Preliminary UVLAMP determinations of argon partition coefficients for olivine and clinopyroxene grown from silicate melts

Chemical Geology 147 Ž1998. 185–200

Preliminary UVLAMP determinations of argon partition coefficients for olivine and clinopyroxene grown from silicate melts Richard A. Brooker

a,)

, J-A. Wartho b, Michael R. Carroll a , Simon P. Kelley b, David S. Draper a

a

b

Department of Geology, Wills Memorial Building, Bristol UniÕersity, Bristol BS8 1RJ, UK Department of Earth Sciences, The Open UniÕersity, Walton Hall, Milton Keynes MK7 6AA, UK

Abstract An ultra-violet laser ablation microprobe ŽUVLAMP. has been applied for the first time to investigate argon partition coefficients for olivines and clinopyroxenes grown from silicate melts at 1 bar argon pressure. These preliminary measurements yield crystalrmelt partition coefficients ranging from 0.138 Ž"0.01. to 0.013 Ž"0.003. for olivine and 0.589 Ž"0.003. to 0.0016 Ž"0.0005. for clinopyroxene. The higher values may indicate sub-microscopic melt inclusions, or some other heterogeneous distributions of ‘non-equilibrium’ argon in the crystals. The lower values are probably more representative of true partition coefficients and fall at least an order of magnitude below the previously reported experimental data. The possibility of anomalous, high argon contents for crystals in previous studies is discussed in terms of surface adsorption, ‘trapped’ argon and early partial melting. q 1998 Elsevier Science B.V. All rights reserved. Keywords: Argon; Olivine; Clinopyroxene; Partitioning; Ultra-violet laser ablation microprobe

1. Introduction The last five to ten years have seen an explosion of studies of noble gas isotope systematics and abundances in volcanic rocks. Due to their chemical inertness and isotopic connections to K and U decay schemes, noble gases have proven particularly useful for investigating the geochemical history of magma source regions in the mantle and the evolution of the atmosphere and crust ŽDymond and Hogan, 1978; Kurz et al., 1982a,b; Staudacher and Allegre, 1982; ` ) Corresponding author. Tel.: q44-117-9289000, Ext. 4789 ŽB ro o k e r.; F a x : q 4 4 -1 1 7 -9 2 5 3 3 8 5 ; E -m a il: [email protected]

O’Nions and Oxburgh, 1983, 1988; Sarda et al., 1985, 1988; Staudacher et al., 1986, 1989; Allegre ` et al., 1986; Ozima and Zahnle, 1993.. While the isotope systematics Že.g. 4 Her 3 He, 40Arr 36Ar, various Xe isotopes. have provided much new information, questions still remain regarding the large variations observed in noble gas abundances and abundance ratios Že.g., HerAr, ArrXe.. In particular, the relative influences of different magma source regions, melting processes, crystal fractionation, and shallowversus deep-level melt degassing are poorly understood. Such questions have implications for understanding the degassing history of the earth, geochemical differences between the upper and lower mantle, and the degree of communication between these

0009-2541r98r$19.00 q 1998 Elsevier Science B.V. All rights reserved. PII: S 0 0 0 9 - 2 5 4 1 Ž 9 7 . 0 0 1 8 1 - 2

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R.A. Brooker et al.r Chemical Geology 147 (1998) 185–200

reservoirs and surface environments Že.g., mantle plumes or recycling of material by subduction: Kaneoka and Takaoka, 1985.. Transport of noble gases in mantle-derived magmas appears to be the major non-catastrophic mechanism for planetary degassing. As magmas rise toward the surface, dissolved gases may exsolve into bubbles and ultimately escape into the atmosphere. Gases may also be retained in the mantle, dissolved in magma or glass Žquenched melt., trapped in bubbles, or contained in crystalline phases. In order to explain the observed variations in atmospheric concentrations it is critical that we understand how igneous processes such as partial melting, magma degassing, and crystal fractionation affect volatile abundances. Although the solubility of noble gases in melt is well characterised, their abundances in magmas are difficult to interpret due to conflicting crystal solubility data and the resulting uncertainty in the equilibrium partitioning behaviour between crystals, melts, and vapours. Despite a lack of reliable data, models have been developed which predict how noble gas contents of mantle-derived magmas can be expected to vary with degree of melting and mantle noble gas concentration Že.g., Zhang and Zindler, 1989; McKenzie and O’Nions, 1991.. Such models are highly dependent on basic assumptions regarding the relative compatibility of noble gases during igneous processes and require accurate values for crystal-melt partitioning. Limited data exist to constrain crystal-melt partition coefficients of noble gases Ž Di s partition coefficient of gas i, expressed as wweight concentration in crystalxrwweight concentration in meltx.. Studies of experimental ŽHiyagon and Ozima, 1982, 1986; Broadhurst et al., 1990, 1992; Shibata et al., 1994. and natural samples ŽBatiza et al., 1979; Kurz et al., 1982a; Kaneoka et al., 1983; Marty and Lussiez, 1993; Valbracht et al., 1994. suggest a wide range of possible behaviours, from highly incompatible Ž D < 1., to compatible Ž D ) 1., with some data indicating an increase in compatibility with increasing size of the noble gas atom ŽHiyagon and Ozima, 1986; Broadhurst et al., 1992.. It is possible that many of the observed variations in partition coefficients are related to analytical problems, as quantification of noble gas at the observed concentrations is notoriously difficult.

UV laser extraction of noble gases can provide precise ‘in situ’ measurements of noble gas abundances ŽNe–Xe. in crystals and melts at spatial resolutions of tens of microns. As a result, noble gas partition coefficients can be directly measured for crystals grown from noble gas-bearing melts. It is theoretically possible to measure the noble gas partitioning at the low concentrations appropriate to natural samples Žsub-ppb.. The combination of these analytical and experimental techniques can circumvent many difficulties inherent to bulk analysis methods used in previous studies, such as incomplete separation of glass from crystals ŽHiyagon and Ozima, 1986. and possible surface adsorption of gases Žatmospheric or experimental. on fine-grained samples Že.g., Broadhurst et al., 1990, 1992.. The main objective of the present study was to test the practical application of the UVLAMP in determining argon partition coefficients for low argon concentrations in experimentally grown crystals.

