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1、微量地球化学 Trace Element Geochemistry,“Geochemistry really is for everyone!” By Fersman (1958),4. 微量元素在各主要地质体中的分布,矿物 岩石 地球化学储库,主要造岩矿物,橄榄石 单斜辉石 斜方辉石 石榴石 尖晶石 斜长石,橄榄石,橄榄石,橄榄石,斜方辉石,斜方辉石,单斜辉石,单斜辉石,单斜辉石,单斜辉石,石榴石,0.01,0.1,1,10,Li,Sc,V,Cr,Co,Ni,Cu,Ga,原始地幔归一化值,Rb,Sr,Y,Zr,Nb,Cs,Ba,La,Ce,Pr,Nd,Sm,Eu,Gd,Tb,Dy,Ho,Er
2、,Tm,Yb,Lu,Hf,Ta,Pb,Th,U,石榴石,尖晶石,斜长石,问题1,如果亏损地幔由Ol+Opx+Cpx+Gt+Sp组成，那么： 元素Yb主要分布在哪些种矿物中？ 元素La主要分布在哪些种矿物中？ 元素Ni主要分布在哪些种矿物中？ 元素Zr主要分布在哪些种矿物中？,问题2,榴辉岩主要由Cpx+Gt组成，那么： 元素Yb主要分布在哪种矿物中？ 元素La主要分布在哪种矿物中？ 元素Ni主要分布在哪种矿物中？ 元素Zr主要分布在哪种矿物中？ 如果该榴辉岩中除了Cpx+Gt以外，还发现少量磷灰石，那么上述元素的分布情况如何？,问题3,如果某麻粒岩由Cpx+Opx+Pl组成，那么： 元素Yb主
3、要分布在哪种矿物中？ 元素La主要分布在哪种矿物中？ 元素Ni主要分布在哪种矿物中？ 元素Sr主要分布在哪种矿物中？ 如果该麻粒岩中除了Cpx+Opx+Pl以外，还发现少量Gt，那么上述元素的分布情况如何？,主要岩石,大洋中脊玄武岩（MORB） 洋岛玄武岩（OIB） 大陆溢流玄武岩（CFB） 岛弧玄武岩（IAB）,Mid-Ocean Ridge Basalts (MORB),Mid-Ocean Ridge Basalts (MORB),Figure 13-1. After Minster et al. (1974) Geophys. J. Roy. Astr. Soc., 36, 541-57
4、6.,MORB Chemistry,MORBs are dominantly olivine tholeiites with distinctively low K2O ( 0.2%). All incompatible trace elements (LREE, K, Rb, Cs, Ba, Pb, Sr, Th, U, Ce, Zr, Hf, Nb, Ta, and Ti) are depleted. No depletion in HREE elements indicating relatively shallow melting within the spinel stability
5、 field (70 depth). Sr and Nd isotope ratios are depleted indicating a depleted upper mantle source region.,Strong depletions in highly incompatible trace elements (REE, LILE) indicating “depleted” mantle source.,No depletion in HREE indicating relatively shallow melting depths (70 km). Why?,MORB Che
6、mistry,MORB Petrogenesis,Decompression partial melting associated with near-adiabatic rise of mantle due to plate separation. N-MORB melting initiated 60-80 km depth in upper depleted mantle where it inherits depleted trace element and isotope signatures.,Oceanic Island Basalts (OIB),Oceanic Island
7、Basalts (OIB),OIB Chemistry,OIBs are range from olivine tholeiites to highly alkaline basalts with relatively high TiO2, Na2O, K2O, and P2O5. All incompatible trace elements (LREE, K, Rb, Cs, Ba, Pb, Sr, Th, U, Ce, Zr, Hf, Nb, Ta, and Ti) are enriched in OIB magmas with respect to MORBs. HREE elemen
8、ts are depleted indicating deep melting within the garnet stability field (70-80 depth). Sr and Nd isotope ratios are relatively enriched with respect to MORBs indicating undepleted (primitive) and/or enriched mantle source components.,Types of OIB Magmas,1. Tholeiitic series (dominant type) “Shield
9、-building stage” - tremendous outpourings of tholeiitic basalts,Types of OIB Magmas,2. Alkaline series (small volumes),“Post-shield stage” - Waning activity. Lavas are more diverse, with a larger proportion of chemically differentiated magmas,Strong enrichment in highly incompatible trace elements (
10、REE, LILE, HFSE) indicating undepleted (primitive) and/or “enriched” mantle sources,Depletion in HREE indicating relatively deep melting depths (80 km) ，Why？,OIB Chemistry,OIBs: 87Sr/86Sr = 0.7035 to 0.710 and Nd = +5 to -5, primitive and/or enriched mantle source(s),Figure 14-6. After Zindler and H
11、art (1986), Staudigel et al. (1984), Hamelin et al. (1986) and Wilson (1989).,OIB Petrogenesis,Adiabatic decompression melting associated with deep upwelling of anomalously hot (+100-200 C) mantle. Partial melting initiated 100 km depth well within garnet stability field (explains common HREE deplet
12、ion). Relatively enriched trace element and isotope signatures indicate enriched mantle source components.,DM,OIB,Continental Reservoirs,EM and HIMU from crustal sources (subducted OC + CC seds),OIB Petrogenesis,Continental Flood Basalts (CFB),Large Igneous Provinces (LIPs) Oceanic plateaus Some rif
13、ts Continental flood basalts (CFBs),Figure 15-1. Columbia River Basalts at Hat Point, Snake River area. Cover of Geol. Soc. Amer Special Paper 239. Photo courtesy Steve Reidel.,Tectonic Setting of CFBs,Continental hot spots Columbia River Plateau Yellowstone Deccan Traps Continental rifting Parana-E
14、ntendeka CAMP (Central Atlantic Magmatic Province),CFB Chemistry,CFBs are mostly tholeiitic and similar to OIB Incompatible trace elements and isotopes are enriched, like OIB BUT show much more variability toward more enriched compositions. Distinctive enrichments of the most highly incompatible ele
15、ments (K, Ba, Rb, Th, Pb, and LREE) over typical OIB. Noticeable depletions in HFSE (Nb, Ta) compared to OIB. Sr and Nd isotope ratios overlap with OIB, but extend to more enriched compositions (higher 87Sr/86Sr and lower Nd).,CFB Chemistry,Strong enrichments in highly incompatible trace elements (R
16、EE, LILE) indicating enriched mantle source(s),No depletion in HREE indicating relatively shallow melting depths (70 km),Distinctive ernirchments in Ba and Pb and depletions in HFSE (Nb, Ta) compared to OIB.,CFBs: 87Sr/86Sr = 0.7035 to 0.713 and Nd = +5 to -10, enriched mantle source(s),CFB Petrogenesis,Basically OIB petrogenesis + added components from conti
