Basalt petrogenesis beneath slow- and ultraslow-spreading Arctic mid-ocean ridges

Abstract

To explore the ability of melting mafic lithologies to produce alkaline ocean-island basalts (OIB), an experimental study was carried out measuring clinopyroxene (Cpx)-melt and garnet (Gt)-melt partition coefficients during silica-poor garnet pyroxenite melting for a suite of trace elements, including U and Th, at 2.5GPa and 1420-1450'C.Partition coefficients range from 0.0083+0.0006 to 0.020+0.002 for Th and 0.0094+0.0006 to 0.024+0.002 for U in Cpx, and are 0.0032+0.0004 for Th and 0.013+0.002 for U in Gt. Forward-melting calculations using these experimental results to model time-dependent uranium-series isotopes do not support the presence of a fixed quantity of garnet pyroxenite in the source of OIB.To use U-series isotopes to further constrain mantle heterogeneity and the timing and nature of melting and melt transport processes, U-Th-Pa-Ra disequilibria, radiogenic isotopes, and trace-element compositions were measured for the slow-spreading Arctic mid-ocean ridges (MOR). A focused case study of 33 young (<10ka) MOR basalts (MORB) from the shallow endmember of the global ridge system, the Kolbeinsey Ridge ACKNOWLEDGMENTSMy deepest thanks, first and foremost, must go to Ken Sims, who taught me so much of what I know about mid-ocean ridge basalts, uranium-series chemistry, sample preparation, and thinking like a scientist. Ken was the most attentive and caring advisor, checking on me regularly to keep me on task and check in, but letting me take time to myself (and telling me to take vacations) when I needed it. Ken sent me all over the world to learn techniques in the best places from some of the best scientists, he challenged me to think things through, and he made sure I always had the resources I needed to excel. I could never have hoped for a more thoughtful, caring mentor.Glenn Gaetani taught me, step by patient step, how to construct and run my own pistoncylinder experiments. Glenn always had words of encouragement to keep me going despite all the difficult failures experimental work can sometimes entail. I got to see firsthand how methods are tested and developed in an experimental lab, and take part in that process. It was an invaluable learning experience, and it has left me with admiring respect for those lifelong experimentalists who love unwrapping their presents after an experiment is done, and who don't mind taking the occasional shower in coolant or finding anti-seize on the back of their elbow in bed at night.I also want to thank the rest of my committee. Fred Frey, my advisor at MIT, was there to teach me and help me think critically about my research throughout my time in the Joint Program. Jian Lin, from Geodynamics project advisor to exam committee chair to a thesis committee member, provided guidance and always reminded me to think beyond chemistry to the big picture questions involved. Peter Kelemen was always excited to talk about Kolbeinsey data and to share his innovative ideas, and he challenged me in committee meetings to think about everything in more depth. Susan Humph...

Effusive and explosive volcanism on the ultraslow‐spreading Gakkel Ridge, 85°E

“…All glasses sampled on Oden and Loke [cf. Shaw et al, 2010] and volcaniclastic samples (Figure 1a) are age-constrained by U-series disequilibria (i.e., 226 Ra/ 230 Th = 3.07 to 3.65 AE 3%) to less than 8000 years, and one volcaniclastic sample (Sample 13-1) is age-constrained (i.e., 210 Pb/ 226 Ra = 0.89 AE 9%) to less than 100 years old [Elkins, 2009]. Thus, we conclude that the large region of high intensity backscatter from SCICEX 1999 represents a zone of seafloor (up to 1 Myr in age) with thin pelagic sediment coverage and only a small area of fresh lavas on the southern lineament may be as recent as the 1999 seismic swarms.…”

Temporal and petrogenetic constraints on volcanic accretionary processes at 9-10 degrees north East Pacific Rise

“…We note that this trend has previously been interpreted as a mixing trend and since the denominators for the X-and Y-axes are not equivalent, we should expect mixing to be best approximated by a hyperbola. However, fitting a hyperbola to these data requires confidence in our knowledge of the end member mixing components [Sohn, 2005], but these end members are poorly constrained by few samples with known initial ( 226 Ra/ 230 Th) [cf., Elkins, 2009;Waters et al, in review]. Thus, we approximate the mixing trend with a line, noting that fitting a binary mixing curve through these "zero-age" data [Sohn, 2005] using two (X,Y) coordinates calculated from the line fit for end member ratios results in a curvature of 0.967 ± 0.07.…”