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JIK¥nalolwk:anology · .. ilnd~resCizdl-.

JournAl of VoiOUialollY and Cieolhcmllll RCICIICh 67 ( 1995) 29-'0· ·

EsR::~atin.g of quartz phenocrysts in the El Cajete and Battleship Rock.Meriibers of Valles· Rhyolite, :Valles Caldera, New -¥exico · ·

. . . Shln Toyoda "·*, Fraser Goff b, Sumiko Ikeda 11, Motoji Ikeya a ..

'Dt{Hlrtmtnt.tl/ EArth and SpGCt Scitnct, Fnc11lty ofScltnct, OmkD Ulliwrslty, M~~ehltllnryoma. To~ Oselk4, $60.Jopan . • &mi!IWI En~Jrottmtntal ScltnctJ Oivlsi011 Lo1 AlnMtu NaliDt1DIIAbortJit1'1• Los A/111110.r. Ntw Multo, B7S4S,US!.

Accepted 2q September 1994

A~:·-.

· . The ~l.cetrOn spin resonance ( ESR) dating method wu cmployccl on quanz phenocrysts sep~~111led from· pumice or the Ei • Cajete and Battleship RocltMembersoflhe Valles Rhyolite in the Vallcscalcleru.New Mexlco. 'T'he resultsofheating experiments ind!CGte lllit 'li Impurity centers hllve two components: a thcnnally stable one and a less stable, ccmpcnnu~ sensitive one. ESR •. ·dates llilng the sUiblcTi center yield eruption 11gcs of S9 :1:6 ka for the Battleship Rock Mcmbennd 53:1: 61ca for the El Cajcte · Membct while recent ••c dates (S. Reneau and J. Oardner, unpub. dasa) from ctrbonlzcd logs In lhc'El Cajeu: pumice Indicate . that hs ·:~gels oldcrthiin 50 lea~ Our results indicate that volcanism In the Valles caldctll is much younger than previously thought ·. ( ~- 130 kil):and that recent revisions to !be post.O.S Ma Slrltlgraphy orValles caldera are probably in enor.Thc results suggest that ~R datin& ·or Cluaru ·m~~y be a useful method for obtaining ages of units In other Quaternary volcanic areas •

. .. ' .. · ~ . ' . .- ..

·• ·~; .• ·.';'. ' ·~ .. ·.·i

· 1. IntrOduction ~

·· Quahz is the. ~est ciommon rock forming mineral in the caitn•s crust lind a common constituent in volcanic rocks and hydrothermal deposits: thus methods that can dnte)hc crysrnllizntion age nnd thermal history of quartt.tmve tremendous applications in geology, vol· canology, and 'archaeology. Three methods developed in' recient ycars::htwc been successfully used to dale quarti;:·(and :.other minerals): Thermoluminescence ('It.).:'inrruicd. optical (10), IUld elec:trcin spin reso­nnnce~(ESR)~ The three methods arc similar in 'that. they niwure the concentrations of trapped elc:cuons in defecis in 'C:rjstal structure$ as functions ortime but the ·theory~ instrumentation. and limitations of the methods

• CorrespondlnaDuthor. Now 111 Depanment ofOeoloiY. McMu· aerUniYC"It~; 1280MalnSL W .. HDmlllon,Ont.,OanlldDLSS4MI •

. ~ '. '

· DJ77:o2'7319,/S09.$0 o 1995 EbcvlerSclcnce B.V. All ri'hll n:semd -S:rD/.0377·_0273( 94 )00093·X

'·'

. \' . ~' ,~ . ·,, :-·

arc different (Aitken, t9ss: t992: Skinner. t98S; Ber·· ger, 1988). . _·

The Valles caldera in nonhem New Mexico (~g.· .. · · 1) fonncd 1.14 Ma·IUld n:prcscnts the culminating magmatic event of the .Jemez volcanic field, which has. been continuously acti'lli: for the last 2::: 13 m.y. (Doell et 41., 1968; Gardner et al •• 1986; Spell et nl., 1990). Valles caldcru also contains many post-cnldcru crup.:. tions of rhyolite domes, flows, and pyrciclastic depositS. that have been dated by numerous methods most recently summarized by Spell and Harrison ( 1993). · . The agcn of the youngest rhyolite laVDS and tcpbras have been the subjcefof c:Onsidcrnblc controversy due. tO SU'UUgt'llpbic: complexities in OUtcfops and drill core, . pcuologie and geochemical similaritieS among units, and non~agrcement of previous ~ting results. In fBc:t. recent revisions to the stl'11tigmphy.ofyoungest Valles .•. rhyolites have been pel'fllexingly numerous (Self c{at.; .. · ·

~-~ ·'

•.•·: .... : ... ·,·:··

11 q . . G

S. Tllytldcl tl ell. I Juunw./of Vltlru"''IIIH)IIItld OttltMrlllul HrJtttrrlt fi7 ( 199!1 29-10

liP IJan'o

BSR Danleahli! Rock lanlmblite EC 1!1 Cajete plinian· deJ!Otlt

rlna dome vent

Fla. I. Localltlca or the samples collec:ted ror ESR dllllnauahown by solid circles. Jwpachl or Ill Oljete pumice fiLII deJIOIIt IU1! ahown In meters. The flaun: Is panllllly modlntd 11ner Sclfe11ll. ( 1988).

1986, 1988,.1991) since the original definitions ofBai· ley.ct u1: (1969). Besides obvious uses tor the mag· mutlc, tccitonlc, and geomorphic evolution of the Vnlles culdera, the age of the El Co jete pumice eruption Is very significant because it forms a widespread blanket or tephrn thioughoutthe southcwncm Jcme~ Mounulins und can construln rhe.age or Quaternary faulting and erosion. (Ciardncr nnd House, 1987: Albrecht ct a1., 1993).

All ·of the ·youngest rhyolites contain quartz as n primary phenocryst (Bniley et ul., 1969: Onrdner et "'" 1986). BCCII.usc ESR doting was used successfully to date quartz·at other Quutemnry tephro deposits (lmui et11l.; 198S::Shimoknwaand lmai, 1987; Imni and Shi· moknwn, 1988) ~ we have determined the thermal sta· bilities of Aland Ti·centers in quartz from pumice of the E1 CajctC and Battleship Rock Members or the Valles ca1dern." Particular emphnsis was given to sam· pies of the .EI. Cajctc pumice because this unll was partially deposited outside the caldcrn where no hydro­

. : , ... · thermaluctivity of post.:CUidernage has occurred. Our ESR dotes indicate thnt the El Cajete and Battleship RockMembersarcofapproximately theso.meage (53-

...... ,.

59 ka) and younger thnn·prcvlously thought using lls· sion-uuck Md 39 Ar-~Ar methods (Self ct at., 1991).

