FORMASI DANAU SANGAT ASAM KAWAH IJEN, JAWA TIMUR

Danau Kawah Ijen merupakan gunung api aktif berukuran 900 m x 600 m, kedalaman ~180 m memiliki air sangat asam (pH ~0,2) dengan volume 30 juta m3, secara administratif terletak di Kabupaten Bondowoso dan Banyuwangi, Jawa Timur. Kawah Ijen merupakan salah satu dari 22 buah kerucut gunung api yang muncul paska pembentukan kaldera Ijen. Ancaman dan bahaya bila terjadi letusan besar maka air danau kawah yang tumpah akan mengakibatkan bencana yang sangat serius. Disamping itu, Air Danau Kawah Ijen merembes melalui dinding bagian Barat membentuk Sungai Banyupahit-Banyuputih sepanjang 40 km hingga Asembagus. Dampak tersier air irigasi, kualitas air sumur gali masyarakat terkontaminasi unsur F. Data survei air sumur menunjukkan bahwa 60 % mengandung F melebihi rekomendasi WHO yang berdampak adanya fluorosis gigi yang signifikan (~70 %). Formasi danau sangat asam Kawah Ijen setidaknya berhubungan dengan tiga kali letusan eksplosif besar yang membentuk tiga kawah utama yang saling memotong berbentuk ellips berukuran 900 m x 600 m. Berdasarkan umur endapan abu hasil erupsi Gunung Ijen paska kaldera, endapan paling tua berumur 5.920 + 200 B.P. terdapat di dinding sebelah Selatan, diikuti umur endapan aliran abu pada berumur 2.170 + 160 B.P. terdapat di dinding sebelah Timur, dan endapan aliran abu paling muda berumur 620 + 120 B.P. berada di sebelah Barat kawah G. Ijen. Stratigrafi endapan-endapan batuan tersebut didukung oleh analisis peta topografi dan morfologi, wilayah puncak. G. Ijen ini yang telah mengalami beberapa kali perpindahan pusat erupsi dalam letusannya, tetapi masih berkisar di sekitar bagian puncak. Kata kunci : Kawah Ijen, eksplosif, abu, fluorosis

Insights from Pb and O isotopes into along-arc variations in subduction inputs and crustal assimilation for volcanic rocks in Java, Sunda arc, Indonesia

New Pb isotope data are presented for Gede Volcanic Complex, Salak and Galunggung volcanoes in West Java, Merbabu and Merapi volcanoes in Central Java and Ijen Volcanic Complex in East Java of the Sunda arc, Indonesia. New O isotope data for Merbabu and new geochemical and radiogenic isotope data (Sr–Nd–Hf–Pb) for three West Javanese, upper crustal, Tertiary sedimentary rocks are also presented. The data are combined with published geochemical and isotopic data to constrain the relative importance of crustal assimilation and subducted input of crustal material in petrogenesis in Java. Also discussed are the significance of limestone assimilation in controlling the geochemical and isotopic characteristics of erupted Javanese rocks and the geochemical impact upon central and eastern Javanese arc rocks due to the subduction of Roo Rise between 105 and 109°E. The negative correlation between Pb isotopes and SiO2, combined with mantle-like δ18O values in Gede Volcanic Complex rocks, West Java, are most likely explained by assimilation of more isotopically-primitive arc rocks and/or ophiolitic crust known to outcrop in West Java. The negative Pb isotope-SiO2 trend cannot be explained by assimilation of the known compositions of the upper crustal rocks. A peak in δ18O whole-rock and mineral values in Central Javanese volcanic rocks (Merbabu and Merapi) combined with along-arc trends in Sr isotope ratios suggest that a different or additional crustal assimilant exerts control on the isotopic composition of Central Javanese volcanic rocks. This assimilant (likely carbonate material) is characterised by high δ18O and high Sr isotope ratio but is not particularly elevated in its Pb isotopic ratio. Once the effects of crustal assimilation are accounted for, strong East to West Java regional variations in Ba concentration, Ba/Hf ratio and Pb isotopic composition are evident. These differences are attributed to heterogeneity in the subducted source input component along the island: a more radiogenic Pb isotopic, lower Ba/Hf component (detrital-rich subducted sediment) in West Java and a less radiogenic Pb isotopic, high Ba/Hf component (attributed to a greater AOC/sediment fluid component and/or dominance of pelagic, clay-rich subducted sediment) in East and possible Central Java. The subduction of the Roo Rise, an area of oceanic basement relief, is thought to contribute significantly to the spatial geochemical source input variations exhibited by Javanese volcanoes