High-resolution decadal-scale eruption age dating of young oceanic basalts at an active hydrothermal vent site

Here we report ( 210 Pb/ 226 Ra), ( 226 Ra/ 230 Th), ( 230 Th/ 238 U) and ( 234 U/ 238 U) disequilibria for eleven lavas from the ABE vent site in the Lau Basin. Most ABE lavas have ( 210 Pb/ 226 Ra) > 1 and ( 226 Ra/ 230 Th) > 3. These results indicate that most of these lavas erupted within the past 100 years. Model ages calculated assuming initial ( 210 Pb/ 226 Pb) = 1.8 – 2.0 further constrain the timing of eruption, suggesting that more than half of the lavas erupted within the past 60 years. When combined with complementary data (side-scan sonar, lava ﬂ ow morphology, tectonic mapping), this high-resolution record provides fundamental time constraints for interdisciplinary studies examining oceanic crustal construction and development of the hydrothermal system in the ABE vent ﬁ eld. Notably the youngest samples cluster around the active vent sites indicating that the ABE vent site ’ s location is a direct consequence of this concentrated young volcanism. This study is the ﬁ rst high resolution U-series study of a sea ﬂ oor vent site and demonstrates the potential of using ( 210 Pb/ 226 Ra) for the determination of lava ages for young submarine lavas in spreading environments with active hydrothermal venting. As such these ( 210 Pb/ 226 Ra) measurements hold the promise for addressing in far greater detail the connections between spreading ridge eruptive and hydrothermal activity on the decadal to century time scales.


Introduction
Time constraints are essential for understanding geological processes at oceanic spreading ridges.Without time constraints, many central questions about how oceanic spreading centers work and how the various aspects of the ridges' volcanic and hydrothermal systems relate to one another are unanswerable.For example, without lava ages it is not possible to relate volcanic activity to tectonics processes, establish temporal changes in lava compositions, and link hydrothermal and biological activities to fluctuations in volcanism.Thus, age constraints are not a detail, they are necessary.Without them, our understanding of ridge systems will be forever limited.
Dating young submarine lavas has proven to be notoriously difficult (e.g., Batiza et al., 1987).Long-lived radiogenic isotope systems like Rb-Sr, Sm-Nd, and U-Th-Pb have half-lives that are too long and lack the temporal resolution for precise ages.Cosmogenic age dating on the ocean floor is not possible.Potassium-Argon dating techniques (e.g., Abdel-Karim and Azzaz, 1995), which have provided crystallization ages in very young sub-aerial lavas, suffer from a myriad of problems when applied to young mid-ocean ridge basalts (MORB), for example: 1) low concentrations of K in MORB yield low Ar (e.g., typically K 2 O in MORB is ~0.1 wt% and so over 10 3 -10 6 years this yields 10 −13 -10 −9 moles/g rock of ingrown 40 Ar); 2) open system behavior of Ar.Although stepwise heating can correct for atmospheric argon, this procedure cannot account for inherited magmatic Ar in MORB glass, the most commonly available sample material.Furthermore, contamination of lavas by seawater K also affects the accuracy of results (Batiza et al., 1987;Duncan and Hogan, 1994;Guillou et al., 2017).Other techniques for dating MORB have had even less success.For instance, fission track dating of MORB glasses is complicated by low U concentrations and consequently low track densities, as well as rapid track annealing (Batiza et al., 1987).Finally, although sediment accumulation has been used to estimate the relative age of seafloor terrain, this method is unreliable for absolute age determinations, as 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 (Klein et al., 2013).
Fortunately, 238 U-series disequilibria can provide robust eruption ages for young oceanic basalts (e.g., Rubin and Macdougall, 1990;Goldstein et al., 1991;Volpe and Goldstein, 1993;Rubin et al., 1994;Lundstrom et al., 1995;Sims et al., 1995;Sturm et al., 2000;Sims et al., 2002;Cooper et al., 2003;Sims et al., 2003;Sims et al., 2008a;Standish and Sims, 2010;Waters et al., 2011;Waters et al., 2013a;Waters et al., 2013b;Haase et al., 2016;Scott et al., 2019;Sims et al., 2021).The 238 U-decay series nuclides have a range of half-lives that are appropriate for dating a wide age range of young basalts, and the application of U-series disequilibria to the dating of young submarine basalts has been well established.While there have been several studies using 238 U-230 Th-226 Ra disequilibria to date oceanic basalts in the range of hundreds of years to hundreds of thousands of years (the half-life of 230 Th is 75 kyr, and 226 Ra is 1.6 kyr), there have been no studies using 226 Ra- 210 Pb to provide high resolution ages on basalts less than 100 years old.
Here we report ( 230 Th/ 238 U), ( 226 Ra/ 230 Th), ( 210 Pb/ 226 Ra) and ( 234 U/ 238 U) disequilibria for eleven lavas from the ABE vent site in the Lau Basin.Because the half-life of 210 Pb is 22 years, the ( 210 Pb/ 226 Ra) measurements provide a temporal record of eruption ages on the order of tens of years.While ( 210 Pb/ 226 Ra) have previously been used to establish age limits for lavas (i.e., less than or older than 100 years, e.g., see Sims et al., 2008a), these results are the first published ( 210 Pb/ 226 Ra) model ages.When combined with complementary data (sidescan sonar, lava flow morphology, tectonic mapping), this highresolution record provides fundamental time constraints for interdisciplinary studies examining oceanic crustal construction and development of the hydrothermal system in the ABE vent field.

