RELATIONSHIPS BETWEEN ROUTING OF MAGMA AND SILICA DIAGENESIS IN THE HOT, SHALLOW SUBSEAFLOOR OF A YOUNG RIFT BASIN (GUAYMAS BASIN, GULF OF CALIFORNIA)

The diagenesis of biogenic silica from amorphous opal-A to crystalline opal-CT is arguably one of the most significant early diagenetic processes in the shallow subseafloor. During silica diagenesis, soft and watery oozes are transformed into harder sedimentary rocks, and this transformation can cause regional-scale differential compaction, subsidence, and the expulsion of pore fluids. Most importantly, it corresponds to changes in sediment rheology from ductile oozes to brittle, fracture- prone, less porous and more permeable sedimentary rocks.

We discuss the potential implications of these rheologic changes on the routing of magma in a young rift basin based on the analysis of off-axis boreholes drilled by the International Ocean Discovery Program Expedition 385 in the actively spreading, intrusive sill-riddled Guaymas Basin at the Gulf of California (Mexico). At Sites U1545, U1546 and U1547 which are characterized by steep geothermal gradients (~135–510 °C/km) and extremely high sedimentation rates (~1 m/kyr), the conversion from opal-A to opal-CT occurs in unexpectedly hot (in situ temperatures of ~74–79 °C) subseafloor conditions. This observation indicates a significantly slower reaction kinetics of biosilica transformation than previously reported.

At Site U1545, where there is no evidence of sill-related metamorphic overprint, XRD data show that the crystallographic ordering (d-spacing) of the opal-CT (101) peak correlates linearly with in situ temperature between (~75 and 110 ºC) throughout the opal-CT zone, thus, providing a silica paleothermometry proxy. We used the paleothermometer to estimate the max temperatures experienced by the sediments in the contact zones at Site U1546 (which includes a ~70 m-thick sill intrusion), and at Site U1547, where the top of a massive sill was recovered at shallower depths.

Finally, we interpret the relationships between the degree of crystallization of the opal-CT sediments in the contact zone and their cross-cutting relationships with the sill to speculate that the sill formation postdates the silica phase change and to hypothesize that the opal-A/opal-CT transition zone acts as major physical anisotropy in the sedimentary column that reroutes magma from vertical to lateral movement, ultimately constraining the magmatic activity to plutonic.