Utilizing Boron to Reveal the Influence of Subducted Slab across Central Java Island Arc, Indonesi

The influence of subducted slab in magma genesis of Sunda Arc (Java Island, Indonesia) has been studied over years. The difference in age of subducted plate and the difference in the nature of the overriding crust are considered as important factors affecting variation in magma compositions along and across this arc. Recent works on this arc interpreted that this variation is most likely related to crustal contamination and sedimentary influx. This study utilized, the most sensitive subduction component, to test the influence of subducted plate on the magma genesis in Central Sunda Arc (CSA), focusing on the across arc variation of magma composition. The collected samples, from a north-south alignment of Quaternary volcanic centers including Merapi, Merbabu, Telomoyo, Ungaran and Muria, represent the CSA. Another two neighboring volcanoes, Dieng and Sindoro, were also analyzed. Basalt and basaltic andesite were analyzed in order to avoid the influence of crustal contamination. In this study, boron concentrations were obtained by Prompt Gamma ray Analysis with JRR-3M reactor at Japan Atomic Energy Agency. The analyzed samples from CSA are sub-alkaline basalt to basaltic andesite, except for samples from Muria, which are alkaline. Samples from Muria are shoshonite, potassic trachy-basalt and tephriphonolite. In general, boron and other incompatible elements show increasing patterns as the SiO2 increases, which can be explained by fractional crystallization. Although Muria samples are high in incompatible elements, B/Nb and B/Zr ratios are low, suggesting the small input of subduction component. Compared with other arcs, B concentrations from CSA are characterized by high values. The subalkaline suites of CSA overlap with those from Kurile, Mariana, Cascade, Northeast Japan arcs. The alkaline suite of Muria is plotted away from sub-alkaline group of CSA, and is closer to but different from OIB field. Across arc variation of subduction component shows a general decreasing trend from trench-side to back-arc side as observed in typical island arcs. In detail, however, the highest B/HFSE ratios of CSA do not appear at the volcanic front like the case of most other volcanic arcs. Instead, it appears 20-30 km behind the volcanic front. The sub-alkaline suites of CSA show the typical island arc characteristic, whilst the back arc magmatism, which is represented by Muria, distinctively shows alkaline characteristic but different from OIB source. The subduction input is observed as represented by B/HFSE all across the CSA. Even the back-arc Muria volcano shows higher B/HFSE values than MORB and OIB and is significantly indicating subduction influence. The decreasing pattern of B/HFSE across arc indicates that fluid input is high near volcanic front and gradually decreases toward back-arc. The maximum subduction input observed a little behind the volcanic front raises a question how the process of (1) dehydration of the subducting slab, (2) metasomatism of the mantle, (3) partial melting of the metasomatised mantle took place at CSA

TINGKAT SULFIDASI FLUIDA PANAS BUMI DITINJAU DARI KANDUNGAN MINERAL SULFIDA DIDALAM KERAK SILIKA PADA PIPA SEPARATOR WELL PAD 7 LAPANGAN PANAS BUMI DIENG

Dieng Geothermal Field is located in Banjarnegara and Wonosobo, Central Java has been utilized for power generation totaling 60 MWe which is extracted from the geothermal energy. During the exploitation and production processes, there is silica scaling deposited, especially in the production pipe. It happens in the separator well pad 7, silica scaling rich-sulphide mineral is deposited inside the separator and reduce the fluid capacity of the pipe. There is a texture similar with quartz vein in the epithermal deposit, crustiform banded and colloform banded. Silica scaling mainly composed of amorphous silica, and also sulphide mineral and silicate mineral. The sulphide mineral of silica scaling are chalcopyrite, pyrite, sphalerite, galena and pyrrhotite. The silicate mineral of silica scaling are half-altered plagioclase, pyroxene, and quartz. The existence of pyrite is indication of an intermediate sulphidation deposit. The existence of pyrrhotite is indication of a low sulphidation deposit. According to the sulfiide mineral composition, the existence of pyrite and pyrrhotite can concluded that the sulphidation state is reducing from intermediate to low. So that, the sulphidation state of geothermal fluid based on the sulphide mineral composition of silica scaling is low-intermediete. Based on the mineragraphy analysis, the first formed of sulphide mineral is chalcopyrite, and then follow by pyrite, sphalerite, galena and pyrrhotite. Crustiform banded and colloform banded of the silica scaling indicate that the sulphide minerals deposited by the presipitation of hydrothermal fluid and the boiling process inside the separator