Lau Spreading Center
Subduction of the Pacific plate at the Tonga trench has led to the formation of the NE-SW trending Tonga volcanic arc and the associated Lau Basin (Figure 1).Opening of the Lau Basin began at ~6 Ma in response to rapid subduction of the Pacific plate along the Tonga, or Tofua, arc (Hergt and Farley, 1994;Hergt and Hawkesworth, 1994).Spreading south of 18 °S is accommodated along the Central, Intermediate, and Eastern Lau Spreading Centers (ELSC).The ELSC runs from 19-22 °S and can be further divided into five northwest stepping subsegments.From south to north these segments are the Valu Fa Ridge (VFR), ELSC IV, ELSC III, ELSC II, and ELSC I (Figure 1).Along the ELSC, the spreading rate increases northward from <60 mm/yr at 22 °S to 90 mm/yr at 19 °S (Zellmer and Taylor, 2001).Based on comparison with open ocean ridges, the ELSC's morphology might be expected to vary from rift valley in the south to crustal plateau in the north.Instead, the slower spreading VFR is at a shallow water depth, has thicker crust, limited faulting, a prominent axial high, and a bright axial magma chamber reflector (AMCR).As spreading rates increase northward, the ridge deepens from 1,700 to 3,300 m, a well-developed rift valley forms, the crust thins, and the AMCR disappears.These changes occur as distance from the arc front increases from 35 km at 22 °S to 100 km at 19 °S (Martinez et al., 2006;Jacobs et al., 2007;Ferrini et al., 2008).The progressive change in distance from the volcanic front suggests that increased influence of the subduction component in the south leads to enhanced magmatic productivity, and that proximity to the arc trumps spreading rate in its influence on ridge morphology (Martinez et al., 2006).Distance from the arc also influences magma composition, as enrichment in mobile trace elements is greater in the southern segments of the ELSC (Escrig et al., 2009).
To take advantage of this unique tectonic setting, the Ridge 2000 program designated the Lau Basin one of three global integrated study sites (ISS).Initial bathymetry and side-scan sonar surveys were conducted in 2004 aboard the R/V Kilo Moana (KM0410; F. Martinez, Chief Scientist).Miniature Autonomous Plume Recorders (MAPRs) were also deployed to search for signs of hydrothermal activity (Baker et al., 2005;2006;Martinez et al., 2006).A second cruise built upon this data by deploying the autonomous underwater vehicle (AUV) ABE and the Woods Hole Oceanographic Institute (WHOI) Towed Camera System (Tow Cam) to generate 1-2 m grid resolution bathymetry of areas where hydrothermal activity was suspected and to collect photos of the seafloor to ground truth previous observations.These data were used to constrain the location of three vent fields: ABE, Tow Cam, and Kilo Moana (Figure 1), with the ABE vent field being selected as the "bullseye" for the Lau ISS Ridge 2000 program.Because of the hydrothermal venting and the apparent young ages of the lavas our study focuses on the lavas of the ABE vent site (Figure 1).

