rocks, Antarctica
著者名(英)
Naoko UENO, Minoru FUNAKI
journal or
publication title
Journal of Toyo University. Natural science
number
50
page range
123-146
year
2006-03
URL
http://id.nii.ac.jp/1060/00002516/
Creative Commons : 表示 - 非営利 - 改変禁止
http://creativecommons.org/licenses/by-nc-nd/3.0/deed.ja
123
Paleointensity study of McMurdo
volcanic rocks， Antarctica
Naoko UENO＊and Minoru FUNAKI＊＊
Abstract Paleointensity of the geomagnetic field was studied on lavas collected from ll sites at McMurdo Sound． These rocks have K−Ar radiometric ages spanning the interval from O．08Ma to 4．2 Ma． The 61 specimens were analyzed， and indicate low paleointensities at Antarctica for the past 5 Ma． Key words：paleointensity， geomagnetic field ， Thelliersl method1．Introduction
To understand the nature of the past geomagnetic field， it is necessary to study both the direction and intensity of the geomagnetic field． Concerning to the McMurdo volcanic rocks， several paleodirectional studies have been reported（e．g． Funaki，1984； Mankinen and Cox，1988）， but paleointensity study has not． The aim of the study is to obtain the paleointensity at Antarctica where a few paleointensity studies have beenperformed．
Various kinds of volcanic rocks， such as kenyte， ankaramite， oliVine basalt， homblende basalt and trachyte on which a directional stlldy（Funaki，1984）and K−Ar ages（Ueno et． aL 1994）had been reported were used in this study．2．Samples
In McMurdo region， volcanism occurred sporadically in the Plio−Pleistocene
（Mankinen and Cox，1988）． K−Ar radiometric ages were determined for the same 25 samples as the samples used in this study from the ll sites． The results were in between O．08 Ma and 4．15 Ma（Ueno et． al．，1994）．Samples were collected by one of the authors（Funaki）in the 1977−1979 austral summer seasons with the support of the U． S． National Science Foundation， from Hut Point Peninsula and Cape Royds on Ross Island and from Taylor Valley（Fig．1）．The sampling sites on the Hut Point Peninsula are illustrated in Fig．2． Petrographic descriptions for the samples of the same sampling area ’上野直子：東洋大学文学部 〒112−8606 東京都．文京区白山52＆20 Faculty of Literature， Toyo University，52＆20， Hakusan， Bunky（ゲ㎞，Tokyo 112−8606 Japan ★★D木實：国立極地研究所 〒1738515 東京都板橋区加賀1−910 National lnstitute of Polar Research，1−910， Kaga， Itabashi−ku， Tokyo 173−8515Japanare listed in Ueno et． al，（1994）． At Cape Royds， samples were collected from north of the Hut． In Taylor Valley， samples were collected from basaltic cones under the Marr Glacier in the middle of the Valley． Polarities of the specimens are normal except those from Observation Hills． 160“E 77’S McMURDO SOUND J ROSS ISLAND CAPE ROYDS
−一・縦・
78°S 160・E O 50km − 170°E Fig．1．Location map of the McMurdo Sound region and the sarnpling sites（modi五ed a丘er Funaki，1984）． HALF ll OO”“ CRATER SmL一
Fig．2． Sampling sites of Hut Point Peninsula（modhied after Funaki，1984）．Paleointensity stUdy of McMurdo volcanic rocks， Antarctica
125
3．EXperimental Procedures
Coe！s version of the Thelliers！method（Coe，1967）was applied to conventional paleomagnetic specimens（25mm in diameter）． Samples varied in length from 10 to 22mm． Samples were treated in a series of paired heating−cooling cycles， repeated in zero and known laboratory field with progressively increasing temperatllre． Nitrogen gas was used at temperatures above 100℃． Initial magnetic susceptibility was measured after each step to detect any magnet−chemical changes． Applied laboratory fields were selected from 10μT，20μT or 50μT to match the paleointensity of the specimens．Partial thermo・remanent magnetization（PTRM）tests were employed to check the
