We use the tephrostratigraphic framework along the Aegean Volcanic Arc established in Part 1 of this contribution to determine hemipelagic sedimentation rates, calculate new tephra ages, and ...constrain the minimum magnitudes of (sub)plinian eruptions of the last 200 kyrs. Hemipelagic sedimentation rates range from ∼0.5 cm/kyr up to ∼40 cm/kyr and vary laterally as well as over time. Interpolation between dated tephras yields an eruption age of ∼37 ka for the Firiplaka tephra, showing that explosive volcanism on Milos is ∼24 kyrs younger than previously thought. The four marine Nisyros tephras (N1 to N4) identified in Part 1 (including the Upper (N1) and Lower (N4) Pumice) have ages of ∼57 ka, ∼63 ka, ∼69 ka, and ∼76 ka, respectively. Eruption ages for the Yali‐1 and Yali‐2 tephras are ∼55 ka and ∼34 ka, respectively. The Yali‐2 tephra comprises two geochemically and laterally distinct marine facies. The southern facies is identical to the Yali‐2 fall deposit on land but the western facies has slightly less evolved glass compositions. Overall, erupted plinian and co‐ignimbrite fall tephra volumes range from <1 to 56 km3 (excluding possible caldera fillings and ignimbrite volumes), and 80% of the eruptions had magnitude 5.5 < M ≤ 7.2 (M = log(m)‐7; m = erupted magma mass in kg). Twenty percent of the tephras represent 3.2 < M < 5.5 eruptions. The long‐term average tephra magma mass flux through highly explosive eruptions of Santorini is estimated at ∼40 kg/s. The analogous data for the Kos‐Yali‐Nisyros volcanic complex is less‐well constrained but similar to Santorini.
Plain Language Summary
Sediment cores from the seafloor of the eastern Aegean Sea contain numerous ash layers from (sub)plinian eruptions from the Aegean Volcanic Arc that were correlated in Part 1. These correlations facilitate determination of sedimentation rates of ∼0.5–∼40 cm/kyr within the hemipelagic sediment bracketing the dated tephras. Sedimentation rates show temporal and lateral variations in the context of climate changes, and regional tectonics. Exceptionally high hemipelagic sedimentation rates within the last 4 kyrs, are linked to the 3.6 ka Minoan and the 1650 AD Kolumbo eruptions that emplaced abundant erodible tephra. Using the sedimentation rates we additionally determine the ages of hitherto undated tephras. We deduce an age of ∼37 ka for a Milos eruption, as well as ∼57 ka to ∼76 ka for marine Nisyros and ∼34 and ∼55 ka for Yali tephras, for which previous dating attempts yielded controversial ages. The ash distribution in the marine realm of up to 105 km2 represents a major fraction of the erupted tephra volumes that range from <1 to 56 km3, placing 60% of the investigated eruptions into magnitude category M6, 20% into M7, and 20% into M3 to M5 classes. Over the past ∼200,000 years, Santorini discharged magmas at an average rate of ∼40 kg/s.
Key Points
Tephrochronology for the Aegean Arc eruptions
Sedimentation rate variability in the Aegean Sea
Eruptive volumes and masses for the major Aegean Arc eruptions
The Milos, Christiana‐Santorini‐Kolumbo (CSK) and Kos‐Yali‐Nisyros (KYN) volcanic complexes of the Aegean Volcanic Arc have repeatedly produced highly explosive eruptions from at least ∼360 ka into ...historic times and still show recent unrest. We present the marine tephra record from an array of 50, up to 7.4 m long, sediment cores along the arc collected in 2017 during RV Poseidon cruise POS513, which complements earlier work on distal to ultra‐distal eastern Mediterranean sediment cores. A unique set of glass‐shard trace element (LA‐ICPMS) compositions complements our major element (EMP) data on 220 primary ash layers and 40 terrestrial samples to support geochemical fingerprinting for correlations with 19 known tephras from all three volcanic complexes and with the 39 ka Campanian Ignimbrite from the Campi Flegrei, Italy. The correlations include 11 eruptions from CSK (Kameni, Kolumbo 1650, Minoan, Cape Riva, Cape Tripiti, Upper Scoriae 1 and 2, Middle Pumice, Cape Thera, Lower Pumice, Cape Therma 3). We identify a previously unknown widespread tephra from a plinian eruption on Milos (Firiplaka Tephra). Near the KYN we correlate marine tephras with the Kos Plateau Tuff, the Yali 1 and Yali 2 tephras, and the Upper and Lower Pumice on Nisyros. Between these two major tephras, we found two tephras from Nisyros not yet observed on land. The four Nisyros tephras form a systematic trend toward more evolved magma compositions. In the companion paper we use the tephrostratigraphic framework established here to constrain new eruption ages and magnitudes as a contribution to volcanic hazard assessment.