ABE vent site
The ABE vent field (20 °45.8′S, 176 °11.5′W), is located 5 km south of the northern terminus of ELSC IV, ~600 m west of the ridge axis, and at a water depth of 2,140 m.The transition from axial plateau to axial valley, as well as the disappearance of the ACMR, occurs just north of the ABE vent field.Within the ABE vent field, hydrothermal activity is localized at five sites, with an additional area of diffuse flow.The area is cut by three NE-SW striking normal faults, with throws of 10-20 m.Three flow fronts were also identified.Lava flows have a pillow/lobate texture, with columnar basalts visible in fault scarps (Martinez et al., 2006;Jacobs et al., 2007;Ferrini et al., 2008).
The ABE vent site, which is the focus of this study, is located on the central segment of the ELSC in the Lau back-arc basin (Figure 1).This portion of the ridge is broad (4.2 km) and tall (200 m) and a NE-SW fault area dominates the field.Several lava different flows with distinct surface morphologies are apparent throughout the ABE survey area.Additionally, remote operated vehicle (ROV) Jason 2 identified three areas of hydrothermal activity along 600 m of the NE-SW trending fault.For a highly detailed description of the ABE vent area the reader is referred to Ferrini et al. (2008).
Because the timescale of mantle circulation is on the order of hundreds of millions to billions of years it can be reasonably assumed that the mantle source starts out with U-series nuclides in a state of radioactive equilibrium.During petrogenetic processes, most significantly during partial melting, U-series disequilibrium is created by fractionation of parent-daughter nuclide pairs.In the absence of secondary processes (e.g., post-eruptive alteration), any disequilibria generated by magmatic processes are "locked in" once the lava has erupted and has solidified.The activity ratio then acts as a "stop-watch," with parent and daughter returning to secular equilibrium after about five halflives of the daughter isotope.Thus, the presence of disequilibria between a parent-daughter pair provides a maximum eruption age.Since 238 U-230 Th-226 Ra- 210 Pb have half-lives ranging from ~20 years to 75 kyr, these techniques are appropriate for dating MORB erupted 0.1-375 kyr.Quantitative estimates of ages by other U-series methods are also possible.Internal isochrons, for example, make use of the differential retention of some radionuclides by different minerals (e.g., Ra is preferentially incorporated into feldspar).However, this technique requires phenocryst-rich samples and is therefore not suitable for MORB glasses.Even when samples contain abundant phenocrysts, the resolution of this method is limited by uncertainties in the phenocryst ages (i.e., have the phenocrysts resided in crustal storage for a significant period of time relative to the half-life of the daughter nuclide).
If the initial extent of disequilibrium at the time of eruption is known, then the difference between the initial activity ratio and the measured activity ratio can be used to determine the lava's eruption age.These "model ages" improve the resolution of the U-series ages by roughly an order of magnitude for each parent-daughter pair; tens of thousands of years resolution for 230 Th model ages and hundreds to thousands of years resolution for 226 Ra model ages.It is important to note that model ages assume a constant source (spatially and temporally) and are the sum of a sample's crustal residence time (i.e., magma storage time) and their eruption age.When determining U-series model ages for a related group of samples, several assumptions must be accounted for (Sims et al., 2003): 1.The lavas reflect melting of source material which has spatially and temporally constant U/Th.2. The initial extent of disequilibria of the lavas is known.This requires similar degrees of melting and melting depths, as well as similar ascent rates (e.g., McKenzie, 1985;Williams and Gill, 1989;Spiegelman and Elliott, 1993;Lundstrom et al., 1995;Sims et al., 1995;1999;2002).3. The U-series disequilibria result from primary magmatic processes, and the lavas remained a closed system with respect to U, Th, Ra, and Pb after eruption.4. The timescales of magma storage and eruption after the processes generating U-series disequilibria ceases must be short relative to the half-lives of 230 Th, 226 Ra, and 210 Pb.
When these conditions are met, U-series model ages can provide constraints for the timing of eruption.This is demonstrated in studies of the East Pacific Rise, where large data sets and the necessary isotopic and geochemical data are available to develop meaningful model ages.In this setting, model ages indicate that midocean ridge magmatism occurs over a wide area and is not limited to the axial trough (Sims et al., 2003;Waters et al., 2013a;Waters et al., 2013b).

Sample collection
Samples were collected during three cruises: KM0417 in 2004 (R/V Kilo Moana; C. Langmuir, Chief Scientist), TUIM05MV in 2005 (R/V Melville; M.K. Tivey, Chief Scientist), and MGLN07MV in 2006 (R/V Melville; C. Fisher, Chief Scientist).Although the samples collected during KM0417 consist of two dredge samples and two rock cores, the remaining seven samples collected during TUIM05MV and MGLN07MV were collected using ROV Jason 2, and thus their locations and geological context are well-known (Table 1; Figure 1).

