Compositional variation of olivine-chromian spinel in Mg-rich magmas as a guide to their residual spinel peridotites

Arc to rift transitional volcanism in the Santa Rosalía Region, Baja California Sur, Mexico

Postspreading volcanism has built large seamounts and volcanic ridges along the short axes of a highly segmented part of the East Pacific Rise crest that ceased spreading at the end of the middle Miocene, offshore Baja California Sur, Mexico. Lava samples from Rosa Seamount, the largest volcano, are predominantly alkalic basalts, mugearites, and benmoreites. This lavas series was generated through fractional crystallization and is compositionally similar to the moderately alkalic lava series in many oceanic islands. Samples from volcanic ridges at three adjacent failed spreading axes include mildly alkalic, transitional, and tholeiitic basalts and differentiated trachyandesites and andesite. The subtle but distinct petrologic and isotopic differences among the four sites may be due to differences in the degree of partial melting of a common, heterogeneous source. Postspreading lavas from these four abandoned axes off Baja California Sur together with those from other fossil spreading axes and from seamount volcanoes that grew on the East Pacific Rise flanks define a compositional continuum ranging from normal mid-ocean ridge basalt (NMORB)-like to ocean island basalt (OIB)-like. We propose that the compositional spectrum of these intraplate volcanic lavas is due to different degrees of partial melting of the compositionally heterogeneous suboceanic mantle in the eastern Pacific. A large degree of partial melting of this heterogeneous mantle during vigorous mantle upwelling at an active spreading center produces NMORB melts, whereas a lesser degree of partial melting during weak mantle upwelling following cessation of spreading produces OIB-like melts. The latter melts have a low (<8 RA) 3He/4He signature indicating their formation is different from that of OIBs from major “hot spot” volcanoes in the Pacific with high 3He/4He ratios, such as Hawaii and Galapagos.

https://agupubs.onlinelibrary.wiley.com/doi/pdf/10.1029/2010GC003319

Petrology and Sr‐Nd‐Pb‐He isotope geochemistry of postspreading lavas on fossil spreading axes off Baja California Sur, Mexico

Resumen es: Se presentan diagramas de discriminacion para magmas basicos (basaltos normativos en hiperstena, basaltos normativos en nefelina, y traquiandesitas basa

Diagrama Ti - V: una nueva propuesta de discriminación paramagmas básicos en cinco ambientes tectónicos

The Nd-Sr-Pb isotopic composition of Pliocene to Recent abyssal tholeiites from the Gulf of California region reflects the interaction of a “plum-pudding” East Pacific Rise MORB source with the continental lithosphere of the North American plate. Tholeiites on the Guadalupe Ridge and at the Gulf Mouth have refractory, depleted mantle and a “plum” component with 143Nd/144Nd = 0.512975, 87Sr/86Sr = 0.7030 as their source components. The heterogeneous nature of the “plum” component in Pb isotopic composition is consistent with an origin from subducted oceanic crust, although the recycling process is likely short and more consistent with the “marble-cake” rather than “mantle-plume” model. Tholeiites in the Guaymas Basin underwent either <3% contamination with a crustal component having 143Nd/144Nd = 0.51230, 87Sr/86Sr = 0.7074, or <4% mixing with a calc-alkaline melt having 143Nd/144Nd = 0.512850, 87Sr/86Sr = 0.7037 generated from remnants of the sources of previous volcanism in the region. Rifting in the Gul...

The Nd-Sr-Pb Isotopic Record in Abyssal Tholeiites from the Gulf of California Region, Western Mexico: No Evidence for a Gulf Mouth Plume

Tholeiitic basalts were obtained from basaltic basement ranging in age from 6 to 17 m.y. on IPOD/DSDP Leg 63. The main rock types encountered at all sites but 473 are basaltic pillow lavas. Although many of these pillow basalts are highly or moderately altered, fresh glass is usually present. At Site 473, we recovered coarse-grained, massive basalts; no clearly defined pillowed forms were observed. Phenocrysts or microphenocrysts present in the Leg 63 basalts are Plagioclase and clinopyroxene at Site 469; olivine, Plagioclase, and spinel at Site 470; and olivine, Plagioclase, and clinopyroxene at Sites 472 and 473. Olivines of the basalts from Holes 470A and 472 (Fo85_88) are generally more magnesian than those of the Hole 473 basalts (Fo77_81). Also, plagioclases of Holes 470A and 472 basalts (An7O_85) are generally more calcic than those of Holes 469 and 473 basalts (An66_72). Geochemical study of the Leg 63 basalts indicates that in all cases they are large-ion-lithophile (LIL) element depleted tholeiites like typical abyssal tholeiites. In particular, they are very similar in composition to those described from the eastern Pacific, although the degree of iron enrichment found in the Leg 63 basalts is not as extensive as in basalts from the Galapagos spreading center. Hence, the geochemical evidence of the Leg 63 basalts is compatible with their formation at a spreading center. Compositional variations in Leg 63 basalts from any single drill hole is small. Major and trace element data indicate that the samples from Holes 469 and 473 are more fractionated in chemical composition than are the samples from Holes 470A and 472; this compositional variation may be largely ascribed to differences in the extent of shallow-level fractional crystallization of similar parental magma. The Hole 472 samples, however, show a LIL element character distinct from the other Leg 63 samples.

/pdf/petrographic-and-chemical-characteristics-of-abyssal-1e2hegwxyj.pdf

P. Lyman

Papers

Petrographic and chemical characteristics of abyssal tholeiites from Deep Sea Drilling Project Leg 63 off Baja California.