Plain Language Summary
The Aegean Volcanic Arc comprises the Milos, Christiana‐Santorini‐Kolumbo and Kos‐Yali‐Nisyros volcanic complexes that present particularly high threats for humans and economy due to abundant highly explosive eruptions in the past. The systematic catalog of how eruption products are dispersed on the seafloor (marine tephras) with time provides information on the number and recurrence of eruptions, on their size, and intensities and is thus essential to quantitatively assess future volcanic hazards and risks. During RV Poseidon cruise POS513 in the Eastern Aegean Sea we recovered 50 sediment cores up to 7.4 m long. More than 220 tephra deposits (e.g., volcanic glass shards) from these eruptions were identified. Glass shard compositions from all layers were used for subsequent geochemical fingerprinting to correlate them with 19 known onshore Aegean eruptions as well as with the 39 ka Campanian Ignimbrite eruption from the Campi Flegrei, Italy. Correlations with 11 eruptions from Christiana‐Santorini‐Kolumbo are established. We identify a previously unknown widespread tephra from an eruption on Milos (Firiplaka Tephra). At the eastern region of the arc, we correlate 7 marine tephras with the Kos‐Yali‐Nisyros volcanic complex.
Key Points
Marine tephrostratigraphy for the Aegean Arc
Chemical fingerprinting to correlate on and offshore tephras
The Tierra Blanca (TB) eruptive suite comprises the last four major eruptions of Ilopango caldera in El Salvador (≤45 ka), including the youngest Tierra Blanca Joven eruption (TBJ; ∼106 km3): the ...most voluminous event during the Holocene in Central America. Despite the protracted and productive history of explosive silicic eruptions at Ilopango caldera, many aspects regarding the longevity and the prevailing physicochemical conditions of the underlying magmatic system remain unknown. Zircon 238U‐230Th geochronology of the TB suite (TBJ, TB2, TB3, and TB4) reveals a continuous and overlapping crystallization history among individual eruptions, suggesting persistent melt presence in thermally and compositionally distinct magma reservoirs over the last ca. 80 kyr. The longevity of zircon is in contrast to previously determined crystallization timescales of <10 kyr for major mineral phases in TBJ. This dichotomy is explained by a process of rhyolitic melt segregation from a crystal‐rich refractory residue that incorporates zircon, whereas a new generation of major mineral phases crystallized shortly before eruption. Ti‐in‐zircon temperatures and amphibole geothermobarometry suggest that rhyolitic melt was extracted from different storage zones of the magma reservoir as indicated by distinct but synchronous thermochemical zircon histories among the TB suite eruptions. Zircon from TBJ and TB2 suggests magma differentiation within deeper and hotter parts of the reservoir, whereas zircon from TB3 and TB4 instead hints at crystallization in comparatively shallower and cooler domains. The assembly of the voluminous TBJ magma reservoir was also likely enhanced by cannibalization of hydrothermally altered components as suggested by low‐δ18O values in zircon (+4.5 ± 0.3‰).
