Abstract
We present new 238U-230Th-226Ra-210Pb and supporting data for young lavas from southwest Pacific island arcs, Eyjafjallajökull, Iceland, and Terceira, Azores. The arc lavas have significant 238U and 226Ra excesses, whereas those from the ocean islands have moderate 230Th and 226Ra excesses, reflecting mantle melting in the presence of a water-rich fluid in the former and mantle melting by decompression in the latter. Differentiation to erupted compositions in both settings appears to have taken no longer than a few millennia. Variations in the (210Pb/226Ra)0 values in all settings largely result from degassing processes rather than mineral-melt partitioning. Like most other ocean island basalts, the Terceira basalt has a 210Pb deficit, which we attribute to ~8.5 years of steady 222Rn loss to a CO2-rich volatile phase while it traversed the crust. Lavas erupted from water-laden magma systems, including those investigated here, commonly have near equilibrium (210Pb/226Ra)0 values. Maintaining these equilibrium values requires minimal persistent loss or accumulation of 222Rn in a gas phase. We infer that degassing during decompression of water-saturated magmas either causes these magmas to crystallize and stall in reservoirs where they reside under conditions of near stasis, or to quickly rise towards the surface and erupt.
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Introduction
Disequilibrium between uranium-series nuclides in lavas, which is created by partitioning of elements between mineral, melt, fluid, and vapor phases and diminishes by radioactive decay, provides unique information about the timescales and processes of magma generation, transport, and differentiation1,2,3,4,5. In terms of just how rapid some of these processes might be, many lavas exhibit disequilibria between 210Pb (T1/2 = 22.6 y), 210Po (T1/2 = 138 days), and 226Ra (T1/2 = 1,600 y), illustrating that magmatic differentiation processes can persist nearly to eruption6,7,8,9.
Activities of 210Pb (indicated by parentheses) are commonly less than those of its long-lived parent (226Ra) in ocean island basalts (median (210Pb/226Ra) = 0.76)10 and in some spreading centers3. These 210Pb deficits commonly have been attributed to Pb-Ra fractionation during melting and rapid transport to the surface3,10. In contrast, the median (210Pb/226Ra) for arc lavas is 1.010, although values range widely, which is typically attributed to Rn loss or gain in the decades leading to eruption5,7,10,11,12,13,14. We explore the hypothesis that 210Pb-226Ra disequilibrium generated by mantle melting generally decays during the time needed for magmas to migrate to the surface in all tectonic settings, and most measured disequilibrium reflects open-system Rn degassing and accumulation. The prevalence of near equal (210Pb) and (226Ra) values in volatile-rich arc magmas, but 210Pb deficits in less-volatile-rich ocean island lavas typically seems paradoxical. Nevertheless, we contend that it is a natural result of the differences in the relative abundances of volatile species and in the modalities of degassing for different tectonic settings.
To explore the origin of 210Pb-226Ra disequilibria in magmas, we analyzed young volcanic rocks from ocean islands and arcs. The requirement for including samples in this study is that they erupted less than a few decades before the date of measurement. The data were acquired over a period of several years (2008–2015) as suitable samples became available. Most of the analyzed samples are from locations where 210Pb data are rare, or represent lava types that erupt infrequently. Several of our samples recently erupted from the Tonga, Kermadec, and Vanuatu arcs in the southwest Pacific, including the Earth’s youngest rhyolite. We also analyzed a “lava balloon” erupted in 1999 offshore of Terceira, providing the first 210Pb data from the Azores. Finally, a sample from the explosive 2010 eruption of Eyjafjallajökull in Iceland was included to investigate 210Pb-226Ra disequilibria in tephras deposited after an eruption that was responsible for extensive disruption to air traffic.
Recent eruptive products
With one exception, all of the samples studied here erupted within 15 years of the analysis date (Table 1). Most samples included in this study are crystal-poor to aphyric. Several of these samples are from the Tonga arc. Samples HH09-01 and -02 are two separate scoria clasts collected during the eruption of Hunga Ha’apai in April 2009. Phenocrysts are rare in these samples, with <1% augite and plagioclase, with trace orthopyroxene in a vesicular matrix consisting of brown glass choked with laths of plagioclase and augite. HR06 is pumice collected from the November 2006 eruption of Home Reef, a near-surface volcanic center 25 km SSW of Late volcano. This sample has about 2% phenocrysts of augite and plagioclase, which form rare glomerocrysts with magnetite. The matrix is highly vesicular colorless glass. Sample F0805 is a crystal poor, vesicular dacite from the summit of Fonualei volcano that is inferred to have erupted between 1979 and 199015.
