Physics
Scientific paper
Dec 2006
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2006agufm.v23e0696n&link_type=abstract
American Geophysical Union, Fall Meeting 2006, abstract #V23E-0696
Physics
1025 Composition Of The Mantle, 1032 Mid-Oceanic Ridge Processes (3614, 8416), 3614 Mid-Oceanic Ridge Processes (1032, 8416), 3619 Magma Genesis And Partial Melting (1037), 8416 Mid-Oceanic Ridge Processes (1032, 3614)
Scientific paper
Trace element studies of residual clinopyroxene (cpx) in abyssal peridotites (AP) [1,2] confirmed the earlier notion that AP are mantle melting residues for MORB [3-5], and also argued that near-perfect fractional melting best explains the cpx trace element data, e.g., progressively more depleted in the lighter rare earth elements (REEs). However, a recent study [6] showed that light REEs in bulk-rock AP samples are more enriched, not more depleted, than in the constituent cpx of the same sample suites previously studied. If the cpx light REEs record sub-ridge mantle melting processes, then the bulk-rock light REEs must reflect post-melting refertilization [6]. The significant correlations of light REEs (e.g., La, Ce, Pr, Nd) with immobile high field strength elements (HFSEs, e.g., Nb and Zr) in bulk-rock AP samples suggest that enrichments of both light REEs and HFSEs resulted from a common magmatic process [6]. The post-melting magmatic refertilization has been suggested previously [7-10] to be important, and is interpreted to take place in the "cold" thermal boundary layer beneath ridges where the ascending melts migrate through and interact with the advanced residues [6]. The apparent problem, however, is why such magmatic refertilization did not affect cpx analyzed for trace elements. Niu [6] interpreted that the ascending melts may not be thermally "reactive", and thus may have only affected "rims" of cpx. These affected rims were later serpentinized together with the rest of the rock, and the analyzed portions of cpx are only "cores" that were unaffected by the ascending melts and also survived subsequent serpentinization. I report here a preliminary LA-ICP-MS study that supports the above interpretation. That is, in some AP samples, while relics of cpx and orthopyroxene (opx) are depleted in light REEs, serpentines (ser) adjacent to cpx and opx relics are less depleted or enriched in light REEs. Below are representative data from three samples. Sample YN1: [La/Sm]N = 0.009 (cpx), 0.014 (opx), 10.0 (ser); [Sm/Yb]N = 0.403 (cpx), 0.081 (opx), 0.741 (ser); [Yb]N = 5.36 (cpx), 1.32 (opx), 0.13 (ser). Sample YN9: [La/Sm]N = 0.007 (cpx), 0.013 (opx), 0.395 (ser); [Sm/Yb]N = 0.648 (cpx), 0.255 (opx), 0.110 (ser); [Yb]N = 7.35 (cpx), 2.15 (opx), 0.09 (ser). Sample YN14: [La/Sm]N = 0.595 (cpx), 1.630 (opx), 4.19 (ser); [Sm/Yb]N = 0.032 (cpx), 0.044 (opx), 0.212 (ser); [Yb]N = 2.62 (cpx), 0.82 (opx), 0.09 (ser). Note also that trace element distributions are quite heterogeneous on various fine scales for all samples (thin sections) studied. These new data and existing observations [6,8-10] point to the significance of grain-boundary porous melt migration in the "cold" thermal boundary layer beneath ocean ridges. References: [1] Johnson et al., J. Geophys. Res., 95, 2661-2678, 1990; [2] Johnson &Dick, J. Geophys. Res., 97, 9219-9241, 1992; [3] Dick et al., Earth Planet. Sci. Lett., 69, 88 -106, 1984; [4] Dick &Fisher, In International Kimberlite Conference vol. 2., 295-308, 1984; [5] Dick, Geol. Soc. Spec. Publ., 42, 71-105, 1989; [6] Niu, J. Petrol., 40, 2423-2458, 2004; [7] Elthon, J. Geophys. Res., 97, 9015 9025, 1992; [8] Niu & Hékinian, Earth Planet. Sci. Lett., 146, 243-258, 1997; [9] Niu, J. Petrol., 38, 1047-1074, 1997; [10] Niu et al., Earth Planet. Sci. Lett., 152, 251-265, 1997.
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