MgO Wt Research Articles

Geological indexes of hydrothermal venting

Hydrothermal venting occurs on mid‐ocean ridge axes with a diverse array of morphological, structural, and petrological characteristics. Consequently, it is not clear if certain geological environments are more conducive than others to hydrothermal activity. This paper uses complementary data sets recently collected on multiple‐tectonic‐segment scales along intermediate to superfast spreading ridge sections to systematically examine this issue. The spatial density of venting is gauged by plume incidence, the fraction of linear ridge crest overlain by hydrothermal plumes. Five geological indexes are compared with plume incidence: ridge‐axis elevation, ridge‐axis cross‐sectional area, the percentage of ridge axis underlain by an axial magma chamber (AMC) reflector, MgO wt% in basaltic glass, and spreading rate. A mean value for plume incidence and each index, where available, is calculated for each of 14 second‐ to fourth‐order tectonic segments from the Juan de Fuca Ridge (44°30′ to 48°45′N) and the northern (9° to 11°50′N) and southern (13°50′ to 18°40′S) East Pacific Rise. Plumes are most common on segments with a cross‐sectional area >3.5 km2, a net elevation >0.35 km, AMC coverage >60%, and MgO >7 wt%. Spreading rate is not deterministic on the segment scale. The data suggest two classes of ridge segments. Segments with relatively low geological index values have a uniformly low plume incidence indicative of a presently feeble magmatic budget. Those with high index values have a plume incidence ranging from low to complete coverage, apparently a consequence of a magma supply rate sufficient to produce frequent dike intrusions and highly variable hydrothermal activity. The best individual predictors of hydrothermal activity are cross‐sectional area and net elevation. Applying these indexes to the East Pacific Rise between 18°N and 33°S suggests that the unexplored segments with the highest probability of activity are those at 15°30′–16°N, 6°–9°N, 5°–6°S, 27°–28°S, and 30°30′–33°S.

Ni-rich olivine in minettes from Two Buttes, Colorado: A connection between potassic melts from the mantle and low Ni partition coefficients

Olivine minettes from Two Buttes, Colorado, contain diverse populations of olivine, some of which is unusually rich in Ni. Olivine compositions range from Fo 83 to Fo 94 with 0.13 to 0.64 wt% NiO. Host minettes have 15.5 to 17.9 wt% MgO and Ni (ppm)/MgO (wt%) of 25 to 32. Ni-rich olivine grains have large homogeneous cores ( 1300–2900 μm diameter, 0.55-0.64 wt% NiO) with thin rims (50–150 μm) that are dramatically poorer in Ni and Mg and richer in Ca, Mn, and Fe. The olivine cores are too rich in Ni to be xenocrysts from normal peridotite. K D olivine-bulk rock Fe Mg values are 0.35 and 0.29, consistent with crystallization of Ni-rich olivine from the host minettes. The association confirms a genetic relationship between Ni-rich olivine and potassic, mafic magmas. High potassium and volatiles may have inhibited polymerization of the melts and caused K D olivine- melt (Ni) to be low during partial melting, resulting in high Ni contents of the magma as suggested for Spanish lamproites by Venturelli et al. (1984). Low oxygen fugacities may also have been important. Following crystallization of phlogopite and loss of volatiles, the Ni-rich olivine may have crystallized. Numerical simulations of Ni diffusion in olivine constrain times for magma evolution and for formation of intricate oscillatory zoning in diopside phenocrysts that coexist with the Ni-rich olivine. The steep gradients at olivine rims can be simulated in less than 1 year at 1250°C, and so the homogeneous high-Ni cores could have existed in the more evolved melt for only a short time. The Ni-rich olivines are evidence that the K- and Mg-rich bulk rock compositions at Two Buttes represent near-primary magmas.