Quantifying water contents in the lithospheric mantle is key to our understanding of global geodynamics, mantle composition, and related physical properties. Most mantle lithologies (peridotite) contain little water (~50 ppm), but petrological heterogeneities such as pyroxenites are more hydrous (~300 ppm) relative to the mantle rocks. Pyroxenites also melt at lower temperatures than peridotites and are thus important to magma genesis. Thus, quantifying pyroxenite water contents provides new information on the distribution of water in the mantle. Here, we present phase-specific FTIR measurements of the water contents in pyroxenite mantle xenoliths from two continental lithospheric domains that experienced intense metasomatism: the French Massif Central (FMC, France) and the Adamawa Volcanic Plateau (AVP, Cameroon). The AVP garnet pyroxenites are more hydrated ([H₂O]_{Clinopyroxene} = 386–685 ppm; [H₂O]_{Orthopyroxene} = 124–155 ppm; [H₂O]_Garnet < 0.5 ppm) than FMC ones ([H₂O]_{Clinopyroxene} = 112–465 ppm; [H₂O]_{Orthopyroxene} = 61–104 ppm; [H₂O]_Garnet < 0.5 ppm). These water concentrations are homogenous at the grain and correlate with equilibrated major element concentrations, indicating that they are representative of lithospheric water, although the FMC pyroxenites were dehydrated during metasomatism by a carbonatitic fluid (based on the correlation between La_N/Sm_N and Ti/Eu ratios); the water contents of AVP pyroxenites were likely not affected by metasomatism. FMC pyroxenites show peculiar FTIR spectra that may reflect the preferential dehydration of specific sites in the pyroxene structure. In both regions, metasomatism modified the light rare Earth element contents (e.g., Ce) of the pyroxenites, resulting in highly variable H₂O/Ce ratios. Therefore, we conclude that the utility of the H₂O/Ce ratio to identify the involvement of pyroxenites in magmas genesis is limited.

量化岩石圈地幔中的水含量是我们了解全球地球动力学，地幔成分和相关物理性质的关键。大多数地幔岩性（橄榄岩）中的水含量很少（约50 ppm），但相对于地幔岩石，岩石异质性（如辉石岩）的含水量更大（约300 ppm）。辉石岩的熔融温度也低于橄榄岩，因此对岩浆成因很重要。因此，定量辉石矿的水含量提供了有关地幔中水分布的新信息。在这里，我们介绍了来自经历了强烈的交代作用的两个大陆岩石圈区域的辉石岩地幔异种岩中水含量的特定阶段FTIR测量：法国地块中部（法国FMC）和阿达玛瓦火山高原（喀麦隆的AVP）。₂ O] _{Clinopyroxene}  = 386–685 ppm; [H ₂ O]_斜 = 124-155 ppm的; [H ₂ O]_石榴石 <0.5ppm的）比FMC酮（[H ₂ O]_单斜 = 112-465 ppm的; [H ₂ O]_斜 = 61-104 PPM; [H ₂ O]_石榴石 <0.5 ppm）表示。这些水的浓度在谷物中是均匀的，并且与平衡的主要元素浓度相关，这表明它们是岩石圈水的代表，尽管在交代过程中，FMC辉石矿被碳酸盐流体脱水（基于La _N / Sm _N之间的关系）。和Ti / Eu比）；AVP辉石的水分含量可能不受交代作用的影响。FMC次辉石显示出独特的FTIR光谱，该光谱可能反映了辉石结构中特定位点的优先脱水。在这两个区域，交代作用改变了辉石岩中轻稀土元素的含量（例如Ce），导致H ₂ O / Ce比率高度可变。因此，我们得出的结论是，H ₂ O / Ce比用于确定辉石岩在岩浆成因中的作用是有限的。