1. In a study of lavas from Vailulu'u and Malumalu in Samoa, Sims et al. [42] demonstrate that the calculated <sup>210</sup>Po age, which provides an eruption date of 8 November 2004 (+62 days/-67 days), falls within a window of bathymetric imaging showing new dome growth occurring sometime between March 1999 and April 2005 (Figure 33).

2. In a study of the EPR, Tolstoy et al. [232] found a 2+ year buildup of seismic activity preceding a sudden drop in seismic activity on 22 January 2006. While this event was interpreted as a singular eruption along the East Pacific Rise, the <sup>210</sup>Po dates in lava samples from this event show a wider range of eruption dates, from June 2005 to January 2006 [232]. The apparent discrepancy between the calculated ages and seismic observations is controversial; however, a simple interpretation that reconciles this discrepancy is that the <sup>210</sup>Po ages may reflect the buildup of seismic activity representing numerous smaller, distant eruptive events rather than precursors to a singular eruptive event.

**5. Timescales of Magma Degassing**

Magma recharge and degassing are directly linked to volcanic activity. Determining the timescales of these shallow-level processes is fundamental to our understanding of physical eruption dynamics and hazard assessment.

At many quiescent but potentially dangerous volcanoes, persistent degassing of volatile constituents dissolved in magma after melting is a common manifestation of volcanic activity. Placing constraints on magma residence times in shallow degassing reservoirs and/or feeding systems is critical to understanding magma dynamics beneath these volcanoes. Measurement of radioactive disequilibria between short-lived <sup>238</sup>U-series isotopes in volcanic gases and lavas can provide constraints on the timescales of shallow-level magma dynamics (e.g., magma recharge and degassing). While there are a few studies that have measured <sup>222</sup>Rn, a noble gas, or its daughter nuclides, <sup>210</sup>Po, <sup>210</sup>Pb, and <sup>210</sup>Bi in gas emissions from actively degassing open-conduit systems [227,228,230], most inferences about magma degassing timescales have come from measurements of ratios between <sup>210</sup>Po, <sup>210</sup>Pb, and <sup>226</sup>Ra in lavas.

Measurements of (<sup>210</sup>Pb)/(<sup>226</sup>Ra) in erupted lavas are ideal for constraining the pro-
cesses and timescales of magma degassing because (1) <sup>222</sup>Rn, which is an intermediary
nuclide between <sup>226</sup>Ra and <sup>210</sup>Pb in the <sup>238</sup>U-decay series (Figures 2 and 34A), is a noble gas
that is highly volatile; and (2) the 22.3 year half-life of <sup>210</sup>Pb is appropriate for monitoring
gas fluxes in magmas over a timeframe from several decades to years before eruption.
Degassing processes generally have a negligible effect on the concentrations and activities
of <sup>210</sup>Pb and <sup>226</sup>Ra themselves, as Ra is not volatile and only about 1% of Pb typically
degasses from magmas [227,228,230]. In contrast, Rn strongly partitions from magma into
gas [195,233]. Hence, if magma degassing endures for years to decades, (<sup>210</sup>Pb/<sup>226</sup>Ra)
values are lowered as a function of the half-life of <sup>210</sup>Pb (Figure 34A,B) and the <sup>222</sup>Rn
degassing efficiency [219,223]. Similarly, if <sup>222</sup>Rn concentrates in the gas phase of a magma
that has stalled somewhere, then <sup>210</sup>Pb generated by decay of <sup>222</sup>Rn can lead to a <sup>210</sup>Pb
excess (Figure 34A,B) in the magma (e.g., [205,220,234]). However, because of the large
differences in the half-lives of <sup>222</sup>Rn (3.8 days) and <sup>210</sup>Pb, the erupted lavas must derive
from a small part of the whole magma system, and the ratio of the magma that is degassing
to the amount of magma that accumulates the <sup>222</sup>Rn and erupts must be on the order of
tens to thousands [220,223]. Lastly, if stored magma is undersaturated in volatiles or has
maintained a steady pressure and temperature for a hundred years or more, (<sup>210</sup>Pb)/(<sup>226</sup>Ra)
may attain a state of radioactive equilibrium [88].
<figure><img src="image_40.png" /><figcaption>**Figure 34.** (A) Schematic illustration showing how accumulation or degassing of <sup>222</sup>Rn can lead to either (<sup>210</sup>Pb) excesses relative to (<sup>226</sup>Ra) or (<sup>210</sup>Pb) deficits relative to (<sup>226</sup>Ra). (B) Global compilation of (<sup>210</sup>Pb/<sup>226</sup>Ra) versus (<sup>226</sup>Ra/<sup>230</sup>Th) for arc and oceanic samples. Data from Gauthier & Condomines [219], Turner et al. [205], Rubin et al. [187], Reagan et al. [220,221]; Berlo & Turner [222]. Note both the <sup>210</sup>Pb excesses and deficits relative to <sup>226</sup>Ra.</figcaption></figure>

