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Lava Geochronology (Focusing on the Shorter Timescales)

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Lava Geochronology (Focusing on the Shorter Timescales) Week 4 – Lava Geochronology (focusing on the shorter timescales) Ultra high resolution methods for submarine eruption verification and within-eruption emplacement histories (weeks to months) Methods for determining eruption frequency, eruption sequences and repose history over multiple eruption cycles (decades to millennia) Observations of Deep Submarine eruptions Eruption Detection . Direct observation . Visual observation soon after the fact (happenstance) . seismic monitoring . T-phase = sound waves . Instruments stuck in a lava flow . water column signatures (particles, heat, gasses) Event Verification . how do I know if lava or tephra were emplaced on the seafloor?: take a picture, or collect a sample and 210Po date it Microbial mats and charred tubeworms at 9° 50’N in Apr-May 1991 new and old lava on the N. AlvinGorda Photos Ridge, WHOI GG 711, Fall 2011, Lect. 4 1 Eruption Age . 210Po- 210Pb dating (weeks to yrs resolution) . 210Pb- 226Ra dating (multiple yrs to decades) . 226Ra-230Th dating (multiple centuries resolution) . paleomagnetic intensity (decades to centuries resolution) Preparing a sample for 210Po- 210Pb analysis in the SOEST Isotope lab The list of known historical eruptions with (a) high resolution date and (b) identified lavas on the deep sea floor is small (most examples are in the table below) Event detection dating resolution Some references JdF Cleft Mounds 1985 Megaplumes Differential seabeam ± 2 years* Chadwick & Embley, 1991 17°S EPR 1990 serendipity Multiple (3He/Heat) ± 2 years* Auzende et al., 1996; Sinton et al., 2002 9°50’N EPR 1991-92 serendipity 210Po ± 2 mo. Haymon et al., 1993; Rubin et al., 1994 JdF CoAxial 1993 seismic seismic ± week* Dziak et al., 1995; Embley et al., 2000 N. Gorda 1996 seismic 210Po/seismic ± 2 mo. Chadwick et al., 1998; Rubin et al., 1998 Loihi Smt 1996 seismic 210Po ± 2 mo¶ Loihi Science Team, 1997 Axial Smt 1998 seismic Seismic, records from Days Chadwick et al., 1999; Dziak et al., 1999 stuck instruments 10°45’N EPR 2003 Serendipity 210Po ± 1 mo. McClain et al., 2004; van der Zander et al., 2004 NW Rota SMT 2004-on serendipity 210Po, direct observation ± 1 mo. Embley et al., 2005 9°50’N EPR 2005-06 serendipity 210Po, seismic (OBS) ± 2 mo. Tolstoy et al., 2006; Soule et al., 2007 210 NELSC 2008-9 Megaplumes Po, H2-heat tbd Rubin et al., in prep, Baker et al., in prep W. Mata 2008-09 Megaplumes Direct observation, Days to Resing et al., in review 210 Po, H2-heat weeks Axial Smt 2011 serendipity 210Po, seismic (both tbd stay tuned underway) * fresh lava observed but no definite link to the event is not established ¶ seismicity occurred 3 to 4 months after lavas were erupted GG 711, Fall 2011, Lect. 4 2 The first time someone made unexpected visual observation of a deep submarine eruption soon after it happened: 9 50N EPR in April 1991 Rachel Haymon, Chief scientist Soon after the 1991-2 N-EPR “tubeworm BBQ” eruption Another time someone made unexpected visual observation of a deep submarine eruption soon after it happened: 10° 45’N, the FIELD expedition (Nov 2003) Janet Voight, Chief scientist Bacterial “Snowblower” vents Lots of diffuse flow venting Bacterial mat on glassy flow surface GG 711, Fall 2011, Lect. 4 3 10° 45’N, 210Po dated to 2003 9° 50’N, 210Po dated to 1991-92; second eruption dated to 2005-6 Axial Co- Axial GG 711, Fall 2011, Lect. 4 4 1996 Gorda and 1993 CoAxial Seismicity Compared (same website source) The general nature of the seismicity appears very similar to that observed from the CoAxial dike injection and eruption of June/July 1993. A rapid series of small earthquakes is observed without a large "foreshock". The following histogram displays the number of recorded events per hour. This level of activity is comparable to the level recorded from CoAxial segment in 1993. The apparent decline in activity midday on Julian Day 62 until late on Julian day 65 is likely due to the loss of the closest array. Data are grouped by days. Loihi 1996 epicenters GG 711, Fall 2011, Lect. 4 5 Loihi 1996 General comments Dating Techniques What is the timescale for tracer change ? How is clock set ? Do all components "close" simultaneously? How big is the initial signature ? How accurate is analysis procedure ? The system MUST remain closed. GG 711, Fall 2011, Lect. 4 6 General