Geological Topics Seminar
Geological Topics Seminar GEOL 5700
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GEOL5700 Class notes Concordia for U Pb One of the things we banter about quite a bit in tectonics is the age of different rock units The numerical ages are all derived from radiometric ages a topic which you have hopefully been exposed to at some point in your education These notes will brie y cover some of the highlights of uranium lead concordia as U Pb dates appear in both sedimentary provenance estimation and hi gh precision dating of igneous rocks Also the U Pb concordia diagrams are a bit different than the more straighforward but practically troublesome systems such as potassium argon K Ar Let us begin with the direct relation between the number of atoms of the 206 isotope of lead 206Pb in some volume of rock at a time t after the system has been closed to loss or gain of lead or uranium 206Pb 206Pbi238UeM 1 1 where the initial amount of 206Pb is denoted mei You can see this last term most easily from 238U 238Ui 8 and that Pb 2 238UI 238U and substituting away Now this equation is useless in practice as counting the absolute numbers of atoms is impractical However the ratios of atoms is practical so we divide by the number of atoms of a non radiogenic isotope of lead 204Pb to get 206 Pb 206 Pb 238 U M 204Pb 204Pb i 204Pble 1 2 We can solve for the time since the system was closed to get zost 206 Pb 1 204 Pb 204 Pb t iln 238U 3 204 Pb The same set of equations can be derived for me and 235U We may rewrite this as 206 Pb 206 Pb 206 Pb A m Pb m Pb i 238U e T 4 204 Pb Or once again in terms of the age of the mineral 1 206 Pb t Elnw 1 5 Now if a mineral has been a closed system we should get the same age from the 206Pb mU system as the 2U713 b 235U system If we designate the decay constants for these as AQOGand A207 respectively we can set the equivalent versions of 5 equal to one another GEOL5700 Uin dating handout 1 206Pb 1 207Pb tQ ln 23 8U 1 AQ ln 23 5U 1 06 07 M 206 Pb 207Pb Mm 1 p2 TU 1 Armed with the values of the decay constants Am 2 155125 10 10 yr 1 and km 2 98485 10 10 yr l we can plot the two ratios of lead and uranium isotopes against one another The curve so plotted represents the locus of possible measurements where the dates from the two radiogenic systems are identical or concordant Hence this curve is called the concordia shown below with points marked in billions of years of age GEOL5700 Uin dating handout GEOL5700 Uin dating handout p 4 This discussion of one particular isotopic system should have indicated some of the pitfalls and advantages of these techniques in general Questions that should be in your mind in evaluating a radiometric age are what if any internal checks exist for loss or gain of isotopes important to the age date What are the conditions under which the system is likely to be open in this manner What assumptions that are geologically questionable have been made in making this age determination and can we evaluate the possibility of these assumptions being violated Similar questions can be asked of other geochronometers as we shall see later in the course GEOL5690 Class notes Paleomagnetism Paleomagnetism is one of the most powerful physical observations of ancient rocks as it is capable of revealing paleolatitude independent of climatic inference rotation of large regions and can provide resolve stratigraphic ties to under 10000 years under the right conditions The idea is that the rocks record the Earth s magnetic field faithfully that field tells of the position of the magnetic pole and the polarity of the Earth s field The problem is sometimes it doesn t work out that way To use this tool correctly you need to know how to figure out when it works and when it does not Reference for this is M W McElhinny Palaeomagnetism and plate tectonics Cambridge Univ Press 1973 or the successor text M W McElhinny and P L McFadden Paleomagnetism Continents and Oceans Academic Press 2000 The Earth s Magnetic Field The basic field of the Earth is that of a dipole thus the strength of the Earth s field is a 313sin2t12 1 where A is the latitude M is the Earth s dipole moment and a is the radius of the Earth The direction of the field is towards the north magnetic pole so the declination of the dipole field is 0 and the inclination of the field I is I tan 12tan2t 2 So far as we know this part of the Earth s field remains pretty sensible over geologic time other than the amplitude and the polarity There are higher harmonics