EGU2020-1405
https://doi.org/10.5194/egusphere-egu2020-1405
EGU General Assembly 2020
© Author(s) 2020. This work is distributed under
the Creative Commons Attribution 4.0 License.

Multi-stage arc magma evolution recorded by apatite in volcanic rocks

Chetan Nathwani1,2, Matthew Loader2, Jamie Wilkinson1,2, Yannick Buret2, Robert Sievwright2, and Pete Hollings3
Chetan Nathwani et al.
  • 1Department of Earth Science and Engineering, Imperial College London, Exhibition Road, South Kensington Campus, London, SW7 2AZ, UK
  • 2London Centre for Ore Deposits and Exploration (LODE), Department of Earth Sciences, Natural History Museum, Cromwell Road, South Kensington, London, SW7 5BD, UK
  • 3Geology Department, Lakehead University, 955 Oliver Road, Thunder Bay, Ontario, P7B 5EL, Canada

The chemical diversity observed in the rock record of volcanic arcs is determined by a multitude of processes operating between the magma source region and the surface. A fundamental step in producing this variability is fractional crystallisation, assimilation and melting in the lower crust which drives magmas to more evolved and hydrous compositions. During extensive fractionation of hydrous magmas in the lower crust, amphibole (± garnet) is stabilized in the fractionating assemblage and plagioclase is suppressed resulting in melts with elevated Sr, an absence of strong negative Eu anomalies (both elements being compatible in plagioclase), and depleted Y (compatible in amphibole and garnet). The high Sr/Y values that result can be used to provide insights into arc magma evolution, evaluate whether a magmatic system has the potential to form a porphyry-related ore deposit and track crustal thickness. However, this deep fractionation history may be obscured due to differentiation and mixing upon ascent to the shallow crust. Since arc rocks are a product of this multi-stage, polybaric process, unravelling the complete history of magmatic evolution using bulk-rock chemistry alone can be challenging. However, accessory minerals such as apatite, are capable of capturing discrete periods of melt evolution during differentiation [1]. For example, apatite has been shown to record the Sr content of the melt at the time of its crystallization which has been used to reconstruct host rock compositions in provenance studies [2, 3].

Here, we use a novel approach to track the petrogenesis of arc magmas using apatite trace element chemistry in volcanic formations from the Cenozoic arc of Central Chile. These rocks formed during magmatism that culminated in high Sr/Y magmas and porphyry ore deposit formation in the Miocene. We use Sr/Y, Eu/Eu* and Mg in apatite to demonstrate that apatite tracks the multi-stage differentiation of arc magmas. We apply fractional crystallization modelling to show that early crystallizing apatite inherits a high Sr/Y and Eu/Eu* melt chemistry signature that is predetermined by amphibole-dominated fractional crystallization in the lower crust. Our modelling shows that crystallisation of the in-situ host rock mineral assemblage in the shallow crust causes competition for trace elements in the melt that leads to apatite compositions diverging from bulk magma chemistry. Understanding this decoupling behaviour is important for the use of apatite as an indicator of metallogenic fertility in arcs and for interpretation of provenance in detrital studies. We suggest our approach is widely applicable for unravelling the composite evolution of arc magmas and studying magmatic processes conducive to porphyry ore deposit formation.

References

[1] Miles, A.J., Graham, C.M., Hawkesworth, C.J., Gillespie, M.R., and Hinton, R.W., 2013, Evidence for distinct stages of magma history recorded by the compositions of accessory apatite and zircon: Contributions to Mineralogy and Petrology.

[2] Jennings, E.S., Marschall, H.R., Hawkesworth, C.J., and Storey, C.D., 2011, Characterization of magma from inclusions in zircon: Apatite and biotite work well, feldspar less so: Geology.

[3] Bruand, E., Storey, C., and Fowler, M., 2016, An apatite for progress: Inclusions in zircon and titanite constrain petrogenesis and provenance: Geology, v. 44, p. 91–94.

How to cite: Nathwani, C., Loader, M., Wilkinson, J., Buret, Y., Sievwright, R., and Hollings, P.: Multi-stage arc magma evolution recorded by apatite in volcanic rocks, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-1405, https://doi.org/10.5194/egusphere-egu2020-1405, 2019

Comments on the presentation

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Presentation version 1 – uploaded on 28 Apr 2020
  • CC1: Comment on EGU2020-1405, Agustin Cardona, 04 May 2020

    Hi, very nice work, you mention on your paper and presentation that Mg# will be key for appropriatly track deep crust signatures. Is there an specific value or interval you are considering as the limit between the two different stages of apatite chemistry fingerprinting.

    • AC1: Reply to CC1, Chetan Nathwani, 04 May 2020

      Hi Agustin, thanks for your comment. I don't think that apatite Mg should be used with an absolute value to discriminate what can and what can't fingerprint deep crustal magma evolution. There are variables e.g. the timing of apatite crystallisation relative to other phases, the melt temperature, crystallising assemblage etc that will cause apatite Mg to vary in different systems. We suggest apatite Mg should be used as a relative indicator of melt differentiation in a single sample, and between different samples of different SiO2 ranges. This can also be used in addition to textural information. Currently there isn't much research that measures apatite Mg in arc rocks, hopefully with more work we can see how good a proxy of melt differentiation apatite Mg is. 

