Magmatic recharge in continental flood basalts

15 Pages·2015·3.3 MB·English

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Geochemistry, Geophysics, Geosystems
RESEARCH ARTICLE Magmatic recharge in continental flood basalts: Insights from
10.1002/2015GC005805 the Chifeng igneous province in Inner Mongolia
KeyPoints: XunYu1,2,Cin-TyA.Lee2,Li-HuiChen1,andGangZeng1
(cid:2)Increasingprimitivenessiscommon
infloodbasalts 1StateKeyLaboratoryforMineralDepositsResearch,SchoolofEarthSciencesandEngineering,NanjingUniversity,
(cid:2)Increasingprimitivenessiscausedby Nanjing,China,2DepartmentofEarthScience,MS-126RiceUniversity,Houston,Texas,USA
magmaticrecharge
(cid:2)Historyofmagmachambersrevealed
bycontinuousbasaltstratigraphy Abstract Eruptivesequencescanbeusedaswindowsintothethermalandchemicalevolutionof
magmachambers.WeexaminedacontinuousverticalsectionoftheBaichahebasaltflowassociatedwith
Correspondenceto:
thelateCenozoicChifengfloodbasaltinInnerMongolia,NorthChina.Fromoldesttoyoungest,MgO
X.Yu,
[email protected] increases,K2O,lightrareearthsandotherincompatibleelementsdecrease,andNb/LaandradiogenicPb
isotopicratiosincrease,allofwhichindicateincreasingprimitivenessanddecreasingcontributionofcrustal
Citation: contaminationwithtime.ThevariablePbisotopeandincompatibleelementratiosrequireacomponentof
Yu,X.,C.-T.A.Lee,L.-H.Chen,and crustalcontamination,mostlikelyofthelowercrust(unradiogenicPb,andlowCe/Pb),intheearliestlavas.
G.Zeng(2015),Magmaticrechargein
Fractionalcrystallizationcanexplainsomeoftheelementalsystematics,butalonecannotexplainvariable
continentalfloodbasalts:Insightsfrom
theChifengigneousprovinceinInner incompatibleelementratiosandPbisotopes,northetemporaltrendtomoreprimitivecompositions.
Mongolia,Geochem.Geophys.Geosyst., Crustalassimilationwithorwithoutfractionalcrystallizationalsocannotexplainalltheelementalsystem-
16,doi:10.1002/2015GC005805.
atics.Wefindinsteadthatrechargebyaprimitivemagma,incombinationwithfractionalcrystallizationand
decreasingratesofcrustalassimilation,isneededtoexplaintheobservedgeochemicalsystematics.Our
Received9MAR2015
observationssuggestthatthedeliveryoffreshbasalttothemagmachambermustincreaseatratesfaster
Accepted4JUN2015
thanthecrustcanbeassimilatedorthattheratesofcrustalassimilationmustdecrease.However,progres-
Acceptedarticleonline9JUN2015
siveadditionofprimitivemagmashouldheatupthecrustandleadtomorecrustalassimilation.Wesuggest
thatduringtheinitialstagesofformingamagmachamber,themagmacoolsanddevelopsanoutercrystal-
linerindofmafictoultramaficcumulates.Thisresultsinathickeningnonconvectingchemicalboundary
layer,whichservestoinsulatethemagmachamberfromfurtherassimilationofcrustandcooling,thelatter
resultinginthereductionofcrystallizationratesandthebufferingofmagmacompositionsatmoreprimi-
tivecompositions.Weshowthatcertainsegmentsofotherlargeigneousprovincesalsodisplayanevolu-
tiontowardmoreprimitivemagmaswithtime,indicatingthatmagmaticrechargemaybeacommon
featureofbasalticmagmachambers.
1.Introduction
Floodbasaltsaremassiveoutpouringsofbasaltsoveranarrowwindowoftime,andbecauseoftheirimpact
oncontinentformationandclimate,theiroriginsareofinterestanddebate[e.g.,Alietal.,2005;Lassiterand
DePaolo, 1997; Lightfoot and Hawkesworth, 1988; Sharma, 1997; White and McKenzie, 1995]. Although it is
generallyagreed that flood basalts derive from the mantle, it is not clear whether they derive by decom-
pression melting in high-temperature plumes, decompression melting driven by lithospheric extension or
meltingoffertilelithologiesinthemantleatotherwisenormaltemperatures[e.g.,Anderson,1994;Kingand
Anderson,1995;Silveretal.,2006;WhiteandMcKenzie,1995].Thegeochemicalcompositionofmagmascan
beusedtoinferthenatureofthemantlesourceoffloodbasalts,allowingonetoevaluatesourcefertility,
mantletemperature,timingofemplacement,etc.However,magmachamberprocessesposeacomplicating
factor. As primitive magmas depart from the mantle and intrude into the cold crust, they may pool as
magmachambers,coolandcrystallize,assimilatecountryrock,anderupt.Effectsoflithosphericcontamina-
tioncanbepronounced,atleastchemicallyandisotopicallyforcontinentalfloodbasalts[e.g.,Arndtetal.,
1993;Carlsonetal.,1981].Finally,magmachambersthatserveasintermediatestagingzonesthatfeedthe
flood basalts likely experience periodic or continuous recharge of primitive magma. Clearly, flood basalts
representcomplexblendsofcrustalmelts,residualmagmas,andrechargingparentalmagmas[e.g.,Light-
footetal.,1990;Sharmaetal.,1991;Woodenetal.,1993].Thepurposeofthisstudyistobetterunderstand
VC2015.AmericanGeophysicalUnion.