Our sample of rhyolite from Havre volcano erupted in July 2012 just west of the Kermadec volcanic front at about 1000 meters water depth. The volume of the eruption is estimated at about 1.5 km3, making it the largest and deepest recorded silicic submarine eruption16. A pumice raft resulting from this eruption spread across the southwest Pacific and Tasman Sea over the ensuing months17. Our sample is a spherical homogenous light gray pumice clast of about 900 cm3 collected from a high-tide accumulation of Havre pumice on a Sydney Harbour beach in February 2014. It consists of frothy fresh glass with about 1% plagioclase and trace augite.
Van A1 is a vesicular lava bomb with abundant mm-scale plagioclase collected shortly after eruption from Yasur volcano, Vanuatu in August 200818.
The Serreta sample is a glassy quenched margin of one of the gas-inflated basaltic pillows that breached the sea surface off of Terceira island in the Azores in February 199919.
Finally, sample EJ-1 is ash deposited during the April 15, 2010 eruption of Eyjafjallajökull in Iceland. It is a portion of sample 2 from Gislason et al.20, where it is described in detail. Sigmarsson et al.21,22 analyzed a separate fraction of this sample. All tephras from this eruption are more than 85% juvenile23. The benmoreite ash is glassy and relatively crystalline, with abundant bimodally zoned crystals of olivine, plagioclase, augite, and magnetite24.
We undertook full compositional characterization of these samples (i.e. major and trace elements, Sr-Nd isotopes and 238U-230Th-226Ra-210Pb disequilibria; Table 1) to distinguish between origins of (210Pb/226Ra) disequilibria by magma generation and differentiation in different tectonic settings and for different magma types.
Results
With the exception of the Serreta sample from the Azores, all of the samples analyzed here have <3.6 wt.% MgO and <50 ppm Cr indicating that they are significantly differentiated from their respective mantle-derived parental magmas (Table 1). The sample from Yasur, Vanuatu is a basaltic trachyandesite (Fig. 1) with a bulk-rock major and trace element composition similar to those of other lavas erupted from this steady-state volcano25,26. Tonga samples from Hunga are low-K andesites with SiO2 between 57.5 and 58 wt.%. Tonga samples from Home Reef and Fonualei samples are low- and medium-K dacites with 64.5 to 66 wt.% SiO2 respectively. The glass in our sample of the Havre, Kermadec pumice is a medium-K rhyolite with 73 wt.% SiO2 and an overall composition matching those of other pumice glass from this eruption27 (Table 1). Our ocean island sample from Eyjafjallajökull, Iceland is a benmoreite with 57 wt.% SiO2 and combined concentrations of Na2O and K2O equal to 7 wt.%. The Serreta sample from Terceira, Azores is relatively primitive alkali basalt with 8.57 wt.% MgO, 235 ppm Cr, 47 wt.% SiO2 and 3.8 wt.% total alkalis.
Data are from Table 1.
All of the samples from western Pacific volcanic arcs (Tonga, Kermadec, Vanuatu) are enriched in fluid mobile elements such as Cs, Rb, Ba, U, K, Pb, and Sr and depleted in Nb and Ta with respect to light rare-earth elements (Table 1; Fig. 2). Th/La ratios for samples from the Tonga arc are in the range of those recorded for mid-ocean ridge basalts, whereas Th/La values are higher for the lavas from Yasur and Havre volcanoes. The two ocean island samples have relatively smooth incompatible element patterns with steep rare earth element slopes and negative K and Pb anomalies. The more evolved samples from both settings have negative Sr and Ti anomalies because of significant plagioclase and titanomagnetite fractionation.
See Fig. 1 and the text for sample locations.
With the exception of the Havre rhyolite, the 143Nd/144Nd radiogenic isotope data for the analyzed samples vary within a restricted range from 0.513030 to 0.512925 (Table 2), whereas the arc samples are offset to higher 87Sr/86Sr ratios (0.703540 to 0.703769) compared to the ocean island samples (0.703257 to 0.703533). The Havre rhyolite has a greater offset, with relatively high 143Nd/144Nd and 87Sr/86Sr ratios (0.513115, 0.703705). Thorium isotopic compositions of the arc samples vary from 1.70 to 0.93, in line with regional variations28,29. Similarly, the (230Th/232Th) ratio in the benmoreite from Eyjafjallajökull is comparable to those of other silicic Icelandic lavas22. In contrast, the Serreta basalt has (230Th/232Th) = 1.40, which is the highest ratio thus far measured for lavas from the Azores30,31.