KARAKTERISTIK GEOLOGI DAN ALTERASI HIDROTERMAL BAWAH PERMUKAAN BLOK GEMURAH BESAR, DAERAH PROSPEK PANAS BUMI LUMUT-BALAI, PROPINSI SUMATERA SELATAN

Gemurah Besar is one of the blocks located in Lumut-Balai Geothermal Prospect Area, South Sumatra Province. The objective of this research is to characterize of subsurface geology and hydrothermal alteration in research area based on data from wells of LMB 1-1, LMB 1-3, LMB 3-1 and LMB 3-3 are located within the Gemurah Besar Block. The research method is focused on the petrographic study and x-ray diffraction (XRD) analysis of cores and cutting samples from research wells. 88 samples of core and cutting rocks have been selected for petrographic study and 41 samples of rocks of cutting sample have been selected for x-ray diffraction analysis. Secondary data will be used is the well temperature data at this time and gross permeability test. The results showed that subsurface lithology composing research area is lapilli-stone, lapilli tuff and andesite. It has weakly � very strongly altered (20 � 95%) and shows the increased alteration intensity as the depth increases. Based on the distribution of hydrothermal alteration mineral formed in certain depth, zonation for hydrothermal alteration may then be made in each well: Smectite-Cristobalite Alteration Zone, Smectite-Quartz Alteration Zone, Illite/Smectite-Chlorite-Quartz Alteration Zone, and Illite±Illite/Smectite-Chlorite-Epidote Alteration Zone. Subsurface temperature shows the presence of decreased temperature (cooling) from paleotemperature to current temperature. Subsurface permeability shows the presence of relatively good permeability zone. Subsurface fluid is dominated by chloride of neutral pH. It is observed from the presence of alteration minerals that can be used as an indicator of temperature, permeability and fluid type. Geothermal systems in the research area is composed of cap rock on smectitecristobalite alteration zones, smectite-quartz alteration zones and illite/smectitechlorite- quartz alteration zone

KARAKTERISASI ENDAPAN MAAR RANU SEGARAN DAN ASOSIASI DENGAN PEMBENTUKAN MATA AIR PANAS DI KECAMATAN TIRIS, KABUPATEN PROBOLINGGO, PROVINSI JAWA TIMUR

Ranu Segaran is one of the Lamongan Volcanic Field�s water-filled maar, located in Tiris Sub-district, Probolinggo District, East Java Province. Several hot-springs as a form of hydrothermal manifestation were found alongside Kali Pekalen, a few hundred meters to the west from Ranu Segaran. Ranu Segaran itself is flanked by Gunung Lamongan on the west and Gunung Argapura on the east, on which the latter is an explored geothermal field owned by PT. Pertamina Geothermal Energy. This fact was then used as the basis of the research, whether the hydrothermal system�s heat source was from Gunung Lamongan or Gunung Argapura. Mineral deposit studies shows that there is a correlation between hydrothermal system and maar deposit. Based on this theory assumptions were made, that if fresh-non-altered rocks are found on the field, it would mean that hydrothermal system�s heat source and maar�s heat source came from the same magma, whereas if altered rocks are found on the field, that would mean hydrothermal system�s heat source and maar�s heat source came from different magmas. Ranu Segaran was chosen as research field based on its spatial proximity with hot springs on Kali Pekalen. Research was first conducted by observing geological features on research field with the support of some literature studies mostly from Carn (2000) and chemistry data by Indarto et al. (2012). After field checking was done, the next step is petrography, grain size, and XRD analysis. The result of this research is that stratigraphically, research field consists of Segaran Porphyry Basalt Unit Segaran Pyroclastic Tuff Unit, and Betok Pyroclastic Tuff. As for Tiris hydrothermal manifestation is shown by hot springs bicarbonate type (HCO3 1374,25�1773,99 mg/l) and carbonate sinter (aragonite). Tiris� hydrothermal system was formed after Ranu Segaran maar was formed. Ranu Segaran itself was interpreted as formed after a small body of magma intruded cold water system (hydrology system). The same magma then formed a hydrothermal system by heating the aquifer that was composed by volcanic rocks. This hydrothermal system then moved out to the surface by passing through weak zones along Kali Pekalen fracture