AllRightsReserved. howthesedifferentmagmachamberprocessesinfluencethecompositionoffloodbasalts.
YUETAL. MAGMATICRECHARGEINFLOODBASALT 1
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Onewaytogaininsightintohowmagmachambersevolveistoinvestigatethe‘‘stratigraphy’’ofcontinuous
lavaflowsinfloodbasaltprovinces.Asacasestudy,weinvestigatedalateCenozoicsequenceofflowsasso-
ciated with the Chifeng continental flood basalt erupted in Inner Mongolia in northern China [Han et al.,
1999;Hoetal.,2008;Hongetal.,2013;Jiaetal.,2002].Theearliestlavasaremoreevolvedandshowgreater
extentsofcrustalassimilationwhilesubsequentlavasbecomemoreprimitivewithtime.Wediscusstheori-
ginofthistrendtowardincreasingprimitivenessinthecontextofrecharge,eruption,assimilation,andfrac-
tionalcrystallization.Weshowthattrendsorcyclesofincreasingprimitivenessareseeninothercontinental
floodbasaltsequences,suggestingthatmagmaticrechargemaybeacommonmechanismintheformation
offloodbasalts.
2.GeologicalBackgroundandSampleDescriptions
TheChifengfloodbasaltfield(CFB)liesonthenorthedgeofNorthChinaCraton(Figure1a)andrepresents
partofa20,000km2Neogenefloodbasaltbelt[Chenetal.,2014;Hanetal.,1999;Liuetal.,1992].Thebasalt
iscomposedofseveralhundredmetersofflowoverlainbyQuaternarysediments[Hanetal.,1999].Ceno-
zoiceruptions intheChifengareayounggradually fromthesoutheastto thenorthwest. AccordingtoJia
etal.[2002],theCFBbasaltscanbedividedintofourstages:a<200mOligocene(33–24Ma)stageinthe
southeasternpartoftheCFB,a<400mlateMioceneflowinterval(10–6Ma)inthecentralpartoftheCFB,
a<100mPlioceneflowinterval((cid:3)5Ma)inthenorth[Jiaetal.,2002],andaPleistoceneflowinterval(0.89–
0.16Ma)asscatteredoutcropsinthenorthwest[Hongetal.,2013;Jiaetal.,2002].Thesefloodbasaltsare
likelyrelatedtotheopeningoftheBohaibasintothesoutheastandassociatedlithosphericextension[Han
etal.,1999;Huetal.,2001].
Here we investigated a Pliocene vertical section (E117857058.1600, N43806052.0000) known as the Baichahe
group (Figure 1a). The eruptions produced a series of layered basalts as thick as 40–70 m [Jia et al.,
2002]. The section consists of at least 10 individual lava flows, identified by the presence of paleo flow
surfaces. The lava layers are thought to be relatively continuous because no soil or sediments were
found between them, suggesting that this sequence was erupted in a short time interval [Jia et al.,
2002]. We obtained fresh samples from nine consecutive flows, totaling 40 m. The total thickness of the
top three layers is similar to the total thickness of the middle three layers which are greater than 15 m,
while the thickness of the bottom three layers is less than 10 meters (Figures 1b and 1c). In hand sam-
ple, the basalts are black and massive and contain a few small (<1 cm) felsic xenoliths. The basalts
exhibit porphyritic to aphyric textures with olivine and clinopyroxene phenocrysts set within ground-
mass composed of fine to medium-grained plagioclase, clinopyroxene, olivine, and Fe-Ti oxides.
3.Methods
Measurementsofwhole-rockmajorelementsweremadeattheStateKeyLaboratoryforMineralDeposits
Research, Nanjing University, China, using a Thermo Scientific ARL 9900 X-ray fluorescence spectrometer
(XRF).Accordingtothemeasuredvaluesofstandards(GSR-1andGSR-3),therelativestandarddeviationis
about61%forelementswithconcentrations>1.0wt%,andabout610%fortheelementswithconcentra-
tions <1.0 wt %. Measurements of whole-rock trace elements were made at the Department of Geology,
Northwest University, China. Trace elements, including the rare earth elements (REEs), were determined
using an ELANG100DRC inductively coupled plasma mass spectrometer (ICP-MS) after acid digestion of
samplesinTeflonbombs.AnalysesoftheUSGSrockstandards(BHVO-2,AGV-2,BCR-2,andGSP-1)indicate
precisionandaccuracybetterthan5%forSc,V,Co,Zn,Rb,Sr,Y,Nb,Cs,Ba,Th,andREEs,and10%forLi,
Cu,Zr,Hf,Ta,Pb,andU.