2. Experimental and analytical techniques 2.1. Starting materials Glasses of appropriate compositions were prepared by decarbonating and melting mixtures of reagent grade oxide and carbonate materials in platinum crucibles Žpre-saturated with Fe for the Febearing experiments.. Electron microprobe analyses of the resulting glasses are presented in Table 1. ES-NT23 is a high-magnesia basalt composition ŽElthon and Scarfe, 1984. melted at 1 bar in a gas Table 1 Electron microprobe analyses of starting glasses Žcompositions in wt%. ES-NT23 BDHB-2Di BDHB-An7 OLDAB DAB SiO 2 TiO 2 Al 2 O 3 FeO tot MgO CaO Na 2 O K 2O Total

45.91 0.95 13.33 9.68 16.04 10.18 1.24 0.61 97.94

54.60 – 11.21 0.19 13.97 18.87 0.10 0.01 98.95

47.36 – 19.60 0.22 14.18 17.10 0.11 0.02 98.59

52.94 0.65 13.57 0.19 12.58 12.13 4.34 2.31 98.71

53.79 0.72 14.83 0.13 8.01 13.17 4.77 2.66 98.71

R.A. Brooker et al.r Chemical Geology 147 (1998) 185–200

mixing furnace with an fO 2 equivalent to the Ni–NiO buffer. BDHB-2Di and BDHB-7An are compositions in the system CaO–MgO–Al 2 O 3 –SiO 2 , claimed by Broadhurst et al. Ž1990. to be in equilibrium with diopside and anorthite at 1 bar. However, BDHB-7An plots in the forsterite field of Presnall et al. Ž1978, see their fig. 3. and we observe forsterite as the liquidus phase for this composition. Composition DAB is a synthetic Fe-free alkali basalt with clinopyroxene as the liquidus phase. Addition of 10 wt% forsterite to DAB produced the composition OLDAB which crystallises olivine on the liquidus. 2.2. Experimental procedures and sample preparation Fragments of glass starting materials Ža few mm in size. were suspended by platinum loops ŽFe presaturated for ES-NT23. in the hot spot of a vertical tube furnace with high purity argon Ž99.98%. flowing at a rate of 10 cm3rmin. A slight positive back-pressure was achieved by bubbling the gas through water on the outflow to maintain an argon pressure of approximately 1 bar. The temperature gradient within the 5 cm working zone of the furnace is "28C and was monitored by a Pt–PtrRh 10 thermocouple adjacent to the samples. Samples were quenched by dropping into cold water. Samples were initially held at a super-liquidus temperature Ž14508C for ES-NT23 and OLDAB, 13208C for other compositions. for ; 6 hrs. This allows diffusional equilibrium to be established for the ; 3–4 mm beads of melt Žassuming DAr of 7 = 10y5 cm2 sy1 ; Lux, 1987.. The temperature was then stepped down to allow crystal growth. Carefully controlled temperature paths are required to grow homogeneous equant crystals which are large enough for the UVLAMP technique but also remain in equilibrium with the melt. In practice, crystals must be several hundreds of microns in size to accommodate ) 50 mm square ablation pits, which are required to provide measurable quantities of argon. This produces some conflict, with small crystals equilibrating rapidly, but larger crystals allowing repeat analyses to determine reproducibility and homogeneity. The ES-NT23 melt was equilibrated in 1 bar Ar for 6 h at 14508C, the temperature was then dropped to 58C below the liquidus temperature of 13508C

187

Žsee Elthon and Scarfe, 1984. in a single step and left for 12 h. For the next 13 h the temperature was decreased by 18C at hourly intervals, then left for 12 h and decreased by 18Crh for a further 12 h. Finally the sample was left at 13208C for 12 h. The resulting crystals were euhedral in outline with a width of ; 200 mm and lengths of 500–1200 mm, although tubes of melt were present along portions of the central axis. Inclusions of melt in these crystals were particularly easy to identify as they are considerably darker than the crystal. Sample OLDAB was run concurrently with ES-NT23 and produced equant olivine crystals 200–300 mm in size. BDHB-2Di, BDHB-7An and DAB were held at 13208C Žsuper-liquidus. for 6 h and then cooled by 28C at hourly intervals for 10 h, left for 10 h and stepped down at 28Crh to 12858C and left for a further 6 h. Sample BDHB-7An contained almost equant euhedral crystals up to 300 = 500 mm and BDHB-2Di contained a few small crystals and one large crystal of 800 = 1800 mm. These samples were removed from the furnace at this stage but DAB, being crystal-free, was returned to the furnace at 12858C and cooled rapidly to 12508C Žthe anticipated liquidus. and then cooled by 58C each hour to 12008C. Possibly as a result of this variable cooling history, the DAB glass contained two populations of crystal morphologies: Ž1. bundles of elongated blades individually 500 mm long and 10–200 mm wide, forming a mass over 1000 mm wide, and Ž2. some elongated ‘feathery’ needles ) 1000 mm long, ; 100 mm wide, with large amounts of included melt Žthe latter crystals were not analysed in this study.. Selected fragments of quenched samples containing both crystal and melt phase were mounted in dental resin and ; 1 mm thick doubly polished sections were prepared. The samples were then analysed by electron microprobe. Suitable samples were thinned further to expose crystals on both sides of the polished section, providing a direct ablation path through the crystal. Prior to UVLAMP analysis crystals were carefully examined by microscope and photographed to help identify areas which appeared to be free of major inclusions. The surface of each sample was examined by backscattered electron ŽBSE. imaging during electron microprobe analysis. Inclusions identified at the crystal surface by the electron probe were also visible by optical mi-

