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Geochemical Heterogeneity Within Mid-Ocean Ridge Lava £Ows: Insights Into Eruption, Emplacement and Global Variations in Magma Generation

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Geochemical Heterogeneity Within Mid-Ocean Ridge Lava £Ows: Insights Into Eruption, Emplacement and Global Variations in Magma Generation Earth and Planetary Science Letters 188 (2001) 349^367 www.elsevier.com/locate/epsl Geochemical heterogeneity within mid-ocean ridge lava £ows: insights into eruption, emplacement and global variations in magma generation K.H. Rubin a;*, M.C. Smith a, E.C. Bergmanis a, M.R. Per¢t b, J.M. Sinton a, R. Batiza a;c a b c Received 5 September 2000; accepted 28 March 2001 Abstract Compositional heterogeneity in mid-ocean ridge (MOR) lava flows is a powerful yet presently under-utilized volcanological and petrological tracer. Here, it is demonstrated that variations in pre- and syn-eruptive magmatic conditions throughout the global ridge system can be constrained with intra-flow compositional heterogeneity among 10 discrete MOR flows. Geographical distribution of chemical heterogeneity within flows is also used along with mapped physical features to help decipher the range of conditions that apply to seafloor eruptions (i.e. inferred vent locations and whether there were single or multiple eruptive episodes). Although low-pressure equilibrium fractional crystallization can account for much of the observed intra-flow compositional heterogeneity, some cases require multiple parent magmas and/or more complex crystallization conditions. Globally, the extent of within-flow compositional heterogeneity is well correlated (positively) with estimated erupted volume for flows from the northern East Pacific Rise (EPR), and the Mid Atlantic, Juan de Fuca and Gorda Ridges; however, some lavas from the superfast spreading southern EPR fall below this trend. Compositional heterogeneity is also inversely correlated with spreading rate. The more homogeneous compositions of lavas from faster spreading ridges likely reflect the relative thermal stability and longevity of sub-ridge crustal magma bodies, and possibly higher eruption frequencies. By contrast, greater compositional heterogeneity in lavas at slower spreading rates probably results from low thermal stability of the crust (due to diminished magma supply and greater hydrothermal cooling). Finally, the within-flow compositional variations observed here imply that caution must be exercised when interpreting MOR basalt data on samples where individual flows have not been mapped because chemical variations between lava samples may not necessarily record the history of spatially and temporally distinct eruptions. ß 2001 Elsevier Science B.V. All rights reserved. eruptions; lava £ows; igneous rocks; geochemistry; magmas; mid-ocean ridge basalts; petrology; submarine volcanoes; volcanism * Corresponding author. Tel.: +1-808-956-8973; Fax: +1-808-956-5512; E-mail: [email protected] 0012-821X / 01 / $ ^ see front matter ß 2001 Elsevier Science B.V. All rights reserved. PII: S0012-821X(01)00339-9 EPSL 5849 17-5-01 350 1. Introduction and background a fraction of those have the geologic information necessary to place them within the context of a Mid-ocean ridges (MORs) are dynamic vol- speci¢c eruption. canic environments. Although remote and di¤cult Since the earliest studies of oceanic volcanism, to observe, MOR volcanism produces lavas that MORBs have been known to be less composition- cover roughly 2/3 of Earth's surface, and MOR ally variable than lavas from other Earthly vol- crust is an important chemo-physical interface be- canic terrains (including subaerial sites of primar- tween the mantle, ocean, and marine biosphere ily tholeiitic extrusion). The lack of broad (e.g. [2^4]). compositional variability coupled with the di¤- To completely understand the physical and culty in obtaining samples with a clearly de¢ned chemical history of MORs, it is important to context has resulted in spatial variation in com- study them over a range of temporal and geo- positions of individual MORB samples commonly graphic length scales, including those related to being used literally to ascribe spatial and/or tem- the emplacement of individual eruptive units. poral variability to petrogenetic processes and/or MOR lava £ows, like those in other volcanic ter- mantle compositions. There has been little at- rains, have complex emplacement histories and tempt to distinguish petrogenetic heterogeneity preserve information about processes from initial between £ows from heterogeneity within a £ow. melt generation to post-emplacement cooling. Yet, the signi¢cance of broader scale spatial and Through comparisons of individual £ows from temporal variations in lava composition cannot be di¡erent MORs, intra-£ow compositional hetero- completely assessed without understanding both geneity is used here to constrain