Like <sup>222</sup>Rn, <sup>210</sup>Po, the immediate daughter of <sup>210</sup>Pb, is volatile at magmatic temperatures. With its short half-life of 138 days, <sup>210</sup>Po is useful for constraining degassing of magmas in the timeframe from months to weeks before eruption. Most lavas erupt with magmatic (<sup>210</sup>Po/<sup>210</sup>Pb) values near zero, indicating that they were degassing Po and other volatiles until eruption [195,220,229–231]. There are exceptions to this rule, however, including at Mount Erebus, which is discussed below. Prior work has attributed elevated (<sup>210</sup>Po/<sup>210</sup>Pb) values to (a) less efficient degassing resulting from a rapid rise of magma to the surface, (b) late-stage magmatic stagnation, or (c) infusion of Po from streaming gasses after magmas reach the near-surface environment (e.g., [202,235]).

In the following, we briefly discuss several examples using (<sup>210</sup>Pb/<sup>226</sup>Ra) and, where available, (<sup>210</sup>Po/<sup>210</sup>Pb) to understand magma degassing processes in mid-ocean ridge (9°03N EPR and the Lau Basin ABE vent site) and ocean island/plume settings (Hawaii, Tristan de Cunha, Mount Erebus Volcano). These examples are chosen to illustrate situations where (<sup>210</sup>Pb/<sup>226</sup>Ra) is greater than one, less than one, and at unity and clearly demonstrate that the relationships between crystallization, recharge, and volatile behavior are complex, but tractable, through the measurement of (<sup>210</sup>Pb/<sup>226</sup>Ra) coupled with other petrological, geochemical, and isotopic constraints.

## 5.1. Magma Degassing in Ocean Ridge Settings

While there is a substantial data set for (<sup>210</sup>Pb/<sup>226</sup>Ra) in young arc volcanic lavas (Figure 34B), there are relatively few studies using (<sup>210</sup>Pb/<sup>226</sup>Ra) to understand magma degassing in MORB. Early measurements of (<sup>210</sup>Pb/<sup>226</sup>Ra) disequilibria in MORB reported predominantly small <sup>210</sup>Pb deficits, with <sup>210</sup>Pb/<sup>226</sup>Ra as low as 0.86 [187,236], which were argued to be a result of melting (Figures 34B and 35B), as Ra is significantly more incompatible in crystals than Pb during mantle melting [54].
<figure><img src="image_41.png" /></figure>

**Figure 35.** (A) ($^{226}$Ra/$^{230}$Th) versus ($^{210}$Pb/$^{226}$Ra) for lavas from the overlapping spreading center (OSC) at 9°03′ N EPR. Note the significant excesses of $^{210}$Pb relative to $^{226}$Ra for the basalts and basaltic andesites. Data from Rubin et al. [187] include samples from Juan de Fuca Ridge, northern and southern EPR, and Loihi. (B) Axis-perpendicular schematic cross-section of the east limb of the 9°03′ N OSC, illustrating how Waters et al. [53] envision the formation of $^{210}$Pb excesses in basalt beneath a dacite magma body and the eruption of an off-axis pillow ridge from an off-axis magma body. (C) The inset illustration is an enlarged view of the sub-axial magma bodies and corresponds to the dashed box above. The illustration is not to scale. Additionally, while the illustration suggests the presence of a dacitic magma body overlying the sub-axial magma sill, the location of dacitic magma is not explicitly known. Detailed images and information on the seismically determined distribution and geometry of subsurface melt can be found in Kent et al. [237] and Singh et al. [238].

