3.8R⊕ , where R⊕ is Earth’s radius (19, 20), MM MD ≈ 1:9 LD MD ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 2:9GM⊕R⊕ p − 1:1 − 1:9 Mesc MD ð1Þ where Mesc is the mass that escapes from the disk as the Moon accretes. To estimate MM , we used Eq. 1 and made the favorable assumption that Mesc = 0. We tracked the origin (impactor versus tar- get) of the particles in the final planet and the disk. To quantify the compositional difference be- tween the silicate portions of the disk and planet, we define a deviation percentage dfT ≡ [FD,tar /FP,tar − 1] × 100 (2) where FD,tar and FP,tar are the mass fractions of the silicate portions of the disk and of the planet derived from the target’s mantle, respectively (21). Identical disk-planet compositions have dfT = 0, whereas a disk that contains fractionally more impactor-derived silicate than the final planet has dfT < 0, and a disk that contains fractionally less impactor-derived silicate than the final planet has dfT > 0. Prior impact simulations (1–3, 14, 15) that consider g ≡ Mimp /MT ≈ 0.1 to 0.2 produce disks with −90% ≤ dfT ≤ −35% for cases with MM > ML , where ML is the Moon’s mass. Results with larger impactors having g = 0.3, 0.4, and 0.45 are shown in Figs. 1 and 2 and Table 1. As the relative size of the impactor (g) is increased, there is generally a closer compositional match be- tween the final disk and the planet. For g ≥ 0.4, some disks have both sufficient mass and an- gular momentum to yield the Moon and nearly identical silicate compositions to that of the final ulation blique, 4 km een an et with ble 1, es with ure in r, with mpera- articles sional plotted. hours, shown . After e plan- erged, Their to the merged a bar- al arms apped persed taining whose n dif- of the s than e near colli- target ibuted ortion- e final sk’s dfT ry ap- stance onal differ- isk and final produced by A) g = 0.3 iangles) and REPORTS on November 25, 2012 www.sciencemag.org Downloaded from “Twins” িಥʁ [Canup, Science, 2012] 20Ne/22Ne 10 11 12 13 40Ar/36Ar 2,000 4,000 6,000 8,000 10,000 21Ne/22Ne 0.07 20Ne/22Ne 10 11 12 13 a Iceland; this study MORB (2ΠD43) Iceland; ref. 18 Air Air Iceland mantle source Iceland mantle source Solar wind b 0.04 0.03 0.05 0.06 Figure 1 | Differences in neon and argon isotopic composition between MORB and the Iceland plume. a, Neon three-isotope plot showing the new analyses of the DICE 10 sample (filled circles) from Iceland in comparison to previously published data for this sample (open circles; ref. 18) and the gas-rich ‘popping rock’ (2PD43) from the north Mid-Atlantic Ridge (open triangles; ref. 17). Error bars are 1s, and forclarity, twoprevious analyses18 with largeerrorbars have not been shown. Step-crushing of a mantle-derived basalt produces a linear trend that reflects variable amounts ofpost-eruptive air contamination in vesicles containingmantleNe.Theslopeofthelineisafunctionoftheratioofnucleogenic 21Ne to primordial 22Ne, with steeper slopes indicating a higher proportion of primordial 22Ne and, thus, a less degassed mantle source. The slope of the Iceland line based on the new analyses is consistent with that obtained previously18. Importantly, 20Ne/22Ne ratios of 12.8860.06 are distinctly higher than the MORB source 20Ne/22Ne of #12.5 as constrained from continental well gases20. b, Ne–Ar compositions of individual step crushes of the DICE 10 sample. 40Ar is generated by radioactive decay of 40K, and low 40Ar/36Ar ratios are indicative of a less degassed mantle. The data reflect mixing between a mantle component and post-eruptive atmospheric contamination. A least-squares hyperbolic fit through the data yields a 40Ar/36Ar ratio of 10,74563,080, corresponding to a mantle solar 20Ne/22Ne ratio of 13.8. This Ar isotopic ratio is used as the mantle source value for Iceland in Figs 2 and 3. Symbols as in a; error bars are 1s. Kinetic fractionation 10 13 Iceland; this study MORB (2ΠD43) a Air 20Ne/22Ne 3He/22Ne 12 11 3He/36Ar 40Ar/36Ar 5,000 10,000 15,000 20,000 25,000 30,000 b Air Iceland mantle source MORB (2ΠD43) mantle source 0.0 0.2 0.4 0.6 0.8 22Ne/36Ar 40Ar/36Ar 5,000 10,000 15,000 20,000 25,000 30,000 Air Sea water c 0.0 0.1 0.2 0.3 0.4 Degassing 0 1 2 3 4 5 6 Figure 2 | Differences in elemental abundances and isotope ratios between MORB and the Iceland plume. Errorbarsare1s.a,3He/22Neversus20Ne/22Ne; b, 3He/36Ar versus 40Ar/36Ar; and c, 22Ne/36Ar versus 40Ar/36Ar. The mantle source composition for 2PD43 (filled grey square in all panels) is based on the 40Ar/36Ar and 20Ne/22Ne ratios as defined in ref. 30, and the mantle source composition for Iceland (filled black square in all panels) is based on Fig. 1. The grey and black arrows at the top