Diamond precipitation and mantle metasomatism – evidence from the trace element chemistry of silicate inclusions in diamonds from Akwatia, Ghana
Diamond precipitation and mantle metasomatism – evidence from the trace element chemistry of silicate inclusions in diamonds from Akwatia, Ghana
复制标题
钻石沉淀和地幔交代作用——来自加纳阿夸蒂亚钻石硅酸盐包裹体微量元素化学的证据
DOI：
10.1007/s004100050328
复制
发表时间：
1997
影响因子：
3.5
通讯作者：
J. Harris
中科院分区：
文献类型：
--
作者：
T. Stachel;J. Harris
摘要 使用 SIMS 测定了产自 Akwatia（加纳 Birim Field）的钻石中四种主要橄榄岩硅酸盐相（石榴石、橄榄石、斜方辉石、单斜辉石）中的微量元素浓度。不相容的微量元素存在于石榴石和单斜辉石中，但在方辉石共生钻石中，Sr 均匀分布在斜方辉石和石榴石之间。二辉橄榄石和方辉橄榄石包裹体之间的分离通常是使用 CaO 与 Cr2O3 图中石榴石的成分场进行的，从橄榄石和石榴石中的 Ti 和 Sr 含量也可以明显看出。二辉橄榄岩包裹体中的钛含量要高得多，而方辉橄榄岩包裹体中的锶含量要高得多。二辉橄榄石榴石的球粒陨石归一化 REE 模式在 HREE (LaN/YbN = 0.02–0.06) 中富集（球粒陨石的 10–20 倍），而方辉石榴石具有正弦 REEN 模式，其中 Ce 和 Nd 浓度最高（球粒陨石的 2–8 倍），Ho 浓度最低（0.2–0.7 倍）球粒状)。单斜辉石包裹体显示出负斜率，La 富集度是球粒状的 10-100 倍，而 Lu 的富集度是球粒状的 0.1-1 倍。具有非常高的角菱沸石含量（分别为 14 和 21 wt% Cr2O3）的二辉橄榄石和方辉橄榄石石榴石，可以通过高得多的轻稀土富集水平轻易地与其共生石榴石区分开来。由二辉橄榄石榴石包裹体计算的熔体成分的 REE 模式属于金伯利岩-菱铁矿和碳酸盐岩熔体的成分领域。由方辉石榴石计算出的稀土元素分馏更加强烈，Ti、Y、Zr 和 Hf 浓度较低，与已知的碱性和碳酸熔体显着不同，需要不同的试剂。方辉橄榄岩包裹体的平衡温度通常低于其共生的 C-H-O 固相线，而二辉橄榄岩包裹体的平衡温度则高于。因此可以排除方斜方晶金刚石从含二氧化碳熔体或流体中结晶的情况。钻石内含物化学和矿物学也与已知的富含水熔体交代作用的例子不一致。因此，我们倾向于通过氧化富含 CH4 的流体来沉淀金刚石，其具有高度分馏的微量元素模式，这可能是由于“色谱”分馏过程造成的。
Abstract Trace element concentrations in the four principal peridotitic silicate phases (garnet, olivine, orthopyroxene, clinopyroxene) included in diamonds from Akwatia (Birim Field, Ghana) were determined using SIMS. Incompatible trace elements are hosted in garnet and clinopyroxene except for Sr which is equally distributed between orthopyroxene and garnet in harzburgitic paragenesis diamonds. The separation between lherzolitic and harzburgitic inclusion parageneses, which is commonly made using compositional fields for garnets in a CaO versus Cr2O3 diagram, is also apparent from the Ti and Sr contents in both olivine and garnet. Titanium is much higher in the lherzolitic and Sr in the harzburgitic inclusions. Chondrite normalised REE patterns of lherzolitic garnets are enriched (10–20 times chondrite) in HREE (LaN/YbN = 0.02–0.06) while harzburgitic garnets have sinusoidal REEN patterns, with the highest concentrations for Ce and Nd (2–8 times chondritic) and a minimum at Ho (0.2–0.7 times chondritic). Clinopyroxene inclusions show negative slopes with La enrichment 10–100 times chondritic and low Lu (0.1–1 times chondritic). Both a lherzolitic and a harzburgitic garnet with very high knorringite contents (14 and 21 wt% Cr2O3 respectively) could be readily distinguished from other garnets of their parageneses by much higher levels of LREE enrichment. The REE patterns for calculated melt compositions from lherzolitic garnet inclusions fall into the compositional field for kimberlitic-lamproitic and carbonatitic melts. Much more strongly fractionated REE patterns calculated from harzburgitic garnets, and low concentrations in Ti, Y, Zr, and Hf, differ significantly from known alkaline and carbonatitic melts and require a different agent. Equilibration temperatures for harzburgitic inclusions are generally below the C-H-O solidus of their paragenesis, those of lherzolitic inclusions are above. Crystallisation of harzburgitic diamonds from CO2-bearing melts or fluids may thus be excluded. Diamond inclusion chemistry and mineralogy also is inconsistent with known examples of metasomatism by H2O-rich melts. We therefore favour diamond precipitation by oxidation of CH4-rich fluids with highly fractionated trace element patterns which are possibly due to “chromatographic” fractionation processes.