variations in the e¡ects of the eruptive process itself (in terms magmatic/petrologic conditions through the glob- of creating or destroying magmatic compositional al ridge system. Furthermore, the spatial distribu- variability) and the extent to which the composi- tion of chemical heterogeneity within a £ow is tional variability in MORB is accounted for by used along with mapped physical features to pre- and post-eruptive petrologic histories of sin- help decipher the cryptic history and range of gle eruptive units. conditions that apply to eruption and emplace- An active MOR eruption has yet to be wit- ment of speci¢c submarine lava £ows. nessed. Still, the range of geomorphic features Numerous studies of MOR basalt (MORB) that constitute an individual submarine lava compositions over the past four decades have pro- £ow have recently begun to be recognized vided insights into the formation of the oceanic through a combination of detailed near-bottom lithosphere and, because MORB is largely unaf- surveying, active monitoring and fortuitous arriv- fected by crustal contamination, how mantle com- al at the scene of recent volcanism (e.g. [8^12]). positions/heterogeneities are sampled by volcan- Such features are easiest to identify in very recent ism (e.g. [5^7] and references therein). However, lavas because weathering and sedimentation can MOR data have mostly been collected at coarse/ rapidly obscure £ow contacts between older lavas. incompletely resolved length and spreading time As a result, the database of mapped discrete sub- scales (10 s of km and kyr), without the bene¢t of marine lava £ows (or £ow sequences) is still small detailed geological maps needed to place volcanic and limited primarily to young £ows (e.g. [7,13]). and other structures in the context of individual We know little of the composition, mode of em- eruptions. MOR volcanoes are relatively inacces- placement or aerial extent of s 99% of the lava sible and are primarily sampled from surface ships £ows within even the most active/youngest region (e.g. wax coring or rock dredging), which is of the global MOR system. In most cases, there broadly equivalent to sampling Hawaiian volca- are inadequate vertical cross-sectional exposures noes from a helicopter £ying at 2 km altitude. with which to identify physical variations arising Only a small percentage of MORB samples in from multiple emplacement lobes [14] and in£a- the literature have been collected using in situ tion/cooling episodes [15]. Instead, MOR £ows observation (by submersible or ROV), and only are typically observed and sampled from the EPSL 5849 17-5-01 351 upper crust or in collapse features. Nevertheless, implies no compositional heterogeneity outside of detailed study of the MOR lava £ow database analytical uncertainty. Higher values indicate an- provides useful, albeit preliminary, insights into alytically signi¢cant compositional variation. HI MOR processes. is referred to here as the `heterogeneity index', since this name seems more appropriate for an index for which larger values correspond to great- 2. Compositional heterogeneity of subaerial and er compositional variance. MORB lava £ows The original HI study included 10 major ele- ments (Si, Ti, Al, Fe, Mn, Mg, Ca, Na, K, P) Physically, most lava £ows are complex, with and seven trace elements (Nb, Zr, Sr, Zn, V, Ni, few, if any, known examples of a simple basaltic Cr) analyzed by XRF on 80^100 g rock powders £ow (one that is not divisible into £ow units hav- of 5^12 samples per Mauna Loa £ow [1]. HI ing distinguishable extrusion and cooling histories ranged from 1.1 to 13.9, except for picrite erup- [14]). Not surprisingly, their compound character tions in 1852 (HI = 30.3) and 1868 (HI = 34.0) that leads to small-scale internal compositional varia- had copiously accumulated olivine. Composition- tions. Early works on subaerial basalt £ows de- al variability was not a simple function of em- ¢ned heterogeneity by variations in phenocryst placement rate or time, but larger area £ows populations (e.g. [14,16]) and liquid chemistry were typically more heterogeneous [1]. Roughly (e.g. [16^18]), and led some authors to caution 50% and V85% of the compositional heterogene- against characterizing lava £ow composition ity in the non-picrites and the picrites could with only one or a few samples. Such heterogene- be mathematically `eliminated' by correcting to ity was attributed to a combination of emplace- constant MgO for low-pressure crystal frac- ment mechanism, assimilation, £ow di¡erentia- tionation or accumulation (with only olivine on tion, in situ fractionation, localized oxidation, the liquidus) [1]. The remaining heterogeneity zoned magma chambers and random mixing. (HI = 1^5) was presumed to re£ect other pro- These early studies also noted that although cesses. Some Kilauea lavas with smaller £ow areas only loosely quanti¢ed, analytical
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