However, small $^{210}$Pb excesses (~10%) were measured in young MORB (Figures 34B and 35B) from the Axial Seamount on the Juan de Fuca Ridge [187,239]. Radon is more incompatible than Pb during mantle melting making $^{210}$Pb excesses more difficult to explain by simple mantle melting. However, because Pb is expected to behave more compatibly than Ra in both plagioclase and sulfide minerals, the $^{210}$Pb excesses could have been generated by preferential melting of plagioclase or sulfide minerals in the mantle or crust (perhaps from melt-rock reactions). However, these minerals would need to be young and have pre-existing $^{210}$Pb excesses. However, if this were the case, then one would expect correlations between geochemical indices of plagioclase or sulfide melting and ($^{210}$Pb/$^{226}$Ra) disequilibria, which are not observed in MORB with $^{210}$Pb excesses.

More recently, larger excesses of $^{210}$Pb have been observed at two spreading-ridge locations, with ($^{210}$Pb/$^{226}$Ra) as high as 1.69 at 9°03 N EPR (Figure 35; [51]) and 1.82 at the Lau Basin ABE vent site (Figure 32; [80]). In both cases, the large $^{210}$Pb excesses are posited to be caused by shallow magma chamber accumulation of $^{222}$Rn produced by deeper
magmatic degassing. In general, buoyancy and viscosity relationships (i.e., Stokes’ Law) suggest that gas bubbles will rise fast enough through basaltic magma (but not through more viscous, SiO₂-rich magmas) to allow differential motion of Rn-bearing gas and melt phases over timescales similar to the half-life of ²²²Rn. However, it is worth noting that, as suggested for Pinatubo [240], Rn-bearing gas may rise even more rapidly through pipe vesicles and fractured viscous magma.

In the case of 9°03′ N EPR (Figure 35), there is a wide range of erupted lava compositions that includes viscous, dacitic lavas and an imaged melt lens [237]. Waters et al. [53] envisioned a scenario in which a relatively large volume of basaltic magma residing within the lower to middle crust is continuously degassing. Radon-bearing, CO₂-rich gas bubbles rise and accumulate within a smaller, shallow reservoir of basalt and basaltic andesite beneath a larger volume of more viscous, dacite magma. ²²²Rn accumulates in the resulting bubble-rich basaltic froth and decays to ²¹⁰Pb. Density instability due to accumulation of this buoyant basaltic froth beneath dacitic magma may also facilitate magma mixing between basaltic and dacitic magma end-members, resulting in eruptions of mafic to intermediate lavas with large ²¹⁰Pb excesses.

In the case of the ABE lavas (Figure 32), which erupted off-axis from the main Lau Spreading center, Sims et al. [80] similarly posit that their ²¹⁰Pb excesses were created by a low-density, high-viscosity, rheological magma barrier which prevented volatile phase migration and caused a build-up of water and ²²²Rn in an underlying magma layer. This hypothesis is supported by two observations about the ABE lavas: (1) they have geochemical signatures suggesting they have been significantly influenced by subduction processes and therefore are relatively water-rich; and (2) they are relatively SiO₂-rich with high viscosities, which would cause a rheological barrier.

## 5.2. Magma Degassing in Ocean Island Plume Settings

### 5.2.1. Kilauea

Kilauea volcano on the island of Hawaii is one of the Earth's most active volcanoes. During the period from 1983 to 2018, Kilauea erupted nearly continuously, predominantly from the Pu'u'O'o vent, with brief eruptions from other vents and fissure systems. Gas emissions at the summit caldera occurred over the entire period [241]. Girard et al. [158] analyzed (²¹⁰Pb/²²⁶Ra) ratios in lavas erupted over this period up until 2008. With the exception of lavas emitted when vents first opened, lavas from Pu'u'O'o, the nearby Kupahianaha vent, and Halema'uma'u within the summit caldera had (²¹⁰Pb)/(²²⁶Ra) = 0.80 ± 0.11. These ²¹⁰Pb deficits were attributed to loss of Rn partitioned into CO₂-rich gasses from the point of volatile saturation at about 30 km depth to the surface over an average of 8 years. These data show that lavas erupted steadily at Kilauea rise to the surface after volatile saturation without significantly stalling within its magma chamber system. In contrast, more crystalline lavas that erupted immediately after the vent openings at Pu'u'O'o and Napau, which is up-rift of Pu'u'O'o, had near-equilibrium (²¹⁰Pb/²²⁶Ra) values, implying that these lavas had been stored in the rift system for several decades or more before being pushed out by newly intruding magmas.

