Magmatic history of the Oldest Toba Tuff inferred from zircon U–Pb geochronology

The magmatic history of the Oldest Toba Tuff (OTT), the second largest in volume after the Youngest Toba Tuff (YTT), northern Sumatra, Indonesia, was investigated using U–Pb zircon dating by LA-ICP-MS. Zircon dates obtained from surface and interior sections yielded ages of 0.84 ± 0.03 Ma and 0.97 ± 0.03 Ma, respectively. The youngest OTT zircon ages were in accordance with the 40Ar/39Ar eruption age of ~ 0.8 Ma, whereas the oldest zircon dates were ~ 1.20 Ma. Therefore, the distribution of zircon U–Pb ages is interpreted to reflect protracted zircon crystallization, suggesting that the estimated 800–2,300 km3 of OTT magma accumulated and evolved for at least 400,000 years prior to eruption. This result is comparable to the volume and timescales of YTT magmatism. The similarities of both magmatic duration and geochemistry between OTT and YTT may indicate that they are similar in size and that the caldera collapse that generated OTT might be much larger previously interpreted.

. Data are shown in Supplementary Table S5.
Scientific Reports | (2020) 10:17506 | https://doi.org/10.1038/s41598-020-74512-z www.nature.com/scientificreports/ of the cauldron block at Lake Toba ( Fig. 1) and the seismically imaged sill-like magmatic complex below it 22 , the associated intrusive volume is likely batholithic in scale 7 . The location of eruptive vents for the second largest OTT eruption are not well constrained. Chesner 18 suggested the OTT vented in the south of the present Toba Caldera (Fig. 1) based on the ignimbrite exposures in the southeastern part of the present caldera. Alternatively, Knight et al. 14 suggested that OTT also vented from the northern part of the caldera based on a magnetic fabric investigation of the volcanic deposits.
Petrologic studies conducted at Toba 17,18 are consistent with a silicic mush model 23 and a compositionally zoned magma chamber 24 . Modeling results from these studies suggest that the quartz-bearing Toba tuffs and associated calderas represent the surface expression of a large granitoid batholith that episodically provides monotonous composition melts to shallow magma reservoirs 18 . All TCC samples have initial 87 Sr/ 86 Sr between 0.71333 and 0.71521 17,18 , suggesting a compositionally restricted source of the parental Toba magmas to the crust, precluding an origin by differentiation of basalt 17 . Chesner 17 suggested that the source rocks of Toba magmas were metavolcanics and metasediments, and that a continental sedimentary source from Paleozoic metasedimentary basement 25 is required. Similarly, Budd et al. 26 suggested that Toba quartz crystals exhibit comparatively high δ 18 O values, up to 10.2‰, due to magma residence within, and assimilation of, local granitic basement.
In this study, two OTT samples were collected from an outcrop along the western margin (caldera wall) of Lake Toba (Fig. 1). Sample 1809-2 was collected ~ 100 m above sample 1809-1 on the same roadcut exposure during a field trip conducted by International Association of Volcanology and Chemistry of the Earth's Interior (IAVCEI) in 2018. The OTT samples are both strongly indurated to welded ignimbrite with fiamme. The samples are crystal rich, containing quartz, plagioclase, sanidine, biotite and amphibole with accessary minerals of Fe-Ti oxides, zircon, apatite and allanite. Sample 1809-2 was assumed to be YTT based on mineralogy (YTT quartz is pale whereas OTT quartz is typically pinkish; YTT has less biotite and has a lower proportion of crystals compared to OTT). However, the sample was determined to be OTT based on new U-Pb dating result in this study.

Results
Ninety-nine zircon rims and 100 zircon interiors (shallow and deep sections) were analyzed for U-Pb isotopic ages by LA-ICP-MS. For some zircon crystals, both the rim and interior sections were analyzed to compare age difference within a single grain. The results are separately shown in Supplementary Tables S2-S4. Data with high (> 75%) common Pb contamination or high (> 70%) uncertainty were excluded for further interpretation because these data are less reliable. In total, the OTT zircon rims yielded a weighted mean age of 0.84 ± 0.03 Ma (95% confidence level; MSWD = 1.7; n = 76), shallow zircon interiors yielded an age of 0.96 ± 0.05 Ma (MSWD = 2.7; n = 66), and deep interiors yielded 0.97 ± 0.03 Ma (MSWD = 3.6; n = 87) (Fig. 3).