Analytical methods
Major elements (Table 2) were measured on glass chips using a Cameca SX-100 electron microprobe (EMP) at Rensselaer Polytechnic Institute (RPI) and a Cameca SX-50 at the University of Massachusetts, Amherst (See Bezós et al., 2009 for analytical details).Trace elements (Table 3) were measured by solution nebulized ICP-MS and laser ablation ICP-MS (SN-ICP-MS and LA-ICP-MS) at Harvard University (See Bezós et al., 2009 for analytical details).Pb, Sr, and Nd isotope ratios were measured on dissolved solutions at the Lamont-Doherty Earth Observatory (LDEO; see Escrig et al., 2009 for analytical details).
Briefly, the 238 U, 234 U, 232 Th, 230 Th and 226 Ra concentrations and isotopic compositions (Table 4) were measured on dissolved and spiked solutions on the WHOI ThermoFisher NEPTUNE following procedures described by Sims et al., 2008a;Sims et al., 2008b), Ball et al. (2008) and Scott et al. (2019).The activities of 210 Pb were measured by analyzing samples for its descendent 210 Po and assuming ( 210 Pb/ 210 Po) = 1 before Pb-Po chemical separation.Analytical details for 210 Po are discussed in Reagan et al. (2005) and Waters et al. (2013).Because the U-decay series measurements are the focus of this paper, the full analytical details for these methods are discussed in full in Supplementary Appendix SA.U-Series abundances and activities for two quality assurance standards, USGS Reference Material Columbia River Basalt (BCR-2) and the U-series community reference material Table Mountain Latite (TML) are also reported in Table 4.