of the modern field and they change with time there is some debate over whether a couple of those terms exist at very long million year time scales These somewhat smaller changes are termed secular variation and the changes in inclination and declination they produce are observed by magnetic measurements over the past few centuries Thus an instantaneous measurement of the Earth s magnetic field will not point to geographic north but one averaged over thousands of years will When using a paleomagnetic direction for estimating paleolatitude or rotations it is necessary that the measurement average enough time that equation 2 holds true While at this point we note the equations governing the position of the apparent pole at latitude N and longitude If from a measurement of the paleomagnetic field declination D and inclination I made at a latitude A and longitude I sinA sin Acosp cosAsinpcosD Bwhen cosp 2 sin Asinn or 180 3 when cosp lt sinAsinA 3 where sin 3 sinpsinD cosit and tan 2cotp where 0 5 p 5180 Note that the latitude of the pole must be between 90 and 90 and that the paleocolatitude is p compare the last equation with eqn 2 GEOL5690 l 39 notes p 2 Magnetic Minerals Almost every mineral has some response to a magnetic field but for nearly all that response is insignificant The few that do have a significant response are the critical minerals for paleomagnetism metamorphic and chemical sometimes goes to hematite above 250 750 C 0ltx50 14 room temperature dehydrates 250 C to FS4 Of these minerals magnetite titanomagnetite and hematite are the main players for paleomagnetism The other minerals are either too uncommon or re ect secondary events such as weathering that are not of interest The Curie point is the temperature above which the mineral will not have a magnetic response of any kind below that temperature the mineral can produce both an induced response and a remanent response An induced response is one that exists only when a magnetic field is applied and goes away once the field is removed as in a shielded lab The remanent response remains when the applied field is removed and it reflects a magentization acquired sometime in the geologic past Why the two responses Well the simplest answer is that it takes some amount of energy for an individual mineral grain to change the direction of its magnetization For very large grains there are domains of uniform magnitization within the mineral that can have differing magnetizations Moving the boundaries domain walls within these multidomain grains is relatively easy to do so the grains will tend to return to a neutral nonmagnetic state in the absence of an external field with time scales of minutes to hundreds of years Smaller grains can be too small to have two domains these single domain crystals are much harder to change as the entire domain has to change direction at once These will tend to hold a direction a lot longer and are often the source of the remanent response we are interested in If these crystals get too small they no longer have much of an energy barrier to changing direction and their behavior is GEOL5690 l 39 notes p 3 superparamagnetic which once again tends to respond to the existing field In practice it is found that grains larger than single domain grains behave much like single domain grains rather than the weak behavior expected of multidomain crystals These pseudo single domain crystals probably carry most of the paleomagnetization from magnetite in igneous rocks There is yet another wrinkle the length of time the crystal can hold on to a direction depends on the temperature This is similar to diffusion of atoms in crystal structures that is related to the closing temperature of minerals for radiometric age dating One way to think of this is quite similar to radioactivity after a time 06937 at a temperature T without an applied magnetic field half of the crystals will have gone to a random state and the magnetic moment of the rock will have dropped by half Expressed mathematically for single domain grains Mt Moe 1 1 g 4 Te kT C where k is Boltzman s constant C is a frequency factor of about 1010 s l v is the volume of the grain and K is an anisotropy constant for a grain K 21ch which is the saturation magnetization J the maximum magnetization the grain gets and H5 the coercivity which is the magnetic force that has to be applied to get the grain to Jamp The relation of this time to the coercivity is used in one style of demagnetization of rocks alternating field demagnetization These