ofthe figure indicate how elemental ratios evolve asaresultofkineticfractionationandsolubilitycontrolleddegassing,respectively. Good linear relationships are observed between isotope ratios and elemental ratios, which reflect mixing between mantle-derived noble gases and post- RESEARCH LETTER 20Ne/22Ne 10 11 12 13 40Ar/36Ar 2,000 4,000 6,000 8,000 10,000 21Ne/22Ne 0.07 20Ne/22Ne 10 11 12 13 a Iceland; this study MORB (2ΠD43) Iceland; ref. 18 Air Air Iceland mantle source Iceland mantle source Solar wind b 0.04 0.03 0.05 0.06 Figure 1 | Differences in neon and argon isotopic composition between MORB and the Iceland plume. a, Neon three-isotope plot showing the new analyses of the DICE 10 sample (filled circles) from Iceland in comparison to previously published data for this sample (open circles; ref. 18) and the gas-rich ‘popping rock’ (2PD43) from the north Mid-Atlantic Ridge (open triangles; ref. 17). Error bars are 1s, and forclarity, twoprevious analyses18 with largeerrorbars have not been shown. Step-crushing of a mantle-derived basalt produces a linear trend that reflects variable amounts ofpost-eruptive air contamination in vesicles containingmantleNe.Theslopeofthelineisafunctionoftheratioofnucleogenic 21Ne to primordial 22Ne, with steeper slopes indicating a higher proportion of primordial 22Ne and, thus, a less degassed mantle source. The slope of the Iceland line based on the new analyses is consistent with that obtained previously18. Importantly, 20Ne/22Ne ratios of 12.8860.06 are distinctly higher than the MORB source 20Ne/22Ne of #12.5 as constrained from continental well gases20. Kinetic fractionation 10 13 Iceland; this study MORB (2ΠD43) a Air 20Ne/22Ne 3He/22Ne 12 11 3He/36Ar 40Ar/36Ar 5,000 10,000 15,000 20,000 25,000 30,000 b Air Iceland mantle source MORB (2ΠD43) mantle source 0.0 0.2 0.4 0.6 0.8 22Ne/36Ar 40Ar/36Ar 5,000 10,000 15,000 20,000 25,000 30,000 Air Sea water c 0.0 0.1 0.2 0.3 0.4 Degassing 0 1 2 3 4 5 6 Figure 2 | Differences in elemental abundances and isotope ratios between MORB and the Iceland plume. Errorbarsare1s.a,3He/22Neversus20Ne/22Ne RESEARCH LETTER contamination processes are ruled out as the reason for the lower 129Xe/130Xe ratios at Iceland. The data in Fig. 3a demonstrate that the Iceland and MORB source mantles evolved with different I/Xe ratios, requiring the two mantle sources to have separated by 4.45Gyr ago with limited subsequent mix- ing between the two. As atmosphere is located near the origin in this plot (Fig. 3a), and mixing in this space is linear, adding subducted atmo- spheric Xe to the MORB source clearly cannot produce the Iceland source, based on its higher proportion of Pu- to U-derived fission Xe, is a conclusion that is independent of the absolute concentrations of noble gases andtherelativepartitioncoefficientsofthenoblegases withrespect to their radiogenic parents. The combined I–Pu–Xe system has been used to constrain the closure time for volatile loss of a mantle reservoir through the 129*Xe/136*XePu ratio1,2,6,25, where 129*Xe is the decay product of 129I decay and 136*XePu is 136Xe produced from 244Pu fission. 129I has a 244 129 136 6.6 6.8 7.0 7.2 7.4 129Xe/130Xe 40Ar/36Ar 2,000 4,000 6,000 8,000 10,000 Iceland mantle 129Xe/130Xe Air b 3He/130Xe 0 200 400 600 800 1,000 Air 129Xe/130Xe 6.6 6.8 7.0 7.2 7.4 7.6 7.8 MORB (2ΠD43) source Iceland mantle source a Figure 3 | Differences in Xe isotopic composition between MORB and the Iceland plume. a, Correlation between 129Xe and 3He in the ‘popping rock’ MORB (2PD43)17 and Iceland (DICE 10). Error bars are 1s. Data points are individual step crushes that reflect different degrees of post-eruptive atmospheric contamination in the vesicles. Air lies near the origin and the mantle compositions at the other end of the linear arrays. The straight lines are robust regressions through the data. Because mixing in this space is linear, the lines also represent the trajectories along which the mantle sources will evolve when mixed with subducted air. The new observations from Iceland demonstrate that the Iceland plume 129Xe/130Xe ratio cannot be generated solely through adding recycled atmospheric Xe to the MORB source, and vice versa. Thus, two mantle reservoirs with distinct I/Xe ratios are required. The mantle 129Xe/130Xe ratio of 6.986 0.07 for Iceland was derived from a hyperbolic least-squares fit through the Ar-Xe data (b) corresponding to a mantle 40Ar/36Ar ratio of 10,745. Note that given the curvature in Ar–Xe space, the 129Xe/130Xe in the Iceland mantle source is not particularly sensitive to the exact choice of the mantle 40Ar/36Ar ratio. LETTER RESEARCH [Mukhopadhyay, Nature, 2012] ٿਂ෦ͷرΨεಉҐମෆۉҰ ٿਂ෦·Ͱ melting ͍ͯ͠ͳ͍