Explosions in March 2008 opened a vent along the lower east wall of Halema'uma'u, ejecting basalts and altered lithic fragments. These explosions were followed by effusion of lavas that filled the crater. This marked the first eruption in recorded history when lavas simultaneously erupted from the summit and the East Rift Zone at Kilauea. Tephra samples with altered rock fragments and Pele's hair had ²¹⁰Pb excesses over ²²⁶Ra resulting from ²²²Rn decay as well as minor Pb condensation and infusion from magmatic gasses (Figure 36). Lava samples erupted during this episode have (²¹⁰Pb/²²⁶Ra) values ranging from 0.8 to 1, suggesting they represented both stored and new lavas [158].
<figure><img src="image_42.png" /></figure>

**Figure 36.** Measured ($^{210}$Po) values in decays per minute per gram (circles with 2σ error bars) and ingrowth curves for two samples erupted from Kilauea plotted against the number of days between the eruption and the date of analysis. Also plotted are ($^{226}$Ra) values for these samples. Samples have been leached with 0.5 M HCl to remove polonium from surfaces. (A) Lava sample erupted in 2005. Partitioning of polonium into the gas phase led to an initial ($^{210}$Po) value near 0. The best fit ingrowth curve yields an initial ($^{210}$Pb) value of 0.204 dpm/g, which is below the measured ($^{226}$Ra) value of 0.244 dpm/g (dashed line). Deficits of $^{210}$Pb with respect to $^{226}$Ra in Kilauea lavas are likely caused by persistent $^{222}$Rn loss during magma degassing before eruption [158]. For this sample, 6 years of persistent Rn-degassing would be needed. (B) Pele's hair erupted in 2008. The leached sample had excess $^{210}$Po, which decayed towards an initial ($^{210}$Pb) value above that of ($^{226}$Ra) suggesting that the Pele's hair fibers contained Pb absorbed from the gas phase. The leachate for this sample had ($^{210}$Po) = 952 dpm/g, illustrating the high ($^{210}$Po) concentrations in sublimated particles generated from cooling magmatic gasses [158].

Several samples erupted between 2005 and 2008 were also serially analyzed for ($^{210}$Po). This serial analysis allows fitting of a radiogenic ingrowth curve and thus determination of both ($^{210}$Po) and ($^{210}$Pb) at the time of eruption. All lavas erupted from Pu'u 'O'o have near-zero initial ($^{210}$Po)/($^{210}$Pb) values (Figure 36). Several samples from Halema'uma'u had similarly low values, including one with an equilibrium ($^{210}$Pb)/($^{226}$Ra) value. Thus, newly arrived magmas as well as some of the shallowly stored magmas must have degassed until eruption. Pele's hair and tephra samples erupted in 2008 from Halema'uma'u, in contrast, had significant excesses of ($^{210}$Po) over ($^{210}$Pb) with measured ($^{210}$Po)/($^{210}$Pb) values as high as 6.5 soon after eruption. This is despite acid leaching of the Kilauea samples to remove $^{210}$Po-bearing sublimates from sample surfaces. These $^{210}$Po excesses were created by the infusion of Po from magmatic gasses into particles making up the fragmental deposits. A presence of sublimated Po on the surfaces of tephra particles was clearly shown by the high ($^{210}$Po) values measured in leachates, particularly from the Pele's hair sample (Figure 36). This illustrates a rarely mentioned, but noteworthy, hazard associated with erupting, as well as passively degassing, volcanoes, namely the presence of short-lived radionuclides, including those that decay by energetic alpha emissions, on ash and smoke particles in volcanic plumes.
### 5.2.2. Tristan de Cunha

