Secret intelligence reports saying that the Soviet Union successfully manipulated the cobalt market just before the Soviet-backed invasion of cobalt-producing Zaire not only provide additional evidence of Moscow's complicity but also contribute to President Carter's hardened mood.

That changed mood could kill a Washington summit meeting with Soviet President Leonid Brezhnev, long planned to precede the November election, at which a new strategic arms limitation treaty, SALT II, would be signed. Behind the president's new mood is the suspicion that the Kremlin is playing fast and loose behind his back, taking advantage of his sincere efforts to meet the Russians half way.

The bloody Soviet-Cuban mischief in Shaba province has hardened Carter's heart. Unlike a few key African specialist in the State Department who warn against provoking Soviet-Cuban retaliation (in Rhodesia, for example), Carter no longer doubts that Moscow and Havana plotted the Shaba invasion. Furthermore, intelligence sources have supplied the White House with a fascinating piece of evidence.

Just before the Soviet-Cuban-directed invasion of May 14 into the mineral-producing heartland of Zaire, Soviet agents quietly purchased nearly 400 tons of scarce cobalt from European brokers. Since Zaire produces half the world's cobalt, one European intelligence bureau estimates that right after the invasion the price of cobalt skyrocketed to three time its pre-invasion value.

Here was a clear case of foreknowledge. The amount of foreign exchange saved by Moscow was a piddling $8 million or so, but that is not the point. What angers Carter is the transparent fraud of assurances that the communists had nothing to do with the Shaba operation, topped by Fidel Castro's personal protest of innocence to the U.S. diplomatic officer in Havana.

The president and his advisers have carefully studied Western intelligence reports about visits to Moscow late last month of both President Agostinho Neto, Marxist boss of Angola, and Isidoro Malmierca, Castro's foreign minister. These two visits set the stage for the Angola-based invasion, making a mockery of communist claims of innocence.

Going well beyond public expressions of dismay over Soviet skulduggery in Africa, private signals from Carter inside his official family show a president losing patience and contemplating major course corrections.

The proposed Washington summit might yet be held. But instead of plotting to bring it off as the high point of his first two years, the president is now analyzing reasons for postponing it. Reason No. 1: Given rising anti-Soviet emotions in Congress, SALT II would have trouble in the Senate, where the required two-thirds vote does not now exist.

Till now Carter has seemed entirely willing to take the chance of signing the treaty this summer, campaign on it during the congressional election campaign, then send it next year to the Senate, where he has always known the reception would be hostile. What has changed, then, is not so much the Senate but Jimmy Carter himself.

Although SALT would be by far the most important sacrifice by a president determined to restrain Moscow, U.S. policy toward black Africa is also in the balance. That policy so far has been set and enforced by U.N. Ambassador Andrew Young, Assistant Secretary of State Richard Moose and the State Department's policy-planning chief, Anthony Lake.

Their aides privately complained last week that Carter's decision to alert elements of the 82nd Airborne Division for possible rescue duty in Shaba province was made without State Department advice or consent.

In fact, Vance not only approved but proposed standby U.S. military power to rescue Europeans in Shaba province if needed. He did so without first checking the department's African bureau - an oversight that had earmarks of careful planning. Taking his cue from the president, Vance is moving toward a tougher U.S. policy on Africa within the State Department in the same way that Carter is moving away from any automatic decision to gift-wrap SALT II and hand it to Brezhnev in late summer.

Late as they come, these possible turns in American policy could yield Carter rich rewards: a new lease on the confidence of his Western European allies and such valuable U.S. friends as Iran and Saudi Arabia, all terrified at the prospect of a rampant Soviet Union unchecked by countervailing American power.