Pb isotopic compositions were obtained using a Neptune plus (Thermo Fisher Scientific) multicollector
inductivelycoupledplasmamassspectrometer(MC-ICP-MS)attheStateKeyLaboratoryforMineralDepos-
its Research in Nanjing University, China. Approximately 200 mg powders were leached for 12 h in warm
2.5NHClbeforedissolvinginanHF-HNO acidmixtureat1208Cformorethan36h.Afterevaporationto
3
dryness,allsampleswererepeatedlytakenupanddriedwithconcentratedHNO withtracesofHFinorder
3
tobreakCaFbonds.Finally,thesamplesweredissolvedinHBr-HNO acidmixture.Pbwasseparatedfrom
3
the rock matrix by chromatographic extraction using an anion exchange resin (200–400 mesh Bio-RAD
resin). Detailed chemical separation procedures are given by Kuritani and Nakamura [2002]. Thallium was
YUETAL. MAGMATICRECHARGEINFLOODBASALT 2
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Figure1.(a)SimplifiedgeologicalmapofeasternChinaand(b)distributionofCenozoicbasaltsinChifengarea.(c)TopfivelayersofBaichaheverticalsection.(d)Bottomfourlayersof
Baichaheverticalsection.
added to correct for mass bias during the measurement. Measured Pb isotopic ratios were corrected for
instrumentalmassfractionationbyreferencetoreplicateanalysesofthestandardNIST-981.Detailedmea-
surementprocedures aregivenbyWhiteetal.[2000].MeasuredvaluesforNIST-981are16.939667(2SE)
for206Pb/204Pb,15.485567(2SE)for207Pb/204Pb,and36.6874616(2SE)for208Pb/204Pb,respectively.The
resultsoftheanalysesofsamplescanbeseeninTable1.
4.Results
Samples from the Baichahe section are alkaline basalt (mainly trachy-basalt): they have moderate SiO
2
(47.4–48.7wt%)andhighalkalis(Na O1K O55.0–6.5 wt%).MgO increasesandK Odecreases upsec-
2 2 2
tion,indicatingatrendtowardmoreprimitivecompositionswithyoungingage.Crust-sensitivetracersalso
decreaseupsection.IncompatibleelementslikeNbandLadecrease.Nb/LaandCe/Pb,whicharegenerally
lowincontinentalcrust,increaseupsection.Leadisotopicratios206Pb/204Pb,207Pb/204Pb,and208Pb/204Pb
increase up section from unradiogenic values more similar to those of lower continental crust from the
NorthChinacraton[Liuetal.,2004](Figure2).Theincreasingprimitivenessanddecreasingcrustalcontribu-
tionupsectioncanalsobeseeninprimitivemantle-normalizedtraceelementdiagrams:basaltsfromlower
layersshowstrongerrelativeenrichmentsofincompatibletomoderatelyincompatibleelements,ascanbe
seenfromthehigherlight rareearthelementenrichmentsignaturesofthelowerlayers comparedto the
upperlayers(Figure3).
5.Discussion
5.1.IncreasingPrimitivenessofMagma
Thegeochemicalandisotopictrendsdescribedaboveindicatethatmagmasbecomeincreasinglyprimitive
with time. These geochemical trends also translate into temperature. Liquid temperatures of each layer
scalewithwhole-rockMgOcontentsifolivineispresent.UsingthethermometryofLeeandChin[2014]and
assuming that the magmas were dry and resided in a midcrustal magma chamber (a pressure of 0.5 GPa
was assumed), we estimate liquid temperatures to vary from 1220 to 12638C up section (Figure 2). These
temperatures represent the temperature of the magma body at the time each layer was erupted. There
may be some absolute uncertainties in the temperatures but the relative variations are likely robust.
