高等微量元素地球化学课件：岩浆作用中微量元素行为

微量元素地球化学岩浆作用中微量元素行为不相容元素主要集中在液相中，它们的行为可以反映结晶作用或者部分熔融过程中熔体的比例Figure9-1b.ZrHarkerDiagramforCraterLake.FromdatacompiledbyRickConrey.FromWinter(2001)AnIntroductiontoIgneousandMetamorphicPetrology.PrenticeHall.岩浆作用中微量元素行为微量元素强烈分布在单一矿物相中Ni–olivine（橄榄石）inTable9-1=14Figure9-1a.NiHarkerDiagramforCraterLake.FromdatacompiledbyRickConrey.FromWinter(2001)AnIntroductiontoIgneousandMetamorphicPetrology.PrenticeHall.岩浆作用中微量元素行为岩浆演化模型Crystallization

（结晶作用）瑞利结晶分异和平衡结晶分异PartialMelting（部分熔融）平衡部分熔融和分离部分熔融岩浆作用中微量元素行为瑞利结晶分异每一个结晶矿物相结晶后从岩浆房中移出 残余液相中某元素的浓度CL

可用瑞利分异方程模拟:eq.9-8 CL/CO=F(D-1)

RayleighFractionation岩浆作用中微量元素行为瑞利结晶分异已知k=Cs/Cl,令m为相的质量，x为摩尔数(n)，则有：

K=岩浆作用中微量元素行为瑞利结晶分异岩浆作用中微量元素行为瑞利结晶分异

岩浆作用中微量元素行为残余岩浆分数瑞利结晶分异

岩浆作用中微量元素行为平衡结晶分异结晶的晶体与残余液体保持平衡状态 残留液体中某些微量元素的浓度CL

可用下列方程进行模拟:eq.9- CL=CO/[D+F(1-D)]其中CO:初始液相浓度

F:残余液体的量分数

D:总分配系数岩浆作用中微量元素行为平衡结晶分异岩浆作用中微量元素行为残余岩浆分数首先确定岩石中某一元素对每个结晶矿物的分配系数，再权重加和得到岩石总分配系数Di=

WAkiAWA=岩石中A矿物质量%ki=元素i在矿物A和熔体间的分配系数A岩浆作用中微量元素行为实例:石榴石辉橄岩=60%橄榄石,25%斜方辉石,10%单斜辉石,5%石榴石,使用表中分配系数:DEr=(0.6*0.026)+(0.25*0.23)+(0.10*0.583)+(0.05*4.7)=0.366例题1：已知有一个玄武质母岩浆，其Rb和Sr的初始浓度分别为79ppm和471ppm.结晶分异作用中分离的矿物相分别为斜长石、橄榄石和单斜辉石RbandSr玄武岩:

1.

换算成矿物的质量分数(WolWcpxetc.)Table9-2.ConversionfrommodetoweightpercentMineralModeDensityWtprop质量分数ol153.6540.18cpx333.4112.20.37plag512.7137.70.45Sum303.91.00RbandSrTable9-2.ConversionfrommodetoweightpercentMineralModeDensityWtprop质量分数ol153.6540.18cpx333.4112.20.37plag512.7137.70.45Sum303.91.00玄武岩:

2.