188

R.A. Brooker et al.r Chemical Geology 147 (1998) 185–200

croscopy and were generally ) 10 mm in size. However, this cannot exclude the possibility that sub-microscopic melt or fluid inclusions are present. Following optical and BSE examination, the samples were ultrasonically cleaned in acetone, ethanol, methanol and finally deionised water before being dried with a heat lamp, then loaded in the ultra-high vacuum Ž- 4 = 10y1 0 mbar. UVLAMP laser port chamber. 2.3. Laser ablation The UVLAMP uses a 10 Hz quadrupled Nd–YAG laser Ž5–10 mm diameter spot size. with a wavelength of 266 nm and 10 ns pulses. A Marzhauser ¨ ¨ MAC computerised X–Y stage, attached to a customised Leica Metallux 3 microscope was used to control the raster speed Ž20 mm sy1 . and the size of the laser pit. Laser ablation times of 3–5 min were employed. Although the power delivered is sufficient to ablate the sample, heating adjacent to the laser pit is insignificant ŽKelley et al., 1994.. The extracted gas was cleaned using two SAES Ž84% Zr–16% Al. AP-10 getters prior to analysis by a high-sensitivity MAP 215-50 noble gas mass spectrometer. The 40Ar extraction line blanks for a five minutes analysis were typically 1.9 = 10y1 2 STP cm3. Analyses were corrected for 40Ar and 36Ar measured blank values. Further details regarding the mass spectrometer and the UV laser technique can be found in Kelley et al. Ž1994. and Arnaud and Kelley Ž1997.. Various ablation techniques were investigated including a static beam, 50–150 mm squares and various elongated zig-zag or line rasters, used to accommodate lath shaped crystals. Examples of the resultant pits for several different techniques are shown in Fig. 1. Pit depths are a function of the laser power setting, the speed of the sample stage, the number of repeat scans and the susceptibility of the sample to ablation. For example, at a power setting of 636 V, the laser ablated ; 13 mmrlayer and ; 20 mmrlayer for 100 mm square rasters in the 2Di crystals and melt, respectively. For other compositions the differences in ablation rate appeared insignificant. It was generally necessary to ablate relatively large pits to extract measurable quantities of argon from the crystals; for some samples several small pits were used to accumulate sufficient argon

Fig. 1. Secondary electron photomicrograph showing laser ablation pits in sample BDHB-2Di. Pits of 50, 100 and 150 mm square are visible as well as 50=150 mm elongated pits from zig-zag rasters.

for analysis. For the doubly polished sample sections Žapproximately 100 mm thick. with individual crystals exposed on both sides, equally sized laser pits can be ablated straight through the crystal and the coexisting melt Žto give holes rather than pits.. This alleviated the need to measure the pit depth and wt% partition coefficients were obtained directly from the measured quantities of argon, requiring only a correction for the density difference between crystals and melt ŽLange and Carmichael, 1987; Deer et al., 1992.. For laser pits which did not ablate all the way through the sections, depths were measured to an accuracy of "1 mm using SEM stereo-imaging facilities. As previously discussed, there was a difference in ablation rates between the clinopyroxene crystals and the melt in sample 2Di. The depths of three clinopyroxene and two melt 100-mm square pits were measured and found to be relatively constant. It was not possible to measure the depths of zig-zag raster pits, but as these did not ablate all the way through the sample, it was possible to apply an ablation ratio Ž1 : 1.538 for crystal : melt. as determined from the measured square pits, to obtain the partition coefficients. The UVLAMP utilises reflected light and a coated lens with a field of view of 1 mm to view the samples during analysis. Therefore the high magnification, higher quality transmitted light photomicrographs, taken with a petrological microscope were

R.A. Brooker et al.r Chemical Geology 147 (1998) 185–200

used to help locate areas of crystals which appeared inclusion-free. Large areas which are clearly well away from major inclusions are the prime sites for analysis. However, repeat analyses are also desirable, particularly to determine crystal homogeneity and analytical reproducibility during the development of various ablation techniques. As a result, analyses were taken close to identified melt inclusions, and in areas where the presence of inclusions had not been totally eliminated by optical examination. In some cases, high argon extraction volumes Žand high DAr values. immediately drew attention to a particular crystal analysis, and post-ablation examination identified intrusion of melt at the edge of the ablation pit. Some of these analyses are included Žand identified. in the results as the data are still useful in distinguishing compatible and incompatible behaviour Žpartition coefficients can be displaced towards, but not past a value of unity for inclusions of melt.. 2.4. Theoretical approach to equilibrium No reversals were performed to establish equilibrium during this preliminary study. However, crystal growth rates have been estimated and the required theoretical diffusional equilibration rate determined for each step between small growth increments. Actual argon diffusion rates have not been measured for the mineral phases of this study. Roselieb et al. Ž1997. suggest an argon diffusion rate of 2 = 10y1 3 cm2 sy1 for quartz at 13008C, although their data could also indicate a rate faster than 1.5 = 10y1 0 cm2 sy1 . Extrapolated data of Foland Ž1974. for K-feldspar would suggest an argon diffusivity of 10y8 cm2 sy1 at 13008C and Amirkhanov et al. Ž1959. estimate a diffusivity of 10y1 0 cm2 sy1 for clinopyroxene. No estimate has been made for olivine. We suggest a theoretical rate approaching 10y9 cm2 sy1 Žindicated by Broadhurst et al., 1990. as a requirement for crystal equilibrium by diffusion in our experiments. However, to err on the side of caution our calculations assume that no argon is incorporated during the growth increment and all argon must be incorporated by diffusion. McKay Ž1986. has demonstrated that even for highly incompatible elements such as Yb in olivine this is not the

189

case and as a result, equilibration can be approached several orders of magnitude faster than by diffusion alone. Our calculations also assume that the crystal rim formed at each step equilibrates via diffusion with the adjacent melt; it is anticipated that the difference in melt composition Žincluding Ar concentration. and temperature between formation of the crystal core and final rim are insignificant for the purposes of this study. However, it must be noted that some postulated incorporation mechanisms involving crystal defects may be highly temperaturedependent. Growth of acceptably euhedral olivines from a natural basalt composition Žas in ES-NT23. proved particularly difficult and the studies of Donaldson Ž1975, 1976. were invaluable for the determination of growthrcooling rates for euhedral crystals and major element equilibration. Unfortunately equivalent information is not available for the other compositions and growth rates were determined by trial and error. For the olivine crystals in experiment ES-NT23 the maximum dimension of 1200 mm suggests growth increments of 24 mm at each of the 25 cooling stages. In the worst case, where growth at each 18C drop is instantaneous without incorporation of argon, a diffusion rate of 1.75 = 10y9 cm2 sy1 would be required for a 24 mm crystal layer to approach equilibrium by argon diffusion in the subsequent hour. However, the olivine crystals also have a tube of melt along much of the long axis, allowing diffusion from two directions and the narrow width of the crystals would allow more rapid equilibration as growth increments are on the order of 4 mmrh on the crystal sides. In reality a diffusion rate slower than 4 = 10y1 1 cm2 sy1 would allow diffusional equilibrium at each cooling interval. Considering the additional isothermal time intervals without growth and argon incorporation during growth, we believe the olivine crystals should approach equilibrium with the silicate melt. The relatively rapid diffusion of argon in the melt ŽLux, 1987. should remove any build up of argon Žassuming incompatibility. at the crystalrmelt interface. Any zonation for other crystal components Žtypical diffusion rates of 10y8 –10y7 cm2 sy1 . should also be insignificant Že.g. Donaldson, 1975.. The composition OLDAB was subject to the same cooling history as ES-NT23, and a few