Plain Language Summary
The collapse of a volcano edifice into its shallow magma chamber can produce one of the most dangerous single events in nature, known as a caldera‐forming eruption. The TBJ eruption in El Salvador is of this kind and occurred around 1,500 years ago, having a profound impact on Maya societies. Because of this, it is crucial to understand the inner workings of caldera‐forming eruptions to assess volcanic risks and their mitigation. Beneath Ilopango caldera, the micrometer‐sized radioisotopically datable mineral zircon grew within different storage levels of a silica‐rich magma reservoir suggesting continuous melt presence for up to ca. 80,000 years prior to eruption. The time information given by zircon contrasts with that extracted from other, more abundant minerals from the same rocks (<10,000 years). We explain this time difference between coexisting minerals by the ability of melt to carry along small zircon crystals, whereas coeval, larger, and more abundant minerals are left behind in the partially solidified portion of the magma reservoir. Once the segregated melt coalesced in a shallower and dominantly liquid magma chamber, major minerals resumed crystallization shortly before eruption. In addition, this new magma incorporated parts of older magmatic rocks from preceding volcanic cycles, thus generating even larger magma volumes.
Key Points
U‐Th zircon ages for the last four explosive eruptions of Ilopango caldera reveal a long‐lived magma reservoir (>80 kyr)
Contrasting residence times for major minerals and zircon suggest extraction of zircon along with evolved melt from crystal residue
Melt extraction from vertically extensive, thermally zoned magma reservoir
•Temporally close-spaced double eruption within a couple of hundreds of years.•Magmas are variably tapped from zoned magma chambers during eruptions due to changing magma discharge rates and/or vent ...migration.•Eruptions started with a series of fallouts featuring stable eruption columns followed by fluctuating and partially collapsing eruption columns.•Eruptive volumes sum up to a total of 25.6km3 and 40.5km3 tephra volume, eruption column heights have been between 20–33 km.•Potential hazards from similar sized eruptions around Coatepeque Caldera are indicated even in the distal regions around San Salvador.
The Coatepeque volcanic complex in El Salvador produced at least four Plinian eruptions within the last 80 kyr. The eruption of the 72 ka old Arce Tephra formed the Coatepeque Caldera and was one of the most powerful explosive eruptions in El Salvador. Hitherto it was thought that the Arce tephra had been emplaced only by one, mostly Plinian, eruptive event that ended with the deposition of a thick ignimbrite.
However, our stratigraphic, geochemical, and zircon data reveal a temporally closely- spaced double eruption separated by a gap of only a couple of hundred years, and we therefore distinguish Lower and Upper Arce Tephras. Both eruptions produced in the beginning a series of fallout units generated from fluctuating eruption columns and turning wind directions. The final phase of the Upper Arce eruption produced surge deposits by several eruption column collapses before the terminal phase of catastrophic ignimbrite eruption and caldera collapse.
Mapping of the individual tephra units including the occurrences of distal marine and lacustrine ash layers in the Pacific Ocean, the Guatemalan lowlands and the Caribbean Sea, result in 25.6km3 tephra volume, areal distribution of 4×105km2 and eruption column heights between 20–33km for the Lower Arce eruption, and 40.5km3 tephra volume, including 10km3 for the ignimbrite, distributed across 6×105km2 and eruption column heights of 23–28km for the Upper Arce eruption. These values and the detailed eruptive sequence emphasize the great hazard potential of possible future highly explosive eruptions at Coatepeque Caldera, especially for this kind of double eruption.
The Toba Caldera on Sumatra, Indonesia is the host of the Young Toba eruption (∼74 ka), globally one of the largest and most recognized eruptions during the Quaternary and regionally concentrated in ...the eastern Indian Ocean. Three older deposits (Middle, and Old Toba Tuff as well as Haranggaol Dacite Tuff) are also attributed to Toba caldera, with their eruption products distributed over the Indian Ocean.