YUETAL. MAGMATICRECHARGEINFLOODBASALT 3
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Table1.GeochemicalCompositionsofBaichaheLayeredBasalts
Layer Layer Layer Layer Layer Layer Layer Layer Layer This This This This
Analysis 1(Top) 2 3 4 5 6 7 8 9(Bottom) Study Referencea Study Reference Study Reference Study Reference
No. 11BCH01 11BCH02 11BCH03 11BCH04 11BCH05 11BCH06 11BCH07 11BCH08 11BCH09 BHVO-2 BHVO-2 AGV-2 AGV-2 BCR-2 BCR-2 GSP-1 GSP-1
MajorElement(wt%)
SiO2 47.6 47.68 47.8 47.39 47.8 48.28 48.74 48.54 48.3
TiO 2.068 2.071 2.148 2.197 2.17 2.234 2.345 2.254 2.257
2
AlO 12.64 12.11 12.71 13.46 13.94 13.84 14.52 13.91 14.04
2 3
Fe2O3Tb 12.34 12.2 12.17 11.79 12.04 11.98 11.45 11.84 11.57
MnO 0.156 0.152 0.153 0.146 0.151 0.148 0.139 0.152 0.153
MgO 10.55 10.39 10.02 8.6 8.3 8.49 7.7 8.73 8.48
CaO 8.54 9.52 8.8 9.18 8.73 8.51 8.02 7.94 8
Na2O 3.95 3.42 3.81 4.12 3.76 3.98 4.4 4.06 3.71
K2O 1.69 1.57 1.68 1.81 1.74 1.81 2.13 2.03 2.08
PO 0.492 0.5 0.508 0.551 0.552 0.563 0.617 0.598 0.615
2 5
LOI 0.04 0.22 0.17 0.89 0.96 0.12 0 0.06 0.41
TOTAL 100.06 99.84 99.97 100.13 100.15 99.95 100.06 100.12 99.61
TraceElement(ppm)
Li 9.10 9.86 8.15 7.86 11.5 8.04 8.12 9.19 10.8 4.79 4.80 11.0 11.0 9.46 9.00 30.6 31.0
Be 1.74 1.58 1.77 1.84 1.85 1.89 2.11 2.04 2.00 1.06 1.00 2.21 2.30 2.09 1.60 1.27 1.40
Sc 19.6 23.3 20.3 17.1 16.9 17.6 16.0 16.8 16.8 32.2 32.0 12.6 13.0 34.5 33.0 6.45 6.20
V 185 203 189 177 177 183 178 179 176 320 317 117 120 414 416 52.8 53.0
Cr 316 461 319 205 182 217 151 207 201 284 280 15.4 17.0 15.6 18.0 9.88 13.0
Co 59.2 56.7 55.3 50.6 52.5 51.3 46.3 51.1 48.8 45.9 45.0 15.4 16.0 38.4 37.0 6.75 6.60
Ni 286 269 254 218 218 220 187 227 217 120 119 18.5 20.0 12.2 13.0 7.17 8.80
Cu 53.3 49.1 55.6 57.5 51.7 57.8 59.3 54.0 52.6 130 127 48.9 53.0 20.9 21.0 30.4 33.0
Zn 113 105 108 109 111 111 111 111 111 102 103 86.3 86.0 128 127 100 104
Ga 19.5 18.4 19.0 20.2 20.4 20.8 22.0 21.0 21.3 21.5 21.7 20.3 20.0 21.7 23.0 26.0 23.0
Ge 1.35 1.36 1.30 1.28 1.30 1.32 1.31 1.30 1.35 1.60 1.60 1.16 1.56 1.87 1.36
Rb 23.5 20.4 22.6 23.9 22.4 24.1 31.1 27.4 28.9 9.06 9.11 66.34 66.30 46.99 46.90 241.09 254.00
Sr 690 538 665 585 627 605 670 645 667 399 396 659 661 340 340 229 234
Y 20.9 20.3 20.6 21.7 21.4 22.0 23.3 22.6 23.2 26.4 26.0 19.8 20.0 36.1 37.0 27.1 26.0
Zr 175 162 171 181 177 186 210 200 206 171 172 231 230 186 184 586 530
Nb 39.9 36.3 39.1 42.2 40.6 43.2 48.3 45.9 46.6 18.6 18.1 14.1 14.5 12.6 12.6 29.1 27.9
Cs 0.41 0.34 0.37 0.37 0.25 0.35 0.46 0.38 0.44 0.11 0.10 1.14 1.16 1.13 1.10 0.98 1.02
Ba 287 269 337 312 374 339 378 352 393 128 131 1134 1130 678 677 1285 1310
La 22.6 20.5 21.7 23.5 23.5 24.3 28.1 26.8 28.0 15.0 15.2 38.2 37.9 24.0 24.9 175.7 184.0
Ce 46.4 42.4 44.5 48.3 47.2 49.6 57.3 54.5 57.2 37.9 37.5 68.3 68.6 52.3 52.9 416.1 399.0