1. Geologic buclcground

The eruptive history of the Jemez volc:Unic field hns been the subject of many investigations but the Valles caldera hns been II focus or pnrtic:ulur interest (Dailey et o.l., ·1969: Smith et al., 1970; Cordner etal •• 1986; · Selret al., 1991 ). The Vallescaldero Is a 22-km-dio.m· etcr collapse depression that formed at I. t4 Ma during eruption of the Tshircgc Member of the BundelierTufr (Smith and Bailey, 1968; Spell ct nl., 1990) ~Soon after caldera formation, n central resurgent (structuml) dome grew in the approximate center of the c:alderu. A sequence of no less than 14 rhyolite domeii, nows, and pyroclastic deposits wns erupted from about l.OS to 0.13 (?)Main thecaldern moat surrounding the resur­gent dome (Doell ct ul., 1968: Ciardncr et al., 1986) • Some of the moat rhyolites arc buried by mont sedi· mcntnry rocks Mel younger moat eruptions (Ooffctul .. 1986). Hydrothermal activity is pervasive in the south·

Fla.: The nnul prop. 111\d(

wes hus I (Oo · of II hydt

· som· OUlC.

upp< stud. sus~

.70"( emt , or t~ (19t (blal

·bert. and (gre: igrar duct .

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..... .......

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. ' 111111"11 shown In·

1ghl using fi.'l· :tal., 1991).

:anic field bus · :x1t the Valles· . r.erest (Bailey ·. r et a1 1986··: ~· ., '' 22·km~iam· 14 Ma during · landellerTurr. · O).Soonofter ··· · ·. (struCtural). the caldera. A cs, ftows, and lbout ·1.05 to : ling lhe resur·;: ·. · et al.,J986),, '

·lY moat sedi· ·. \S (Ooffetad., ·· e in the soulh·

S. T1•ytlda tl al.l Jmmwl•ifVulca""'''I:Y alld GtothrrmtJI Rtltllrch 67 ( 1995) 29-JO 31

Original VC-1 StratigraphY Stratigraphy

(Bailey ct al., 1969) (Cloer & Oudncr, 19117>

Revised Stratigraphy II I (Sclfet &1.,1988)

Revised Stratigraphy 112 (Selrctal,.l991l

Proposed Slnlugraph)l rtbiareport)

r------"""'1 I • .. •- •-- • • ... t r------""1 ,-------, · · I Banco Bonito M. 1 1 Banco Bonilo M. Baneo Boni1o M. .___c_IJ_o kl_l _ _, : Banco Bonito M. : (17Q.%40 ka) Banco Bonito M.

El Ca~e M. l : Banleship Rock M. : : .... ~-~-;;, .. ~ .. - ~ I Battleship Rock M.l (>42 ka) · I I I • )'O IU: I (59 ka) · I I I I .__ ______ _,

I El ,._. M I I I

I I r------.... 1 '""jelc • 1 , Bllltlcahlp Rucll. M. 1 ,r~~~~....,

DanJcahlp Rock M. Blllleahlp Rock M. 1 - - - ----- •• ' 1 1 E1 Cajetc M. _ (278 ka) I (5(). 73 ka) El Cajctc Scrica : 1 ..._..-~~-..._. (1J0.170ka) 1 EICajeu:M. 1 ,....----.,

Soulh Mountain Rhy. . (490 ka)

VC·I RhyaliiC (365 ka)

VC·l Tuffi

Volcanlclutlc . Breccia

South Mountain Rhy . (490ka)

1 ............. 1 VC-1 RbyoliiC ReWed (1) Series

(<:!00.360 ka)

(<365 ka)

VC·I Tulla

Volcaniclaatic II Volcanlclulic Breccia · ,_ __ B;;.;.rccc.;.;.;.;..la;...._.

Soulh MOW\tain Rhy. (521 ka)

Soulh MOW!IIln Rhy. (490-520 ka)

Fig. 2. Chlllt ahowins 1ho evolution or strallgraphlc lntcrpreuuion' for geologic units In the 50Uthem moat zone or Valles C41dcm, New Mulco. The VC·I COle& c:onmln tephm unh5 whose age unll corrclllllon wllh surf'occ CJpolurcs arc not reiOivcd. The ESR d4tea lndiCIIle lhAt the Bonleshlp Rock Dnd 1!1 Cljete Memben arc opproldmJilely the umc 1£1: u lndiCIIIed by Self et Dl. ( 19881 bularc considerably younger lb4n proposed by Self cllll. ( 1\191 ) • The ltl'llllgraph)' proposed In thll l'q!On relllllll most or the concepti developed by BDIIC)' ct &1. ( 1969). aorr and Clllldncr ( 191f7), and Selfct Ill. ( 1988),

western sector or the cnldern ( Ciofr et al., I 992) and has been continuously uctive forthc lust I million yenrs ( Cioff und Shcvenell, 1987). ln their curly K·Ar study of the Valles l!nldern, Doell et :11. ( 1968) noted that hydrothermal alteration was u possible: influence on some of their (inconsistent!) results. Although most outcrops and· cores· of the youngest rhyolites do not appear to be hydrothermally altered, lhe initial ESR study of the Valles Rhyolite by Ogoh etnl. ( 1993) suggested· that modest tempcmture anomalies (about 70°C) have occurred at very shallow levels in the south· em cnldeni mont in the recent past.

The origin Ill strntigruphy of the three youngest units of the Valles Rhyolite was defined by Builcy ct ul. ( 1969) I1S (lop to bottom) Banco Bonito Member (block, glassy, porphyritic obsidian), El Cojete Mem· ber (white lo very pule omnge, pyroclaslic fall, flow, and surge depositS), and Bnllleship Rock Member · (grey to tan, lithic-rich ignimbrite). The original strat· lgmphic definitions changed drumaticully ufler 1986 due to numerous reinterprclutions of field rc:lntions,thc

impact of discoveries in corchole VC-1, and the acqui· sition of various dntes (Fig. 2).

VC-1, the firsr. corehole drilled in Vnlles cnlderu under the U.S. Continenllll Scientific Drilling Progrnm, WllS sited in the southwestern moat zone of the colderu and provided 856 m of continuous core of intrn-enlderu volcanic and sedimcnUll')' rocks (333 m) and pre-cal· dern Paleozoic and Precumbrinn rocks (S23 m) (Ciofl' ctlll., 1986; Ooff and Oudncr, 1987: Cieissman, 1988; Hulen and Nielson, 1988). A key observation regnrd· ing the site of VC-1 is that it is looued in a sector of the caldera moat where the ring-fracture system and the pre-caldera lcmez fault zone intersect. As'' result, there has been considerable erosion and deposition of · cnldem-fill rocks in this sectorthal WllS previously unre· cognized {Cioff eta!., 1986). About300 m south of VC-1, 35m of Banco Bonito obsidian directly overlies the nonwelded top of the Battleship Rock ignimbrite. However, In VC-Ithe Banco Bonito is 149m thick and overlies 11.9 m of tephrn deposits of debated origin und strntigmphic llSsignmcnt ( Ooff and Onrdncr, 1987:

.1 q q • .-:!) ., 0 0 1 • 3

.. :::

•"',.·,. .. .. ,· .·... . ..