We present the Quaternary marine tephra record from an array of 14 sites and 28 holes from deep ocean drilling programs, complementing earlier work on distal to ultra-distal Indian Ocean sediment cores and terrestrial distribution data of Toba deposits. A unique set of major and trace element glass-shard compositions on 115 primary ash layers together with glass shard morphologies, core pictures and statistical analysis support geochemical fingerprinting between marine tephra layers and known deposits from Toba and five so far unidentified medium to large eruptions assigned to northern Sumatra. Additionally, zircon crystallization ages have been determined for the Haranggaol Dacite Tuff resulting in a new maximum eruption age of 1.42 ± 0.034 Ma.
Tephra volumes and magma masses for the (co-ignimbrite) fallout are estimated based on the compiled marine tephra distribution that are complemented by published proximal ignimbrite volumes. For YTT the resulting tephra and DRE volumes of 5600 km3 and 3600 km3, respectively, are in between the previous estimates. For MTT (253 km3 DRE), ODT (1550 km3 DRE), HDT (129 km3 DRE), and the five additionally identified eruptions from Northern-Sumatran volcanoes, new magma volumes have been determined. Overall, the Indian Ocean tephra record reveals in one large eruption every 200 kyr in the Quaternary that is derived from northern Sumatra.
The Toba Caldera on Sumatra, Indonesia, is a famous spot for volcanism since it is the host of one of the largest and most recognized eruptions on the world, the ∼74 ka Young Toba supereruption. It produced widespread volcanic particles (volcanic ash) that were distributed around the globe by winds, but most deposits are concentrated in the eastern Indian Ocean. Next to the Young Toba Tuff (YTT), three older deposits from probably similar large eruptions are known from this caldera that are also found as ash layers in the Indian Ocean.
We investigated this marine tephra record, including 28 locations from the three phases of the deep ocean drilling program (DSDP, ODP, IODP), and created a unique set of glass-shard trace element (LA-ICP-MS) compositions that complements our major element (EMP) data on 115 primary ash layers. Comparing these compositions between the different marine locations and to known terrestrial deposits of the Toba eruptions is done by geochemical fingerprinting where the characteristic chemical “fingerprint” of marine ash layers is correlated between oceanic and to on-land deposits to find out from which eruption it originated. Glass shard morphologies, core pictures and statistical analysis complement this approach to support the correlations to four known tephras from Toba (Young, Middle, and Old Toba Tuff, as well as Haranggaol Dacite Tuff) and five so far unidentified eruptions from Northern Sumatra.
The distribution of all tephras in the Indian Ocean allows to estimate erupted volumes by mapping out the occurrence and thicknesses of the tephras on the seafloor using drill cores. The resulting magma volumes amount to, for example, between 3600 km3 and 129 km3 magma for Young, Middle, and Old Toba Tuff as well as the Haranggaol Dacitic Tuff. Together with the five additionally identified (northern)Sumatra eruptions, these results indicate that one large eruption has happened every 200 kyr in northern Sumatra over the last 2.4 Ma.
•Quaternary marine tephrostratigraphy of very large eruptions for the Eastern Indian Ocean•Chemical fingerprinting of marine tephras between each other and to terrestrial Toba deposits•Revised and new volume estimations for large Northern Quaternary Sumatran eruptions and respective repose times
Provenance studies of widely distributed tephra deposits are important to deduce systematic changes in the source, size, distribution, and temporal variation of regional explosive volcanism. ...Long-term deep ocean drilling sedimentary records are particularly useful for these kind of studies. In this study, we establish a robust tephrochronostratigraphy for 235 primary marine tephra layers collected during International Ocean Discovery Program Expeditions 353, 354, and 362, complemented by older drill cores from Deep Sea Drilling Program and Ocean Drilling Program Legs 22, 119, 120, 121, and 183. We infer at least two major phases of highly explosive arc volcanism during the Early Miocene to Pleistocene, as well as three episodes related to explosive ocean island volcanism located in the Kerguelen plateau, the Broken Ridge, and close to Réunion reaching back to the Paleogene.