reproducibility of the PTRM． Specimens were set in the furnace horizontally or perpendicularly to the applied laboratory field． Variation on applied field magnitude and direction， and size of the specimens， produced no difference in results．Natural Remanent Magnetizations（NRM）determined after demagnetization in zero
field， and TRMs acquired in known field are plotted with the NRM on the ordinate and the TRM on the abscissa． The slope of a line is calculated by the method of Kono and Tanaka（1984），in which the least squares fitting of Williamson（1968）was used．4．Res111ts and Discussions
The experiments were made on 61 samples（Tablel）．NRM−TRM diagrams， Zijdervelddiagrams and PTRM tests were mainly used to find linear segments in NRM−TRM
diagrams to calculate paleointensity． Summaries of the experiments for each sampling site are given below．4−1Cape Royds
Two Curie temperatures，100−120℃and 550−570℃， were reported for the samples from this site（Funaki，1984）．Results of this study are illustrated in Fig．3（Fig．3−1， Fig．3− 2）．Large changes appeared around 100℃and 500℃in measurement of initial magnetic susceptibility（X）during heating and cooling cycles in air． Low unblocking temperatures and the decrease of pTRM（partial TRM）in 300−400℃was the characteristics of this area． Initial susceptibility in room temperature after each heating−cooling step varied within 20％．Averaged paleointensity of four samples， which have averaged age of O．080±O．02 Ma（Ueno et al，1994），is 13±2μT．4−2Hut Point Peninsula
（1）Crater Hi11：At this site， straight line segments were obtained both in NRM−TRM and Zijderveld diagrams（Fig．4−（1））． Curie temperatures of these rocks are approximately 310℃and 560℃． Averaged paleointensity of five samples， which have the age of O．39±0．04 Ma， is 27±9／， T． （2）TWin Crater：Specimens from this area had straight line in NRM−TRM diagrams under the low temperatures as those from Cape Royds（Fig．4−（2））． Large changeson initial susceptibility occurred during cooling． Averaged paleointensity of two samples， which have the age of O．47±0．01 Ma， is 18±9μT．
（3）Between Cape Armitage and Scott Base：Two Curie temperatures were
observed， around 22（｝330℃and 520℃（Fig．4−（3））．Both NRM and pTRM change only in the high temperature steps． Averaged paleointensity of two samples， which have the age of O．50±0．05 Ma， is 19±9μT． （4）Nor血of Hut Point：Good results were obtained from this site． Reversible curve was obtained in Z／X。 measurement， that ran low at 540℃（Fig．4−（4））．Averaged paleointensity of two samples， which have the age of O．68±0．03 Ma， is 19±1μT． （5）Hut Point：Although good linear segments were obtained from the steps over 400℃（Fig．4−（5）），both pTRM test and initial susceptibility measurement seem not so good． Averaged paleointensity of two samples， which have the age of L32± 0．63Ma， is 15±0μT．（6）Castle Rock：Double Curie temperatures were expected fromγ／X。 curve
（Fig．4−（6））． Both pTRM test and initial susceptibility measurement seem not so good． Averaged paleointensity of two samples， which have the age of O．73±0．02 Ma， is 80±2μT．（7）Black Knob：Complicated NRM−TRM diagrams were obtained at temperature