Pr 5.72 5.26 5.47 5.91 5.82 6.10 6.95 6.68 6.94 5.26 5.35 8.01 7.84 6.63 6.70 55.64 52.00
Nd 24.0 22.4 23.1 24.9 24.5 25.5 28.5 27.5 28.6 24.8 24.5 30.3 30.5 28.2 28.7 207.2 196.0
Sm 5.64 5.39 5.50 5.79 5.68 5.95 6.44 6.28 6.43 6.13 6.07 5.47 5.49 6.47 6.58 25.4 26.3
Eu 1.81 1.76 1.78 1.86 1.88 1.92 2.08 2.02 2.07 2.08 2.07 1.56 1.54 1.94 1.96 2.25 2.33
Gd 5.21 4.97 5.04 5.34 5.27 5.51 5.91 5.71 5.87 6.05 6.24 4.67 4.52 6.40 6.75 14.5 12.1
Tb 0.78 0.75 0.76 0.80 0.78 0.81 0.87 0.84 0.85 0.93 0.92 0.63 0.64 1.02 1.07 1.38 1.34
Dy 4.23 4.13 4.12 4.34 4.28 4.45 4.77 4.54 4.68 5.31 5.31 3.47 3.47 6.25 6.41 6.23 5.50
Ho 0.75 0.73 0.72 0.76 0.76 0.79 0.83 0.80 0.83 0.98 0.98 0.65 0.65 1.26 1.28 0.94 1.01
Er 1.91 1.79 1.81 1.91 1.88 1.98 2.12 2.02 2.07 2.55 2.54 1.82 1.81 3.59 3.66 2.50 2.70
Tm 0.24 0.23 0.23 0.25 0.25 0.26 0.27 0.26 0.27 0.33 0.33 0.25 0.26 0.51 0.54 0.29 0.38
Yb 1.42 1.33 1.34 1.43 1.40 1.47 1.56 1.50 1.55 2.01 2.00 1.62 1.62 3.32 3.38 1.67 1.70
Lu 0.19 0.18 0.19 0.20 0.19 0.20 0.21 0.21 0.21 0.28 0.27 0.24 0.25 0.49 0.50 0.24 0.21
Hf 3.93 3.70 3.79 4.02 3.94 4.16 4.61 4.39 4.53 4.38 4.36 5.03 5.00 4.73 4.90 14.6 15.5
Ta 2.15 1.98 2.10 2.29 2.19 2.33 2.60 2.49 2.55 1.18 1.14 0.84 0.87 0.77 0.78 0.89 0.97
Pb 2.91 2.38 2.48 2.73 2.57 2.92 3.67 3.50 3.86 1.67 1.60 13.1 13.2 10.1 11.0 59.5 55.0
Th 3.18 2.81 3.01 3.28 3.19 3.40 4.09 3.92 4.11 1.23 1.22 6.05 6.10 5.77 5.70 106 106
U 0.95 0.74 0.95 1.00 1.04 1.01 1.20 1.16 1.23 0.42 0.40 1.84 1.86 1.62 1.69 2.34 2.54
Isotope
206Pb/204Pb 18.0064 18.0345 18.0323 18.0164 18.012 18.0161 17.9536 17.9476 17.9348
2SEc 0.0006 0.0007 0.0007 0.0007 0.0012 0.0006 0.0005 0.0006 0.0007
207Pb/204Pb 15.4848 15.495 15.4986 15.4818 15.4845 15.504 15.4764 15.4751 15.4758
2SE 0.0006 0.0008 0.0006 0.0006 0.0011 0.0005 0.0004 0.0005 0.0007
208Pb/204Pb 38.0448 38.0843 38.0812 38.0478 38.058 38.0867 38.0059 38.0023 38.0047
2SE 0.0015 0.0022 0.0017 0.0017 0.0026 0.0014 0.0012 0.0014 0.0017
Ce/Pb 15.95 17.85 17.97 17.71 18.37 17.00 15.64 15.56 14.82
Nb/La 1.77 1.77 1.80 1.79 1.73 1.78 1.72 1.71 1.66
Tm(8C)d 1263 1260 1255 1234 1229 1232 1220 1236 1232
aReferencedataforBHVO-2,AGV-2,BCR-2,andGSP-1aretakenfromGovindaraju[1994].
bFe2O3T,totalironasFe2O3.
cSEreferstostandarderror.
dLiquidtemperaturesforlavasfromdifferentlayers[LeeandChin,2014].HereweassumedHO50wt%,andP50.5Gpa.
2
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Figure2.Spatialvariationsforrepresentativemajor,traceelements,andisotopes.Verticalaxisshowstheheightsofverticalsection.Hori-
zontalaxisshowsvariationsofwt%MgOandKO,ppmNb,La,andNi,Nb/La,206Pb/204Pbandthetemperature(8C)ofmagma.Dotted
2
andstraightcurvesrepresenttheboundariesofdifferentlayers.
Collectively,ourobservationsindicatethatthemagmachamberbecamemoreprimitive,hotter,andassimi-
latedlesscrustalwallrockwithtime.