Twenty-two widespread arc-derived tephra layers from individual eruptions can be correlated by geochemical fingerprinting between multiple holes. These provide nine Quaternary and 13 Neogene temporal tie points in the sedimentary sequence including four new UPb zircon ages. Provenance analysis of the marine tephra layers, which is based on glass composition, assign eleven of these layers to a Toba-like source, ranging from 24 Ma to 75 ka, with the youngest correlative being Young Toba Tuff. Based on distribution pattern, thickness decay, and compositional evidence another eleven tephra layers can be assigned to a northern Sumatran Arc or to an Andaman Arc provenance. First-order minimum eruptive volume estimates for the Neogene tephra layers imply eruptive magnitudes ranging from M = 6.5 to M = 7.5, proving a continuous history of large explosive eruptions from the Sumatran/Andaman Arc since the Neogene, as previously known from the Quaternary.
Knowing the origin of large volcanic eruptions is important to deduce systematic changes in the source, size, distribution, and changes of regional explosive volcanism. In this study we use widely dispersed marine volcanic deposits in the Indian Ocean, so-called marine tephra layers, to reconstruct the history of a) Sumatran highly explosive eruptions over the last 24 Ma, and b) ocean island volcanism frequently observed in the Indian Ocean. The marine deposits are often the only evidence of major eruptions in the past because respective volcanic edifices and terrestrial deposits are often destroyed by subsequent eruptions, eroded or overlain by younger deposits. Deep ocean drilling provides the unique possibility to overcome these problems by recovering long, tephra-bearing marine sediment sequences in the drill cores. We present the results for 235 primary marine tephra layers that have been collected during eight deep drilling campaigns in the Indian Ocean. Twenty-two widespread connections of arc-derived tephra layers are established by comparing distinctive geochemical fingerprints of the tephras between multiple locations. These provide nine Quaternary and 13 Neogene temporal tie points in the sedimentary sequence. The origin of these marine tephra layers is determined based on the unique glass compositions of individual volcanoes and eruptions, assigning 11 of these layers to a Toba-Caldera-like source ranging from 24 Ma to 75 ka, with the youngest correlative being the iconic Young Toba Tuff. Another eleven tephra layers can be assigned to a northern Sumatran Arc or to an Andaman Arc provenance.
Our record demonstrates a continuous history of large explosive eruptions from the Sumatran/Andaman Arc since the Neogene. We additionally infer (1) at least two major phases of highly explosive arc volcanism from the Sumatra-Andamen volcanic chain during Early Miocene to Pleistocene times, and (2) three episodes related to explosive ocean island volcanism found in the Kerguelen plateau, the Broken Ridge, and close to Réunion.
•Cenozoic marine tephrostratigraphy and provenance of large explosive eruptions for the Eastern Indian Ocean•Long-term eruptive history for large explosive eruptions•Temporal compositional variations in explosive arc and ocean island volcanism of the Indian Ocean tephra inventory
Defining a precise timeline for past eruptions from explosive volcanoes in continental arcs is imperative to forecast future hazards and mitigate volcanic disasters in these often densely populated ...regions. However, establishing reliable ages for Quaternary eruptions in the Central American Volcanic Arc has been challenging due to the common lack or alteration of suitable K-rich phases for 40Ar/39Ar geochronology, but also from their position in time beyond the reach of 14C dating. This especially holds for the active Amatitlán caldera in Guatemala, from which at least six explosive silicic eruptions have produced tephra blanketing neighboring regions that are today inhabited by millions of people. Zircon, a common datable accessory mineral in Amatitlán caldera magmas, is used here to retrieve eruption ages by applying the novel zircon double-dating method (ZDD) that integrates 238U–230Th disequilibrium dating and (U–Th)/He thermochronology. This approach yielded the first-ever radioisotopic ages of 24 ± 3 ka and 48 ± 6 ka (1σ), respectively, of two of Amatitlán caldera's most recent eruptions (J-tephra and E-tephra). Remarkably, both zircon crystallization and ZDD eruption ages for the older and voluminous T-tephra and L-tephra units significantly post-date existing plagioclase 40Ar/39Ar dates by ca. 26 and 70 kyr, respectively. The ZDD eruption age for T-tephra is 93 ± 4 ka, whereas zircon crystallization ages for L-tephra yield a maximum model eruption age of ca. 124 ka. The strong eruption age divergence between ZDD and plagioclase 40Ar/39Ar dating argues for the presence of inherited or xenocrystic plagioclase in Amatitlán caldera eruptive products. Statistical analysis based on the updated eruptive history suggests a recurrence interval of ca. 17 kyr, which is significantly shorter than previously estimated. The new age data, thus, suggest a more frequent eruptive activity of Amatitlán caldera than formerly thought and underscores the necessity to better understand the current underlying magmatic system and to constrain its past eruptive history more precisely.