relations reveal a lot about how paleomagnetism works For a grain of specific volume we might define a blocking temperature TB where 39c 100s or so this represents a temperature below which the grain s magnetization is essentially frozen in VK klnC100s 5 TB Note that smaller grains will have a lower blocking temperature also note that this temperature is frequently well below the Curie temperature A second relation is that the effect of temperature over time can be mimicked at higher temperatures over a longer time If we rearrange 3 to put the constants on one side we find that T1 In C11 T2 lnCIz 6 This means that the same effect on remanence is obtained by staying at temperature T1 for time 71 or temperature T2 for time 72 Using a typical value of C of 1010 s l being at 150 C for 106 yr is the same as being at 450 C for about 5000 seconds note that T is in K This relation is critical to thermal demagnetization or rock samples Acquisition of remanence The overall magnetization a rock sample has is termed the NRM or Natural Remanent Magnetization The NRM usually combines several sources of magnetization found in rocks thermal remanent magnetization TRM acquired as a rock cools chemical remanent magnetization CRM acquired as a rock s minerology changes GEOL5690 l 39 notes p 4 detrital remanent magnetization DRM acquired as sedimentary rocks form induced remanent magnetization IRM usually caused by flow of large currents from lightening strikes All but IRM are potential signals to be studied Thermal Remanent Magnetization TRM This is acquired as a rock cools down Mineral grains rapidly fix their magnetic directions as the temperature falls through the blocking temperature for those grains When cooled rapidly the whole rock will record the magnetic field nearly instantaneously Thus surface volcanic rocks which cool in days will accurately record the secular field and not the average dipole field In order to recover the dipole field enough different flows covering a long enough time span gt103 104 yr must be sampled In contrast plutonic rocks and metamorphic rocks might cool so slowly that they span several reversals in the Earth s magnetic field while these will average secular variation pretty well they may also average changes in the magnetic direction of geologic interest A flavor of the TRM termed a partial TRM or pTRM is the acquisition of a magnetization in the low blocking temperature grains either by a rock being at a low temperature for a long time as happens as a rock is slowly unroofed Frequently low temperature magnetizations are called overprints and the lowest temperature one is often aligned parallel to the modern day field Detrital Remanent Magnetization DRM This is a magnetization acquired as minerals float down out of suspension in a uid water generally and then lithify into sediment The grains have a magnetic field acquired prior to becoming sediment Thus as they float in water they are subjected to torques from the Earth s field that tend to align the grains with the Earth s field as they float in water When the grains land on the base of the fluid their magnetic moments tend to point along the field some further readjustment of the grain orientation can occur until the sediment is lithified enough to prevent grains from rotating As the sediment compacts it is possible that the grains will tend to rotate towards the horizontal if the magnetic grains are not equant this would tend to decrease the inclination of the remanent magnetic field and is as we shall see a serious concern When deposition of sediments is very rapid gt1 cm 1000 yrs it is possible to preserve a record of the secular variation of the Earth s field Usually secular variation is easily averaged in sediments by sampling a small thickness of sediments Chemical Remanent Magnetization CRM Chemical changes in magnetic minerals produces CRM most commonly weathering of rocks will produce the hydrated minerals like goethite and can oxidize magnetite to hematite Hydrothermal alteration and metamorphism can also produce these changes The timing of acquisition of this remanence is more problematic than for TRM not only might it be acquired over a long time or uncertain age but the minerals produced might at any given time span a large range of sizes and coercivites making it difficult to isolate a CRM from a previous TRM or DRM Induced Remanent Magnetimtion IRM Electrical currents generate magnetic fields intense fields are produced naturally by lightening strikes which produces currents that