Increasing primitiveness with time can be explained by a change in the nature of mantle source or by
magmachamberprocesses.Forexample,variationsinfloodbasaltsequencesarecommonlyexplainedby
earlymeltingofenrichedmantlecomponentsandlatermeltingoflessenrichedcomponents[e.g.,Lassiter
etal.,1995;LassiterandDePaolo,1997;Woodenetal.,1993].However,wenotethatthe1220–12638Cliquid
temperaturescalculatedforthebasalticlavasarewellbelowprimarymantle-derivedmagmatemperatures,
indicating that the effects of magmatic differentiation in the crust must be considered. Below, we thus
explore whether the compositional trends displayed in the Chifeng series can be explained by magma
chamberprocesses.
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5.2.SimultaneousRecharge,
Eruption,Assimilation,and
FractionalCrystallization(REAFC)
The processes that influence the com-
positionalevolutionofamagmacham-
ber are magma recharge (R), crustal
assimilation (A), fractional crystalliza-
tion (FC), and eruption (E) [e.g.,
DePaolo, 1981; Lee et al., 2014; Spera
and Bohrson, 2001, 2002]. For the
Chifeng series, fractional crystallization
(FC) alone cannot change Pb isotopes
orincompatibleelementratioslikeNb/
Laanddoesnotexplaintheincreasing
Figure3.Primitivemantle-normalizedtraceelementdiagramofbasaltsfrom primitivenesswithtime(Figure4).Simi-
Baichaheverticalsection.PrimitivemantlevaluesarefromMcDonoughandSun larly,crustalassimilation(A)alonedoes
[1995].
not match the observed data because
itpredictsadecreaseinNb/LaandradiogenicPbisotopeswithdecreasingMgOcontentbecausetheconti-
nental crust end-member is characterized by low Nb/La and unradiogenic Pb (206Pb/204Pb in the crust of
theNorthChinacratonareaslowas16.5)[Gaoetal.,1998;Liuetal.,2004];althoughNb/Laand206Pb/204Pb
are indeed low for the samples with the lowest MgO contents (<8 wt %), these ratios remain constant
between8and11wt%MgO(Figure4).Simultaneousassimilationandfractionalcrystallization (AFC)can
potentiallyexplainsomeoftheelementalandisotopictrends,butcannotexplaintheincreaseinprimitive-
nesswithtime(Figure4).Insummary,theincreasingprimitivenessofthemagmaswithtimerequiressome
componentofrecharge(R)byaprimitiveparentalmagma.
We now consider the relative importance of recharge, eruption, assimilation, and fractional crystallization
(REAFC) on the Chifeng basalts. Different approaches can be explored. One approach is to follow the
energy-constrainedapproachofSperaandBohrson[2004],whereintheamountofassimilationislimitedby
energyavailabilityfromthehostmagmaitselfandtheefficiencyofheatingupthecountryrock.Whilesuch
Figure4.Modelingresultsofpurefractionalcrystallizationwith(blacklines)orwithoutrecharge(blackdottedlines),assimilationwith(bluedottedlines)orwithoutfractionalcrystalliza-
tion(reddottedlines),andsimultaneousrecharge,eruption,assimilation,andfractionalcrystallizationmodel(redlines)forBaichahebasalts.Rayleighfractionalcrystallizationequation
[Gast,1968;Shaw,1970]andRecharge-Eruption-Assimilation-FractionalCrystallizationmodels[Leeetal.,2014]areusedhere.SymbolsarethesameasFigure2.
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amodelisintheorymorerealisticbecausephysics,inthesenseofenergyconservation,isconsidered,the
physicsofheattransport,magmamixing,andassimilationarenotexplicitlyconsidered,andassuch,these
modelsmaybetoorestrictiveintermsofthevariablespace.Wehavethuschosentorelaxthephysicallimi-
tationsandinsteadexploredifferentcombinationsofR,E,A,andFCthatmightmatchthetemporalevolu-
tion of composition following a box model approach of Lee et al. [2014]. Without initially worrying about
physical and energetic limitations, we identify the family of REAFC scenarios that can best reproduce the
observedChifengtrends(Figure4).Wethenevaluatethephysicalplausibilityofsuchscenarios.