•New and updated ages for explosive eruptions from Amatitlán caldera.•Zircon double-dating suggests a much younger eruptive history.•Existing plagioclase Ar eruption ages are ca. 26 to 70 kyr older than ZDD.•New eruption history suggests a shorter recurrence interval of ca. 17–24 kyr.
Perturbations in stratospheric aerosol due to explosive volcanic eruptions are a primary contributor to natural climate variability. Observations of stratospheric aerosol are available for the past ...decades, and information from ice cores has been used to derive estimates of stratospheric sulfur injections and aerosol optical depth over the Holocene (approximately 10 000 BP to present) and into the last glacial period, extending back to 60 000 BP. Tephra records of past volcanism, compared to ice cores, are less complete but extend much further into the past. To support model studies of the potential impacts of explosive volcanism on climate variability across timescales, we present here an ensemble reconstruction of volcanic stratospheric sulfur injection (VSSI) over the last 140 000 years that is based primarily on terrestrial and marine tephra records. VSSI values are computed as a simple function of eruption magnitude based on VSSI estimates from ice cores and satellite observations for identified eruptions. To correct for the incompleteness of the tephra record, we include stochastically generated synthetic eruptions assuming a constant background eruption frequency from the ice core Holocene record. While the reconstruction often differs from ice core estimates for specific eruptions due to uncertainties in the data used and reconstruction method, it shows good agreement with an ice-core-based VSSI reconstruction in terms of millennial-scale cumulative VSSI variations over the Holocene. The PalVol reconstruction provides a new basis to test the contributions of forced vs. unforced natural variability to the spectrum of climate and the mechanisms leading to abrupt transitions in the palaeoclimate record with low- to high-complexity climate models. The PalVol volcanic forcing reconstruction is available at https://doi.org/10.26050/WDCC/PalVolv1 (Toohey and Schindlbeck-Belo, 2023).
Felsic magmas from the Izu‐Bonin rear arc have compositions that resemble average continental crust in some respects. In order to understand their origin, we studied 1.1–4.4 Ma tephras in a rear‐arc ...drill core from International Ocean Discovery Program Expedition 350, Site U1437. They provide a well‐dated record of changing magmatic compositions during the early stages of the most recent episode of Izu‐Bonin arc rifting. Based on our comprehensive recontextualization of published analyses of <7 Ma regional dredged rocks across the arc, basalts to rhyolites are shown to vary in coherent chronological and spatial trends and can be classified into three series: light rare earth element‐depleted volcanic front series; a rift‐related series with nearly flat rare earth element patterns; and light rare earth element‐enriched rear‐arc seamount chain‐type (RASC‐type) series, which are abundant in the studied Site U1437 tephra record. All three series erupted simultaneously between 4.4 and 1.1 Ma, including the RASC‐type rhyolites which erupted until 1.1 Ma in significant quantities. Remarkably, trace element and radiogenic isotope ratios are similar between rhyolites and basalts from the same region. This recontextualization of rhyolite affinity represents a significant departure from existing frameworks. Geochemical modeling shows that fractional crystallization can largely explain <4.4 Ma RASC‐type rhyolites with some additional open system processing evident in rhyolites with >73% SiO2. However, trace element and Hf isotope ratios preclude rear‐arc rhyolite derivation by partial melting of the Oligocene‐Eocene arc basement. Thus, we favor a model where fractional crystallization is more important than crustal melting in producing intra‐oceanic arc rhyolites in this region.