Uranium, Th, and Ra concentrations for the samples analyzed here vary over wide ranges (0.13 to 1.68 ppm, 0.13 to 5.05 ppm, and 116 to 654 fg/g respectively; Table 2). Uranium isotope ratios are within error of secular equilibrium for most samples. The exception is the Fonualei dacite with (234U/238U) = 1.016, which is attributed to seawater alteration. The arc samples have significant 238U and 226Ra excesses over 230Th, with (230Th/238U) ranging from 0.54 to 0.93 and (226Ra/230Th) from 1.38 to 4.80 (Fig. 3). The Havre rhyolite has the least U-Th-Ra disequilibria of these samples. Sample Van A1 from Yasur also has relatively low (238U/230Th) and (230Th/232Th) values (0.83 and 1.46 respectively). These values are similar to those reported for lavas erupted from Yasur in 1975 and 199329.
In contrast with the arc lavas, but like ocean island basalts in general, the Serreta basalt has 230Th and 226Ra excesses ((230Th/238U) = 1.19 and (226Ra/230Th) = 1.37). Excesses of 230Th and 226Ra are less pronounced in the relatively fractionated benmoreite from Eyjafjallajökull ((230Th/238U) = 1.09 and (226Ra/230Th) = 1.07).
Measured 210Po activities in leached samples range from 0.10 to 1.4 dpm/g (see Supplementary Information Table S1). This contrasts with the 3 to 14 fold higher activities in the leachates of the youngest samples (1.5 to 6.5 dpm/g) resulting from significant Po in sublimates adhered to vesicle walls and ash particles during eruption37. The inferred (210Pb/226Ra)0 ratios of leached samples are all within analytical error of secular equilibrium with the exception of a 14% 210Pb excess in the Home Reef sample and a significant 210Pb deficit in the Serreta sample ((210Pb/226Ra)0 = 0.71; Table 2).
Our (210Pb/226Ra)0 data for Iceland sample EJ-1 is within analytical error of the value published in Sigmarsson et al.22, although our measured concentrations of U, Th and Ra are lower by 6.5–15%, suggesting our fraction of the bulk tephra sample had higher proportions of mineral particles versus glass.
Discussion
The data for young samples presented above are discussed in terms of geographical area and tectonic setting below. We go on to compare these data to the global dataset and address wider implications of short-lived radionuclide disequilibria in lavas.
Southwest Pacific arc samples
The samples from Hunga, Home Reef, and Fonualei in the Tonga arc have large 238U and 226Ra excesses that are typical for, indeed only found in, oceanic island arcs of the western Pacific27,28,31,32. These disequilibria, as well as the trace element patterns and elevated Sr isotope values, are inferred to reflect fluid addition of U, Ra, and Sr to the mantle wedge source from the subducting Pacific plate, in keeping with previous interpretations of U-series data for Tonga arc lavas28,32. The subducting Pacific plate beneath the Tonga arc is relatively cold38 and the high H2O/Ce39, and low Th/La ratios for these lavas (Fig. 4) imply that fluids released from the Pacific plate are water rich and the subducting plate minimally melts. The Havre rhyolite marks an end-member isotopic composition for the Tonga-Kermadec system, with its unusual pairing of low (230Th/232Th) and high 143Nd/144Nd compared to oceanic basalts34 and typical subducted lithologies40,41. These observations are consistent with derivation of most Nd from the local mantle, and most Th from subducting sediment.
For samples from western Pacific arcs, the low (230Th/238U) and high (226Ra/230Th) values associated with samples with high Th/La suggests that these ratios largely reflect the degree to which Th is mobilized from the subducting plate. Symbols as in Fig. 1. Error bars are illustrated for values larger than the sample symbols.