Tristan de Cunha island is a remote oceanic volcanic center in the South Atlantic Ocean at the west end of the Walvis Ridge, which has been attributed to mantle melting in a long-lived mantle plume. On 19–20 July 2004, a period of intense seismic activity was recorded by newly installed Comprehensive Test Ban Treaty Organization/International Monitoring System seismometers on Tristan da Cunha, indicating that there had been an eruption on a rift zone flanking the nearby Nightingale Island. A few days later, pumice began to wash up on local beaches. Samples of the pumice were collected by the British Geological Survey, and one was analyzed for ($^{210}$Pb/$^{226}$Ra), as reported by Reagan et al. [221]. The pumice was phonolite, a highly differentiated lava produced by crystal fractionation from an alkaline basalt. $^{210}$Pb was enriched over $^{226}$Ra by about 50% in the phonolite, despite accompanying evidence for degassing of volatile elements such as Po, and thus Rn, associated with the eruption. Instead of gas accumulation, the $^{210}$Pb excess in this case was attributed to the extensive fractionation of feldspars and amphiboles required to produce phonolite from basalt, as both of these phases preferentially incorporate Ra over Pb into their crystal structures. Modeling of these and other data showed that the original basaltic magma was low in volume and had intruded into the rift zone no more than two centuries before eruption. The significant $^{210}$Pb excess in the pumice showed that the crystal fractionation to generate the phonolite was likely ongoing until the time of eruption. Serial analysis of $^{210}$Po in the same sample (Figure 37) showed that it had an initial ($^{210}$Po)/($^{210}$Pb) value of 0.15 upon eruption, illustrating that the pumice in question was indeed ejected during the July 2004 eruption [221].

<figure><img src="image_43.png" /></figure>

**Figure 37.** Measured ($^{210}$Po) and ingrowth curves for an acid-leached sample of phonolite pumice collected from a beach on Tristan da Cunha in September 2004. Also plotted is the measured ($^{226}$Ra) value for this sample (dashed line). The illustrated $^{210}$Po deficit with respect to $^{210}$Pb shows that this pumice erupted within a few months of the date of collection. The most likely origin of the pumice was a subsea eruption associated with an earthquake swarm offshore of Nightingale Island on 29 and 30 July 2004. The ($^{210}$Pb) excess over ($^{226}$Ra) calculated from the ($^{210}$Po) ingrowth curve was attributed to rapid crystal fractionation of Ra-bearing minerals from the magma over a period of several decades to a century before the eruption [221].

### 5.2.3. Mount Erebus Volcano

Mount Erebus Volcano has a HIMU radiogenic Sr, Nd, Hf, and Pb isotopic sig-nature [242] and a thermal structure suggestive of deep mantle upwelling [243,244]. Mount Erebus also has the most persistent, continuously degassing lava lake on Earth [88,89,242,245]. The Erebus lava lake degasses both by quiescent emissions and
strombolian eruptions with distinct gas compositions [246]. These originate from different depths within the magma conduit, with the quiescent emissions coming from shallow passive degassing, and deeper eruptions deriving from deep within the system ([89], and references therein).

Measurements of U and Th decay series nuclides in young Erebus lava bombs (collected at the time of eruption) show ($^{210}$Pb/$^{226}$Ra) of unity and are thus in radioactive equilibrium [88]. This observation, that $^{210}$Pb is in equilibrium with $^{226}$Ra, is rather surprising as one would expect that removal of $^{222}$Rn by magma degassing would have a significant impact on the ($^{210}$Pb/$^{226}$Ra) of the magma. To explain the measured ($^{210}$Pb/$^{226}$Ra) of unity in the young, known-age Erebus phonolite bombs, Sims et al. [88] posited that magma degassing for the phonolitic magma occurred deeper and hundreds of years before eruption and therefore ($^{210}$Pb) and ($^{226}$Ra) had reached a state of radioactive equilibrium. In this scenario, the anorthoclase-rich phonolite circulating in the upper part of the magmatic system is made of old, degassed magma that does not communicate with gas emissions derived from deeper parts of the magma chamber. These ongoing emissions are rising to the surface and causing the periodic strombolian eruptions (see, e.g., Sims et al. [89] and references therein for an overview of degassing processes on Erebus volcano).

This scenario is consistent with (1) the observation that the melt inclusions from anorthoclase megacrysts are highly degassed compared to melt inclusions in olivine from parental basalts from around the flanks of the volcano [247–249]; (2) calculated magma residence times, using measurements of all relevant $^{238}$U, $^{235}$U, and $^{232}$Th decay nuclides from phonolite glass and anorthoclase megacrysts in known-age lava bombs, are on the order of a thousand years (Figure 29); and (3) for the 2005 lava bomb, whose eruption date (16 December 2005) is known explicitly, serial analysis showed ($^{210}$Po)/($^{210}$Pb) = 0.28, which is significantly higher than observed in most zero-aged lavas, indicating that $^{210}$Po was not completely degassed from the lava at the time of eruption. This ($^{210}$Po)/($^{210}$Pb) value was modeled to show that the shallow magma/lava lake system, on average, degasses about 1% of its Po per day [88].

