高等微量元素地球化學(xué)課件：巖漿作用中微量元素行為

微量元素地球化學(xué)巖漿作用中微量元素行為不相容元素主要集中在液相中，它們的行為可以反映結(jié)晶作用或者部分熔融過(guò)程中熔體的比例Figure9-1b.ZrHarkerDiagramforCraterLake.FromdatacompiledbyRickConrey.FromWinter(2001)AnIntroductiontoIgneousandMetamorphicPetrology.PrenticeHall.巖漿作用中微量元素行為微量元素強(qiáng)烈分布在單一礦物相中Ni–olivine（橄欖石）inTable9-1=14Figure9-1a.NiHarkerDiagramforCraterLake.FromdatacompiledbyRickConrey.FromWinter(2001)AnIntroductiontoIgneousandMetamorphicPetrology.PrenticeHall.巖漿作用中微量元素行為巖漿演化模型Crystallization

（結(jié)晶作用）瑞利結(jié)晶分異和平衡結(jié)晶分異PartialMelting（部分熔融）平衡部分熔融和分離部分熔融巖漿作用中微量元素行為瑞利結(jié)晶分異每一個(gè)結(jié)晶礦物相結(jié)晶后從巖漿房中移出 殘余液相中某元素的濃度CL

可用瑞利分異方程模擬:eq.9-8 CL/CO=F(D-1)

RayleighFractionation巖漿作用中微量元素行為瑞利結(jié)晶分異已知k=Cs/Cl,令m為相的質(zhì)量，x為摩爾數(shù)(n)，則有：

K=巖漿作用中微量元素行為瑞利結(jié)晶分異巖漿作用中微量元素行為瑞利結(jié)晶分異

巖漿作用中微量元素行為殘余巖漿分?jǐn)?shù)瑞利結(jié)晶分異

巖漿作用中微量元素行為平衡結(jié)晶分異結(jié)晶的晶體與殘余液體保持平衡狀態(tài) 殘留液體中某些微量元素的濃度CL

可用下列方程進(jìn)行模擬:eq.9- CL=CO/[D+F(1-D)]其中CO:初始液相濃度

F:殘余液體的量分?jǐn)?shù)

D:總分配系數(shù)巖漿作用中微量元素行為平衡結(jié)晶分異巖漿作用中微量元素行為殘余巖漿分?jǐn)?shù)首先確定巖石中某一元素對(duì)每個(gè)結(jié)晶礦物的分配系數(shù)，再權(quán)重加和得到巖石總分配系數(shù)Di=

WAkiAWA=巖石中A礦物質(zhì)量%ki=元素i在礦物A和熔體間的分配系數(shù)A巖漿作用中微量元素行為實(shí)例:石榴石輝橄巖=60%橄欖石,25%斜方輝石,10%單斜輝石,5%石榴石,使用表中分配系數(shù):DEr=(0.6*0.026)+(0.25*0.23)+(0.10*0.583)+(0.05*4.7)=0.366例題1：已知有一個(gè)玄武質(zhì)母巖漿，其Rb和Sr的初始濃度分別為79ppm和471ppm.結(jié)晶分異作用中分離的礦物相分別為斜長(zhǎng)石、橄欖石和單斜輝石RbandSr玄武巖:

1.

換算成礦物的質(zhì)量分?jǐn)?shù)(WolWcpxetc.)Table9-2.ConversionfrommodetoweightpercentMineralModeDensityWtprop質(zhì)量分?jǐn)?shù)ol153.6540.18cpx333.4112.20.37plag512.7137.70.45Sum303.91.00RbandSrTable9-2.ConversionfrommodetoweightpercentMineralModeDensityWtprop質(zhì)量分?jǐn)?shù)ol153.6540.18cpx333.4112.20.37plag512.7137.70.45Sum303.91.00玄武巖:

2.

使用表中提供的每個(gè)礦物的分配系數(shù)計(jì)算巖石分配系數(shù):Di=

WAkiDRb=0.045DSr=0.848FCRb=79CSr=4710.0513817430.107126680.154846280.203676020.252975810.302495660.352155520.401905410.451695320.501535230.551405160.601295090.651195030.701114970.751044920.80984870.85924830.90874790.9583475平衡結(jié)晶分異FCRb=79CSr=4710.058525500.105625460.154205410.203355360.252785320.302385270.352085230.401855180.451665140.501515100.551395060.601285010.651194970.701114940.751044900.80984860.85924820.90874780.9583475瑞利結(jié)晶分異平衡結(jié)晶分異作用和瑞利分離結(jié)晶模擬單礦物分配系數(shù)RbSrLaScPla0.0711.8300.0100.070Gar0.0420.0120.0014.000Amp0.2900.4600.5444.200Cpx0.0310.0600.0562.700Opx0.0220.0400.0300.120Oli0.0100.0140.0070.170分離結(jié)晶模擬F(斜輝石)CSr=471CSc0.0583562510.1042961370.152911960.202208740.251782610.301496520.351290450.401135400.451014360.50916330.55836300.60769280.65712260.70663250.75621230.80584220.85551210.90521200.95495191.0047118F(角閃石)CSr=471CSc0.0523750.0010.1016330.0110.1513120.0420.2011230.1040.259960.2130.309020.3820.358300.6260.407730.9590.457251.400.506851.960.556502.660.606213.510.655944.540.705715.750.755507.170.805318.810.8551410.70.9049912.80.9548415.31.0047118.0F(斜長(zhǎng)石)CSr=471CSc0.05392920.10701530.15981050.20124800.25149650.30173550.35197480.40220420.45243380.50265340.55287310.60308290.65329270.70350250.75371240.80391220.85412210.90432200.95451191.0047118單礦物瑞利分離結(jié)晶模擬部分熔融作用實(shí)比過(guò)程：巖石按原有礦物的比例進(jìn)行熔融的過(guò)程。在整個(gè)熔融進(jìn)程中，固相礦物的比例不變，從而總體分配系數(shù)也不改變。這是一種為了計(jì)算方便而采取的簡(jiǎn)化處理，或因難以獲得巖漿結(jié)晶歷史中實(shí)際礦物組成變化的信息而采取的一種近似方法非實(shí)比過(guò)程：指巖石的不按照源巖礦物組成的比例進(jìn)行熔融的過(guò)程，即在熔融的不同階段，固相礦物的組成是變化的?？傮w分配系數(shù)也相應(yīng)改變。這顯然更符合實(shí)際的情況。但是需要用其它手段確定熔融過(guò)程中礦物組成的變化

