Comparisons of z values between fine-grained titanomagnetites within interstitial glass and large-grained titanomaghemtes in older ocean-floor basalts

Transmission electron microscopy observations and rock magnetic measurements reveal that alteration of fine- and large-grained iron-titanium oxides can occur at different rates. Fine-grained titanomagnetite occurs as a crystallization product within interstitial glass that originated as an immiscible liquid within a fully differentiated melt; in several samples with ages to 32 Ma it displays very little or no oxidation (z = ca. 0). In contrast, samples with ages of 10 Ma or older are observed to also contain highly oxidized (z >/= 0.66) large-grained titanomaghemite. These large grains, having originated by direct crystallization from melt, are associated with pore space. Such pore space can serve as a conduit for fluids that promote alteration, whereas fine grains may have been \"armored\" against alteration by the glass matrix in which they are embedded. Apparently, alteration of oceanic crust is a heterogeneous process on a microscopic scale. The existence of pristine, fine-grained titanomagnetite in the interstitial glass of older ocean-floor basalts that have undergone significant alteration implies that such glassy material is capable of carrying original thermal remanent magnetization and may be suitable for paleointensity determinations.

透射电子显微镜（Transmission electron microscopy）观测与岩石磁学测量结果显示，细粒与粗粒铁钛氧化物的蚀变作用速率存在差异。细粒钛磁铁矿（titanomagnetite）作为结晶产物赋存于填隙玻璃中，该玻璃源自完全分异熔体中的不混溶液相；对多个年龄可达32 Ma的样品进行分析，其氧化程度极低甚至未发生氧化（z ≈ 0）。与之相反，年龄≥10 Ma的样品中还存在高度氧化（z ≥ 0.66）的粗粒钛磁赤铁矿（titanomaghemite）。这些粗粒颗粒直接由熔体结晶形成，与孔隙空间伴生。此类孔隙空间可作为流体运移通道，促进蚀变作用；而细粒颗粒则被其所赋存的玻璃基质包裹防护，免受蚀变影响。由此可见，洋壳的蚀变作用在微观尺度上具有非均质性。经强烈蚀变的老洋底玄武岩的填隙玻璃中，仍存在未受蚀变的细粒钛磁铁矿，这表明此类玻璃质物质能够保留原生热剩磁，且可用于古强度测定。