S. Tr~y,Jda rt nl. I JtJurrtllltl/ V,t,·an~~lnR)I Ulld CirothrrniCJI Hmurclr67 I /IJ9$) 29--40

Goffet al., 1989:·Sclfct ul., 1988, 1991 ), Much of this lephru dc~sil resembles nonwelded Battleship Rock ignimbrite. No fnll, now, and surge deposits similar to the El Cnjetc Member occur in this horizon.

One of'the significant finds in VC-1 wus discovery of another. block; glussy, porphyritic obsidian now (VC-1 Rhyolite) .having u thickness of 20.0 m und underlying a poorly-developed soil ncnr the base of the (debated) tephra deposits. The VC·I Rhyoli\c has no known surface exposure in the c:aldern; thus it is impos· sible to relate its sU'Iltigrnphic position to other young rhyolites except in VC-1. VC-1 obsidian breccia clcurly overlies a well-developed soil that formed at the top or 11 sequence or three pyroclastic Oow deposits (VC·I Tuffs). The VC-1 Tuffs overlie another well-devel­oped soil at the top of u volcaniclllStic sequence which includes Oow-brcccin of South Mountuin Rhyolite at the bllSe; The contact of calderu-fill rocks with under· lying Permian Abo Formation occurs ut 333 m depth.

Tephra deposits in VC-1 nrc not distinct enough to make positive correlations with surface outcrops. The youngest rhyolites including VC·I Rhyolite and VC·1 Tuffs have surprisingly similarpctrogrnphy and major/ truce clement chemisll)' (Gardner et nt., 1986). All of the youngest rhyol itcs include xenocrystic mnteriol that produces spurious K-Ar und '" Ar-<10 Ard11tcs C Spell and Harrison, 1993). As n result of these uncei'Ulintics und different intcrpretntions, the strutigraphy hilS evolved a.'> shown in Fig. 2 but no interprelDtion is accepted by all geologists who work in the cnlderu.

3. Previous age dctrrmlnatiorL<i or the youngest rhyolites.

The youngest post-caldera rhyolites nrc located in the southern moat zone of Valles caldern 11nd consist of domes, flows, and pyroclastic deposits < O.S Ma (Doell etnl .. 196M~ Bailey eL at •• 1969~ Gardner ct al., 1986; Gcissmon, 1988: Goff et al., 1989: Spell and Harrison, \993). The nges of these youngest rhyolites hove been previously invcstigllted by '"C, fission-truck, K·Ar, l

9Ar-40Ar, U-Th discquilibrium,11nd ESR meth· ods (above reference.'>; Marvin nnd Dobson, 1979; Miyoji et nl., 19RS: Self et nl .• 1988, 1991: Ogoh ct ul., \993) but none of these methods yield consistent nor gcologicully rcllsonnblc resuiL'>.

Doell et nt. ( 196~) reported u K·Ar age of 490 ± IS kn for South Mountain Rhyolite but did not report any ugcs for the overlying youngest rhyolites, presumably bcc:11USC they were too young for K·Ar dating. Goff und · · Gordner (1987) oblllincd u K·Ar nge of600± 100 ka on South MounlDin Rhyolite breccia from the bottom of the mont volcanic section ofVC-1 (Fig. 2}. A more recent K-Ar dote of 507 ±IS kn hilS been dctennined by Spell nnd Kyle ( \989) und successive 311Ar-40Ar ages or 517 ± 14 and 521 ±4 ka have been listed by Spell et nl. (1990) and Spell and Harrison ( 1993), respectively. The consistent ages reponed for this dis· tinc:tive unit uc imporlllnt because the youngest rhyo· Iiles overlie South Mountuin Rhyolite in severn! key outcrops in the caldera mont (Smith ctal., 1970; Self et nl .. 1988: Cioff etal .• 1989).

The VC-1 Rhyolite hllli n K·Ar date of 365 ± 61 ka ( Gnrdner et at., 1986) and 3,. Ar· 40 Ar dates of S 18 to 63 I ko (n • S, Spell and Hlll'Tison, 1993). The range or 39Ar-411Ar ages suggests that xenocrystic: material is present in this unit. The Battleship Rock Member bus 11 K-Ar date of 278±2 ka (Goff et at., 1989), but fiss!on-trnck ages of 180±70 to 130±70 ure nlso reponed for this unit {Miynji ct ol., 1985).

Bnilcyetnl. ( 1969) found ac:nrboniz.cd log in a fresh rondcut through pumice of the El Cnjcte Member ulan~ slDtc highway 4 but the 1•c age was >42 ka. More recently, S. Rencnu 11nd J.N. Gurdner (unpubl. datu, I 992} have dated additional logs found in a pumice mine 200m north of highway 4 and obtnined ages of >SO kn. Miyaji el at. ( 1985) obtained a. fission-truck age of 170 ± 70 ka. Self et ul. ( 1988) incorrectly reported a U-Th disequilibrium date of about 150±40 kn. Self et a!. ( 1991) and Spell and Harrision ( 1993) report 311 Ar·"" Ar uges of S 19 to 923 kn ( n • 4) indicat· ing thnl xenocryslic material is also present in this unit.

Marvin nnd Dobson ( 1979) obtnincd n fission-tn1ck age of 130 ± 100 ka for obsidian of the Bnnco Bon itLi Member, undisputlbly the youngest of all post-Valles cnldem eruptions. Miyaji eta!. ( 1985) reponed another fission·trnck age of 140 ±50 ku. Sclfetlll. ( 1991) und Spellund Harrison (1993) list nine: ~"Ar-411Ar dates rnnging from 205 to 1300 ku. This latter nge range is perhaps the best evidence for xenocrystic material being present in this unit (a.'l welln.o; other youngest rhyolites) nnd the problem is discussed by Spell and Harrison ( 1993).

• .,­.!!)

iJu qunAz a&;\1i th~o Quert~ · th&~< Memt (Goff it wm­yicldc thus ir and/c ESR1 lee ted Jocuti· hydro perf or cente1

4. El

ES elccu minct defcc 1993 elect I 1954 form ESR unpa mine dose ment ofth1 dosia accu· byE natua

11 de tel llS Sl

arag. hydr shell 1981 rock

I'

. . .

