealed rapid zircon crystallization prior to the 12.9ka LSE....zircons from the tephra and syenitic nodules gave an isochron age of 17.1ka (+-1.3ka) (1 sigma). Axel Schimitt thus argued that the LLST zircons shortly predate the eruption...in this paper, the zircons are interpreted as having been scavenged from a precursor syenite intrusion (and are not part of the phonolite crystallization sequence)...

Anyhow, what is argued in Schmitt (2006) apears consistent to your suggestions.

That study is based on 20 individual zircons from LLST (mm) and 4 large zircon from 3 individual subvolcanic nodules.

Many thanks for sharing your insights,

Best wishes,

Gerald  

Hi everybody...

I have crushed many kg's of pumice and found not many zircons on Laacher See Tephra. In highly evolved LLST pumice with 2600 ppm Zr I found < 1mm euhedral bipyramidal zircons (no prisms), which is a typical for zircons from alkaline melts. So, at 4-5 % water and 780 °C and 2600 ppm the phonolite is J U S T  saturated in zircon. The estimate that Marlina made suffers from the problem that Role said about the fact that the Laacher See phonolite (almost 17,3 % alkalies) is outside the range to give reasonable results by the Harrison and Watson saturation method. Most of the zircons in Laacher See phonolite, however, are derived from the syenite - carbonatite crystal mush / carapace. These are cognate and many are close to eruption age. However, after a more comprehensive study, we (including Schmitt) revised the Schmitt (2006) statement that zircons (in general) are close in age to eruption time. Quite the contrary.  Many zircons are much older (>20 ka prior to eruption.

Wetzel F, Schmitt AK, Kronz A, Wörner G (2010) In situ U-Th disequilibrium dating of pyrochlore at sub-millennial precision. Am Min 95: 1353-1356

Schmitt AK, Wetzel F, Cooper KM, Zou HB, Wörner G (2010) Magmatic longevity of Laacher See Volcano (Eifel, Germany) indicated by intrusive carbonatites. J Pet 50: 1053-1085

This is no surprise since the mush and carapace records the prior evolution of the evolved phonolite magma and the phonolite has entrained older crystals.  

So, the most evolved (only) LLST phonolite magma that erupted from the (top of the) core of the mush / carapace complex crystallized zircon at the time of eruption. However, older zircons exist in abundance in the cumulate / syenite carapace. These crystallized also from highly evolved (but older) Laacher See phonolite and resided at lower temperature (

Hi Gerhard,

Thanks for the input - I immediately admit that my calculation was just very quick and dirty, just to get an idea. It seems that a lot of magmatic systems are good at recycling their older mushes!

Hi Gerhard, Hi Marlina,

Thanks indeed for sharing all these insights and info; and thanks to everyone for the stimulating discussions.

Gerhard, the 2 papers you mention do not download from my PC on your RG page (formating issue ?).

Would you be so kind as to attach them here as PDFs, in a reply on this forum ?

Also, there are other occurrences of zircon, large ones this times (although the crystals seem to roughly conserve their length to width ratios) from different lavas at small volcanoes around (and related indirectly to) the Laacher See Volcano (eg. red-brown zircons, this time, up to 3cm long from Ettringer Bellerberg tephrite).

As those tephrite lavas are also strongly silica-undersaturated and alkali rich I am wondering how one can rationalize the crystallization of such large (rare) zircons, considering all we have just been discussing. There, at least locally, one seems to be far beyond just having reached saturation (perhaps with the surroundings being just at zircon saturation or below it ?).

Is it related to strong (huge ?) chemical heterogeneity at a local level in the crystal mush, possible contact or reaction with a silica-rich country rock (locally assimilated) or local reaction with cognate syienitic nodules, and/or to very long residence time of those crystal mushes giving plenty of time for very slow cooling and large crystal growth (a situation in contrast with that of the LLST magma or not as you seem to be suggesting from your latest research ?) ?

Strickingly the tiny LLST zircons are tiny and colourless whereas these can be very large and tend to have a vivid red brown colour. They appear to be found in 100-150ka old tephrite lava at the proximity of xenoliths/inclusions/enclaves (?) according to mineralogists' reports (and the Geol Map for the approximate age of those lavas).

Could you share your expert opinion on how you think those large colourful zircons in the Ettringer Bellerberg 100-150ka tephrite lavas may form ?

If you could also attach any relavant reference pertinent to understanding the dynamics of those Ettringer /Mayener small volcanic systems, at your best convenience, this would also be helpful to guide me to some understanding.

Thank you in advance.

Beste Grüsse,

Gerald

Really big brown zircons can be found in some pegmatitic nepheline syenites of, for instance, the Oslo Rift. The trick is that zircon shouldn't crystallise early on in the magmatic development, so that there is enough zirconium around towards the last pegmatitic stage; and the melt should not be too alkaline that other weird agpaitic  Zr-bearing minerals start to crystallise. So it may be that the big ones you describe are also a kind of xenocrysts (or 'antecrysts') derived from an intrusive rock type, closely related geochemically to the later magma that picked them up.