FollowingLeeetal.[2014],therateatwhichthemassMofthemagmachamberchangesisgivenby
dM dM dM dM dM
5 re1 e1 cc1 x (1)
dt dt dt dt dt
wheretheratesofrecharge(R),eruption(E),assimilation(A),andfractionalcrystallization(FC)aredM /dt,
re
dM /dt, dM /dt, and dM/dt. Eruption and fractional crystallization rates are negative quantities because
e cc x
massisremovedfromthemagmachamberwhilerechargeandassimilationarepositivequantitiesbecause
massisadded.Elementalmassbalanceisthengivenby
dm 5dM C 1dM C 1dM C 1dM C (2)
ch x x e ch cc cc re re
wheredm isthechangeinmassofagivenelementinthemagmachamber.C ,C,C ,C arethecon-
ch ch x cc re
centrationsoftheelementinthemagmachamber,crystallizedminerals,crustalwallrock,andrecharging
magmas.C isrelatedtotheconcentrationintheresidualliquidinthemagmachamberC byanequilib-
x ch
riumpartitioncoefficientbetweencrystalsandmelt,D(cid:4)C/C .Theactualcompositionoftherecharging
x ch
magma is not known, so we assume that the recharging magma is represented by the most primitive
magma sampled in this study (C 510.5 wt %, C 520.8 ppm, C 537.0 ppm, C 52.50 ppm,
MgO La Nb Pb
206Pb/204Pb518.03).Wealsoassumethatthecrystallizingphasesareolivineandclinopyroxenewitharatio
of(cid:3)1.WethusadoptbulkpartitioncoefficientsforMg,Nb,La,andPbasD (cid:3)2,D (cid:3)0.03,D (cid:3)0.028,
Mg Nb La
and D (cid:3)0.001based on McKenzie andO’Nions [1991] andO’Neill and Jenner [2012],althoughthe exact
Pb
values of partition coefficients are not critical because we are only interested in general behaviors with
theseboxmodels.CompositionsofthelowercontinentalcrustfromNorthChinaCratonareselectedfrom
Gaoetal.[1998](C 54.8wt%,C 520.0ppm,C 58.00ppm,C 515.0ppm,206Pb/204Pb517.00).
MgO La Nb Pb
Below,wepresenttheresultsofnumericalmodelsusingequations(1)and(2)fordifferentscenarios.Inall
models,wehavenormalizedthemassofthemagmachamberandallmassinputsandoutputstotheinitial
massof themagmachamber. Thus,themassof themagmachamberinitiallyis unityandtheinputs and
outputseffectivelyrepresentfractionalquantities,thatis,theproportionofmassaddedorremovedrelative
to the original mass of the magma chamber. Because we do not have constraints on the lifespan of the
magmachamberoractualmassinputsandoutputsfromthemagmachamber,weusetheintegratedmag-
ðdM
maticrechargemassM 5 redtasaprogressvariableinsteadoftime,althoughM neednotscaleline-
re dt re
arly with time. We then modify the ratios of the inputs and outputs relative to the recharge rate, which
eliminatestimeasanexplicitvariable.Thus,weexplorehowthecompositionofthemagmachambervaries
asinputsandoutputs,dM/dM ,changewithtimeorintegratedmagmachambermassM .Althoughour
i re re
approach treats all inputs and outputs as continuous, any stepwise changes in magma chemistry can be
modeledasrapidchangesinthemagnitudesofcertaininputsoroutputs.Wealsoassumethatallmagmas
derivefromthesamemagmachamber.Multiplemagmachambersarepossiblebutwehavenoreasonto
modelsuchscenarioswithoutanysupportingorrelevantdata.
5.2.1.NonsteadyStateMagmaChambers
Herewepermitthemagmachambertochangemass.WeassumeconstantrechargeratedM /dtandvary
re
oneofthefluxes.InFigure5a,weallowcrystallizationratesdM/dttochangewhileallotherinputs,dM /dt,
x re
dM /dt, and dM /dt, remain constant. When fractional crystallization increases faster than recharge, e.g.,
e cc
whendM/dM increaseswithtime(line1inFigure5a),magmacompositionsbecomedepletedinMgbut
x re
morequicklycontaminatedbycrustalcomponents,asshownbyrapiddecreasesinNb/LaandPbisotopes
comparedtoMgOdecreases.WhenrelativefractionalcrystallizationratesdM/dM decreasewithtime(line
x re
3inFigure5a),magmacompositionsinitiallybecomemoreevolvedbuteventuallyreturntotheirprimitive
character in terms of elemental abundances. In this case, signatures of crustal assimilation in the form of
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Figure5.EffectofdMre/dt,dMe/dt,dMcc/dt,anddMx/dtonevolutionofmagmaandmassofmagmachamber.(a)Modelingresultsofvariablefractionalcrystallizationrates.Line1shows
decreasingdM/dM whilelines2and3showconstantandincreasingdM/dM .(b)Modelingresultswithvariableassimilationrates.Line1showsincreasingdM /dM whilelines2
x re x re cc re
and3showconstantanddecreasingdM /dM .(c)Modelingresultswithvariableeruptionrates.Line1showsdecreasingdM/dM andlines2and3showconstantandincreasingdM/
cc re e re e
dMre.REAFCmodelsareassumed[Leeetal.,2014].SymbolsarethesameasinFigure2.
trace element and radiogenic isotope ratios (e.g., Nb/La or 206Pb/204Pb) could be preserved even though
majorelementsystematicsreturntoprimitivecompositions.Thisreflectsthefactthatthecrustaltraceele-
mentandisotopicsignaturesusedherearefundamentallybasedonincompatibleelements,whichconcen-
trate in the magma as the magma crystallizes. Thus, when the rate of fractional crystallization decreases
relativetotherateofrecharge,butcrustalassimilationratesremainconstant,themagmabecomesmore
primitiveintermsofmajorelementsbutcanretainasignatureof crustalcontamination intermsof trace
elementsorradiogenicisotopes(line3inFigure5a).Ourdata,however,showthatcrustalassimilationsig-
naturesdecreasewithincreasingMgOcontent,suggestingthatincreasingrechargeratesalonemaynotbe
sufficient.