The tight negative and positive correlations respectively between (226Ra/230Th) and (230Th/238U) versus Th/La for our arc samples (Fig. 4) suggests that much of the variation in 226Ra-230Th-238U disequilibria relates to the degree to which Th is mobilized from the subducting plate rather than bulk assimilation of crust or time-scale of fractionation. The Home Reef dacite and Hunga andesites have similarly high (226Ra/230Th) ratios, consistent with similar century to millennium time frames of fractionation for these lavas32. The smaller 226Ra excess but similar U-Th systematics for the Fonualei dacite compared with the other Tonga samples imply a modestly extended time-frame of fractionation, perhaps a few thousand years. The Havre rhyolite has comparatively low 238U and 226Ra excesses, a low (230Th/232Th) value, and a comparatively high Th/La value with respect to the Tonga lavas. We attribute all of these observations to a more significant transfer of Th from subducting sediment28, which resulted in less initial 238U-232Th-226Ra disequilibria in the parental magmas for the Havre rhyolite compared with the Tonga arc, and a time frame of fractionation that also did not exceed several thousand years. Similarly, the incompatible trace element enriched basaltic trachyandesite from Yasur has moderate excesses of 238U and 226Ra over 230Th compared with the Tonga arc samples, which might also reflect transfer of Th from subducted sediment29.
Most lavas from the Tonga-Kermadec and Vanuatu arcs have (210Pb/226Ra)0 ratios within analytical error of 1 (Table 2, Fig. 5). The U-Th-Ra and trace element systematics outlined above indicate that the parental magmas for these lavas were water rich. The equilibrium (210Pb/226Ra)0 values indicate that disequilibrium created by any process including fluid transfer from the subducting plate, melting, and deep degassing decayed away before eruption9. Moreover, the last stage of 222Rn degassing must have occurred over a time period short enough to be undetectable using 210Pb-226Ra disequilibria12. The small 210Pb excess in the Home Reef pumice is can be explained by Rn transfer in a gas phase to the dacite from un-erupted, more mafic recharge magma that may have triggered the eruption14,48.
The phonolite whole rock from Tristan da Cunha8 and phonolite glass separates from Mount Erebus9 are shown separately because they demonstrably have had (226Ra) lowered by crystal fractionation of K-rich feldspars or amphiboles. Data were excluded for samples with unusually high Pb concentrations and high (210Pb/226Ra) values that likely reflect the assimilation of young 210Pb-enriched sublimates5,36,47. Dashed lines denote secular equilibrium. Arcs have wide-ranging (210Pb/226Ra) values but have a median value of equilibrium, whereas ocean island basalts have a median value of 0.76, and rarely have 210Pb excesses. We attribute these variations to differences and volatile profiles and modes of degassing in these different tectonic settings. See text for further explanation.
Atlantic Ocean island samples
The Serreta alkaline basalt from Terceira is characterized by an unusually high (230Th/232Th), but 230Th and 226Ra excesses that are similar in scale to those generally observed for alkaline ocean island basalts elsewhere, which we interpret to reflect the dynamics of melting garnet-bearing mantle49,50,51. The enrichment of U over Th in the source of this basalt that lead to the high Th isotopic composition, therefore, likely took place more than 350,000 years before magma genesis. This ancient U enrichment could have resulted from mantle carbonation52.
The benmoreite we analyzed from Eyjafjallajökull has a major element, trace element, and isotopic composition that is consistent with it representing a mixture between an alkaline basalt and an alkali rhyolite or trachyte21,24. Its low (226Ra/230Th) value (1.066) compared to a basalt erupted in 2010 from Fimmvörðuháls on Eyjafjallajökull’s flank (1.368)22 is most readily explained by several thousand years of aging during differentiation and storage of the silicic mixing end-member.
The initial (210Pb/226Ra)0 ratio of 0.71 for the Serreta sample is similar to the values observed for most10,34,35 but not all48 ocean islands and basalts erupted in continental rift settings45,46. Basalts from Surtsey, Eldfell, and Fimmvörðuháls in Iceland have comparable 210Pb deficits22,46. The origin of these 210Pb deficits is a focus of the next section.
In contrast with Icelandic basalts, the benmoreite from Eyjafjallajökull has a (210Pb/226Ra)0 ratio within error of equilibrium, whereas both the alkaline basalt and trachyte erupted at Eyjafjallajökull have 210Pb deficits22, and mixing between these particular magmas could not have resulted in the equilibrium (210Pb/226Ra)0 value of the benmoreite. Therefore, like the arc samples discussed above, the benmoreite erupted in 2010 appears to have resided in the crust long enough for any original 210Pb-226Ra disequilibrium to decay away. These data also suggest that the basalt that triggered the 2010 eruption was different than the one inferred to have mixed to generate the benmoreite (see also ref. 24).