Results
On a total alkali silica diagram (Figure 2), the ABE site lavas are basaltic andesites, with silica contents ranging from 53.62-54.88weight percent, total alkalis (Na 2 O plus K 2 O) ranging from 2.88 to 3.77 weight percent and molar Mg numbers ranging from 32 to 40 (Table 2).The ABE samples are light rare earth element (LREE) depleted with CI normalized (Palme and O'Neill, 2014) (La/Dy) n ranging from 0.49 to 0.52 and have flat heavy rare earth (HREE) element patterns with (Dy/Yb) n ranging from 0.99 to 1.01 (Table 3; Figure 3).These ABE samples' LREE are more depleted than average N-MORB and their HREE are slightly more enriched and show a limited range in composition compared to the rest of the ELSC.On an extended trace element diagram normalized to primitive mantle (Palme and O'Neill, 2014), these samples have distinct enrichments of Ba, U and K and relative depletions of Nb and Ta (Table 3; Figure 3).Overall, these lavas are tholeiitic based on the high FeO*/MgO at approximately 54% SiO 2. Their depleted LREE patterns and enrichment in fluid-mobile elements, indicate that they are very similar to classic arctholeiites.Compositionally, it is important to note that one sample, RC-086, has a distinctive chemistry with lower LREE and other trace element abundances, most notably Pb, than the rest of the ABE samples.Thus, sample RC-086 is plotted separately on all figures to highlight its chemical and isotopic distinction.
Radiogenic isotope ratios are only available for three ABE samples: two dredge samples (DR50-1 and DR51-1) and one rock core sample (RC-086) (Table 3).Results for the two dredge samples (DR50-1 and DR51-1) were previously reported in Escrig et al. (2009)  87 Sr/ 86 Sr ranges from 0.703008-0.703265.The compositional range of long-lived radiogenic isotopes observed within the ABE vent field is limited, especially when compared with the whole of the ELSC (Figure 4).
U-Series abundances and activities for the ABE samples and two quality assurance standards (BCR-2 and TML) are reported in Table 4. Recommended values for these U-series standards are tabulated in Sims et al. (2008b) and Scott et al. (2019).For a full discussion of the pedigree and site locations for each of these reference materials see Sims et al. (2008b).In the following discussion we have listed the results of RC-086 separately.
• ( 234 U/ 238 U) ranges from 1.000 to 1.004, with an average ( 234 U/ 238 U) of 1.002 ± 0.001 (2SD).Both standard reference samples, TML and BCR-2, have ( 234 U/ 238 U) of 1.001 ± 0.002.Although all samples measured have one to two per mil ( 234 U) excesses we note that these measurements were not conducted with abundance sensitivity (RPQ) filtering, so there is likely some down mass contribution from 235 U on 234 U (Scott et al., 2019 Since secular equilibrium is achieved after ~5 half-lives, the presence of disequilibria between a parent-daughter pair provides an eruption age limit.All ABE lavas have ( 230 Th/ 238 U) < 1, indicating they were erupted less than 350 kyr ago and ( 226 Ra/ 230 Th) > 1, indicating they were erupted less than 8 kyr ago.Far more stringent constraints on their age are that all the ABE lavas (except RC-086) have ( 210 Pb/ 226 Ra) > 1, implying that these lavas erupted within the past 100 years.
In practice, eruption ages are better constrained than these age limits because samples with large disequilibria are likely much younger than the maximum age limit.To further constrain ages, if the initial extent of disequilibrium at the time of eruption is known or can be reasonably approximated, then the difference between the initial activity ratios and the measured activity ratios can be used to determine the lava's eruption age.These "model ages" significantly improve the resolution of the U-series ages.For these ABE lavas, we use our ( 210 Pb/ 226 Ra) measurements to provide decadal constraints on lava eruption ages.
Frontiers in Geochemistry frontiersin.org For the following reasons we argue that these ABE lavas are ideal for determining ( 210 Pb/ 226 Ra) model ages.
(1) The lavas share a common source.Although long-lived radiogenic isotope ratios are only available for three ABE samples, the isotopic and major and trace element compositional ranges observed within the ABE vent field are small.For example, compared to the ELSC (Figure 4) range of 206 Pb/ 204 Pb of 18.13-18.66, 206Pb/ 204 Pb within ABE is 18.46-18.49,or roughly 6% of the total range in ELSC lavas (Escrig et al., 2009).Therefore, it is reasonable to conclude that the ABE lavas share a common mantle source.
(2) The ABE lavas reflect similar extents of melting, with similar melting and ascent rates.Except for sample RC-086, the ABE lavas with 210 Pb excesses have a limited range of major and trace element abundances (Tables 2, 3; Figures 2, 3) and have similar extents of ( 230 Th/ 238 U) and ( 226 Ra/ 230 Th) disequilibria (Figure 5).(3) The ABE lavas' U-Th-Ra-Pb disequilibria result from primary magmatic processes.Although ( 234 U/ 238 U) of the ABE samples are not exactly within secular equilibrium, it is highly unlikely that these slight excesses of 234 U are a result of secondary alteration processes because of the likely slight tailing of 235 U on 234 U. Furthermore, these  Given that all the ABE samples except RC-086 meet the above criteria, we calculate (  Turner, 2010;Reagan et al., 2017).To model these samples' eruption ages, we use two possible and reasonable end-member scenarios as bracketing starting conditions.Although, somewhat arbitrary, we have chosen the given endmembers {( 210 Pb/ 226 Ra) 0 = 1.82 to 2.0 and ( 226 Ra/ 230 Th) 0 = 3.02 to 3.55} because they closely bracket the observed range in the observed lavas (Figure 7; Table 4).We then interpolate a model zero-age isochron between the two endmembers (Figure 7).Given this model zero-age isochron we use the samples' ( 226 Ra/ 230 Th) to establish their initial ( 210 Pb/ 226 Ra) and calculate the time of decay toward ( 210 Pb/ 226 Ra) since eruption.Because of the large difference in the half-lives of 210 Pb (t 1/2 = 22 years) and 226 Ra (t 1/2 = 1,600 years), the decay trajectory in ( 226 Ra/ 230 Th) versus ( 210 Pb/ 226 Ra) space (Figure 7) is essentially vertical.Over a period of one-hundred years, ( 226 Ra/ 230 Th) decreases by only 3%.For all samples except for RC-086, we calculate model eruption ages for all samples of 0-100 years, with an average eruption age for the vent field being roughly 53 ± 29 years at the time of analysis in 2013.It is important to note that because the model age isochrons get closer together over time the uncertainties in the sample ages increase.
In contrast to all other ABE vent site samples, RC-086 has a ( 210 Pb/ 226 Ra) that is slightly below unity but within error of equilibrium at the one-sigma level of uncertainty, and its measured ( 226 Ra/ 230 Th) is much lower than all other samples (Figure 7).The simplest explanation for this difference is that RC-086 is much older than the other samples; its equilibrium ( 210 Pb/ 226 Ra) indicates it erupted more than 100 years ago and, if one assumes the initial ( 226 Ra/ 230 Th) was the same as the young ABE samples (on the order of 1.8-2.0),then its measured ( 226 Ra/ 230 Th) provides a model eruption age of 1,800-2,500 years.However, some caution is warranted with this simple older eruption interpretation.RC-086, which is located on the northern edge of the vent field, is compositionally distinct from other ABE lavas (Figures 1-6), including having different ( 230 Th/ 238 U).Additionally, secondary alteration and post-eruptive Pb loss could be responsible for the lower 210 Pb in this sample, since RC-086 has lower Pb concentration than the other samples and higher ( 234 U/ 238 U), which is also suggestive of secondary alteration.Therefore, RC-086 is either older than the other ABE samples, or the model ages for this sample are not valid because of either alteration of different initial starting conditions, or both.
To first order our model ages indicate that, at the 68% uncertainty (1σ), all the ABE samples except RC-086 were erupted less than 100 years before the time of measurement (2013), with an average eruption age of 53 years.These very young model ages for the ABE vent field are consistent with both visual observations of these samples (Figure 8) as well as roughness analysis from side-scan sonar (Figure 9),  suggesting that there is little sediment cover (Ferrini et al., 2008).Not surprisingly, the youngest of these samples cluster around the active vent sites.We thus infer that the ABE vent site's location is a direct consequence of this concentrated young volcanism.Additionally, it is notable that the oldest eruption ages were measured for flows on the northern and southern ends of this vent site, consistent with the observations of Ferrini et al. ( 2008) that noted the heaviest sedimentation was observed in the extreme northern and southern extents of the survey area.
The fact that all the ages of lava flows associated with the vent field are so young raises interesting questions about the timing relationships between eruptions and hydrothermal activity.One implication of the very young ages would be that recent eruptive activity leads to, or is at least associated with, active hydrothermal venting.Are all the hydrothermal vents along the ELSC also associated with very recent hydrothermal activity?Are eruptions on-going at this location?Is the hydrothermal activity at this site perhaps declining?Would lavas from portions of segments without recent activity be significantly older?The measurements we report hold the promise for addressing in far greater detail the connections between eruptive and hydrothermal activity on the decadal to century time scales.