R.A. Brooker et al.r Chemical Geology 147 (1998) 185–200

190

small equant crystals Ž200–300 mm. were grown. The exact liquidus temperature and hence the time taken for the growth of these crystals is unknown, but the near perfect euhedral morphology indicates slow, constant growth. Even if all the argon diffused into the crystal during the final 12-h isothermal period, a rate of 5.2 = 10y9 cm2 sy1 would be sufficient for equilibration. For the euhedral crystals in BDHB-2Di and BDHB-7An, the maximum observed dimensions are 500 = 1800 mm and 500 = 300 mm, respectively. These compositions are known to be close to the liquidus at the initial temperature of 13208C and as a result, growth should have taken place throughout the cooling interval. This suggests a maximum growth rate of 14 mm for each growth increment of the BDHB-7An olivines, requiring a diffusion rate of 5.3 = 10y1 0 cm2 sy1 . However, the calculated growth increments for the large clinopyroxene grown in BDHB-2Di are 51 mm, requiring a diffusion rate of 7.2 = 10y9 cm2 sy1 . Unlike the ES-NT23 olivines, this is only slightly reduced for the shorter axis dimensions, as the crystals are more equant. Sample DAB has the most uncontrolled growth and equilibration history. The ‘feathery’ acicular needles are typical of rapid growth but it is not clear if the ‘bundles’ of laths represent individual crystals originally surrounded by equilibrated melt, or a single crystal with included Ži.e. isolated. planes of melt. It should be noted that there was no final, isothermal equilibration period for this sample.

On a theoretical basis it would appear that: Ž1. an argon diffusion rate of 10y9 cm2 sy1 is sufficient for equilibration by diffusion for all samples except DAB and possibly BDHB-2Di; Ž2. a rate several orders of magnitude slower is acceptable if argon is incorporated during crystal growth ŽMcKay, 1986.. However, these are both unknown variables and if either of these assumptions are incorrect, then the crystals may be deficient in argon to some extent. It is also possible that growth has not been at the constant rate used in calculations. Initial supersaturation could be followed by a period of relatively rapid growth.

3. Results The final temperatures and compositions of the phases produced in the experiments are shown in Table 2. The origin of increased K 2 O and Na 2 O in all run products is unknown, but this is not expected to affect the conclusions of this study. The oxidation state in the ES-NT23 experiment and the resulting Fe 2qrFe 3q ratio of the melt and olivine depends largely on the impurities ŽH 2 , O 2 and H 2 O. in the high-purity argon used Žeach claimed to be - 10 ppm.. This was not measured, but the melt is brown in colour compared to the green starting glass, suggesting an fO 2 above the Ni–NiO buffer used for the starting material.

Table 2 Electron microprobe analyses of run products Žcompositions in wt%.

SiO 2 TiO 2 Al 2 O 3 FeO tot MgO CaO Na 2 O K 2O Total Est. density Žgrcm3 . % crystalsa a

BDHB-2Di 12858C melt Cpx

BDHB-An7 12858C DAB 12008C melt Ol melt Cpx

OLDAB 13208C melt Ol

ES-NT23 13208C melt Ol

54.66 – 14.13 0.23 12.21 16.96 0.74 1.33 100.26 2.61 25

47.33 – 20.83 0.19 12.04 17.59 0.48 1.85 100.31 2.64 5

54.46 0.68 14.08 0.21 12.55 12.23 4.05 3.00 101.26 2.55 -2

46.29 1.02 14.32 10.61 13.04 11.26 1.51 1.83 99.88 2.75 5–10

54.55 – 1.79 0.09 20.31 23.37 0.14 0.05 100.31 3.22

As wt% calculated from change in melt composition.

42.29 – 0.17 0.07 56.61 0.51 0.05 – 99.70 3.3

55.34 0.80 19.36 0.16 4.65 9.47 6.18 4.03 99.99 2.45 25

53.10 0.52 3.80 0.15 18.25 24.65 0.36 – 100.83 3.22

43.05 – 0.06 0.07 57.39 0.53 – – 101.10 3.3

42.25 – – 4.79 54.12 0.29

101.45 3.3

R.A. Brooker et al.r Chemical Geology 147 (1998) 185–200

3.1. Partition coefficients Volumes of extracted argon and derived partition coefficients are shown in Table 3. A range of ablation techniques were developed during this study Žsee Section 2.3. but the calculated concentration of argon in a given melt was relatively constant Ž"10%. for each type of ablation pit. As a result, average melt values were used to calculate partition coefficients for some samples Žsee Table 3.. It is clear from Table 3 that the range of partition values reflects a significant variation in the argon released from the crystals. 3.1.1. BDHB-2Di A large number of suitable analysis sites were found in the large BDHB-2Di clinopyroxene. However, this crystal was of only moderate quality in terms of the accurate determination of melt inclusion locations and several sets of parallel fractures were present. In some cases, these fractures may have represented planes of included melt Žas seen in DAB. or sites of trapped argon fluid. This may have contributed significantly to the range of partition coefficients obtained. The distribution of derived coefficients is illustrated in Fig. 2. It should be noted that the lower values are dominated by the elongated zig-zag rasters which were oriented parallel to the fractures. A partition coefficient of 0.0016 Ž"0.0005. for BDHB-2Di Žaverage of five lowest values. is judged to be representative. As mentioned in Section 2.4, it is possible that these crystals may show low, heterogeneous non-equilibrium argon concentrations if calculated equilibration rates were underestimated and the lowest value of 0.0001 Ž"0.0005. may represent either an unequilibrated zone of the crystal, or a particularly inclusion-free area. It is not possible to distinguish between these options from the data of this study, but there is no obvious spatial relationship between sites of low argon content and the crystal morphology to indicate zonal argon distributions. 3.1.2. DAB The clinopyroxene crystals in DAB are the most likely to show low, non-equilibrium argon contents as they were grown at the fastest rates and without a final isothermal equilibration period. However, these crystals repeatedly yielded some of the highest argon