210Pb in gas-laden magma
Magma migration and degassing can strongly affect the (210Pb/226Ra) values of magmas because of the volatility of Rn. This volatility is illustrated by the near absence of 222Rn in freshly erupted lavas from ocean island and arc settings53. In contrast, Pb is only weakly volatile5. Thus, to produce measurable 210Pb deficits, magmas must continually lose Rn for a minimum of about two years. After a century of degassing, magmas will have (210Pb/226Ra) values close to zero. Comparatively longer durations of degassing are required to produce 210Pb deficits of a particular magnitude if Rn loss is less than 100% efficient5 or is discontinuous14. Excesses of 210Pb over 226Ra can be generated relatively rapidly by transfer of Rn-bearing gasses or fluids from greater volumes of magma and into smaller volumes of magma48. In this section, we explore whether Rn-Ra fractionation is the principal cause of variations in (210Pb/226Ra) values for magmas from both ocean island and arc settings.
The relative concentrations of the volatile species CO2 and H2O in parental magma impact how and when a magma degasses and the consequences of this degassing on its physical properties. Both CO2 and H2O concentrations in primary ocean-island basalts have been estimated to be from several tenths of a percent up to 1 wt.%54,55. Volatile concentrations in basalts from the Azores and Iceland appear to be in keeping with these values. Olivine-hosted melt inclusions from the basalt lava balloons erupted in 2001 from Terceira, Azores preserve water contents of up to 0.9 wt.% H2O and 1500 ppm CO219. Olivine-hosted glass inclusions in basalts from Pico island near Terceira have as much as 0.4 wt.% CO2, implying even higher concentrations in parental magmas52. Melt inclusions in olivines from basalts erupted from Fimmvörðuháls in 2010, which is a flank vent for Eyjafjallajökull, have up to 1 wt.% water56. Olivine-hosted glass+bubble inclusions in basalts erupted from Laki volcano in AD 1783–1784, 130 km to the northwest of Eyjafjallajökull in Iceland, have up to 0.5 wt.% CO257.
CO2 concentrations of primitive arc basalts, including those of the western Pacific arcs, are poorly known because of extensively degassing. Using global volatile and magma fluxes as guides, CO2 concentrations have been estimated to be 0.3 to 1.3 wt.% CO258. We presume that CO2 concentrations in lavas from the western Pacific arcs are in this range. H2O concentrations are significantly higher in primitive arc magmas compared with ocean island basalts, with many mafic lavas, including those for the Tonga arc, recording water concentrations in the 4–6 wt.% range59,60. Based on gas emissions and compositions of olivine-hosted glass inclusions, parental basaltic magmas for Yasur volcano, Vanuatu have approximately 1–2 wt.% H2O and 0.25 wt.% CO225.
Primitive basalts with several tenths of a percent CO2 in both ocean island and arc settings would typically saturate in a CO2-rich vapor phase and potentially begin open-system degassing at deep crust or upper mantle depths55,58. CO2 dominated degassing has little effect on mineral stability and magma viscosity61,62. H2O largely remains dissolved in magmas until the upper several km of crust, where it becomes considerably less soluble63. Open-system loss of H2O from silicate magmas in the upper crust strongly affects viscosity, because it lowers magma temperatures, enhances polymerization, and causes bubble and mineral growth. Latent heat released by crystallization can mitigate the viscosity increase by raising the temperature of a magma64, but overall, the loss of a water-rich volatile phase from magmas typically increases viscosity by orders of magnitude65.
Volatile phase saturation also pressurizes the system by bubble growth, creating a driving force towards eruption that is moderated by open system loss of the gas phase65. It is the unsteady interplay between changes in viscosity, pressurization, and conduit/reservoir geometry that determines whether water loss will cause a magma to freeze and stall out in the crust, or to migrate rapidly to the surface and erupt66,67,68.
Magmas with relatively low H2O/CO2 ratios, such as those found in most ocean islands, typically arrive to the surface environment sparsely crystalline and only modestly differentiated. The subaqueously erupted Serreta basalt is an example of such a magma. If its 210Pb deficit resulted from a steady loss of Rn in a CO2-rich gas phase, then its (210Pb/226Ra)0 ratio of 0.71 suggests a rise time from the depth of CO2-saturation to the surface of 11 years assuming that Rn degassed with perfect efficiency5. The required maximum average magma velocity would depend on the initial CO2 content of the magma. Assuming 1 wt.% H2O, 0.5 wt.% CO2 and a crustal density of 3000 kg/m3, saturation of the gas phase would occur at about 20 km69. The maximum average velocity of Serreta magma in this case would have been about 1.8 km/yr. The (210Pb/226Ra)0 value for the Serreta basalt is nearly identical to the median value for ocean island basalts in general10, suggesting that this magnitude of rise time and magma velocity is typical for ocean island basalts in general.
