Abstract Details
(2020) The Individuality of Melting Regimes in Small Basaltic Systems: Variations in Space and Time
McGee L, Smith I & Morgado E
https://doi.org/10.46427/gold2020.1761
The author has not provided any additional details.
05d: Room 2, Friday 26th June 05:45 - 05:48
Lucy E McGee
View all 5 abstracts at Goldschmidt2020
Ian E M Smith View all 4 abstracts at Goldschmidt2020 View abstracts at 2 conferences in series
Eduardo Morgado View abstracts at 4 conferences in series
Ian E M Smith View all 4 abstracts at Goldschmidt2020 View abstracts at 2 conferences in series
Eduardo Morgado View abstracts at 4 conferences in series
Listed below are questions that have been submitted by the community that the author will try and cover in their presentation. To submit a question, ensure you are signed in to the website. Authors or session conveners approve questions before they are displayed here.
Submitted by Georg F. Zellmer on Monday 22nd June 04:37
Hi Lucy, great talk, thanks. I like the concept of the complexity threshhold, as it really provides us with a way of addressing the important transition between monogenetic and polygenetic volcanism. Given the complexity of phenocrystic crystal cargo that you mentioned, and also the effects of assimilation (how did this happen? - partial melting of crust/granite or disagregation of crystals into the ascending mantle melts), would it be worthwhile looking at glass/groundmass geochemical variations rather than whole-rock variations? Like Ruadhan Magee did for Etna in this session? I think it may well be worthwhile! Also, I am wondering how much influence a upper asthenospheric low-degree partial melt, postulated by Sebastien Pilet in session 5f, may have in providing a geochemical imprint on individual edifices in distributed monogenetic volcanic fields? Any comments on that? Thanks! Georg
Thanks Georg! Absolutely the crystal cargo gets in the way, as we can see from the numerous studies showing that these can come from a variety of storage zones, even in so-called 'simple' systems. That's why tephras are so valuable - the small cones from the Carran Los Venados field which I show are wonderfully glassy, and a good estimate of 'melt' although I agree it would be better to pick groundmass as I see Ruadhan shows in his talk in this session. Auckland lavas are often aphyric too so that helps, but they certainly contain some crystals. I will check out Sebastien Pilet's talk (it's on my list!) and edit this answer to address your second question!
Hi Lucy, great talk, thanks. I like the concept of the complexity threshhold, as it really provides us with a way of addressing the important transition between monogenetic and polygenetic volcanism. Given the complexity of phenocrystic crystal cargo that you mentioned, and also the effects of assimilation (how did this happen? - partial melting of crust/granite or disagregation of crystals into the ascending mantle melts), would it be worthwhile looking at glass/groundmass geochemical variations rather than whole-rock variations? Like Ruadhan Magee did for Etna in this session? I think it may well be worthwhile! Also, I am wondering how much influence a upper asthenospheric low-degree partial melt, postulated by Sebastien Pilet in session 5f, may have in providing a geochemical imprint on individual edifices in distributed monogenetic volcanic fields? Any comments on that? Thanks! Georg
Thanks Georg! Absolutely the crystal cargo gets in the way, as we can see from the numerous studies showing that these can come from a variety of storage zones, even in so-called 'simple' systems. That's why tephras are so valuable - the small cones from the Carran Los Venados field which I show are wonderfully glassy, and a good estimate of 'melt' although I agree it would be better to pick groundmass as I see Ruadhan shows in his talk in this session. Auckland lavas are often aphyric too so that helps, but they certainly contain some crystals. I will check out Sebastien Pilet's talk (it's on my list!) and edit this answer to address your second question!
Submitted by Maurizio Petrelli on Monday 22nd June 16:20
Hi Lucy, thanks for accepting our invitation. It is a capturing talk. I was involved in the study by Jankovics et al. (2019) you cited, and I was intrigued by the complexity of the system unravelling many concurrent dynamics acting at different degrees in space and time. Could you start providing some answers to the questions you raised during your talk? Particularly, any timescale estimation of storages at different crustal levels or magma transfer rates? How do these timescales affect the chemical variations within the system?