191

contents observed, due in part to the presence of unavoidable, thin planes of included melt possibly induced by rapid growth. The actual partition coefficient for this sample would be below the lowest measured value of 0.2727 Ž"0.0014. in Table 3, although this may not represent an equilibrium situation. 3.1.3. BDHB-An7 and OLDAB For samples BDHB-An7 and OLDAB, the olivine crystals did appear to be inclusion-free, but were small and limited in number. As a result, the reproducibility of these results is unknown and it is particularly unclear if a larger data base Žas for BDHB-2Di. would have revealed even lower partitioning values. The lowest observed partitioning coefficients of 0.0141 Ž"0.0018. for OLDAB and 0.0473 Ž"0.0036. for BDHB-7An may only represent maximum values. 3.1.4. ES-NT23 The olivines in ES-NT23 are probably the most reliable crystals in this study in terms of an approach to equilibrium and they also contained large areas free of any Židentifiable. inclusions. It became apparent during initial analysis that some argon was being released from the stainless-steel backing plate as the beam ablated through the sample Žsamples 1–5 in Table 3.. This excess argon was not released during instrumental blank determinations and therefore not accounted for in the background correction. Although this additional argon is less significant for the glass analyses, it appears to represent a 4-fold increase in the argon content of the crystals and increases the measured partition coefficient range from 0.01–0.02 up to 0.05–0.06. The average of the two lowest partition coefficients appears to be most representative for this sample at 0.0133 Ž"0.0023.. 3.2. Absolute solubilities The most accurate estimate of pit size and therefore measurement of concentrations are for the BDHB-2Di melts which consistently produced values of 5.8 Ž"0.5. = 10y5 STP cm3 gy1 bary1 Ž8 determinations.. Argon concentration for the ESNT23 melts suggests solubilities of 4.68 Ž"0.02. = 10y5 STP cm3 gy1 bary1 for four of the five

192

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Table 3 Extracted argon volumes and derived partition coefficients Sample

BDHB-2Di Glass

BDHB-2Di Clinopyroxene

DAB Glass DAB Clinopyroxene

Ablation technique, pitrhole area and depth where measured Ž m m.

40 Ar 10y1 2 STP cm3

Error Žqry .

zig-zag raster pit 150 = 50 zig-zag raster pit 150 = 50 zig-zag raster pit 150 = 50 zig-zag raster pit 150 = 50 AVERAGE for zig-zag raster 50 square pit 100 square pit = ; 107 deep 100 square pit = ; 107 deep 100 square pit = ; 107 deep 100 square pit = ; 107 deep 100 square pit = ; 107 deep 100 square pit = ; 107 deep AVERAGE for 100 square 150 square pit = 147 deep

89.50 92.20 93.50 94.50 92.42 86.70 172.00 155.00 158.00 148.00 175.00 160.00 161.30 310.60

Ž0.22. Ž0.30. Ž0.22. Ž0.31. Ž0.26. Ž0.23. Ž0.40. Ž0.51. Ž0.47. Ž0.36. Ž0.38. Ž0.39. Ž0.42. Ž0.89.

40.50 14.00 0.39 0.16 0.18 0.01 0.84 61.60 23.70 2.77 15.10 53.20 10.20 71.60 3.60 0.11 3.94 9.61 11.40 0.66

Ž0.18. Ž0.14. Ž0.08. Ž0.06. Ž0.07. Ž0.06. Ž0.09. Ž0.28. Ž0.22. Ž0.20. Ž0.13. Ž0.20. Ž0.11. Ž0.30. Ž0.08. Ž0.09. Ž0.11. Ž0.11. Ž0.11. Ž0.06.

line raster pit 127 = 50 = 145 deep line raster pit 86 = 50 = 145 deep

109.00 150.00

Ž0.32. Ž0.34.

line raster pit 127 = 50 = 151 deep c line raster pit 127 = 50 = 151 deep c line raster pit 86 = 50 = 137 deep c line raster pit 86 = 50 = 137 deep c line raster pit 86 = 50 = 137 deep c line raster pit 86 = 50 = 137 deep c line raster pit 86 = 50 = 137 deep c line raster pit 86 = 50 = 137 deep c

81.70 50.40 90.70 105.00 94.30 68.50 50.80 70.70

(0.34) (0.29) (0.17) (0.30) (0.16) (0.32) (0.160 (0.20)

zig-zag raster pit 150 = 50 Ža.r.. zig-zag raster pit 150 = 50 Ža.r.. zig-zag raster pit 150 = 50 Ža.r.. zig-zag raster pit 150 = 50 Ža.r.. zig-zag raster pit 150 = 50 Ža.r.. zig-zag raster pit 150 = 50 Ža.r.. 50 square pit Ža.r.. 100 square pit =; 69 deep 100 square pit =; 69 deep 100 square pit =; 69 deep 100 square pit =; 69 deep 100 square pit =; 69 deep 100 square pit =; 69 deep 100 square pit =; 69 deep 100 square pit =; 69 deep 100 square pit =; 69 deep 100 square pit =; 69 deep 100 square pit =; 69 deep 100 square pit =; 69 deep 150 square pit =95 deep

Uncorrected partition coefficient

Corrected partition coefficient a

Maximized error b Žqry .

0.4382 0.1515 0.0042 0.0017 0.0019 0.0001 0.0097 0.3819 0.1469 0.0172 0.0936 0.3298 0.0632 0.4439 0.0223 0.0007 0.0244 0.0596 0.0707 0.0021

0.5459 0.1887 0.0053 0.0022 0.0024 0.0001 0.0121 0.4758 0.1830 0.0214 0.1166 0.4109 0.0788 0.5530 0.0278 0.0008 0.0304 0.0742 0.0880 0.0026

Ž0.0026. Ž0.0016. Ž0.0007. Ž0.0005. Ž0.0006. Ž0.0005. Ž0.0009. Ž0.0022. Ž0.0014. Ž0.0010. Ž0.0008. Ž0.0017. Ž0.0007. Ž0.0024. Ž0.0004. Ž0.0004. Ž0.0006. Ž0.0007. Ž0.0007. Ž0.0001.

0.7495 0.4624 0.6047 0.7000 0.6287 0.4567 0.3387 0.4713

0.5476 0.3378 0.4869 0.5637 0.5063 0.3678 0.2727 0.3796

(0.0040) (0.0030) (0.0019) (0.0027) (0.0019) (0.0024) (0.0014) (0.0018)

R.A. Brooker et al.r Chemical Geology 147 (1998) 185–200

193

Table 3 Žcontinued. Sample

Ablation technique, pitrhole area and depth where measured Ž m m.