•age ~r 490 ± ts.: In u previous ESRdoting study ( Ogoh et ul., 19~13), id not report any.; quartz sepiU'utes obtained from outcrops of the Bun,~o itcs, presumubly·; Bonito, EICujetc, und Buuleship Roc:k Mem~r.~ withm dating; Goff und' . the southern moot of the colderu yieldetlugcs s 45 k.1. of600:f:'IOO Ia{· Quunz scpurutes were ulso obtained from pumice of

. ·from the bollon{i.·. the VC·l core in uniL'I identified us the Buuleship Rnck ~Fig. 2);A·moie: ' Mcmber,thc Upper VC-1 Tul'f, und Middle VC·I "rurr oecn determined . (Goffund Gardner, 1987: Oeissmun, 1988). How·Jvcr, :ssivc y)Af.MJA/·. it wus found that successively deeper unit.~ in VC·I c been llstcd'b)' :·. yielded successively :younger 11gcs (us young us 19 ku); !arrison ( 1993), · thus it wus suspected that thermulannculins from pust •rtod for this dis·:· und/ or present hydrothermal ucli vity was cuusing the :youngest myo- ... ESR ages to be too young. For these n:a!ions, we col· c in scvcrul key·.' lecled udditionol samples of El Cujete pumice form :tal., 1970: Self loc:utions ouwide the culdcru to eliminate effects of

I hydrothc:rrnol ullcrution nnd thermal ttnnealing und to

c or365 ± 61 ko • perform hcutlng expcrimenl.'i of AI und Ti impurity · dates of 518 ta < . centers. •3). Thcrungc·ar I ystic material is .. l xk Member h115·:. 14. Ehlc:tron ~pin resonance dutlng

al., 1989), but 1 l0±70 are al!o" 1 ESR (electron spin resonance) detects unpaired lB5). · , tel~ctrons in s:sm(lles. U~p~ired electrons arc creutcd in wllo in afresh·, ·I mmeruls by naturul rod111Uon and urc trnpped ut some tc Mc!ber along' ·· defect nnd impurity sites in the crystul structure ( lkeya, >4'2. ka More· 1993): for example, nn AI atom rc(llucins Sltrnps un

' (un ubi dAUI:: electronic hole to form un AI center (Griffith ct al .. .r d l p 'umlc~~· 1954), nnd uTi utorn replacing Si trups un electron to :tai n ~: es · r .• form o Ti center (Wright ct ul., 1963) in quartz. An ld : . ~ltts~k· ESR signal intensity corresponds to the umount of JB:). ~:o"rrcc:tly unpaired ~lec~ons which hn~e been uccumulotcd .in ~he f abo t 150 ±40 . mineral Stncc II WUS crystulllzcd, The natural rudtutton ' . u · .

1 3 . dose rotc produced by U, Th, and their daughter ele·

IatriC sl:n) ·~ r :~: ments, nnd K is calculated by using the concentrntions 1 n •. 1~ 1~ :', 1of these elements or mcn.sured by themtoluminesccncc esenl tn ~hts Unll·: ~dosimeters. ·n1c uge is obtained by dividing the toUt! ~" flsston-~k .uc:cumuluteddose (or equivalent dose, Du) detcrminct.l ,,c Banco Donato ··by ESR measurements by the unnuol dose (D) from If aU post· Valles jnuturul rudiation. ·reported another .i The ESR method hns been previously upplied to :ul. (1991 > a~ the a"e or many minernls and dcJlOSits such "AI· .oAr d11t~ ·. stalactites of calcite (e.g., lkcyn, 1975), cor:.ls of

lltc:ragcmngeas. te (e.g., Ikeda ct ul., l991),tooth enamel or IQ')'stic motcrinl. te (e.g. GrOn und Invernuti, l9RS), nnd • othct· youngest, of culcitc and of aragonite ( lkeyu and Ohmuru, ~by ·Spell and 98 t), us well ns qunnz and plugioclnse in volcanic

·~· .:· .. ·:·· . . ,.

and IL'Ihc:s (lmni ct at., 1985: Shimokuwn und

itnui, 1987). '1111: thcnnulstubility of the dc:fccl.'i lirnit'i the age range to be dlltc:d up to about 2 Mn (e.g., Shimokawo nnd lmai, 1987}. The younger ngc limit depends on the sensitivity of the ESR spectrometer, being o.round severulthousund yenrs for recent com· merciol ones (lkeyu, 1988).

The signals of AI centers in qunrtz were used to obtain ESR uges or volc:llnic tcphra.'l in the first study (lmui ct al., 1985). The signals ofTi centers in quartz were also used in subsequent studies (lmui und Shi· moknwu, 1988). Some fundamental results on thermal stubi II tics of purumognctie defects und impurity centers were reported byToyodu und lkeyu ( 1991 ). However, churucteristics of all the centers, such u.~ thermnl stu· bill ties, response for irradiation, nnd reaction between the centers, hove not been well established yet.

S. Sample shes and Knmplc prcparutlon

Wr. collected 2 kg of pumice from the type loculily or the: Duttlcship Rock Member ( BSR04) um.l 2 kg ol' rock from the caslcmmost roadcut of South Mountain Rhyolite c"posed nlons highway 4 ( SMROl) os shown in Fig. I. We nlso collected 2 kg of pumice from five locnllties (EC02, ECOS, EC06, EC07, nnd ECOH) or El Cnjete Member. Thc:!lc locutions were chosen to he both Inside ond outside the culdcra to eli minute nny bia.'l due to post·culdern hydrothermal ulterution (Smith et ul., 1970: Clofrct ut., 1989). Original pumice nnd rhy· olite were crushed in n mortar und were sieved. Porous grulns larger thun I mm size were removed by using u heavy liquid (sodium polytungstnte solutiun). A (lort· able mug netic separator removed the grains containing magnetic minerals. Quartz gruins of I to 2 mm were handpicked. Finally, lhe extracted gruins were etched in 20% hydrofluoric acid for 2.S hours to remove defects ncar the surruce caused by ulpho cmiucrs in the original rock matrix. Six to eight uliquots of90 to 120 mg from ench snmple were prepared after the gruins were crushed to 75 to 250 p.m.

6. Experln1ental proct.'fiure for ESR dutln~

ESR signuls from AI oml Ti centers were observed with a commerclnl X·bund ESR spectrometer (JEOI.. RE·IX) ut liquid nitrog~:n temperature (77K) with u

l. q q •

..

.... ~'. :· ' : ., '

S. Tuyttda tl aLI J1111mal tt/ VlllttUtolnRY tUtd Gtntlatmu~/ Rt1tt1rch 67 ( 199$) 2!J-IO

· ... ,;

, ... ~­.!)

(AJo.r . be~ib~·-. orale>~ P~i:iu:: .. : ....

~·

II

' 2000 r:n :. ' ; Radioat :' -.

· · w~ne{: . Tablei·<:·

The ralit ': ·

. 2mT

• ..