Thank you Maurizio! Yes Eva Jankovics' paper really made me realise that the petrology has a huge part to play in looking at the fine detail of these small basaltic systems. Going back to the model of 'transient storage' from my colleague Eduardo Morgado's work - I think examining volcanic sequences where we have a good control on the stratigraphy (such as Mirai Takebe and Jane Scarrow have done in this session) is really important in this case to pinpoint at what stage true storage may have begun (if it did at all). I work a lot with U-series isotopes and these are good at giving relative timescales to some of these aspects, for example the presence of large Ra-excess in magmas suggesting processes occurred within a few thousand years (and sometimes this varies with volcanic stratigraphy, as we saw in a recent study from Southern Chile where we used this approach, https://doi.org/10.1007/s00410-019-1643-x) but really crystal scale studies looking at what's in equilibrium and when crystallisation occurred (such as in Charline Lourmand's excellent talk from this session) are the way to address this. Really important questions! I look forward to discussing this more if people are interested. Lucy
Hi Lucy, thanks for accepting our invitation. It is a capturing talk. I was involved in the study by Jankovics et al. (2019) you cited, and I was intrigued by the complexity of the system unravelling many concurrent dynamics acting at different degrees in space and time. Could you start providing some answers to the questions you raised during your talk? Particularly, any timescale estimation of storages at different crustal levels or magma transfer rates? How do these timescales affect the chemical variations within the system?
Thank you Maurizio! Yes Eva Jankovics' paper really made me realise that the petrology has a huge part to play in looking at the fine detail of these small basaltic systems. Going back to the model of 'transient storage' from my colleague Eduardo Morgado's work - I think examining volcanic sequences where we have a good control on the stratigraphy (such as Mirai Takebe and Jane Scarrow have done in this session) is really important in this case to pinpoint at what stage true storage may have begun (if it did at all). I work a lot with U-series isotopes and these are good at giving relative timescales to some of these aspects, for example the presence of large Ra-excess in magmas suggesting processes occurred within a few thousand years (and sometimes this varies with volcanic stratigraphy, as we saw in a recent study from Southern Chile where we used this approach, https://doi.org/10.1007/s00410-019-1643-x) but really crystal scale studies looking at what's in equilibrium and when crystallisation occurred (such as in Charline Lourmand's excellent talk from this session) are the way to address this. Really important questions! I look forward to discussing this more if people are interested. Lucy
Submitted by Michael Rowe on Tuesday 23rd June 00:38
I like your mention of a "complexity threshold" but I'm wondering how you could quantify it in regard to the different settings as you've proposed?
Hi Mike - as you can see this is rather a theoretical concept at the moment, I am currently working on putting some numbers on these ideas. In Auckland we had the advantage of some excellent volume calculations provided by Kereszturi et al (2013). For the Carran Los Venados volcanic field some estimates exist from a previous study but not for individual cones. However, as you suggest, this is in a very different setting - I am pretty sure that Mirador's large extrusion rate (1 month for all of that!) and potentially larger melting event was tectonically assisted due to its position on an arc-scale fault system. So these things must be taken into account before a quantitative model can be produced: possibly a model needs to include a 'simple scenario' where no external factors are considered (an Auckland-style?) and layers of complexity induced by external factors such as tectonics. Best wishes, Lucy
I like your mention of a "complexity threshold" but I'm wondering how you could quantify it in regard to the different settings as you've proposed?
Hi Mike - as you can see this is rather a theoretical concept at the moment, I am currently working on putting some numbers on these ideas. In Auckland we had the advantage of some excellent volume calculations provided by Kereszturi et al (2013). For the Carran Los Venados volcanic field some estimates exist from a previous study but not for individual cones. However, as you suggest, this is in a very different setting - I am pretty sure that Mirador's large extrusion rate (1 month for all of that!) and potentially larger melting event was tectonically assisted due to its position on an arc-scale fault system. So these things must be taken into account before a quantitative model can be produced: possibly a model needs to include a 'simple scenario' where no external factors are considered (an Auckland-style?) and layers of complexity induced by external factors such as tectonics. Best wishes, Lucy
Submitted by Emily Johnson on Wednesday 24th June 20:52
Hi Lucy, excellent talk! To also ask about the complexity threshold: I really like this idea and it seems to make a lot of sense for longer-lived monogenetic eruptions where shallow storage occurs. Do you have any sense of the timescales of storage required to homogenize the melts? And would this thus be more likely at long-lived monogenetic volcanoes, like Jorullo and Paricutin, that erupted for 9+ years?