Hi again,

yes, I know of (not about) these large brown zircons in tephrites. The nicest one is on display in the local Museum collection in Mayen (Mayener Burg) , but I dont know if it is still on display.

Marlina is exactly on the right track. There are abundant and famous megacrysts (sanidine, magnetite, each often many cm in size) in some explosive phonolite deposits of the Rieden center (e.g. Volkesfeld). These megacrysts are derived as xenocrysts from pegmatites of "phonolitic" composition. I am convinced that the large zircons come from similar disrupted evolved alkaline pegmatites. Thus, they are foreign to the tephrites but are large enough to survive without considerable dissolution. It would be interesting to see wether or not their crystal faces have sharp edges from growth or are resorbed and rounded, as I would predict

Hi again Marlina, Gerhard and folks,

Thanks for these further insights. On principle it makes perfect sense.

I also note that Gerhard is suggesting resorbed Brown-red zircons. Well I do not want to spoil your joy but please have a look at the photo of a perfectly euhedral large zircon (Ettringer Bellerberg Teprite lava; no pegmatite in sight anywhere either; just looks like a lovely tephrite lava to me that it is on) in the following amateur mineralogist book, if you may:

H. Schmeltzer (1977) Mineral-Fundstellen. Rheinland-Pfalz und Saarland. Christian Weise Verlag

Photo of pristine 3cm red-brown zircon pheno is on p. 65.

No pegmatite, no resorption in sight ! At least outside shape is perfect for such zircons it seems to me; perhaps the Inside textures would show a complex story ?

So also your suggestion of alkaline pegmatite makes sense, how to reconcile this logical idea with such a basic observation ?

Best,

Gerald

Hi again,

Well, these alkaline pegmatites are below the surface. We also find granulite, granite,  and mica schist xenoliths and none of these rocks are anywhere in sight near these tephrite volcanoes.

The more pristine these zircons appear in shape the less time they have spent in the tephrite lava, I presume. Also, what looks like a perfectly shaped crystal ma not be as perfect when looking at the edges in thin section.

In fact, I see perfectly shaped crystals in these nice pictures, however, with slightly rounded edges...

Gerhard

Hi Gerhard,

Thanks. Yes, it may well be that the large zircons have been picked up rapidly by the tephrite lava.

Do you know if anyone has done similar work on those zircons from Ettringer Bellerger lavas to the work you did on Laacher See zircons ?

Thanks again,

Beste Grüsse,

Gerald  

Dear Gerald,

in the Eifel, nobody has studied these zircons ever. SInce the host lavas have ages well in excess of 100.000 years, the application of the U-series dating method will be impossible.

I think it would be well worth doing a detailed geochemical study of chemical zonation and inclusions in these zircons.

There is an abstract on similar zircons in basanites from the Lausitz volcanic field by Büchner et al (2011). Their interpretations is in accord with what we have been thinking about these zircons in the East-Eifel: They are xenocrysts derived from evolved alkaline intrusives (I'd say pegmatites)  at depth and have resided only short time in the mafic magmas that brought them to the surface.

Cheers

Gerhard

Dear Gerhard,  Thanks a lot for all these info. Regarding the large Laacher See zircons, I fully agree with your pegmatite hypothesis and I have scanned a photo of a zircon that supports this - indeed you have already suggested similar observations you have made yourself. On the other hand for the large zircons in the older tephrite lavas I am less convinced that the same process could work (no obvious resorbption features). I'll attach the two scans to illustrate the contrast asap, in the coming days most probably.

Thanks again for the discussion and for the ref.

Beste Grüsse,

Gerald

I am also having problem with my Syenite rock samples, I did mineral separation on it to separate zircons....but the zircons i got doesnt have the distinct-zircon shape. I have alot of brown minerals with the adamantine luster, occurring with some opaque white minerals(Probably apatite)..I took the brown ones for CL imaging, and they weren't fluorescing under CL. I am a little confused and stuck on my project work because of this..your advice and suggestions will be highly appreciated on this.... thank you very much Gerhard Wörner G. G. J. Ernst Marlina Elburg

Josiah Oluwaleye , could your brown minerals perhaps be titanite (sphene)? They should be slightly more magnetic than zircon, so if you used a Frantz magnet for your separation, then my suggestion is not such a good one. Quite a few syenites contain titanite though....

Marlina Elburg , Thanks for your suggestion. I used the Frantz magnet for separation, some of the brown ones could probably be titanite, because i got a few titanite minerals under my sections. I observed brown ones with the adamantine luster and a bit rounded, also some yellowish ones and opaque ones.. and they all are mostly rounded.

Josiah Oluwaleye then perhaps titanite is the most likely culprit. So if you wanted the zircons for dating, then you could use titanite instead!