Pla •. 3. ESR alanals obtcrYed at liquid nluoaen tcmpenllum (77K) In qllll'tZ CJ~Inleled rmm SMROI. ESR alcnaJ• due ro rwo lmpurtry ccnrc:n · 11n: oblaoled;AJ center, (AIO.J0, where 111 Alarom replac:lns Slalom rrapaan electronic holc,lllld Tl ocnterwhh u•, !TI04/U• )0, where ia' Tl atom replai:ln1 Sl11on1 lnlps An clcclton.

·loQ::Jc}{z.fi~j~ ·~ixtullition of the amplitude at 0.1 mT · Annual dose. (D) was calc:ulaled for.cnc:h sample· and with a microwave puwcrofS inW (Fig; 3). These using the concentrations ofK20, U,1111d Th (Table I) signals: were .C:llLISed by_ AI centers, [AI0,.]0 (Me~ that were determine,!' by instrumentnl neutron activu~· MorriS:-1971), and by lithium compensated Ti centers; tion analysis.ofthe whole rock rhyolite from which the [TiO~/LI~J~ CI'oy0da,'.J992). quanzwasscpanucd (J.N. Gardner, unpubl. duta). The

· A60Co~:~ourccwns·Used to irmdiatesamples:uudose decrement of pray dose wu5'considefcd for the diam~ rnte of34' Gy/h: Signal growth wus fitted to a straight· etcr of quartz grains of I.S mm to be 46% for 23znt line· or:. 10· ·a.5atunuion. curve obtained by the least chain, SO% for 231U c:huin,1111d 52% for.coK using fig. squaicS.,mcthcxr:using a: fortmn prognun packllgc A·2 in Grlln (1989) (original datn by.Mcjdahl, 1979):. named.~SALS (Nilkngnwu and 0yMngi,l982). Total The error in the annual dose is cstimaled to be 6% by. accumulaleddosC:s.(D8) were determined by extrap- considering the errors. In the: concentrations of K. U, ointing the growth line/curve to the zero ordinate. The Md Th. Cosmic dose rat.c WllS cstiiiUlted to be 0.30

. error.inDI.~ cstlmutcd by considering the errors of. mGy/y for four samples of EJ Cajetc:pumjcc (ECOS, . the pmmeters of.: the line/curve 1111d the correlation EC06, EC07, and ECOB)~ which have been in outcrop, betwceri them.· · ··.. and to be 0.21 mOy/y for other samples which have· Tllblc·l:~· .. .

· Conoeitltldons or l'lldiOGcllvc element~ obclllned by ncuuon octlvatlon ..Wysla (J.N. Oardnct, unpubllihed daiu). Wlltcrc:onlent. 1111d clllcullled dille l'llln for the pi'CICilt wnplea

··;... . U (ppm) 111 cppin>. . KaO(._,). Walel' contcnt Cosml dose nuc (m0y/y) D (mCiy/y)

sOuth r.1oiln1Aln Rli~n.C a.l E1 Cljcta Pumloc ceca:> ' 5.4' . e1 CAJcte Pumlca <Oihorsl . 5.4 Danlahlp l_tock :"~. · 5.4

29.9 4.67 1.8 0.21 5.06 19.? 4.22 3.6 0.21 ' 3.87 19.7 4.22 3.6 0.30 3.96 19.1 4.00. 3.5 0.21 3.72

: . . ~

'•

Sllmplc· ;

SMROI ·

ECD2·

ECOS'

EC06'

EC07.

EC08.

BSRP

IIDn 1111 . , 11 obuu

~151 ·.e

al ' . .... ...

'0' -?;. 1/1 .. c 5 ·GI £. a: (/)· w·

: Fia.·" tilef'W aroun· · ltep II lhcnn

:·.·:.:;.: ... : .... ·. S. T11y,lllu 11 ul.l Juurnul ti}'Vult·utmluJf1 Ullll Gtuthrrmrll Hrsrurrll67 ( /1195) 29-IU

' ••• ~~ '!. '

·.·:~ been buried CSMROt; EC02. BSR04) using fig. A-4 in 7. Resull.!l and dlscu.ulon

.. , . . GrUn' ( 1989)' (original data by Aitken, 1985 and by

Prescott.llnd:Stephan; 1982) where the elevation is 2000 m. ~Rn und ~.n loss were us~umcd to be SO%. Radiooc:livc':discquillbrium between mu und l.\1~ was neglected. . ' .

Table2 · · · . .

7, I. ESR agts with no lrtat trtatmtnt

Signal enhancements or AI und Ti centers by urtifi· cial 'Y"'Y irrndintion ure shown in Fig. Su~ where totul

,.· ''· Tho n:sulll ofBSR dalln1 u•ln11hc •11111411 of AI amJ TI c:cnu:r~ and liable c:omponcnl orn a:n1er1 for Vlllln Khyollle

',

' ' .. ,.I.·, \ ""·'=·. ~. ·,.; : .. \ .

. ., ...... t two lmpurily c:en~ T104/U•J0, wt~en~a · ·

~·.;. :·"";·;>-•·

. ::. ~:~:~:··;,> j:" 3.96 ~ . "

.:::, 3.12:~.~ ... :·>· ~ ' . . . . ~ ·: . :: ·.

Samt•le' bcfon: he:ltln& arter helllln11

AI center ncen1er Tl center

Dn(Oy) A&e (Ita) Dti(O)') Ar:c (Ita) Du (Oy) Aile (kD)

SMROI' 1'700 +300 330 +60 2200 +:ZOO 440 +SO II It

-200 -so -170 -40 EC02 103 ' +58 52 +IS 3311 +17 117 ±'1 :m +43 73 +II

-47 -13 -16 -27 -7 ECOS 233 +37 !IIJ +10 109 +34 7R +10 I '.II +23 49 +6

.:.J:z -9 -32 -9 -16 -4 EC06 . ·163 +18 41 ±S 290 +Ill 73 ±6 m +34 S3 +9

_.16 ' -17 -24 -6 EC07 230. +41 SK +II ~~K +20 65 :!:6 17K +23 4S +6

.~36 -10 -19 -17 -4 llC08 Ill I +24 46 +7 297 +II 75 ±!I llK +26 57 +6

~22 -6 -7 -18 -s DSRP 127 +II 34 +4 170 :!:6 46 :!:3 no +24 59 +6

-10 -3 -19 -!I

11 Dnlllld apm nOt obllllncd bcCIIU!IC the IUibh: com('KIIIent WDI nol enlwnc:ed by 'Y ni,Y lmdlllllon. Ill minimum age would be 440 ka, which . Is obtained fiVm Tl centera whit no heattn:alment.