## 6. Brief Overview of Techniques for Measuring U-Series Nuclides in Oceanic Volcanic Rocks

The U and Th decay series isotope systematics (see Section 2) make the measurement of U and Th decay series nuclides quite challenging for two reasons:

1. Because the equilibrium abundances of the U and Th decay series nuclides are proportional to their respective half-lives ($a = \lambda n$, where $\lambda$ = activity, $\lambda = \ln(2)/\text{half-life}$, and $n$ = number of atoms), the short-lived progeny of $^{232}$Th, $^{238}$U, and $^{235}$U occur in very low concentrations. For example, in most oceanic basalts, $^{232}$Th and $^{238}$U are generally found in weight concentrations of ng/g to ug/g, $^{234}$U and $^{230}$Th in concentrations of pg/g, $^{226}$Ra in concentrations of fg/g, and $^{228}$Ra and $^{210}$Pb in concentrations of ag/g.

2. Isotope ratios involving long-lived and short-lived nuclides are often quite extreme as the equilibrium atomic ratios of the various elements are set by the ratio of their decay constants; as such, the measured ratios are typically on the order of $10^{-5}$ for $^{234}$U/$^{238}$U, $10^{-6}$ for $^{230}$Th/$^{232}$Th, $10^{-3}$ for $^{226}$Ra/$^{228}$Ra, and $10^{-8}$ for $^{210}$Pb/$^{208}$Pb.

Early measurements of U decay series nuclides relied exclusively on alpha- and gamma-spectrometric methods that counted individual decay events [182,195,229,231,250]. These counting methods provided seminal and cutting-edge U and Th-decay series data and are still used today for short-lived nuclides such as $^{210}$Pb, $^{210}$Po, $^{228}$Th, and $^{227}$Ac [44,52,80,87,88,158,202,251,252]. However, for the longer-lived U and Th decay series isotopes $^{232}$Th, $^{238}$U, $^{230}$Th, and $^{226}$Ra, these decay counting techniques have very large uncertainties due to their low count rates. Thus, mass spectrometric methods that
count atoms rather than decay events, such as TIMS, SIMS, and MC-ICPMS, significantly improve sensitivity and precision for the measurement of the longer-lived isotopes such as $^{238}$U-$^{234}$U-$^{230}$Th-$^{226}$Ra and $^{235}$U-$^{231}$Pa, while simultaneously decreasing analysis times significantly [42,163,206,253–261]. Readers are referred to the above papers for details on chemical separations and purification methods and counting and mass spectrometric techniques. For a one-to-one comparison of samples measured by both alpha and gamma versus mass spectrometry, see Sims et al. [259].

Finally, quality assurance needs careful attention, particularly when calculating U-series disequilibria, as these ages involve multiple measurements and therefore propagation of many and varied sources of analytical uncertainty. This includes determining the uncertainties of each individual factor leading to the analytical measurement. Such uncertainty propagation can become complicated when there is covariance between measurement errors. Several papers explicitly address the proper handling of errors associated with U and Th decay series chronological measurements (e.g., [262] and references therein). It is important to note that reference materials, run as unknowns, are essential for establishing both the precision and accuracy of the measurements. One crucial quality of U and Th decay series measurements is that the equilibrium decay principles establish the relative abundances of the parent-daughter pairs. Determining both the precision and accuracy of these abundances requires use of unaltered quality assurance materials (e.g., well characterized international rock standards) that are old relative to the half-lives of the isotope system of interest (e.g., [259,260,262,263]).

## 7. Summary

This paper demonstrates the potential of U and Th decay series for unraveling the temporal complexities of oceanic magmatism. While there admittedly are remaining uncertainties limiting the results that are obtainable from U and Th decay series measurements, there are several robust results.

