Deeply dredged submarine HIMU glasses from the Tuvalu Islands, Polynesia: Implications for volatile budgets of recycled oceanic crust

Abstract

Pb). Until now, deeply dredged submarine HIMU glasses have not been available, which has inhibited complete geochemical (in particular, volatile element) characterization of the HIMU mantle. We report major, trace and volatile element abundances in a suite of deeply dredged glasses from the Tuvalu Islands. Three Tuvalu glasses with the most extreme HIMU signatures have F/Nd ratios (35.6 6 3.6) that are higher than the ratio (21) for global OIB and MORB, consistent with elevated F/Nd ratios in end-member HIMU Mangaia melt inclusions. The Tuvalu glasses with the most extreme HIMU composition have Cl/K (0.11-0.12), Br/Cl (0.0024), and I/Cl (5-6 3 10 25 ) ratios that preclude significant assimilation of seawater-derived Cl. The new HIMU glasses that are least degassed for H 2 O have low H 2 O/Ce ratios (75-84), similar to ratios identified in end-member OIB glasses with EM1 and EM2 signatures, but significantly lower than H 2 O/Ce ratios (119-245) previously measured in melt inclusions from Mangaia. CO 2 -H 2 O equilibrium solubility models suggest that these HIMU glasses (recovered in two different dredges at 2500-3600 m water depth) have eruption pressures of 295-400 bars. We argue that degassing is unlikely to significantly reduce the primary melt H 2 O. Thus, the lower H 2 O/Ce in the HIMU Tuvalu glasses is a mantle signature. We explore oceanic crust recycling as the origin of the low H 2 O/Ce (50-80) in the EM1, EM2, and HIMU mantle domains.

Seawater cycled throughout Earth’s mantle in partially serpentinized lithosphere

“…The similarity of these elements relative mantle incompatibilities is illustrated for the silicate melts investigated in Fig 3 (see also Fig S3, electronic supplement) 4,23,27 . This figure also shows that F, which has a significantly higher compatibility, is more similar to Pr than to Nd, which has been widely used for comparison with F previously 11,18,19,26 (Fig 3). The implied water-and halogen-enrichment of some HIMU sources is at odds with these sources, which are depleted in most fluid mobile elements and characterised by high Nb/U and Ce/Pb (Fig 4c), being generated solely by dehydration of subducted oceanic crust (ref.…”

“…Elements with similar mantle incompatibility like Cl, Br, I and K (or Nb) are not easily fractionated by mantle melting, so if assimilation processes are shown to be minor (Fig 2), the relative abundances of these elements in a mantle melt can be considered representative of the mantle source 4,13,[17][18][19][20]23,25 .…”

“…Data was collected over four 20 sec integration periods in static mode (total measurement time 80 sec). The measured 19 F signal was calibrated using 18 O as the internal standard and a suite of glasses previously measured for F at the Carnegie Institute of Washington 26,52 , and three widely available glasses (SRM 610, 612 and BHVO) ( Fig S5). The reported measurements for each sample are the average of 3-5 measurements on different glass chips.…”

“…New glasses were also selected from the East Pacific Rise, Mid Atlantic Ridge, Southwest and Southeast Indian Ocean Ridges, Ontong Java Plateau and Reunion Island. The previously published data include enriched MORB from Macquarie Island, HIMU-influenced MORB from the equatorial Atlantic, HIMU-type OIB from Mangaia and Tuvalu, EM-type OIB from Samoa, Society and Pitcairn, and primitive 'OIB' with high 3 He/ 4 He from Baffin Island, Samoa and the NW Lau Basin (Fig 1) 11,12,[17][18][19][20][21] . In total we have evaluated data from ~400 glasses enabling us to define global baselines for mantle halogen and H 2 O abundance ratios and elucidate previously unrecognised processes involved in the generation of HIMU and globally variable MORB mantle reservoirs.…”

“…The volatile, isotope and trace element systematics of the mantle. a,b) The glasses H 2 O/Ce is shown as a function of 208 Pb/ 207 Pb and 87 Sr/ 86 Sr enrichment ( 208 Pb/ 207 Pb is used to enable plotting of Tuvalu and Mangaia data18,19 ). c) Nb/U-Ce/Pb plot separating HIMU (high Nb/U and Ce/Pb) and EM-type reservoirs that are closer to Primitive Mantle (PM) and Continental Crust (CC).…”