~ .. · ...... . ~,500 ·~..;.---~ ... § ' . \ . \ j • ...... I

' \

• AI-(SMIICII 3QO 'iii' o AI- II!CG:r! •"= • Tl 1*1111 (SioiiiCI) ~

!. \

•• I I

~~ . ~. 'o. ~

·. b 0 400 °

a: (/) w

Fla. 4 •. The: change. In 111!1111 lntcinsltla or AI and Tl centerA on acpwise heutna of' SMRO lud o( EC02. Roch lnrenslrlea d«reued around 200"C. HoWC\'er,·the lnlcnlll)' orTI a:ntci'A lhoWII D MlCond

l1Cp above lSO"C lndlcallna lhll Tl cenam hll~e two c:ocnponenc~: · lherinlllty llllbtc one lllld lcuatablc (tempcracun: acnslllve) one,

accumulated doses ( 0 8 ) were determined. 'fnble 2 summarizes the results of ESR dating. The ages obt11ined by AI centers are systematically younger than the ones by Ti centers. Such systematic differences, between ESR ages were also observed by Shimokawa and lmai (1985), No effect of hydrothennalactivily wns observed because there is no systematic change in : the ESR ages due to the sampling locnlities orEI Cnjete pumice (see Fig. I) .

7.2. Thtrmal stabilities of AI and Ti ctnttrs

Two aliquots of qu:utz grains from SMROl nnd EC02 were isochronally heated betwr:en·l 00 and 400"C with a durulion of 15 minutes to exnmine thermal sta· bility or the paramagnetic centers (Fig • .4), The signal intensities or AI and ·n centers were measured nrter each temperature step. . . .

Both signal il\lcnsitics or AI und Ti centers in SMROl dccrcnscdon hentingaround 200"C (Fig.4 ). '111e figure

I:

". ··.

-------------------~ 'q ·q

S. Toytldu tl u/.1 Jt•urnul 11{ Volrunt~lti/I,Y 11nd Crt11htr111ul Rt1rurrlr 67 ( 1005!29-10

also shows that the decrements of the signal intensities ofTi centers nrc less around 2SO"C than those below 200"C making a clear second step. This behavior indi· cates thut Ti centers have two components; u less stnhle Ctempenuuresensitivc) one and n stable one. The same tendency wns observed forTi centers in EC02 where the amount of decrement in the first step is small.

The cocfficienl'i of the signal decrease in the tern· peruture steps were calculated and plotted on an Arrhe· nius dingrum for AI centers and forTi centers of both less stable and stable components assuming second order decay kinetics (Toyoda und lkcya, 1991). The potential ·signal decrement in the geologic time scnlc: was estimnted by extrapolating the correlation to the ambient temperature of the samples. Considering the error in the cxtrupolation, it is possible thnt the signal intensities of AI center and of less stuble component of Ti center may hnve been decrensed considerubly, while no decrease must have occurred for stable component ofTi centers.

If purl of both AI and less stable Ti centers have decreased while the sumples hud been buried, the ESR uge oblllined for AI center would be less thun the one for Ti centers bcc:uusc Ti centers huvc 11 stuhlc com· poncnt which has never been affected. The systematic age difference observed between AI center und Ti ccn· ters might indicate such decrement of the centers.

The dc:cuy on hcnting of less stable component ofTi center appears to be corrcluted with the dc:c:uy of AI cc:nter us shown in Fig. 4, It may indicate that electronic holes lnlpped ut AI centers nrc released on hcuting to recombine· with electrons trapped ut Ti centers. The stable component of Ti center decuys ul higher tern· perature when electronic holes nrc unstubitized and relc:nsed from some other centers. In other words, nat· urul and artiOc:iul rndhuion may creutc two types of electron-hole pairs. One is Ti center (electron) und AI center (hole) and the other is Ti center (electron) nnd unknown hole center. The former pair is thermally less stable than the latter.

Using this logic, thermal stabilities of pnrumugnetic centers depend not only on their own churucteristics butnlso on that of other centers with which electrons/ holes nt the centers recombine. It implies that thermal stabilities or pol'Dmognctic centers in quartz ure differ· ent ror individual samples. For example, a part of Ti center Is more stublc thun AI centers in the present

sample, while th~ opposite ~labilities were observed in a case of granite {Toyoda and lkcyu. 1991 ).

7 . .3. ESR dating wit/a slablt compmumr of ri ctmtrs

TI1c above experimental results and discussions indi· cute thnt only the stuhle component ofTi cen1ers would be useful for ESR dntin$ ur the quartz in Valles Rhy­olite. The experimental procedures for ESR dating were repented for all s;~mples after they were hen ted nt 260"C to extract the stable component. The signals or the AI centerdisuppeured on hcnting for nil snmples, TI1c ESR intensities of Ti centers nrc plotted a.o; u function of artificial y ray dose (Fig. Su·c). Obtained Dn's and uges nrc shown in Table 2.

The intensity of Ti centers in SMROI shows no enhancement uftcr henting (Fig. Su) resulting in no Dn and age obtained. This would be because the intensity of stable Ti centers is suturutc:d in this dose runge. Indeed, after SMRO I wa.o; heAted nt 420°C for 1 S minutes to anneal ull ESR signuls,the stable component wus regenerated by y rny irrndiution bellow I kOy but wns saturated ubove I kOy. Pun of AI and Ti centers in this sample mny have: decreased while the sample had been buried, us implied by the difference between ages for AI and Ti centers before hencing. Although the amount of decoy in the pust cannot be estitnuted, the minimum estimated uge would be 440 ka for SMRO I by D11 ohtained forTi center before heuting. If there hnd been no decay for Ti centers in the pn;;t, the observed intensity should h11Ye been lnrger, tbereforc, we should hove obtained !urger value of Da.

The intensity of the stuble component ofTi ccmers was enhanced by 'Y roy irrndintion (Fig. Sb, e) for BSR04 nnd EC samples, resulting in D8's and uges as shown in Table 2. nu: stable component would huve never been affected by ambient temperature even in the honest days In summer. The age of BSR04 11sing the stable component is older than the one ob1.1incd by using total amount orTi centers. It is because the less sUiblc component, part or which may have decrensed while the snmple hud been buried, wus removed by heating the sample!!. On the other hattd~ the uges of EC samples obtnlned after hcnting nrc younger tlllln the ones before heating. The following experiment wus mnde to check the thermul behavior ofTi center!!. Arter an aliquot of EC02 was heated at 42CJ4C for 15 111inutes · to unnculnll ESR signllls, the snmple wus il'l'lldiuted by

• 5 ., D. o· !t "'cii'.

i;t:' •:§~ € .!'2.-

~ .!!!1 .5 a: f/J w

Fig.~. Tl (a) In Sl IICCUII\Utl•

'Y ruy u WCtll rr 260"C I slightly result i1 transfer· to Ti rc1 II prOCe!o pumice m11tion• ment aft llS prop' process

:.-, S. Ttl)'fJdd tl t~/.1 Jt1urMI11/ Vt,/Canlllfll/.11 anJ Gtothtntllll Rtuarth 67 ( l99.f) 2li-IO.