travel mainly along the ground s surface The directions acquired have to do with the peculiarities of the lightning s current often an IRM in a rock can be recognized by having a much higher moment than any other magnetization as well as being quite different than other samples directions IRMs cannot be removed by thermal cleaning but are ameniable to AF cleaning since they represent a resetting of the lower coercivity grains in the rock see below Because the their occurance this is most a problem for GEOL569U Pa m n n m n V p 5 exposed for qulte awhrle m a cllmate conduclve to eleomoal slorrns Measuring the Hemanenl Direction and Lhelr reasons for usrng themln exammmg Lhe magneuzauon ofrocks Key to ths ls ml v1 ba u up rlme theory of magneuc behayror of sngle domaln grarns surnrnanzed aboye solvean 391 remove ohernroally generated phases has been used but rarely works so wlll be rgnored below We shall nrsl surnrnanze Lh baso concepts for Lhermal emagneuzauon Lhen see how m pmcuce they are executed and dlscuss A dernag allesser leng Thermal Demag netization As we saw aboye spendrng an houror so alone dernagneuzauon and apply an allernaung fleld ofpmgresslvely hrgher magnltude sa 250 ll allygn erh the magneuc fleld berng applled m Lhermal demagneuzauon Lhere lsno fleld b w Ml d ml only laln lnl nl l Mn Thu a a l h randhl h r lernperamres all Lhatls len are the hrgherand hrgher bloolnng temperature grarns For a rock wth a ths rsreyersrng the process by whlch ltacqulred lts magneuz u n or rnslanoe conslder a rook where Lhere rsa hlghrEmpemmre componentacqulred pnmary r componentacqulred over a long urne as the rock salbuned bverprlntquot gray llne below Na Overprlrll ml We39dllke r r p oooled As we progressryely heaLLhe rock the rnagnrmde of the overpnntdecreases GEOL569EI w m nttmnt p6 I 200cc Overprinl If we wxll Some t the man of that dnectton In thts case the duecuon would be biased somewhat towards the noth A masurements but would have chosen a specmc step my the 400 C and averaged the mmr at L MOSL 19 Lumpon nt vow on up Of course the magiaJc Vectors are Lhraa dimensionaL so gures In paleomagiaJc papers show both honzontal and vemcal components someumes mlled zndemeld or Orthographc ddagrams CEOLSESEI peleumeguetlsm notes p 7 sa lam N t tb ttb lwl Lheopen updown axes Thus the oyerprlrltpolrlts rlonll and down steeply wlllle tlle prlmary oomporlerltpolrlts rlonlleastarlol down gently Tllls can also be plotted on a stereorlet 39 39 lmllar ooerelylty equ 4 In tllls oase ltls the lower ooerelylty grams Lhatfall by the myslde A magn tlo eld ls applleol to the rock wltll a la lol Change lrl 3012er e g H slrltrllre y Math 6 p l a where lu gt apperls ls tllattlle applloatlorl of the fleld oauses grams wltll lower COElelty Lhe gram freezes ln wth the dlrecuon of Lhe fleld at LhatlnstanL Slnce small opposlte polarlty of the sllgh y larger grams ata tlrrle wlulater tllls wlll cancel the momentofeach set Tllls oorlurlues on to the lowest ooerolylty gralrls Thus only grams wltll Because magueute grams have a lower ooerolylty tllarl llematlte tllls usually works better on rmgnemle bearlng rocks lt ls m pracuce dlfflcult to clean hemamgrbearmg looks wltllout oauslrlg other problems by 5mg AF demagneuzauon gramsl 1 E GEDLESQU Palemagnelm nmzs p z demagneuzanun se semetrmes a eemhrnatren er thermal andAF are used ta remave hydmus mrnerals predueedhy weathering and then rermve the xemammg luwrcuemlvny gmns leferemes hetween AF and thermal dermgneuzaunn behavmrs are lmkediu dmerent magneun mmemlugles that mlght he present frequently the eharee ls hased en tnal and enuranalysrs Dfsnms prlet samples Onee dlreetrens are aedurre Bra numbernfsamples theyare averaged usmg spheneal staushes The ularuneervarnty nfthe mean ls ex ressed as d n ean he eenverued td a pele pesrhan The paleedlreetrens ean he eenverued ta paleepele usmg 3 The angular uneervarnty e that pele ls termed AV ln erder ta ealeulate the paleepele anydefuxmauun nfthe mcks has ta he remaved Usually thrs ls as srmple as m mcks er plutanre melts Lhmgs ean he pretty eernplex and petentrally a seuree er ermr Field Tests ufMayn sizatiun Many umes the theery desenhed aheve umrks very well thrs ls eennrmed uhen fur rnstanee measurements are made mm lil erent hthnlngles Dfmcks mm lil erent pans Ufa teetanre plate hut represenung asmgle ume andthe r erred apparent pele ls rdentreal There are tests that ean he made umhrnan rndlvrdual srte and hetweena numbeer srtes The elassre tests fur stahrhty Dfpalenmagneuc slgml are 1 Fold