ES-NT23

Static beam ablading 15 = 20 eliptical holes completely through sample, repeated as indicated same number in crystal and adjacent melt d Glass1. Žn.r.. d Olivine1. Žn.r.. d Glass2. Žn.r.. d Olivine2. Žn.r.. d Glass3. Žn.r.. d Olivine3. Žn.r.. d Glass4. 6 holes= ; 98 deep d Olivine4. 6 holes= ; 98 deep c d Glass5. 14 holes = ; 98 deep d Olivine5. 14 holes = ; 98 deep Glass6. 6 holes = ; 98 deep Olivine6. 6 holes = ; 98 deep c Glass7. 6 holes = ; 98 deep Olivine7. 6 holes = ; 98 deep Glass8. 6 holes = ; 98 deep Olivine8. 6 holes = ; 98 deep Same technique as ES-NT23. Žn.r.. Glass Olivine Glass Olivine c Glass Olivine Same technique as ES-NT23 Žn.r.. Glass Olivine Glass Olivine c

BDHB-7An

OLDAB

40

Ar 10y1 2 STP cm3

Error Žqry .

16.79 1.19 8.88 0.66 14.82 0.86 23.92 4.85 55.47 3.94 23.70 1.45 18.07 0.42 23.70 0.34

Ž0.16. Ž0.14. Ž0.11. Ž0.18. Ž0.14. Ž0.08. Ž0.13. (0.11) Ž0.46. Ž0.14. Ž0.13. (0.09) Ž0.11. Ž0.09. Ž0.13. Ž0.08.

10.52 1.82 19.65 15.03 24.52 1.45

Ž0.14. Ž0.11. Ž0.11. (0.12) Ž0.20. Ž0.10.

34.98 0.64 38.79 2.41

Ž0.22. Ž0.08. Ž0.24. (0.110

Uncorrected partition coefficient

Corrected partition coefficient a

Maximized error b Žqry .

0.0709

0.0588

Ž0.0074.

0.0743

0.0617

Ž0.0174.

0.0580

0.0482

Ž0.0049.

0.2028

0.1683

(0.0047)

0.0710

0.0590

Ž0.0026.

0.0612

0.0508

(0.0034)

0.0232

0.0193

Ž0.0042.

0.0143

0.0119

Ž0.0029.

0.1730

0.1384

Ž0.0101.

0.7649

0.6119

(0.0083)

0.0591

0.0473

Ž0.0036.

0.0183

0.0141

Ž0.0018.

0.0621

0.0478

(0.0025)

a

Corrected for density difference andror differential ablation rate. Analytical error from Ar analysis. c Glass inclusions identified in crystals Žalso indicated by italics.. d Ar released from backing plate, not corrected by background blank Žsee text.. a.r.s ablation ratio used to determine partition coefficient Žcrystal : melt ratio for BDHB-2Dis 1.54.. n.r.s number of holes andror sample thickness not recorded. b

determinations and two measurements for DAB produced values of 4.8 and 9.8 = 10y5 STP cm3 gy1 bary1 . The BDHB-2Di, ES-NT23 and the higher DAB measurements are in good agreement with values predicted by theoretical models Žsee Carroll and Stolper, 1993.. Very approximate estimates of pit volumes for other melts suggest solubilities of a similar order of magnitude. The lowest and highest concentrations estimated for the BDHB-2Di clinopyroxene were 0.005 and

3.2 = 10y5 STP cm3 gy1 bary1 , respectively, although the amount of argon released for the lower value was near the detection limit. All DAB clinopyroxenes contained melt inclusions and the lowest measured concentration of 1.6 = 10y5 STP cm3 gy1 bary1 represents a maximum value. The lowest argon content measured for a ES-NT23 olivine Žfree of excess argon; see Section 3.1.4. suggests a solubility of 0.06 = 10y5 STP cm3 gy1 bary1 , comparable with sub-ppb estimates for natural mantle olivine

194

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Fig. 2. Histogram of partition coefficients derived for BDHB-2Di. Analyses producing values in the range 0.6–0.4 probably reflect significant inclusions of melt in the analysed crystal volume. Ablation techniques are listed in Table 3. Note partition ranges on the x-axis are variable.

concentrations Ž10y6 –10y7 STP cm3 gy1 , e.g. Dymond and Hogan, 1978; Kaneoka et al., 1983; Staudacher et al., 1986; Poreda and Farley, 1992.. It was not possible to determine argon contents for the other crystals.

4. Discussion 4.1. Comparison with preÕious studies Fig. 3 compares the range of DAr partition coefficients reported in this study with Di values reported by previous workers for experimental ŽHiyagon and Ozima, 1986; Broadhurst et al., 1990, 1992; Shibata et al., 1994. and natural samples ŽMarty and Lussiez, 1993; Valbracht et al., 1994.. The most striking feature of these data is the ; 100-fold range in reported Di values. In particular, it is not clear whether DAr Žas well as D Kr and D Xe . values are greater or less than unity for olivine and clinopyroxene. Batiza et al. Ž1979. suggested partitioning values above unity Žnot included in Fig. 3. for all noble gases except Ne, but assumed this compatible be-

haviour reflected disequilibrium in their natural samples. However, the experimental data of Broadhurst et al. Ž1992. also indicated that heavy noble gases are more compatible than commonly assumed, with DAr , D Kr and D Xe possibly greater than unity, and Di weakly but positively correlated with the size of the noble gas atom Ži.e., larger gases were more compatible.. In addition, Shibata et al. Ž1994. have also reported values above unity for DAr and D Kr . If correct, these results have significant implications for models of mantle degassing and crust–mantle–atmosphere evolution ŽAzbel and Tolstikhin, 1990., most of which have assumed uniformly incompatible behaviour for the noble gases during mantle melting Že.g., Zhang and Zindler, 1989.. The data reported in this study strongly suggest that DAr values are well below unity. Our results for olivine are at the lower end or below the previously reported experimental data range, but in agreement with highly incompatible behaviour inferred from mineral-melt pairs in natural basaltic samples ŽMarty and Lussiez, 1993; Valbracht et al., 1994.. As previously noted, the lower partition coefficients in Table 3 may only represent maximum DAr values, as melt inclusions may have been included in the crystal analyses. Whilst it is possible that slight compositional differences could be responsible for the variation in partition coefficients between different studies, it is more likely that experimental or analytical errors contribute significantly to the range of results in Fig. 3. As the argon concentration in melts appears relatively consistent between studies, it is the crystalline phases which appear responsible for the large range of values observed within, and between studies. The most common source of error identified by other workers relates to melt or fluid inclusions in the crystalline phase Že.g. Hiyagon and Ozima, 1986; Marty and Lussiez, 1993, 1994; Hiyagon, 1994; Valbracht et al., 1994. or incomplete separation of crystals and melt Že.g. Hiyagon and Ozima, 1986.. Fluid inclusions are potentially far more significant than inclusions of melt Že.g., a 1 mm3 argon inclusion in a 10 6 mm3 analysis volume can easily yield a DAr above unity.. However, a range of other effects may also be important, especially for the powdered samples commonly used for bulk analytical techniques.