InFigure5b,wevarycrustalassimilationdM /dt,butholdallotherfluxes,dM /dt,dM /dt,anddM/dt,con-
cc re e x
stant.WhenrelativecrustalassimilationratesdM /dM increasewithtime(line1inFigure5b),themagma
cc re
initiallybecomesmoreevolvedwithminimaleffectofcrustalassimilation,buteventuallythemagmadevel-
opscontinentalcrust-likeelementalandisotopicsignatureswithlittlechangeinMgOcontent.Whenrela-
tivecrustalassimilationratesdM /dM decrease(line3inFigure5b),themagmainitiallyevolvestolowMg
cc re
and high Nb contents, but gradually returns to its primitive character in terms of elemental and isotopic
ratios,whichcanexplainourobservations.
InFigure5c,wealloweruptionratedM /dttochangebutholddM /dt,dM /dt,anddM/dtconstant.Vary-
e re cc x
ingeruptionrates,withoutchanginganyotherrelativefluxes,doesnotchangeelementalorisotopicvaria-
tion diagrams. This is because it is assumed that erupted magmas have the same composition as the
magmachamber.However,eruptionstronglyinfluencestherateatwhichmagmasevolvebecauseeruption
decreases the size of the magma chamber, reducing the times for mixing and turnover. Thus, eruption
alonecannotexplainthereturntoprimitivecharacterofthemagmawithtime.
5.2.2.SteadyStateMagmaChambers
Wealsoconsideraconstantmassmagmachamber(dM /dt50)undergoingrecharge(Figure6).Asshown
ch
above, decreasing fractional crystallization (dM/dM ) and crustal assimilation rates (dM /dM ) relative to
x re cc re
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Figure6.REAFCmodelresultsforconstantmassmagmachamber.(a)Decreasingassimilationrate.(b)Increasingassimilationrate.MgOandNb/Laareusedinthemodeling.Magma
chamberisnormalizedtooriginalmass.Tickmarksoncurveswithlabelednumbersrepresentoverturnnumbers.SymbolsarethesameasinFigure2.
magmatic recharge rates is required for a magma to become more primitive and less crustally contami-
natedwithtime.Foraconstantmassmagmachamber,weconsidertwoextremescenarios.Inthefirst,the
geochemicalevolutionofthemagmachambercanbegeneratedifweholdrechargeratedM /dtconstant
re
andallowcrustalassimilationratedM /dtandcrystallizationratedM/dttodecrease(Figure6a).Tomain-
cc x
tainasteadystatemagmachamberundertheseconditionsrequireseruptionratedM /dttoincrease.If,on
e
the other hand, we attempt to reproduce the geochemical trends by allowing crustal assimilation rate to
increase,therechargeratemustincreaseatamuchfasterratetocompensatefortheeffectsofassimilation
(Figure6b).Inthisscenario,steadystateisonlymaintainediferuptionratesarehigh,andasaconsequence,
inordertoreturnthemagmachambertoprimitivevalues,highermagmachamberturnoverratesarealso
required. However, anincreasing eruption rate seems atodds with thefact that eruption diedjust as the
magmawasbecomingmoreprimitive.Thus,themostplausiblesteadystatescenarioisthatshowninFig-
ure6a,whereincrystallizationratesandcrustalassimilationratesdecreaseduringrecharge.
5.2.3.Summary:DecreasingAssimilationandCrystallizationisRequiredDuringRecharge
Based on the above analysis, the increasing primitiveness and decreasing crustal component require
rechargewithaprimitivemagmaandadecreaseintheratesofcrustalassimilationandfractionalcrystalli-
zationrelativetorechargerate.Wealsoshowedthatassimilationandfractionalcrystallizationalonecannot
explainthistrend,anderuptionhasnoeffectotherthanslowingdowntherateofgeochemicalevolution
for a given recharge rate. While our analysis cannot distinguish between steady state or nonsteady state
magmachambers,therequirementfordecreasingcrustalassimilationrelativetorechargemaynotbeintu-
itivegiventhattheadditionofmoremagmasshouldgraduallyheatupthecountryrock,makingiteasier
to assimilate. We will return to this problem later, but to evaluate whether the magmatic recharge
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Figure7.
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