Discussion
The eruption age for OTT is based on previously reported 40 Ar/ 39 Ar analyses; 789 ± 12 ka (2σ) on sanidine 27 , 799.7 ± 19.9 ka (2σ) on biotite 28 , and 798.8 ± 23.7 ka (2σ) on glass shards 4 . They are all in agreement with the cyclostratigraphic estimate of 788.0 ± 2.2 ka, using data from marine boreholes 3,28 . Therefore, I assume that the OTT erupted at ~ 0.8 Ma. An allanite Th-Pb crystallization age of 0.83 ± 0.04 Ma (2σ) reported for OTT 29 is also in agreement with the interpreted eruption age.
All the youngest weighted mean ages from the three different OTT zircon depth sections overlap the eruption age of 0.8 Ma within 2σ uncertainty (Fig. 3), indicating that zircon crystallization continued just before the eruption. The 0.84 ± 0.03 Ma from the rim section is resolvably younger than the ages measured from zircon interiors, reflecting continued overgrowth of zircon crystallization. Moreover, the fact that the deep section age of 0.97 ± 0.03 Ma is identical to the shallow section age of 0.96 ± 0.05 Ma indicates that inheritance of older material is rare. In fact, there were three inherited zircons out of 151 analyzed zircons in OTT (i.e., zircon 1809-1-2-30 yielded a ~ 573 Ma rim, zircon 1809-1-40 was ~ 44 Ma and ~ 68 Ma in shallow and deep sections, respectively, and zircon 1809-2-1 yielded ~ 4 Ma and ~ 48 Ma in shallow and deep sections, respectively). These inheritance ages were omitted in calculated weighted mean ages.
The small proportion of inherited zircon in OTT is similar to results from the YTT. Reid and Vazquez 7 suggested that the paucity of inherited zircon in YTT is notable because relatively old crustal material several kilometers-thick clearly played an important role in melt petrogenesis. They postulated crustal material was incorporated without significant entrainment of zircons and/or country rock zircons were nearly quantitatively resorbed during crustal melting, whose conditions are favorable in the lower crust. Similar assimilation conditions are also interpreted for OTT magma.
Comparison of the ages from rim, shallow, and deep sections obtained on the same zircon grains is not straightforward to interpret (Fig. 4a). In general, zircon ages should become younger toward crystal surface but this trend is not statistically distinguishable due to the relatively low analytical precision of LA-ICP-MS analyses. For example, zircons 1809-2-10 and 1809-2-11 show younger and older rim ages than their interior ages, respectively. Cathodoluminescence (CL) image of 1809-2-10 ( Fig. 5a) shows clear oscillatory zoning which is indicative of progressive and punctuated crystallization and consistent with its younger rim age. On the other hand, CL image of 1809-2-11 (Fig. 5b) show patchy sector zoning. Although it is difficult to explain the older rim age, the CL image may indicate a different growth history from zircon 1809-2-10. Zircon 1809-2-29 has a darker rim in CL than its interior (Fig. 5c), which corresponds with the higher uranium content in the rim (Fig. 4b).
The fact that some rim ages are as old as ~ 1.2 Ma (Fig. 3a) and some interior ages are as young as ~ 0.8 Ma (Fig. 3b,c) may indicate that zircons grew at different times in different parts of the magma reservoir due to intra-reservoir convection and remobilization processes, similar to YTT petrogenesis 26 (Fig. 3c). From this, I interpret that OTT zircon crystallization started at ~ 1.20 Ma, coeval with the HDT eruption, and that the duration of OTT zircon crystallization prior to eruption suggests a magmatic residence time of > 400,000 years for OTT magma. This is comparable with the 500,000 years of YTT magmatism 7 . Although evidence for the vents or caldera margin associated with the OTT is scarce because the YTT presumably engulfed them 30 , the similar whole rock and zircon geochemistry of OTT and YTT 3,17,18,31,32 (Fig. 2) and similarly long magmatic duration may indicate that they are similar in size. The commonly accepted caldera size of YTT is at least 2 times larger than OTT in dimensions (Fig. 1), whereas the actual caldera size of OTT might be larger than that shown in Fig. 1. New results from this study suggest the distribution of YTT might be overestimated, as I have obtained U-Pb ages that are consistent with OTT for an outcrop locality that was previously assumed to be YTT. The Pusuk Buhit Lava Flows, located near the sites of the OTT sampled in this study (Fig. 1), has a reported zircon U-Pb age of 0.91 ± 0.10 Ma (2σ) 33 , which also may indicate some of the OTT vents are located near Pusuk Buhit.
Finally, it is evident from this study and Matsu'ura et al. 13 that LA-ICP-MS can be used to obtain information on the latest phase of zircon crystallization (material analyzed predominantly shallower than 5 μm depth), www.nature.com/scientificreports/ which is comparable to the spatial resolution of SIMS analyses 7,34 . LA-ICP-MS is more convenient compared to SIMS and when applied to sample surfaces, as in this study, it has the potential to yield robust U-Pb ages from the latest phase of zircon crystallization.

