Mid-ocean ridge peridotites record significantly greater variability in major and trace elements, isotopic compositions, and thermodynamic properties such as oxygen fugacity (f_O2) than do their basaltic counterparts. This variability may derive from modern ridge processes related to melting and melt-rock interaction or from long-lived source heterogeneity related to recycled material or ancient melting events. In this study, we investigate variations in spinel geochemistry as well as silicate major and trace element chemistry and oxygen fugacity of a suite of peridotites from a single segment of the Southwest Indian Ridge (SWIR). We present new petrographic analysis and trace element data for samples with previously-published f_O2 results and combine this with new data for a suite of SWIR gabbro-veined peridotites. We find that SWIR residual lherzolites record low spinel Cr# (Cr# = 100*Cr/(Cr+Al) < 30) and represent low to moderate degrees of melting (∼5-8%) beneath the ridge axis, with no change in oxygen fugacity during melting. In contrast, a subset of SWIR peridotites with high spinel Cr# (Cr#>30) record both melt extraction as well as melt-rock interaction. In these samples, spinel Cr# has been substantially elevated by reaction of spinel to form plagioclase during melt addition, complicating the use of spinel Cr# in mid-ocean ridge peridotites as a proxy for degree of melt extraction alone. While spinel Cr# remains a robust proxy for melt extraction within residual, non-melt-influenced samples, mid-ocean ridge peridotites must first be evaluated to ensure that modification by melt-rock reaction has not occurred. Although addition of MORB melt to a peridotite residue modifies spinel Cr#, this melt addition does not result in significant changes to the f_O2 recorded by the peridotite. Residual SWIR lherzolites record f_O2 of 0.66±0.39 relative to the quartz-fayalite-magnetite buffer (QFM), statistically indistinguishable from melt-influenced and veined SWIR samples (QFM+1.13±0.61). In contrast to other tectonic settings, such as subduction zones, ocean islands, and continental cratons—locations where peridotite is oxidized by petrogenetically unrelated, presumably high-f_O2 melts/fluids—ridge peridotites interact with MORB, which has little to no oxidizing power over its own mantle residues. Thus, modern processes beneath the ridge modify peridotite major and trace elements, but do not generate variability in oxygen fugacity.

中海洋脊橄榄石的玄武岩中主要元素和微量元素，同位素组成以及热力学性质（如氧逸度（f _O2））的变化明显大于其玄武岩。这种变异性可能源自与熔融和熔融岩石相互作用有关的现代山脊过程，也可能源自与回收材料或古代熔融事件有关的长寿命烃源异质性。在这项研究中，我们调查了西南印第安海岭（SWIR）单个区段中一组橄榄岩的尖晶石地球化学以及硅酸盐主要和微量元素化学和氧逸度的变化。我们为以前发布的f _O2样品提供了新的岩相分析和痕量元素数据结果，并将其与新数据结合在一起，获得了一套SWIR辉长岩脉橄榄岩。我们发现SWIR残留的锂铁矿记录了低尖晶石Cr＃（Cr＃= 100 * Cr /（Cr + Al）<30）并代表了脊轴下方的中低熔融度（〜5-8％），没有变化在熔化过程中的氧气逸度。相反，具有尖晶石Cr＃（Cr＃> 30）的SWIR橄榄岩的一个子集既记录了熔体抽采，又记录了熔体与岩石的相互作用。在这些样品中，尖晶石Cr＃在熔体添加过程中通过尖晶石反应形成斜长石酶而显着升高，这使尖晶石Cr＃在大洋中脊橄榄石中的使用复杂化，从而只能单独提取熔体。尽管尖晶石Cr＃仍然是残留物（不受熔体影响的样品）中熔体萃取的有力替代品，中洋脊橄榄岩必须首先进行评估，以确保没有发生熔岩反应引起的变质。尽管将MORB熔体添加到橄榄岩残余物中会改变尖晶石Cr＃的含量，但这种熔体添加并不会导致熔体的显着变化。f橄榄石记录的_O2。相对于石英-铁橄榄石-磁铁矿缓冲液（QFM），残留的SWIR锂铁矿的f _O2记录为0.66±0.39，与受熔体影响和脉动的SWIR样品（QFM + 1.13±0.61）在统计上没有区别。与俯冲带，大洋洲和大陆克拉通等其他构造环境相反，橄榄岩被岩石成因无关的，可能是高f _O2熔体/流体氧化的位置，山脊橄榄岩与MORB相互作用，而MORB几乎没有氧化能力覆盖其自身的地幔残留物。因此，山脊下面的现代过程改变了橄榄岩的主要元素和痕量元素，但不会产生氧气逸度的变化。