comments Resolution for a chronometer is defined as T/T. The more precisely we can determine time increments the higher the resolution Memory Resolution Weeks Typical See a doctor days hours Great parts of hours Last year a month ago yesterday this morning General comments Resolution of 230Th-234U-238U radiometric dating of corals using high precision mass spectrometry Different application but the shape of the error envelope is the same t½ is 75ka. Notice that best ages are within ca 0.1x to 10x of the half life. Resolution is NOT the same thing as error. Resolution depends on measurement precision but absolute errors can shift age dates and their resolution bands up or down GG 711, Fall 2011, Lect. 4 7 Actinide decay chains 238U decaying to 206Pb There are 3 naturally occurring actinide isotope half life = 4.46 x 109 yrs radioactive decay chains that we exploit in Geochronology 235U decaying to 207Pb half life = 0.7 x 109 yrs 232Th decaying to 208Pb half life = 14.1 x 109 yrs Age of Earth = 4.55 x 109 yrs Systematics 238U → → → 234U → 230Th → 226Ra → 222Rn → → → 210Pb → → 210Po t½: 75200 yr 1600 yr 3.8 day 22 yrs 138.4 days All are incompatible in silicate minerals found in basalts and ultra mafics Rn is a noble gas Pb is moderately chalcophile Po is volatile above ca 100 C U and Ra aqueous solubility >> Th - 238 2 t - 2 t The U series A 2 = A 2 e + A 1(1-e ) Excess o water soluble Not water b cd soluble Excess o Gas 2 I 2 (A /A )-1 /A (A a Equilibrium volatile 0 100 10000 1000000 Time Since Fractionation (years) a ( 210 Po/ 210 Pb) b ( 210 Pb/ 226 Ra) cd( 226 Ra/ 230 Th) ( 230 Th/ 238 U) GG 711, Fall 2011, Lect. 4 8 6 5 4 3 2 Log years 1 0 ( 230 Th/ 238 U) ( 226 Ra/ 230 Th) ( 228 Ra/ 232 Th) -1 ( 231 Pa/ 235 U) ( 210 Pb/ 226 Ra) ( 210 Po/ 210 Pb) Tracer Surface Flow Chronology First Cro-Magnons Helen abducted by Trojans Ice Man takes his last alpine hike Atilla the Hun invades Rome ridge axis Pele arrives at Kilauea sea floor Spanish Inquisition begins Kamehameha unifies Hawai'i Millard Filmore elected president Radium discovered by the Curies Elvis joins the army Disco Obama elected president Mantle & Crustal Processes mantle 100 years ? -X 0 X 10,000 years ? distance a ( 210 Po/ 210 Pb) b ( 210 Pb/ 226 Ra) 226 230 230 238 cd( Ra/ Th) ( Th/ U) GG 711, Fall 2011, Lect. 4 9 210 Po-210 Bi- 210 Pb Systematics 210Po-210Pb dating t ½ =138.4 days clock set on eruption ( 210 Po/ 210 Pb) > 100 (T = T ) 210 210 sf ( Bi/ Pb) ~ 20 analysis: counting 210 Po, 210 Bi ( 210 Po/ 210 Pb) ~ 0 210 Pb 210Pb Direct Degassing Behavior of Metals During Volcanism Subaerial Shallow Submarine Deep Submarine (extensive literature) (limited literature) (only Po data) Po ~100% degassed ~100% degassed ~100% degassed Pb ~1-2% degassed ~1-2% degassed ? Bi ~20% degassed ? ? Macdonald Seamount – Rubin and Macdougall, 1989 Single Sample Evolution 1.8 1.6 210 = 210 Po f Pb 1.4 1.2 210 Po Activity 1.0 (dpm/g) 0.8 clock set on eruption 0.6 0.4 0.2 210 = 2 error Po i 0 0 200 400 600 800 1000 Days Since Eruption (October 22, 1987) Multiple analyses of 210Po with time in a single sample GG 711, Fall 2011, Lect. 4 10 1.2 0.14 Time Since Zero-point: secular equilibrium 210 1.0 0.12 ( Pb) 5 4 1.9 yrs 0.1 0.8 3 1.5 yrs "zero-point" of 0.08 2 1.1 yrs ingrowth curve 0.6 210 ( Po) 0.06 0.8 yrs 0.4 1 0.04 210 210 ( Po/ Pb) 0.4 yrs 0.2 0.02 ingrowth t sample collection date 1/2 0 0 half lives 0 500 1000 0 500 1000 Time (days) Time Since Eruption: 1.00 5 690 days 1.9 yrs 4 3 552 days 1.5 yrs 0.75 2 414 days 1.1 yrs ( 210 Po/ 210 Pb) 0.50 276 days 0.8 yrs 1 0.25 138 days 0.4 yrs 0 days 0 GG 711, Fall 2011, Lect. 4 11 eruption window collection date ( 210 Po) 100 % 75 % degassing degassing maximum age error in maximum age Method details • Po degasses upon eruption • “grows in” to grandparent 210Pb • time-series 210Po analyses in a lava • regression to ingrowth curve • “maximum” age is intercept • Conservative minimum age is lower of sample collection date or 210Po = 0.25 * 210Pb Sources of Error 0.15 a. Extent of Intial Degassing b. Regression Intercept Projection 0.10 asy mp t ot e: 210 21 0 ( Po t)=( Pb) 0.05 210 ( Po) dpm/g maximum age (100% Po d egassin g ) maximum age mi nim um age minimum age (75 % Po de gass ing ) 0 0.15 c. Time between eruption and d. Chemical Analysis Error 1st Po analysis resolution loss 0.10 0.05 210 ( dpm/g Po) max imum age maximum age minimum age m i nimum age -250 0500100005001000 Time (days) GG 711, Fall 2011, Lect.
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