Plain Language Summary
Silicic magmatism is common on Earth's continental crust but comparatively rare within the oceanic crust. The main geological processes that create silicic magmas are debated. It is well established that the chemical characteristics of magmatism in the Izu‐Bonin oceanic subduction zone have changed over the last 7 Myr. By examining published geochemical data from volcanic rocks from the Izu‐Bonin subduction zone and comparing them to new geochemical data from volcanic rocks from IODP Drill Site U1437, we showed that basaltic‐through‐silicic composition volcanic rock have matching geochemical signatures when comparing rocks of the same age and/or volcano location and represent magmas that share a common origin. This suggests that silicic magmas within the Izu‐Bonin are not created by melting preexisting crust. Understanding what processes create silicic magmas is important in understanding the evolution of subduction zones over time, as well as the geologic evolution and creation of Earth's continental crust.
Key Points
Explosive felsic eruptions were a significant output of the Izu‐Bonin rear arc
7 to 1 Myr old Izu‐Bonin rocks fall into three geochemical categories, each with distinct basalt to rhyolite associations
Magmatic differentiation via fractional crystallization can explain geochemical trends of Izu‐Bonin rear‐arc glasses
The Santa María-Santiaguito volcanic complex collectively is one of the most active and dangerous volcanoes in the entire Central American volcanic arc. Within 20 years of the destructive Plinian ...eruption of the Santa María stratocone (Guatemala) in 1902, the Santiaguito volcanic dome complex started to form by parasitic vent eruptions which have lasted over nearly a century. Because of the frequent eruptions of Santiaguito, it offers unique opportunities to study secular changes in its subvolcanic magma reservoir at high temporal resolution. Ash collected days and hours after eruptions of Santiaguito during an unusually explosive interval in August 2016 represents some of the least evolved compositions (60–62% SiO2) with the lowest 87Sr/86Sr isotope ratios ever reported for the system. This extends the multi-decadal trend of Santiaguito to progressively less evolved magma compositions with time, relative to the 1902 event which erupted overwhelmingly dacitic pumice. Santiaguito's geochemical changes are indicative of increasingly larger contributions of fresh basaltic andesite magma recharge mixing with dacitic magma that remained after the 1902 eruption. High-resolution secondary ion mass spectrometry (SIMS) 238U-230Th disequilibrium dating and geochemistry of zircon from Santiaguito ash and Santa María 1902 pumice reveal the presence of evolved and zircon-saturated melts over the entire ca. 100 kyr interval of basaltic andesite eruptions that constructed the Santa María central cone. Quartz diorite basement xenoliths (~55% SiO2) with Eocene zircons are common in the Santa María 1902 deposits, and a few equally old zircon xenocrysts within Santiaguito ash suggest that vestiges of assimilated Eocene crustal rocks are present in the modern magma system. Zircon crystals in andesitic Santiaguito tephra are identical in age and trace element signature to those in the 1902 pumice. This is direct evidence for the persistence of residual evolved magma with crystals dating back to the Santiaguito cone-forming stage between ca. 100 and 25 ka and the pre-1902 eruptive quiescence which lasted for ca. 25 kyr. It also indicates that residual dacite still contributes to Santiaguito andesitic magmas. Zircon survival in Santiaguito andesite requires comparatively brief timescales of magma mixing between basaltic andesite and resident dacite magmas because otherwise zircon would become resorbed.
•2016 Santiaguito ash has the least evolved composition in the volcano's history.•Whole-rock Sr isotopes among the most primitive ever reported for Santiaguito.•U-series zircon ages suggest evolved magma coeval to Santa María basaltic magmatism.•Zircon dating reveals Eocene (ca. 41 Ma) crustal basement below Santiaguito.•Andesite originated by mixing of dacite leftover and basaltic andesite recharge.
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