TSK 11 Göttingen 2006 Vahle et al. Discrimination of different volcanic rock units by mag- netic properties — geother- mal field at Reykjanes penin- sula (SW-Iceland) Poster C. Vahle1 A. Kontny1 F. Dietze1 H. Audunsson2 The geothermal field at Reykjanes peninsula is located at the boundary where the submarine Reykjanes Ridge passes over into the rift zone of south- western Iceland. The geothermal field coincides with a magnetic low in the aeromagnetic anomaly map and is situ- ated within a dense NE–SW fissure and fault zone. Surface geology is charac- terized by different historic fissure erup- tions (youngest from 1226AD), shield lava (12.5–14.5 ka) and intercalated pil- low basalt–hyaloclastite ridges probably formed during the last glacial episode (14.5–20 ka). During a field magnetic study in the vicinity of the geother- mal field in summer 2005 different volcanic rock units have been sam- pled to correlate rock magnetic and magneto-mineralogical properties with magnetic field intensity. Additionally, measurements on a dense dolerite intru- sion, recovered from the RN–19 bore- hole (2245–2248m depth) in May 2005 within the frame of IDDP, should shed light on the influence of crustal rocks on the total magnetic field intensity. Generally, the natural remanent mag- netization and magnetic susceptibility, measured on rock specimen, is high, ranging between 2.5 and 33.6Am−1 and 2–37 ×10−3 SI, respectively. The high NRM coincides with the mag- 1 Geol.-Pal. Institute, Ruprecht-Karls-Uni- versity, INF 234, 69120 Heidelberg, Germany 2 Technical University of Iceland, Hofdabakka 9, 110 Reykjavik, Iceland netic high outside the geothermal field. The Koenigsberger ratios (Q) are also high (12–132) for all surface samples, indicating the predominance of rema- nent magnetization. Most of the study area is covered by strongly magnetic Stampahraun (1226AD) and Skalafell (8–11.5 ka) pahoehoe and block lava stemming from fissure eruptions. The rock magnetic characteristics of the- ses flows are quite similar, whereas the older flow (Skalafell) shows stronger scattering. The pillow lava and espe- cially the picritic Haleyjabunga shield lava show lower NRM intensity and magnetic susceptibility. The NRM of the doleritic dike sample from RN-19 drilling is rather low (5.4–8.8Am−1) but susceptibility is high (32.5–34.5 ×10−3 SI), indicating large grain sizes, formed during typically slow cooling of an intrusion. First temperature de- pendent magnetic susceptibility data in- dicate homogeneous Ti-rich titanomag- netite (Tc = 60–240°C), pure magnetite (Tc = 580°C) and an irreversible ti- tanomaghemite with Tc at about 450°C in the area of the magnetic low. The occurrence of magnetite and the low- temperature behavior of kT curves be- low −150°C indicate exsolution tex- tures typically forming during high- temperature oxidation. Our observa- tion, that high crustal magnetization is related to the youngest flow along the rifting axis is in agreement with obser- vations of the central anomaly magneti- zation high from mid-ocean ridges (e.g. Schouten et al. 1999). References Schouten H, Tivey MA, Fornari DJ & Cochran JR (1999) Earth and Planetary Science Let- ters 169, 37–50 1