Major-Element Chemistry of Basaltic Glasses Recovered during Deep Sea Drilling Project Leg 64

Abstract

Fourteen microprobe analyses of basaltic glasses from three drill sites plus two dredged samples from the seafloor at the mouth of the Gulf of California suggest that magmatic liquids that cooled to form the volcanic crust in the Gulf of California have been fractionated, whereas other, more primitive submarine basalts from the Mid-Oceanic Ridge (MOR) have not. The limited chemical diversity among drill sites suggests that magmatic liquids, which erupted along the accretionary boundary in the Gulf of California, are grossly similar. Major-element variations in the glasses from Hole 474A may, in part, however, be attributed to simple crystal-liquid fractionation in a subridge magma chamber. Additionally, the analysis of a glass from Hole 475B, near the eastern continental slope of southernmost Baja California, suggests that magmatic liquids which produced submarine basalts associated with initial opening of the Gulf may have been chemically less evolved than present ones. Trace-element concentrations (Saunders et al., this volume, Pt. 2 [hereafter "this volume"]) in whole rocks from Hole 475B suggest that this magma may have experienced minor sialic contamination on its ascent through lower crustal continental material. Whole-rock and glass major-element concentrations, however, suggest that early Pliocene(?) basalts that erupted in Hole 475B were derived from a magmatic liquid of (deep) mantle origin. The liquid quickly rose to the surface with little shallow-level fractionation.

Petrology of Alarcon Rise lavas, Gulf of California: Nascent intracontinental ocean crust

“…Sediment assimilation may have also created additional complexities in the composition of Guaymas Basin magmas although this has not been investigated in detail [cf. Saunders et al , 1982a, 1982b; Fornari et al , 1982]. The facts that some of the Isla Tortuga lavas may have assimilated Na 2 O from halite deposits and that their 87 Sr/ 86 Sr ratios generally increase with fractionation [ Batiza , 1977] are consistent with this interpretation.…”

“…A part of the scatter in the data for Alarcon samples, however, is difficult to quantify using simple melting and crystallization modeling techniques. Specifically, the scatter of major element oxides in Figure 3 and trace elements in Figure 4 requires multiple liquid lines of descent or magma mixing processes or both, similar to those invoked for the wide compositional variation of Guaymas Basin lavas [e.g., Fornari et al , 1982; Perfit et al , 1982]. These additional processes may be due to the fact that magmatic differentiation in small, nascent oceanic basins is different from that in open ocean ridge systems or mature marginal basins.…”

“…Sediment assimilation may have also created additional complexities in the composition of Guaymas Basin magmas although this has not been investigated in detail [cf. Saunders et al, 1982aSaunders et al, , 1982bFornari et al, 1982]. The facts that some of the Isla Tortuga lavas may have assimilated Na 2 O from halite deposits and that their Graham et al, 1988] and field for the EPR MORB Ito et al, 1987;Mahoney et al, 1994;Castillo et al, 1998Castillo et al, , 2000 are shown for reference.…”