Hello Emily and thank you! I think this is definitely more applicable to those longer-lived monogenetic eruptions, and it seemed to me that Paricutin and Jorullo were potentially examples of this - although I know there have been various theories which have been recently updated and revised to explain the geochemical trends. It's a really good question about the timescale of storage (also asked by Maurizio) - this sounds like a job for crystal-scale zoning studies! My feeling is that in these very small magma batches the tephra contains the initial complexity and the lavas record the beginnings of storage (although this is certainly the 'whole rock chemistry' view and crystal experts may have other ideas). Seeing as tephras in these kinds of eruptions can erupt over very short timescales this would suggest the construction of a mini-mush could be very rapid? I would be really interested to hear what people think about this! Looking forward to the discussion, Lucy
Hi Lucy, excellent talk! To also ask about the complexity threshold: I really like this idea and it seems to make a lot of sense for longer-lived monogenetic eruptions where shallow storage occurs. Do you have any sense of the timescales of storage required to homogenize the melts? And would this thus be more likely at long-lived monogenetic volcanoes, like Jorullo and Paricutin, that erupted for 9+ years?
Hello Emily and thank you! I think this is definitely more applicable to those longer-lived monogenetic eruptions, and it seemed to me that Paricutin and Jorullo were potentially examples of this - although I know there have been various theories which have been recently updated and revised to explain the geochemical trends. It's a really good question about the timescale of storage (also asked by Maurizio) - this sounds like a job for crystal-scale zoning studies! My feeling is that in these very small magma batches the tephra contains the initial complexity and the lavas record the beginnings of storage (although this is certainly the 'whole rock chemistry' view and crystal experts may have other ideas). Seeing as tephras in these kinds of eruptions can erupt over very short timescales this would suggest the construction of a mini-mush could be very rapid? I would be really interested to hear what people think about this! Looking forward to the discussion, Lucy
Submitted by Emily Johnson on Wednesday 24th June 20:54
One other question: I'm interested in the shift you say at Carran-Los Venados to a more fluid-dominated melting regime: do you have a theory for why this may have happened (any tectonic link)? And what is the timescale over which the shift happened?
Thanks for being interested in the talk to ask two questions! :) We have been thinking about this, and we know that the basal lavas are thought to be interglacial and the monogenetic field is very recent (to support that I analysed Ra isotopes on some of the basal lava samples and they are in equilibrium, which means they are older than 8000 years BP). We did wonder if this change from drier to wetter source melting could be due to glacial rebound - there was a study invoking that process from Southern Chile (doi:10.1130/G37504.1 Rawson et al. 2015) and also a great study by Sims et al 2013 from Iceland. But there is obviously active tectonism in this area due to subduction and arc-parallel faults so these could also have an effect - the tectonic link to magmatism in this area was investigated in a study of the nearby Puyuehue volcano and the CLV field by Bucchi et al 2015 dx.doi.org/10.1016/j.jvolgeores.2015.10.013. Thanks!
One other question: I'm interested in the shift you say at Carran-Los Venados to a more fluid-dominated melting regime: do you have a theory for why this may have happened (any tectonic link)? And what is the timescale over which the shift happened?
Thanks for being interested in the talk to ask two questions! :) We have been thinking about this, and we know that the basal lavas are thought to be interglacial and the monogenetic field is very recent (to support that I analysed Ra isotopes on some of the basal lava samples and they are in equilibrium, which means they are older than 8000 years BP). We did wonder if this change from drier to wetter source melting could be due to glacial rebound - there was a study invoking that process from Southern Chile (doi:10.1130/G37504.1 Rawson et al. 2015) and also a great study by Sims et al 2013 from Iceland. But there is obviously active tectonism in this area due to subduction and arc-parallel faults so these could also have an effect - the tectonic link to magmatism in this area was investigated in a study of the nearby Puyuehue volcano and the CLV field by Bucchi et al 2015 dx.doi.org/10.1016/j.jvolgeores.2015.10.013. Thanks!
Submitted by Beñat Oliveira on Friday 26th June 02:38
Truly inspirational talk Lucy! Are you planning to do any modelling? If so, of what kind?
Wow thanks Beñat! I certainly do want to try some kind of modelling - see my answer above to Mike Rowe. I actually just downloaded your recent JPet paper as the topic looks really relevant to this work!
Truly inspirational talk Lucy! Are you planning to do any modelling? If so, of what kind?
Wow thanks Beñat! I certainly do want to try some kind of modelling - see my answer above to Mike Rowe. I actually just downloaded your recent JPet paper as the topic looks really relevant to this work!
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