R.A. Brooker et al.r Chemical Geology 147 (1998) 185–200

195

Fig. 3. A comparison of partition coefficients obtained in this study and previously reported values. Broadhurst et al. partitioning values for argon are from the 1990 paper; the average of the 1Di and 2Di melts Ž1.25 = 10y5 STP cm3 gy1 bary1 . is used for clinopyroxene and the 7An melt value Ž1.2 = 10y5 STP cm3 gy1 bary1 . is used for the olivine, as this composition has forsterite as the liquidus phase Žsee Section 2.1.. The Broadhurst et al. data for other gases are from the 1992 paper. Other data from Hiyagon and Ozima, 1986 and Shibata et al., 1994. For the data of this study, the symbols represent the lower measured coefficients and the arrows represent the range of values obtained or inferred. Data from selected natural olivine samples ŽMarty and Lussiez, 1993; Valbracht et al., 1994. are included for comparison.

4.2. Surface adsorption Surface adsorption of noble gases is a problem commonly noted for finely powdered samples used in bulk analysis techniques Že.g. Hiyagon and Ozima, 1982, 1986; Broadhurst et al., 1990, 1992; Shibata et al., 1994; Roselieb et al., 1997.. Niemeyer and Leich Ž1976., Niedermann and Eugster Ž1992., R.C. Wiens Žpers. commun., 1992. and Roselieb et al. Ž1997. have demonstrated that adsorbed Ar, Kr and Xe may be more tightly bound than commonly assumed, remaining attached to samples at temperatures above 10008C. Hiyagon and Ozima Ž1986. have suggested that adsorption of noble gases by the crystalline phase may have increased partition coefficients in their study, with adsorption effects being

strongest for the heavier gases. This could explain the increase in partition coefficients with noble gas size seen in Fig. 3 for both the Hiyagon and Ozima Ž1986. and Broadhurst et al. Ž1992. data. Roselieb et al. Ž1997. have demonstrated that the amount of adsorbed argon released at lower temperatures is variable between different types of the same mineral Žquartz. and can be drastically reduced by etching a sample prior to argon exposure. Some adsorbed atmospheric argon was evident for the exposed polished surfaces analysed in this study as samples were not heated in vacuum prior to analysis. For the total quantities of argon typically extracted from the melts the surface argon contribution is negligible, as long as several layers are ablated deep into the sample. However, for the crystals

196

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with the lowest argon contents and smaller pit volumes, the surface argon may become significant. With the UVLAMP it is theoretically possible to exclude argon released from the first layer and adjust the calculated pit volume to correct for this surface anomaly. 4.3. ‘Trapped’ argon T he distinction betw een physisorbed, chemisorbed, trapped and dissolved noble gases can be difficult to establish Žsee Broadhurst et al., 1992.. Bernatowicz Ž1980. suggests that an increase in adsorption noted as samples are crushed, may be related to surface reconstruction. A heterogeneous surface effect several microns deep Žmuch deeper than surface-absorbed gases. has been identified by UVLAMP analysis during depth profiling of Kfeldspar crystal exposed to 1–2 kbar argon for diffusion rate determinations ŽCarroll et al., 1994; Wartho et al., a and b, submitted.. This effect appears more pronounced if the crystal surface is polished rather than cleaved, prior to argon exposure. Although damage during cleaving may be significant, a more severe effect may be related to the heterogeneous distribution of damage induced by polishing, especially for crystal surfaces. A system of fractures Žthe Bilby layer. or dislocations may allow increased contamination Žor surface area and adsorption.. This polishing effect has also been observed during studies of hydrogen diffusion in feldspars ŽGraham and Elphick, 1991.. In high-pressure experiments it may be possible that high concentrations of gas are forced into the pre-existing surface fracture system to become trapped, perhaps by annealing during the experiment. For a given fracture system the severity of this effect may be related to the argon pressure, but the volume of fractures connected to the grain surface and the exposed surface area of powders may also be significant in low-pressure experiments. Roselieb et al. Ž1997. performed electron microprobe analyses on polished sections of quartz grains exposed to 8 kbar of argon at 13008C and identified local argon concentrations inside grains as high as 4000 ppm against a background of ‘true solubility’ below 30 ppm Ž- 10y6 STP cm3 gy1 bary1 . which was apparent for ; 90% of the grain section. This

feature was variable between different quartz types suggesting some extrinsic control. This may explain the large variation observed for the various clinopyroxenes of Broadhurst et al. Ž1990. who suggest that a solubility mechanism is required for the crystals ‘‘that can vary significantly amongst samples of almost identical major element compositions’’. The average of up to 100 microprobe analyses ŽK. Roselieb, pers. commun., 1997. over the sectioned grain surfaces in the Roselieb et al. Ž1997. study agreed well with the bulk argon extraction measurements Ž) 10y5 STP cm3 gy1 bary1 .. Roselieb et al. Ž1997. suggest that the source of high concentrations may be argon trapped by fractures, dislocations or fluid trails and point out the difficulties in identifying such heterogeneous features even by transmission electron microscopy, as well as the unreliability of the derived bulk extraction data. If small fluid inclusions are involved, these may not be removed by equilibration during the experiment Žand may only be slightly modified during a reversal.. Small fluid inclusions may also be retained during the low-temperature phase of gas extraction during bulk analysis. Marty and Lussiez Ž1993. have demonstrated the persistence of fluid inclusions by showing that heating a sample to 6008C may release less noble gas than crushing at room temperature. 4.4. Early partial melting Another phenomenon which might increase the noble gas content of heated bulk crystal samples is early partial melting ŽEPM.. EPM has been reported in clinopyroxene at temperatures 2508C below the melting point Žsee Ingrin et al., 1991; Doukhan et al., 1993.. EPM was originally noted in strained clinopyroxene crystals, but also occurs in unstrained crystals ŽIngrin et al., 1991.. The amount of melting can be as high as 0.2% with 0.1 mm droplets at 12508C ŽIngrin et al., 1991.. The initial microdroplets Ž0.01 mm. are silica-rich ŽDoukhan et al., 1993. with a potentially high argon solubility and as a result it may be possible to increase the partitioning value past unity. Similar ‘pre-melting’ effects have been noted for olivine Že.g. Kohlstedt and Mackwell, 1987.. These problems are obviously not encountered if crystals are grown from melts as per the experiments of this study, but could be important for