above 300℃（Fig．4−（7））． PTRM test were performed for the steps under 300℃ where linear segments were obtained． Averaged paleointensity of two samples， which have the age of O．55±0．25 Ma， is 4±0μT．（8）Half moon Crater：Two segments with successful pTRM tests were obtained
from both the low and the high temperature components（Fig．4−（8））．The initial susceptibility curves were reversible with temperature， and changes of initial susceptibility after each step were small except those for one specimen． We had no reason to reject both of them． From four samples， which have the averaged age of O．83±0．12 Ma， two averaged paleointensities obtained are 27±6μT加m the lower temperature portion in NRM−TRM diagrams， and 47±9μT from the higher portion．（9）Observation HMs：Good segments were obtained（Fig．4−（9））from the high
temperature steps． Averaged paleointensity of five samples， which have the age of 1．28±0．03Ma， is 20±4μT．4−3Taylor Valley
Very low paleointensities were obtained from this site． A sharp decrease between 450 ℃and 550℃and reversibility on cooling are the distinctive features in measurement of temperature dependent susceptibility（Fig．5）．Averaged paleointensity of three samples， which have the age of 3．51±0．64 Ma， is 5±1μT．4−4Magnedc mineralogy
The magnetic mineralogy of pilot samples from Crater Hill and Half Moon Crater isPaleointensity stUdy of McMurdo volcanic rocks， Antarctica
127
dominated by hematite． The main magnetic minerals for samples from other sites are titanomagnetite and magnetite． Among the samples dominated by titanomagnetite and magnetite， those from North Hut Point， Observation Hill and Taylor Valley had ilmenite lamellae． Coercive force（Hc）， saturation magnetization（Ms）and ratio of saturation remanence（Mrs）to saturation magnetization were also determined by vibrating samplemagnetometer for some specimens（Table 1）． Magnetic coercivity was over 20 mT
except for Crater Hill and Haif Moon Crater． All of the Mrs／Ms values were over O．12， acceptable value for sample selection（Thomas，1993）． x／Xo 1．1 1．0 0．9 0．80950
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Sampling site Cape Royds O．080±0．002Ma Crater HM O．39±0．04Ma Twin Crater O．47±0，01Ma Beセvveen Oepe Arm．＆Scott B． 0．50±0．05Ma North of Hut Point O．68ゴヒ0．03Ma HUt Peint 1．32±0．63Ma Castle Reek O．73±0．02Me Bledtt Knob O，55±O．25Ma HeSf Meen Crater O．83±0．12Ma （high temp． segrnent Observation Hill 1．28±0．03Ma Taylor V311ey S、mpl。 N。． J（E−3 Am2／kg）X（E−6mユ／kg）H・mT M・（E−1 Am2／kg）M・s／M・T℃ 79010950 79011013 丁9010907 790110ハt 77121607 77121610 77121612 77121613 77121614 77112710 77112715 7801210了 78012105 7BOI1806 78011808 77121602 77121603 77121911 7712tgO7 7SO12215 78012216 78012203 78012204 78012205 78012206 78012203 78012204 78012205 78012206 77121404 77121503 78122305 78122310 78122306 79010401 ？？122iO2 77122106 0．46 0．62 0．43 0．98 751う︰721311
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03267
5334﹁3
128 63 234 28．5 3，0 20．3 42、3 12．9 4．6 29．4 23．1 20．5 24．9 2．0 7．9 2．1 1．6 O．1 2．2 1．3 2．0 14．5 2．7 N 102−254 6 30−147 4 102−254 6 158−253 4 402−−552 4 415−581 6 333−562 7 254−562 8 0．40 503−581 4 fraction FμT 0．4 15 0．4 12 0．4 10 0．2 13 averaged F 12．5±2，1 0．4 34 0．7 31 0．丁 34 0．7 14 0．5 23 averaged F 27．2±8，6 0．14 30−2056 0．7 26 109−265 6 0．7 9 averaged F 17．5±8．5 0．23 453−532 3 0、5 27 487−552 3 0．2 10 averaged F 1臥5±8．5 0．37 253−552 7 0，7 18 354−562 5 0．9 20 averaged F 19．0±1．0 0．29 403−552 4 0．7 15 403−502 3 0，3 15 averaged F 15．0：ヒ0，0 452−531 3 0．3 78 415−503 3 α6 82 averaged F 80．0±2．0 158−253 4 0．2 4 158−253 4 0，2 4 averaged F 4．0：ヒ0．0 205−453 6 0，19 158−402 6 253−453 5 253−353 3 0．18 e．16 0．18 o．32 453−532 3 402−531 4 453−531 3 353−531 5 102−351 403−562 354−562 402−531 451−542 452−531 355−554 354−52775644