test lrthe dlreehans fmma srte er lucalltysamplmg a smgle unrteluster slgmfmandy hetteramerunreldlng the dlreetrens than befnre the rmgneuzaunn ls eensrdered ta predate reldl A wnnkle rntmdueedrn reeent years ls a pamal unfnldmg w cwucgu hkamagnzimn mes 2 Revemlleg Many umes a sedimen39ary sequence will span qne qr mare we icnqw qrumes when the eld pqrnted the Ems furlung pennds qr ume The mere rsqmewhat pqstdepqsmqnai magnetic drrectrqns The mean drrecuqn qr the reversed q samples shquid he anupqdai ta that qr the nqrmai Empires irtheyare nqt it means there is pmhahiy andtherdrrecuqn that ms nqt fully remqved qurnstance m qur gures aisu he a little tn the nqrvh rimary determined fUNemoved Overprim Aciual Normal Primary Em Reversed P determined Unremoved Overprmt i39 A mild cqmiiary qr the reveml ESL IS qne qr nun suemy hetween nearby sedimentary sequences H mm qrmqre sequences cqntarn rdenucai sequences qr revered and nqrmai p stra lanues r e the same magnets ugmphy rt rsrrequenuy Laken ta mean that the rred very nearthe ume qr depusmun qr the mcis Actu Reversed Primary Thisisuf usedtqshqw tmagneuzauqnsmeasuredeisewherern qns ten the secuqn were nqttqtauy qverpnnted hy iatermagneuzau E2 5 qusgu Paleamagnznnn mus A Baked contact tea The heat Bf ah mtruswe hhdy dmps h mpldlymiu the wallka Ifgmu measure the magnauzaunn m aseetmh fmmthe mtrusrhh hut mth the eh trymek Wu ear see u the magheuzatmhs predahe hrphstdahe the mtrusrhh quotthe magheuzatmhs predahe the mtrusrhh Wu unu see a Lmnsmunfmm magneuzaunns the ehurrtry mek farther amy Frequently these hess are assuerahed unthfaxrly well de ned stausueal measures Bf sueeess pamculariy the chi Est Ennis anunmbigmies If a stahle pnmary magheuzatmh Is rhuhd n umuld seem that mherpretatmh umuld he pretty strarghtruruard fmmequauuns early 011th ddut There are sume my th gehemhe spunhus measurements m the 1ah e g havmg a eld unthmthe hetrz huh area Curing me ththe Ems hhehtatmh we unu nut umn39y IUD ahhve ould ehrreeuy rdehtuy these p 1ems genlngm aspeets Bf mherp Elf d tha pass h ye drahely vamus these are men the eruxurdrspuhes As these are eehtral th many genlugm argumen39s mhst vamusly the drspuhe dyer BajaanushCDlumbxa quot we heed th he auare thhs phsahle eauses nftmuble Paleohoramtal Itmay seem ample hut hhhunrrg pdehhhnzuhtal ear he Lmublesume Furmstahee Emplmg pluthme meta usually requrres SDmB mrerehee Bf m m that pluthme meks were hnermed as they are th s has heeh challenged m several eases xhshme rare eases strau edmeks ear a1sh have an uheertamty m palehhhnzuhtal Lhs rshmmed th meks that mrght have a hurrzem dephsrtmhal dlp e g lam aws If Wu have a palenhnnzun39al and a gDDd measure unhehhatmh theh Wu shhdd have a mhust esumahe Dfpaleulamude Hemisphere ambiguity vhu may hhtkrmur urhrehhemrsphere 3mm rueks eame mm m mm mstahees Wu dhh39t knuw whmh my 15 Wu earmht rule hut18u Bf In these eases GEOL5690 l 39 notes p 11 20 N with a normal polarity would be identical to those from 20 S with a reversed polarity if the rock was either upside down or rotated 180 about a vertical axis This problem most frequently arises within tectonically dismembered strata melanges Flattening As we noted above as sediments compact they thin and the magnetic minerals present within the sediments can be systematically rotated towards horizontal This will make the paleomagnetic inclination too low shallow This is presumably more of a factor within facies that compact more e g shales recall the backstripping analyses from a few weeks ago Sometimes people test for this by measuring the magnetic anisotropy of the sedimentary rock and there have been attempts to directly correct for flattening with burial depth of a sediment Some workers will not use sediments for paleolatitude studies because of this Note that a rock unit with flattening will pass fold reversal conglomerate and consistency tests Deformation of the rock Perhaps the most ignored potential problem is if the rock has been deformed This is somewhat similar to flattening Finite strain of a rock causes the minerals to rotate differently than the lithologic boundaries used to estimate paleohorizontal Perhaps the most extreme example is simple shear parallel to bedding Small grains within the rock will rotate at a rate proportional to the amount of shear Alpine geologists have been exploring these effects for some time but identifying and correcting these effects has been less of a front bumer issue for Cordilleran geologists
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