Anand, M.

Franchi, I. A.

Potts, N. J.

Tartèse, R.

van Westrenen, W.

English

Open Research Online (The Open University)

Open Research OnlineThe Open University’s repository of research publicationsand other research outputsApatite-Melt Volatile Partitioning Under LunarConditionsConference or Workshop ItemHow to cite:Potts, N. J.; van Westrenen, W.; Tarte`se, R.; Franchi, I. A. and Anand, M. (2015). Apatite-Melt VolatilePartitioning Under Lunar Conditions. In: 46th Lunar and Planetary Science Conference, 16-20 Mar 2015, TheWoodlands, Texas, USA.For guidance on citations see FAQs.c© [not recorded]Version: Version of RecordLink(s) to article on publisher’s website:http://www.hou.usra.edu/meetings/lpsc2015/pdf/1372.pdfCopyright and Moral Rights for the articles on this site are retained by the individual authors and/or other copyrightowners. For more information on Open Research Online’s data policy on reuse of materials please consult the policiespage.oro.open.ac.ukAPATITE-MELT VOLATILE PARTITIONING UNDER LUNAR CONDITIONS.  N. J. Potts1,2*, W. van Westrenen2, R. Tartèse1, I. A. Franchi1, M. Anand1,3.  1Department of Physical Sciences, The Open University, Mil-ton Keynes, UK. (*nicola.potts@open.ac.uk). 2Faculty of Earth and Life Sciences, VU University Amsterdam, Am-sterdam, NL. 3Department of Earth Sciences, The Natural History Museum, London, UK.    Introduction: The volatile component of apatite [Ca5(PO4)3(F,Cl,OH)] makes it a popular choice for studying the abundance and distribution of indigenous lunar volatiles [e.g. 1-5]. The thermodynamic behavior of volatile partitioning into apatite is, however, poorly constrained. Limited experimental studies of apa-tite/melt partitioning have focused on terrestrial sys-tems [6], chlorine-brine systems [7], or Martian condi-tions [8]. Recent studies applicable to basaltic magmas have varied parameters in each experiment [9] making it difficult to extract individual effects of temperature, pressure, and melt composition. Modeling of apa-tite/melt partitioning of volatiles in crystallizing melts [10] demonstrated the importance of quantifying abun-dances of all of the main volatiles, as they are major structural components in apatite. At present, experi-mentally-derived partition coefficients remain unde-termined for lunar compositions and conditions. In this study, fluorapatite, depleted in OH and Cl, was grown in equilibrium with silicate melt to provide a baseline for further experiments. These experiments were conducted with bulk compositions similar to that of late-stage mesostasis regions in Apollo lunar basalt samples [11] in which apatite is typically found. Once partitioning behavior in a nominally ‘dry’ fluorapatite system is constrained additional experiments will add in varying amounts of OH and Cl. The aim is to enable accurate and precise tracking of the effects of volatiles on partitioning. Once these effects are known robust back-calculations of volatile contents in lunar melts, from which apatite crystallized, could be attempted. Methods: Experimental methods. Experiments were conducted in an end-loaded piston cylinder at VU University Amsterdam using a talc-pyrex assembly. For each experiment a graphite bucket was filled with starting material and closed with a graphite lid. The bucket was inserted into a triple crimped and welded Pt capsule. The double-capsule was then placed in an alumina inner tube. A W97Re3-W75Re25 thermocouple (T/C) was used to measure and control T throughout the experiment. The sample was placed in the hotspot of the assembly, 2 mm from the T/C tip end, shown to be 10 C hotter than the T/C reading [12]. Runs were performed using a “hot piston-in” technique to achieve the desired P-T conditions. Each mixture was heated to 1550 C (super liquidus) at 100 C/minute, dwelled for 10 minutes, and subsequently cooled to final experi-mental temperatures at 50 C/hr.  During the heating cycle P was kept low, ~300 psi, until the final T was reached and then increased to ~715 psi (equivalent to 1 GPa). Experiments were left to dwell for 21 hrs at final T. Runs were quenched by turning off the power. Experiments were conducted for 7, 18, 46, and 72 hrs to assess equilibrium. Figure 1: Back-scattered electron (BSE) image of experi-mental sample with representative apatites.   