355
O．4 20 0．3 34 0．3 29 0．2 26 avereged F 27．3±5．9 0．5 57 0．6 45 0．5 51 0．7 35 averaged F 47．0±9．4 0．4 0．8 0．9 0．3 0．3 averaged F 0．4 0．B O、4 averaged F 2i 18 2i 25 13 19．6±4，4 4 6 5 5．0±1．0 Table 1 Summary of data J：NRM intensity x：initial susceptibility Hc： coeciVity Ms：satUration magnetiZation Mrs：saturation remanence T：temperatUre range in which F was calculated N： number of data points over which F was calculated F：calculated paleointensity from NRM−TRM diagram．5．Summary and Conclusions
Asummary of the results is shown in Table 1． Calculated VDMs from averaged paleolntensities and the inclination data of the same samples obtained by Funaki（1984） are listed in Table 2． While VDMs for the past 5 my have been reported as being close to the present−day value（McFadden and McElhinny，1982；Prevot et al．，1990；Otake et．aL，1993），VDMs from the successful data of this study are very low，0．86 to 3．8 xlO22 Am2． The reason for this is unknown， but the effects of local non−dipole fields are probable．Sampling site
Crater HillCape Arm−Scott B
North of Hut PtObsevation HM
Taylor Vally
Age Ma
0．39±0．04 0．50±0．05 0．68±0．03 1．28±0．03 3．51±0．64N
5
2
2
5
3
FμT
27．2±8．6 18．5±8．5 19．0±1．0 19．6±4．4 5．0±1．0VDM 1022 Am2
3．8±1．2 2．5±1．1 2．5±0．1 2．6±0．6 0．86±0．09 Table 2 Vrirt）ual Dipole Moment calculated N：number of specimens F：site mean paleointensi｛y VDM：Virtual Dipole Moment ln unit of 1（戸2Am2， In conclusion， Coels version of the Thelliers｝method was applied to volcanic rocks from the McMurdo region which have K−Ar ages covering the interval from O．08 to 4．2 Ma． We obtained successful results from samples that meet the following criteria；linearsegments in NRM−TRM diagrams and Zijderveld diagrams， PTRM tests， initial
susceptibility after each demagnetization steps， initial susceptibility with temperature，microscopic observations， Hc and Mrs／Ms． Low geomagnetic paleointensity values
were characteristic of the units sampled at McMurdo Sounds for the interval from O．08− 4．2Ma．References
Armstrong， R． L．（1978）K−Ar dating−Late Cenozoic McMurdo volcanic group and Dry Valley glacial history， Victria Land， Antarctica． N． Z． J． Geol． Geophys．，21，685−698． Coe， R． S．（1967）Paleo−intensities of the earth’s magnetic field determined from Tertiary and Quaternary rocks． J． Geophy． Res．，72，3247−3269． Cole， J． W．， P． R． Kyle and V． E． Neall（1971）Contributions to Quarternary geology of Cape Crozier， White Island and Hut Point Peninsula， McMurdo Sound region， Antarctica． N． Z． J． GeoL Geophys．，14，528−546． Forbes， R B．， D． L． Turner and J． R． Carden（1974）Age of trachyte from Ross Island， Antarctica．Geology，2，297−298．Funaki， M．（1984）Paleomagnetic investigation of McMurdo Sound region， South
Victoria Land， Antarctica． Mem． Natl． Inst． Polar Res． Jpn．， Ser． C， Earth Sci．，16，1−81． Kono， M． and H． Tanaka（1984）Analysis of the Thelliers’method of paleointensity determination，1． Estimation of statistical errors． J． Geomag． Geoelectr．，36，267−284． Kyle， P． R． and S． B． Treves（1974）Geology of Hut Point Peninsula， Ross Island． Antarct L． U． S．，9（5），232−234． Mankinen， E． A． and A． Cox（1988）Paleomagnetic investigation of some volcanic rocksPaleointensity study of McMurdo volcanic rocks， Antarctica