部分熔融作用CL=液相中某微量元素的濃度；CO=熔融前某元素在巖石中的濃度F=重量熔體分?jǐn)?shù)[=熔體/(熔體+巖石]（熔融度）分離熔融作用熔融殘留分離熔融作用累積部分熔融作用

批式部分熔融部分熔融的熔體與熔融殘留體始終保持平衡，直到這些熔體被移走部分熔融一定量的熔體整批上升離開(kāi)母體，每一批上升熔體有一個(gè)熔體量分?jǐn)?shù)（F）批式部分熔融eq.9-5CL=液相中某微量元素的濃度；CO=熔融前某元素在巖石中的濃度F=重量熔體分?jǐn)?shù)[=熔體/(熔體+巖石]CC1Di(1F)FLO=-+巖漿作用中微量元素行為部分熔融過(guò)程模擬RbSrLaSc熔融殘留相熔融殘留相Pla0.0711.8300.0100.070Gar0.0420.0120.0014.0000.150.30Amp0.2900.4600.5444.2000.500.35Cpx0.0310.0600.0562.7000.350.35Opx0.0220.0400.0300.120Oli0.0100.0140.0070.170部分熔融源巖濃度794711718巖石分配系數(shù)（D1)0.1620.2530.2923.645巖石分配系數(shù)（D2)0.1060.1730.1932.247微量元素定量模擬曲線FCRb=79CSr=471CSc=18Cla=170.0597344150.0071420.1054426300.041870.1538719430.120650.2030415670.256530.2525213270.462450.3021711580.748400.3519010321.124360.401709341.599330.451548552.183300.501417902.884280.551307363.709260.601216904.668240.651136505.768230.701076157.015220.751015848.418210.80955569.982200.859153211.716190.908651013.626180.958248915.71918FCSc=18Cla=170.055.13550.105.33500.155.55450.205.78420.256.04380.306.32360.356.62330.406.96310.457.34290.507.76280.558.22260.608.75250.659.35240.7010.04230.7510.84220.8011.78210.8512.89200.9014.24190.9515.9019部分熔融結(jié)晶分異微量元素定量模擬曲線批式熔融使用不同分配系數(shù)，CL/CO

對(duì)F做圖Di=1.0Figure9-2.Variationintherelativeconcentrationofatraceelementinaliquidvs.sourcerockasafiunctionofDandthefractionmelted,usingequation(9-5)forequilibriumbatchmelting.FromWinter(2001)AnIntroductiontoIgneousandMetamorphicPetrology.PrenticeHall.巖漿作用中微量元素行為Di

?1.0(相容元素)在熔體中濃度很低尤其是對(duì)低度部分熔融(lowF)Figure9-2.Variationintherelativeconcentrationofatraceelementinaliquidvs.sourcerockasafiunctionofDandthefractionmelted,usingequation(9-5)forequilibriumbatchmelting.FromWinter(2001)AnIntroductiontoIgneousandMetamorphicPetrology.PrenticeHall.巖漿作用中微量元素行為高度不相容元素強(qiáng)烈富集在初始部分熔融很少量的熔體中隨著部分熔融程度的增加而明顯降低Figure9-2.Variationintherelativeconcentrationofatraceelementinaliquidvs.sourcerockasafiunctionofDandthefractionmelted,usingequation(9-5)forequilibriumbatchmelting.FromWinter(2001)AnIntroductiontoIgneousandMetamorphicPetrology.PrenticeHall.巖漿作用中微量元素行為當(dāng)F

1液相中每種微量元素的濃度=等于其源巖中的濃度(CL/CO

1) AsF1

CL/CO

1CC1Di(1F)FLO=-+Figure9-2.Variationintherelativeconcentrationofatraceelementinaliquidvs.sourcerockasafiunctionofDandthefractionmelted,usingequation(9-5)forequilibriumbatchmelting.FromWinter(2001)AnIntroductiontoIgneousandMetamorphicPetrology.PrenticeHall.巖漿作用中微量元素行為AsF

0CL/CO

1/Di如果我們知道很低度部分熔融巖漿中某元素的CL

和它的總分配系數(shù)Di

我們就能估計(jì)出源區(qū)該元素的濃度CC1Di(1F)FLO=-+Figure9-2.Variationintherelativeconcentrationofatraceelementinaliquidvs.sourcerockasafiunctionofDandthefractionmelted,usingequation(9-5)forequilibriumbatchmelting.FromWinter(2001)AnIntroductiontoIgneousandMetamorphicPetrology.PrenticeHall.對(duì)極不相容元素，Di

0

equation9-5 變成:

eq.9-7

C