.. werco~~cd_in:;:; '199lk<<. ,::. ·.

.. j' . . .. ' ~ ' . ~ :'

-~ ;n, of ti; cent~~~;;·: .. •' ·~ ·· .. ..... <:·.·~ ·. : ... ::: .. ~: ... . 1 dlscussi~ indi~ · · . rn centerS would'~~ . :u in·.v~Jlcs;Rliy~: :, ESRdlWng Were::·. ; rc hc:IUCd at26a'.c .~. ,:, signals of.thc.At: .. ,;wnples;:Thc esR.· ·. ·:,::as a func:tion;o[ <btaincd.~Dit's'_on~·-

: ~, • r ~

~MRO(~ow(no · ; resulting i~ ric>:' DB~· :·.:ausc lhc'intensity>:. , lhii d05c ;nmsc>

··• 1 42tre· for: 15': .... ·''· . .,,.·.~ .. ' ·' i stable component·

3000.

~ 52000

~ -~ :!! $1000 .s a: en w

0

a Alcentr)r . oTI cantor · • Tl cent(ir'

·: . . . (alter n..allng)

.. ,.' ,. 1-<. . .. .,.

,. ·' ,. .. ' ,.. ,'. .~·

.. ·" . .. , .. ,. ,~ ·'· .•

-;rooo -1000 .0

(II

¢"

0A"~"t .. ·" . o"tf R'~

.. *' .o" o· ,.. .

.. .. .. 1000 2000

y ray dose (Gy) .

1500

-~ Gl ssoo a: (/l w

20001~--~----~------~~'~bl a AI center ,0 o Tl center ,./ • Tl center ,o

(alter heating) /

,cl,d,

o' r~'

o" ,/

o~~~--~~s~oo=---~,~o~

y ray doao (Gy)

a AI center ~/ o Tl center • T1 center /

(altor heating) k , .l ,¢

, d'" . .¢ ,. t·:p-"

/p" ., ,

·benow 1UcOY:bui~ :'At and~:ri centers . '·whilc;ihe siunpti '·iitrerenc:C \letWcen llins.AllhouSli.~.~. t be estiina~:thc i4okl'forSMR0l. ·c beatlnii . ..Jf:~crc·. s jn.'tbo.:pasC·;&hc. -~o o 500 1000

· :n tarier~'iherer~.: y ·ray dose \Gy) · • .IC _of l?a;:., _;_.~ '.;:;:;;;' · ,.,, $. The en~mcnt by anlftcilll .., my lmdilltlon of I!SR lntensltliis of AI cemen 111111 ofTi centm both before llftd after headns at 260"C ~cnt,o~Ti CCJ!~) In SMROI~ (b) In BSR04,1lftd (c) In EC08. The n:sraslon ll11e 11nd lllwatlon CUI'YCI an: obcalned by the lc:ut sq~ 1\alng. Tocal _ta · (fiJ,;5b,' for~mullllcd dOIICS .< 0.) 1111 Obtained IJy CllntpOitdng the line/ CUI'Ye CD the :WO ordllllle, ~nDa'a · · ·lOnent ray nt 200 Oy. The;signal intensities of Ti centers

· mcnsurCd · both.; bCfore nnd ·after re-heating at usinll:.me.!W'·~ for 15'minu~;'Thc intensity afrer henting was

· largerthtu\ bCforc heating. This experimentnl the-•c:~~ .... jndicntesJhat''an·.unknown _clccrronic process

:dccrcascat.'l!ftiric. · electrons from some unstable stntes on heating rcplacinat· Si to· forin Ti centers. If it is true,·such

~lrnt:!~c may. have occurred in the quwtt in El Cajete 1 "·~., .... wtiile it had bcien.buried; resulting in ovcresti-

,.yrlflri:mcrrn .of D8by the conventional method. Heat trent· after '-1 nly-irradilition prior to ESR mCASurcmcnt.

~~r ......... ·,. ·A in this paper, cnn·simulntc this electronic aS well BS remove the less Stllble component.

' .. . . . .

·,.J

. ,;..,.·:· ... ··· '•

8, Geologic lmpllc:atfnns

The workofOgoh ctal. { 1993) hnsshown that some ESR signllls nrc thermnlly ·unstnblc: thus great cnrc must be tnkc to insurcthatsumplesuscd forESRstudics have not been subjected to thermal t'r hydrothermal stress. Temperature.~ ns low ns 6tr'C for 10 kn'could effect ESR dnte5 on quartz as much as 30%, lis implied

. by the present study. . . . By cvllluntion of the thennal effects on qu&rtz sep­

arates, we h11ve dcrennincd ESR ages of 4S .to 73 len for the El Cajctc Member and 59 :t: 6 len for the Battle· ship Rock Member of the Valles Rhyolite. Considering

• s .. · .. ., :~

o·, .. · 0. '1.: .. . ..

• 4=1:

',•'

'., . ~

·!· '

. ''

'•

.... ',. • ~I ·•

I • .• ,•

311 .'i. TIIYfldll "' 111.1Jmmml11/ VIIIC'IIMII•JI.V 1111d Grmhm1111/ Hru11rth 611 /99SJ 2U-4tl

Rcnenu.and Gardner's 1~C age of >SO kn (unpubl. Battleship Rock Members arc closely associated inti me duta),1hc cruption·ugc of El Cujcte Member would be with the overlying Danco Bonito Member us originally · SO to 73ku (Fig. 2).1'hc weighted avcrugc of the five proposed by Builcy ct at. ( 1969) nnd Selfet ul. ( 1988). El Cujetc·samplc.~ yields an age of S3±6 ka where In addition, the Ballll'.ship Rock ignimbrite is younger ISOPLOTprogrnm of K.R. Ludwig is used ( D.A. Saw· und dilitinct from the VC·I Tuffs ItS ('lroposed by Goff ycr, 1994,:personal communication). These: ages nrc and Oardncr ( 1987) and the El Cujctc und Banco considcrubly younger thun most of the other ugc deter· Bonito Members arc both younger thun indicutcd by minutions made on this unit. By compnrison, the ESR thcmostrccentrcvisionsofSelfet ul. ( 1991 ), although age or the South Mountuln Rhyolite is >440 ku (Ti some observations in the tephra deposiL'I remain to be center before heating) which is rcusonably close to the clariticd. presently accepted usc of 520 ku (Spell and Harrison, 111c ngc or most recent volcanism in the JcmC'J. 1993). Mounrains is probably much younger than previously