Methods
Zircons from OTT samples (each ~ 1 kg) were separated using standard heavy liquid and magnetic separation techniques, yielding a few hundred zircons from each sample. Most zircons are euhedral to subhedral, including large (> 400 μm in length) and elongated zircons ( Supplementary Fig. S1). Many zircons contain inclusions of glass and euhedral apatites of various size (Fig. 5). Most zircons exhibit oscillatory growth zoning in CL, although some are sector zoned (Fig. 5, Supplementary Fig. S1). Zircons were embedded in a PFA Teflon sheet and unpolished surface was targeted for analyses. Zircon U-Pb dating was performed at the Central Research Institute of Electric Power Industry, using LA-ICP-MS (a Thermo Fisher Scientific ELEMENT XR magnetic sector-field ICP-MS coupled to a New Wave Research UP-213 Nd-YAG laser) with experimental conditions following Ito et al. 35 and Ito and Danišík 36 (Supplementary Table S1). Zircon interior ages were obtained as follows: A 30 μm laser beam with ~ 7 J/cm 2 energy density and 10 Hz repetition rate was used to ablate the sample for 30 s following a 30 s background measurement. U-Pb ( 238 U-206 Pb) ages were obtained using the 10-20 s laser ablation data for shallow section, and using the 20-30 s data for deep section. Note that the first 10 s data were discarded due to signal instability. Because a 30 s laser ablation creates a pit of ~ 27 μm in depth in zircon (Supplementary Figs. S1, S2), I assumed that the shallow section U-Pb age is derived from material ablated between ~ 9 and 18 μm depths and ~ 18 to 27 μm depths for deep sections. Note that this procedure cannot exclusively sample age information at the targeted depths because mixing from shallower depths happens to some extent 37 .
In order to obtain zircon surface (or near surface) ages, the following conditions were also employed: a 40 μm laser beam with ~ 7 J/cm 2 energy density, 5 Hz repetition rate and a horizontal scan speed of 2 μm/s for 20 s. In this condition, a shallow (maximum depth: ~ 8 μm) triangle-shaped pit (in profile view) approximately 80 μm in length was created ( Supplementary Fig. S2). Therefore, > 80% in volume analyzed from the surface is shallower than 5 μm in depth. The surface (or rims) U-Pb ages were obtained using the 10-20 s laser ablation data. U and Th concentrations were quantified by comparing counts of 238 U and 232 Th for the sample relative to the standard 91500, which is assumed to have homogeneous U and Th concentrations of 80 and 30 ppm respectively 41 , followed by a correction relative to NIST SRM 610 glass standard. The uncertainty of U and Th were not quantified but should be < 20% considering the measured U and Th concentrations of secondary standards (Plešovice and Bishop Tuff) (Supplementary Tables S2-S4) and NIST SRM 610 glass standard.
No down-hole isotopic (Pb/U, Th/U) fractionation correction was performed because data from the same depth range (or time span) was used for standards and unknowns in each analysis. Representative raw timeseries U-Pb signal data are shown in Supplementary Figs. S3-S8.
The rim, shallow, and deep section U-Pb ages from the Plešovice zircon were between ~ 334 and 339 Ma, consistent with the reference value of 337 Ma (Supplementary Tables S2-S4). All the Bishop Tuff ages (~ 0.73 Ma) were slightly younger (~ 40,000 years younger) than the reference value of 0.77 Ma (Supplementary Tables S2-S4), which does not affect the main conclusions.

Data availability
Data are available in Supplementary information.