C．， ｡．プ
9． Result and Conclusion
Sakurajima volcanic rocks were found to show self－reversal partial thermal re－
manent magnetization（pTRM）in narrow ranges of temperature． Detailed pTRM
experiment at intervals of 10℃～15℃was conducted on raw and annealed samples．Self－reversal pTRM was observed at 460℃～490℃in annealed samples of An－ei pumice， and at double ranges of 245℃～260℃and 330℃～340℃in raw samples of An－ei Iava and Osumi pumice． In the former cases， pTRM carrier could not be hemo－ilmenite because it appeared at higher temperature than the Curie tempera－
ture（Tc）of hemo－ilmenite which was reported under 400℃， generally 200℃～300
℃．The possible mechanism is the model of two phases with different blocking temperatures：One phase with the higher blocking temperature becomes magnetized first， parallel to the external field， and when the magnetic field of the first phase swamps that of the external magnetic field， the second phase with the lower block－
ing temperature subsequently becomes magnetized anti－parallel to the first．
Proceeding experiments supported this two phases model、 Thermal initial suscepti－
bility of Arei pumice changed after anllealing from the single phase with Tc of 470℃to three phases with different Tc of 340℃，470℃and 580℃． Besides， self－
reversal pTRM which appeared in the field of under 100μT disappeared after upon the stronger field from 190μT to 520μT according to the samples． Yu et al．（2001）
reported a reversal remanence observed during high thermal demagnetization（500
℃～580℃）in the samples Kurokami pumice in Sakuralima， and they suggested it to be originated by chemical alteration of chromites． It seems it was not the pre－
sent case．
Samples used in this study are plotted in pseudo－single domain or multi domain area in Day diagram， Self－reversal in narrow temperature range must be one of the reasons of incorrect paleo－magnetic intensity obtained from pseudo－single domain or multi domain sarnple．
Acknowledgements
We thank students of the Geological Department， Kagoshima University， notably
130 Naoko UENO et al，
D．Fukushima， A Higashi， Y． Fujisawa， M． Muraoka and M． Morikawa for assis－
tance in sampling． We thank Prof． M． Funaki， NIPR for advice on Bitter Pattern observation．
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要 旨
上野直子・鄭 重・上野宏共：狭い温度範囲でおきる自己反転残留磁化の検証
岩石が地球磁場中で冷却する際，通常は地球磁場方向の自然残留磁化を獲得する．地球 磁場と逆向きの磁場を獲得したとき，自己反転残留磁化と呼ぶ．この現象は自然界で榛名
軽石をはじめとして，いくつか報告されている．自己反転のメカニズムは磁化獲得温度が違う強磁性鉱物（A，B）が接しているとき，
まず高温で磁化を獲得する鉱物Aが，ある温度で地球磁場方向の磁化を獲得する．温度 が下がると低温で磁化を獲得する鉱物Bが磁化を獲得する．このとき鉱物Bがさらされ ている磁場の向きは，鉱物Aがつくる磁場（反地球磁場方向）と地球磁場の和になって いる．鉱物Aによる反磁場が地球磁場よりも大なら鉱物Bは自己反転残留磁化を持つ．
鉱物が獲得する磁化の大きさは鉱物の磁化率と外部磁場によるので鉱物Aと鉱物Bがそ れぞれ獲得した磁化の大きさよっては岩石全体として自己反転残留磁化をもつことになる．
今まで，榛名軽石をはじめとして自己反転磁化が見られる岩石にはヘモイルメナイトが 確認されている例が多いために，自己反転磁化の原因はヘモイルメナイトの存在であると 言われてきた．しかし，上記の獲得モデル（2相モデル）で考えると特定の鉱物が存在す
る必要はない．磁化獲得温度や磁化率に差のある鉱物が接していればよい．この考えを検 証するために，部分熱残留磁化（pTRM）の変化率に正負がある試料を選び，磁化が減少 する温度範囲について狭い温度区分でpTRMを直接測定する実験をおこなった．試料と して桜島の溶岩・軽石と大隈軽石を用いた．今回，狭い温度範囲でpTRM実験を行った すべての試料について自己反転を獲得する温度範囲が特定できた．また，生試料では自己 反転が確認できない試料でも，焼き鈍すと自己反転温度が確認できることがわかった
（Fig．11， Fig．12， Fig，14）．なお，これらの試料はヘモイルメナイトではなく，チタン