  • CC2: Comment on EGU2020-1405 - Zoning and Apatite Stability, Nicholas Barber, 04 May 2020

    Good morning, and thank you for providing this excellent contribution to the study of hydrous, high pressure arc magmas! This sounds like a really promising approach. One question I had related to the textures vs. the composition of the apatites. Are the compositions presented in this display bulk compositions of each apatite? I'm curious whether you have any sense of what changes in Sr/Y, Eu/Eu*, and Mg occur across zoned apatites. If they are good records of deep arc magma evolution, would the rims of the crystals be recording that decoupled, shallow evolution you talk about near the end of your display? 

    Another question I had relates to the stability of apatite. I'm not as familiar with igneous apatite as I am with other common minerals (amphibole, plag, cpx, etc.), so this probably something you could set me straight on. How common should apatite be as an accessory in arc systems? I'm thinking of arcs that may not be as thick as the Andes (SE Asia) or more primitive (Tonga or Marianas). 

    The display was excellent, and I look forward to seeing your work on this topic going forward. Thank you again! 

    • AC2: Reply to CC2, Chetan Nathwani, 04 May 2020

      Hi Nicholas - thanks for your questions and kind comments.

      Regarding your first point, we analysed the apatites by laser ablation ICP-MS with a 35 micron spot size. In these volcaninc rocks, generally the crystals were not large enough to permit multiple analyses (i.e. core-rim) on the crystals. However, CL imaging shows there's definitely some zoned crystals around, and I agree, you would expect to see the cores vs rims showing a more deep vs shallow signature respectively. In future work, where crystals are large enough (i.e. in plutonic rocks; if diffusion is slow enough) this would be interesting to pursue.

      From my experience apatite is ubiqutous in arc rocks. However, they were only large and abundant enough for this study in intermediate to felsic samples. I think maybe it would be a challenge to find coarse enough apatites in any mafic volcanic rocks, apatite does saturate later in these systems too. Studies in thinner arcs and more primitive arcs may have to rely on evolved products. But certainly, I think there is scope for this type of study in all arc settings, and am aware of people studying apatites in thinner arcs and more primitive arc rocks.

       

  • CC3: Comment on EGU2020-1405, Rebecca Morris, 07 May 2020

    Hi Chetan,

    I unfortunately could not attend your presentation on Monday, and have a few questions from your 'apatite habits & texture' slide (#7):

    1. To confirm, the large euhedral grain in image A is apatite, and the zircon is included? Was curious as I am more used to seeing apatite followed by zircon saturation in arc magmas, and this is opposite to what I would expect.  

    2. Which of those images shows the 'minor secondary apatite'? I'm curious as to what texture/habit you are considering as 'secondary'.

    3. In image E, is the apatite primary (the one that is surrounded by magnetite)? Curious as to what type of rock Image E is from, I have similar apatite + magnetite textures in some very strange gabbros.

    If it is easier to correspond with me via email, I am at: ramorri@uvic.ca

    Very interesting stuff & thanks for sharing/doing the work!

    Rebecca

    • AC3: Reply to CC3, Chetan Nathwani, 07 May 2020

      Hi Rebecca - many thanks for your questions. Also, there's further information on the petrography available in the Supplementary Material for the paper on the GSA website. 

      1. Yes, this is a zircon crystal as an inclusion within an apatite. I agree, this is less usual as typically the opposite is expected, and in this sample (La Copa Rhyolite), there are quite a few occurrences of it. I am not quite sure of why this occurs, there could be a few reasons. Perhaps the melt was (for whatever reason) enriched in zirconium or depleted in phosphorous causing either phase to saturate earlier/later than expected. Could these zircon inclusions be a form of antecryst? Perhaps during protracted magma storage in the shallow crust, several magma recharge episodes caused zircon or apatite saturation to occur several times. 

      2. Image D is the secondary apatite, this is associated with clays that are replacing feldspars. These secondary apatites typically form a series of contiguous subhedral crystals assoicated with clays. You also see secondary apatite associated with clays in the propylitic zone of porphyry deposits.

      3. These apatites are primary magmatic (rock is an andesite). Apatite associated with magnetite in igneous rocks seems to be really common from my experience, in fact, usually if I am looking for apatite under the microscope, I look for the magnetites. Not sure exactly why that is but I think it might be that magnetite saturation causes phosphorous concentrations to increase to the point where apatite saturation occurs.

      Hope this helps. Feel free to email at cln14@ic.ac.uk if you have any further questions.

      Chetan

      • CC4: Reply to AC3, Rebecca Morris, 08 May 2020

        Hi Chetan,

        Thank you for the info, and good to know about the Supplementary Materials in the paper - I will definitely have a look!

        Cheers, Rebecca