Stéphane Dibacto

Pierre Lahitte

David Karatson

Daniel Veres

Ralf Gertisser

Sequence of construction of Ciomadùl dacitic lava domes, East Carpaathians, by unspiked K-Ar Cassignol-Gillot technique in comparison with other radiometric methods

Located at the southern tip of the Intra-Carpathian Volcanic Range in Romania, the Ciomadul (Csomád) volcanic complex (CVC) is the youngest eruptive center of the Carpatho-Pannonian Region (CPR). It consists of a dozen of dacitic lava domes that encompass two youngest explosive craters. Whereas, in the last decade, the Ciomadul explosive history for the last 50 ka was well constrained by numerous studies, the chronology of the dome sequence still lacks robust chronological constraints, since it is based on biased K-Ar ages. Here, we provide a detailed K-Ar dating approach to re ne the chronology of the domes, using the unspiked K-Ar Cassignol-Gillot techniques for the rst time in Carpathian Range. New eruption ages were performed on groundmass, the only material the cooling of which is doubtlessly contemporaneous with the eruption. Each age obtained is found to be younger than those proposed on previous whole rock K-Ar datings. Our analysis shows that the K-Ar ages in the previous studies su er from the unsuitability of the material used to obtain eruptive ages, since dating whole rock gives a mixed age due to various mineral fractions with di erent individual ages. A correlation between excess argon from these whole-rock measurements and the abundance of glomerocrysts is established. we also investigated the sequence of crystallisation of phenocrysts present in the lavas, with respect to the modi cation of the eruptive ages. We dated di erent phases containing potassium, i.e. feldspar, amphibole, biotite, plagioclase microlites, and groundmass. Ages obtained on feldspar, amphibole, biotite are systematically older than those obtained on the groundmass illustrating that these phenocrysts were formed before (or well before) the eruption, and brought up by the magma during the eruption. On the other hand, plagioclase microlites display a complex range of ages, from the age of groundmass to the age of plagioclase phenocrysts.The newly applied methodology, mainly consisting in a double full preparation rst at larger grain size and next at < 100 micrometres, allows us to propose an appropriate procedure to separate the adequate material in order to obtain the K-Ar age of the eruption, i.e. the groundmass, in which there is no risk of presence of older, inherited phenocrysts.

Main issue is to separate by using heavy liquid in association with magnetic separator the groundmass from the important presence of inherited phenocrysts such as: biotite, amphibole, feldspars (plagioclase).

Standard point

Extrapolated curve

0 0 1 1,2 1,4 1,6 1,8 2

Absorbance intensity

Potassium concentration (ppm)

Potassium content

Intensity measurement due to analyse sample separate from the groundmass any fraction of inherited mineral having an age not contemporaneous to Whole rock: crushed and sieved at 250-500µm 200 g of this is cleaned and separated using heavy liquid in association with magnetic separator in order to isolate with typical narrow density range of density for the dacitic domes of 2.46-2.51 of groundmass but not only.

Observations

Remaining grain from step (B) was crushed and sieved again at: 63-125µm Presence of mixed grains i.e grains composed with groundmass/plagioclase and/or groundmass biotite, amphibole Groundmass density is to close to density of plagioclase: this could biaised our results For example grain with 2.50 of density could be composed of 2/3 of a 2.45 dense groundmass and a 1/3 of 2.60 dense plagioclase. the dome eruption. It also allows to minimize the potential risk to keep grain a ected by uid circulation.

Results

O cial name

Dealul Cetătii (15CIO01)

The only sample where W.R. and G.M. ages are equal. It is also the only sample in which this is no glomerocryst.

  rock: Cal-akaline to alkalin rock Very stable analytic condition reached by our mass spectrometerGroundmass age = last phase to crystallize during eruption Eruption ageDensity range of groundmass in this study is between 2.40-2.51; felspar (2.55-2.65); Biotite/amphibole (2.8-3.17).

  Grains are then separated by heavy liquid a second time in order to separate the pure G.M. fraction (C), typically (2.46-2.50 of density) from the remaining plagioclasic grains (D).

  Biot.This range of density (2.46-2.51) eliminates from G.M. (in black) most of grain made of pure minerals, particularly inherited phenocrysts present. Mixed grains of Amp-Bte/G.M. are mainly excluded because of high density of mafic mineral. Mixed plg/G.M. grain and peripheric part of some phenocrysts remain To avoid such problems, we proceeded to a double full steps separation Goals: Conclusion The work based on whole rock need to be proscribed in Carpathians volcanism context. It is even necessary to carry out for such samples a double full steps separation due to the presence of an abundant amount of minerals inherited from glomerocryst magmatic clots.

  age simulation from Image analysis of thin section 'Pure' groundmass Age (a i ): 184±5ka Proportion (P i ): 60%; K% (K i ): 3.72 Pure Plagiolase Age : 1132±22 Ma Proportion: 11%; K% (K i ): 0.82% Mixed grains Proportion (P i ): 29% Thin section image (A) is binarized on gray scale range (B1) and pixelated (B2) to simulate a 200 µm size grains Determination of each grain composition from their color (C): Blue (pure G.M. grain), Yellow (pure plagioclase grain), Range of red (Mixed grain) Estimation of age of whole rock and for each grains (D): Age = Σa i *p i *K i / Σp i *K i age, the proportion and K content of each phase present in each grain i obtained age: 320 ka 74% elder than groundmass age Age previously obtained on traditional K/Ar whole rock on the same lava dome (Dealul Cetăţii): 400±160 ka S-E andN domes Known parameters Generally, fractionation occurs during rock crushing with a larger proportion of groundmass than phenocrysts being crushed at lower size than 200 µm. For instance if a third of G.M. is removed during crushing and no Plg is lose, the resulting proportion between G.M. and Plg would induce an age of 405 ka In order to test the whole rock incidence and show ,that, ages obtained on whole rock are biased. An image simulation of grain composition have been done from the thin section of 15CIO01 sample coming from Dealul Cetatii (A) in di erent steps:

  ages from this study (in ka) Time scale (in ka) for ages from previous studies O cial name Rock chemistry in correlation with our age What can explain di erence between our ages and previous whole rock age? New K/Ar ages obtained in this study based on the Un-spiked Cassignol-Gillot technique Analyse of the origin of the biaising of whole-rock age by presence of phenocysts Presence of exces argon: 40 Ar total > 40 Ar * produced by in situ decay of 40 K Inherited argon contamination from older grains incorporatedQuanti cation of the abundance of glomerocrysts on whole-rock ageGlomerocrysts are aggregates of phenocrysts (> 1 mm and up to 5 mm in size,), with cumulate texture containing mainly plagioclases and amphibole and secondarily Fe-Ti oxides, apatite, biotite and zircon.Mineralogical analysesEvaluation of percentage of glomerocrysts in each rock (thin section) dated on both whole-rock (previous studies) and pure separated groundmass (this work)Assuming that the principal reason of di erence of our age with traditional K/Ar dating is due to inherited argon. on traditional K/Ar dating(Whole rock = WR) and dating performed on Zircon by (U-Th)/He method according to the litterature Presence of inherited Ar = presence of phenocryst such as plagioclase, biotite, amphibole (minerals abundantly present in Ciomadùl rock).