Dccuuse of the Bunco Bonito Member undiputubly thought. If so, our results luwe implications for the overlies the El Cajctc Member, the Banco Bonito obsid· evolution of post• Valles caldera mugmas, for the geo· ian now is < 73 ku. his worth noting that this obsidian morphic evolution of the caldera mout zone, and for now has a very youthful appearing upper surface with the uge of faulting in this tcctonicnlly 11ctive region. well-defined pressure ridges up to 20 m deep. The ESR ages arc con!dstent with the present state of preservation (Fink and Munley, 1987). If the ages urc correct, it is Acknowledgcmenl'i very apparent that the recent 311Ar·40Ar ogcs of both the El Cnjete nnd Banco Bonito Member.~ presented by Self We would like to thank Dr. T. Nugntomo ( Nnn1 ct ul. ( I \'191) urc: too old and that their recent revisions University ofEducution) for 110Co ')'ray irrudiution. We to the post·0.5 Ma uniL~ of the caldcrn ure again In error. ulso express our grulitude to Dr. J.N. Gordner (Los These older ages arc so variable that they must be Alumos National Laborulory) for giving us the results cau.~d by ubundlint xenocrysts in the units derived ofiNAA for the chemical compositions of the members from older rocks beneath the culdcns (Spell und Har· of Valles Rhyolite. We also would like to thunk Dr. rison, 1993). D.A. Sawyer for reviewing our ESR ages and for pro·

Additional efforts arc still needed lo clarify lhe ESR vidlng the result or calculmion for uvcruging. The mun· uges by determination of the rndon loss rute as wella.o1 useript wus reviewed by Dr. A.F. Skinner (Williams by revealing lhe chnructcrislics of AI ond Ti cc:nlers in College) und by Dr. J.N. Ourdner. This research wu.t; quartz during heating and irradiation. Rndon loss ofO% supported in pun by Gmnt·in·Aid for Encouragement or 100% yields about 30% younger or older ages, of young scientists (No. 03!!S4034, 04!!54028) from respectively. the Ministry or Education and Culture and in part by

Inoue Foundation for Science. The second author was supported by the U.s. Department of Energy, Office of

9. Conc:lu!llon~ Basic Encr~y Sciences.

Qunrtz phenocryst-; in Quuternary tcphms and pos-Rderenceli sibly other types of volcanic deposits can be used to

obtain ESR dates. Sampling sites must be evaluated to Aitken, MJ., 19115. 'rtlCmKllurniner.ccll(c Owing, Academic Pn:n, eliminate effects of thermal and hydrothermal stress.

Ti centers in quartz have a thermally Jess stable com· London, 359 pp.

Aitken, MJ .. I992. OptiCAl dutlna. Qwu. Sci. Rev .. II: 127-131. poncnt nnd a stable component for at least three mem· AlbR~C:ht, A .. Herzoa. O.P., Klein, J., Dczfouly·t\rjo111411dy, D. and bcrs of the Valles Rh yolitc. The ESR signul of the stubh: Oofr, F., 1993. QullleiTUII)' erosion 1111Ll cotrnic·ray-apo•ure hlh• component ofTi centers yields un age of 53± 6 ka for lory derived frorn 1Goe and ,.,.1 produce In altu: an exAmple from

l'lljDrito platuu, Valle~ culdera re;lon. OeoloJY, 21: SSt- S!iol: the El Cujete Member (Its •~c age indicuteli <SO kn.) Bailey, R.A., Smith, R.I.. lind Rou, c.s.. 1969. Stnularaph~e and nn age of 59± 6 ka for the Bnllleship Rock Mem- nomemclature or volcanic rock• In the Jemez Mounuun5, New bcr. If these dates nrc: correct, then the El Cnjcte and Medco, U.S. Oeol. Sun-., null. 1274·P, 19 I'll•

.. ~-,!:)

'7 0 !fe)te '10, • AI Dbell. :1ru Qllll

CH Fink,.

(!:I

21 Ollllln

Ill N1

O~~rLln

llll tie

Oel~~~

Ql hu tlz

oorr, 1

D.• oorr,t

N( of' 99

Ooff,l It, fro ho 17

Ooff.l Vo Vr. Je1 46

a orr. WI IIIII ten Ill

Orifllll on1 17:

Ortln, · l:t

Ortln,l ILAo Nu

Bulen. the Coo ph~

Ikeda, I'll: 71.

5 ., f)

S. T11y.Jdu ~~ ul.l JtJUrfUJI 11[ V11/tu""'"IIY 11rul Gtotht-rlfllll RrJtllrt:h 67 ( 1995) 29-IO 39 0 :1 ely associalcd in 1isnc

..tcmbcras onK\nAl\y

.nd Selfct"at: c 1988). gnimbrilc'is younger as proposed by~Ooff

··I Cajcac and 'Banco . er than incticalcd by . al. ( 199]) ~ ahhough leposils remain to be

anism in the :Jemcl. :nger than previously imolic:Ations: for: the

:nagma5.;for thegca· , mont zone, and for :ally ncdve rcg~ori.

T. Nagatomo. (Nora , oyray irradiation: We • l.N. Ovdncr (Los ; giving us the teStllts ;itions or the members uld l!ke 10 ·thank Dr. ~R ngcs .IUld for pro­:~averagins. Thcmnn· :: Skinner (Williams . ~~.This rescarCh.WDS ,d for &couragemcnt >34, 048.54028) .from ~ulturc and in·'pan by he seeond author wBS

1tofEncrgy. Office or

lktj:er,O.W .. 1988. Dllling Qw11emaryevencs by luminescence. In: OJ.Euu:rbrook (Edllor),DDiinJ:QIIlllemaryEvenii.Geoi.Soc. Am., Spec. PAp .. 227; 13-.SO.

Doell, R.l'., Dalrymple, 0.8., Smith, RJ.. 1111d Ualley, tv,., 19611, l'llleol'!lllllnetism, poiiiSSIUm•III'JlDn llges, 1111d geology of myollles 1111d ll.nccllued rocu olrhe Valles COJ!dm, New Me,lco. Mem. Geol. Soc. Am., 116: 211-248 .

Fink, J.H.1111d Manley, C.R .. 1987. Origin or pumlceouund glwy rexllln:sln rhyolire llows and domes. Cleo I. Soc:. Am., Spec. PAp .• 212:71-88.

Oudner, J.N. and House, L., 19117. Seismic hlll.llnls invesriaarlons at I.e» Alalll()f. Nllllonal Labonuory, 1984 to 1985, Los Allunos NAil. LAb. Rept.,I.J .. II072·MS: 76 w/maps.

Gardner, J.N., Goff, P., GarciA, 5, 1111d Haaon. R.C., 1986, St111ti· er;phrC n:llul0t11 and lithologic varilllions In lhe Jemez volcanic !kid, New Mc:xlco.J, Oeophys. Res., 91: 1763-17711,

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