R.A. Brooker et al.r Chemical Geology 147 (1998) 185–200

isolated crystals. Broadhurst et al. Ž1990. report partial fusion and compaction of crystal powders and Shibata et al. Ž1994. demonstrate a substantial increase in gas solubility in crystals Žand the partition coefficient; see Fig. 3. as temperatures are increased from 1300 to 16008C. Dislocations which relieve stress around EPM droplets offer another potential site for increased solubility, perhaps with a preference for larger noble gases as required by some partitioning trends Žsee Fig. 3.. 4.5. Effect of dislocations and composition Compared with theories related to solubility in melts Že.g. Roselieb et al., 1992; Carroll and Stolper, 1993; Carroll and Webster, 1994. noble gas solubility in crystals is poorly understood. It is particularly difficult to identify a reliable range of crystal solubility data which might provide a useful comparison and yield pressure, temperature and compositional dependence information. However, identification and elimination of heterogeneous concentrations of argon which do not represent true incorporation Žor solubility. into the crystal structure may reveal a range of equilibrium concentrations which are much less diverse. The scale at which the heterogeneous entrapment approaches a true solubility mechanism is a subject for debate. However, it should be noted that the entrapment of ‘heterogeneous argon’ in crystals during experiments may also reflect a tendency for similar processes in nature. Various types of defect have been suggested as sites for noble gas solubility in crystals. As defects are difficult to study in complex compositions, the noble gas sites associated with intrinsic point defects have been examined by Tsuchiyama and Kawamura Ž1994. using a molecular dynamic simulation of the simple compound MgO. The results suggest that small He atoms may be in interstitial sites, whilst Ne can substitute for Mg or O Ži.e. in vacancies.. Ar and Kr are too large to fit in the O 2y site Žlargest of the two vacancies., and instead require associated vacancies where the atoms are in sites replacing a pair of Mg and O ions. The number of intrinsic defects in a crystal has a thermodynamically defined, highly temperature-dependent equilibrium constant Žpossibly explaining the 16008C data of Shibata et al., 1994., but this is overshadowed by extrinsic defects associ-

197

ated with impurities Žespecially aliovalent ions. at a lattice site. For olivine, trace elements such as Fe 3q, Al 3q and Ca2q may be important, with Fe 3q, Al 3q also important in clinopyroxene. It is also possible that trace elements may exclude noble gases by competing for favourable sites Že.g., Navrotsky, 1978.. However, it should be noted that for noble gases, an increasing size may increase compatibility in crystals Žsee Fig. 3., and this strongly contrasts with ‘solid’ trace elements suggesting that different sites would be favoured. Tsuchiyama and Kawamura Ž1994. suggest that there are probably more than enough suitable intrinsic and extrinsic defects in silicate crystals at magmatic temperatures to explain concentrations of noble gases observed in 1-bar experiments, even for the relatively high concentrations of Broadhurst et al. Ž1990, 1992.. Navrotsky Ž1978. suggests that extended defects such as edge- and screw-dislocations, characterised by larger size and greater abundance than point defects, would offer more highly favoured sites for trace elements, especially those excluded from lattice sites due to size or charge incompatibility. Similar arguments may also be applicable to the noble gases in minerals, but it should be noted that dislocation densities depend on strain rates which are generally low in magmatic source regions Že.g., Mercier, 1979.. Broadhurst et al. Ž1990, 1992. present various arguments supporting the idea that noble gases are sited in lattice vacancy defects rather than interstitial defects or sites related to dislocations. However, they admit that estimated equilibrium defect populations for olivine at their experimental conditions are not sufficient to accommodate the measured noble gas concentrations and suggest that some extended defects are also required. Perhaps these could start to represent the areas of high heterogeneous argon concentrations noted by Roselieb et al. Ž1997.. For the crystals in this study which are grown in a ‘low stress’ environment, the dislocation density may be lower than the Broadhurst et al. Ž1990. metamorphic samples. In terms of vacancies related to impurity or trace element concentrations, the Fe-bearing olivine in ES-NT23 may have a high concentration of vacancies associated with Fe 3q Žhigh fO 2 ., but argon concentrations appear similar for Fe-bearing and Fefree olivines. In general it might be considered that the ES-NT23 igneous-type olivine compositions used

198

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in this study Žand Hiyagon and Ozima, 1986; Shibata et al., 1994. are more appropriate that the metamorphic crystals used by Broadhurst et al. Ž1990.. It is clear from this study that a clinopyroxene which is actually grown in equilibrium with a BDHB-2Di type melt can result in a much lower partition coefficient value than the pyroxene actually used by Broadhurst et al. Ž1990, 1992.. This may reflect differences in defect densities or the distribution of competing trace elements which cannot re-equilibrate in the Broadhurst et al. Ž1990, 1992. experiments. However, the other explanations discussed are perhaps more plausible.

Most of the problems suggested for previous experimental techniques give high partition coefficients and it is possible that DAr for all samples is less than 0.01

5. Summary

References

We have shown that the UVLAMP analytical technique can be used to measure noble gas contents in melts and coexisting crystals at concentrations approaching natural abundance. It is important to stress the preliminary nature of the data obtained in this study, as the main objective was to develop and test the suitability of the UVLAMP for this type of analysis. However, argon appears to show incompatible behaviour for all olivine and clinopyroxene samples examined. Although an approach to equilibrium has not been definitively demonstrated, it is unlikely that partition coefficients would be greater than unity. The high spatial resolution of the UVLAMP, is particularly useful for avoiding any inclusions of melt of fluid which are identified within crystals and may also be used to identify and eliminate surface related anomalies. However, a range of DAr values approaching but not exceeding unity is observed for most of the samples in this study, which may suggests the presence of an unidentified population of melt inclusions in the crystals. This situation contrasts with other experiments where crystal surfaces are exposed to gaseous argon and fluid may be incorporated perhaps in annealed ‘fractures’, to give partition values which exceed unity. All previous studies of argon partitioning in Fig. 3 have at least some, if not all of their values below unity. The melt argon solubilities determined in this and other studies are relatively consistent and the most likely source of error in determining the partition coefficient is the gas content of the crystals.

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