An alternative explanation for 210Pb deficits in ocean island basalts and MORB is Ra-Pb fractionation during melting. This explanation is based on weak correlations between (210Pb/226Ra) and geochemical parameters that vary with changes in degree of melting, such as (226Ra/230Th)3,10. However, recent studies of (210Pb/226Ra) variations in lavas from individual ocean islands35,36, continental rifts44,45, and mid-ocean ridges44 have not supported melting as the primary cause of (210Pb/226Ra) variations. These studies have illustrated, for example, that although mineral-melt partitioning during magma generation should only produce 210Pb deficits, some lavas from all tectonic settings have 210Pb excesses (Fig. 5). Such excesses can result from crystal fractionation of K-feldspar and amphibole, but only in highly differentiated magmas such as phonolite8 (Fig. 5), not in magmas that crystallize olivine, augite, and plagioclase like the Serreta basalt19.
Only rarely does shallow degassing of water from an ocean island magma cause it to stall out in shallow magma reservoirs for periods of time long enough to equilibrate (210Pb/226Ra) values. Portions of Iceland offer a physical setting where such magma stagnation and differentiation is common because of its rather water rich parental magmas, glaciation, thicker and somewhat older crust70. The benmoreite from Eyjafjallajökull appears to have stagnated at a depth in the crust of about 1.7–5.0 km71 without persistent Rn degassing as evidenced by its equilibrium (210Pb/226Ra) ratio.
As is the case for ocean island basalts, primitive arc magmas must typically begin to saturate in a CO2-rich volatile phase at deep crust or mantle depths. The rare arc basalts that erupt with 210Pb deficits10 reflect this process. However, most arc magmas stall in the crust and differentiate significantly. For parental magmas with 2–6 wt.% water, open-system degassing of water-rich vapor phases begins at depths of 1.5–12 km, which match the observed storage depths for magmas in volcanic arcs60. The Yasur lava is likely at the shallow end of this range based on the probable lower water contents25 for its parental magmas compared with those from Tonga.
Once arc magmas stall, they must typically maintain (222Rn/226Ra) equilibrium for most of the time spent in the reservoir system, which means these magmas must not undergo persistent open-system degassing. Pulses of 222Rn loss by degassing as magmas resided in the crust are allowed, so long as these pulses were spaced in time long enough to prevent measurable decay of 210Pb. Numerical modeling of steady-state systems undergoing periods of complete 222Rn loss followed by periods of repose with (222Rn) = (226Ra) indicates that such systems will have: (210Pb/226Ra) = 1/(1 + td/tr) where td is the time the system spends degassing and tr is the time spent in repose. In this circumstance, values of (210Pb/226Ra) remain within analytical error of equilibrium only if td/tr is less than about 0.03, indicating that any magma with equilibrium (210Pb/226Ra) must not lose Rn for the vast majority of its residence time in the crust.
An alternative explanation is that the fluxes of Rn in and out of the magma are equal. Of these two scenarios, we favor the former because a fortuitous balance between 222Rn lost and gained seems unlikely to have occurred in multiple systems. Another alternative is that that 222Rn supplied from recharge magmas mutes any 210Pb deficit that exists in a resident magma due to degassing. However, this explanation requires magma renewal rates approaching 1 magma volume/year5, which basically just explains why equilibrium (210Pb/226Ra) values are maintained in magmas as they flow rapidly through conduits to the surface. Thus, we conclude that the near equilibrium (210Pb/226Ra) values for eruptives from Yasur, Hunga Ha’apai, Home Reef, Havre, Fonualei and Eyjafjallajökull require that their parental magmas stalled and differentiated in the crust without persistent open-system degassing for more than a century before eruption. For all of these systems, any open-system loss of 222Rn to a gas phase during the final rise toward the surface must have occurred over a period of less than two years to prevent (210Pb) from being measurably less than (226Ra).