Lau ridge construction
A notable aspect of the ABE vent field is that it is located ~600 m west of the ridge axis.If one assumes that the lavas erupted on axis and that the average spreading rate at the latitude of ABE is ~80 mm/ year, then the expected age of these lavas is ~7,500 years (Martinez and Taylor, 2002).There are two possible ways to reconcile the much younger apparent and model ages (Table 5) compared to the spreading rate ages.The first possibility is that magmatism in the ELSC is not localized at the ridge axis, and that off-axis magmatism has occurred quite recently.The second possibility is that the flows originated at the axis and have traveled as surface flows or through lava tubes to distances >600 m (e.g., Sims et al., 2003 for discussion on flows that likely originated at the axis and traveled off-axis at 9-10 °N East Pacific Rise).Given the patterns of faulting, the bathymetry showing shallower depths and inflation adjacent to the vents, and the observed flow fronts, this later possibility is highly unlikely.Thus, we conclude that the ABE vent site is a locus of off-axis volcanism.

Petrological implications
These ABE lavas are notable in that they are highly enriched in ( 210 Pb) relative to ( 226 Ra).Such excesses of ( 210 Pb) over ( 226 Ra) have been reported for subsets of MORB and arc lavas and are commonly attributed to decay of 222 Rn within an accumulated gas phase (e.g., Reagan et al., 2006;Sims et al., 2008a;Berlo and Turner, 2010;Condomines et al., 2010;Reubi et al., 2015), typically near a density or viscosity barrier within the magmatic system (e.g., Kayzar et al., 2009;Waters et al., 2013).In an arc setting, magmas are enriched in water from subducting plates.Loss of this water causes crystallization, which may result in rheological barriers to volatile-phase migration from below (e.g., Reagan et al., 2017).In a spreading center environment, magma differentiation away from a melt lens can also create a lowdensity, high-viscosity magma barrier (Waters et al., 2013).Thus, we similarly posit, for the following reasons, that these ABE lavas' ( 210 Pb) excesses were also created by a low-density, high-viscosity, rheological magma barrier which prevented volatile phase migration and caused a build-up of a 222 Rn-rich magma, which subsequently resulted in an ingrowth of 210 Pb: (1) These ABE lavas were erupted off-axis.(2) These lavas have geochemical signatures suggesting that they have been significantly influenced by subduction processes and are therefore relatively water-rich.(3) These lavas are significantly differentiated and are thus high viscosity and volatile rich.
7 Conclusion indicates that they erupted within the past 100 years.However, given the magnitude of their 210 Pb-226 Ra disequilibria, model ages suggest that many of the lavas erupted within the past 60 years (Table 5).• In combination with high-resolution bathymetry, this highresolution record provides fundamental time constraints for interdisciplinary studies examining oceanic crustal construction and the development of hydrothermal systems on in the ELSC.• The young ages, shallower bathymetry, and localized faulting with emanating flow fronts at the ABE vent site represents a region of off-axis volcanism.• While it may be possible to argue that sample RC-086 has an eruption age of 1,800-2,500 years, it is important to keep in mind that, although this sample is indeed likely to be much older, it has also undergone post-eruptive, secondary alteration thereby rendering its exact model age as uncertain.
• Lastly, we argue that these ABE lavas' ( 210 Pb) excesses were created by a low-density, high-viscosity, rheological magma barrier which prevented volatile phase migration and caused a build-up of a 222 Rn-rich magma, which consequently resulted in an ingrowth of 210 Pb.

