11:30 〜 11:45
[SVC30-10] Multiple Eruptions and Magma Types of The Old Ijen Caldera Forming Eruption: Insight from Stratigraphy and Petrological Characteristics of Ignimbrite
キーワード：Ijen Caldera Complex, Caldera-forming eruption, Ignimbrite, Multiple eruptive event, Magma type
Caldera-forming eruption remains a subject of ongoing discussion, particularly the pre-eruptive condition, characteristics of magma capable of generating such a big eruption, and whether the eruption resulted from single or multiple events. The Ijen Caldera Complex in East Java, Indonesia, exhibits an enormous caldera up to 25 km in diameter. This feature serves as evidence of the past catastrophic explosive eruption of the Old Ijen Volcano. Ignimbrite, as the key product of Old Ijen caldera-forming eruption, was found extensively on the northern flank of the Ijen Caldera rim. Our study aims to investigate the mechanism of Old Ijen caldera-forming eruption and its magma characteristics. To achieve these objectives, we conducted stratigraphic measurements, component identification, petrographic analysis, and whole-rock geochemistry.
We identified seven ignimbrite units, from bottom to top: Lithic-rich ignimbrite (LI), Scoria-rich Ignimbrite (SI), Fine-grained Ignimbrite (FI), and Coarse-grained Ignimbrite (CI) 1-4. Units SI and CI are the most widely distributed deposits, indicating that these units may serve as the main product or the most voluminous product of Old Ijen caldera. Seven types of juvenile components make up the ignimbrite deposits: Dark Gray Pumice (DGP), Light Gray Pumice (LGP), Banded Pumice (BDP), Yellow Pumice (YP), Brown Pumice, and Scoria (SC).
Based on petrography observation, plagioclase and orthopyroxene are present as major minerals. Apatite and oxide are minor minerals, along with rare olivine and clinopyroxene.
Major element geochemistry shows that Old Ijen products have dacite composition with SiO2 ranges from 63 to 67 wt%. Based on trace element data, we identify at least five distinct magma groups:
Type 1: a typical subduction-related magma with negative Eu anomaly. Four distinct trends in Harker diagram were recognized within this type: 1a: SC from SI unit; 1b: DGP, LGP, and BDP from CI-1 unit; 1c: DGP, LGP, and BDP from CI-2 unit; 1d: SC from CI-4 unit.
Type 2: enriched in REE, exhibits negative Eu and Ce anomaly. This magma produced the DGP of SI unit.
Type 3: REE-poor magma with positive Eu and negative Ce anomaly. This magma produced YP in CI-4 unit.
Type 4: the most REE-depleted magma with positive Eu and negative Ce anomaly. This magma produced SC from CI-1 unit.
Type 5: this magma resulted from the mixing of Type 1d and Type 3 magma, producing BDP and DGP of CI-4 unit.
Our results indicate that the caldera forming eruption of Old Ijen was originated from multiple magma chamber during multiple eruptive events, reflects the highly complex magma chamber system beneath Ijen Caldera Complex.
We identified seven ignimbrite units, from bottom to top: Lithic-rich ignimbrite (LI), Scoria-rich Ignimbrite (SI), Fine-grained Ignimbrite (FI), and Coarse-grained Ignimbrite (CI) 1-4. Units SI and CI are the most widely distributed deposits, indicating that these units may serve as the main product or the most voluminous product of Old Ijen caldera. Seven types of juvenile components make up the ignimbrite deposits: Dark Gray Pumice (DGP), Light Gray Pumice (LGP), Banded Pumice (BDP), Yellow Pumice (YP), Brown Pumice, and Scoria (SC).
Based on petrography observation, plagioclase and orthopyroxene are present as major minerals. Apatite and oxide are minor minerals, along with rare olivine and clinopyroxene.
Major element geochemistry shows that Old Ijen products have dacite composition with SiO2 ranges from 63 to 67 wt%. Based on trace element data, we identify at least five distinct magma groups:
Type 1: a typical subduction-related magma with negative Eu anomaly. Four distinct trends in Harker diagram were recognized within this type: 1a: SC from SI unit; 1b: DGP, LGP, and BDP from CI-1 unit; 1c: DGP, LGP, and BDP from CI-2 unit; 1d: SC from CI-4 unit.
Type 2: enriched in REE, exhibits negative Eu and Ce anomaly. This magma produced the DGP of SI unit.
Type 3: REE-poor magma with positive Eu and negative Ce anomaly. This magma produced YP in CI-4 unit.
Type 4: the most REE-depleted magma with positive Eu and negative Ce anomaly. This magma produced SC from CI-1 unit.
Type 5: this magma resulted from the mixing of Type 1d and Type 3 magma, producing BDP and DGP of CI-4 unit.
Our results indicate that the caldera forming eruption of Old Ijen was originated from multiple magma chamber during multiple eruptive events, reflects the highly complex magma chamber system beneath Ijen Caldera Complex.