Conclusions
Parental magmas in arc and ocean island settings typically have concentrations of 0.3–1.3 wt.% CO2, which leads to the onset of degassing of magmas in the upper mantle or deep crust. This degassing leads to a steady rise of these magmas and a persistent Rn loss that results in the common 210Pb deficits in ocean island settings. The Serreta basalt analyzed here has a near-median (210Pb/226Ra)0 value for an ocean island basalt implying a minimum of 8.5 years of rise to the surface at a velocity as high as about 2.4 km/year after it first saturates in a gas phase. For ocean island basalts, significant H2O degassing typically occurs too shallowly to significantly impede their progress toward the surface. An exception appears to be Eyjafjallajökull, where magmas ponded and differentiated to trachytic to rhyolitic compositions within the relatively thick and glaciated crust. Its system of magma chambers must be relatively complex, because the equilibrium (210Pb/226Ra)0 value for the benmoreite suggests that basaltic and silicic endmembers mixed more than a century before it erupted.
Arc magmas typically have significant subducted H2O58,60, and open-system degassing of this water in the middle to upper crust typically will cause magmas to freeze and stall65. The commonplace equilibrium (210Pb/226Ra) values in arc magmas suggest that once these magmas stall, they must not persistently lose or gain Rn for long enough periods or at high enough rates to affect (210Pb) values. The further implication is that water-saturated magmas do not generally undergo persistent open-system degassing. Remobilization of these stalled magmas might occur by melt-crystal separation or by pressurization caused by magma recharge72.
Water-rich magmas must rise to the surface while degassing over periods of less than 2 years from their final staging reservoir to maintain (210Pb/226Ra) values within analytical error of secular equilibrium. Rise times for water-saturated magmas between staging reservoirs could be similarly rapid. In this circumstance, equilibrium (210Pb/226Ra) values will be maintained throughout magma systems that are saturated in a water-rich volatile phase. Quiescent final staging reservoirs and rapid rise times to the surface implied by equilibrium (210Pb/226Ra) values in water-laden magmas suggests an inherent lack of long-term predictability of explosive eruptions.
Methods
Major elements
Hand specimens were crushed using a stainless steel jaw press and the chips were then ultrasonically washed in MilliQ H2O to remove any seawater contamination. The Havre rhyolite was washed with weak HCl and water. The washed chips were subsequently dried and then powdered in an agate mill. Major element concentrations were determined on a Siemens® SRS300 XRF at the University of Auckland, following standard techniques73. Fresh glass was analyzed for the Havre rhyolite. A polished thin section was carbon coated and analyzed by a Cameca SX100 electron microprobe at Macquarie University. A defocused beam was used with an accelerating voltage of 15 kV and a beam current of 15 nA. Counting times of 10 s were used for both peak and background measurements. The analysis in Table 1 represents the average of 5 spots. Spectrometer calibration was achieved using the following standards: Jadeite (Na), Fayalite (Fe), kyanite (Al), olivine (Mg), chromite (Cr), spessartine garnet (Mn), orthoclase (K), wollastonite (Ca, Si) and TiO2 (Ti).
Trace elements
Sample powders were dissolved in a HF-HNO3 mixture for trace element analysis. Most samples were analyzed at Macquarie University using an Agilent® 7500CS ICP-MS following procedures outlined in Eggins et al.74. Detection limits during the period of study are listed in the Supplemental Materials Table S2. Sample EJ-1 was analyzed using a Thermo X-series II ICP-MS at the University of Iowa using methods described in Peate et al.75.
Radiogenic isotopes
Sr-Nd isotope analysis was performed on ~100 mg of powdered sample that was digested in a HF-HNO3 mix in heated Teflon beakers. Sr and REE fractions were separated using a cationic column containing Biorad® AG50W-X8 (200–400 mesh) cationic exchange resin, after which Sm and Nd were separated using EIChrom® LN-spec resin following the column procedure given by Pin et al.76. Samples were loaded on to out-gassed single (Sr) and double (Nd) rhenium filaments using 2 μl of TaCl5 + HF + H3PO4 + H2O2 and 5 μl of 1 N HCl, 0.35 N H3PO4 activator solutions, respectively. Analyses were performed in static mode on a ThermoFinnigan Triton® TIMS at Macquarie University. Instrument mass fractionation was accounted for by normalizing 87Sr/86Sr and 143Nd/144Nd to 87Sr/86Sr = 0.1194 and 143Nd/144Nd = 0.7219, respectively. Analysis of SRM-987 yielded an 87Sr/86Sr value of 0.710272 ± 4. Values for USGS rock standard BHVO-2 are listed in Supplementary Information Table S2. Sr and Nd blanks were lower than 1000 and 80 pg, respectively. No corrections for blanks were made. Repeated measurements of standard materials during the data acquisition period yielded the following values: 87Sr/86Sr of 0.710259 ± 0.000038 (2 SD; n = 42) for the SRM-987 standard; 87Sr/86Sr of 0.703493 ± 0.000068 (2 SD; n = 52) for BHVO-1; 143Nd/144Nd of 0.511112 ± 0.000017 (2 S.D.; n = 32) for the JMC Nd (321) standard and 143Nd/144Nd of 0.512971 ± 0.000025 (2 SD; n = 45) for BHVO-1.