Sample imaging. Samples were imaged with The Open University FEI Quanta 3D dual beam secondary electron microscope (SEM). BSE images were ac-quired, using an electron beam with a 20 kV accelerat-ing voltage and a 0.6 nA beam current. NanoSIMS methods. Samples were coated with 30 nm of gold and were transferred into the Cameca Na-noSIMS 50L, at The Open University. During analysis a 20 × 20 μm area was pre-sputtered using a ~700 pA beam for 1 minute. Apatite and glass targets were iden-tified in these areas, using real time imaging of Si and F secondary ions. Once they were identified an 8 × 8 μm area was further rastered using a 110 pA beam for 1 minute. For analysis, secondary ions of 16O 1H, 18O, 28Si, 19F, 31P, and 37Cl were collected simultaneously from the central area of a 4 × 4 μm area using a 30 pA beam current for 3 minutes. F, OH and Cl abundances were calibrated using 19F/18O, 16O1H/18O and 37Cl/18O ratios measured on apatite [13] and glass [14] stand-ards with known volatile abundances. Results: Temperatures in this study are higher than those in the real lunar systems due to experimental restrictions. To perform ‘dry’ experiments at natural temperatures (~900 C) prohibitively long run times would be required (>>72 hrs). In our run products vol-1372.pdf46th Lunar and Planetary Science Conference (2015)atile loss is observed in runs of 48 hrs and longer. Ex-periments at 1500 C did not grow apatite (or any other solid phase) but instead displayed silicate liquid im-miscibility. Experiments run at 1300 C produced small (<3 μm) apatites with a small melt fraction (close to the solidus of the dry system) and were not measur-able. All apatites in the measured samples were euhe-dral and varied in size from ~5 - 20 μm (Figure 1). Measurements were made on a variety of apatite sizes across the samples. Standard deviations on volatile abundances in apatite and glass analyses were low in-dicating homogenous, unzoned phases. Figure 2: F and H2O abundances of experimental apatites and glasses. Cl abundance ~0.03 wt.% for all experiments except L2a OH 1450 with 0.19 wt.% in apatite and 0.07 wt.% glass. Error bars smaller than points.   Initial series L2a experiments have intermediate H2O abundances in apatites (~0.11 – 0.2 wt.%) and melt (~0.2-0.5 wt.%; Figure 2). Series L3a experiments were prepared and then stored at 110 C for a mini-mum of 2 days prior to the experiment which reduced the apatite H2O content to ~0.02 – 0.1 wt.% and melt H2O content to ~0.1 - 0.3 wt.% (Figure 2). For the se-ries L2a_OH experiments, 3 ml of DI water was added to the L2a powder, which was stored at 110 C for 20 minutes then prepared. This translated into an in-creased H2O content in apatite at ~0.3 - 0.5 wt.% and an increase in the melt OH content to 0.3 – 0.6 wt.%. As discussed in previous studies [9, 10] the volatile content of apatite is a function of the availability of all three volatiles as F + Cl + OH = 1 (apfu) in apatite. As such, using the exchange coefficient (KD), as defined in Equation 1, is required.         𝐾𝐷𝐹−𝐻2𝑂 =  (𝑋𝐴𝑝𝐹  /  𝑋𝑚𝑒𝑙𝑡𝐹 )(𝑋𝐴𝑝𝐻2𝑂 / 𝑋𝑚𝑒𝑙𝑡𝐻2𝑂  )                      (Eq.1) Similar to the experiments of [9] there appear to be no strong relationships between KD and intrinsic varia-bles (T, composition, etc.) in our experiments. Our experiments, and those of [9], appear to suggest a rela-tion between KDCl-H2O and KDF-H2O, however, the lack of overlap in P-T-composition between the two experi-mental sets makes this difficult to constrain further (Figure 3). No trends for H2O-Cl/F-Cl or H2O-F/Cl-F are seen with experiments from this study or [9]. Large variations in KD values are seen in Figure 3, from both experiments here and those of [9].  Variations in KD values within both data sets are similar (factor of 10) even though experimental conditions differ. These variations are, therefore, controlled by experimental conditions. Figure 3: KDAp-melt for F-H2O and Cl-H2O plotted for ex-periments in this study and [9].Error bars are propagated errors and generally smaller than points. No error bars giv-en for data from [9].  Ongoing experiments will increase the H2O con-tents in the system to provide insight into whether these trends and variations continue. Acknowledgements: This work was funded by an STFC Studentship awarded to NJP, research grant to MA (Grant no. ST/I001298/1), and a NWO Vici grant to WvW. Francis McCubbin is thanked for providing apatite standards. Thanks also to Edgar Streenstra for lab assistance.  References: [1] Boyce J.W. et al. (2010) Nature, 466, 466-486. [2] McCubbin F.M. et al. (2011) GCA, 75, 5073-5093. [3] Greenwood, J.P. et al. (2011) Nature Geosci., 4, 79-82.  [4] Barnes J.J. et al. (2013) Chem. Geol., 337-338, 48-55. [5] Tartèse R. et al. (2013) GCA, 122, 58-74. [6] Doherty A.L. et al. (2014) Chem. Geol., 384, 94-109 [7] Mathez, E.A. and Webster, J.D. (2005) GCA, 69, 1275-1286.  [8] Vander Kaaden, K.E. et al., (2012) LPSC XLIII, #1247. [9] McCubbin, F.M. et al., (in review) Am. Min. [10] Boyce J.W. et al. (2014) Science, 344, 400-402  [11] Potts, N.J. et al. (2014) LPSC XLIV, #1946. [12] Watson, E. et al. (2002) EPSL, 51, 322-335. [13] McCubbin F.M. et al. (2012) Geology, 40, 683-686. [14] Jochum, K.P. et al. (2007) GGR, 29, 285-302.  1372.pdf46th Lunar and Planetary Science Conference (2015)

Apatite-Melt Volatile Partitioning Under Lunar Conditions

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