1. **Defining the timescales of magma genesis during decompression mantle melting.**

The formation and transport of basaltic magma in the mantle during adiabatic decompression can be characterized in terms of the velocity of the upwelling solid mantle; the rate of melting; the melt velocity; melt transport processes (timescale, focused and diffuse, closed or open system); and the delay time, or storage time, of magma before eruption. While the velocity of upwelling mantle and the corresponding melting rate can be estimated from models coupled with thermodynamic data and geophysical observations (e.g., [29,30,146,264]), the melt velocity and magma storage times are difficult to estimate from first principles and require measurement. Fortunately, because the daughter isotopes $^{230}$Th and $^{226}$Ra of the $^{238}$U decay series and the daughter isotope $^{231}$Pa of the $^{235}$U decay series have half-lives that bracket the timescales over which melting and melt extraction are thought to occur, measurements of ($^{230}$Th/$^{238}$U), ($^{226}$Ra/$^{230}$Th), and ($^{231}$Pa/$^{235}$U) ratios in young basaltic lavas provide unique information on the timescales and modes of melt generation and transport.

2. **Determining the eruption ages of oceanic Quaternary volcanism.** Without lava ages, it is not possible to relate volcanic activity to tectonics processes, establish the timing and duration of changes in lava compositions, and link hydrothermal and biological activities to fluctuations in submarine and subaerial oceanic volcanism. Thus, age constraints are not a detail, they are necessary. Without them, our understanding of oceanic systems will remain limited. Dating young submarine lavas has proven to be notoriously difficult—long-lived radiogenic isotope systems such as Rb-Sr, Sm-Nd, Lu-Hf, and U-Th-Pb have half-lives that are too long and hence lack the temporal resolution for precise ages; cosmogenic age dating on the ocean floor is not possible;
potassium-argon dating techniques suffer from a myriad of problems when applied to young oceanic basalts; and sedimentation rates are known to vary widely over short spatial scales. Thus, the visual appearance of lavas provides only a subjective, semi-quantitative estimate of age [265]. Fortunately, <sup>238</sup>U and <sup>232</sup>Th decay series disequilibria can provide robust constraints on the eruption ages of young oceanic basalts. The application of the <sup>238</sup>U, <sup>235</sup>U, and <sup>232</sup>Th for dating eruption ages of volcanic rocks uses three different approaches: (1) *age limits*, whereby the absence or presence of disequilibria can place bounds on the eruption age of a lava; (2) *internal mineral isochron methods*, which establish the crystallization age of a volcanic rock; and (3) *U-Th-Ra 'model' ages*, which couple measurements of U and Th disequilibria with geological, geochemical, and isotopic constraints to estimate the eruption age of an individual lava or a sequence of lavas.

3. **Establishing the timescales of magma recharge and degassing.** Processes operating within magma reservoirs affect both the physical state and the chemical composition of magmas within them. Measurements of (<sup>210</sup>Po), (<sup>210</sup>Pb), and (<sup>210</sup>Bi) in gas emissions from actively degassing open-conduit systems and (<sup>210</sup>Pb)/(<sup>226</sup>Ra) and (<sup>228</sup>Ra/<sup>228</sup>Th) in young known-age erupted lavas provide otherwise unobtainable constraints on the timescales of magma recharge and degassing. These timescales are directly linked to a volcano's activity and are thus fundamental to our understanding of physical eruption dynamics and hazard assessment.

**Author Contributions:** Conceptualization, K.W.W.S.; Writing—Original Draft Preparation, K.W.W.S., G.J.S., L.J.E. and M.K.R.; Visualization, K.W.W.S., G.J.S. and L.J.E.; Writing—Review and Editing, K.W.W.S., G.J.S., L.J.E., M.K.R., P.B.K. and J.B.-T. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research was funded by the National Science Foundation (NSF), grant numbers EAR-0960270, OCE-0603838, OCE-0627053, EAR-0307123, OCE-0137325, EAR-0001924, and OCE-9730967.

**Data Availability Statement:** No new data were created or analyzed in this study. Data sharing is not applicable to this article.

**Acknowledgments:** The insightful discussions over the past three decades with many of the researchers cited in this review have significantly deepened our understanding of U-series processes. This work has been generously funded by NSF. Lastly, I (K.W.W.S.) would like to express my gratitude to Bill White for inviting me to undertake this task and for his editorial guidance throughout.

**Conflicts of Interest:** The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as potential conflicts of interest.

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