Funding
The author(s) declare that financial support was received for the research, authorship, and/or publication of this article.This work was funded by National Science Foundation NSF OCE-0732480 (KS), NSF OCE-0732449 and OCE-0751844 (CL).
FIGURE 1 (A) Bathymetric map of the ABE vent field showing sample locations.(B) The large-scale map shows the segments of the Lau Spreading Center (Valu Fa, and the center and northern ELSC) and the various hydrothermal vents listed from south to north: Vai Lili, Mariner, Tui Malila, ABE (denoted with a red star), Tahi Moana, Tow Cam and Kilo Moana.The small inset shows the global context of the Lau Spreading Center.See Ferrini et al. (2008) for more details on imaging and location.

FIGURE 3 (
FIGURE 3 (A) CI normalized (Palme and O'Neill, 2014) rare-earth element plot.The green field is the ABE vent site and the dark green is the average of the ABE samples, excluding RC-086.Grey field is the Eastern Lau field with dark grey line being the average, with data for Eastern Lau field from Bezos et al. (2009).Sample RC-086 is plotted separately on all figures to highlight its chemical distinction.The mean composition of MORB comes from Gale et al. (2013).Average oceanic arc is from Kelemen et al. (2014).Average arc tholeiite comes from a weighted average of data from Vanuatu and Palau, Schmidt and Jagoutz (2017).(B) Primitive mantle normalized (Palme and O'Neill, 2014), extended trace element plot.

FIGURE 8
FIGURE 8Sample photos taken by ROV Jason 2, arranged in order of model age (left to right with top row being the youngest).

TABLE 1
ABE hydrothermal vent site sample description.

TABLE 4 (
Continued) ABE hydrothermal vent site U-series abundances and isotopes.
210Pb/ 226 Ra) model ages for the time (T) since sample eruption using: Pb 226 Ra m − 1 210 Pb 226 Ra o − 1 where ( 210 Pb/ 226 Ra) m is the measured ratio, ( 210 Pb/ 226 Ra) o is the calculated initial ratio at the time of the lavas' eruption (Table 5), and λ 210 is the decay constant of 210 Pb.Calculating these ( 210 Pb/ 226 Ra) model eruption ages requires an assessment of the initial extent of 210 Pb excess at the time of these samples' eruption.Although ( 210 Pb/ 226 Ra) as high as seven has been measured, ( 210 Pb/ 226 Ra) greater than two is extremely rare (Berlo and

TABLE 5 (
210Pb/ 226 Ra) Model Ages.Age uncertainties are the upper and lower limits in years from the mean model age assuming 1σ uncertainty for210Pb and 2σ uncertainty for 226 Ra measurements.For example, RC-083 ranges from 22 to 35 years, with a mean model age of 28 years.Uncertainties are propagated as described under Table4.Ages are reported relative to the date of analysis (2013). a • 210 Pb-226 Ra-230 Th disequilibria presented here provide robust decadal scale age constraints for the ABE vent field lavas.While these results are specific to the ABE vent field, they demonstrate the potential of using 210 Pb-226 Ra disequilibria as a high-resolution chronometer for young lava fields of unknown age.• All the samples except RC-086 have ( 210 Pb/ 226 Ra) > 1, which