GEOLOGI DAN KARAKTERISTIK ENDAPAN MANGAN TIPE SEDIMEN DI DAERAH SUPUL KABUPATEN TIMOR TENGAH SELATAN PROVINSI NUSA TENGGARA TIMUR

Sedimentary manganese layers have been discovered in Supul, South Central Timor Regency, East � Nusa Tenggara Province. The manganese layers is associated with deep sea sedimentary rock and interbedded with redish to redish brown claystone. The deposit shows the spatial linkage with mud volcano intrusion. Physically, the manganese layers range from 2 mm to 4 cm in width, compact, lenticular, solid, and strongly deformed. Mineralogically, it is composed of manganite mineral (MnO(OH)) as primary mineral, pyrolusite (MnO2), lithiophorite (Al,Li) MnO2(OH)2, and associated with gangue minerals including calcite (CaCO3), silica (SiO2), limonite (FeO(OH), hematite (Fe2O3) and Barite (BaSO4). There are two form types of manganese ores that found in study area, that is manganese nodule and manganese layers. Mineralogically, the manganese nodule composed of manganite that associated with limonite. It has grade of 62.72 and 69.42 wt.% MnO. Whereas manganese layers classified into three form types. The first type is pyrolusite and has grade of 66.05 wt.% MnO. The second and third have different in the hardness. Mineralogically, the second and third types of manganese layer composed of manganite as primary manganese mineral, and also lithiophorite and pyrolusite. It has grade 63.33%-71.57 wt.% MnO. In general iron in Mn Ore is very low ranging from 0.2 to 1.54 wt.% Fe2O3, hence, Fe / Mn ratio is very low of 0.0025-0.0691%, which typically indicates sedimentary origin. This sedimentary origin is supported by petrologic and petrographic data showing layered structure of manganite and lithiophorite, as well as the degradation of crystal/grain size manganite. Geochemical analysis shows that manganese ore is non hydrothermal and was precipitated in reduction condition according to REE normalization graphic that revealing similar distribution pattern of REE with timor nodule, pacific hydrogenous and nodule hydrogenous that is the existing of Ce positive anomaly, graphic data of Co+Ni vs. As+Cu+Mo+Pb+V+Zn and the calculation of Ceanomaly. Moreover, this nodule manganese views hydrogenous deposit based on Al and Si concentration, and supported by the positive correlation of Mn with Cu, Ni and Zn, whereas the manganese layers is detrital diagenic deposit (remobilization of manganese in the water column of the ocean, precipitated and sedimented on the deep sea bottom) as well as reveals the effect of hydrothermal, which is the positive correlation of Mn and As. This is proven by the presence of quartz and barite veinlets cutting the Mn layers, manganite recrystallization along vei layers and the presence of pyrite. Geochemically supports the analysis mineralogically that is manganite which is one type of mineral manganese is relatively stable and in the solid phase has a balance of the sea water and are often not stable in oxidizing conditions so it was replaced by pirolusit. Based on field data and analysis of laboratory data, it shows that the formation of manganese layers deposit in study area is assumed due to the remobilization of manganese in the water column of the ocean, while the manganese nodules are hidrogenous deposits, formed by the chemical reaction within sea water shaping unsolvable particle in sea water so that it will be sink into the bottom of sea floor/precipitation of metals from sea water