U-series
Samples were spiked with 236U-229Th and 228Ra tracers and dissolved using an HF-HNO3-HCl mix in heated teflon pressure bombs. Samples were brought into solution, dried, and U, Th and Ra were separated chromatographically76. U and Th concentrations and isotope ratios were measured in dynamic mode on a Nu Instruments® MC-ICP-MS at Macquarie University. Mass bias was determined assuming 238U/235U = 137.88 and the IC0 gain was determined during interspersed dynamic analyses of CRM145 assuming a 234U/238U ratio of 5.286 × 10−5 77,78. Radium aliquots were loaded onto degassed Re filaments using a Ta-HF-H3PO4 activator solution78 and 228Ra/226Ra ratios were measured to a precision typically ~0.5% in dynamic ion counting mode on the TIMS at Macquarie University. Most of the samples were analyzed with TML and BCR-2 standards79,80 between 2008 and 2011 (see Supplementary Information Table S2). For the Serreta and Havre Ra analyses that were performed in 2015, the 228Ra concentration of the spike was determined at the same time as the unknowns by analyzing the TML rock standard and assuming secular equilibrium. Analysis of BCR-2 performed at the same time yielded values that were within 3% of secular equilibrium (see Supplementary Information Table S2). Estimated total analytical errors (2σ) for activity ratios listed in Table 2 include errors on the precision of each analysis and the uncertainties of spike values based on replicate analyses of standards.
For the 210Pb data, 1–2 g aliquots of sample powder were leached in 0.25 N HCl for 20 minutes before decanting the supernatant solution. The residual powder was then rinsed 3 times in MilliQ H2O before drying. The leachates and the leached powders were then spiked with 209Po and analysed by α-counting on an EGG Ortec® system at the University of Iowa in 2010 and 201112,13,43. The (210Pb/226Ra)o values used in Table 1 are based on (210Po) and assume secular equilibrium between 210Pb and 210Po (t1/2 = 138 days) for samples analyzed more than 2 years after eruption. For samples Van-A1 and HH09-01, multiple measurements of 210Po in-growth over about 3 years allowed for extrapolation to equilibrium (210Pb)o values (see Supplementary Information Table S1). Errors quoted in Table 2 are 2σ. (210Po) values measured for USGS standards BCR- 2 and RGM-2 during sample analysis are in Supplementary Information Table S2.
Additional Information
How to cite this article: Reagan, M. et al.210Pb-226Ra disequilibria in young gas-laden magmas. Sci. Rep. 7, 45186; doi: 10.1038/srep45186 (2017).
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Acknowledgements
We are grateful to Nicole Keller, Jim Gill, Victor Hugo Forjaz, Kelipi Mafi, Ian Smith, Marco Brenna, and Simon Kremers for their generosity in providing samples or sample information. Thanks go to Peter Wieland for assistance in the lab, JC Afonso for Matlab code used for fitting radioactive equilibration curves to 210Po data, and Ken Sims for many insightful discussions. Jim Gill, Julie Prytulak, two anonymous reviewers, and the Scientific Reports editorial board are thanked for their comments, which significantly improved the manuscript. This work was directly funded by an Australian Research Council (ARC) Discovery Project DP0988658 to S.T. and M.R. and an ARC Future Fellowship FT120100440 to H.H.
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M.R. and S.T. wrote the main text and prepared figures for the manuscript. H.H., C.B., D.P., M.T. and J.C. reviewed and edited the manuscript. All authors participated in data collection.
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Reagan, M., Turner, S., Handley, H. et al. 210Pb-226Ra disequilibria in young gas-laden magmas. Sci Rep 7, 45186 (2017). https://doi.org/10.1038/srep45186
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