使用表中提供的每个矿物的分配系数计算岩石分配系数: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平衡结晶分异FCRb=79CSr=4710.058525500.105625460.154205410.203355360.252785320.302385270.352085230.401855180.451665140.501515100.551395060.601285010.651194970.701114940.751044900.80984860.85924820.90874780.9583475瑞利结晶分异平衡结晶分异作用和瑞利分离结晶模拟单矿物分配系数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分离结晶模拟F(斜辉石)CSr=471CSc0.0583562510.1042961370.152911960.202208740.251782610.301496520.351290450.401135400.451014360.50916330.55836300.60769280.65712260.70663250.75621230.80584220.85551210.90521200.95495191.0047118F(角闪石)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(斜长石)CSr=471CSc0.05392920.10701530.15981050.20124800.25149650.30173550.35197480.40220420.45243380.50265340.55287310.60308290.65329270.70350250.75371240.80391220.85412210.90432200.95451191.0047118单矿物瑞利分离结晶模拟部分熔融作用实比过程：岩石按原有矿物的比例进行熔融的过程。在整个熔融进程中，固相矿物的比例不变，从而总体分配系数也不改变。这是一种为了计算方便而采取的简化处理，或因难以获得岩浆结晶历史中实际矿物组成变化的信息而采取的一种近似方法非实比过程：指岩石的不按照源岩矿物组成的比例进行熔融的过程，即在熔融的不同阶段，固相矿物的组成是变化的。总体分配系数也相应改变。这显然更符合实际的情况。但是需要用其它手段确定熔融过程中矿物组成的变化

部分熔融作用CL=液相中某微量元素的浓度；CO=熔融前某元素在岩石中的浓度F=重量熔体分数[=熔体/(熔体+岩石]（熔融度）分离熔融作用熔融残留分离熔融作用累积部分熔融作用

批式部分熔融部分熔融的熔体与熔融残留体始终保持平衡，直到这些熔体被移走部分熔融一定量的熔体整批上升离开母体，每一批上升熔体有一个熔体量分数（F）批式部分熔融eq.9-5CL=液相中某微量元素的浓度；CO=熔融前某元素在岩石中的浓度F=重量熔体分数[=熔体/(熔体+岩石]CC1Di(1F)FLO=-+岩浆作用中微量元素行为部分熔融过程模拟RbSrLaSc熔融残留相熔融残留相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部分熔融源岩浓度794711718岩石分配系数（D1)0.1620.2530.2923.645岩石分配系数（D2)0.1060.1730.1932.247微量元素定量模拟曲线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部分熔融结晶分异微量元素定量模拟曲线批式熔融使用不同分配系数，CL/CO

对F做图Di=1.0Figure9-2.Variationintherelativeconcentrationofatraceelementinaliquidvs.sourcerockasafiunctionofDandthefractionmelted,usingequation(9-5)forequilibriumbatchmelting.FromWinter(2001)AnIntroductiontoIgneousandMetamorphicPetrology.PrenticeHall.岩浆作用中微量元素行为Di

»1.0(相容元素)在熔体中浓度很低尤其是对低度部分熔融(lowF)Figure9-2.Variationintherelativeconcentrationofatraceelementinaliquidvs.sourcerockasafiunctionofDandthefractionmelted,usingequation(9-5)forequilibriumbatchmelting.FromWinter(2001)AnIntroductiontoIgneousandMetamorphicPetrology.PrenticeHall.岩浆作用中微量元素行为高度不相容元素强烈富集在初始部分熔融很少量的熔体中随着部分熔融程度的增加而明显降低Figure9-2.Variationintherelativeconcentrationofatraceelementinaliquidvs.sourcerockasafiunctionofDandthefractionmelted,usingequation(9-5)forequilibriumbatchmelting.FromWinter(2001)AnIntroductiontoIgneousandMetamorphicPetrology.PrenticeHall.岩浆作用中微量元素行为当F

1液相中每种微量元素的浓度=等于其源岩中的浓度(CL/CO

1) AsF1

CL/CO

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

0CL/CO

1/Di如果我们知道很低度部分熔融岩浆中某元素的CL

和它的总分配系数Di

我们就能估计出源区该元素的浓度CC1Di(1F)FLO=-+Figure9-2.Variationintherelativeconcentrationofatraceelementinaliquidvs.sourcerockasafiunctionofDandthefractionmelted,usingequation(9-5)forequilibriumbatchmelting.FromWinter(2001)AnIntroductiontoIgneousandMetamorphicPetrology.PrenticeHall.对极不相容元素，Di

0

equation9-5 变成:

eq.9-7

C